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.
