FUNDAMENTAL STUDIES AND POTENTIAL APPLICATIONS OF CLOUD POINT

EXTRACTION

By

MELISSA ENSOR FREIDERICH

A dissertation submitted in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

WASHINGTON STATE UNIVERSITY Department of Chemistry

DECEMBER 2011

To the Faculty of Washington State University:

The members of the Committee appointed to examine the dissertation of MELISSA ENSOR FREIDERICH find it satisfactory and recommend that it be accepted.

______Kenneth L. Nash, Ph.D., Chair

______Sue B. Clark, Ph.D.

______James O. Schenk, Ph.D.

______Glenn Fugate, Ph.D.

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ACKNOWLEDGMENT

“There are two possible outcomes: if the result confirms the hypothesis, then you've made a measurement. If the result is contrary to the hypothesis, then you've made a discovery.”

Enrico Fermi

“Failure is simply the opportunity to begin again, this time more intelligently.”

Henry Ford

“A man who carries a cat by the tail learns something he can learn in no other way.”

Mark Twain

The preceding quotes, to me, sum up my graduate school experience. The past five years have not been easy and often times seemed impossible. I need to acknowledge and thank the people who helped me get to where I am today because without them I would not have completed this journey.

First of all, I want to thank my husband, John. He has been my rock and sometimes only friend and source of comfort during my time in Pullman. John never wavered in his support of me and usually believed in me more than I believed in myself. I would not have completed my Ph.D. without his unwavering love and encouragement. John, for putting up with me the last four years, you totally deserve a Camaro.

If it were not for my parents, I would not be here at all. Additionally, they have been a source of constant support and encouragement the past five years. Thank you, Mom and Dad, for everything you have done for me. I hope this makes you proud. Also, thank you for not pushing or trying to encourage me to go into chemistry. That would almost certainly have backfired and then I would not have met John or be writing this, so thanks for that.

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Of course I cannot forget my advisor, Kenneth Nash. While we have had our differences over the years, graduate school would not have been what it was without him. I have learned and grown a lot as a scientist and person during the years I have spent working in the Nash group.

Thank you, Ken, for your assistance and guidance over the past five years.

I would like to thank Linfeng Rao, Guoxin Tian, Sergey Sinkov, and Greg Lumetta for allowing me to come to their respective laboratories and perform experiments that would have otherwise not been possible.

Finally, I would like to thank the members of my committee Ken Nash, Sue Clark, Jim

Schenk, and Glenn Fugate, for agreeing to be on my committee and committing to reading this thesis in its entirety.

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FUNDAMENTAL STUDIES AND POTENTIAL APPLICATIONS OF CLOUD POINT

EXTRACTION

Abstract

by MELISSA ENSOR FREIDERICH, Ph.D. Washington State University December 2011

Chair: Kenneth L. Nash

Separation of the components of nuclear fuel provides an interesting and complicated separation challenge as the trivalent lanthanides and actinides must be separated from each other.

The difficulties of this separation have prompted exploration of novel separation techniques, such as cloud point extraction (CPE). CPE is an aqueous separation technique that utilizes a surfactant, in place of an organic solvent, for phase separation. The technique has shown utility for transition metal separations and some applicability to lanthanide separation. While numerous separation schemes have been demonstrated with CPE, very little fundamental research has been done to expand understanding of the mechanism of extraction and phase separation in the system. Current CPE systems have been developed using a “trial and error” approach with no insight into how to improve the separation.

The focus of this dissertation was to develop a better fundamental understanding of the mechanism of extraction in CPE systems, as a means of improving separation system design. To achieve this goal a CPE system for lanthanide and actinide separation was designed and studied.

The influence of electrolytes on the extraction behavior and system behavior was examined along with the influence of the surfactant on metal ligand complexation using radiometric

v techniques, potentiometric titrations, uv-visible spectroscopy, and fluorescence spectroscopy.

These studies combined to help give insight into a seemingly simple extraction system, showing

CPE is in fact much more complicated than initially expected. The goal of these studies was to provide a better understanding of the CPE system to facilitate improved system design.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS ...... iii

ABSTRACT ...... v

LIST OF TABLES ...... xiii

LIST OF FIGURES ...... xv

I. INTRODUCTION ...... 1

I.1 CLOUD POINT EXTRACTION (CPE) ...... 2

I.2 CLOUD POINT TEMPERATURE (CPT) AND ELECTROLYTE EFFECTS ...... 6

I.3 CPE OF METAL IONS ...... 9

I.4 RESEARCH SCOPE ...... 16

I.5 LANTHANIDE CARRIER LIGANDS ...... 17

II. EXPERIMENTAL PROCEDURES AND METHODS ...... 19

II.1 MATERIALS ...... 19

II.2 INITIAL CPE EXPERIMENTS ...... 20

II.3 PMBP CPE EXPERIMENTS ...... 22

II.4 CPT DETERMINATION ...... 23

II.5 KARL FISCHER ANALYSIS ...... 24

II.6 FT-IR ANALYSIS ...... 24

II.7 pCH MEASUREMENTS ...... 25

II.8 POTENTIOMETRIC TITRATIONS ...... 25 vii

II.9 UV-VISIBLE SPECTROSCOPY ...... 26

II.10 FLUORESCENCE MEASUREMENTS ...... 27

III. RESULTS AND DISCUSSION

III.1 EXPLORATIONS OF CLOUD POINT EXTRACTION AND APPLICATION TO LANTHANIDES AND ACTINIDES

III.1.1 RESULTS ...... 29

III.1.1.18-HQ ...... 29

III.1.2 SYSTEM DESIGN ...... 31

III.1.2.1 Surfactant Selection ...... 31

III.1.2.2 Ligand ...... 33

III.1.2.3 Salt Effects ...... 36

III.1.3 CLOUD POINT TEMPERATURE ...... 37

III.1.3.1 Alcohol Study ...... 37

III.1.3.2 Electrolytes Study ...... 38

III.1.4 CONCLUSIONS...... 41

III.2 DEVELOPMENT OF A CPE SEPARATION TECHNIQUE USING 1-PHENYL-3- METHYL-4-BENZOYL-5-PYRAZOLONE TO EXTRACT EU(III) AND AM(III)

III.2.1 SYSTEM OPTIMIZATION ...... 47

III.2.2 SLOPE ANALYSIS ...... 52

III.2.3 ACTINIDE PARTITIONING ...... 54

III.2.4 METAL LOADING ...... 55

III.2.5 CONCLUSIONS...... 57

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III.3 EFFECTS OF STRUCTURE-BREAKING AND STRUCTURE-MAKING ELECTROLYTES ON A NONIONIC CLOUD POINT EXTRACTION SYSTEM

III.3.1 RESULTS ...... 60

III.3.2 DISCUSSION ...... 69

III.3.2.1 Cloud Point Temperature ...... 69

III.3.2.2 CPE of Eu(III) ...... 71

III.3.3 CONCLUSIONS...... 75

III.4 FT-IR SPECTROSCOPIC STUDY OF THE SURFACTANT RICH PHASE (SRP) FROM A CLOUD POINT EXTRACTION SEPARATION IN THE PRESENCE OF ELECTROLYTES

III.4.1 RESULTS ...... 78

III.4.2 DISCUSSION ...... 84

III.4.2.1 Neat Triton X-114 and SRP ...... 84

III.4.2.2 SRP Formed with Salts ...... 84

III.4.3 CONCLUSIONS...... 88

III.5 PROTONATION CONSTANTS OF DIGLYCOLIC ACID (DGA) IN MICELLAR SOLUTIONS OF THE NONIONIC SURFACTANT, TRITON X-114

III.5.1 RESULTS ...... 91

III.5.1.1 Pseudophase Model ...... 91

III.5.1.2 Low Surfactant Concentration at Low Temperature ...... 92

III.5.1.3 High Surfactant Concentration at High Temperature ...... 93

III.5.2 DISCUSSION ...... 93

III.5.3 CONCLUSIONS...... 95

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III.6 COMPLEXATION OF LANTHANIDES WITH DIGLYCOLIC ACID IN AQUEOUS MICELLAR SOLUTIONS CONTAINING THE NONIONIC SURFACTANT, TRITON X-114

III.6.1 RESULTS ...... 98

III.6.1.1 UV-visible Titrations ...... 98

III.6.1.2 Potentiometric Titrations ...... 102

III.6.2 DISCUSSION ...... 105

III.6.2.1 UV-visible Spectra ...... 105

III.6.2.2 Potentiometric Titrations ...... 107

III.6.3 CONCLUSIONS...... 109

III.7 FLUORESCENCE OF EU(III) WITH DIGLYCOLIC ACID IN SURFACTANT SOLUTIONS: INSIGHTS INTO CPE

III.7.1 BACKGROUND ...... 110

III.7.1.1 Lifetime Data ...... 110

III.7.2 RESULTS ...... 111

III.7.2.1 Fluorescence ...... 111

III.7.2.2 Features of Emission Spectra ...... 117

III.7.3 DISCUSSION ...... 123

III.7.3.1 Surfactant and Metal Behavior without Complexation ...... 123

III.7.3.2 Electrolyte and Surfactant Behavior without DGA ...... 126

III.7.3.3 Eu and DGA Complexation in Water ...... 127

III.7.3.4 Eu/DGA/Surfactant ...... 129

III.7.3.5 Eu/DGA/Surfactant/Salts ...... 130

III.7.4 CONCLUSIONS...... 131

IV. CONCLUSIONS ...... 132

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V. REFERENCES...... 139

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LIST OF TABLES

Page

I. INTRODUCTION ...... 1

Table I.1. Examples of successful CPE systems for transition metals; APDC (ammonium pyrrolidine dithiocarbamate), 5-Br-PADAP (2-(5-bromo-2-pyridylazo)-5- (diethylamino)phenol), DDTC (diethyldithiocarbamate), DDTP (o,o- diethyldithiophosphate), 8-HQ (8-hydroxyquinoline), PAN (1-(2-pyridylazo)-2-naphthol, PMBP (1-phenyl-3-methyl-4-benzoyl-5-pyrazolone), PONPE 7.5 (polyethylene glycol mono-p-nonylphenyl ether), TAN (2-(2-thiazolylazo)-2-naphthol.) ...... 10

Table I.2. CPE systems for lanthanide and uranium separation/pre-concentration reported in the literature using a variety of surfactants and chelating agents ...... 14

III.1. EXPLORATIONS OF CLOUD POINT EXTRACTION AND APPLICATION TO LANTHANIDES AND ACTINIDES ...... 29

Table III.1.1. Alcohol study with 2% Triton X-114 and water ...... 38

III.3. EFFECTS OF STRUCTURE-BREAKING AND STRUCTURE-MAKING ELECTROLYTES ON A NONIONIC CLOUD POINT EXTRACTION SYSTEM ...... 59

Table III.3.1. 24Na radiotracer salt partitioning study with 2 wt % Triton X-114, with and without PMBP; error represents the standard deviation of triplicate measurements ...... 65

Table III.3.2. Slope analysis of Eu3+ extraction with varying PMBP and 2 wt % Triton X- 114 in the presence of 0.5 m salt...... 67

Table III.3.3. Karl Fischer analysis: wt % of H2O present in the SRP after separation in the presence of 0.5 m salt and 2 wt % Triton X-114 ...... 69

III.4. FT-IR SPECTROSCOPIC STUDY OF THE SURFACTANT RICH PHASE FROM A CLOUD POINT EXTRACTION SEPARATION IN THE PRESENCE OF ELECTROLYTES ...... 77

Table III.4.1. Frequency (cm-1) and assignments of FTIR bands of neat Triton X-114 and Triton X-114 SRP with electrolytes ...... 83

III.5. PROTONATION CONSTANTS OF DGA IN MICELLAR SOLUTIONS OF THE NONIONIC SURFACTANT, TRITON X-114 ...... 90

Table III.5.1. Protonation constants for DGA with varying surfactant (Triton X-114) concentrations at temperatures below the CPT, μ = 0.1 m ...... 96 xii

Table III.5.2. Protonation constants for DGA in the presence of varying high concentrations of Triton X-114 at temperatures above the CPT, μ = 0.1 m ...... 97

III.6. COMPLEXATION OF LANTHANIDES WITH DIGLYCOLIC ACID IN AQUEOUS MICELLAR SOLUTIONS CONTAINING THE NONIONIC SURFACTANT, TRITON X-114 ...... 98

Table III.6.1. Metal ligand stability constants determined by UV-visible titration with DGA and Triton X-114, 20 mm Nd3+ and Ho3+, 1 mm Am3+, μ = 0.1 m NaCl, 25°C ...101

Table III.6.2. Lanthanide stability constants with DGA determined potentiometrically in the presence of Triton X-114, μ = 0.1 m NaCl ...... 104 Table III.6.3. Lanthanide stability constants with DGA determined potentiometrically in the presence of Triton X-114, μ = 0.1 m NaCl ...... 104

Table III.6.4. Lanthanide stability constants with DGA determined potentiometrically in the presence of Triton X-114, μ = 0.1 m NaCl ...... 105

III.7. FLUORESCENCE OF EU(III) WITH DIGLYCOLIC ACID IN SURFACTANT SOLUTIONS: INSIGHTS INTO CPE ...... 110

Table III.7.1. Decay lifetimes and waters of hydration: 1 mm Eu(ClO4)3, varying background electrolyte, varying Triton X-114 wt%, pH ~ 5 ...... 112

Table III.7.2. Decay lifetimes and waters of hydration: 1 mm Eu(ClO4)3 , 0.1 m NaCl, varying Triton X-114 wt%, varying DGA, pH ~ 3 ...... 115

Table III.7.3. Decay lifetimes and waters of hydration: 1 mm Eu(ClO4)3, 0.5 m salt, Triton X-114, varying DGA ...... 116

Table III.7.4. Speciation calculations with Eu3+ DGA stability constants. All calculations done in Hyss 2009 ...... 129

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LIST OF FIGURES

Page

I. INTRODUCTION ...... 1

Figure I.1.1. Schematic of metal ion extraction in a CPE system ...... 3

Figure I.1.2. General structures and types of surfactants ...... 4

Figure I.1.3. General diagram of a micelle formed with a nonionic surfactant ...... 5

Figure I.1.4. Structure of the Triton X nonionic surfactant series, n = 7.5 average for Triton X-114 ...... 5

Figure I.1.5. Change in CPT as the concentration of Triton X-114 is increased; line represents phase splitting ...... 6

Figure I.1.6. Figure from Reference 27 by Schott demonstrating the influence of electrolytes (NaSCN, NaI) on the CPT of a 2 wt % solution of Triton X-100 as a function of concentration and anion...... 8

Figure I.1.7. Structures of ligands used in previous CPE systems ...... 11

Figure I.1.8. Structure of diglycolic acid (DGA) ...... 18

III. EXPLORATIONS OF CLOUD POINT EXTRACTION AND APPLICATION TO LANTHANIDES AND ACTINIDES...... 29

Figure III.1.1. (Top Graph) Recovery of total 152/154Eu activity as a function of pH (Bottom Graph) Eu extraction with 8-HQ as a function of pH. D value (blue circles); KD values (red triangles) ...... 32

3+ Figure III.1.2. Line indicates the KD value for Eu with no ligand; 2 wt % Triton X- 114 with tracer concentrations of Eu3+ and excess of ligand (varied) 1. Pyrogallo red, 2. Murexide, 3. Arsenazo (III), 4. Pyrocatechol Violet, 5. Pyridylazo, 6. Xylenol Orange, 7. Ethylenediaminetetraacetic acid (EDTA), 8. Phenylphosphinic acid, 9. Dipicolinic acid, 10. Phosphonoacetic acid, 11. Triphenylphosphine, 12. Diaminocyclohexane tetraacetic acid, 13. Sulfanilic acid, 14. Pyromellitic acid, 15. Diethylenetriamine pentaacetic acid, 16. Acetylacetone, 17. 1,10 phenanthroline, 18. Picolinic acid, 19. Isoascorbic acid ...... 34

Figure III.1.3. Eu3+ extraction at various pHs as a function of PMBP concentration. Lines do not represent a model fitting and errors are the standard deviation of triplicate measurements...... 35

xiv

Figure III.1.4. Extraction of Eu3+ using 0.001 M PMBP in ethanol and 2 wt % Triton X- 114 with various salts over a range of salt concentrations. Errors represent the standard deviation from triplicate measurements ...... 36

Figure III.1.5. Change in the CPT of 2 wt % Triton X-114 solution with increasing salt concentration, Salting-in electrolytes ...... 40

Figure III.1.6. Change in the CPT of 2 wt % Triton X-114 solution with increasing salt concentration, Salting-out electrolytes ...... 40

Figure III.1.7 Change in the CPT of 2 wt % Triton X-114 solution with increasing salt concentration ...... 41

III.2 DEVELOPMENT OF A CPE SEPARATION TECHNIQUE USING 1-PHENYL-3- METHYL-4-BENZOYL-5-PYRAZOLONE TO EXTRACT EU(III) AND AM(II) ...... 47

Figure III.2.1. Extraction of Eu3+ with 0.001 m PMBP at varying pHs. Solutions contained 2 wt % Triton X-114 and incubation time was 60 minutes at 60 °C. Error represents one standard deviation from measurement of three replicate samples ...... 49

Figure III.2.2. Extraction of Eu3+ with 0.001 m PMBP and 2 wt % Triton X-114 at pH 3 over a range of incubation times in a 60oC water bath. Error represents one standard deviation from measurement of three replicate samples...... 49

Figure III.2.3. Extraction of Eu3+ with varying concentrations of PMBP using 2 wt % Triton X-114 at pH 3. Incubation time of 20 minutes at 60°C. Error represents one standard deviation from measurement of three replicate samples ...... 51

Figure III.2.4. 152/154Eu3+ in the aqueous phase after phase separation with 2 wt% Triton X-114 and 0.001 m PMBP at pH 3. A: 152/154Eu in 0.4 mL aliquots of the 4 mL aqueous phase. Samples were taken from the top down to the aqueous phase in contact with the SRP. B: 152/154Eu in 0.2 mL aliquots of the aqueous phase taken over an hour while the aqueous phase remained in contact with the SRP. Error represents one standard deviation of triplicate measurements...... 52

Figure III.2.5. Distribution of Am3+ and Eu3+ into the SRP over varying PMBP concentrations with 2 wt% Triton X-114 at pH 3. Error represents one standard deviation of triplicate measurements...... 55

Figure III.2.6. Distribution of varying macroscopic amounts of Eu(NO3)3 with 0.001 m PMBP and 2 wt % Triton X-114 at pH 3. □ - Eu3+ extracted with first contact of the aqueous phase with 0.001 m PMBP in Triton X-114. Δ – Eu3+ extracted from remaining aqueous phase contacted with additional PMBP and Triton X-114. ○ – Total Eu3+ extracted after both contacts ...... 57

xv

III.3. EFFECTS OF STRUCTURE-BREAKING AND STRUCTURE-MAKING ELECTROLYTES ON A NONIONIC CLOUD POINT EXTRACTION SYSTEM ...... 59

Figure III.3.1. Cloud point temperatures of 2 wt % Triton X-114 solutions with salts as a function of their concentration (□NaCl, ○KCl, ▲NH4Cl, ▼NaSCN, ◊ KSCN, ◄ NH4SCN, ► NaNO3, KNO3, * NH4NO3) ...... 61

Figure III.3.2. Partitioning of Eu3+ with 2 wt % Triton X-114 at pH – 3, in the presence of 0.5 m salt as a function of the individual salts’ cloud point temperature. Line represents partitioning of Eu3+ in the absence of salt ...... 62

Figure III.3.3. Partitioning of Eu3+ with 2 wt % Triton X-114 and 0.001 m PMBP, pH – 3, in the presence of 0.5 m salt as a function of the individual cloud point temperatures. Line represents partitioning of Eu3+ in the absence of salt ...... 63

Figure III.3.4. Distribution of Eu3+ with 0.001 m PMBP as a function of salt concentration with 2 wt % Triton X-114 at pH 3 ...... 64

Figure III.3.5. Distribution of Eu3+ with 2 wt% Triton X-114, varying concentrations of PMBP at pH 3 ...... 66

Figure III.3.6. Partitioning of Eu3+ with 0.001 m PMBP, 2 wt % Triton X-114, and 0.5 m salt, pH 3. Samples were heated at temperatures a specified amount above the CPT .....68

III.4. FT-IR SPECTROSCOPIC STUDY OF THE SURFACTANT RICH PHASE FROM A CLOUD POINT EXTRACTION SEPARATION IN THE PRESENCE OF ELECTROLYTES ...... 77

Figure III.4.1. FTIR spectra of neat Triton X-114, a 2 wt % aqueous solution of Triton X- 114, and a SRP formed from a 2 wt % aqueous solution of Triton X-114 ...... 79

Figure III.4.2. FTIR spectra of the SRP formed from a 2 wt % Triton X-114 solution containing varied NaNO3 concentrations ...... 80

Figure III.4.3. Close up of finger print region of the FT-IR spectra of the SRP formed with NaNO3 ...... 80

Figure III.4.4. FTIR spectra of the SRP formed from a 2 wt % Triton X-114 solution containing varied NaCl concentrations ...... 81

Figure III.4.5. Close up of the finger print region of FTIR spectra of a SRP formed with 2 wt % Triton X-114 and varying NaCl concentrations ...... 81

Figure III.4.6. FTIR Spectra of the SRP formed from a 2 wt % Triton X-114 solution containing varied NaSCN concentrations ...... 82

xvi

Figure III.4.7. Close up of the finger print region of FTIR spectra taken of a SRP formed with 2 wt % Triton X-114 and varying NaSCN concentrations ...... 82

III.5. PROTONATION CONSTANTS OF DGA IN MICELLAR SOLUTIONS OF THE NONIONIC SURFACTANT, TRITON X-114 ...... 90

Figure III.5.1. Pseudophase model for micellar systems with a monoprotic acid (m – micellar pseudophase; b – bulk phase) [i]m – micellar molar concentration of species, i, with respect to the volume of the micellar pseudophase, [i]b – bulk molar concentration with respect to the volume of the extramicellar bulk solvent phase ...... 92

III.6. COMPLEXATION OF LANTHANIDES WITH DIGLYCOLIC ACID IN AQUEOUS MICELLAR SOLUTIONS CONTAINING THE NONIONIC SURFACTANT, TRITON X-114 ...... 98

Figure III.6.1. UV-vis spectra of 20 mm Nd(ClO4)3 in 20 wt % Triton X-114 at 0.1 m NaCl ionic strength with 60 mm DGA; Additions of 1 M NaOH were made to adjust the pH ...... 99

Figure III.6.2. UV-vis spectra of 20 mm Ho(ClO4)3 in 10 wt % Triton X-114 at 0.1 m NaCl ionic strength with 60 mm DGA; Additions of 1 M NaOH were made to adjust the pH ...... 100

Figure III.6.3. UV-vis spectra of 1 mm Am(NO3)3 in 10 wt % Triton X-114 at 0.1 m NaCl ionic strength with 60 mm DGA; Additions of 1 M NaOH were made to adjust the pH ...... 101

III.7. FLUORESCENCE OF EU(III) WITH DIGLYCOLIC ACID IN SURFACTANT SOLUTIONS: INSIGHTS INTO CPE ...... 110

Figure III.7.1. Emission spectra for 1 mM Eu(ClO4)3 in water and Triton X-114 wt % solutions ...... 117

Figure III.7.2. Emission spectra of 1 mM Eu(ClO4)3 in a 20% Triton X-114 solution with addition of 0.5 m electrolytes ...... 118

Figure III.7.3. Emission spectra of 1 mM Eu(ClO4)3 in a 40% Triton X-114 solution with addition of 0.5 m electrolytes ...... 119

Figure III.7.4. Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA concentration and 0.1 m NaCl in water ...... 120

Figure III.7.5. Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA and 0.1 m NaCl in water, 10, 20, and 40 wt % Triton X-114 ...... 121

xvii

Figure III.7.6. Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA and 0.1 m NaCl in water, 10, 20, and 40 wt % Triton X-114; 690 nm peak ...... 121

Figure III.7.7. Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA in 20 wt % Triton X-114, varying electrolytes (0.5 m) ...... 122

Figure III.7.8. Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA in 40 wt % Triton X-114, varying electrolytes (0.5 m) ...... 123

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DEDICATION

For my parents, Dale and Ginger, for always believing in me and For my husband, John, who is the best husband I could have ever dreamed of.

This is for y’all.

xix

I. INTRODUCTION

The components of used nuclear fuel represent a complicated separations challenge.

Approximately a third of the elements of the periodic table is contained within nuclear fuel after use due to fission and activation products.1 Of these components, the most important and challenging separation involves the lanthanides and actinides. All lanthanides and the actinides after plutonium exist predominantly in the trivalent oxidation state; additionally, they have similar ionic radii, eliminating use of the two properties most frequently employed by separation methods.2-4 However, this separation is necessary for reprocessing as lanthanides have a very large neutron capture cross section, causing them to act as neutron poisons in a fuel matrix.

Lanthanides are created from the fission processes of uranium and plutonium and grow in as the fuel is burned. This causes the fission process to become less and less efficient due to the capture of neutrons by the lanthanides.5,6 In particular, separation of americium from the lanthanides is important and difficult due to the comparable charge density of Am and the lanthanides. This particular separation becomes important if fuel cycles are operated to transmute trans-plutonium actinides.

Solvent extraction techniques have been developed and used to achieve lanthanide/actinide separation. This method uses two immiscible liquid phases, an aqueous/water phase and an organic solvent phase. For metal ion partitioning, complexants are typically used to achieve separations. The aqueous solution containing the metal ions to be separated is contacted with an organic phase containing a chelating agent or solvating ligand. The complexing agent is not water soluble and typically interacts with the metal at the interface between the two phases. The metal is extracted into the organic phase by complexation with the interfacially active chelating

1 agent. The chelating agent can be selective based on oxidation state, metal ion size, or a number of other factors.

