Towards an Understanding of the Mutual Solubilities of Water and Hydrophobic Ionic Liquids in the Presence of Salts: The Anion Effect

The understanding of the specific interactions between salt ions and ionic liquids (ILs) in aqueous solutions is relevant in multiple applications. The influence of a series of anions on the solubility of 1-butyl-3-methylimidazolium tricyanomethane in aqueous environment was here studied. This study aims at gathering further information to evaluate the recently proposed(1,2) mechanisms of salting-in- and salting-out-inducing ions in aqueous solutions of ILs and to provide insights at the molecular-level on the phenomena occurring in these systems. The observed effect of the inorganic species on the aqueous solubility of the ionic liquid qualitatively follows the Hofmeister series, and it is dependent on the nature and concentration of the anions. The liquid-liquid equilibrium data and H-1 NMR results here reported support a model according to which salting-in- and salting-out-inducing ions operate by essentially different mechanisms. While salting-out is an entropically driven effect resulting from the formation of hydration complexes and the increase of the surface tension of cavity formation, the salting-in phenomena is a consequence of the direct binding of the ions to the hydrophobic moieties of the IL. Further evidence here obtained suggests that the interactions of the inorganic ions are not only established with the cation of the IL, but also with the anion, with the observed solubility effect the result of a balance between those two types of interactions.