Salting-In and Salting-Out of Water-Soluble Polymers in Aqueous Salt Solutions

To obtain further experimental evidence for the mechanisms of the salting effect produced by the addition of salting-out or sating-in inducing electrolytes to aqueous solutions of water-soluble polymers, systematic studies on the vapor–liquid equilibria and liquid–liquid equilibria of aqueous solutions of several polymers are performed in the presence of a large series of electrolytes. Polymers are polyethylene glycol 400 (PEG400), polyethylene glycol dimethyl ether 250 (PEGDME250), polyethylene glycol dimethyl ether 2000 (PEGDME2000), and polypropylene glycol 400 (PPG400), and the investigated electrolytes are KCl, NH₄Cl, MgCl₂, (CH₃)₄NCl, NaCl, NaNO₃, Na₂CO₃, Na₂SO₄, and Na₃Cit (tri-sodium citrate). Aqueous solutions of PPG400 form aqueous two-phase systems with all the investigated salts; however, other investigated polymers form aqueous two-phase systems only with Na₂CO₃, Na₂SO₄, and Na₃Cit. A relation was found between the salting-out or sating-in effects of electrolyte on the polymer aqueous solutions and the slopes of the constant water activity lines of ternary polymer–salt 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 effects, the constant water activity lines had a convex slope. The effect of temperature, anion of electrolyte, cation of electrolyte, and type and molar mass of polymers were studied and the results interpreted in terms of the solute–water and solute–solute interactions. The salting-out effect results from the formation of ion (specially anion)–water hydration complexes, which, in turn, decreases hydration, and hence, the solubility of the polymer and the salting-in effect results from a direct binding of the cations to the ether oxygens of the polymers