The Trivalent Actinide-Lanthanide Separations by Phosphorus reagent Extraction from

Aqueous Komplexes, or TALSPEAK process, is currently utilized for lanthanide/actinide separations.7-9 The slight difference in covalent nature of the bonding of lanthanides and actinides is exploited in this process. While this method can successfully separate lanthanides and actinides, it uses an organic solvent for extraction creating a mixed waste stream.

Elimination of organic solvents from a nuclear reprocessing scheme could greatly lessen the environmental impact of reprocessing and potentially reduce the amount of hazardous waste for disposal. Because of the potential benefits of an all aqueous separation system, there is a drive for new, aqueous separation techniques for lanthanide/actinide separations. One such technique that has shown promise is cloud point extraction (CPE).

I.1. Cloud Point Extraction

CPE is a simple and straightforward extraction procedure (Figure I.1.1).10 A surfactant, typically nonionic, is added to an aqueous solution containing the species of interest. The concentration of the surfactant must be above the critical micelle concentration (CMC) of the surfactant which allows the surfactant to organize and form micelles. For metal ion extraction, a suitable chelating agent is added. In principle, the complexant should facilitate partitioning of the metal ion into the micelle by forming a hydrophobic complex with the metal. Once homogenized by agitation, the properties of the aqueous solution are changed to reach the cloud point temperature (CPT). This can be done by addition of salt or other surfactants or with heating. Upon reaching the CPT, the solution becomes cloudy and eventually separates into two

2 phases: the aqueous phase and the surfactant rich phase (SRP). The aqueous phase contains surfactant at a concentration less than or equal to the CMC, while the SRP contains the majority of the surfactant. The SRP is significantly smaller in volume than the aqueous phase, allowing for preconcentration of the target analyte. The phase separation is reversible upon mixing and cooling below the CPT.

Figure I.1.1: Schematic of metal ion extraction in a CPE system10

Watanabe, et. al., first published a CPE separation procedure in 1976 using the nonionic surfactant, Triton X-100, with the ligand, 1-2-thiazolylazo-2-naphthol (TAN), to extract Ni(II) .11

Since then, the number of publications relating to CPE has risen steadily, demonstrating various separations targeting metals, proteins, and organic molecules.10 The increased interest in CPE arises from its potential advantages, including the ability to concentrate a variety of analytes with near quantitative recovery, high preconcentration factors, separation using mild conditions, and use of a surfactant rather than an organic solvent.10

There are a number of factors to consider in a CPE system. Optimization of a CPE procedure involves choosing an appropriate concentration and type of surfactant, temperature, ionic strength, equilibration time, chelating agent, and pH. Typically, this has been done using a trial and error approach and systematically optimizing the procedure.

3

I.1.1. Surfactants

There are four types of surfactants: anionic, cationic, zwitterionic (amphoteric), and nonionic (Figure I.1.2).12 In CPE, the focus has been on nonionic surfactants or mixtures of nonionic surfactants with anionic and cationic surfactants.13 Nonionic surfactants are used most often because they are inexpensive, readily available, and have relatively low CPTs and CMCs.13

They are amphiphilic molecules with polar, hydrophilic heads, and hydrophobic tails. The tails are hydrocarbon chains which may be branched or linear and contain aromatic rings. The head groups contain polar groups, such as ethers and alcohols. When present in concentrations above the CMC, the surfactants associate in solution to form normal micelles. The surfactant molecules orient so their hydrophobic hydrocarbon chains are at the core of the micelles with the hydrophilic oxyethylene chains oriented towards the water (Figure I.1.3).13 Water should be excluded from the hydrophobic core but may be incorporated into the outer layers of the micelle.

Nonpolar species can be solubilized within the nonpolar cores of the micelle. Above the CPT, the micelles aggregate to form a separated phase, the SRP.

Figure I.1.2: General structures and types of surfactants12

4

Figure I.1.3: General diagram of a micelle formed with a nonionic surfactant13

Triton X-114, the surfactant used in the studies found within, belongs to one of the most commonly used classes of nonionic surfactants, t-octylphenoxy polyoxyethylene ethers (Figure

I.1.4).14 These surfactants are produced from the polymerization of octylphenol with ethylene oxide and differ only by the number of oxyethylene groups. Triton X-114 has an average of 7.5 oxyethylene units giving it an average molecular weight of 537. The CMC for Triton X-114 is

0.2 mM and the CPT is 23 °C for a 1 wt % solution.15

Figure I.1.4: Structure of the Triton X nonionic surfactant series, n = 7.5 average for Triton X-

11415

5

I.1.2. Cloud Point Temperature and Electrolyte Effects

The CPT for each surfactant is different and is determined by several factors. First of all the CPT of a surfactant depends on its structure. CPT increases with decreasing length of the hydrocarbon chain (tail) or increasing length of oxyethylene chain (polar head).14 For constant oxyethylene content the CPT of a nonionic surfactant can be reduced by decreasing the molecular mass or branching of the hydrophobic moiety. The CPT also depends on surfactant concentration and increases with increasing concentration for the nonionic surfactant, Triton X-

114 (Figure I.1.5).14

40

35

2 L

C o 30

25

20 L CloudPoint, 15

10 0 2 4 6 8 10 12 [Triton X-114], (wt. %)

Figure I.1.5: Change in CPT as the concentration of Triton X-114 is increased; line represents phase splitting14

6

The influence of ionic strength on traditional solvent extraction systems has been extensively studied. Different electrolytes, through salting-in and -out effects, can change the distribution of metal ions by increasing or decreasing metal partitioning.16 Previous, studies on the influence of ionic strength on the extraction efficiency in a CPE system have been inconclusive.17-19 Studies have been done on the effect of electrolytes on the CPT, CMC, and micelle formation in CPE systems and have revealed significant influences by electrolytes on these properties of the aqueous surfactant system.20-24

Schott, et. al., published a series of papers detailing the influence of electrolytes on the

CPT of the nonionic surfactant Triton X-100 (Figure I.1.6).25-32 An increase in the CPT indicates salting-in (increasing the water miscibility of the surfactant) by the electrolyte. A decrease in the

CPT is caused by salting-out, decreased water miscibility of the surfactant. For the addition of electrolytes, the effect of anions on the CPT is generally greater than that of the cations. Anions change water structure by either promoting or disrupting the organization of water molecules, in essence shifting the equilibrium n H2O  (H2O)n. Structure-making anions salt-out by increasing structure in the solution, thus increasing the viscosity and surface tension of water. When the viscosity and the surface tension of the water are increased, the amount of energy required for pocket formation within the structure of water is increased. The greater energy requirement makes it harder for surfactant molecules and micelles to create pockets of space in the water and

- - 2- effectively pushes them out of the water. Structure-making anions include Cl , F , SO4 , and

2- 25 CO3 . Structure-breaking anions salt-in by disrupting the structure of the surrounding water, increasing the relative concentration of non-hydrogen bonded water molecules. As the concentration of free water molecules is increased, more water molecules are available to hydrogen bond with the ether groups of the nonionic surfactants. This increases the interaction

7 of the surfactant with water, effectively keeping the surfactant molecules from interacting with

- - - 25 each other and forming micelles. Structure-breaking anions include SCN , I , and ClO4 . As electrolytes change the CMC and CPT of surfactants through solution structural changes, it is logical to assume they may also influence extraction behavior.

Figure I.1.6: Figure from Reference 27 by Schott demonstrating the influence of electrolytes (NaSCN, NaI) on the CPT of a 2 wt % solution of Triton X-100 as a function of concentration and anion.

8

I.1.3. Cloud Point Extraction of Metal Ions

The CPE systems reported in the literature use a relatively narrow range of ligands for metal extractions.33 For the hydrated polyvalent metal ions that are the typical targets of extraction, complexation with hydrophobic ligands is needed to allow their partitioning to the

SRP. Some classes of ligands that have been successfully used in CPE include derivatives of carbamates, pyridylazo, quinoline, and napthol. An effective ligand must have some solubility in an aqueous solution, but also form adequately hydrophobic metal complexes that prefer the less polar SRP. In most of the CPE literature a trial and error approach has been employed to identify ligands that will chelate the metal ion of interest and preferentially transfer to the SRP.

In general, CPE procedures in the literature address separations of transition metals

(Table I.1.1).33 A typical CPE procedure for transition metal separation starts with an aqueous solution containing the metal. Ligand and surfactant are added to the aqueous solution in suitable quantities. Depending on the solubility of the ligand, it may be added pre-dissolved in the neat surfactant or to the aqueous solution. The solution is mixed to homogenize and then heated above the cloud point temperature to promote phase separation. Once removed from heat, the sample is either placed on ice to increase the viscosity of the SRP or centrifuged to enhance phase separation. The SRP is typically on the bottom and the aqueous phase can be easily removed from the SRP for analysis. High performance liquid chromatography (HPLC), ICP-

AES/OES, flame atomic absorption spectroscopy, UV-visible spectroscopy, and graphite furnace atomic absorption spectroscopy have all been used to analyze the aqueous phase in CPE.34 Due to its viscosity the SRP is difficult to analyze and must be diluted for analysis with ICP or HPLC.

With preconcentration through CPE, detection limits of ppt are more easily achievable for metals.34 Phase ratios of 20:1 aqueous phase:SRP and concentration factors over 100 have been

9 seen in CPE systems.35 Successful back extraction of the metal ions from the SRP has been demonstrated using a dilute acid.36 The acid is mixed with the SRP and another phase separation is induced, extracting the metal ion back into the aqueous phase.

Metal Ions Chelating Agent Surfactant

Al, Cd, Cr, Pb, Mn, Ni, PMBP Triton X-114, X-100 Fe

As, Sb, Cd, Cr, Co, Hg, APDC Triton X-114, X-100 Se

Bi, Cu, Pb, Hg, Ag, Cd, Dithiozone Triton X-114, X-100 Ni

Bi, Cd, Cr, Hg, Ag, Sb, DDTC Triton X-114, X-100 Se

Cd, Hg, Ag, Pb DDTP Triton X-114

Cd, Co, Mn, Hg, Rh, PAN Triton X-114, X-100 Sn, V, Cu, Ni, Zn

Cd, Pb, Hg, Ni, Cr, Co, 5-Br-PADAP Triton X-114, PONPE 7.5 Fe, Pt

Cd, Pb, Ag TAN Triton X-114

Al, Zn, Sc, Y, Mo, Sn 8-HQ Triton X-114, X-100

Table I.1.1: Examples of successful CPE systems for transition metals: APDC (ammonium pyrrolidine dithiocarbamate), 5-Br-PADAP [2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol], DDTC (diethyldithiocarbamate), DDTP (o,o-diethyldithiophosphate), 8-HQ (8- hydroxyquinoline), PAN [1-(2-pyridylazo)-2-naphthol], PMBP (1-phenyl-3-methyl-4-benzoyl-5- pyrazolone), PONPE 7.5 (polyethylene glycol mono-p-nonylphenyl ether), TAN [2-(2- thiazolylazo)-2-naphthol].35

Table I.1.1 gives a brief overview of some of the CPE systems that have been developed and applied to transition metal ion separation. CPE has been applied to the majority of transition

10 metals successfully. A comparatively small number of ligands have been used repeatedly for different metal systems and their structures are shown below (Figure I.1.7). No conclusive evidence as to the key characteristics of a chelating agent for a successful CPE procedure has been presented.

Structures

PMBP APDC

Dithizone

DDTC DDTP

PAN 5-Br-PADAP

11

TAN 8-HQ

Figure I.1.7: Structures of ligands used in previous CPE systems

The extraction efficiency for metal ions has been calculated several ways. There is no consensus in the literature as to how distribution or extraction percentages should be reported for cloud point data. Due to the viscosity of the SRP, analysis is usually done by measuring the aqueous phase before and after phase separation. Some authors have calculated extraction (E) and distribution ratios by taking the difference in volume into account using the following equation:37

(1)

The concentration factor (CF) compared to an equal phase volume system was calculated by the equation:

(2)

Ci - initial concentration of metal in the homogenous aqueous solution Cf - concentration of metal remaining in the aqueous phase after separation Vi - initial volume of the homogenous aqueous solution prior to phase separation Vf - volume of the aqueous phase after phase separation

Another approach to defining percent metal extraction does not take into account the difference in volume, but only compares the concentration of metal ion in the aqueous phase before and after extraction.38

12

(3)

E % = Extraction percentage Ci – initial concentration of metal in solution Cf – concentration of metal in aqueous phase after separation

This approach must assume mass balance as the SRP is not analyzed. The SRP can be diluted with acid for direct analysis using GFAAS, ICP, or Flame AA. If the SRP is analyzed the % extracted is:39

(4)

Mn+ - Concentration of the metal ion

CPE has been applied to the partitioning of La3+, Gd3+, Dy3+, Er3+, Ce4+, Eu3+, and Yb3+ from various systems, but not to their mutual separation (Table I.1.2).36,38,40-49 The only actinide whose partitioning in a CPE system has been reported is uranium, most commonly with 8- hydroxyquinolinol for complexation prior to separation.37,50,51

CPE has also been used to separate different oxidation states of some metals and metalloids, namely Cr (VI, III) and Sb (V, III).52-56 The methods, using a ligand and pH controlled processes, extracted the trivalent oxidation state, leaving behind the pentavalent and hexavalent oxidation states. As many of the proposed extraction schemes for nuclear fuel reprocessing include separation of elements by oxidation state (i.e., III from IV,V,VI). This approach is very promising for application of CPE to reprocessing.

13

Metal Ligand Surfactant Analysis Gd(III), La(III) p-sulfonato- Triton X-100 UV-vis thiacalyx[6(8)]arenes

La, Nd, Eu, APDC Triton X-114 ICP-MS Tm(III)

La(III), Eu(III), HDEHP Triton X-100 ICP-AES Lu(III)

Ce(IV) n-TBHA Triton X-114 ICP-OES

+ UO2 Metal chelating X-ray fluorescence thermo-responsive spectroscopy surfactants

U(VI) CTAB Triton X-114 UV-vis

U(VI) Br-PADAP Triton X-114 Molecular absorption spectrometry La(III), Gd(III), Water-soluble Triton X-100 UV-vis Yb(III) calixarenes

Ln(III) PAN PONPE 7.5 and NAA PONPE 20

Dy(III) Br-PADAP PONPE 7.5 Flow injected analysis with CE

Gd(III) 2(3,5-dichloro-2- PONPE 7.5 UV-vis pyridylazo)-5- dimethylaminophenol

Er(III) 2(3,5-dichloro-2- PONPE 7.5 Absorptiometric pyridylazo)-5- determination dimethylaminophenol

La(III), Gd(III) 8-HQ Triton X-114 ICP-AES

CE – capillary electrophoresis; NAA – neutron activation analysis Di(2-ethylhexyl)phosphoric acid (HDEHP), n-p-tolylbenzohydroxamic acid (n-TBHA), cetyl trimethylammonium bromide (CTAB)

Table I.1.2: CPE systems for lanthanide and uranium separation/pre-concentration reported in the literature using a variety of surfactants and chelating agents38-53

14

There have only been a limited number of reports thus far which have gone beyond the typical system optimization for CPE (i.e., equilibrium time, temperature, pH, surfactant concentration, ligand type and concentration). A few researchers have attempted to apply traditional solvent extraction techniques to ascertain the stoichiometry of the metal ligand complex. Ohashi, et. al., used HDEHP to extract lanthanum, europium, and lutetium with Triton

X-100 in a CPE system.41 HDEHP is not water soluble and the authors did not indicate how the ligand was added into the aqueous system. When performing a slope analysis, the authors calculated a slope of three, indicating that a neutral complex was formed and extracted into the

SRP. The authors suggested that the mechanism for extraction of lanthanides by HDEHP was the same as that seen in a traditional organic aqueous extraction system. This seems highly unlikely as both phases are still aqueous in a CPE system. Very little research has been done directly addressing the composition of the SRP, but one paper by Perez-Gramatges, et. al., did look at the water content of a SRP formed using low concentrations of the surfactant PONPE

7.5.57 They reported the SRP contained as much as 81 % water. Even if the SRP they created with 1 wt % Triton X-100 only contained 50 % water, it is highly unlikely that a neutral lanthanide HDEHP species would be soluble. The discrepancies and lack of research on CPE systems leave a wide field of research open.

Not only are the mechanisms of metal ion extraction unclear in CPE, the process of phase separation itself is not well understood. Many theories have been advanced to explain how and why phase separation occurs in CPE. It has been speculated that phase separation is caused by an increase in micellar size and dehydration of outer micellar layers with the increase in temperature.35 Theoretically, at higher temperatures, the ethyl ether (EtO) segments dehydrate and their mutual attraction increases, promoting micellar growth and phase separation. Another

15 theory is that each surfactant is surrounded by a lattice of water molecules hydrogen bonded to the polar group of the surfactant.10 As temperature increases, the water molecule lattice is destroyed by entropy, allowing weak van der Waals forces to prevail among the surfactant molecules. These theories are supported by very little experimental work and further investigation is needed to fully elucidate the mechanism of cloud point separation.

I.1.4. Research Scope

To fully exploit the potential of CPE for lanthanide and actinide separation, it is important to understand what characteristics of a ligand will increase its tendency to preferentially partition into the SRP, while still interacting strongly with the metal cation. Thus far, no systematic studies of the fundamental drivers for these systems have been completed.

How or why particular ligands accomplish transport of lanthanides and actinides to the surfactant rich phase (SRP) is not understood. At present, knowledge of what characteristics of the system can be manipulated to improve the selectivity of the SRP for a lanthanide over an actinide or vice versa is unavailable. The primary objectives of this work were to develop a better understanding of the mechanism of metal complex extraction in CPE and to gain insight into what drives phase separation and extraction.

To help accomplish these goals, a wide variety of analytical techniques have been employed. Initially, CPE studies using PMBP to extract radiotracer Eu3+ under a variety of conditions in the presence of electrolytes were done to monitor the influence of electrolytes on the partitioning of Eu3+. Karl Fischer analysis, FT-IR spectroscopy, and 24Na+ radiotracer studies were all used to examine the composition of the SRP formed in the presence of various electrolytes. The CPT of surfactant solutions was determined in the presence of electrolytes.

16

Traditional solvent extraction techniques, such as slope analysis, were used to help elucidate the mechanism of extraction of PMBP and Eu3+ into the SRP in CPE. Surfactant solutions ranging from 2 wt % Triton X-114 to 40 wt % were examined with lanthanide DGA complexes using

UV-visible spectroscopy, potentiometric titrations, and fluorescence spectroscopy. The combination of techniques allowed insight into metal-ligand complexation and the metal inner hydration sphere in the presence of large quantities of surfactants, similar to the composition of the SRP.

I.1.5. Lanthanide carrier ligands investigated

1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP)

PMBP was used in the initial studies of a CPE system to extract europium. It has been used previously for CPE of transition metals and for traditional solvent extraction of lanthanides and actinides.58-65 PMBP has very limited solubility in water and, thus, limited solubility in the

CPE system and the SRP. In a previous article by Sun, et. al., PMBP was used with Triton X-

100 to extract manganese (II) from water samples.59 The PMBP was dissolved in ethanol and added as an ethanol solution. The authors found that a PMBP concentration of 5 x 10-4 M at a pH between 3 and 7 was sufficient for quantitative extraction. For subsequent studies which required higher concentrations of ligands, a different ligand was chosen because of the solubility constraints with PMBP.

Diglycolic Acid (DGA)

Results in the early stages of this investigation pointed toward the general inadequacy of guiding principles for the identification of appropriate structural characteristics of carrier ligands

17 for lanthanides in CPE. It was determined that a potentially productive pathway for advancing this objective was to examine the properties of a well-known complexing agent that might offer a variety of strategies for further functionalization to modify phase compatibility. For these studies, diglycolic acid (DGA) was used (Figure I.1.8). It is tridendate and the chemistry of its lanthanide aqueous phase chemistry has been extensively investigated, ensuring that reliable thermodynamic data describing aqueous complexes between DGA and lanthanides is available.66-72 In addition, DGA has high solubility in aqueous solutions and can be derivatized to make amides and esters, allowing for eventual investigation of the effect of ligand properties and solubility on partitioning in CPE.

Figure I.1.8: Structure of diglycolic acid (DGA)

18

II. Experimental Procedures and Methods

II.1. Materials

The surfactants, Triton X-114, Tergitol Type 15-S-7, and Tergitol Type NP-7, were ordered from Sigma Aldrich and used as received. The ligands, including 1-phenyl-3-methyl-4- benzoyl-5-pyrazolone (PMBP) and diglycolic acid (DGA), were used as received from Sigma

Aldrich without further purification. Stock solutions of PMBP in surfactant were made up by weighing solid PMBP and dissolving it in Triton X-114.

All salts, with the exception of NaNO3 and NaClO4, were used as received without further purification and were of analytical grade or better. NaNO3 and NaClO4 crystals (ACS reagent grade) purchased from GFS chemicals, were dissolved in deionized water, filtered through a fine glass frit filter, then recrystallized from hot water. The solutions were then standardized using ion-exchange chromatography (Dowex 50x beads, H+ form) and potentiometric titrations.

The Eu152/154 tracer was prepared at Washington State University (WSU) via neutron irradiation of 99.999 % europium oxide from Arris International Co. at the 1 MW TRIGA reactor at WSU Nuclear Radiation Center. The 24Na tracer was prepared via irradiation of the

3+ nitrate salt. For macroscopic Eu studies Eu(NO3)3 was used.

The lanthanides used for this study included La3+, Nd3+, Eu3+, Ho3+, and Lu3+. The lanthanide perchlorate stocks were prepared from 99.999 % lanthanide oxides from Arris

International Co. The lanthanide oxides were mixed with HClO4 (70 %, Trace Metal Grade acids, Fisher Scientific) and heated to promote the dissolution of the lanthanide solids. The acidity of the lanthanide stock solutions was adjusted to pH 3.0. The solutions were standardized

19 to determine metal concentration, perchlorate concentration, and H+ concentration using ICP-

MS, ion-exchange chromatography (Dowex 50x8, H+ form), and potentiometric titrations. The

243Am was prepared from the oxide by dissolution in concentrated nitric acid. A stock solution of 0.019 M Am(NO3)3 was prepared at Pacific Northwest National Laboratory and used for all experiments.

All solutions were prepared by mass in 18 MΩ water. Surfactant solutions were prepared by weight percent in distilled water. Concentrations were defined on a molality scale.

II.2. Initial CPE Experiments

For the nonradiometric CPE experiments solutions of Cu(NO3)2 and Eu(NO3)3 were studied. The samples were 45 mL total volume and consisted of 450 mg of Triton X-114 (1 wt

%), 10-3 M 8-HQ, and the metal solution (Cu – 0.001 M; Eu – 0.002 M). The pH was not controlled or monitored in initial experiments. The samples were homogenized by hand shaking.

Once homogenized the samples were placed in a 60°C water bath for 15 minutes, centrifugationed at 3000 rpm for 10 min, and allowed to sit in an ice bath prior to phase separation. The ice bath increased the viscosity of the SRP and eased phase separation. A sample of the aqueous phase was taken for analysis by ICP-OES. A similar method was employed with uranium, using a 10-4 M uranium stock solution and the same ligand (10-3 M) and surfactant conditions (1 wt %). The pH was controlled for these samples and varied from 3 to 5 prior to equilibration.

152/154 The 8-HQ system was examined with Eu(NO3)3 to facilitate detection of the metal in both phases. Application of radiotracer methods has the important advantage of enabling an

20 assessment of mass balance which is difficult with other techniques. The difficulty is due to the viscosity of the SRP and the incompatibility of the surfactant with the ICP-OES analysis method.

Radiotracer 152/154Eu was used in 4.5 mL samples with 1 wt % Triton X-114, 10-3 M 8-HQ, and water over a range of pHs. The samples were prepared and pH adjusted before the spike of radiotracer europium was added. For ligand selection, the samples were also 4.5 mL with 2 wt

% Triton X-114, 1 M NaNO3 to maintain constant ionic strength, and water. The ligand was in large excess (0.2 mM) as the metal in the system came solely from the tracer spike (10-5 or 10-6

M approximate concentration). The pH was adjusted to between 3.3 - 3.7 to avoid metal hydrolysis using 1 M HNO3. A small volume, 10 μL, of the radiotracer solution was introduced into the combined sample which was vortex-mixed for 3 min, placed in a water bath for 1 hour at

60°C, removed from the water bath, and placed on ice without centrifugation. The phases were then separated and 200 μL aliquots were taken by pipette from each phase for analysis. The samples were counted on the Packard Cobra 5003 Autogamma counter.

The distribution ratio was defined as:

(1)

The partitioning of Eu3+ was defined as:

(2)

CPM – Counts per minute after background correction

Percent extracted was calculated as:

(3)

21

Concentration factor compared to an equal phase volume system:

(4)

Ci - initial concentration of metal in the homogenous aqueous solution CAP - concentration of metal remaining in the aqueous phase after separation

II.3. PMBP CPE Experiments

For metal distribution studies, samples were 4.5 grams total mass, including the surfactant, 2 wt % Triton X-114. Unless otherwise noted the PMBP concentration was 0.001 m, salt concentration was 0.5 m, and the pH was adjusted to 3 with acid. All samples were prepared in triplicate. Due to its low solubility in water PMBP was dissolved in Triton X-114 and introduced with the surfactant during sample preparation. Eu152/154 was introduced into the samples from an aqueous stock solution after pH adjustment. The samples were vortex-mixed for approximately 10 minutes to ensure homogeneity, and then placed in a 60oC water bath for

20 minutes. The equilibration temperature of 60oC was chosen as a temperature well above the

CPT of the system. At the conclusion of this procedure, the system was clearly biphasic; the

SRP was at the bottom of the reaction vessel. Upon removal from the water bath the samples were placed on ice to enhance phase separation by increasing the viscosity of the SRP. The sample remained quiescent during this time. After approximately 10 minutes, complete phase separation was achieved, as defined by a clear aqueous phase, and the aqueous phase was removed. Samples of 100 μL of each phase were taken by pipet for radiometric analysis using a

NaI(Tl) detector (Packard Cobra 5003). To more accurately determine the distribution and partitioning of the Eu3+, the SRP and aqueous phase (AP) were weighed before and after separation to determine the phase ratio and before and after sampling to determine the sample

22 weight. The viscosity of the SRP makes accurate sampling by volume difficult. Equations 1-4 were used for all calculations. Samples used for the sodium distribution study were prepared and treated in the same manner as described above, but 24Na replaced europium.

