Tuning the miscibility of water in imide-based ionic liquids

Liquid-liquid phase behavior measurements were performed for binary mixtures of water and ionic liquids (ILs) containing the same 1-ethyl-3-methylimidazolium ([C₂mim]⁺) cation and different imide-based anions, having symmetric (bis(fluorosulfonyl)imide ([FSI]^-)) or asymmetric structures (2,2,2-trifluoromethylsulfonyl-N-cyanoamide ([TFSAM]^-) and 2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide ([TSAC]^-)). An inversion of phase behavior was observed: while below ∼298 K, the miscibility of water in the studied ILs increases according to the order [C₂mim][TSAC] < [C₂mim][FSI] < [C₂mim][NTf₂], for temperatures above ∼303 K, the reverse trend is observed [C₂mim][NTf₂] < [C₂mim][FSI] < [C₂mim][TSAC]. In turn, above ∼306 K the [C₂mim][TFSAM] is completely miscible with H₂O in all ranges of concentrations. The obtained results also revealed an unusual water solubility variation of 11% in [C₂mim][FSI], and 20% in [C₂mim][TSAC], when the system temperature was changed by less than 1 K, around 298 K and 301 K, respectively. Molecular Dynamics (MD) simulations were used to understand the IL-water interactions and rationalize the experimental observations. These results suggested that the miscibility trends are mainly related to the ability of the water molecules to form water-anion and water-water aggregates in solution.