Subambient Behavior of Mannitol in Ethanol–Water Co-solvent System

Abstract

The purpose of this study is to characterize the freezing behavior of mannitol in ethanol-water co-solvent systems in comparison with the corresponding aqueous solution. Subambient differential scanning calorimetry (DSC) and microscopy techniques were used to investigate the freezing behavior of mannitol in aqueous solutions and in ethanol-water co-solvent systems. The DSC thermogram of the frozen aqueous solution, which was warmed after cooling at 5.0 degrees C/min, consisted of a glass transition, an endothermic transition, and a crystallization exotherm from mannitol, respectively. The thermograms of ethanol-containing solutions were different in view of including some thermal events attributable to ethanol hydrates. The glass transition of amorphous mannitol was also observed in the thermograms, but became unclear with increasing ethanol in the co-solvent system. The microscopy experiments enabled understanding of the subambient behavior of mannitol. Ethanol was largely removed by vacuum drying rather than freeze-drying. In addition, such manipulations as annealing during the freezing process and slower cooling (0.5 degrees C/min) enhanced the crystallization of mannitol in the frozen system. In the presence of ethanol, crystallization of mannitol was inhibited under subambient conditions. Annealing or slower cooling promoted the crystallization of mannitol during the freezing process.