To determine whether quantitative extraction of europium into the SRP was possible, the

CPE procedure was repeated with residual aqueous phase and fresh surfactant (with PMBP) using the same separation procedure. For the CPE experiments performed in this paper, the aqueous phase samples were taken from the aqueous phase after separation from the SRP.

To verify that the biphasic samples in the ice bath remained stable until the samples were removed, an experiment was done to assess the possibility of thermal diffusion of Eu3+ from the

SRP into the aqueous phase with gradient studies. These studies monitored the effects of time and volume on Eu3+ partitioning and confirmed optimum sampling technique. For the volume study ten 0.4 mL aliquots of the aqueous phase were taken from the top of the aqueous fluid to give a total of 4 mL, representing the majority of the aqueous phase. The samples were taken starting at the top of the aqueous phase and ended with the last sample being the aqueous phase in direct contact with the SRP. For the time study 0.2 mL samples of the aqueous phase were taken over a time period (1 – 60 min) as the sample sat on ice. The phases remained in contact during the time study.

II.4. Cloud Point Temperature Determination

Samples for CPT determination were 12 grams total mass and contained 2 wt % Triton

X-114, salt (0.01 m to 1 m), and 0.001 m PMBP when present. For CPT determination, samples were placed in a water bath at a temperature below their CPT. This temperature was defined by the solution being clear and monophasic as determined by visual inspection. Samples were

23 allowed to equilibrate for 20-30 minutes as the temperature was raised in 1oC increments. Once the solution became cloudy upon visual inspection, the sample was removed from the water bath and the temperature recorded as the CPT. The error associated with this method is ± 1°C.

II.5. Karl Fischer Analysis

Samples for Karl Fischer analysis were prepared in a similar manner, containing 12 grams total mass with 2 wt % Triton X-114 and 0.5 m salt. The SRP was separated and used for the Karl Fischer analysis. The titrator contained Aquastar CombiCoulomat fritless reagent from

EMD and was calibrated in triplicate using Aquastar 0.1 % water standards, also from EMD.

Average calibration value was 1000 ppm ± 3 % error. A small sample (~0.01 g) of the separated

SRP was introduced by syringe into the titrator for water analysis (Mettler Toledo Coulometric

Karl Fischer Titrator DL 50). The mass delivered was determined by weight of the syringe before and after injection. All samples were prepared, run, and measured in triplicate.

II.6. FT-IR Analysis

Samples for IR analysis were 12 grams total and contained 2 wt % Triton X-114 and salt

(0.1 m to 6 m) in DI water. The samples were homogenized by shaking, heated to 60°C for 20 minutes in a water bath, removed, and placed in an ice bath to increase the viscosity of the SRP, easing phase separation. With the exception of high salt concentrations (> 2 m), the SRP was on the bottom and the aqueous phase was removed from the SRP. At high salt concentrations, the

SRP was the top phase and removed from the aqueous phase. IR spectra were taken of the separated SRP or neat surfactant on a Nicolet 6700 FTIR with a diamond 30,000 – 200 cm-1 attachment from Thermo Corporation. The samples were placed on the instrument using a pipet

24 post-extraction without further manipulation. Scans were taken from 400 to 4000 cm-1 up to 64 scans.

II.7. pcH Measurements

An Orion Ross semi-micro glass electrode with 3 M NaCl filling solution was used for all titrations. The high concentration of NaCl was used to minimize junction potential variations.

The electrode was standardized daily with a strong acid/base titration of 0.01 m HCl with 0.4 m

NaOH at controlled temperature and ionic strength. Gran analysis of the strong acid - strong base titration data provided a calibration curve (pcH vs. mV) and a conversion equation.73 The solutions used for the standardization contained the same percentage of Triton X-114 used in the experiments.

II.8. Potentiometric Titrations

A Mettler Toledo DL50 Graphix auto-titrator was used to conduct the potentiometric titrations. The ionic strength was held constant at 0.1 m with NaCl for all experiments and the temperature ranged from 5 to 45°C ± 0.1C for the titrations. The temperature was maintained with a jacketed beaker and a circulating water bath (VWR). Separate titrations were completed to determine the pKa for DGA in 0.1 m NaCl and stability constants for Ln·DGA (β101, β102, β103) complexes. A stock titrand solution containing 0.01 m DGA, 0.01 m HCl, 0.1 m NaCl, and appropriate amount of surfactant was prepared for each surfactant concentration. One equivalent of HCl was added to ensure the full protonation of DGA at the start of the titration. The stock was standardized for ligand concentration and was then used for all metal ligand potentiometric titrations. For stability constant determinations, known amounts of Ln(ClO4)3 stocks were

25 spiked into the DGA titrand solution to achieve 1:1, 1:2, and 1:3 metal to ligand ratios.

Titrations of DGA with Ln3+ were done at all three metal to ligand ratios to ensure the ability to fit all three Ln3+ DGA species. Densities were determined for all components of the titration solutions and all samples were prepared by mass. Duplicate titrations were completed for each experimental condition. All titrations were conducted using 0.4 M NaOH as the titrant with 0.1 m NaCl. The titration data were analyzed using the fitting program PSEQUAD.74 The data were fit up to pcH 3.0-4.0 for the Ln·DGA complexation titrations to reduce added uncertainties from lanthanide hydrolysis products in the fitting routines.

The weight percent of Triton X-114 ranged from 0.01 wt % to 40 wt % for the titrations of 0.01 m DGA with 0.4 m NaOH. At 0.01 wt % the solution was approximately equal to the

CMC of Triton X-114. Duplicate static titrations (constant 0.05 mL additions of base) were run for fitting with the PSEQUAD program.74 A large range of surfactant concentrations were examined to evaluate the influence of the surfactant concentration, near and above the CMC, on the protonation constants of DGA. The potentiometric titrations were run at 25, 35, and 45°C with 0, 2, 10, 20, and 40 wt % Triton X-114, μ = 0.1 m NaCl, these temperatures were all above the CPT. Additionally, titrations were run at 5, 10, and 18°C with 0.02, 0.2, and 2 wt % Triton

X-114, these temperatures were all below the CPT.

II.9. UV-vis Spectrophotometry

Absorption spectra of Nd(III), Ho(III), and Am(III) were measured for solutions with a constant metal to ligand ratio (1:3 for Nd and Ho; 1:5 for Am) and varying pH. The ionic strength was held constant at 0.1 m with NaCl. The Nd and Ho solutions contained 20 mm of the respective metal, 60 mm of DGA, and an appropriate amount of surfactant (Triton X-114, 0 –

26

40 %) at 0.1 m ionic strength. The Am solution contained 1 mm of 243Am and 5 mm DGA. Due to the increased viscosity only 2 and 10 wt % Triton X-114 were used for the Am experiments.

The initial pH was not adjusted, but was allowed to vary with metal and surfactant concentrations. The pH was adjusted by adding small increments of 1 m NaOH and spectra were recorded after each addition. The pcH was determined in a separate experiment with the same solutions after calibration of the electrode with a Gran titration. All Nd3+ and Ho3+ spectra were taken on a Varian Cary 50 Bio UV-visible spectrometer from 350 – 800 nm with a resolution of

0.5 nm in a 1 cm pathlength quartz cell. The Am3+ spectra were taken using an Ocean Optics

S2000 spectrophotometer with a resolution of 0.25 nm also in a 1 cm pathlength quartz cell. All spectra were baseline corrected and fit using HypSpec.75 The hypersensitive bands for Nd3+ and

Ho3+ (~575 nm and ~450 nm respectively) were used to calculate metal ligand stability constants. The transition at 503 nm was used for Am3+ calculations.

II.10. Fluorescence Measurements

For the fluorescence measurements, surfactant solutions of 10, 20, and 40 wt % Triton X-

114 with 1 mm Eu(NO3)3, varying DGA concentrations (0.6 – 10 mm), and 0.1 m NaCl were prepared. To examine salt effects, the 20 and 40 wt % Triton X-114 solutions with 1 mm Eu and varying DGA concentrations were also prepared with 0.5 m NaCl, NaNO3, and NaSCN.

Luminescence emission spectra and lifetime measurements of Eu3+ in aqueous solutions were collected using a HORIBA Jobin Yvon IBH FluoroLog-3 fluorometer adapted for time- resolved measurements. The light source was a sub-microsecond Xenon flash lamp (Jobin

Yvon, 5000XeF) coupled to a double grating excitation monochromator for spectral selection.

The detector, a single photon detection module (HORIBA Jobin Yvon IBH, TBX-04-D),

27 incorporates a fast rise time PMT, a wide bandwidth preamplifier, and a picosecond constant fraction discriminator. Samples were analyzed at 25°C in 1 cm quartz cells. Data were collected using an IBH Data Station Hub, and analyzed using the DAS 6 decay analysis software package from HORIBA Jobin Yvon IBH. The lifetime data was fit using multi-exponential decay curves.

The emission spectra of Eu3+ complexes were acquired on the same samples with

3+ 5 excitation at 464 nm. At that wavelength, the Eu ion is excited to the D2 excited state and

5 undergoes a rapid, nonradiative relaxation to the D0 state. Emission occurs during the

5 7 subsequent relaxation from the D0 to the FJ manifold. Changes in the inner coordination environment of the Eu3+ metal ion result in different intensities for the peaks in the emission

5 7 76 spectra which represent the D0  FJ (J = 0, 1, 2, 3, …) electronic transitions.

28

III. Results and Discussion

III.1. Explorations of Cloud Point Extraction and Application to Lanthanides and

Actinides

The goal of this investigation was to develop a new CPE separation system for lanthanide and actinide separations. Initial studies followed procedures used in previous CPE investigations and were based primarily on studies reported by Draye, et. al.36-38,40 Draye and coworkers used the ligand 8-hydroxyquinoline (8-HQ) with the surfactant Triton X-114 to extract UO2(NO3)2,

Gd(NO3)3, and La(NO3)3. Due to its hydrophobicity, 8-HQ was dissolved in the surfactant and the complexant/surfactant solution was introduced into the aqueous solution containing uranium or lanthanide. Various surfactant and ligand concentrations were examined to optimize extraction. For the optimized conditions a maximum extraction efficiency of nearly 100 % was reported. In addition to developing a new CPE system for lanthanide extraction, the influence of various electrolytes was examined. It had been reported previously that the ionic strength of the solution does not impact the extraction in a CPE system.18 In this thesis, the influences of a wide variety of salts over a range of concentrations on the CPTs and extractions of a CPE system are reported.

III.1.1. Results

III.1.1.1. 8-HQ

The results of attempts to repeat previous literature references were surprising. For both copper and europium, a powder precipitate was visible after extraction and phase separation.

The precipitate was at the bottom of the SRP and contained within the phase. The solid precipitate was bluish-green for the samples containing copper and white for those containing

29 europium. For low concentrations of Cu, almost 99 % of the copper was removed from the aqueous phase, so the copper must have been contained within the precipitate or the SRP.

However, no analysis was done of the SRP to determine whether the precipitate or the SRP contained the majority of the copper. If the copper were contained within the precipitate, the separation cannot be attributed to a successful CPE procedure and, instead, would be attributed to precipitation. At higher copper concentration, 0.002 M, a 50 % reduction in the copper in the aqueous phase was seen. This would be expected as the metal and ligand are approximately equal to each other at that concentration, indicating formation of Cu(HQ)2. For europium at constant metal concentration, ~ 70 % of the europium was removed from the aqueous phase. No analysis was done of the SRP or the precipitate so the europium could have been either in the

SRP or the solid precipitate. Lack of pH control could have contributed to the precipitation, as both europium and copper have known hydrolysis products at higher pHs.77,78 To support the hypothesis of hydroxide precipitation, speciation calculations were performed. The stability constants for europium with 8-HQ are not recorded in the NIST database, but the constants for lanthanum with 8-HQ have been reported.79 Using the stability constants for lanthanum with 8-

HQ, speciation calculations were done using Hyss.80 The speciation calculations indicated a high degree of hydrolysis at higher pHs caused by the low metal to ligand ratio, supporting the idea that the precipitant could be caused by hydrolysis, if the pH were high enough. If the pH were above 6 it can be assumed that the precipitate seen in the SRP contained the majority of

“extracted” metal ion.

Under the conditions used for uranium extraction, no SRP formation was seen, but an orange precipitate was observed in all samples. The aqueous phase was analyzed with ICP and contained no detectable uranium, even without the formation of the SRP, indicating all the metal

30 ion was in the precipitate. Speciation calculations using formation constants from NIST database indicated formation of hydrolysis products under the metal to ligand ratios used at pHs greater than 5. As the pH was not monitored, it cannot be said for certain that hydrolysis occurred. It is still unclear why there was no SRP formation.

3+ The KD and DEu values calculated from radiotracer Eu extraction with 8-HQ were plotted as a function of pH (Figure III.1.1) and can be seen to increase with increasing pH.

However, as pH increases, a loss in total activity is seen, suggesting hydrolysis and precipitation of the metal ion. At pH 6, only 70 % of the total activity added to the sample was recovered.

The pH required for maximum extraction of Eu3+ with 8-HQ in a CPE system is above the pH that hydrolysis beings. These results indicated that a different system was needed for lanthanide and actinide extraction.

III.1.2. System Design

III.1.2.1. Surfactant Selection

The surfactants examined were Tergitol Type 15-S-7, Tergitol Type NP-7, PONPE 7.5, and Triton X-114. Tergitol Type 15-S-7 formed a SRP on top of the aqueous phase, making for a more challenging separation compared to the other surfactants which form a SRP as the lower phase. Tergitol Type NP-7 and PONPE 7.5 did not demonstrate any notable advantages over

Triton X-114. As Triton X-114 had a suitable CPT (23°C) and literature precedent, it was selected and used for all further experiments.15,52,81

31

100

80

60

40

20 % of total activity recovered activity total of %

30 D 25 Eu K D

3+ 20

15

10

Distribution Eu Distribution 5

0

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 pH

Figure III.1.1: (Top Graph) Recovery of total 152/154Eu activity as a function of pH (Bottom Graph) Eu extraction with 8-HQ as a function of pH. D value (blue circles); KD value (red triangle)

32

III.1.2.2. Ligand Selection

Ligand selection was done using the only available method of trial and error. The radiotracer method allowed analysis of both phases, aqueous and SRP, providing a check on mass balance. All ligands were examined at the conditions previously mentioned and the partitioning of Eu3+ is shown in Figure III.1.2. Only tracer amounts of Eu3+ were used with large excess of ligand. The following ligands were tested: pyrogallo red, murexide, arsenazo III, pyrocatechol violet, pyridylazo, xylenol orange, ethylenediamine tetraacetic acid (EDTA), phenylphosphinic acid, 2,6-pyridinedicarboxylic acid, phosphonoacetic acid, triphenylphosphine oxide, diaminocyclohexane tetraacetic acid, sulfanilic acid, pyromellitic acid, diethylenetriamine-pentaacetic acid, acetylacetone, 1,10-phenanthroline, picolinic acid, and isoascorbic acid. The structures for all the ligands can be found in the attached appendix. Equal partitioning of the radiotracer 152/154Eu was seen without added chelating agent, resulting in a very small percentage of the total metal present partitioning into the SRP (phase ratio of 12:1)

(Figure III.1.2). None of the ligands tested demonstrated any appreciable ability to concentrate the metal ion in the SRP. Some of the ligands (EDTA and 1,10-phenanthroline) were tested over a range of pHs with no change in partitioning noted (data not shown). Additionally, different salts were tried as supporting electrolytes at different ionic strengths. In doing this it was noted that salts influenced metal ion distribution leading to later studies. The conventional solvent extraction reagent, 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) was found to facilitate partitioning of Eu3+ into the SRP (Figure III.1.3) significantly at low pHs, avoiding hydrolysis concerns.

33

0.10

0.08

0.06

D

K 0.04

0.02

0.00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

3+ Figure III.1.2: Line indicates the KD value for Eu with no ligand; 2 wt % Triton X-114 with tracer concentrations of Eu3+ and excess of ligand (varied) 1. Pyrogallo red, 2. Murexide, 3. Arsenazo (III), 4. Pyrocatechol Violet, 5. Pyridylazo, 6. Xylenol Orange, 7. Ethylenediaminetetraacetic acid (EDTA), 8. Phenylphosphinic acid, 9. Dipicolinic acid, 10. Phosphonoacetic acid, 11. Triphenylphosphine, 12. Diaminocyclohexane tetraacetic acid, 13. Sulfanilic acid, 14. Pyromellitic acid, 15. Diethylenetriamine pentaacetic acid, 16. Acetylacetone, 17. 1,10 phenanthroline, 18. Picolinic acid, 19. Isoascorbic acid

34

100

80

pH 3.0 60 pH 2.5

pH 2.0

40

% Eu % SRP in 20

0 0.0000 0.0005 0.0010 0.0015 0.0020 [PMBP] (m)

Figure III.1.3: Eu3+ extraction at various pHs as a function of PMBP concentration. Lines do not represent a model fitting and errors are the standard deviation of triplicate measurements.

PMBP has very limited solubility in water but is sparingly soluble in ethanol and was dissolved in ethanol for introduction into the CPE solution. A stock solution of 0.1 M PMBP in ethanol was prepared and used for all extractions. The system was first optimized by examining the effect of pH, ligand concentration, and incubation time on the extraction efficiency. The phase ratio for all studies remained constant at 12:1 (aqueous phase:SRP). The ligand concentration chosen for more detailed studies was 0.001 M PMBP. At low ligand concentrations partitioning of Eu3+ was highest at pH 3 (Figure III.1.3). All samples were adjusted to pH 3 prior to extraction in subsequent experiments.

35

III.1.2.3. Salt Effects

Having established that PMBP was able to facilitate Eu3+ partitioning, this system was employed to assess the influence of salts on the extraction efficiency. A range of 1:1 and 1:2 electrolytes comprised of various cations and anions were introduced into the PMBP extraction system over the concentration range 0.01 – 1 M (Figure III.1.4). At low salt concentrations very little change in the extraction efficiency was seen. Above 0.1 M, the influence of the electrolytes began to become apparent. The only salts which do not decrease the partitioning of Eu3+ at 1 M

- salt concentration were Na2SO4, (CH3)4NCl, and NH4Cl. The NO3 salts, with the exception of

3+ NH4NO3, dramatically decreased Eu partitioning at 0.5 and 1.0 M concentrations. This finding led to further investigation of the influence of salts on other aspects of the CPE system, in particular the CPT.

100

90 LiClO 4 KNO Eu) 80 3 NaNO 3 NaClO

152/154 4

70 NaI

NH NO 4 3 NaCl 60 Na CO 2 3 KCl NH Cl 4 % Extracted ( Extracted % 50 (CH ) NCl 3 4 Na SO 2 4 40 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 log[Salt] (M)

Figure III.1.4: Extraction of Eu3+ using 0.001 M PMBP in ethanol and 2 wt % Triton X-114 with various salts over a range of salt concentrations. Errors represent the standard deviation from triplicate measurement.

36

III.1.3. Cloud Point Temperature

III.1.3.1. Alcohol Study

Since PMBP was added as an ethanol solution, the effect of alcohol solutions on the CPT was of interest. Additionally, for future system design, most ligands known to extract lanthanides and actinides in conventional solvent extraction have limited water solubility.

Having some miscibility with water is a requirement for a ligand in a CPE system, as the ligand must partition into the still mostly aqueous SRP. One approach to altering this property was to modify the system to decrease the water content and allow for solubility of traditional solvent extraction reagents (i.e., HDEHP, TOPO). The CPT of 2 wt % Triton X-114 in the presence of alcohols was examined. Alcohols have been reported to have a significant effect on the CPT by increasing it dramatically; this was also seen in this work (see Table III.1.1).82 The surfactant/alcohol/water system was miscible in alcohol-water solutions as high as 40 % ethanol, but no SRP was formed, even at temperatures up to 95 °C. At 15 wt % ethanol, the CPT was near the boiling point of water and no SRP was seen above 20 wt % for methanol or ethanol.

However, it was useful to note that the SRP did still form in the presence of small amounts of alcohol, i.e. < 20 wt %. This opened the possibility that ligands soluble in alcohol could be applied in future investigations.

37

% Methanol Ethanol

5 CPT < 60 CPT > 60

10 CPT < 60 CPT > 60

15 85 > CPT > 60 CPT > 85

20 No SRP Formation No SRP Formation

30 No SRP Formation No SRP Formation

40 No SRP Formation No SRP Formation

Table III.1.1: Alcohol study with 2 wt % Triton X-114 and water

III.1.3.2. Electrolytes Study

It has been reported in literature that the CPT of certain nonionic surfactants changes with the addition of electrolytes.30,32 Similar results were seen for 2 wt % Triton X-114 with most salts (Figures III.1.5-III.1.7). The CPT of a 2 wt % solution of Triton X-114 with no additives is

25 °C.15 The addition of increasing amounts of salt resulted in an increase in the CPT by as much as 30 °C. In general, the nature of the anion dominated the effect of the salt, although some differences were seen among the cations, in particular the tetramethyl/butyl ammonium cations.

The tetrabutyl ammonium salts have limited aqueous solubility, but, even at 0.1 M, they caused a large increase in the CPT, indicating a large degree of salting-in by the cation (Figure

III.1.5). The anion influence in this case was smaller than that of the cation, but still present.

The (C4H8)4NCl salt did not increase the CPT as much as the (C4H8)4NBr. This is expected

38 behavior as the Cl- anion is known to salt-out and negates some of the salting-in influence of the large cation. The influence of the anions which salt-in is apparent for the smaller cations, such

+ + + - - - - as Na , K , and NH4 . The I , SCN , and ClO4 anions all salt-in to varying degrees with SCN salting-in slightly more than the other anions. Other cation effects are seen with Li+ and Al3+ and both appear to salt-in.

Several anions and cations salt-out, decreasing the CPT (Figure III.1.6). In some cases at higher salt concentrations the CPT was below the freezing point of water and was unable to be accurately determined. Those CPTs are listed as 0°C. The divalent and trivalent anions salt-out very strongly compared to the singly charged anions. All the divalent and trivalent anions decrease the CPT to below the freezing point of water, regardless of the cation. Of the two singly charged anions, F- and Cl-, the fluoride anion salted-out more strongly. The cation influence can be seen clearly for the chloride salts at 1 M salt concentration. Na+ and K+ are

+ approximately equal with NH4 salting-in slightly and raising the CPT relative to NaCl and KCl,

+ with (CH3)4N salting-out and lowering the CPT relative to NaCl and KCl. Finally, some of the salts were seen to have no influence on the CPT of Triton X-114. The nitrate and bromide anions did not salt-in or salt-out the surfactant. This agrees with previous literature.30 The slight changes seen in the CPT could be attributed to the cations.

39

56 LiClO

4 C) o (C H ) NBr NH SCN 52 4 8 4 4

48 KSCN (C H ) NCl 4 8 4 44 NaClO NaI 4

40 KI 36 Al(NO ) 3 3 (CH ) NI 3 4 32

28

24

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Cloud Point Temperature ( Temperature Point Cloud Concentration (M)

Figure III.1.5: Change in the CPT of 2 wt % Triton X-114 solution with increasing salt concentration; Salting-in electrolytes

28 C)

o 24

20 NH Cl K CO 4 K HPO 2 3 2 4 16 NaF Na SO 2 4 12 KCl NaCl NaCH CO 8 3 2 (CH ) NCl KF 3 4 4 (NH ) CO 4 2 3 Na CO 0 2 3

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Cloud Point Temperature ( Temperature Point Cloud Concentration (M)

Figure III.1.6: Change in the CPT of 2 wt % Triton X-114 solution with increasing salt concentration; Salting-out electrolytes

40

40

C) o 35

(CH ) NNO 30 3 4 3 NaNO NH NO 3 4 3

25 KBr

NaBr KNO 20 3

15

10

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Cloud Point Temperature ( Temperature Point Cloud Concentration (M)

Figure III.1.7: Change in the CPT of 2 wt % Triton X-114 solution with increasing salt concentration

III.1.4. Conclusions

The preliminary work helped to shape further studies by revealing some irregularities in previous CPE research in the literature. Several important lessons were learned that were utilized in additional studies. The addition of PMBP as an ethanol stock increased the CPT in addition to the increase caused by salts. For later studies, PMBP was made up as a stock solution in the neat surfactant, Triton X-114. When introduced in this manner, PMBP had an almost negligible effect on the CPT. This preliminary work also was done using concentrations defined on the molarity scale which is poorly suited to an experiment in which the temperature is varied.

All subsequent work was completed using solutions whose concentrations were prepared on a molality basis. A large range of salts were examined in initial work, as well. Based on these early results, the scope was narrowed to consideration of only nine salts for the rest of the work:

+ + + - - - 1:1 salts of Na , K , and NH4 with Cl , NO3 , and SCN anions. These salts were chosen based

41 on preliminary studies and the fact that they encompass the range of changes induced by electrolytes according to the Hofmeister series.

