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

Abstract

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][NTf]) shows stronger interactions with water at low water concentrations, with the analogous PIL N,N-dimethylethanolammonium bis(trifluoromethanesulfonyl)imide ([DMEtA][NTf]) 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][NTf], 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][NTf] 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.

The properties of residual water molecules in ionic liquids: a comparison between direct and inverse Kirkwood–Buff approaches

“…In contrast to beneficial properties like low flammability, low volatility and low melting points, the use of ILs for practical purposes is often limited by their high hygroscopicity. Thus, even low amounts of water in the IL crucially influence the performance and modify thermophysical properties in addition to the structural organization of the solution when compared to pure ILs [8][9][10][11][12][13][14][15][16][17][18][19][20][21] . As a prominent example, the influence of water significantly diminishes the broad electrochemical window known for pure ILs, 19 .…”

“…In agreement with previous results, it was reported that hydrophilic ILs show a tendency of pronounced water-anion network formation 31,32 and that even hydrophobic ILs attract high amounts of water 8,33 . The pronounced hygroscopicity of hydrophobic ILs was explained by the occurrence of water clusters at low and intermediate water mole fractions 20,21 . Although hygroscopic properties are typical for all IL classes, ranging from protic to aprotic ILs, a recent publication highlighted slight differences 21 .…”

“…The pronounced hygroscopicity of hydrophobic ILs was explained by the occurrence of water clusters at low and intermediate water mole fractions 20,21 . Although hygroscopic properties are typical for all IL classes, ranging from protic to aprotic ILs, a recent publication highlighted slight differences 21 . Thus, the strength of ion-water interactions changes for different ILs with varying water content whereas ion-ion interactions are only weakly affected by the presence of water 21 .…”

“…Although hygroscopic properties are typical for all IL classes, ranging from protic to aprotic ILs, a recent publication highlighted slight differences 21 . Thus, the strength of ion-water interactions changes for different ILs with varying water content whereas ion-ion interactions are only weakly affected by the presence of water 21 . As a conclusion, the formation of water clusters 1-15 | 1 was often inferred for intermediate concentrations in apolar and thus hydrophobic ILs 20,21 .…”

“…Thus, the strength of ion-water interactions changes for different ILs with varying water content whereas ion-ion interactions are only weakly affected by the presence of water 21 . As a conclusion, the formation of water clusters 1-15 | 1 was often inferred for intermediate concentrations in apolar and thus hydrophobic ILs 20,21 . For low content of water, it was shown that even homologues of common IL cations, i. e. dialkylimidazolium, show a distinct water affinity 20,24,30 .…”