Influence of Environmental Conditions on the Kinetics and Mechanism of Dehydration of Carbamazepine Dihydrate

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Abstract
The object of this project was to study the influence of temperature and water vapor pressure on the kinetics and mechanism of dehydration of carbamazepine dihydrate and to establish the relationship between the dehydration mechanism and the solid-state of the anhydrous phase formed. Three experimental techniques were utilized to study the kinetics of dehydration of carbamazepine dihydrate (C15H12N2O.2H2O)-thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and variable temperature powder X-ray diffractometry (VTXRD). These techniques respectively provide information about the changes in weight, heat flow and solid-state (phase) during the dehydration process. The instrumental setup was modified so that simultaneous control of both the temperature and the water vapor pressure was possible. The experiments were carried out at different temperatures, ranging from 26 to 64 degrees C. In the absence of water vapor, the dehydration followed the 2-dimensional phase boundary controlled model at all the temperatures studied. In the next stage, the water vapor pressure was altered while the studies were carried out at a single temperature of 44 degrees C. The dehydration was 2-dimensional phase boundary controlled at water vapor pressures < or = 5.1 torr while the Avrami-Erofeev kinetics (3-dimensional nucleation) was followed at water vapor pressures > or = 12.0 torr. In the former case, the anhydrous phase formed was X-ray amorphous while it was the crystalline anhydrous gamma-carbamazepine in the latter. Thus a relationship between the mechanism of dehydration and the solid-state of the product phase was evident. The dehydration conditions influence not only the mechanism but also the solid-state of the anhydrous phase formed. While the techniques of TGA and DSC have found extensive use in studying dehydration reactions, VTXRD proved to be an excellent complement in characterizing the solid-states of the reactant and product phases.