Surfactant Structures

Tergitol Type 15-S-7

Tergitol Type NP-7

Triton X-114

Ligand Structures

Pyridylazo resorcinol

42

Phenylphosphinic acid

8-hydroxyquinolinol (8-HQ)

1,10 Phenanthroline

1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP)

PAN

Ethylenediaminetetraacetic Acid (EDTA)

43

Isoascorbic acid

Pyromellitic acid

Diethylenetriamine pentaacetic acid

Diaminocyclohexane tetraacetic acid

Pyrogallol Red

44

Pyrocatechol Violet

Murexide

Dipicolinic Acid

Phenylphosphinic acid

Phosphonoacetic acid

Triphenyl phosphine oxide

45

Sulfanilic acid

Picolinic acid

Acetyl Acetone

Arsenazo III

Xylenol Orange

46

III.2. Development of a cloud point extraction (CPE) separation technique using 1-phenyl- 3-methyl-4-benzoyl-5-pyrazolone (PMBP) to extract europium and americium

PMBP has been used previously in CPE systems for extraction of Cd and Mn59,60 and has been used in traditional solvent extraction systems for the separation of lanthanides and

61-63,65 84 actinides. The pKa of PMBP is 4.01, making it suitable for lanthanide extraction. PMBP contains hydrophobic moieties, forms hydrophobic complexes, and is only sparingly soluble in water. It does not appear that PMBP has been applied to lanthanide or actinide CPE systems in any prior investigations. In this work, an optimized procedure for extraction of radiotracer Eu3+ and Am3+ using PMBP was developed. The use of radiotracers enabled analysis of both phases without the need for dilution of the SRP.

In addition to the development of a successful CPE system for extraction of Eu3+ and

Am3+, the potential for process-scale applications of the system have been explored. The processing capabilities of CPE have been demonstrated previously for removal of pollutants from waste water.83 However, no such system has been applied to metal separations in CPE. The effect of repeated contacts with macroscopic quantities of Eu was also examined in this research.

Back extraction has been previously demonstrated with U and 8-HQ using a nitric acid contact with the SRP and then re-separating the phases.37

III.2.1. System Optimization

Optimization of extraction conditions in CPE requires a different focus from system setup in solvent extraction as there are additional factors, such as temperature, incubation time, separation technique, and phase ratios to consider to optimize the CPE system. The CPT of

Triton X-114 is 23°C, meaning heating to 60°C is more than enough to induce the stabilization of

47 the separate phases. There is some debate in the literature as to the degree of overheating needed for efficient separation. Previous work with CPE systems using Triton X-114 deemed 60°C a suitable temperature to induce sufficient phase separation without the need for extreme temperatures.39

Extraction experiments were conducted as a function of pH (1.5 – 4.5) (Figure III.2.1).

The pH was kept below 5 to eliminate any chance of hydrolysis of the metal ion.77 The pH of the homogenous solution was tested before separation and the pH of the aqueous phase was tested after extraction. The pHs in the figure represent the initial pH adjusted with hydrochloric acid. Almost no partitioning was seen below pH 2. Above pH 2, partitioning of the metal ion into the SRP increased and appeared to level off with a slight decrease above pH 4, correlating to the pKa of PMBP. Very little difference in the extraction was seen between pH 2.5 and 3.5. The highest partitioning of Eu3+ occurred at pH 3 (~95 %); this pH was chosen for all subsequent experiments. The phase ratio did not change with varying pH. The optimum pH for extraction is the same pH used for lanthanide PMBP extraction in a traditional solvent extraction system.65

The incubation time in the water bath had little impact on the metal partitioning after 20 minutes (Figure III.2.2). The study indicated that the kinetics of phase transfer and separation are not particularly fast: with a 10 minute incubation period, only 80 % extraction is seen, compared to 95% at an hour. For a five minute incubation period, only 45 % of the europium was extracted. Twenty minutes was selected as the most appropriate time for equilibrium for further experiments as the reaction is ~ 95% complete in this shorter time frame.

48

100

80

60

40

Extracted 3+

20 % Eu %

0 1 2 3 4 5 pH

Figure III.2.1: Extraction of Eu3+ with 0.001 m PMBP at varying pHs. Solutions contained 2 wt % Triton X-114 and incubation time was 60 minutes at 60 °C. Error represents one standard deviation from measurement of three replicate samples.

100

80

60

40

Extracted 3+

20 % Eu %

0 0 10 20 30 40 50 60 Time (minutes)

Figure III.2.2: Extraction of Eu3+ with 0.001 m PMBP and 2 wt % Triton X-114 at pH 3 over a range of incubation times in a 60oC water bath. Error represents one standard deviation from measurement of three replicate samples.

49

Equilibrations were also done at a range of soluble ligand concentrations (Figure III.2.3).

Due to its limited solubility in water, the maximum concentration of PMBP was limited to 0.002 m; at higher concentrations of PMBP, a precipitate was observed in the SRP. This was caused by the incorporation of water into the surfactant when it formed the SRP, decreasing the solubility of PMBP relative to that in the neat surfactant. There was a very sharp increase in extraction up to a concentration of 0.0005 m PMBP. Above this concentration, there was no further increase in extraction. The system appeared to saturate at this maximum extraction value.

It is likely that the PMBP reaches a solubility limit inhibiting further extraction of a europium

PMBP complex. The phase ratio remained constant at 12:1 over the range of ligand concentrations.

Gradient and time sampling studies were performed to optimize the sampling technique employed. In a CPE system, the two phases are miscible, making it important to ensure no mixing of the phases is occurring prior to sampling. The results from sampling the aqueous phase showed that there was little change in the 152/154Eu activity from the first sample to at least the eighth (3.2 mL). There was some evidence for higher concentrations in the two samples closest to the aqueous-SPR interface (Figure III.2.4). The fraction of aqueous phase in contact with the SRP showed a larger amount of 152/154Eu activity, indicating some slow mixing of the

SRP and the aqueous phase at the interface while the phases were in the ice bath. For other studies, a sample of the aqueous phase was taken after the phase was separated from the SRP and homogenized. The gradient study indicates that this was an appropriate method and minimized error.

The purpose of the time study was to determine whether and/or how quickly back-mixing of the SRP with the aqueous phase occurred while the samples were on ice. For experiments in

50 this research samples typically were allowed to sit on ice 10-30 minutes prior to phase separation

(Figure III.2.4B). Over this time frame there was no apparent mixing as the amount of Eu3+ present in the aqueous phase did not change. The sample taken at one minute showed a lower activity than samples taken later, indicating some time was needed for the separation to reach an equilibrium/steady state condition. For the purposes of this study, the results supported the method and timing of the separation technique employed as no change was seen from 10 minutes to 1 hour.

100

80

60

40

Extracted 3+

20 % Eu %

0 0.0000 0.0005 0.0010 0.0015 0.0020 [PMBP] (m)

Figure III.2.3: Extraction of Eu3+ with varying concentrations of PMBP using 2 wt % Triton X- 114 at pH 3. Incubation time of 20 minutes at 60°C. Error represents one standard deviation from measurement of three replicate samples.

51

1600 900 A 1400 800 B

1200 700 600 1000 500

800

400 600 300 400 200

200 100

0 0 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 0 10 20 30 40 50 60 CPM in Aqueous Phase Sample Phase Aqueous in CPM Aqueous Phase Fraction (mL) Time (minutes)

Figure III.2.4: 152/154Eu3+ in the aqueous phase after phase separation with 2 wt % Triton X-114 and 0.001 m PMBP at pH 3. A – 152/154Eu in 0.4 mL aliquots of the 4 mL aqueous phase. Samples were taken from the top down to the aqueous phase in contact with the SRP. B – 152/154Eu in 0.2 mL aliquots of the aqueous phase taken over an hour while the aqueous phase remained in contact with the SRP. Error represents one standard deviation of triplicate measurements.

III.2.2. Slope Analysis

In an attempt to identify the Eu:PMBP stoichiometry of the complex extracting into the

SRP, a slope analysis experiment was done. Slope analysis is employed in traditional solvent extraction by varying ligand concentration while holding the metal concentration constant.

When the log of metal ion distribution ratio is plotted versus the log of ligand concentration, the slope usually indicates how many ligand molecules are partitioning with the metal into the organic phase. Slope analysis of PMBP with europium in traditional solvent extraction systems has been reported.65 In those studies, a slope of 4 was observed, indicating the dominant extracted complex is M(PMBP)3∙HPMBP, a neutral species with a fourth ligand added without exchanging H+ to the aqueous phase, presumed to be in the diketone form of PMBP. The slope obtained for the partitioning of Eu3+ with PMBP into the SRP was 1.7 (± 0.1) (Figure III.2.5).

52

This slope indicates that a different partitioning mechanism may be operable in CPE compared to that observed in traditional solvent extraction systems.

Very little previous work has been done to elucidate the extraction mechanism employed in CPE in metal ion partitioning. Research has mainly been limited to system optimization studies. Some papers have presented slope analysis results for a CPE system.41,42 The distribution values for those studies were calculated taking into account the differences in volume of the SRP and aqueous phase. The calculated distribution, therefore, represents D values that would be expected if the SRP and aqueous phase were the same volume, but does not represent the actual distribution seen. It does not appear that slope analysis results completed in this manner will yield information on the actual CPE system and metal ligand stoichiometry.

The results obtained using this method matched slopes reported for a traditional aqueous organic solvent extraction system which is highly unlikely considering the “organic” phase still contains a considerable amount of water.85 The assumption that the ligand partitions completely into the

SRP cannot be made if the ligand has any water solubility. In this study, the ligand, PMBP, had virtually no solubility in water and was assumed to partition into the SRP.

The result seen in this research, a slope of 1.7, indicate a very different mechanism, requiring either partitioning of a charged species into the SRP (with complementary exchange of cations into the bulk aqueous phase) or the inclusion of background electrolyte ions in the extracted lanthanide complex (e.g., Eu(PMBP)2(NO3)). This result is perhaps not very surprising if the composition of the SRP is taken into consideration. It has been demonstrated with Karl

Fischer analysis that the SRP composition can be up to 75 % water, making the SRP a significantly more polar “organic” phase than is seen in traditional solvent extraction.86 The polar nature of the SRP would certainly allow for the partitioning of charged species and, in fact,

53 would likely suggest that the partitioning of an extremely hydrophobic metal ligand complex containing four PMBP molecules is unlikely. The organization of the aqueous and surfactant phases in the SRP is not known, but it is reasonable to assume that the SRP is comprised of nonionic micelles and water arranged in a stable microemulsion. The following equation is one proposed extraction mechanism that could account for the slope seen.

3+ - Eu (aq) + (NO3 )(aq) + 2 PMBP(surfactant) ↔ Eu(PMBP)2(NO3)(SRP)

No pH change was seen in the aqueous phase after extraction. In solvent extraction systems with acidic extractants, a change in the pH of the aqueous phase is often seen as the protons migrate to the aqueous phase. This is not seen in CPE, an observation that is consistent with the conclusion that the behavior of the SRP is not analogous to the behavior of traditional “organic” phases.

While there is no observed proton exchange, the possibility of Na+ exchange from the SRP with the partitioning of the Eu:PMBP complex cannot be discounted and should be considered, as charge neutrality must be maintained.

III.2.3. Actinide Partitioning

In addition to Eu3+, the distribution of radiotracer Am3+ with PMBP was examined

(Figure III.2.5). The distribution of Am3+ and slope correlate well with the distribution and slope seen with Eu3+. While little or no selectivity for europium over americium is seen, this could be modified with use of a different ligand. PMBP is not a suitable extractant for separations of lanthanides from trivalent actinides.

54

2.5

Am3+ 2.0 Eu3+

1.5

1.0

LogD 0.5

0.0

-0.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 Log [PMBP]

Figure III.2.5: Distribution of Am3+ and Eu3+ into the SRP over varying PMBP concentrations with 2 wt % Triton X-114 at pH 3. Error represents one standard deviation of triplicate measurements.

III.2.4. Metal Loading

A metal loading study was performed with macro amounts of europium (Figure III.2.6).

Metal to ligand ratios from 1:0.5 – 1:10 were employed with 0.001 m PMBP and corresponding

152/154 amounts of Eu(NO3)3. Tracer Eu was used to monitor partitioning, assuming that the system was in isotopic equilibrium. With increasing ligand to metal ratio the partitioning of Eu3+ increased up to ~93 % at a M:L of 1:10. At lower ratios of PMBP to Eu3+, extraction in the first contact was reduced, though not to the degree that the radiotracer limit of 1:2 (Eu:PMBP) stoichiometry would seem to indicate. At the extreme limit of metal ion concentration (0.002 M),

0.001 M PMBP should partition no more than 25 % of Eu3+ into the SRP, yet the observed fractional partitioning was 34 %. Either the stoichiometry is reduced under these extreme metal loading conditions or is perhaps continuously variable as conditions change.

55

To determine whether Eu3+ could be quantitatively extracted into the SRP with multiple contacts, a fresh sample of surfactant / PMBP solution was added to the residual aqueous phase containing the remaining metal ion for a second contact. After a second contact, ~98 % of the residual Eu3+ was transferred to the SRP. The ratio of the aqueous phase to the SRP decreased with the second contact of fresh surfactant from 11:1 to 7:1. In two contacts 99.8 % of the Eu was extracted with PMBP in this CPE system and contained in a much smaller volume, with an overall concentration factor of ~ 10. This result demonstrated the possible applicability of CPE metal separation for processing.

Scamehorn, et. al., have described an online CPE procedure for water purification.83 A nonionic surfactant was added to an aqueous solution containing various phenols and other pollutants. The solution was stirred and heated. The SRP was allowed to form and sink to the bottom, containing 80-90 % of the pollutants. The upper aqueous phase was removed and was mostly contaminant-free. The process continued with the addition of fresh surfactant. It is not unreasonable to think that a process like this could also work for metal separations. One problem is the limited solubility of PMBP, so a different ligand that can achieve similar distribution ratios and dissolve at higher concentrations would be beneficial. Design/identification of such a ligand must await increased detailed information on the structure of the SRP and on the location and stoichiometry of the metal complexes that are contained in that phase.

56

First Contact 100 Second Contact Total Extraction

80

60

Extracted 3+

40 % Eu %

20 0.0000 0.0005 0.0010 0.0015 0.0020 [Eu] (m)

Figure III.2.6: Distribution of varying macroscopic amounts of Eu(NO3)3 with 0.001 m PMBP and 2 wt % Triton X-114 at pH 3. □ - Eu3+ extracted with first contact of the aqueous phase with 0.001 m PMBP in Triton X-114. Δ – Eu3+ extracted from remaining aqueous phase contacted with additional PMBP and Triton X-114. ○ – Total Eu3+ extracted after both contacts

III.2.5. Conclusions

The development of a new CPE system for lanthanide and actinide separation using

PMBP has been reported. This is the first work reported applying radiometric analysis to CPE, allowing for metal analysis of the aqueous and SRP. The application of radiotracer techniques provides an advantage over analysis using ICP-OES or AES wherein the SRP is not easily analyzed due to its viscosity and composition.

The formation of the PMBP Eu3+ complex and its partitioning into the SRP was found to be reasonably rapid, with maximum extraction reached after only 10 minutes. The optimum pH for extraction was the same pH used for lanthanide/PMBP extraction in a traditional solvent extraction system. The gradient study indicated that some back-mixing occurs near the interface of the aqueous phase with the SRP. This was not surprising since CPE systems are reversible

57 upon cooling and the SRP is miscible with the aqueous phase. As the time study shows, the remixing of the SRP with the aqueous phase is very slow, if the system is not disturbed.

Quantitative extraction of macroscopic amounts of europium can be achieved in only two contacts with Triton X-114 and PMBP. This observation promises potential application of CPE at a process scale.

It was also shown that the mechanism of extraction in this CPE system was not the same as that seen in traditional solvent extraction systems. This is likely due to the difference in composition of the “organic” phases, as the SRP is composed of more than 60% water, hence far more polar than traditional organic diluents. A mixed complex is not seen in extraction of Ln3+

PMBP complexes in nonpolar solvent systems, as the nonpolar organic phase cannot support charged species partitioning. More information and insight into the composition of the SRP are needed to further explain this finding.

58

III.3. Effects of structure-breaking and structure-making electrolytes on a nonionic cloud point extraction (CPE) system

Since CPE involves the rearrangement of water structure, it is appropriate to consider the effects of dissolved salts and their order making/order breaking effects on the properties of water to probe the system. Salt effects on a surfactant system can be determined through a change in the CPT or CMC.25 As the CMC for nonionic surfactants is very low (i.e., 0.2 mM for Triton X-

114)15, changes in the CMC can be difficult to detect, leaving changes in the CPT as the preferred method for monitoring salt effects. Schott, et. al., published a series of papers detailing the influence of electrolytes on the CPT of the nonionic surfactant, Triton X-100.25-32 An increase in the CPT indicates salting-in by the electrolyte, increasing the water miscibility of the surfactant. A decrease in the CPT is caused by salting-out, decreasing water miscibility of the surfactant. For the addition of electrolytes, the effect of anions on the CPT is generally greater than that of the cations. Anions change water structure by either promoting or disrupting association of water molecules, shifting the equilibrium n H2O ↔ (H2O)n to the left or right.

Structure making anions salt-out by increasing structure in the solution, thus increasing the

- - 2- 2- 30 viscosity and surface tension of water. These anions include Cl , F , SO4 , and CO3 .

Structure breaking anions salt-in by disrupting the structure of the surrounding water, increasing the relative concentration of non-hydrogen bonded water molecules. These anions include SCN-,

- - 30 I , and ClO4 .

In the following studies, the influences of different electrolytes on metal partitioning or

SRP composition were examined as they have not been previously reported and can provide useful insight. A system previously developed using PMBP to extract europium with the

59 nonionic surfactant Triton X-114 was investigated in conjunction with a series of lyotropic electrolytes.86

III.3.1. Results

The effects of some of the salts in this study on the CPT (Figure III.3.1) have been previously demonstrated and reported in the literature with analogous nonionic surfactants such as Triton X-100.32 Chloride salts are known to salt-out the surfactant. In CPE this effect is demonstrated through a decrease in the CPT. The degree of salting-out increased linearly with increasing salt concentration between 0 – 1 m, which agrees with previous reports. There is a superimposed effect of the cation on this pattern for chloride, with the CPT being successively

+ + + lower for NH4 > K > Na . A similar effect is not seen with the nitrate or thiocyanate anions.

The nitrate salts had a minor effect on the cloud point temperature. As the nitrate anion has been reported to have no effect on the CPT, any effect seen can be attributed to the cations.30

The temperatures seen are not significantly different from the CPT for a 2 wt % Triton X-114 solution (23°C) in the absence of any additives.15 Increasing the concentration of nitrate salt has a slight impact on the CPT and is dependent on the cation. For KNO3 the CPT decreases from

27 to 24°C, while for NH4NO3 the CPT increases from 26 to 29°C. NaNO3 had a constant CPT of 26°C.

Thiocyanate salts raise the CPT through salting-in and the increase in the CPT correlates with increasing salt concentration. As the salt concentration is increased the differences among the three cations becomes more apparent. At 1 m salt, KSCN had a CPT of 46°C, whereas

NaSCN and NH4SCN had CPTs of 52°C. The CPTs were also determined for all salts in the

60 presence of 0.001 m PMBP (data not shown). PMBP uniformly lowered the CPTs for all the salts by 2-3oC.

NaCl KCl NH Cl 50 4 NaSCN KSCN NH SCN 40 4 NaNO 3 KNO 3

C) NH NO 4 3

o 30

CPT ( CPT 20

10

0.0 0.2 0.4 0.6 0.8 1.0 Concentration (m)

Figure III.3.1: Cloud point temperatures of 2 wt % Triton X-114 solutions with salts as a function of their concentration (□NaCl, ○KCl, ▲NH4Cl, ▼NaSCN, ◊ KSCN, ◄ NH4SCN, ► NaNO3, KNO3, * NH4NO3)

Salts were also found to impact the distribution and partitioning of europium with and without PMBP. In the absence of PMBP, there is minimal partitioning of europium (Figure

III.3.2). The line in the figure represents the extraction of europium by the surfactant alone, which is low, representing at most 10 % of the Eu3+. Addition of salts did not dramatically increase the partitioning of europium, but caused noticeable changes. The partitioning of europium increased with thiocyanate salts to a KD ~ 0.13 (~ 12 % in the SRP) and decreased with chloride salts to a KD ~ 0.04 (~ 4 % in the SRP) compared to a system with no salt (KD ~ 0.06 or

6 %). Nitrate salts had no effect on the partitioning.

61

0.16 NH SCN 4 0.14 KSCN

0.12

0.10

D NaSCN K NaNO 0.08 3 KNO 3 0.06 NH NO 4 3 KCl 0.04 NaCl NH Cl 4 0.02 15 20 25 30 35 40 45 Cloud Point Temperature oC

Figure III.3.2: Partitioning of Eu3+ with 2 wt % Triton X-114 at pH 3, in the presence of 0.5 m salt as a function of the individual salts’ cloud point temperature. Line represents partitioning of Eu3+ in the absence of salt.

When PMBP is added, the extraction of europium increases and a different trend is seen with salts (Figure III.3.3). The line again represents the partitioning of Eu with PMBP in the absence of salt. The trend observed is opposite the trend seen without PMBP. The chloride salts increase partitioning (KD ~ 25, ~96 % in the SRP), thiocyanate salts decrease partitioning (KD ~

5, ~82 %), and nitrate salts slightly increase (KD ~12, ~92 %) partitioning of europium into the

SRP compared to a KD of 10 (~90 %) in the absence of salt.

62

30 NaCl NH Cl 4 25

20 KCl

NaNO

3 D 15

K NH NO 4 3 10 KNO NH SCN 3 4 5 KSCN NaSCN 0 15 20 25 30 35 40 45 Cloud Point Temperature (oC)

Figure III.3.3: Partitioning of Eu3+ with 2 wt % Triton X-114 and 0.001 m PMBP, pH 3, in the presence of 0.5 m salt as a function of the individual cloud point temperatures. Line represents partitioning of Eu3+ in the absence of salt.

To determine if salt concentration would further increase or decrease europium extraction with PMBP, a distribution study with varying salt concentrations was done with sodium salts

(Figure III.3.4). The concentration of salt influenced europium distribution linearly. NaCl increased Eu3+ distribution in proportion to the salt concentration, but NaSCN decreased Eu3+ distribution as salt concentration was increased. The concentration of NaNO3 in the system had no effect on the distribution of europium. The phase ratio for the salt samples in the distribution study did change. The phase ratio for all samples, except those containing thiocyanate, averaged around 14:1 (Aq:SRP). For the thiocyanate salts, the phase ratio dropped to an average of 10.5:1 due to an increase in the size of the SRP. The difference in the phase ratios was taken into account when calculating the KD in the system. The KD values (not shown) still followed the same trend as the DEu values.

63

2.8

2.6

2.4

2.2

2.0 Eu

1.8

1.6 Log D Log NaCl 1.4 NaNO 3 1.2 NaSCN

1.0 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Log [Salt]

Figure III.3.4: Distribution of Eu3+ with 0.001 m PMBP as a function of salt concentration with 2 wt % Triton X-114 at pH 3

Radiotracer sodium was used to monitor partitioning of sodium salts between the SRP and the aqueous phase in the presence and absence of PMBP. The concentration of the salt did not impact the distribution of the Na+ (results not shown). No concentration of the salt in the

SRP was evident, meaning the relative ionic strength was maintained in both phases. The phase ratio remained constant at 13:1 for all samples in the study, with the SRP representing ~ 7 % of the total volume. Without PMBP, NaSCN has the highest partitioning of salt, NaNO3 the lowest partitioning, and NaCl lies in between. The addition of PMBP did not significantly affect

NaNO3 partitioning and only slightly influenced that of NaSCN. The partitioning of NaCl to the

SRP was increased by 3% with the addition of PMBP.

64

Salt (0.5 m) % SRP no PMBP % SRP with PMBP

NaCl 4.2 ± 0.7 7.5 ± 0.8

NaNO3 3.8 ± 0.1 4.7 ± 0.3 NaSCN 7.1 ± 0.4 6.7 ± 0.3

Table III.3.1: 24Na radiotracer salt partitioning study with 2 wt % Triton X-114, with and without PMBP, error represents the standard deviation of triplicate measurements

To determine the stoichiometry of the extracted Eu:PMBP complex, slope analysis experiments were completed. In solvent extraction systems using aqueous and organic phases,

Jordanov, et. al., reported that the variation of lanthanide distribution ratio as a function of

PMBP concentrations yielded a slope of 4.65 The authors proposed the following extraction reaction:

3+ + Ln + 4 HPMBP  Ln(PMBP)3∙ HPMBP + 3H

PMBP extracts lanthanides into the organic phase as a neutral complex; three PMBP ligands exchange H+, while the fourth solvates as the diketone. Due to solubility limits of PMBP in the aqueous CPE system, the slope analysis was performed over a smaller ligand range (1.0 x 10-5 –

0.002 m) in this work. As seen in Figure III.3.5, the distribution ratio increased between 10-5 –

10-4m PMBP before reaching a saturation point above which no further increases in extraction were seen.

65

2.5

2.0

1.5

Eu

1.0 Log D Log

0.5

0.0 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 Log [PMBP]

Figure III.3.5: Distribution of Eu3+ with 2 wt % Triton X-114, varying concentrations of PMBP at pH 3

For slope analysis, the slope of the points prior to saturation was taken, i.e., points below

0.001 m. The slope was 1.7 ± 0.1 and differed significantly from the slope of 4 seen in the traditional organic/aqueous solvent extraction system. The slope analysis was repeated in the presence of nine salts (Table III.3.2), and variations in the slopes were observed. The Cl- salts increased the PMBP slopes to 2, and the SCN- salts decreased the slopes, bringing them close to

- 1. The NO3 salts had a slope similar to that seen with no salt. SRP charge balance must be maintained and different numbers of anions from the supporting electrolytes are partitioning, but how many and from which phase remains unclear.

