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Ii ION-PAIR BEHAVIOR BETWEEN POLYOXOMETALATES ANION

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ION-PAIR BEHAVIOR BETWEEN POLYOXOMETALATES ANION AND ALKALI METAL CATION A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirement for the Degree Master of Science Songtao Ye May 2018 ii ION-PAIR BEHAVIOR BETWEEN POLYOXOMETALATES ANION AND ALKALI METAL CATION Songtao Ye Thesis Approved Accepted: ______________________________ ____________________________ Advisor Dean of the College Dr. Tianbo Liu Dr. Eric Amis ______________________________ ____________________________ Committee Member Dean of the Graduate School Dr. Toshikazu Miyoshi Dr. Chand Midha ______________________________ ____________________________ Department Chair Date Dr. Colleen Pugh iii ABSTRACT Ion-pair behavior describes the partial association of oppositely charged ions in electrolyte solutions. Previous study mainly focused on the ion-pair behavior between simple ions, such as ion pairing in NaCl solution as well as ion-pair interactions in supramolecular complexes and biological associations. However, very few attentions have been placed on the solution system with particle sizes in between. Recently, a group of well-defined, huge anionic cluster named polyoxometalates (POMs) have been synthesized and well characterized. The size of POMs is around nanometer scale, which is exactly between simple ions and large colloids. The solution behavior for POMs is much different form simple electrolyte solutions or large colloids. As a result, it is interesting to study the ion-pair behavior for POMs in solution. Herein, ion-pairs between Lacunary Keggin type POMs and alkali metal cations are investigated. The result showed that ion-pairs are formed between alkali cations and the “pocket” area on the surface of Lacunary Keggin type POMs – K7PW11O39. Electrostatic interaction and the entropy gain during the solvation shell lost were considered major driving forces during the ion-pair formation. Smaller alkali cations (e.g., Li+ and Na+) tended to form contact ion- pair (CIP) which result in an elevated enthalpy change measured by Isothermal Titration Calorimetry (ITC). Larger alkali cations (e.g., Rb+ and Cs+) favored a loose type of ion-pair – solvent separated ion-pair (2SIP) and solvent shared ion-pair (SIP). Size exclusion between the “pocket” area on K7PW11O39 POM surface and alkali cation also played a significant role in determining the ion-pair structure. Results were further confirmed by Nuclear Magnetic Resonance Spectroscopy (NMR). iv ACKNOWLEDGEMENT First, I would like to express my gratitude to my advisor Prof. Tianbo Liu for his patient guidance and strong support for my study at the University of Akron. He brought me this area and offered me great opportunities to study the complex solution behavior. Meanwhile, I also want to thank my committee member Dr. Toshikazu Miyoshi for his advice to my research and constructive feedback to my dissertation. Besides, I would like to appreciate Jiancheng Luo, a senior PhD student in our group. He is so generous to share precious experience and useful methods with me. This dissertation would never be finished without his help. And I also want to all the group members for their assistance during my research. Finally, I would like to extend my appreciation to my family and my friends, especially my parents. I appreciate all your support from both mentally and physically. v TABLE OF CONTENTS LIST OF TABLES .......................................................................................................... vi LIST OF FIGURES ....................................................................................................... vii CHAPTER ...................................................................................................................... 1 I. INTRODUCTION AND BACKGROUND ................................................................. 1 1.1 Introduction of ion-pair behavior ..................................................................... 1 1.2 Experimental methods for studying ion-pair .................................................... 5 1.3 Solution behaviors of macroions ...................................................................... 7 1.4 Study motivation ............................................................................................. 11 II. EXPERIMENT.......................................................................................................... 12 2.1 Sample preparation .............................................................................................. 12 2.1.1 Synthesis of Na3[PW12O40] ·13H2O .......................................................... 12 2.1.2 Synthesis of K7[PW11O39] ·13H2O ............................................................... 