Cation/Anion Associations in Ionic Liquids Modulated by Hydration and Ionic Medium

In order to understand the unique solvation and conduction properties of ionic liquids (ILs), we explore their interionic associations modulated by hydration level and ionic medium. Pulsed-field-gradient NMR allows sensitive measurement of separate cation and anion diffusion coefficient, which combine to reflect ionic aggregation. With increasing hydration of ILs, the anomalous ration of cation to anion diffusion coefficients reverses, then plateaus to values consistent with expected hydrodynamic radii ratios (r(cation)/r(anion) = 1.4 for [C-2 mim][BF4]). When ILs diffuse inside an ionic polymer, ion associtions are modulated by ionic interactions between mobile cations and anions, and drag from fixed -SO3 lilning the polymer's hydrophilic channels. Surprisingly, cations diffuse substantially faster (<= 3x) at low hydration inside membranes, revealing prevalent anion aggregates. At high hrdration isolated anions diffuse faster (<= 4x), than cations. Probing ionic interations provides pivotal insight into these subtle fluids with quntitative implications for electrolyte applications such as batteries and artificial muscle mechanical acturtors.