66

Salt Slope No Salt 1.7 ± 0.1 NaCl 2.1 ± 0.1 KCl 2.1 ± 0.1

NH4Cl 1.9 ± 0.1

NaNO3 1.7 ± 0.1

KNO3 1.7 ± 0.1

NH4NO3 1.7 ± 0.1 NaSCN 1.3 ± 0.1 KSCN 1.2 ± 0.1

NH4SCN 1.1 ± 0.4

Table III.3.2: Slope analysis of Eu3+ extraction with varying PMBP and 2 wt % Triton X-114 in the presence of 0.5 m salt

All the salt samples had different CPTs, but all were heated to 60°C, resulting in different amounts of overheating. To determine whether the overheating (heating over the CPT in °C) influenced KD, an overheating study was performed with NaSCN, NaCl, and NaNO3. The samples were heated to give 10, 20, 30, and 40 degrees above the CPT (Figure III.3.6). As the degree of overheating increased there was an increase in the partitioning of Eu in the presence of

NaCl. However, for NaNO3 and NaSCN, the amount of overheating did not influence the

o partitioning and KD remained constant. At 30 and 40 C overheating, NaCl produced the highest

KD, NaSCN the lowest, and NaNO3 fell in between.

67

20 18 NaCl NaNO 3 16 NaSCN 14 12

10

D

K 8 6 4 2 0 10 15 20 25 30 35 40 Overheating above CPT

Figure III.3.6: Partitioning of Eu3+ with 0.001 m PMBP, 2 wt % Triton X-114, and 0.5 m salt, pH 3. Samples were heated at temperatures a specified amount over the CPT.

The water content of the SRP has not been reported or discussed in earlier studies. Karl

Fischer analysis indicated that, though the SRP is the “organic” phase in a CPE system, it is still mostly aqueous. Karl Fischer analysis indicated the water content of the SRP resulting from a 2 wt % Triton X-114 solution is ~ 60 % by mass. The water content of the SRP also changed with varying salts (Table III.3.3), increasing from 59 % to > 73 % in the NaSCN system. Adding

PMBP to the salt solutions did not change the water content of the SRP (data not shown). The effect of the anions dominated any effect from the cations. The water content of the nitrate and chloride salts were comparable, whereas the SRP in the presence of thiocyanate salts contained significantly more water.

68

Salt wt% H2O No Salt 59 ± 3 NaCl 60 ± 3 KCl 57 ± 2

NH4Cl 59 ± 2

NaNO3 62 ± 3

KNO3 59 ± 2

NH4NO3 67 ± 2 NaSCN 75 ± 1 KSCN 74 ± 1

NH4SCN 77 ± 3

Table III.3.3: Karl Fischer analysis: wt % of H2O present in the SRP after separation in the presence of 0.5 m salt and 2 wt % Triton X-114

III.3.2. Discussion

III.3.2.1. CPT

The CPT study results correlate with previous literature studies on the analogous nonionic surfactant, Triton X-100.30,32 Triton X-100 and Triton X-114 differ only by the number of ethyl ether units in their hydrophilic tails, i.e., n = 9.5 and 7.5 respectively, resulting in different CPTs for the two surfactants.15 Triton X-100 has a much higher CPT, 65°C, compared to Triton X-114, 23°C.7 In a review paper by Hinze and Pramauro, the difference in the CPT was attributed to the change in the length of the polyether chain.14

The change in the CPTs of surfactants, induced by the addition of electrolytes, involves various equilibria in solution. These equilibria have been described by Schott et.al.26,27

1. (H2O)n ↔ n (H2O) (Destructuring/Structuring of H2O)

69

2. + 2 H2O ↔ • 2 H2O (Solvation/Dehydration)

Equation 1 describes increasing or decreasing water structure. Structure breaking anions (such as SCN-) shift Equation 1 to the right and increase the concentration of non-hydrogen bonded water molecules in solution, effectively destructuring water. The “free” (non-hydrogen bonded) water molecules are capable of salting-in the surfactant (Equation 2) by forming hydrogen bonds with the ether groups of the surfactant. Salting-in the surfactant increases its solvation by H2O resulting in an increase in the CPT. Structure-making anions (such as Cl-) push Equation 1 to the left, ordering water, and salt-out by competing with the surfactant for waters of hydration.

Salting-out lowers the CPT as the surfactant becomes less soluble in the aqueous phase. The decrease in solubility is caused by increased water molecule association resulting in dehydration of the surfactant. Nitrate has little effect on the CPT and presumably little or no influence on the structure of water (Equation 1). The anion has a weak tendency for complexation, does not compete with ether groups of the surfactant, and does not mask the salting-in or -out of cations.

Schott, et. al., have reported that certain cations can salt-in surfactants through complexation with the ether oxygen atoms or, if no complexation occurs, cations can

30 + + + alternatively salt-out the surfactant. The cations used in this study, K , Na , and NH4 , do not complex the surfactant and K+ and Na+ are reported to be approximately equally effective as

30 + + + salting-out ions. In this study, K and Na were seen to decrease the CPTs relative to NH4 ; a similar effect is not seen in the strongly de-structuring thiocyanate salts. The effects of K+ and

Na+ in the present study were not equal, as has been suggested in previous reports.30 The effect of the cation is related to the nature of the anion. The K+ cation salts-out more strongly in the presence of thiocyanate and nitrate anions, while Na+ salts-out more strongly in the presence of the chloride anion. As the CPT of NH4NO3 does not change and the nitrate anion does not mask

70

+ + cation effects, it appears that NH4 does not salt-in or -out. This is not surprising as NH4 can engage in hydrogen bonding with water, fitting into the normal water structure.87

A linear relationship between salt concentration and degree of salting-in or -out has been previously reported.32 As the concentration of the salt increases, the effect on Equation 1 increases linearly. The salting-in or -out effect of a salt is concentration dependent and the surfactant is hydrated or dehydrated in proportion to the degree of salting-in and -out. This was demonstrated in this study as the change in the CPT was related to the degree of salting-in or -out caused by electrolyte concentration.

The effect of PMBP on the CPT was not influenced by the supporting electrolyte or electrolyte concentration. Reducing the CPT, in the case of a hydrophobic compound such as

PMBP, demonstrated an increased interaction between micelles and facilitation of phase separation through salting out.88 This result could indicate increased ordering of the water structure which opposes the formation of “cavities” in water that will accommodate the micelles thus pushing the micelles into the SRP, forcing a transition.

III.3.2.2. CPE of Eu3+

In the absence of PMBP, there is minimal partitioning of Eu3+ into the SRP (Figure

III.3.2). The surfactant alone is ineffective at extraction of europium, indicating it has minimal direct interaction with Eu3+ absent PMBP, and that the solvating ability of the surfactant is not strong enough to overcome the metal cation hydration. The europium is neither excluded nor concentrated in the SRP; equal partitioning of the europium is seen. The majority of the europium remains in the larger aqueous phase with only a small percentage in the SRP in parallel with the phase ratio. The differences in KD seen among the salts correlated with the amount of

71 water in the SRP as determined by Karl Fischer titrations. The amount of water in the SRP when thiocyanate salts were present was increased by approximately 15 %. As the partitioning of Eu3+ is very small, the increase observed with the thiocyanate anion can be explained by the increase in water content and size of the SRP and resulting phase ratio decrease (10:1 compared to 14:1).

The difference in the partitioning with nitrates and chlorides is much smaller than the difference between nitrate salts and thiocyanates. The chlorides and nitrates were within error of each other. They also contained the same amount of water in the SRP and did not change the phase ratio of the system. In the absence of PMBP, the partitioning of Eu3+ was directly correlated to the water content of the SRP; thus, it might be concluded that this is an indication of the inherent solubility of Eu3+ in the aqueous portion of the SRP.

When PMBP was introduced, metal partitioning was greatly enhanced and showed an opposite trend with the chloride salts enhancing extraction and the thiocyanate salts decreasing extraction efficiency. The water content and size of the SRP did not change with the addition of

PMBP; however, a hydrophobic metal ligand complex must partition in place of the water

- - soluble metal ion. The SRP in the presence of Cl and NO3 salts contained less water, making it less polar than the SRP formed in the presence of SCN- salts and leading to increased solubility of PMBP and its complexes with Eu3+. The increased solubility accounted for the larger Eu3+

PMBP partitioning seen and was further supported by the slope analysis results.

In contrast with comparable solvent extraction systems, slope analysis results in this

+ study indicated that either charged complexes (e.g., Eu(PMBP)2 ) or mixed ligand species were partitioning into the SRP. Only one or two PMBP molecules partitioned with the metal ion, as compared to four PMBP molecules per europium ion seen in a traditional solvent extraction system.65 In conventional solvent extraction systems, the metal complex must be hydrophobic

72 and charge-neutral for partitioning to a nonpolar organic phase to occur. Though charge neutrality must be maintained in each phase, it appears that this same requirement is not necessary in CPE. This phenomenon may arise from the reality that the counter phase is apparently a micellar “solution” that is mostly water (60-75 % water by mass).

There is evidence in the literature to support the partitioning of charged species in a CPE system. Doroschuk, et. al., demonstrated that charged species of Arsenazo I and III can partition

89 2- into a SRP formed from a nonionic surfactant. They found that 18% of the H4R form of

-3 -4 Arsenazo I transferred to the SRP. Smaller quantities of the H3R and H2R species were also extracted, proving the SRP can incorporate even highly charged ions. In the slope analysis studies of this investigation, a concentration of PMBP was reached above which no further increase in extraction of europium was seen. It is conceivable that this represents saturation of the SRP, suggesting that the solubility of the Eu PMBP complex in the SRP is limited due to

PMBP’s hydrophobic nature.

The thiocyanate salts promoted formation of a 1:1 Eu:PMBP complex, as evidenced by slope analysis, leaving a doubly charged species to partition. The chloride salts promoted the 1:2 complex with a singly charged species partitioning. The nitrate salts did not change the slope analysis and showed a mixture of 1:1 and 1:2 complexes. The DEu values suggest it is easier for the singly charged 1:2 Eu PMBP complex to partition into the SRP than the doubly charged 1:1 complex. The increased partitioning of europium into the SRP promoted by the chloride salts can be attributed to the formation of the 1:2 complex. Doroschuk reported that a higher degree of partitioning was seen with lesser charged species and decreasing partitioning was observed as the charge on the species increased.89

73

The slope analysis results led to a suggestion that the following species are likely predominant in Eu3+ partitioning as facilitated by PMBP:

3+ - + Eu + HPMBP + 2SCN  Eu(PMBP)(SCN)2 + H

3+ - + Eu + 2HPMBP + NO3  Eu(PMBP)2(NO3) + 2 H

3+ - + Eu + 2HPMBP + Cl  Eu(PMBP)2(Cl) + 2 H

To maintain charge neutrality in the SRP, the salts are likely partitioning with the metal ligand complex. The Na24 study did show that some amount of salt partitions into the SRP. This leaves

3-n open the alternative hypothesis that the Eu(PMBP)n species actually partitions with the

+ + + exchange of n medium cations (Na , K , NH4 ) exchanging to the aqueous phase.

The slope analysis provided insight into the differences in DEu seen with the addition of salts showing different mechanisms of extraction. The different mechanisms of extraction correlated with the water content of the SRP. PMBP is sparingly soluble in water and, as such, prefers a less polar environment. An increase in water in the SRP resulted in fewer PMBP molecules partitioning with the metal ion. As the water content increased, the solubility of

PMBP in the SRP decreased. In another study, Doroschuk, et. al., measured the partitioning and relative affinity of the hydrophilic COOH moiety and hydrophobic methylene moiety (CH2) into a SRP.17 This study used monobasic aliphatic carboxylic acids of the general formula

CnH2n+1COOH (n = 3-6) and measured acid extraction. When these acids are considered as bifunctional molecules containing a hydrophobic (methylene) and a hydrophilic (COOH) character, an additive scheme of extraction of the carboxylic acids allows for measurement of the

ΔG of extraction. With increasing water content of the SRP, the extraction of the hydrophobic methylene moiety decreased, while the extraction of COOH increased. The overall ΔG for methylene partitioning into the SRP was higher than that seen in traditional solvent extraction

74 systems, indicating it was harder for the hydrophobic moiety to partition to the “organic” phase in a CPE phase. The ΔG for COOH was much smaller. This is further evidence to support the idea that water content of the SRP plays a critical role in metal ligand partitioning in CPE. The requirements for partitioning of metal ligand complexes in CPE are markedly different from the requirements in traditional solvent extraction.

Water content of the SRP does not completely explain the differences in distribution

- - ratios and mechanisms seen, as Cl and NO3 have the same water content in their respective

- SRPs. However, the NO3 salts do not increase the partitioning of the Eu PMBP complex or the slope compared to a salt free system; the Cl- salts do both. To explain this phenomenon, another aspect of the system should be considered: the salting-in or -out of the surfactant by salts. From the CPT studies, it was determined that the chloride anion salts-out, whereas the nitrate anion neither salts-in or -out. Salting-out decreases the solubility of nonpolar molecules and strengthens hydrophobic interactions, driving PMBP to the less hydrophilic SRP.90 Salting out has also been attributed to the enhancement of the structure of water. The enhancement of the structure of water could help the Cl- salts increase partitioning and explain the difference in partitioning between the chloride and nitrate salts. The distribution difference among the salts cannot be attributed to the amount of overheating as the trend is still apparent even with all the samples being overheated by the same amount. The anions themselves influence the structure.

It is not just a dehydration effect.

III.3.3. Conclusions

The influence and effect of electrolytes on metal ligand distribution and SRP composition have been reported. Electrolytes can salt-in or salt-out the surfactant in a CPE system, which

75 results in a correlated change in the CPT. The water content of the SRP is an important factor in metal ion partitioning in the europium PMBP CPE system and could be influenced by different electrolytes. The change in the water content also resulted in a change in the extraction mechanism, as evidenced by the slope analysis results reported. Understanding how electrolytes influence the SRP and metal distribution of a metal ligand complex in CPE is an important feature for design of future CPE systems and ligand design for this application.

76

III.4. FT-IR Spectroscopic Study of the Surfactant Rich Phase (SRP) from a Cloud Point

Extraction Separation in the Presence of Electrolytes

An understanding of aqueous surfactant solutions with high concentrations of surfactant, such as that found in the SRP, could aid in developing further understanding of CPE systems.

However, there is currently very little research on concentrated aqueous surfactant solutions (>

25 wt % surfactant). Most research on aqueous surfactant behavior has been done at lower surfactant concentrations. Even at relatively low concentrations, interesting behavior has been observed for aqueous surfactant solutions in the presence of electrolytes. A series of papers by

Schott, et.al. on the behavior of the nonionic surfactant Triton X-100 with electrolytes revealed

26-32 - - - significant surfactant electrolyte interactions. Chaotropic anions, such as ClO4 , I , and SCN ,

- - 2- were found to salt-in the surfactant while kosmotropic anions, such as Cl , F , and SO4 , salted- out the surfactant. This behavior resulted in changes in the cloud point temperature of the surfactant solution. Additionally, there have been reports that the electrolytes also changed other properties of the solution, such as the CMC and micelle size and shape.91,92 It is unclear at this time how these effects and electrolyte influences translate to the structure and organization of concentrated surfactant solutions.

It is known that the SRP contains the majority of the surfactant after phase separation and a significant amount of water (as much as 75 % water).86 Previous studies have demonstrated that the water content of the SRP changes with addition of different electrolytes. Salting-in anions, such as SCN-, increase the water content and salting-out electrolytes, such as Cl-, decrease the water content.86 The changes occurring in the structure and organization of the SRP with the change in water content as electrolytes are added are currently unknown. Influences of

77 salts on IR spectra have been reported for neat surfactants and aqueous solutions of surfactants.91-93 Previous work with FTIR and surfactants has examined surfactants and surfactant solutions (up to 15 %), but not concentrated aqueous surfactant solutions such as the

SRP from a CPE procedure.93-102 The purpose of this study was to examine the influence of type and concentration of electrolytes on the formation and structure of a SRP formed from a 2 wt %

Triton X-114 solution using infrared (IR) spectroscopy. Understanding of the fundamental structural changes induced by electrolytes in the SRP would greatly aid in design of new CPE systems.

III.4.1. Results

Spectra of neat Triton X-114 (a liquid), a SRP formed with no salt, and a 2 wt % Triton

X-114 aqueous solution are shown in Figure III.4.1. The spectrum of the neat Triton X-114 indicated the nearly anhydrous nature of the neat liquid surfactant by the very small peak present

-1 97 at ~3500 cm (O-H stretch indicative of the presence of H2O). Triton X-114 exhibited a significant number of peaks in the fingerprint region from 900 to 1600 cm-1 and a split peak at ~

2900 cm-1. The peaks are assigned in Table III.4.1. The SRP spectrum retained some of the peaks in the finger print region, but the majority of the peaks are no longer visible and all the peaks decreased in intensity. Two prominent peaks are visible, at 1639 cm-1 and 3350 cm-1, which can be attributed to the presence of water and O-H stretching.97 The O-H stretches are the only two peaks seen in the spectrum of the 2 wt % Triton X-114 aqueous solution and no peaks in the fingerprint region indicating the presence of the surfactant.

The IR spectra of the SRP were seen to change with increasing salt concentration and for

- - - + different anions, Cl , SCN , and NO3 , with Na (Figures III.4.2-III.4.7, Table III.4.1). The

78 biggest differences were seen in the O-H stretches around 3400 cm-1 and the finger print region from 1750 to 900 cm-1. For all the salts a red shift and decrease in the intensity at ~3350 cm-1 was observed with increasing salt concentration. The shift and change in intensity was greatest

-1 for NaNO3 and smallest for NaSCN. The intensity of the bands in the 1750 – 900 cm region increased for NaCl and NaNO3 (Figure III.4.3 & III.4.5) as salt concentration was increased.

The NaSCN spectra did not demonstrate as many peaks or show as much change in the peaks from 1750 – 900 cm-1 compared to the other salts (Figure III.4.7). Growth of a peak at ~2050 cm-

1 was observed and attributed to the presence of SCN-.97 The peak appeared with increasing

NaSCN concentration, indicating an increased presence of the salt in the SRP. The peaks in the fingerprint region did not shift with an increase in salt concentration, but increased in intensity or changed shape.

100

80

60

40 Intensity

20 neat Triton X-114 Triton X-114 aqueous solution SRP, 2 wt% Triton X-114 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1)

Figure III.4.1: FTIR Spectra of neat Triton X-114, a 2 wt % aqueous solution of Triton X-114, and a SRP formed from a 2 wt % aqueous solution of Triton X-114

79

100

80

60

40 0.01 m NaNO 3 Intensity 0.1 m NaNO 3 1 m NaNO 20 3 3 m NaNO 3 6 m NaNO 3 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1)

Figure III.4.2: FTIR Spectra of the SRP formed from a 2 wt % Triton X-114 solution containing varied NaNO3 concentrations

90

80

70 NaNO 3 0.01 m 60 0.1 m 0.5 m 50 1 m 2 m 3 m

% Transmittance % 40 4 m 5 m 6 m 30 1500 1400 1300 1200 1100 1000 900 Wavenumber (cm-1)

Figure III.4.3: Close-up of finger print region of the FT-IR spectra of the SRP formed with NaNO3

80

100

80

60

40

0.01 m NaCl Intensity 0.1 m NaCl 20 1 m NaCl 3 m NaCl 5.88 m NaCl 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1)

Figure III.4.4: FTIR Spectra of the SRP formed from a 2 wt % Triton X-114 solution containing varied NaCl concentrations

90

80

70

NaCl 0.01 m 0.1 m 60 0.5 m 1 m 3 m

50 4 m % Transmittance % 5 m 6 m 40 1500 1400 1300 1200 1100 1000 900 Wavenumber (cm-1)

Figure III.4.5: Close-up of the finger print region of FTIR spectra of a SRP formed with 2 wt % Triton X-114 and varying NaCl concentrations

81

100

80

60

40

Intensity 0.01 m NaSCN 20 0.1 m NaSCN 1 m NaSCN 6 m NaSCN 0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1)

Figure III.4.6: FTIR Spectra of the SRP formed from a 2 wt % Triton X-114 solution containing varied NaSCN concentrations

90

80

70

NaSCN 0.01 m 60 0.1 m 0.5 m 1 m 50 2 m

% Transmittance % 5 m 6 m 40 1500 1400 1300 1200 1100 1000 900 Wavenumber (cm-1)

Figure III.4.7: Close-up of the finger print region of FTIR spectra taken of a SRP formed with 2 wt % Triton X-114 and varying NaSCN concentrations

82

Triton X-114 NaCl NaNO3 NaSCN Assignment

828 828 CH2 rock, C-O-C stretch

944 946 947 CH2 rock

1100 1096 1093 1102 C-O-C stretch

1186 1186 1186

1245 1246 1245 1249 CH2 twist

1294 1294 CH2 twist

1349 1349 CH2 wag

1364 1364 CH2 wag, C-C stretch

1457 1457 1455 CH2 scissor

1511 1511 1511 1515 C-C stretch

1609 C-C stretch

1641 1636 1634 O-H scissoring

2057 CN stretch

2868 2876 CH2

2948 2952 2951 2949 CH2

3351 3355, 3387 3336, 3416 3356, 3395 O-H stretch

Table III.4.1: Frequency (cm-1) and assignments of FTIR bands of neat Triton X-114 and Triton X-114 SRP with electrolytes97

83

III.4.2. Discussion

III.4.2.1. Neat Triton X-114 and SRP

The changes in the FTIR spectra that were seen demonstrated a large difference between the SRP and the neat surfactant. The neat surfactant has been reported to be crystalline; the changes observed in the spectra of the SRP, when compared to the neat surfactant, showed a change from crystalline to amorphous structure.97 This was likely caused by the large increase in the water content. The crystalline neat surfactant has little-to-no water, as evidenced by the IR spectrum and Karl Fischer data, whereas the SRP has a significant amount of water, approximately 60 wt % water.86 The change in the shape and intensity of the peaks in the fingerprint region of the spectra also indicated a shift from crystalline to amorphous structure.96

The neat surfactant displayed very sharp, high intensity peaks, while the SRP peaks were less intense and broad. The spectrum of the aqueous phase containing 2 wt % surfactant demonstrated no indication of surfactant. Apparently, the surfactant must be present at a more significant concentration for its presence to change the FT-IR spectrum.

III.4.2.2. SRP Formed with Salts

3000 – 3600 cm-1

A variety of changes in different peaks were observed in the spectra of the SRP once salts were added. A shift in the O-H stretch between 3200 and 3400 cm-1 was seen. As the amount of salt was increased, a red shift in the O-H peak was observed. This was indicative of a change occurring in the hydrogen bonding of the water in the SRP. The O-H stretching in the 3000 –

3700 cm-1 region has been attributed to different water environments.99 An O-H peak at 3300 cm-1 indicates water is fully hydrogen bonded. A peak closer to 3600 cm-1 indicates water is

84 participating in less hydrogen bonding and free O-H groups are present. In all the SRP spectra, the O-H peak was close to 3300 cm-1, thus a high degree of hydrogen bonding within the SRP would be expected. As salts were added, the red shift of the O-H stretching indicated a disruption in the hydrogen bonding of the water in the SRP, suggesting more free O-H groups existed as the salt concentration was increased.99 This effect was most dramatic and noticeable

-1 for NaNO3 (Δ 80 cm ), indicating NaNO3 decreased the hydrogen bonding of water in the SRP

- the most at high salt concentrations. This was interesting when considering that the NO3 anion has an intermediate position in the Hofmeister series and does not change the CPT of a CPE system at lower concentrations.26 This is compared to NaSCN and NaCl which represent opposite extremes of the Hofmeister series, representing structure-breaking and structure-making anions. Both anions change the CPT of surfactant solutions, even at low concentrations.26

NaSCN and NaCl caused a shift in the peak, showing both anions disrupted the hydrogen bonds, but to a much lesser degree than the nitrate anion (Δ 39 and 32 cm-1, respectively). In addition to a shift in the peak, a decrease in the intensity of the peak was observed, most noticeably for

-1 NaNO3 (Figure III.4.2). The decrease in intensity of the O-H stretch at ~3400 cm with increasing salt concentration is analogous to decrease in intensity seen for surfactant solutions as the temperature is increased and is likely caused by a dehydration of the surfactant.98 If dehydration of the surfactant is responsible for a decrease in the intensity of the O-H stretching, that would also explain the decrease in hydrogen bonding seen in the NaNO3 system.

2700 – 3000 cm-1

In addition to changes in the hydrogen bonding of water, changes in the interaction of the surfactant with itself were evident. For characterization of changes in the structure and interaction of PEO block copolymers in aqueous media, the wavenumber shifts of the CH3

85 stretching and bending bands ( 2980 and 1370 cm-1) and 1100 cm-1 C-O stretch have been reported as the significant vibration bands.96 The bands in the 2760 – 3060 cm-1 region are

94 associated with asymmetric and symmetric CH2 stretching vibrations of the surfactant. In previous literature the FTIR spectra of a triblock copolymer, Pluronic P105, were taken at different temperatures to examine both crystalline and molten structures of the surfactant.96 For the neat surfactant the strongest band for the crystalline state in the C-H stretching region was the

-1 CH3 symmetric stretching band at 2884 cm . This band appeared with two weak shoulders at

-1 2860 and 2929 cm which represent the CH2 symmetric and asymmetric stretching, respectively.

The symmetric stretching indicated a more crystalline nature, while the asymmetric stretching was attributed to an amorphous surfactant structure.96 Both shoulders were present in the neat surfactant spectrum for Triton X-114, with the peak near 2860 cm-1 being significantly larger since the neat, anhydrous surfactant is mostly crystalline in nature. The spectrum of the SRP formed from a 2 wt % Triton X-114 has a very small peak near 2920 cm-1 caused by asymmetric stretching of CH2. As there is no evidence for symmetric stretching it is assume the SRP is present in an amorphous conformation.

The addition of salts changed the nature of the SRP and this was seen in the IR spectra.

The split/double peak that was present in neat surfactant was also seen in the SRP formed at high salt concentrations for NaNO3 and NaCl. At lower salt concentrations for NaNO3 and NaCl, only a single peak was evident and only a single peak was seen at all concentrations for NaSCN.