12 2.2 Isothermal Titration Calorimetry (ITC) ............................................................... 13 2.3 Nuclear Magnetic Resonance Spectroscopy ........................................................ 15 III. RESULT AND DISCUSSION ................................................................................ 16 vi 3.1 Ion-pair formation monitored by Isothermal titration calorimetry (ITC) ............ 16 3.2 Ion-pair formation monitored by Nuclear Magnetic Resonance Spectroscopy (NMR) ........................................................................................................................... 22 IV. CONCLUSION........................................................................................................ 24 REFERENCE ................................................................................................................. 25 vii LIST OF TABLES Table Page 1. Types of POMs which can form Blackberry in aqueous solution.1 (Reprinted from ref. 34, copyright ACS Publication.) ........................................................................... 8 2. Radius of bare alkali cations and alkali cations in water ............................................. 18 viii LIST OF FIGURES Figure 1 The Eigen–Tamm scheme for stepwise formation from free solvated cations Xx+ and solvated anions Yy− of 2SIP ion pairs, then SIP ion pairs, and finally CIP ion pairs, with elimination of solvent molecules from the solvation shells of the ions.5 (Reproduced from ref. 5, copyright IUPAC 2008) ..................... 1 Figure 2 Dielectric loss (ε") spectrum for 0.37 M NiSO4(aq) at 25 °C. (Reproduced from ref. 1, copyright ACS Publications) ............................................. 6 Figure 3 “Giant Wheel” POMs self-assemble into Blackberry like structure in aqueous solution. (Reprinted with permission from ref. 35, copyright 2003 Nature Publishing Group.) ..................................................................................................... 9 n- Figure 4 Structure of ɑ-Keggin [XM12O40] anion. Red balls represent oxygen atoms, blue balls represent M atoms and green ball represents X atom. The overall POM anion carries n negative charges. (Reprinted from ref. 39, copyright ACS Publications) ............................................................................................................ 10 n- Figure 5 Structure of ɑ-Lacunary Keggin [XM11O39] anion. One tetrahedral unit is removed from ɑ-Keggin type structure ............................................................ 10 Figure 6 Basic configuration of an isothermal titration calorimetry42 .............. 13 n- Figure 7 Structure of ɑ-Lacunary Keggin [XM11O39] anion. ........................... 15 ix 7- Figure 8 Titrating 10 mM KCl solution into 0.5 mM [PW11O39] POM solution. ................................................................................................................................. 16 7- Figure 9 Titrating 10 mM RbCl solution into 0.5 mM [PW11O39] POM solution. ................................................................................................................................. 17 7- Figure 10 Titrating 10 mM RbCl solution into 0.5 mM [PW11O39] POM solution. ................................................................................................................................. 17 x 7- Figure 11 Titrating 10 mM LiCl solution into 0.5 mM [PW11O39] POM solution. ................................................................................................................................. 19 7- Figure 12 Titrating 10 mM NaCl solution into 0.5 mM [PW11O39] POM solution. ................................................................................................................................. 20 Figure 13 Contribution form enthalpy and entropy to the Gibbs free energy during ion- pair formation. ...................................................................................... 21 Figure 14 NMR spectrum for titrating Li+ into K7[PW11O39] POM solution. 22 + Figure 15 NMR spectrum for titrating K into K7[PW11O39] POM solution ...... 23 xi CHAPTER I INTRODUCTION AND BACKGROUND 1.1 Introduction of ion-pair behavior Ion-pair behavior describes the partial association of oppositely charged ions in 2 electrolyte solutions to form distinct chemical species called ion-pairs. Typically, an ion pair can be classified as 3 types, a double solvent-separated ion pair (2SIP), if the primary solvation shells of the both ions remain intact, a solvent-shared ion pair (SIP), if both ions share one solvent layer, or a contact ion pair, which two ions contact directly with each other.3 The development in understanding the relationships between the 4-5 various types of ion pairs was based on ultrasonic absorption data
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