The Rules of Ion Pairing in Aqueous Solution
Restricted (Penn State Only)
- Author:
- Myers, Ryan
- Graduate Program:
- Chemistry (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 13, 2023
- Committee Members:
- Paul Cremer, Chair & Dissertation Advisor
Kenneth Knappenberger, Major Field Member
John Asbury, Major Field Member
Philip Bevilacqua, Program Head/Chair
Ralph Colby, Outside Unit & Field Member - Keywords:
- Ion Pairing
Vibrational Spectroscopy
Polymer Solubility - Abstract:
- Ion pairing is a ubiquitous phenomenon that impacts a wide range of fields including biology, water purification, chromatography, and more. Despite its importance, accurately predicting the molecular-level details of ion-ion interactions remains a challenge. In this dissertation, we investigate the rules governing ion pairing to better understand when ions will bind to other ions, polymers, and small molecules. By leveraging vibrational spectroscopy and thermodynamic measurements, we gain deeper insights into the intermolecular interactions of ions in aqueous solutions. Specifically, we aim to distinguish between contact ion pairs and water-mediated ion pairs and explore how hydration and charge density impact the ion pairing motif. We also examine how ion pairing affects polymer solubility, with solvent-shared ion pairs increasing solubility and ions that bridge functional groups decreasing it. While previous studies have focused on Hofmeister salts, our research delves into how hydronium and hydroxide ions interact with polymers in aqueous solutions. We find similar, yet different results of these ions at the polymer/water interface than what has been found at the air/water interface. By analyzing the solvation shell around molecules, we uncover evidence of intermolecular interactions that may not be readily observable through solute modes. Through our comprehensive spectroscopic and thermodynamic analysis, we aim to uncover the underlying principles that govern ion pairing in aqueous solutions. Here within, we report several key findings. In Chapter 2, we find that both H3O+ and OH- ions are excluded from a hydrophobic polymer interface, however, hydronium is less excluded than hydroxide. Chapter 3 shows that the polymer chemistry and specific ion effects can show differences in how ion pairing impacts the behavior of the polymer solubility by leading to a polynomial shape that shows either enhancing or diminishing effects on the polymer solubility. Then in Chapter 4, it is shown that divalent metal cations will have varying impacts on polypeptide solubility based on a competition between hydration and electrostatics where Ca2+ dehydrated and bridged amide oxygens whereas Mg2+ paired one to one and maintained its hydration shell. Chapter 5 shows that an increase in the charge density of phosphates in a pyrophosphate moiety allows for Mg2+ to shed its hydration shell and form CIPs with biologically relevant phosphate molecules at low millimolar concentrations. Finally, Chapter 6 shows that alkali metal cations are able to make or break local water structures around pendant functional groups leading to a cationic kosmotropic and chaotropic effects.