Phase equilibria and energetics of binary mixtures of water with highly hydrophilic [EMIM]-based ionic liquids: Methanesulfonate, methylsulfate, and dimethylphosphate

Abstract

As a follow-up to our systematic studies on the aqueous solutions of 1-ethyl-3-methylimidazolium-based ([EMIM]) ILs, this work deals with phase equilibria and energetics of other three aqueous [EMIM] ILs whose anion is methanesulfonate ([MeSO3]), methylsulfate ([MeSO4]), and dimethylphosphate ([DMP]). We present here new and extensive experimental VLE, SLE and excess enthalpy data, our aim being to provide truly reliable information that would substantially upgrade that existing in the literature. After investigations of the phase behavior and properties of the pure ILs, we performed for the IL solutions systematic measurements of the water activity in the span from 0.05 to 1 and excess enthalpy in the highly dilute IL region (xIL < 0.003) as functions of composition and temperature in the range of 30 K. The freezing temperatures of solutions were further measured in the water-rich region and, in case of [EMIM][DMP], also in the IL-rich region. The VLE and excess enthalpy data were then simultaneously correlated by a modified NRTL equation whose ability to reliably capture VLE, SLE and the energetic properties of the systems was subsequently demonstrated in detail. This also comprised due comparison and analysis of the available literature information which lead to the identification of some incorrect literature data. The calculated excess Gibbs energy and excess enthalpy of the systems revealed for all three systems large negative deviations from ideality that increase in the sequence [EMIM][MeSO3] < [EMIM][MeSO3] < [EMIM][DMP], the values exhibited by the (water + [EMIM][DMP]) belonging to the largest negative deviations from the ideal mixture behavior ever observed. In contrast to other aqueous [EMIM] ILs, the aqueous [EMIM][DMP] exhibits an enormously negative excess entropy which at xw > 0.5 corresponds to an entropy loss upon mixing and indicates thus an unprecedented structuring effect of the system. The excess Gibbs energy and enthalpy values at extrema for aqueous solutions of five hydrophilic [EMIM] ILs studied here and in our previous work were further found to closely correlate with the solvatochromic H-bond basicity parameters of the ILs anions, which proved that the water – IL affinity is governed by the H-bonding of the water to the anions of the ILs. Finally, simple MD simulations gave insight into the way and the number of H-bonds between the water molecules and the anion of the IL in the water-rich region.