The salting-out effect and phase separation in aqueous solutions of electrolytes and poly(ethylene glycol)

The compositions of coexisting phases have been determined for aqueous two-phase systems containing poly(ethylene glycol) (PEG) of nominal molecular weight 8000 and a series of eight electrolytes: NaOH, Na₂CO₃, Na₂SO₄, Na₂HPO₄, Na₃PO₄, (NH₄)₂SO₄, MgSO₄, ZnSO₄. The salting-out effects of the electrolytes on the polymer were obtained by fitting a Setschenow-type equation to the tie line data to derive a salting-out coefficient for each electrolyte. For the sodium salts the relative effectiveness of the anions in forming biphasic mixtures follows the Hofmeister series. The relative salting-out effectiveness of the sulphate salts, however, is probably influenced by complexation of the cations by PEG. The values of the salting-out coefficients reflect the extent of preferential hydration of the ethylene oxide units in the polymer chains.