Aqueous solutions of imidazolium ionic liquids: molecular dynamics studies

Abstract

Molecular dynamics (MD) simulations have been carried out at room temperature on a series of aqueous 1-n-alkyl-3-methylimidazolium bromide ([C_nmim][Br]) solutions with alkyl chain-lengths ranging from ethyl to octyl. The computed cation distributions have been found to be inhomogeneous, with the degree of organization of their tails increasing with the length of the alkyl chain. Cation diffusion decreases with increasing chain length partly due to the formation of aggregates. Anions, which are partially associated with cations, also show a similar behavior despite being the same in all of the solutions. Aqueous [C₂mim][Br] solution has been found to remain isotropic even at high concentration of ionic liquid (IL). While cations in [C₄mim][Br] solution weakly associate to form small clusters, those in [C₆mim][Br] solution form small aggregates. Definite aggregate formation has been observed in [C₈mim][Br] solution. Aggregates are poly-disperse and their size varies from 10 to 25 cations. The most probable value of the aggregation number agrees with experimental results. Aggregates are quasi-spherical objects with the alkyl tails forming the core and the imidazolium head-groups exposed to water. The structure observed in the MD simulations does not support the stair-like model proposed based on a nuclear magnetic resonance study on [C₈mim][Br] solution. The van der Waals interactions between the alkyl tails are responsible for the aggregation of cations.