-1 The 2860 cm peak grew in with increasing salt concentration for NaNO3 and NaCl, while the second peak at 2950 cm-1 increased in intensity. For NaSCN the largest peak was at 2949 cm-1 showing CH2 asymmetric stretching. No symmetric stretching was indicated for NaSCN even with an increase in the salt as the symmetric peak was not present. For NaNO3 and NaCl both

86 asymmetric and symmetric stretching was indicated, but the symmetric stretching did not appear until high salt concentration. The presence of the symmetric stretching at high NaNO3 and NaCl concentrations could indicate an increase in the crystalline structure of the SRP. The chloride anion is known to be structure-making accounting for an increase in the crystallinity of the

SRP.90 The nitrate anion is not well known for being structure-making or -breaking. However, in this situation, it appeared the nitrate anion had an influence on the structure of the SRP and increased the structure. Thiocyanate is known to be “destructuring,” or structure-breaking and did not increase the structure of the SRP.90 The IR spectra indicated the prevalence of the amorphous structure, even at high salt concentration. The differences and presence or absence of peaks in this region gave an indication of changes in the conformation and mobility of the surfactant, likely representing a change in the overall structure of the SRP, such as a change in the packing and/or packing density of the surfactant.96

1350-1365 cm-1

In the neat surfactant there was a very small sharp peak at 1364 cm-1 which has previously been attributed to the crystalline structure of an anhydrous surfactant.96 This peak

-1 shifted to 1350 cm in the SRP, with NaNO3 and NaCl, and disappeared entirely with NaSCN.

The intensity of the band also changed with salt concentrations for NaCl, and was a large broad peak with shoulders in the presence of NaNO3. In previous literature of surfactants and aqueous surfactant solutions, the FTIR band at 1349 cm-1 was assigned to the amorphous phase of the surfactant.91 The disappearance of the peak in the presence of NaSCN correlated with the absence of any CH2 symmetric stretching and helped confirm the amorphous nature of the SRP formed with NaSCN. The presence and shift of this peak with NaNO3 and NaCl could be attributed to both amorphous and crystalline characteristics of the surfactant being present in the

87

SRP. This also correlated with the presence of both symmetric and asymmetric stretching of the

CH2 peaks.

1100 cm-1

A shift in the 1100 cm-1 peak (C-O stretch) indicated a change in the degree of water hydrogen bonding with the surfactant backbone. A shift toward lower frequency was indicative of an increase in surfactant/water hydrogen bonding.95 Little-to-no shift of the peak was observed for any of the salts. However, the shapes and intensities of the peaks changed with increasing concentration for all three salts. The band increased in intensity with as the salt concentration increased. Some degree of change in the C-O stretch must have occurred to explain the differences seen in the spectra. The changes in the other peaks had demonstrated the changes occurring with both water hydrogen bonding and surfactant interaction. The changes in this peak were present for all salts and could indicate that there is a change in surfactant/water interactions for all salts with increased concentration.

III.4.3. Conclusions

The FTIR spectra of neat Triton X-114 and SRPs formed from aqueous solutions of

Triton X-114 with salts were examined. The SRP demonstrated a number of changes from the neat surfactant and was amorphous compared to the crystalline structure of the neat surfactant.

Once different salts were added the SRP was changed. This was apparent in the differences in the FTIR spectra. These differences became more obvious with increasing salt concentration.

The amorphous and crystalline structure of the SRP could be changed with the addition of different salts. NaNO3 had the most dramatic spectral changes and influenced the structure of the surfactant and interactions of the surfactant within the SRP. NaCl influenced the structure of

88 the SRP to a lesser degree and NaSCN had minimal structural influence compared to the other salts. How or why NaNO3 promoted the most change in the SRP is unclear and further studies are needed. Investigations into the effect of salts on micelle shape and size would be helpful and possibly elucidate the cause of the changes observed in the FTIR spectra.

89

III.5. Protonation Constants of Diglycolic Acid (DGA) in Micellar Solutions of the Nonionic

Surfactant, Triton X-114

Aqueous micellar media have garnered increased attention in recent years. This is partly due to its applicability to separation challenges through the use of techniques like cloud point extraction (CPE). Diglycolic acid (DGA) is a dicarboxylic acid that has been previously used for lanthanide extraction. While DGA is too water soluble to be of use in a CPE system, DGA is easily derivatized to increase its hydrophobicity.103,104 With future ligand design in mind, specifically for lanthanide CPE systems, DGA is an appealing choice for study in micellar systems, as its derivatives already have a demonstrated ability to extract lanthanides in conventional solvent extraction.103 Additionally, the protonation constants of DGA under varying aqueous conditions have been well documented.105-108 The thermodynamics of DGA reactions have not, to our knowledge, been previously examined in aqueous micellar media.

Protonation constants of some acids have been determined in micellar aqueous media.109-

116 Authors have examined the influence of anionic, cationic, and nonionic surfactants on the protonation constants of a variety of acids. Previously the nonionic surfactant, Triton X-100 has been used at relatively low concentrations with various acids.114-116 The authors of these papers noted an increase in the pKa with increasing surfactant concentration. In this study an analogous nonionic surfactant, Triton X-114, which differs only in the length of the polyoxyethylene chain, will be used over a range of concentrations with DGA to examine the effect of the nonionic surfactant on the protonation equilibria of DGA.

90

III.5.1. Results

III.5.1.1. Pseudophase Model

Initially the pKas were determined at different temperatures in the hope that van’t Hoff analysis could be performed to monitor changes in the enthalpy of protonation as surfactant concentration was increased. However, the nature of the system prevents such analysis from being used. The pKa values determined experimentally in the presence of micellar media are an observed pKa and can be different from the actual pKa. Other equilibria must be considered to calculate the pKa of an acid in micellar media as discussed below.

To account for the different equilibria in micellar media, the pseudophase model is a simple thermodynamic model that has been developed and widely used in the literature to describe micelle formation and micellar solubilization phenomena (Figure III.5.1).117 In this model micelles are described as a separate phase into which the acid partitions in both the acid and base form. The acid dissociation is not only occurring in the bulk phase but also in the micellar pseudophase. The partitioning of the acid and base form into the micellar phase is not necessarily equal. A difference in the partitioning can shift the acid base equilibrium, modifying the total proton concentration. For example, if the acidic form is more easily incorporated into the micelles, the equilibrium shifts so that the acid appears weaker and the apparent pKa increases. To account for this equilibrium and to convert the apparent pKa, the ratio of the micellar pseudophase volume to the extramicellar bulk volume must be known. A knowledge of the CMC, volume of a micelle, and the micelle aggregation number is required for those

116 calculations. In this study only the apparent pKas were measured, as the corrections necessary require information indicated above is considered to be beyond the scope of this experiment.

91

Figure III.5.1: Pseudophase Model for Micellar Systems with a monoprotic acid116 (m – micellar pseudophase; b – bulk phase)

[i]m – micellar molar concentration of species, i, with respect to the volume of the micellar pseudophase [i]b – bulk molar concentration with respect to the volume of the extramicellar bulk solvent phase

III.5.1.2. Low Surfactant Concentration at Low Temperature

Table III.5.1 lists the protonation constants for DGA in low concentration solutions of

Triton X-114 at temperatures below the CPT. The surfactant concentrations were at or above the

CMC (≥ 0.2 mM). The experiments were done at low temperatures to prevent micelle aggregation and secondary phase formation. The literature values for the protonation constants of DGA in μ = 0.1 m at 25°C are 3.9 and 6.7.105,107 The protonation constants determined in low surfactant concentrations at temperatures below the CPT are lower than those seen in pure water at 25 °C for all temperatures and surfactant concentrations. An interesting trend is the decrease in both protonation constants as the surfactant concentration is increases up to 2 wt %.

For pKa1 an increase with increasing temperature is seen for all surfactant concentrations.

This is not true for pKa2. In 0.02 wt % Triton X-114 pKa2 does not demonstrate a temperature dependence as the values do not increase or decrease linearly and are nearly constant within the

92 uncertainty limits of the fit parameters. For 0.2 wt % a large jump is seen from 10 to 18°C for pKa2 and in 2 wt % a larger jump is seen from 5 to 10°C in pKa2 with no further change.

III.5.1.3. High Surfactant Concentration at High Temperature

Table III.5.2 lists the protonation constants for DGA in more concentrated surfactant solutions well above the CMC at temperatures above the CPT. This allows for the formation of a biphasic system, likely complicating the system further. For 2 wt % the pKas are lower than in pure water at 25°C. The first protonation constant, pKa1, increases and becomes larger than the literature value at 35 and 45°C, but pKa2 remains lower at all temperatures. When comparing the

2 wt % values to values at lower temperatures, a large increase in pKa2 can be seen to occur from

18 to 25°C. The major difference from 18 to 25°C is that the temperature at 25°C is above the

CPT and micelle aggregation is more probable. This may account for the increase in the pka observed. At 25°C 10 wt % pKa1 is higher than literature while pKa2 is lower until 35 and 45°C.

For 2 and 10 wt % pKa2 does not demonstrate a linear relationship with temperature with the pka2 at 45°C being slightly lower than the one at 35°C. For the other surfactant concentrations (20 and 40 wt %) there is an increase in both pKa1 and pKa2 as the temperature increases. Both protonation constants in 20 and 40 wt % Triton X-114 are higher than the constants reported in water with the 40 wt % value being significantly higher.

III.5.2. Discussion

A change in the pKas of weak acids in the presence of nonionic surfactants has been

109-116 previously reported at 25°C. The change in the pKa was attributed to a difference in the dielectric constant and polarity in a micelle and is analogous to the change seen with the use of

93 different solvents such as dioxane.118 It has been experimentally determined and demonstrated that the interior of a micelle has a different dielectric constant than that seen in bulk water. The dielectric constant for the interfacial microenvironment of nonionic Triton X-100 was reported to be 32 by Drummond, et. al.111,112 This is significantly different from the dielectric constant in pure water.119

The differences seen in the pKas determined at low and high surfactant concentration are not surprising for a number of reasons. First of all the low surfactant concentration experiments were done at lower temperatures preventing a secondary phase formation. Additionally, as surfactant concentration is increased, the size and shape of the micelle has been reported to change.120 This could influence the polarity of the micellar core and the dielectric constant within the micelle further changing the pKa. For water soluble acids, such as DGA, the pKa observed is likely a composite value from species within the interfacial phase and the bulk aqueous phase. At higher surfactant concentrations this becomes less of a concern and could be reflected by the change in the pKa values. For the pKa determinations at high surfactant concentration, it is possible that impurities in the surfactant come into play. It has been previously reported that at high surfactant concentrations the impurities contained within the surfactant (i.e., starting materials) can influence pKa determination as they are reported to have some buffering capacity.113

The increase in pKa with increasing surfactant concentration at high surfactant concentrations was consistent with a previous report. Jaiswal, et. al., noted that in the analogous nonionic surfactant, TX-100, acids tend to dissociate less than in pure water resulting in an

114 increase in the observed pKa. The authors attributed this to the large number of electron- releasing polyoxyethylene head groups possessed by surfactants in the Triton series. Due to a

94 decrease in the dielectric constant caused by the presence of the surfactant, the electron density on the carbon atom in the acid molecule increases which increases the electron density on the adjoining C-OH bond. This increases the electron density in the O-H bond making it harder for the acid to release the proton, increasing the pKa.

This is not observed at lower surfactant concentrations and temperatures as the pKa decreases under those conditions. Jaiswal, et. al., also made the observation that it could be possible that the presence of the ethoxylated oxygen atoms on the surfactant could increase the

114 number of hydrogen bonds and promote dissociation, effectively lowering the pka. The results seen at low surfactant concentrations in this paper perhaps support the idea that at low temperatures and low concentration the ethoxylated oxygen atoms on the surfactant could increase the hydrogen bonding resulting in lower observed pKas.

III.5.3. Conclusions

The nonionic surfactant, Triton X-114, was found to influence the protonation constants of diglycolic acid over a range of concentrations and temperatures. The various equilibria present in a micellar pseudophase prevented the determination of anything other than apparent pKas, limiting analysis of the protonation constants. At low surfactant concentrations and at temperatures below the CPT, the protonation constants were seen to decrease. This could possibly be attributed to an increase in hydrogen bonding caused by the ethoxylated oxygen atoms on the surfactant. At higher surfactant concentrations and at temperatures above the CPT, the protonation constants were seen to increase relative to that seen in pure water. This correlated with previous results in literature and is caused by a change in the dielectric constant within the micelle.

95

Triton X-114 (wt %/ mM) Temperature (°C) Log K1 Log K1K2

0.01 / 0.2 5 3.66 ± 0.01 5.93 ± 0.01

10 3.75 ± 0.00 6.10 ± 0.01

18 3.84 ± 0.01 6.22 ± 0.01

0.02 / 0.4 5 3.66 ± 0.01 6.00 ± 0.01

10 3.71 ± 0.01 6.05 ± 0.01

18 3.76 ± 0.01 6.02 ± 0.01

0.2 / 4 5 3.64 ± 0.01 5.75 ± 0.01

10 3.68 ± 0.01 5.73 ± 0.01

18 3.76 ± 0.01 5.91 ± 0.01

2 / 40 5 3.45 ± 0.01 5.27 ± 0.01

10 3.55 ± 0.01 5.60 ± 0.01

18 3.60 ± 0.00 5.61 ± 0.01

Table III.5.1: Protonation constants for DGA with varying surfactant (Triton X-114) concentrations at temperatures below the CPT, μ = 0.1 m

96

Triton X-114 (wt % / M) Temperature (°C) Log K1 Log K1K2

2 / 0.04 25 3.81 ± 0.01 6.26 ± 0.01

35 3.98 ± 0.01 6.62 ± 0.01

45 4.03 ± 0.01 6.58 ± 0.02

10 / 0.2 25 3.97 ± 0.02 6.56 ± 0.02

35 4.07 ± 0.01 6.85 ± 0.01

45 4.09 ± 0.01 6.76 ± 0.01

20 / 0.4 25 4.12 ± 0.02 6.89 ± 0.02

35 4.19 ± 0.00 7.18 ± 0.01

45 4.31 ± 0.01 7.33 ± 0.01

40 / 0.75 25 4.31 ± 0.03 7.48 ± 0.03

35 4.34 ± 0.01 7.57 ± 0.01

45 4.46 ± 0.01 7.66 ± 0.01

Table III.5.2: Protonation constants for DGA in the presence of varying high concentrations of Triton X-114 at temperatures above the CPT, μ = 0.1 m

97

III.6. Complexation of Lanthanides with Diglycolic Acid (DGA) in Aqueous Micellar Solutions Containing the Nonionic Surfactant, Triton X-114

While complexation of some metal ligand complexes in the presence of surfactants has been reported, the complexation of DGA with lanthanides or actinides in the nonionic surfactant,

Triton X-114, has not been investigated.121-127 In this study, complexation of DGA with La3+,

Nd3+, Eu3+, Ho3+, Lu3+, and Am3+ was studied at varied surfactant concentrations using potentiometry and UV-vis spectroscopy. The pKas of DGA in the different surfactant solutions have previously been determined and reported.86

For these studies the surfactant concentration was calculated as a weight percent of the total solution. From the weight percent, an approximate concentration of the surfactant can be calculated and is shown along with the weight percent in Tables III.5.1 and III.5.2. The lowest concentration of surfactant in this study is approximately equal to the CMC of Triton X-114 and thus can be considered analogous to the aqueous phase after extraction in CPE. At the highest surfactant concentration (> 20 wt %) the surfactant solution is analogous to the SRP generated during a CPE separation.

III.6.1. Results

III.6.1.1. UV-visible

No change was observed in the initial spectra of Nd(ClO4)3 or Ho(ClO4)3 as the surfactant concentration was increased. Once DGA was added, the spectra shifted indicating metal complexation. For Nd(ClO4)3 the hypersensitive peak at 575 nm showed a new peak and a shoulder appeared to the right of the metal ion peak indicating complexation and formation of the 1:1 and some 1:2 complexes with DGA (Figure III.6.1).

98

0.18 20 mm Nd(ClO ) 4 3 0.1 m NaCl 60 mm DGA, pH - 1.2 0.16 20 wt % Triton X-114 pH - 1.4 pH - 1.7 0.14 pH - 2.0 0.12 pH - 2.2 pH - 2.5 0.10 pH - 2.9 pH - 3.8 0.08

0.06

Absorbance 0.04

0.02

0.00 550 560 570 580 590 600 610 Wavelength (nm)

Figure III.6.1: UV-vis spectra of 20 mm Nd(ClO4)3 in 20 wt % Triton X-114 at 0.1 m NaCl ionic strength with 60 mm DGA; Additions of 1 M NaOH were made to adjust the pH

As the pH was increased, further shifts and the appearance of peaks were seen, indicating further complexation. At the highest pH three peaks and a small shoulder were visible,

3+ demonstrating the presence of four species in solution, assumed to represent uncomplexed Nd aq and the three metal ligand complexes. As the pH was increased some differences in the spectra were seen at different surfactant concentrations indicating slight changes in the speciation in the presence of surfactant. This could be seen in the calculated stability constants which increased for all three Nd:DGA species as the surfactant concentration was increased (Table III.6.1). For

3+ the Ho aq spectra with DGA and changing pH, the spectral changes were less dramatic than those seen for Nd3+ (Figure III.6.2). With the addition of DGA, the absorbance of the hypersensitive peak at 453 nm increased and began to split. As the pH was adjusted the peak splitting became more pronounced, indicating the change in the Ho:DGA species present. The

HypSpec fitting of the Ho spectra also show an increase in the stability constants with increasing

99 surfactant concentration (Table III.6.1). At 40 wt % surfactant the stability constants for DGA complexes with Nd3+ and Ho3+ were significantly larger than that seen in water. Due to the viscous nature of the high surfactant concentration solutions, the titrations with Am3+ were only completed in water, 2 % and 10 % Triton X-114. The peak at 503 nm experienced a red shift with the addition of DGA and subsequent pH increase (Figure III.6.3). A decrease in the stability constants in 2 wt % Triton X-114 compared to water was seen for Am3+. At 10 wt % the 1:1 and 1:2 metal ligand complexes have larger stability constants than in water, but this was not true for the 1:3 complex.

Ho(ClO ) 0.18 4 3 0.1 m NaCl 60 mm DGA, pH - 1.18 0.16 10 wt % Triton X-114 pH - 1.34 pH - 1.51 0.14 pH - 1.69 pH - 2.07 0.12 pH - 2.54 pH - 8.71 0.10

0.08

0.06

Absorbance 0.04

0.02

0.00 430 440 450 460 470 480 490 500 Wavelength (nm)

Figure III.6.2: UV-vis spectra of 20 mm Ho(ClO4)3 in 10 wt % Triton X-114 at 0.1 m NaCl ionic strength with 60 mm DGA; Additions of 1 M NaOH were made to adjust the pH

100

0.5 Am Stock 1 mM 243Am(NO ) 3 3 5 mm DGA, pH - 2.16 0.1 m NaCl pH - 2.39 0.4 10 % Triton X-114 pH - 2.75 pH - 3.39 pH - 9.07

0.3

0.2

Absorbance 0.1

0.0 490 500 510 520 Wavelength (nm)

Figure III.6.3: UV-vis spectra of 1 mm Am(NO3)3 in 10 wt % Triton X-114 at 0.1 m NaCl ionic strength with 60 mm DGA; Additions of 1 M NaOH were made to adjust the pH

Metal Wt % Triton X-114 1:1 1:2 1:3 Nd 0 6.39 ± 0.03 11.19 ± 0.07 15.09 ± 0.41 2 6.42 ± 0.10 11.16 ± 0.17 15.13 ± 0.29 10 6.59 ± 0.03 11.36 ± 0.05 14.98 ± 0.12 20 7.05 ± 0.05 11.84 ± 0.08 15.81 ± 0.28 40 7.55 ± 0.07 12.39 ± 0.13 17.43 ± 0.27

Ho 0 6.02 ± 0.02 11.01 ± 0.07 14.54 ± 0.15 2 6.16 ± 0.12 11.60 ± 0.51 15.76 ± 0.70 10 6.82 ± 0.10 12.24 ± 0.16 16.16 ± 0.24 20 6.84 ± 0.02 11.78 ± 0.08 15.78 ± 0.25 40 6.95 ± 0.02 12.23 ± 0.17 17.37 ± 0.48

Am 0 6.25 ± 0.19 10.85 ± 0.25 13.82 ± 0.34 2 6.24 ± 0.11 10.61 ± 0.14 13.53 ± 0.18 10 6.40 ± 0.10 10.92 ± 0.12 13.60 ± 0.15

Table III.6.1: Metal ligand stability constants determined by UV-visible titration with DGA and Triton X-114, 20 mm Nd3+ and Ho3+, 1 mm Am3+, μ = 0.1 m NaCl, 25°C

101

III.6.1.2. Potentiometric Titrations

The stability constants for all three DGA metal complexes were determined for La3+,

Eu3+, and Lu3+ in 2, 10, 20, and 40 wt % Triton X-114 at 25, 35, and 45°C. The temperature study was initially done with the purpose of performing van’t Hoff analysis of the metal ligand complexation in surfactant to evaluate the enthalpic and entropic contributions to complex stability. However, this proved impossible without further information on the system. This aspect of the system will be addressed in the discussion section. The data at 25, 35, and 45°C are presented in Tables III.6.2-III.6.4 along with literature values for the Ln:DGA complexes at 0.1 and 1 M ionic strength (25°C).

25°C

The literature values show that the stability constants for the Lu3+ and Eu3+ DGA complexes are approximately equal and much stronger than those for La3+ in water.134 In this study, as the surfactant concentration increased the stability constants increased up to a maximum value at 40 wt % surfactant. The general trend seen was that Lu3+ formed the strongest complexes, followed by Eu3+ > La3+ in the surfactant solutions. At most surfactant concentrations, the stability constants determined were less than the literature values in water.

The values in 40 wt % surfactant were larger than literature values and indicated an increase in the strength of all three metal ligand complexes for La3+, Eu3+, and Lu3+. Both La3+ and Lu3+ were calculated to have stability constant values greater than that seen in water at 2 wt % Triton

X-114. This number decreased for 10 and 20 wt % Triton X-114 for the 1:1 and 1:2 complexes with La3+ and all three Lu3+- DGA complexes before a final increase at 40 wt %. Only at 40 wt

% were the Eu3+- DGA stability constants higher than those seen in water.

102

35°C

Almost no correlation could be made with the stability constants at different temperatures so the data were treated separately for the purposes of this paper. In general for all metals and metal ligand species, the stability constants were higher than those seen at 25°C at lower surfactant concentrations, but not for 40 wt %. For La3+, Eu3+, and Lu3+ a steady increase in the stability constant for the 1:1 species was seen as the surfactant concentration increased. This did not hold true for the 1:2 and 1:3 species. For Eu3+ and Lu3+, the 1:2 species stability constants appeared to increase with increasing surfactant concentration with some deviation. The 1:2 species for La3+ did not demonstrate any trend or much change. The stability constants for the

1:3 species actually decreased as the surfactant concentration increased for Lu3+ and for La3+ with the exception of the 2 wt % data. The same was not true for Eu. No trend was observed for the 1:3 Eu:DGA species.

45°C

A general increase in the stability constants with increasing surfactant concentration was seen for all metals and metal ligand species. The stability constants also tended to be slightly larger than the stability constants observed at 35°C.

103

Metal Wt % Triton X- 1:1 1:2 1:3 (25 °C) 114 La 0 4.93 8.41 10.25* μ = 1 2 5.63 ± 0.03 9.34 ± 0.04 11.89 ± 0.2 10 4.92 ± 0.05 8.97 ± 0.04 12.20 ± 0.06 20 5.13 ± 0.07 9.16 ± 0.07 12.31 ± 0.15 40 6.82 ± 0.12 11.53 ± 0.13 14.06 ± 0.25

Eu 0 5.53 10.04 13.20* μ = 1 μ = 0.1 6.41 11.28 14.23* 2 5.09 ± 0.05 9.57 ± 0.03 13.48 ± 0.04 10 5.43 ± 0.06 9.78 ± 0.05 13.90 ± 0.06 20 5.39 ± 0.08 10.37 ± 0.04 13.71 ± 0.09 40 7.52 ± 0.14 12.76 ± 0.14 15.67 ± 0.17

Lu 0 5.64 10.55 13.16* μ = 1 μ = 0.1 6.41 11.61 14.20* 2 6.47 ± 0.09 11.65 ± 0.08 14.84 ± 0.17 10 5.30 ± 0.12 10.09 ± 0.06 14.36 ± 0.09 20 5.53 ± 0.08 10.71 ± 0.04 14.30 ± 0.08 40 7.91 ± 0.70 14.95 ± 0.60 19.65 ± 0.60

Table III.6.2: Lanthanide stability constants with DGA determined potentiometrically in the presence of Triton X-114 at 25°C, μ = 0.1 m NaCl, * indicates values from literature28

Metal Wt % Triton X- 1:1 1:2 1:3 (35 °C) 114 La 2 5.23 ± 0.04 8.98 ± 0.05 12.03 ± 0.06 10 5.47 ± 0.04 9.57 ± 0.04 12.70 ± 0.06 20 5.63 ± 0.04 9.85 ± 0.05 12.47 ± 0.09 40 5.95 ± 0.10 9.55 ± 0.15 12.25 ± 0.33

Eu 2 5.66 ± 0.08 10.05 ± 0.07 13.46 ± 0.09 10 5.79 ± 0.03 10.47 ± 0.02 14.02 ± 0.03 20 5.70 ± 0.09 10.40 ± 0.08 12.85 ± 0.17 40 6.37 ± 0.08 11.43 ± 0.07 14.20 ± 0.12

Lu 2 5.74 ± 0.14 11.13 ± 0.08 15.21 ± 0.10 10 5.88 ± 0.09 10.94 ± 0.07 14.62 ± 0.11 20 6.01 ± 0.18 11.57 ± 0.11 14.35 ± 0.16 40 6.46 ± 0.16 11.71 ± 0.14 13.85 ± 0.39

Table III.6.3: Lanthanide stability constants with DGA determined potentiometrically in the presence of Triton X-114 at 35°C, μ = 0.1 m NaCl

104

Metal Wt % Triton X- 1:1 1:2 1:3 (45 °C) 114 La 2 5.34 ± 0.07 9.32 ± 0.07 12.28 ± 0.12 10 5.41 ± 0.03 9.27 ± 0.03 11.90 ± 0.05 20 5.75 ± 0.07 9.72 ± 0.09 12.54 ± 0.17 40 5.78 ± 0.07 9.83 ± 0.08 12.43 ± 0.18

Eu 2 5.73 ± 0.07 10.16 ± 0.06 13.60 ± 0.07 10 5.78 ± 0.04 10.33 ± 0.04 13.92 ± 0.05 20 6.14 ± 0.04 10.87 ± 0.04 13.79 ± 0.09 40 6.48 ± 0.07 11.47 ± 0.07 14.83 ± 0.11

Lu 2 5.45 ± 0.19 10.50 ± 0.09 14.30 ± 0.17 10 5.69 ± 0.08 10.64 ± 0.05 13.85 ± 0.12 20 6.31 ± 0.14 11.59 ± 0.11 14.30 ± 0.18 40 6.40 ± 0.29 12.11 ± 0.21 15.81 ± 0.28

Table III.6.4: Lanthanide stability constants with DGA determined potentiometrically in the presence of Triton X-114 at 45°C, μ = 0.1 m NaCl

III.6.2. Discussion

III.6.2.1. UV-vis Spectra

The hypersensitive peaks of Nd3+ and Ho3+ have been well documented in the literature and are known to change as the metal ion environment changes.135 Nd3+ has two hypersensitive peaks; the 580 nm peak was chosen for fitting in this study. The peak at 580 nm represents the

4 4 2 3+ transitions from the I9/2 ground state to G5/2 and G7/2. Ho has potentially three hypersensitive peaks, but only one that demonstrates extreme hypersensitivity. This peak occurs at 450 nm and

5 5 5 was used for analysis in this study. The peak represents the I8  G6, F1 transition. No visible change was seen in the hypersensitive peaks with the addition of surfactant for either metal. This likely indicates that the surfactant does not directly influence the metal ion symmetry as any perturbations of the symmetry would change the observed spectra. Fluorescence emission studies on the inner coordination sphere of Eu3+ in the presence of Triton X-114 supported this

105 idea.86 Once DGA was added to the metal ion surfactant solutions some subtle changes were visible, caused by the slight differences in speciation. This was represented in the changes in the stability constants.

