Ion dissociation and transport behavior in ionic liquids and ionic liquid solutions
The study of ionic liquids (ILs) has increased significantly over the past two decades as their favorable physiochemical properties have allowed them to be applied in a range of separation, electrochemical, and biological processes. During this time, researchers have repeatedly demonstrated the high customizability of ILs using careful cation/anion pairing to design ILs for highly specific tasks. More recently, focus has shifted to the study of IL/solvent mixtures to increase the range and efficacy of IL-applications. The diverse nature and complexity of these IL systems requires researchers to develop new techniques and metrics for understanding IL behavior by way of thermophysical properties. This work aims to further the present understanding of IL behavior in neat IL and IL/solvent systems using combined experimental and theoretical approaches. We place a central focus on ion dissociation, which describes the extent to which cations and anions in ionic liquids (ILs) and ionic liquid solutions exist as individual “free” species. Ion dissociation is of both fundamental scientific interest and practical importance because ion dissociation has been shown to impact viscosity, density, surface tension, volatility, solubility, chemical reactivity, and many other important chemical and physical properties. In this work, we introduce a novel framework for estimating ion dissociation using easily obtained density, viscosity, and conductivity measurements in conjunction with ion Stokes Radii estimates. This framework is used to evaluate a number of ILs and IL solutions and used to establish a comprehensive overview of anion, cation, and solvent effects on ion transport properties. In addition to estimating ion dissociation, we consider the effects of ion-solvent interactions, including hydrogen bonding and microstructure formation, on ion transport properties.