Unveiling the salting effects of choline chloride on aqueous polymer solutions

Abstract

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 10,000 (PEG10000), polypropylene glycol 400 (PPG400), and polyvinylpyrrolidone 10,000 (PVP10000). Deviations of water isoactivity 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 isoactivity 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.