Isothermal Cold Crystallization Kinetics Study of Sildenafil

Isothermal cold crystallization of sildenafil, which is used as API of Viagra, was investigated by differential scanning calorimetry, optical microscopy, and X-ray diffraction. To analyze the kinetics of crystallization Avrami and Avramov approaches were used. These two models enabled us to characterize basic parameters such as crystallization constant rate (k), dimensionality (n) and activation energy of the crystallization process. It was found that the value of parameter n only slightly changes with temperature, indicating the same mechanism of crystallization. Moreover, we found that the activation energies for the crystallization process obtained from the all applied methods are comparable. From the optical measurements, it was possible to estimate the time of the nuclei formation, the nucleation density, sizes of the forming embryos and crystals at each temperature as well as the crystal morphology and habit. Additionally, monitoring the crystallization process by optical microscopy, we observed unexpected corrugations of the sample surface before the nucleation begins. This behavior can be related to the phenomenon of H-bond formation between molecules of sildenafil.