Understanding the Effect of the C2 Proton in Promoting Low Viscosities and High Conductivities in Imidazolium-Based Ionic Liquids: Part I. Weakly Coordinating Anions

Abstract

1,3-Disubstituted imidazolium ionic liquids have been the subject of numerous theoretical and experimental studies due to their low viscosity-often the very lowest for any given cation/anion family. One of the mysteries in the imidazolium family of salts is the sharp increase in viscosity that is observed on methylating at the C2 position in the ring. In the nonmethylated case, the C2 proton is observed to be distinctly acidic and, where this is undesirable, substitution of the C2 position removes the problem, but produces an unexpected increase in viscosity. Methylation at other positions on the ring does not produce such a significant effect. In this study, two possible structural or energetic sources of the increased viscosity were investigated: (1) ion association, as probed by the Walden rule, and (2) differences in the potential energy surface profiles that favor ionic transport in the non C2-methylated imidazolium ionic liquids. The second hypothesis was investigated using high-level ab initio theory. The higher viscosity of C2-methylated imidazolium ionic liquids is shown to be a result of high potential energy barriers (significantly above the available thermal energy) between the energetically preferred conformations on the potential energy surface, thus restricting movement of ions in the liquid state to only small oscillations and inhibiting the overall ion transport.