Thermodynamic aspects of freeze-drying A case study of an organic solvent-water system

Aqueous solutions of glycine and tetrahydrofuran (THF) were frozen under variable conditions and subsequently annealed. The phases that resulted from this process were studied by X-ray diffraction, DSC and cryotemperature SEM. Under conditions achieved in common laboratory freeze-dryers, the THF clathrate hydrate of cubic structure II was formed in near-quantitative yields. The temperature-composition state diagram for the THF-water-beta-glycine systems suggests that the critical temperature of the primary drying stage is the temperature of the four-phase peritectic reaction (269 K), and not the temperature of the congruent melting of the THF hydrate in the THF-water system (278 K). Freeze-drying is shown to go much faster if aqueous solutions are substituted for THF-water solutions. This finding is of great importance for practical applications, including pharmaceutical drug formulation.