MICROCALORIMETRIC INVESTIGATION OF PHASE-TRANSITIONS .1. IS WATER DESORPTION FROM THEOPHYLLINE-CENTER-DOT-HOH A SINGLE-STEP PROCESS

The dehydration of theophylline.HOH has been followed via isothermal c alorimetry at 40 and 47 degrees C, The power (P)-time relationship fou nd at 47 degrees C can be fitted as a single Gaussian distribution whe re the cumulative area up to time t relative to the total area measure s fractional dehydration (alpha). An Avrami-Erofeev plot for solid sta te nucleation reactions with power 0.25 best linearizes the alpha vs t ime relationship yielding a rate constant which corroborates that foun d by others who used a gravimetric method. At 40 degrees C, endothermi c heat flow in the microcalorimeter, temperature difference between sa mple and reference in differential thermal analysis (DTA), and weight loss in thermogravimetric analysis (TGA), each operated in the isother mal mode indicate that water loss is a multi-step process. Power (P)-t ime data from the thermal activity monitor at 40 degrees C were deconv oluted into two Gaussian relationships yielding rate constants via Avr ami-Erofeev plots for each step with excellent reproducibility, The ra te constants for the two steps are about equal at 40 degrees C but the re are significant differences in lag times.