Effects of Polarity and Hydrogen Bonding on Physical Properties and Ion Dissociation in 1-Ethyl-3-methylimidazolium Ionic Liquid + Non-aqueous Solvent Systems

摘要

Previously, we have shown that the dissociation of ions in ionic liquids (ILs) and IL solutions can be estimated from measurements of ionic conductivity, viscosity, and density. Here, we explore the limitations of this method by studying the effects of solvent polarity and hydrogen bonding on ion dissociation in mixtures of 1-ethyl-3-methylimidazolium ethyl sulfate ([emim][EtSO_(4)]), trifluoroacetate ([emim][TFA]), and triflate ([emim][TfO]) with ethylene glycol, triethanolamine, and glycerol. New density, viscosity, and ionic conductivity measurements reveal significant reductions in viscosity at dilute IL concentrations due to the disruption of solvent OH–OH hydrogen bonding. This is apparent because the pure solvent viscosities are comparable to or greater than those of the pure ILs. As a result, we are able to show that macro-scale viscosity measurements taken experimentally do not accurately capture the micro-viscous forces acting on diffusing ions, leading to unrealistic dissociation values that exceed unity. Finally, both ion dissociation and the energetic favorability of mixing (as measured by H ~(E)) increase with anion hydrophilicity for all three solvents.