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 (Ts). Two Ts 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 Ts values into a single transition, indicating the formation of a single amorphous phase. A single Ts 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.