The salting-in and salting-out phenomena occurring in aqueous systems composed of choline chloride (ChCl) and water-soluble polymers were deeply scrutinized in this work. For this purpose, four ternary systems of {polymer + ChCl + water} were subjected to vapor-liquid equilibrium study by the isopiestic measurements at 25 ◦C. The polymers used are polyethylene glycol 400 (PEG400), polyethylene glycol 10000 (PEG10000), polypropylene glycol 400 (PPG400), and polyvinylpyrrolidone 10000 (PVP10000). Deviations of water iso-activity curves from the linear isopiestic relation (LIR) were considered as a benchmark to identify the dominant interactions and the salting effects occurring in the systems under investigation. Ternary aqueous solutions of PEG400/PEG10000/PVP10000 + ChCl showed positive deviations from the LIR, indicating the favorable polymer-ChCl interactions in these systems, and thus the occurrence of the salting-in effect in the concentration range investigated in this work. The strength of the co-solvency effect of ChCl on the aqueous polymer solutions decreases in the order PVP10000 > PEG10000 > PEG400. On the other hand, the aqueous system of PPG400 + ChCl exhibited negative deviations from the LIR in the monophasic region, demonstrating the unfriendly and incompatible behavior of ChCl with PPG in aqueous media resulting in the salting-out effect. The binodal curve determining the boundary between the monophasic and biphasic regions was obtained for the PPG400-ChCl aqueous biphasic system. It was found that the water iso-activity curves in the biphasic region of this system have positive deviations from the LIR due to the partial dehydration of the solutes by phase splitting.
