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 coefficients, which combine to reflect ionic aggregation. With increasing hydration of ILs, the anomalous ratio 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][BF(4)]). When ILs diffuse inside an ionic polymer, ion associations are modulated by ionic interactions between mobile cations and anions, and drag from fixed -SO(3)(-) lining the polymer's hydrophilic channels. Surprisingly, cations diffuse substantially faster (≤3×) at low hydration inside membranes, revealing prevalent anionic aggregates. At high hydration, isolated anions diffuse faster (≤4×) than cations. Probing ionic interactions provides pivotal insight into these subtle fluids, with quantitative implications for electrolyte applications such as batteries and "artificial muscle" mechanical actuators.