The dehydration processes of cephalexin phases IV, II, III-1/2 H2O, V-H2O and the noncrystalline solid dihydrate (NC-2H2O) were studied by means of various thermal kinetic analyses using differential thermal analysis (DTA) and differential scanning calorimetry (DSC) instruments. The activation energy and mechanism of dehydration were determined by using approximate thermal kinetic analyses according to Kissinger's and Barton's methods, as well as the nonisothermal kinetic method of Criado and the isothermal DSC method. The dehydrations of phase IV and NC-2H2O were first-order reactions as determined by all methods. The dehydration of phase III-1/2 H2O followed first-order kinetics under nonisothermal conditions and two-dimensional diffusion kinetics under isothermal conditions. The dehydration of phase V-H2O followed three-dimensional diffusion kinetics under nonisothermal conditions, and 1/2 order kinetics under isothermal conditions. The dehydration mechanisms of phases III-1/2 H2O and V-H2O, obtained by allowing organic desolvates to absorb water, depended on the heating conditions. Phase transition induced by the dehydration was measured by X-ray diffractometry. Phases IV, III-1/2 H2O and V-H2O were transformed into phases I, III and V, respectively, by dehydration at 130°C. Phase II was transformed into phase IV after heating at 40°C. NC-2H2O remained in an amorphous state at 130°C.
