Water Sorption and Glass Transition Behaviors of Freeze‐Dried Sucrose–Dextran Mixtures

Abstract

The water sorption and glass transition behaviors of freeze‐dried disaccharide–polysaccharide mixtures at various contents were investigated at relative humidities (RHs) of 0, 11, 23, and 33%. Sucrose and three types of dextrans, which differ in molecular weight, were used as model di‐ and polysaccharides, respectively. The relationship between the dextran and water contents of the sucrose–dextran mixture at different constant RHs indicated that a mixture of sucrose and dextran was lower than that calculated by the Lang and Steinberg mass balance equation. In the RH range of 0–23%, the glass transition temperature, Tg, increased to a considerable extent when the dextran content was equal to or higher than 50%, while the increase in Tg at dextran contents lower than 50% was small. A marked increase in Tg was observed at RH 33% for dextran contents of 0–25% as well as in the range above 50%. This suggests that the physical stability of the highly hydrated amorphous disaccharide is effectively strengthened by the addition of a small amount of polysaccharide. These tendencies were similar for the three dextrans of different molecular weights. Furthermore, it was demonstrated that the addition of a small amount of dextran is quite effective in preventing the collapse of amorphous sugar during freeze drying. © 2002 Wiley‐Liss Inc. and the American Pharmaceutical Association J Pharm Sci 91:2175–2181, 2002