Isopiestic Investigations of the Interactions of Water-Soluble Polymers with Imidazolium-Based Ionic Liquids in Aqueous Solutions

To provide insight into the salting effects produced by the addition of hydrophilic ionic liquids to aqueous solutions of water-soluble polymers and to obtain a relation between the vapor–liquid and liquid–liquid equilibrium behavior of ionic liquid–polymer aqueous–biphasic systems, the isopiestic equilibrium molalities of some ternary ionic liquid–polymer–water systems in both the one- and two-phase areas together with the liquid–liquid equilibrium phase diagrams for systems capable of inducing phase separation were determined at 298.15 K. The polymers are poly­(ethylene glycol) 400 (PEG400), poly­(ethylene glycol) 2000 (PEG2000), poly­(ethylene glycol) 6000 (PEG6000), poly­(ethylene glycol) 10 000 (PEG10 000), poly­(propylene glycol) 400 (PPG400), and poly­(propylene glycol) 725 (PPG725), and the investigated ionic liquids are 1-butyl-3-methylimidazolium bromide ([C₄mim]­[Br]) and 1-butyl-3-methylimidazolium hydrogen sulfate ([C₄mim]­[HSO₄]). It was found that aqueous solutions of [C₄mim]­[HSO₄] form aqueous biphasic systems with PPG400 and PPG725 (salting-out effect); however, other investigated systems do not form aqueous biphasic systems (salting-in effect). A relationship was found between the salting-out and salting-in effects of ionic liquids on aqueous polymer solutions and the slopes of the constant water-activity lines for ternary ionic liquid–polymer aqueous solutions so that in the case of the salting-out effect the constant water-activity lines had a concave slope, but in the case of the salting-in effect the constant water-activity lines had a convex slope