Electrolyte-induced changes in glass transition temperatures of freeze-concentrated solutes.

Abstract

Addition of electrolytes to solutions of non-crystallizing solutes can cause a significant decrease in the glass transition temperature (Tg') of the maximally freeze-concentrated solution. For example, addition of 2% sodium chloride to 10% solutions of dextran, PVP, lactose, and sucrose causes a decrease in Tg' of 14 degrees to 18 degrees C. Sodium phosphate has a smaller effect on Tg' and is unusual in that 1% to 2% sodium phosphate in 10% PVP causes a second glass transition to be observed in the low-temperature thermogram, indicating a phase separation in the freeze concentrate. Comparison of DSC thermograms of fast-frozen solutions of sucrose with and without added sodium chloride shows that electrolyte-induced reduction of Tg' is not caused by a direct plasticizing effect of the electrolyte on the freeze concentrate. Measurement of unfrozen water content as a function of temperature by a pulsed nmr method shows that the most likely mechanism for electrolyte-induced changes in Tg' is by increasing the quantity of unfrozen water in the freeze concentrate, where the unfrozen water acts as a plasticizer and decreases Tg'. The correlation time (tau c) of water in the freeze concentrate is in the range of 10(-7) to 10(-8) seconds. The results underscore the importance of minimizing the amount of added salts to formulations intended for freeze drying.