On the Miscibility and Immiscibility of Ionic Liquids and Water

Although the "like-dissolves-like" rule is often invoked to explain why sodium chloride dissolves in water, hidden behind this explanation is the delicate balance between the very large cohesive energy of the ionic crystal and large solvation energies of the ions. Room-temperature ionic liquids (ILs) are liquid analogues of ionic crystals and, as dictated by a similar energetic balance, may either fully mix with water or be immiscible with water depending on ion type and cation/anion combination. In this work, we study three hydrophobic and three hydrophilic ILs to examine whether a priori prediction of water miscibility is possible based on analysis of bulk properties alone. We find that hydrophilic and hydrophobic ILs exhibit distinct signatures in their (reciprocal space) Coulomb interactions that indicate predisposition to water mixing. Hydrophilic ILs exhibit a prominent peak in their electrostatic interactions at ∼5–8 Å length scale, largely due to repulsion between neighboring anion shells. When mixed with water, this peak is significantly reduced in magnitude, indicating that electrostatic screening by water molecules is an important driving force for mixing. In contrast, hydrophobic ILs show no such peak, indicating no predisposition to mixing. In addition to this analysis, we compute and compare solvation free energies of the six different anions in water, ion-pairing free energies at "infinitely" dilute concentration, and water absorption free energies in the different ILs. Analyzed within the context of empirical data, our calculations suggest that hydrophobicity trends of different ILs are very sensitive to precise water content at dilute conditions. For example, we predict that bis(fluorosulfonyl)imide-based ILs exhibit anomalously large water absorption free energies at zero water content, with increasing hydrophobicity as preferential absorption sites within the IL become saturated.