Molecular dynamics simulation of the structure and dynamics in mixtures of ionic liquids and alcohols

Abstract

Molecular dynamics simulations were conducted on mixtures of ionic liquid (IL) and alcohols, specifically methanol, ethanol, and 1-propanol. Two different ILs, [Mmim][MeSO4] and [Bmim][MeSO4], were used at varying alcohol mole fractions to investigate the impact of alkyl chain length of cation, alcohol type, and alcohol concentration on different structural and dynamic properties. The unique characteristics of the ILs were observed due to the varying polarity of solvents and the creation of diverse local environments surrounding the ILs. The alcohol weakens the hydrogen bond network of the IL. A better anion-cation packing was observed for [Mmim][MeSO4]-alcohol than for [Bmim][MeSO4]-alcohol mixtures. The free volume and clustering of alcohol molecules in [Mmim][MeSO4]-alcohol mixtures were stronger than those in [Bmim][MeSO4]-alcohol mixtures; these effects were stronger for mixtures containing longer-chain alcohols. Alcohol accelerates the dynamics of IL, examined in terms of the anion/cation diffusion coefficients, dynamics of ion-pair formation/rupture, and dynamics of hydrogen bond formed between anion-cation. This effect is more pronounced for mixtures of ILs with shorter-chain alcohols. These findings are consistent with the results on the structural perturbations in ILs in the presence of alcohol, i.e., weakening of hydrogen bond network in ILs upon addition of (shorter-chain) alcohols to the ILs, and reduced alcohol clustering.