Investigation of aqueous biphasic systems formed from solutions of chaotropic salts with kosmotropic salts (salt–salt ABS)

Abstract

A study of salt–salt aqueous biphasic systems (ABS) was conducted to increase our understanding of solutions of kosmotropic vs. chaotropic salts, especially since most ionic liquids (ILs) fall within the latter class. The salting-out strength of the kosmotropic salts follows the well established Hofmeister series, as observed in polymer–salt ABS, and can be directly related to the ions' Gibbs free energies of hydration (ΔG_hyd). Most currently studied ILs are designed to have chaotropic cations and are thus salted-out by kosmotropic salts. Here, we describe the phase diagrams for imidazolium-, pyridium-, and quaternary ammonium- and phosphonium-based chloride salts (all chaotropic salts) salted-out by K₃PO₄, K₂HPO₄, K₂CO₃, KOH, and (NH₄)₂SO₄ (all kosmotropic salts). The Gibbs free energy of methylene transfer (ΔG_CH2) was also determined for 1-butyl-3-methylimidazolium chloride ([C₄mim]Cl)/K₃PO₄, K₂HPO₄, and K₂CO₃ ABS. The latter results are in the range of an ethanol–water to a chloroform–water system, and can be controlled predominately by the system composition.