KINETICS OF THE SOLID-STATE PHASE-TRANSFORMATION OF FORM-BETA TO FORM-GAMMA OF SULFANILAMIDE USING TIME-RESOLVED ENERGY-DISPERSIVE X-RAY-DIFFRACTION

The kinetics of the solid-state phase transformation of form beta to g amma 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 o f 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 p henomenon suggests a distribution of activation energies in the powder ed samples. Despite this complication the data have been analyzed by c onsidering only the fraction transformed. Of the various kinetic model s 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 m odels, however, was not totally satisfactory. The Avrami-Erofeyev mode l was found to depart increasingly from the observed data at high alph a values. The Cardew model, being specific for powdered or polycrystal line 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 microscop y has revealed that the transformation always proceeds from a single n ucleation event in each crystallite and that coalescence of growing su rfaces and ingestion of potential nuclei are unimportant, which is con sistent with the Cardew model. Also, extinction studies using polarize d light have shown that the transformation in the crystallites is gene rally of the type single crystal to single crystal but does not exhibi t any orientational relationship. The overall activation energy and th e 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 activati on energy based on the Avrami-Erofeyev model was 89 +/- 8 kJ/mol. Thes e magnitudes are within the expected range for molecular crystals.