ION TRANSFER ACROSS LIQUID/LIQUID INTERFACES Yuanhua Shao Deyree of Doctor of PhIIOSOPhV University of Edinburgh 1991 Acknowledgement I wish to thank Dr.Hubert Girault for his guidance and encouragement throughout the past three years. I am also grateful for the help and advice of the members of the Electrochemistry Group in Edinburgh University: A.Brown, C.Belmont, J.Campbell, M.Osborne, B.Seddon, A.Stewart, S.N.Tan. G.Taylor, l(Taylor. J.Tietje-Girault. I would like to thank SBFSS, The British Council, MediSense (U.K) and Edinburgh University for financial support. Also thanks to Profs. Z.Zhao and P.Li of Wuhan University for their encouragement and help in making my visit to Edinburgh possible. My thanks also goes to the technical staff in the Department of Chemistry, especially Mr.A.King. I also wish to acknowledge the friends in this department and my Chinese friends for their hosptiality, their support and their friendship. Finally, I wish to express my sincere gratitude to my family for their support and patience in the past three years. ,~G4 1~~ rLiJ I 4U ABSTRACT Double step potential chronocoulometriC measurements have been set up for the investigation of kinetics of charge transfer across the liquid/liquid interface and to study adsorption at these interfaces. Semioperation analysis of chronoamperometric and chronocoulometric data were also developed and used to evaluate the kinetic parameters. The transfer of the acetylcholine ion across the water/1,2-dichlorOethafle, water/nitrobenzene+tetraChlOrOmethane, water/dichloromethafle, and water/1,2-dichlorObeflzefle interfaces have been studied as a function of the viscosity of both phases and the composition of the organic phases respectively. The results showed that the standard rate constant is inversely proportional to the viscosity of either phase. The potential dependence of the rate of ion transfer follows the Butler-Volmer equation. It is shown that the main difference between ion transfer and ion transport is a consequence of an entropy effect. The kinetics and thermodynamics of sodium and potassium transfer facilitated by dibenzo-18-crown-6 have been studied by chronocoulOmetry and its convolution technique and cyclic voltammetry. Three possible mechanisms have been examined carefully and a new terminology is being proposed. A simple way to measure the ratio of the diffusion coefficients of ionophore and its complex ion in the organic phase by using micropipette has been developed. Lithium and proton transfer across water/1,2-dichloroethafle interface facilitated by ETH1810 have been studied and a mechanism has been proposed. Potassium transfer across water/1,2-dichloroethafle interface facilitated by DLADB18C6 has also been observed. The adsorption of hexadecyl sulphonate anion (HDS) on the water/1,2-dictiloroethane interface has been investigated by classical methodology. Finally, a new method to measure the half wave potential has been applied to the measurement of formal Gibbs energies of transfer of ions across the water/1,2-dichloroethafle interface. The validity of TATB (TetraphenylarSOfliUm Tetraphenylborate) assumption is also discussed. IV Declaration I declare that work presented in this thesis is my own unless otherwise stated by reference. List of courses attended Physical departmental seminars (1988-1991). Physical evening meeting (1988-1991). Convolution for kinetic studies. Dr.H.Girault (Edinburgh University, 1989). Picosecond chemistry and Spectroscopy with lasers. Dr.S.Meech Heriot-Watt University, 1989). Molecular Mechanics and modelling. Dr.P.Taylor (Edinburgh University, 1990). Medicinal Chemistry. Prof.R.Baker et al (Merck, Sharp and Dohme, 1990). A look at solids, surfaces and surface films. Drs.S.Affrossman (Strathclyde University), C.S.McDougall (Edinburgh University), P.John (Heriot-Watt University), 1991. Butler meeting on Electrochemistry (1989-1991). Informal spring meeting on Electrochemistry (1989, 1991). Bioelectrochernistry workshop in Oxford (1990). New biosensors (150th RSC congress in London, 1991). Common Abbreviations Roman Alphab . a 1 activity of species A electrode area Ac acetylaholine cation b backward, reverse or bulk quantity C, c concentration or capacitance CE counter electrode CV crystal violet 0 diffusion coefficient DB18C6 dibenzo-18croWfl 6 1,2-DCB 1,2_dichlorobeflZefle 1,2DCE 1,2-dichiorOethafle DCM dichioromethafle E electrode potential ETH181O see text forward F Faraday constant o Gibbs energy AG ° standard Gibbs energy of a process interfacial current ITIES interface between two immiscible electrolyte solutions VI k Boltzman constant or electrochemical rate constant M unspecified cation NB nitrobenzene o organic phase o oxidised form of unspecified species Q charge r electrode radius or ion radius R Gas constant or reduced form of unspecified species RE reference