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 KirkwoodBuff theory of solutions, which can quantify the interactions between species in ILwater 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][NTf 2]) shows stronger interactions with water at low water concentrations, with the analogous PIL N,Ndimethylethanolammonium bis(trifluoromethanesulfonyl)imide ([DMEtA][NTf 2]) having stronger waterion interactions at higher water contents, despite waterion interactions weakening with increasing water content in both systems. (2) Water has little effect on the average ionion interactions in both protic and aprotic ILs, aside from the AIL [Ch][NTf 2], which shows a strengthening of ionion interactions with increasing water content. (3) Selfassociation of water in both PILwater systems leading to the presence of large aggregates of water in ILrich compositions has been inferred. Waterwater interactions in [DMEtA][NTf 2] were found to be similar to those of dialkylimidazolium AILs, whereas these interactions were much larger in the PIL N,Ndimethylethanolammonium propionate ([DMEtA][Pr]), attributed to the change in anionwater interactions.