The effect of temperature on water vapor sorption by some amorphous pharmaceutical sugars

To determine the effect of temperature on the water vapor sorption behavior of some amorphous pharmaceutical sugars, aqueous solutions of sucrose, lactose, trehalose, and raffinose were freeze-dried using a conventional laboratory lyophilizer. The amorphous sugars formed were stored for several months at 5, 30, and 50 degrees C and at a range of relative humidities (0-90% RH). After equilibration the extent of water vapor sorption was determined gravimetrically, and the presence of any crystalline material was determined. A significant amount of water was sorbed by each of the amorphous sugars even at moderate humidities. In every system studied, lowering the storage temperature at any given relative humidity caused an increased quantity of water to be sorbed. This indicated the predominance of an exothermic water vapor sorption process. Spontaneous crystallization of all the sugars occurred at elevated RHs, and the onset of crystallization did not necessarily coincide with attainment of the water content of the final crystalline forms(s) or the reduction of the sugars' glass transition temperature to ambient conditions. Notably, the amorphous and crystalline forms of some sugars were able to coexist in a quasi-equilibrium state under certain temperature and humidity conditions.