The role of water in cholinium carboxylate ionic liquid's aqueous solutions

Binary systems composed of water and cholinium carboxylate ionic liquids, namely cholinium lactate ([Ch][Lac]), cholinium propanoate ([Ch][Prop]) and cholinium malonate ([Ch][Mal]) were studied from the neat ionic liquid to very diluted aqueous solutions. Densities and viscosities were measured and atypical behaviors were observed, such as the increasing density of the binary [Ch][Prop] + H2O mixtures with increasing water content. In order to get molecular level insights on the IL + H2O solvation schemes, H-1 NMR studies were performed. Large deviations were obtained in the anions resonances when compared to those of the cation suggesting that water interacts preferentially with the anion counter-part of the ionic liquid. The increasing density of [Ch][Prop] + H2O system with increasing water content can be related to the orientation of the alkyl chains, as a result of their nanoscale organization. This behavior was confirmed through the study of the thermophysical properties of [Ch][Hex] + H2O mixtures, where this phenomenon is known to occur. (C) 2015 Elsevier Ltd. All rights reserved.