Study on the Unfrozen Water Quantity of Maximally Freeze-Concentrated Solutions for Multicomponent Lyoprotectants

The concentration of maximally freeze-concentrated solutions W′g and the corresponding glass transition temperature T′g and ante-melting temperature T′m of lyoprotectant solutions, are critical parameters for developing lyophilization process. Usually, the lyoprotectant solutions are multicomponent solutions composed of electrolytes, sugars, proteins, polymers, and other chemicals. In this article, the Wg′ values of several multicomponent solutions including trehalose/NaCl, bovine serum albumin/NaCl, and hydroxyethyl starch/NaCl with water were determined by differential scanning calorimetry. A linear relationship between the unfrozen water fraction Wun and the initial solute concentrations Wi was found: Wun = ∑(ai·Wi), which suggested that in the multicomponent solutions each solute could hydrate a certain amount of water ai (g water/g solute) that could not be frozen. The hypothesis was compared with more literature data. For the same solute in different solutions, variation in the fitted coefficient ai is noticed and discussed. If a “universal” value ai for each solute is adopted, both W′g and T′g for a multicomponent solution could be predicted if Couchman-Karasz equation is adopted for calculating glass transition temperature at the same time. The prediction discrepancies for T′g with experimental data were less than 2°C. The finding is discussed about its molecular basis and applicability.