Room temperature ionic liquids (ILs) have been the subject of intense investigation over the last decade or so. Negligible vapor pressure, excellent solvation properties, high thermal and electrochemical stability coupled with the flexibility to design ILs with unique properties make them ideal candidates as solvents in novel product and process development. Liquid-liquid extraction of products from aqueous phase of a biological media is one of the many promising processes that can take advantage of the unique properties of ILs. One of the challenges in development and optimization of such a process is the solubility of ILs in water leading to water contamination problems. Our experimental collaborators have demonstrated that the solubility of ILs in water is dependent on nature and concentration of inorganic salts. In their work, the salting-in/salting-out ability of inorganic salts was found to qualitatively follow a trend known as the Hofmeister series that explains the changes in solubility of the solute in terms of salts as "water structure makers" or "water structure breakers". However, 1H NMR studies conducted by our collaborators suggested that the salting-in/salting-out of ILs was a direct consequence of the unexpected interaction of salt ions with the hydrophobic moieties of the IL cation.
In this talk, we present the results of molecular dynamics simulations of aqueous solutions of the IL, 1-butyl-3-methylimidazolium bis(trifuoromethylsulfonyl)imide [bmim][Tf2N] in the presence of inorganic salts NaCl, Na2SO4 and NaClO4 to obtain molecular level understanding of salting-in/salting-out behavior of ILs. The analysis, in terms of the radial distribution functions, clearly supports the existence of preferential specific interactions between the low electrical charge-density ("apolar moiety") parts of the IL cation and the inorganic salts. Predictions of the Kirkwood-Buff theory of solutions are also in agreement with the salting-in/salting-out behavior observed experimentally.