Interactions in Water-Ionic Liquid Mixtures: Comparing Protic and Aprotic Systems.

The sensitivity of ionic liquids (ILs) to water affects their physical and chemical properties, even at relatively low concentrations, yet the structural thermodynamics of protic IL- (PIL-) water systems at low water concentrations still remains unclear. Using the rigorous Kirkwood-Buff theory of solutions, which can quantify the interactions between species in IL-water systems solely from thermodynamic data, we have shown the following: (1) Between analogous protic and aprotic ILs (AILs), the AIL cholinium bis(trifluoromethanesulfonyl)imide ([Ch][NTf2]) shows stronger interactions with water at low water concentrations, with the analogous PIL N,N-dimethylethanolammonium bis(trifluoromethanesulfonyl)imide ([DMEtA][NTf2]) having stronger water-ion interactions at higher water contents, despite water-ion interactions weakening with increasing water content in both systems. (2) Water has little effect on the average ion-ion interactions in both protic and aprotic ILs, aside from the AIL [Ch][NTf2], which shows a strengthening of ion-ion interactions with increasing water content. (3) Self-association of water in both PIL-water systems leading to the presence of large aggregates of water in IL-rich compositions has been inferred. Water-water interactions in [DMEtA][NTf2] were found to be similar to those of dialkylimidazolium AILs, whereas these interactions were much larger in the PIL N,N-dimethylethanolammonium propionate ([DMEtA][Pr]), attributed to the change in anion-water interactions.