Influence of alcohols on the inter-ion interactions in ionic liquids: A molecular dynamics study

Abstract

Despite a growing number of research reports on neat room temperature ionic liquids (RTILs) and their mixtures with molecular solvents in recent years, understanding and rationalising of such systems is still a challenge. In this work, we performed a classical molecular dynamics simulation study of the pure components - 1-ethyl-3-methylimidazolium thiocyanate (C2C1imSCN), methanol, and ethanol - and their binary mixtures at room temperature. Thermodynamic (density and heats of vaporization), transport (viscosity and self-diffusion coefficients) and structural (in terms of radial, angular and spatial distributions) properties were analysed. It was found, that with the decrease of RTIL content, the ions self-diffusion coefficients notably increase, reaching higher values in the C2C1imSCN-MeOH system. Density and viscosity follow the opposite trend, reaching their minimum at lower RTIL mole fraction. Negative deviations of excess molar volume from ideality in the studied mixtures with minima at ~ 0.2–0.3 mole fraction of RTIL suggest the strongest ion-molecular interactions at this mixture composition. A careful analysis at the molecular level revealed that introducing of alcohols to both systems weakens the inter-ionic H-bonding network, particularly, at low RTIL content. The cation-cation arrangement was found to lose its characteristic above/below orientation in neat RTIL and become disordered at low RTIL content. As to the tail length of the selected alcohols, this was found to have an insignificant effect on the structural properties of the addressed systems.