Kinetics of the Solid-State Phase Transformation of Form β to γ of Sulfanilamide Using Time-Resolved Energy-Dispersive X-ray Diffraction

Abstract

The kinetics of the solid-state phase transformation of form beta to gamma of sulfanilamide in powdered samples have been investigated using energy-dispersive X-ray diffraction (EDXRD) combined with synchrotron radiation. The beta to gamma transformation which is relatively fast has been followed in real time, courtesy of the high time resolution of the EDXRD method. The data obtained yield alpha-time curves of high accuracy and precision. The observed kinetics are atypical in that the transformation does not always proceed to completion but plateaus off, the rate and extent being higher with increasing temperature. This phenomenon suggests a distribution of activation energies in the powdered samples. Despite this complication the data have been analyzed by considering only the fraction transformed. Of the various kinetic models considered, the Avrami-Erofeyev (n = 3.5) and the Cardew model were found to best describe the data. The data fitting with both of these models, however, was not totally satisfactory. The Avrami-Erofeyev model was found to depart increasingly from the observed data at high alpha values. The Cardew model, being specific for powdered or polycrystalline samples, was significantly better, but only up to alpha values of about 0.85. Above this point the Cardew model deviates markedly from the observed data. Direct visual observation using hot-stage microscopy has revealed that the transformation always proceeds from a single nucleation event in each crystallite and that coalescence of growing surfaces and ingestion of potential nuclei are unimportant, which is consistent with the Cardew model. Also, extinction studies using polarized light have shown that the transformation in the crystallites is generally of the type single crystal to single crystal but does not exhibit any orientational relationship. The overall activation energy and the individual nucleation and growth activation energies for the beta to gamma transformation based on the Cardew model were determined to be 101 +/- 7, 142 +/- 14, and 70 +/- 4 kJ/mol, respectively. The activation energy based on the Avrami-Erofeyev model was 89 +/- 8 kJ/mol. These magnitudes are within the expected range for molecular crystals.