Structure, Dynamics, and Interfacial Behavior in Ionic Liquid-Alcohol Binary Mixtures: A Molecular Dynamics Simulation Study.

Abstract

In this study, molecular dynamics (MD) simulations were conducted to elucidate the influence of cation type, anion type, and alcohol type on the bulk and interfacial properties of binary mixtures of ionic liquids (ILs) and alcohols. Simulations were performed at different mole fractions of ILs in the presence of methanol (MeOH) and butanol (BuOH). Four ILs were considered, comprising the cations 1,3-dimethylimidazolium ([MMIM]) and 1-butyl-3-methylimidazolium ([BMIM]) paired with the anions methyl sulfate ([MeSO₄]) and octyl sulfate ([OcSO₄]). The systems were examined in both the bulk phase and at the liquid-vapor interface. A range of analyses was employed to characterize structural, dynamics, and surface properties across mole fractions, including radial distribution functions, combined radial-angular distributions, density profiles, mean square displacements, and calculations of diffusion coefficients, surface tension, and molar electrical conductivity. The results show that the probability of cation-anion, cation-alcohol, and anion-alcohol interactions increases as the IL mole fraction decreases. The dominant hydrogen-bond interaction occurs between the oxygen atom of the sulfate anion and the hydroxyl hydrogen of the alcohol, with a closest distance of 0.18 nm and an angle of approximately 180°. Across all mole fractions, alcohol molecules exhibited higher mobility than the ionic species. Increasing the IL mole fraction resulted in a nonlinear decrease in the diffusion coefficients of all system components. Interfacial density profiles revealed that, at low mole fractions of [MMIM][MeSO₄] and [BMIM][MeSO₄], alcohols─particularly BuOH─are more prevalent in the vapor-phase region, while increasing IL concentration enriches the interface with ions. For [BMIM][OcSO₄], the anion consistently occupied the outermost layer toward the vapor phase at all concentrations. Surface tension variations with IL mole fraction were found to be strongly dependent on cation and anion identity. In the presence of MeOH, changes followed an approximately linear trend for all three ILs, whereas with BuOH, the trend was nonlinear, exhibiting a breakpoint associated with the onset of aggregation and micelle formation.