Effect of salts and sugars on phase separation of polyvinylpyrrolidone[ndash ]dextran solutions induced by freeze-concentration

Abstract

Phase separation of frozen aqueous solutions of polyvinylpyrrolidone (PVP)–dextran and the effects of co-solutes on this process were studied. Thermal analysis of frozen PVP or dextran solutions shows softening temperatures of the non-ice phase (T_s). Two T_s values resulting from polymer phase separation are observed in frozen 1–5% PVP 10000–dextran 9300 solutions. Solutions containing less than 13% PVP–dextran are miscible at -10°C, strongly suggesting that the phase separation in frozen 1–5% PVP–dextran solutions occurs during freeze-concentration.

Addition of sucrose or certain salts melds the two T_s values into a single transition, indicating the formation of a single amorphous phase. A single T_s value is obtained in solutions containing 40–50 mg ml^-1 of sucrose. The effect of salts follows Hofmeister's lyotropic series. Salting-in salts (e.g., NaSCN, KSCN, KI, KBr) inhibit the phase separation at initial concentrations less than 50 mM. Salting-out salts (e.g., Na₂SO₄, Na₂HPO₄, KH₂PO₄) show little or no effect. The concentrations of solutes required to inhibit phase separation suggest that disaccharides ‘dilute’ the polymers in the amorphous phase. Salting-in salts might inhibit phase separation by a dilution effect and by increasing the binodal for polymer phase separation.