electrode SCE saturated calomel electrode T absolute temperature TCM tetrachioromethafle TBA tetrabutylammOflium TEA tetraethvlamm 0n m TMA tetramethvlammonium TPAs tetraphenylarsoniurn TPB tetraphenylbOrate TPBCI tetra-kis[4-chlorophenyl]bOrate w aqueous phase z charge number Greek Alphabet charge transfer coefficient or phase denoted ct VII B phase denoted B y activity coefficient scan rate TI viscosity c dielectric constant r surface concentration Galvani potential difference VIII CONTENTS Title Acknowledgement Abstract Declaration Common Abbreviations Contents Pages No. Chapter One. Introduction 1 1.1 Historical background 1 1.2 Electrochemical approaches 2 1.3 Solvents and base electrolytes 3 1.4 Interfacial structure 7 1.5 Charge transfer reactions 10 1.5.1 Ion transfer reaction 10 1.5.2 Facilitated ion transfer reaction 11 1.5.3 Electron transfer reaction 12 1.6 Kinetics 12 1.7 Adsorption 13 1.8 Oscillation 17 1.0 Microelectrodes 17 1.10 Fluctuation analysis of ITIES 18 1.11 Gel and membrane 18 1.12 Computer simulation 19 1.13 Photoelectrochemistry 19 1.14 The scope of the present work 20 Chapter Two. Fundamental concepts of Electrochemistry at liquid/liquid interface 22 2.1 Equilibrium conditions and the Nernst poten tial 22 2.2 Single ion Gibbs energy of transfer 25 2.3 Solvation of ion 28 2.4 Interfacial structure and the ion transfer mechanism 31 2.4.1 MVN model 32 2.4.2 GS model 37 Chapter Three. Methodology and theoretical background 40 3.1 Chronoamperometry and Chronocoulometry 40 3.1.1 Theory 41 3.1.2 Adsorption 45 3.1.3 Electrochemical measurement 46 3.1.3.1 Experimental .. 48 3.1.3.2 Results and discussion 48 3.2 Convolution techniques of chronoamperometry and chronocoulometry 54 3.2.1 Theory 54 KI 3.3 Cyclic Voltammetry 56 3.4 Experimental techniques 57 3.4.1 Large planar ITIES .57 3.4.2 Micro-ITIES 58 61 3.4.3 Reference electrodes 3.4.4 Measurement of potential of zero charge (PZC) 62 3.4.5 Chemicals 62 Chapter Four. Kinetics and mechanism of acetyicholine ion transfer across the liquid/liquid interface 66 4.1 Introduction 66 4.2 Kinetics of the transfer of AC across the water/1,2-DCE interface 68 4.2.1 Variation of diffusion coefficient and Gibbs transfer energy as a function of viscosity of aqueous phase 68 4.2.2 Kinetics of Ac 4 transfer across the water/1,2-DCE interface 73 4.3 Kinetics of Ac across the water/NB-TCM interface 83 4.3.1 Variation of Gibbs energy and diffusion coefficient as a function of the dielectric constant of the aqueous phase 83 4.3.2 Variation of the kinetics of Ac transfer as a function of the dielectric constant of the aqueous phase 89 4.4 Kinetics of Ac transfer across the water/DCM and water/1,2-DCB interface 4.5 Discussion 101 4.5.1 Linear Gibbs energy relationship 101 4.5.1.1 Linear Gibbs energy relationship for diffusion 101 4.5.1.2 Linear Gibbs energy relationship for transfer 103 4.5.2 Butler-Volmer relationship 107 4.5.3 Activation energy for ion transfer 109 4.5.4 Ion transfer and ion transport 110 Chapter Five. Kinetics and mechanisms of facilitated ion transfer across the water/1,2-DCE interface 113 113 5.1 Introduction 5.2 Kinetic studies on Na and K transfer facilitated by DB18C6 at the water/1,2-DCE interface 116 5.2.1 Results 116 5.3 Facilitated ion transfer at micro-ITIES supported at the tip of micropipette 127 5.3.1 Introduction 127 5.3.2 Results and discussion 128 5.4 Lithium and proton transfer across the water/1,2-DCE interface facilitated by ETH1810 135 5.4.1 Introduction 135 5.4.2 Results and discussion 135 5.5 iC transfer facilitated by DLADB18C6 across the water/1,2-DCE interface 146 XII 5.5.1 Introduction 146 5.5.2 Results and discussion 146 Chapter Six. The adsorption of hexadecyl suiphonate anion on the water/1,2-DCE intertace 152 6.1 Introduction 152 6.2 Results and discussion 153 6.2.1 HDS in aqueous phase 153 6.2.2 HDS in 1,2-DCE phase 162 6.3 Conclusion 164 Chapter Seven. Determination of the half wave potential of the species limiting the potential window 167 7.1 Introduction 167 7.2 Experimental 169 7.3 Results 170 7.3.1 Determination of the half wave potential 170 7.3.2 Test of the proposed method 172 7.4 Conclusion 179 Appendix I Tables for the standard Gibbs energies of transfer from water to organic phases for various ions 183 Appendix II Computer Programs 197 Appendix Ill Lists of publications 210 References 211 Xlii Chapter One INTRODUCTION The electrochemical study of charge transfer reactions across liquid/liquid interfaces (or, electrolysis at the Interface between Two Immiscible Electrolyte Solutions [ITIES], or Oil/Water [01W] interface) is a relatively new branch of electrochemistry. Although the first electrochemical study was carried out nearly 90 years ago [1], our understanding of these processes remains still very rudimentary compared to electron transfer reactions at metallic electrodes [2].