Liquid phase behavior of imidazolium-based ionic liquids with alcohols: effect of hydrogen bonding and non-polar interactions

Abstract

Ionic liquids (ILs) have been suggested as potential “green” solvents to replace volatile organic compounds in reaction and separation processes due to their negligible vapor pressure. In order to develop ILs for these applications, it is important to understand the factors that control their phase behavior with other liquids. In this work, we continue our study of the effect of different factors on the phase behavior of imidazolium-based ionic liquids with alcohols, focusing on ILs with somewhat longer alkyl chains (hexyl and octyl) on the cation. All systems exhibit upper critical solution temperatures (UCSTs), with low solubility of the IL in the alcohol and high solubility of the alcohol in the IL. Increasing the alkyl chain length on the alcohol causes an increase in the UCST of the system. By contrast, increasing the alkyl chain length on the cation results in a decrease in the UCST of the system. Decreasing hydrogen bonding opportunities with the cation by replacing a hydrogen atom with a methyl group at the C2 position on the imidazolium ring results in an increase in the UCST. The degree to which the anion affects the UCST diminishes with increasing length of the cation alkyl chain.