Overall, the increase in the surfactant concentration tended to increase the strength of the metal ion interactions with DGA. Previous literature reports have indicated the formation of different species in the presence of surfactant micelles owing to their hydrophilic and hydrophobic nature, but these papers did not report or indicate formation of stronger metal ligand complexes. Mendez, et. al., reported the complexation of Nd3+ with 1-(2-pyridylazo)-2-naphthol as the concentration of Triton X-100 was increased.121 The authors reported a decrease in the stability constant with increased surfactant concentration, but only looked at very dilute surfactant solutions, less than 0.1 wt %. The decreased stability was attributed to incorporation of the ligand in the micelle core, artificially decreasing the ligand concentration in the bulk water, thus making complexation more difficult. The increased stability seen with Nd3+ and

Ho3+ with DGA seemed to indicate that DGA is not partitioning into the micelles, which is reasonable as DGA is very water soluble (can reach concentrations > 1 M in water) and would not likely prefer the hydrophobic core of a micelle to the bulk aqueous phase.136

The increase in metal ligand stability with increasing surfactant concentration can be attributed to a decrease in the dielectric constant and change in the polarity of the solution.

Similar increases in stability constants were seen for other metal ligand systems in solvents with decreasing dielectric constant or polarity.119 The dielectric constant in a Triton X-100 micelle has been reported to be approximately 32, very different from water alone which has a dielectric constant of 80.113,120

106

The increase in the stability constants, when the surfactant concentration changed from

20 wt % to 40 wt %, was significantly larger compared to the other changes observed. This could be explained by considering the structure of the micellar phase. Kimura, et. al., used polarized FT-IR to examine the Triton X-100 and water system.137 In their study, they saw a sudden breakdown in the viscosity of the surfactant water solution at ~ 40 wt % surfactant. This was explained as a phase transition occurring due to a change of the structured hydrogen-bonded water network among the micelles. The oxyethylene chains of the surfactant below this concentration are completely surrounded by water and isolated from each other. The break down in viscosity and change in hydrogen bonding at 40 wt % changes the interaction among the micelles and allows for fluidity of the system.137

The stability constants determined for Am3+ did not appear to exhibit a trend. The stability constants at 2 wt % were slightly smaller but, within error limits, the same as stability constants determined in water. At 10 wt % Triton X-114 the stability constants increased slightly. The increase in the stability constants seen for Nd3+ did not occur until higher surfactant concentrations. The same was true for La3+, Eu3+, and Lu3+ in the potentiometric titrations.

III.6.2.2. Potentiometric Titrations

To consider the stability constants determined in surfactant solutions by potentiometric titration, the micellar pseudophase model must be considered (Figure III.5.1). The presence of micelles inhibited the ability to perform van’t Hoff analysis on these systems using potentiometric titrations due to the various equilibria that were not accounted for in this study.

107

For La3+, Eu3+, and Lu3+ at 25°C there was a large increase in the metal ligand stability constants from 20 wt % to 40 wt % Triton X-114. In a study of a Triton X-100 aqueous system by Kimura, et. al., a change in the behavior of the surfactant was observed at 60 wt % water.137

This change was described as a phase transition and is accompanied by a breakdown in the viscosity of the surfactant. The fluidity of the system increases allowing for more mobility. The increase in mobility can help explain the sudden increase in the stability constants. Prior to 40 wt %, there is an overall decrease in the stability constants. The increase in the stability constants at 40 wt % is of interest because this is the approximate concentration of surfactant in the SRP in CPE.

A decrease in metal ligand stability constants has been previously reported for low concentration surfactant solutions.121 Mendez, et.al. attributed the decrease to solubilization of the ligand in the micelle interior, effectively decreasing the ligand concentration.121 As previously noted in this paper, it is not likely that DGA would partition into a hydrophobic micelle core due to its water solubility and this decrease was not seen in the spectrophotometric titration data. This could indicate that potentiometric titrations may not be the most precise way to determine stability constants in solutions with high surfactant concentrations. With potentiometric titrations, the micellar psuedophase must be considered along with potential viscosity effects. The electrode can be influenced by both factors and is submerged in a heterogeneous system as the surfactant solutions were above the CPT allowing for micelle aggregation. This would also explain the lack of trend for the stability constants observed with increasing temperature. The nature of the information required for any potential stability constant corrections goes beyond the scope of this report.

108

III.6.3. Conclusions

The stability constants for DGA with five lanthanides (La3+, Nd3+, Eu3+, Ho3+, Lu3+) and

Am3+ were determined in surfactant solutions ranging from 2 to 40 wt %. The stability constants for all five lanthanides were significantly increased at 25°C and 40 wt % Triton X-114. This could indicate an increased metal ligand interaction in the SRP in a CPE system as the SRP composition is generally around 40 wt % surfactant. Spectrophotometric titrations proved to be a superior method for stability constant determination compared to potentiometric titrations as no corrections for the micellar pseudophase or electrode behavior must be made. Potentiometric titrations at different temperatures did not yield stability constants that could be used for van’t

Hoff analysis. This is likely because of other factors, such as the micellar pseudophase, which should be considered.

109

III.7. Fluorescence of Eu3+ with Diglycolic acid in Surfactant Solutions: Insights into

Cloud Point Extraction

In this work, features of the coordination chemistry of the well characterized europium diglycolic acid (DGA) system have been examined in CPE “surrogate” systems, solutions containing significant amounts of surfactant, to compare metal ligand complexation in the presence and absence of the nonionic surfactant, Triton X-114.66-69 DGA was chosen not only because of its extensive study with lanthanides in the literature, but also for its easy derivatization and the potential for future ligand design.138,139 Europium was chosen as the metal ion for its fluorescent properties which allowed the use of fluorescence emission spectroscopy and time resolved fluorescence spectroscopy (TRFS) to examine the inner coordination sphere of the metal ion. The hydration number of the inner coordination sphere and details of the coordination environment of Eu3+ can be directly measured using TRFS, allowing for determination of ligand coordination numbers and, in this case, to characterization of the influence of surfactant.76

III.7.1. Background

III.7.1.1. Lifetime data

The waters of hydration in the inner coordination sphere for Eu can be calculated from luminescence lifetime decay, kobs, determined from TRFS. This is done by examining the differences in the lifetime in both D2O and H2O solutions. This method precludes any de- excitation lost through ligand contribution. However, if it can be assumed that there is no de- excitation energy lost through the complexing ligands, the lifetime measurements in H2O can be used with the following equation to determine hydration numbers76:

110

-3 NH2O = 1.05 x 10 kobs (Eu) – 0.44 (or 0.7) (± 0.5 water)

This expression assumes that all non-radiative de-excitation occurs through the vibration of the

O-H groups associated with waters of hydration. Since DGA has no O-H groups for non- radiative de-excitation, it is reasonable to expect that changing emission lifetimes are attributable to the presence of inner-sphere water molecules in the emitting species. Previous reports support the premise that carboxylic acids do not contribute.140,141

III.7.2. Results

III.7.2.1. Fluorescence

Hydration of Eu3+ in the absence of complexant

A single exponential decay model was sufficient to fit the lifetime data of Eu3+ in aqueous solutions containing no surfactant. The uncomplexed hydrated metal ion in water contained nine waters in the primary coordination sphere (Table III.7.1), in agreement with previous literature results for Eu3+ in aqueous solution.142 Choppin et al. reported a hydration number of nine for Eu in 0.1 m NaCl and water. For all Triton X-114 solutions, the lifetime data of the Eu3+ was fit with a double exponential decay model, indicating the presence of two Eu3+ species that do not exchange rapidly in solution (Table III.7.1). In this study, the hydration number of Eu3+ did not change in aqueous solutions of 10 and 20 wt % Triton X-114; however, the hydration number decreased to eight in aqueous solution containing 40 wt % Triton X-114.

The species accounting for the second slower decay has a significantly longer emission lifetime.

This species is almost completely dehydrated, containing only one or two waters in its inner coordination sphere. Interestingly, an increase in surfactant concentration does not affect the hydration of this species.

111

Results for the Eu3+ fluorescence decay for equivalent samples containing sodium salts as supporting electrolyte are shown in Table III.7.1. Again, two species were seen. The lifetime/ hydration of the long-lived species appeared to be largely independent of the identity and concentration of the salt. In contrast, there was a statistically significant variation in the hydration of Eu3+ species with short fluorescence lifetimes. Cation hydration appears to be constant for a given salt type (independent of surfactant concentration), increasing in the

- - - apparent order NO3 < Cl < SCN . By changing from NaSCN to NaNO3, a decrease of two waters was seen. There was a slight difference between the hydration number in 20 wt % and 40 wt % Triton X-114 for the hydrated metal ion, with the hydration number being smaller in the 40 wt %, indicating less hydration. However, when considering that the error associated with these calculations is 0.5 water molecules, the difference was not necessarily statistically significant.

Triton Salt Concentration Lifetime n H2O Lifetime n H2O X-114 % (m) (μsec) (μsec) 0 NaCl 0.1 9 - - 10 NaCl 0.1 113 ± 0.3 8.7 453 ± 6 1.7 20 NaCl 0.1 114 ± 0.6 8.7 431 ± 6 1.8 40 NaCl 0.1 121 ± 1.2 8 474 ± 3 1.6 20 NaSCN 0.5 103 ± 0.4 9.6 467 ± 4 1.6 40 NaSCN 0.5 109 ± 0.4 9.1 464 ± 1 1.6 20 NaCl 0.5 113 ± 0.6 8.7 465 ± 5 1.6 40 NaCl 0.5 119 ± 0.8 8.3 488 ± 3 1.5

20 NaNO3 0.5 128 ± 1.2 7.7 478 ± 11 1.5

40 NaNO3 0.5 131 ± 1.5 7.5 481 ± 9 1.5

Table III.7.1: Decay lifetimes and waters of hydration: 1 mm Eu(ClO4)3 , varying background electrolyte, varying Triton X-114 wt%, pH ~ 5

112

Eu DGA Complex Fluorescence

Water

3+ + - 3- DGA is known to form three complexes with Eu : ML , ML2 , and ML3 , and has been shown in a previous report to displace approximately three waters for each ligand binding in aqueous systems.134 Lis and Choppin reported a hydration number of 6.4 for the 1:1 complex,

3.5 for the 1:2 complex, and 0 for the 1:3 complex at pHs 5, 6, and 7, respectively. Their

3+ determinations were done using the lifetime of the Eu DGA species in H2O and D2O. The data

3+ obtained in this current work was obtained by examining the lifetime decay of Eu in H2O under different experimental conditions (i.e., lower pH). To compare the data with previous work the n bar values were calculated for each metal ligand ratio using stability constants for Eu3+ and DGA determined in water.86 The n bar values were calculated using the equation reported by Choppin and Chopoorian which is based on speciation calculations and the pH of the solution (See

Appendix for values).143 The n bar values are shown in Table III.7.2 and represent the average number of ligands bound to the metal under the given conditions.143 The lifetimes observed for the Eu3+ DGA complex in water (present work) correspond well to reported literature values

(Table III.7.2).134 For a n bar value of one, meaning one ligand bound to the metal, the hydration number is around six, agreeing with Choppin’s report of displacement of three waters per DGA binding. Once n bar is two, the hydration number drops to three or four waters, correlating with the loss of five or six waters of hydration. The speciation and n bar calculations do not predict a predominance of the 1:3 species even at metal ligand ratios of 1:4 and 1:10 because of the pH of this experiment (pH ~ 3), thus complete dehydration is not expected or observed.144

113

Surfactant

Once the surfactant is added, the system changes and the n bar values must be recalculated using stability constants determined in surfactant solutions.86 Some variation in n bar was seen as the surfactant concentration changed (Table III.7.2). Slight variation in the hydration of the metal ligand complex was also observed as the surfactant concentration was changed. However, the major change was that the lifetime data of Eu3+ with DGA in aqueous surfactant solutions was fit with a double exponential decay model, indicating two Eu3+ emitting species at all DGA ratios when surfactant is present. In the 10 and 40 wt % solutions the second species (Species II) was slightly hydrated at the lowest metal to ligand ratios, containing perhaps

1-2 water molecules. The long lived species became completely dehydrated as the ligand concentration increased. For 20 wt % Triton X-114, all secondary species were dehydrated with very long lifetimes. No difference was seen in the hydration of the primary Eu DGA complex

(Species I) in water and 20 wt % Triton X-114. In 40 wt % Triton X-114 a slight decrease in the hydration ( ≥ 0.5 waters) was seen at the 1:0.6 and 1:1 ratios and in 10 wt % Triton X-114 a slight increase in hydration was seen at all metal ligand ratios.

Na+ Salts

The influence of added sodium salts on the inner coordination sphere was examined in 20 and 40 wt % Triton X-114 solutions with Eu3+ and DGA (Table III.7.3). Two emitting species were indicated. While few differences were seen in the presence of varying surfactant and 0.1 m

NaCl for the hydration of the metal ligand complex, addition of 0.5 m of NaCl, NaSCN, and

NaNO3 changed the observed hydration. For all salts, there was a notable decrease in the hydration of the metal ligand complex in 40% relative to the 20% solutions. There was some evidence for such an effect in the 0.1 m NaCl data but the difference was small and values were

114 still within the level of uncertainty in the measurement which made it difficult to draw a conclusion. The difference seen with the addition of more complex salts was more dramatic and statistically distinct.

Eu Species I Eu Species II

Triton X- Eu:DGA n bar Lifetime n H2O Lifetime (μsec) n H2O 114 % Ratio (μsec) 0 1:0.6 0.6 150 ± 0.4 6.4 - - 0 1:1 1.0 164 ± 0.4 5.8 - - 0 1:2 1.7 198 ± 0.3 4.7 - - 0 1:4 2.1 231 ± 0.5 3.9 - - 0 1:10 2.2 268 ± 0.4 3.2 - - 10 1:0.6 0.6 142 ± 1.8 7.0 450 ± 20 1.7 10 1:1 0.9 157 ± 0.7 6.3 15000 ± 700 0 10 1:2 1.6 187 ± 0.6 5.2 5600 ± 160 0 10 1:4 2.3 217 ± 0.7 4.4 3800 ± 75 0 10 1:10 2.7 244 ± 1.2 3.9 2700 ± 53 0 20 1:0.6 0.6 149 ± 0.8 6.5 1500 ± 8 0 20 1:1 1.0 165 ± 1.0 5.8 2200 ± 20 0 20 1:2 1.7 192 ± 0.6 4.9 7040 ± 240 0 20 1:4 2.1 216 ± 1.0 4.3 3000 ± 60 0 20 1:10 2.3 248 ± 1.0 3.6 3700 ± 90 0 40 1:0.6 0.6 159 ± 1.5 6.0 800 ± 30 0.6 40 1:1 0.8 179 ± 2.0 5.3 810 ± 30 0.6 40 1:2 1.4 205 ± 1.0 4.5 1600 ± 120 0 40 1:4 1.9 229 ± 0.7 4.0 3400 ± 570 0 40 1:10 2.1 258 ± 0.8 3.5 3200 ± 560 0

Table III.7.2: Decay lifetimes and waters of hydration: 1 mm Eu(ClO4)3 , 0.1 m NaCl, varying Triton X-114 wt %, varying DGA, pH ~ 3

Among the salts there was also a difference in hydration. NaNO3 decreased the hydration in both 20 and 40 wt % surfactant at all metal to ligand ratios. The most pronounced decreases occurred in 40 wt % and at low metal to ligand ratios. NaSCN increased the waters of hydration for the 1:0.6 and 1:1 ratio in the 20 wt %. In 40 wt % NaSCN decreased the hydration by 1-2 waters for all metal ligand ratios except 1:0.6. Little difference was seen with NaCl in 20 wt %

Triton X-114, but in 40 wt % a decrease was seen at low metal ligand ratios.

115

A secondary species was also seen in 20 wt % Triton X-114. The second species was completely dehydrated at all metal to ligand ratios with all salts. However, for 40 wt % samples, the second species was not completely dehydrated at lower metal to ligand ratios and contained between 1 and 2 waters of hydration. The salts influenced the hydration of this secondary species in the 40 wt % solution. With NaSCN, the second species retained a water molecule up to a metal to ligand ratio of 1:10. For NaCl, water was retained up to a ratio of 1:2 and, for

NaNO3, only the 1:0.6 ratio demonstrates any hydration.

Eu Species I Eu Species II

Triton X- Salt Eu:DGA Lifetime (μsec) n H2O Lifetime (μsec) n H2O 114 (%) (0.5 m) Ratio 20 NaNO3 1:0.6 164 ± 0.4 5.8 1500 ± 45 0 1:1 173 ± 0.4 5.5 3000 ± 200 0 1:2 198 ± 0.5 4.7 6000 ± 2000 0 1:10 266 ± 0.5 3.3 9500 ± 1500 0 20 NaCl 1:0.6 153 ± 0.7 6.3 1400 ± 60 0 1:1 158 ± 0.5 6.1 2000 ± 120 0 1:2 198 ± 0.6 4.7 3040 ± 700 0 1:10 256 ± 0.6 3.5 2800 ± 400 0 20 NaSCN 1:0.6 139 ± 0.9 7.0 1400 ± 30 0 1:1 158 ± 1 6.0 1500 ± 40 0 1:2 196 ± 0.7 4.8 5700 ± 700 0 1:10 288 ± 1 3.0 6200 ± 1000 0 40 NaCl 1:0.6 184 ± 9 5.1 520 ± 20 1.3 1:1 198 ± 4 4.7 540 ± 7 1.3 1:2 205 ± 1 4.5 1400 ± 75 0 1:10 266 ± 1 3.3 1800 ± 190 0 40 NaNO3 1:0.6 180 ± 14 5.2 480 ± 50 1.5 1:1 211 ± 1 4.4 2000 ± 80 0 1:2 222 ± 0.8 4.1 3800 ± 230 0 1:10 283 ± 0.8 3.1 5500 ± 700 0 40 NaSCN 1:0.6 162 ± 3 5.9 530 ± 7 1.3 1:1 247 ± 6 3.6 610 ± 4 1.0 1:2 283 ± 9 3.1 660 ± 5 0.9 1:10 329 ± 2 2.5 2700 ± 250 0

Table III.7.3: Decay lifetimes and waters of hydration: 1 mm Eu(ClO4)3, 0.5 m salt, Triton X- 114, varying DGA

116

III.7.2.2. Features of Emission Spectra

Results without DGA

5 7 All spectra were normalized to the 592 nm peak ( D0  F1), a magnetic dipole transition generally considered to be insensitive to the cation coordination environment.145 Four peaks were visible in the emission spectra of Eu3+ metal ion without ligand (Figure III.7.1). The

5 7 3+ absence of the D0  F0 forbidden transition indicates that all Eu species present were high

5 7 symmetry. The changes in the hypersensitive 618 nm peak, D0  F2 transition, indicate a

5 change in the metal ion coordination. In addition the changes seen in the peak at 690 nm ( D0

7 146  F4) indicated variation in the coordination environment of the metal ion.

1.2 10 % Triton X-114 20 % Triton X-114 1.0 40 % Triton X-114 Eu(ClO ) 4 3 0.8

0.6

0.4

0.2

NormalizedIntensity 0.0 560 580 600 620 640 660 680 700 720 Wavelength (nm)

Figure III.7.1: Emission spectra for 1 mM Eu(ClO4)3 in water and Triton X-114 wt % solutions

The addition of different salts resulted in changes in the emission spectra (Figures III.7.2 and III.7.3). For NaSCN, a fifth peak at 580 nm was visible. NaSCN and NaNO3 both also increased the intensity of the hypersensitive peak from that seen with surfactant only. Increasing the concentration of NaCl from 0.1 m to 0.5 m had no effect on the observed emission. The salts

117 followed the same trend in 20 and 40 wt % surfactant. Intensity in the hypersensitive band increased when going from 20 wt % to 40 wt % with NaCl, but, with NaNO3 and NaSCN, intensity of the hypersensitive band did not change with an increase in surfactant.

1.2 No DGA, 20 % Triton X-114 1.0 0.1 m NaCl 0.5 m NaCl 0.5 m NaNO 3 0.8 0.5 m NaSCN Eu(ClO ) 4 3

0.6

0.4

0.2 NormalizedIntensity 0.0 560 580 600 620 640 660 680 700 720 Wavelength (nm)

Figure III.7.2: Emission spectra of 1 mM Eu(ClO4)3 in a 20 wt % Triton X-114 solution with addition of 0.5 m electrolytes

118

1.2 No DGA, 40 % Triton X-114 1.0 0.5 m NaCl 0.5 m NaNO 3 0.5 m NaSCN 0.8 Eu(ClO ) in H O 4 3 2

0.6

0.4

0.2

NormalizedIntensity 0.0 560 580 600 620 640 660 680 700 720 Wavelength (nm)

Figure III.7.3: Emission spectra of 1 mM Eu(ClO4)3 in a 40 wt % Triton X-114 solution with addition of 0.5 m electrolytes

Results with DGA

Five peaks were visible in the emission spectra for Eu3+ in samples also containing DGA

5 7 (Figure III.7.4). The fifth peak that appeared around 580 nm was the D0  F0 forbidden transition which indicated species of low symmetry.146 It is traditionally a peak with very low intensity and was seen at all metal to ligand ratios. There was an increase in the intensity and

5 7 shape of the hypersensitive 618 nm peak ( D0  F2) with increasing ligand, indicating complexation and (probable lowering of symmetry) change in the inner coordination sphere.

5 7 The D0  F3 peak at 650 nm was present but very weak as expected for a forbidden transition.

5 7 The environment sensitive peak at 690 nm ( D0  F4) exhibited change as ligand was added.

The emission spectra for the Eu DGA complex in 10, 20, and 40 wt % surfactant solutions also contained the peak at 580 nm, indicating low symmetry (Figure III.7.5). Changes in the intensity or shape of the hypersensitive band with increasing surfactant concentration were not obvious,

119 even at the highest concentration, 40 wt % Triton X-114. However, the environment sensitive transition at ~690 nm showed dramatic changes with the increase/addition of surfactant (Figure

III.7.6). The shoulder at ~ 686 nm grew and became a second peak with increasing surfactant with the changes largest in 40 wt % surfactant. While little change was seen with increasing ligand concentration in water or even 10 wt % surfactant large changes were seen as ligand was added to the 40 wt % solution. The peaks decreased in intensity with addition of ligand, indicating a change in the metal environment not seen in water as the metal ligand complex forms.

1.8 Eu:DGA 1.6 5D 7F 1:0.6 0 2 1:1 1.4 1:2 1.2 1:4 5D 7F 0 1 1:10 1.0 Eu

0.8

0.6 5D 7F 5D 7F 0 4 0.4 0 0

0.2 5D 7F 0 3 0.0

560 580 600 620 640 660 680 700 720 NormalizedIntensity / Counts Wavelength (nm)

Figure III.7.4: Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA concentration and 0.1 m NaCl in water

120

1.8 1.6 Water Eu:DGA 10% Triton X-114 1:0.6 1.4 1:1 1.2 1:2

1.0 1:4

0.8 1:10 0.6 Eu 0.4 0.2 0.0

1.8 1.6 20% Triton X-114 40% Triton X-114 1.4 1.2

1.0

Normalized Intensity / Counts / Intensity Normalized 0.8 0.6 0.4 0.2 0.0 560 580 600 620 640 660 680 700 720 560 580 600 620 640 660 680 700 720 Wavelength (nm) Wavelength (nm)

Figure III.7.5: Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA and 0.1 m NaCl in water, 10, 20, and 40 wt % Triton X-114

0.8 Water Eu:DGA 10% Triton X-114 1:0.6 0.6 1:1 1:2 1:4

0.4 1:10

Eu

0.2

0.0 670 680 690 700 710 720 670 680 690 700 710 720

0.8 20% Triton X-114 40% Triton X-114

0.6

0.4

Normalized Intensity / Counts / Intensity Normalized 0.2

0.0 670 680 690 700 710 720 670 680 690 700 710 720 Wavelength (nm) Wavelength (nm)

Figure III.7.6: Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA and 0.1 m NaCl in water, 10, 20, and 40 wt % Triton X-114; 690 nm peak 121

Na+ Salts

The addition of salts, 0.5 m NaNO3, NaSCN, and NaCl, in 20 wt % and 40 wt % surfactant solutions showed some changes (Figures III.7.7 and III.7.8). The 580 nm peak was still visible for all spectra. No change in the hypersensitive peak was observed in 20 wt % surfactant for any of the salts. The only change seen for the spectra in 20 wt % was in the 690 nm peak with NaSCN. More changes were visible in 40 wt % surfactant as the intensity of the hypersensitive peak decreased for all salts from the intensity previously observed with 0.1 m

NaCl. The decrease was largest for NaSCN and there was a more distinct shoulder present in the hypersensitive transition. Some subtle changes were also seen in the 690 nm peak.

