EFFECT OF SALTS AND SUGARS ON PHASE-SEPARATION OF POLYVINYLPYRROLIDONE-DEXTRAN SOLUTIONS INDUCED BY FREEZE-CONCENTRATION

Phase separation of frozen aqueous solutions of polyvinylpyrrolidone ( PVP)-dextran and the effects of co-solutes on this process were studie d. Thermal analysis of frozen PVP or dextran solutions shows softening temperatures of the non-ice phase (T-s). Two T-s values resulting fro m polymer phase separation are observed in frozen 1-5% PVP 10000-dextr an 9300 solutions. Solutions containing less than 13% PVP-dextran are miscible at -10 degrees C, strongly suggesting that the phase separati on in frozen 1-5% PVP-dextran solutions occurs during freeze-concentra tion. Addition of sucrose or certain salts melds the two T-s values in to a single transition, indicating the formation of a single amorphous phase. A single T-s value is obtained in solutions containing 40-50 m g ml(-1) of sucrose. The effect of salts follows Hofmeister's lyotropi c series. Salting-in salts (e.g., NaSCN, KSCN, KI, KBr) inhibit the ph ase separation at initial concentrations less than 50 mM. Salting-out salts (e.g., Na2SO4, Na2HPO4, KH2PO4) show little or no effect. The co ncentrations of solutes required to inhibit phase separation suggest t hat disaccharides 'dilute' the polymers in the amorphous phase. Saltin g-in salts might inhibit phase separation by a dilution effect and by increasing the binodal for polymer phase separation.