Understanding of structures, dynamics, and hydrogen bonds of imidazolium-based ionic liquid mixture from molecular dynamics simulation

Abstract

Here we employ molecular dynamics simulation to study structures, dynamics, and hydrogen bonds of the imidazolium-based ionic liquid mixture, which consists of equimolar [Emim][BF4] and [Bmim][BF4]. Our simulation results demonstrate that the imidazolium rings of both different cations almost show identical structures with [BF4]− anions regardless of the alkyl chain length. Meanwhile, both kinds of cations almost display the same association/dissociation dynamics with anions. By comparison, the order of diffusion speed is [Emim]+ > [Bmim]+ > [BF4]− while that of rotation speed is [BF4]− > [Emim]+ > [Bmim]+ in the ionic liquid mixture. By analyzing the average numbers of hydrogen bonds (HBs) and the relevant HB dynamics between cations and anions, we find that [Bmim]+ cations have more and stronger HBs with anions than [Emim]+ cations, which should be partly responsible for slower diffusion and rotation of [Bmim]+ cations than those of [Emim]+ cations. Therefore, this simulation study provides a molecular-level understanding of the role of alkyl chain and HB on the relevant structure and dynamics properties in the imidazolium-based ionic liquid mixture.