1.6 1.4 0.5 m NaSCN 1.2 1.0

0.8

0.6 0.4 0.2 0.0 560 580 600 620 640 660 680 700 720 1.6 0.5 m NaNO 1.4 3 1.2 1.0

0.8

0.6

Intensity (A.U.) Intensity 0.4 0.2 0.0 560 580 600 620 640 660 680 700 720

1.6 1.4 0.5 m NaCl Eu:DGA 1:0.6 1.2 1:1 1.0 1:2 1:10 0.8 Eu(ClO ) 4 3 0.6 0.4 0.2 0.0 560 580 600 620 640 660 680 700 720 Wavelength (nm)

Figure III.7.7: Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA in 20 wt % Triton X- 114 varying electrolytes (0.5 m)

122

1.6 1.4 0.5 m NaSCN 1.2 1.0

0.8

0.6 0.4 0.2 0.0

1.6 0.5 m NaNO 1.4 3 1.2 1.0

0.8

0.6 0.4

Intensity (A.U.) Intensity 0.2 0.0

1.6 0.5 m NaCl 1.4 Eu:DGA 1.2 1:0.6 1:1 1.0 1:2 0.8 1:10

Eu(ClO ) 0.6 4 3 0.4 0.2 0.0 560 580 600 620 640 660 680 700 720 Wavelength (nm)

Figure III.7.8: Emission spectra for 1 mM Eu(ClO4)3 with increasing DGA in 40 wt % Triton X- 114 varying electrolytes (0.5 m)

III.7.3. Discussion

III.7.3.1. Surfactant Metal Behavior without complexation

Fluorescence studies of Eu3+ with ligands in the presence of nonionic surfactants have been reported for surfactant concentrations < 1 wt %.147-151 Surfactants were used at low concentrations to enhance fluorescence intensity through an increase of luminescence quantum yields. The enhanced yields were attributed to the increase in viscosity and decrease in polarity found in a micelle compared to bulk aqueous solution.147 Double exponential models were used to fit lifetime data in the presence of nonionic surfactants, implying the existence of a second species.147 However no attempt was made to identify and explain the presence of the second species.

123

When surfactant was present in this current study, two species were always indicated in the fluorescent lifetime data, as evidenced by fitting the lifetime decay data with a double exponential decay model. The two species must have significantly different rates of exchange in their inner coordination sphere to be seen as two distinct species. If the exchange rates in the inner hydration sphere were similar, only one species with an average lifetime and hydration number would be observed.152 The shorter lifetime species, the aqueous metal ion, showed a slight decrease in hydration at a concentration of 40 wt % surfactant and 0.1 m NaCl, as the primary hydration number dropped from nine to eight. Significantly less water was present in the 40 wt % surfactant solutions and decreased water activity may account for the slight dehydration. The presence of a second species with a longer lifetime indicated the probable solvation of Eu3+ by the surfactant. The difference in exchange requirement also lent support to this conclusion. As surfactant molecules were significantly larger than water molecules, it would be expected that the exchange rate would be different, especially if specific orientation of the surfactant were required for interaction with Eu3+.

The presence of a second dehydrated species could indicate formation of an inner-sphere complex with Eu3+ and surfactant. However, emission spectra for Eu3+ surfactant solutions

(Figure III.7.1) do not show a peak at 580 nm, indicating the symmetry of the species is

5 7 maintained. The appearance of a peak at 580 nm is indicative of the D0  F0 transition, normally a forbidden transition. Inner sphere complex formation typically destroys the center of

3+ 5 7 inversion of Eu and partially allows the D0  F0 transition, which shows up as a weak peak around 580 nm.153 This was demonstrated in this study upon the addition of NaSCN or DGA to

Eu3+ in solution, each of which will be discussed below. It was difficult to determine whether the surfactant was binding directly to the inner sphere of the Eu3+ ion based on the emission

124 spectra. If an inner sphere complex were formed, the surfactant would bind the Eu3+ in a way that retained high symmetry as seen for the aqueous Eu3+. The symmetry could be dictated by packing constraints around the metal center owing to the large size of the surfactant molecule and presence of micelles. There have been reports of nonionic surfactants, such as Triton X-114, binding cations through their ether groups.80 In most CPE systems little to no metal ion extraction was seen in the absence of a chelating agent, seeming to indicate a surfactant does not complex metal ions.38 No conclusive evidence of a surfactant metal complex has been presented and very little work has been reported at high concentrations of surfactant.

If no inner-sphere complex is formed, how is the Eu3+ ion being dehydrated? In previous literature, it was noted that micelle formation increased the observed lifetime and fluorescence of Eu3+ due to protective effects of the micelle.148 Another study noted that micelle formation led to the exclusion of water from metal ion proximity as evidenced by longer fluorescence lifetimes.150 In this study, the surfactant was present at concentrations significantly above the CMC, allowing micelle formation and possibly explaining the increased lifetimes observed for Eu3+ with surfactant. Additionally, Eu3+ could be dehydrated by the ether oxygen atoms from one or more Triton X-114 molecules, resulting in low hydration numbers. Each surfactant molecule has an average of 7.5 ethylene oxide units per surfactant monomer, correlating with the hydration number of ~ 1.5 seen for the dehydrated species and accounting for a total hydration number of nine. It was unclear from the current data whether this type of dehydration could occur with the Eu3+ ion retaining its symmetry.

Change in the environment of the metal ion was indicated by changes in the 618 nm hypersensitive peak and 690 nm environment sensitive peak of the emission spectra.146

Fluorescence intensity increasing with increasing surfactant has been reported, and is expected,

125 as fluorescence intensity is enhanced by asymmetry surrounding the metal ion.148 The shapes of the peaks also changed which could indicate changes in the coordination of the metal ion. Since the formation of an inner sphere complex is uncertain, the peak shape change could be attributed to a change in micelle structure or organization of surfactant surrounding the metal ion outside of the inner coordination sphere. The lifetime of the dehydrated metal species did not change, supporting the idea that no interactions occurred within the inner coordination sphere. The environment and orientation of the surfactant molecules and micelles around the metal could have changed without increasing or decreasing the water in the metal ion inner sphere. More information is needed to begin to speculate on the structural changes occurring and was beyond the scope of this work.

III.7.3.2. Electrolyte and Surfactant Behavior without DGA

NaCl did not change the emission spectra or lifetimes observed at either 0.1 or 0.5 m.

This is expected behavior as the literature indicates the Cl- ion does not form inner-sphere complexes with lanthanides and the concentration of the ion does not affect the hydration number.154 In contrast, both lifetimes and emission spectra were seen to change in 0.5 m NaSCN and NaNO3. The appearance of the 580 nm peak with NaSCN indicated a change in the inner coordination sphere and possible SCN- complexation.155 The lifetime data showed an increase in the number of waters in the inner coordination sphere, 9.6 or 9.1 compared to 8.7 or 8. One explanation for this observation was that the primary hydration number was relatively constant but the effective fluorescence quenching per water molecule was increased with addition of

- electrolyte. Choppin, et. al., saw higher hydration numbers than expected for ClO4 , with values up to 11 reported.142 As electrolyte concentration increased, more solvent water is involved in

126 hydration of the ions, decreasing secondary hydration and allowing the Eu3+ to interact more with water in the inner coordination sphere. The Hofmeister series and its explanation of electrolyte effects could also provide an answer. The thiocyanate anion is known to be a chaotropic or “distrupting” anion and influences the structure of water by increasing disorder, as does perchlorate.156 The disordering of water surrounding the metal ion could also account for an increased hydration number by increasing the ligand exchange rate.

- There has been debate in the literature as to whether NO3 forms an inner sphere or outer sphere complex.155 The emission data in this study does not indicate a loss of symmetry caused by an inner sphere complexation as the 580 nm peak is absent. However, the lifetime of the aqueous Eu3+ increased, suggesting dehydration and possibly an inner sphere complex. The

3+ 154 increase in lifetime for Eu in the presence of NaNO3 has been previously reported.

Both NaSCN and NaNO3 increased the intensity of the hypersensitive band beyond the effect of the surfactant on its own. The increase was the same in 20 and 40 wt % surfactant, thus could be attributed to the electrolytes. Electrolytes are known to influence surfactants, in particular in CPE systems. It has been hypothesized that electrolytes can cause changes in the shape of the micelle formed by the surfactant.157 A change in the micelle structure could explain the increase in the fluorescence intensity seen as fluorescence intensity is enhanced by asymmetry around the Eu3+ center.

III.7.3.3. Eu DGA complexes in water

The hydration numbers seen at the highest metal to ligand ratios were slightly higher than predicted with speciation and n bar calculations as predicted by previous literature. DGA binds in a tridentate fashion displacing three waters for each ligand complexing. Due to the low pH of

127 the experiment both the ML2 and some ML3 species were predicted by speciation calculations at the 1:10 metal to ligand ratio (Table III.7.4). A lower hydration number than that observed was expected according to n bar calculations (Table III.7.2) which predicted two or more DGA molecules per metal ion. A previous paper by Maupin, et. al., explored Eu and DGA complexation using fluorescence and saw a weaker association than predicted by potentiometric stability constants.158 The 1:2 complex persisted at metal to ligand ratios up to 1:5, even though speciation calculations with potentiometric data predicted no 1:2 complex at that ratio. No clear explanation for this finding was given. However, the authors did allude to a pH dependence of the fluorescence data, perhaps accounting for the discrepancies.

The pH conditions of this experiment were significantly lower than those used for previous experiments (~3 vs. 5-7) and a pH dependence of lifetime data has been reported with more waters being observed at pHs < 3. Brittain, et. al., reported the pH dependence of the number of waters bound to the inner coordination sphere for Tb (III) in the presence of NTA.159

At pHs > 4.5, the number of coordinated water molecules was constant at one water molecule.

At lower pHs, < 3, as many as four water molecules were reported. This difference was attributed to incomplete complex formation below pH 4.5. No speciation calculations were presented in the paper.

128

% Triton X- Eu:DGA pH % Eu % EuDGA % EuDGA2 % EuDGA3 114 Ratio 0 1:0.6 3.64 42.26 55.63 2.11 0 1:1 3.46 13.17 74.61 12.20 0.02 1:2 3.19 0.32 27.83 69.74 2.10 1:4 3.02 0 6.89 81.5 11.60 1:10 2.87 0 2.45 72.04 25.51 10 1:0.6 3.73 46.05 49.33 4.40 0.23 1:1 3.40 23.04 61.51 13.66 1.79 1:2 3.18 4.75 44.54 34.75 15.96 1:4 3.03 0.6 16.11 35.98 47.32 1:10 2.88 0.05 3.65 21.54 74.76 20 1:0.6 3.58 52.04 37.42 10.47 0.07 1:1 3.40 30.22 44.25 25.21 0.33 1:2 3.22 4.02 26.12 66.04 3.83 1:4 3.03 0.37 8.46 75.67 15.51 1:10 2.87 0.05 2.87 64.21 32.87 40 1:0.6 3.69 40.29 59.39 0.33 0 1:1 3.44 5.64 89.12 5.24 0 1:2 3.22 0.05 30.45 67.61 1.89 1:4 3.07 0 8.76 81.65 9.58 1:10 2.91 0 4.03 77.31 18.65

Table III.7.4: Speciation calculations with Eu3+ DGA stability constants. All calculations done in Hyss 2009.

III.7.3.4. Eu/DGA/Surfactant

The lifetimes for the Eu DGA complex in solutions containing increasing amounts of surfactant showed a slight increase at lower metal to ligand concentrations. Change was also seen in the emission spectra in the 690 nm peak, which increased in intensity at lower ligand to metal ratios. At high ligand to metal ratios, 1:10 ratio of Eu:DGA, the emission spectra and lifetimes were the same without regard to surfactant concentration. The changes seen in the emission spectra supported the slight decrease seen in the lifetimes for low metal to ligand ratios.

The changes could be attributed to interaction of the surfactant with the metal ion. At higher

129

DGA concentrations, more ligand is available to interact with the metal ion, reducing surfactant interaction with the inner coordination sphere of the metal. However evidence of the influence of the surfactant on the metal ion environment was present and seen at low concentrations of ligand.

III.7.3.5. Eu/DGA/Surfactant/Salts

Changes in the lifetimes and emission spectra were seen for all three electrolytes for the observed fluorescence species and were dependent on DGA concentration. The changes in the emission spectra were limited mainly to the hypersensitive band at 618 nm. In 20 wt % surfactant there was little change in the hypersensitive band upon the addition of electrolyte, indicating no environment or coordination changes for the Eu DGA complex. In 40 wt % surfactant there was an overall reduction in the fluorescence intensity, which could have been a result of reduced ligand exchange dynamics. There was an increase in viscosity with an increased percentage of surfactant, likely slowing exchange rates and decreasing the observed intensity. The intensity of the hypersensitive band with NaSCN in 40 wt % decreased the most and exhibited a more pronounced peak shoulder. The changes in the NaSCN emission spectra were accompanied by a decrease in the hydration of the metal ligand complex, at higher metal ligand ratios, and an increase in hydration of the second complex. In the absence of DGA, there was evidence of a NaSCN inner sphere complex with Eu3+, as the emission spectra showed a peak at 580 nm, indicating a loss of symmetry with the addition of the NaSCN. This could

n explain the lower hydration number as formation of a Eu(DGA)x(SCN)y complex cannot be ruled out. Eu3+ complexation by SCN- would also explain the changes seen in the emission spectra.

130

As the increase in the fluorescence intensity provided by surfactants was attributed to incorporation of the metal ion into the micelle, an assumption could be made that the decrease in intensity was caused by a lack of micelle incorporation. This did not occur in 40 wt % surfactant without salt. These findings provide some insight into the influence of electrolytes but further studies are needed.

III.7.4. Conclusions

Fluorescence spectroscopy was used to examine the coordination environment around

Eu3+ in solutions containing the nonionic surfactant, Triton X-114, different supporting electrolytes, and DGA. It was found that, in the presence of surfactant, two species were always observed in the lifetime data and an increase in the intensity was observed for the emission spectra. However no clear evidence for an inner sphere metal surfactant complex was seen. The second longer lifetime species was attributed to the incorporation of the metal into a micelle.

The addition of electrolytes changed the emission spectra and lifetimes of the Eu3+ DGA species and was attributed to the ability of electrolytes to influence surfactant organization.

131

IV. CONCLUSIONS

Separation of lanthanides and actinides remains a major challenge for nuclear fuel reprocessing. Processes have been developed for lanthanide/actinide separation, such as

TALSPEAK.7 Solvent extraction-based separations have the advantage of near continuous operation and high throughput, but require organic solvents that can be expensive and occasionally create waste disposal challenges. Looking toward the future of nuclear fuel separations, the current emphasis on a science-based approach encourages unconventional approaches to accomplishing these separations. In this climate of exploration, aqueous based separation systems, such as cloud point extraction (CPE), have added attractiveness. CPE is an entirely aqueous system for separation that utilizes surfactants as the “organic” phase, eliminating organic solvents with an inexpensive aqueous soluble replacement.10 The separation relies on a reversible shift in temperature to drive partitioning.

CPE has been used successfully for oxidation state separation of some transition metals and for lanthanide separation and pre-concentration, indicating it could be useful for the lanthanide/actinide separation challenge.40,52 While CPE systems have been developed and utilized for various preconcentration and separation schemes, there is at present only minimal understanding of the fundamental chemistry involved in the separation process. To exploit the full potential of CPE for lanthanide/actinide separations, increased fundamental understanding of the mechanism of extraction, the composition of the “organic” phase, the factors that influence metal ligand partitioning, and the behavior of the metal and metal ligand complex in surfactant solutions must be realized. Previous CPE research has not adequately addressed these issues.

A few CPE papers have attempted to characterize the extracted metal ligand species, but have not been able to adequately describe or elucidate the extraction mechanism.17,41,57

132

Additionally, the analysis of the SRP has been hindered by its viscosity and incompatibility with instruments typically used for analysis, such as ICP-OES/AES or Flame Atomic Absorption.

Lack of the ability to account for mass balance has been another area in need of improvement in

CPE systems. Limited research on metal ligand complexes in surfactant solutions has been reported previously.116,160,161 Most of the research has been done in low concentration surfactant solutions; a distinguishing feature of this work is the attempt to explore solution chemistry in media that resemble the more concentrated phases that separate in CPE. While it has been fairly well established that surfactants interact with a variety of compounds in different media, the behavior of a concentrated surfactant solution, analogous to the SRP in CPE, in the presence of metal ligand complexes, has not been previously studied. The primary goal of this research was to improve understanding of the behavior of metal ligand complexes in CPE systems.

The preliminary studies revealed some irregularities and inadequacies in previous CPE research and discussion in the literature. Several important lessons were learned and utilized in additional studies. Preliminary studies were done using concentrations defined on the molarity scale which is poorly suited to an experiment in which the temperature is varied and all subsequent work was completed using solutions whose concentrations were prepared on a molality basis. Additionally, it was discovered that ligands that work in traditional solvent extraction systems do not always work in a CPE system. Following conventional wisdom in

CPE (i.e., trial and error system design), PMBP was chosen as the lanthanide carrier ligand for partitioning studies. Being a highly lipophilic ligand, PMBP was initially added as an ethanol stock solution. The addition of ethanol in turn increased the CPT and may have changed the composition of the SRP. It was later discovered that PMBP was soluble in the neat surfactant,

Triton X-114, and could be introduced with the surfactant. When introduced in this manner,

133

PMBP had an almost negligible effect on the CPT and any influence from the ethanol was eliminated. The PMBP Eu3+ system described here is the first CPE report applying radiometric analysis to CPE, allowing for metal analysis of both phases and a check on material balance.

The application of radiotracer techniques provided an advantage over analyses using ICP-OES or

AES where the SRP is not easily analyzed due to its viscosity and composition. The lessons learned in the initial studies were applied in all subsequent work.

The formation of the PMBP Eu3+ complex and its partitioning into the SRP was found to be reasonably rapid at pH ~ 3, and maximum extraction was reached after only 10 minutes.

Sampling and gradient studies did indicate some back-mixing occurred near the interface of the aqueous phase with the SRP which was not unexpected as CPE systems are reversible and the

SRP and aqueous phase are miscible under suitable conditions. The system can remain relatively stable with little remixing if it is undisturbed. With the optimized CPE extraction system quantitative extraction of macroscopic amounts of europium was achieved in two contacts with

Triton X-114 and PMBP. It was also shown that the mechanism of extraction in this CPE system was not the same as that seen in traditional solvent extraction systems. This is thought to be caused by the difference in composition of the “organic” phases. The SRP contains water and as such is far more polar than traditional organic diluents which cannot support charged species partitioning. More information and insight into the composition of the SRP were needed to further explain this finding.

As the influence of electrolytes on the CPT and other properties of surfactant systems had been previously reported a range of salts and their effects on Eu3+ distribution and CPT were examined. Electrolytes can salt-in or salt-out the surfactant in a CPE system, which results in a correlated change in the CPT. Whether salts also change the distribution and partitioning of

134 metals in a CPE system was previously unclear. The salts chosen for the experiments performed encompassed the range of changes induced by electrolytes according to the Hofmeister series.

The electrolytes did influence and change the metal ligand distribution. They changed the composition of the SRP by altering the water content, which then changed the metal partitioning.

The water content of the SRP was found to be an important factor in metal ion partitioning in the europium PMBP CPE system. The change in the water content of the SRP resulted in a change in the extraction mechanism, as evidenced by slope analysis results. With the thiocyanate salts, the increased water content of the SRP inhibited partitioning of the hydrophobic PMBP. The decreased partitioning was correlated with a decreased slope that indicated only one PMBP molecule was partitioning with each Eu3+. The chloride salts increased the Eu3+ partitioning and promoted the formation and partitioning of two PMBP molecules per Eu3+. The water content in the SRP of the chloride systems was also less than that seen in the thiocyanate systems. These results showed that understanding how electrolytes influence the SRP and metal distribution of a metal ligand complex in CPE is an important feature for design of future CPE systems and ligand design for this application.

The FTIR study of the SRP in the presence of various concentrations of electrolytes revealed some interesting structural changes. First the SRP, even in the absence of electrolytes, demonstrated a number of changes from the neat surfactant and was more amorphous in nature when compared to the crystalline structure of the neat surfactant. Once salts were added, the

SRP also changed and the changes increased with increasing salt concentration. NaNO3 caused dramatic spectral changes and influenced the structure of the surfactant and interactions of the surfactant within the SRP. Evidence of changes in the hydrogen bonding of water with the surfactant was seen as the salt concentration was increased. NaCl and NaSCN were also seen to

135 influence the structure of the SRP, but to a lesser degree. How or why NaNO3 promotes the most change in the structure of the SRP remains unclear and further studies are needed.

Investigations into the effect of salts on micelle shape and size would be helpful and could possibly elucidate the cause of the changes observed in the FTIR spectra.

To explore the direct influence of the surfactant on the ligand and metal ligand complex in CPE, further studies employed the well characterized ligand, diglycolic acid (DGA), with lanthanides in solutions of the nonionic surfactant, Triton X-114. Triton X-114 influenced the protonation constants of DGA over a range of concentrations and temperatures. At low surfactant concentrations and temperatures below the CPT, the protonation constants decreased.

This could be attributed to an increase in hydrogen bonding caused by the ethoxylated oxygen atoms on the surfactant. At higher surfactant concentrations and temperatures above the CPT, the protonation constants increased relative to those seen in pure water. This correlated with previous results from the literature and was likely caused by a change in the dielectric constant within the micelle. However the various equilibria present in a micellar pseudophase prevented the determination of anything other than apparent, or observed, pKas. This prohibited application of techniques such as van’t Hoff analysis, to determine entropic or enthalpic data, which was the initial objective of the temperature variation investigation.

The stability constants for DGA with five lanthanides (La3+, Nd3+, Eu3+, Ho3+, Lu3+) and

Am3+ were determined in surfactant solutions ranging from 2 to 40 wt % using two different analytical methods, potentiometry and spectrophotometry. The stability constants for all five lanthanides were significantly increased at 25°C and 40 wt % Triton X-114. This result may indicate an increased metal ligand interaction in the SRP in a CPE system as the SRP composition is generally around 40 wt % surfactant. Spectrophotometric titrations proved to be

136 a superior method for stability constant determination compared to potentiometric titrations, as no corrections for the micellar pseudophase or electrode behavior must be made. Unfortunately, only a comparatively small number of lanthanide ions exhibit the requisite hypersensitivity in their spectra that is needed to enable this technique.

The coordination environment and inner-hydration sphere around Eu3+ with Triton X-

114, different supporting electrolytes, and DGA were examined with fluorescence spectroscopy.

In the presence of Triton X-114, at concentrations above the CMC, two species were always observed in the lifetime data and an increase in the intensity was observed for the emission spectra indicating an interaction of the surfactant with the Eu3+. The emission spectra did not allow relative quantitation of Eu partitioning between conventional DGA complexes and the surfactant phase, but they did indicate that both species were present. No clear evidence for an inner sphere metal surfactant complex was seen. The second, longer lifetime species, was attributed to the incorporation of the metal into a micelle. The addition of electrolytes resulted in changes in the emission spectra and lifetimes of the Eu3+ DGA species. The spectral changes correlated with the ability of electrolytes to influence surfactant organization as was seen in the

FT-IR studies.

Overall, the studies performed revealed useful information on the nature of the SRP

(mostly aqueous), the mechanism of extraction in CPE (apparent reduced M:L stoichiometries relative to solvent extraction analogs), and the interactions of the surfactant with the metal ion and metal ligand complexes. Knowledge on the influence of electrolytes on the SRP will be useful in future CPE system design. For example, increasing the water content of the SRP is advantageous when attempting to partition a more water soluble metal ligand complex.

However, while some questions were answered, many questions were also raised. It is still

137 unclear what specific characteristics of a ligand would make it suitable for a CPE system. The knowledge that the composition of the SRP can be changed will be important in future studies.

The changes in the micelle shape, size, and aggregation caused by electrolytes were almost certainly the cause of any structural changes in the SRP and future SANS experiments are needed. Information from SANS experiments could also help correct for the micellar pseudophase in the pKa and stability constant data and provide more insight into the changes caused by the surfactant. It is likely that theories used to describe stable microemulsions will prove fruitful in establishing improved understanding of cloud point extraction phase separation.

CPE is a useful separation technique with potential, but is a more complicated system than initially expected.

Acknowledgments

Funding: U.S. Department of Energy, Office of Nuclear Energy, Nuclear Energy Research

Initiative Consortium (NERI-C) and University Nuclear Energy Research Initiative (U-NERI) programs.

Linfeng Rao and Guoxin Tian hosted studies on fluorescence spectroscopy (Chapter III.7) at

Lawrence Berkeley National Laboratory

Sergey Sinkov and Greg Lumetta hosted studies on spectrophotometric determination of lanthanide and Am DGA complex stability (Chapter III.6) at Pacific Northwest National

Laboratory

138

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