DEHYDRATION OF THEOPHYLLINE MONOHYDRATE POWDER - EFFECTS OF PARTICLE-SIZE AND SAMPLE WEIGHT

From isothermal thermogravimetric analysis, the dehydration mechanism and activation energy of dehydration of theophylline monohydrate powder were found to be dependent on both the particle size and sample weight. With a large sample weight (i.e., 17.0 mg), the dehydration process was best described by the Avrami-Erofeev equation (n = 1/4) for both the unfractionated samples (needles of length less than or equal to 3 mm) and the < 500 mu m sieve fraction. In contrast, dehydration for the < 150 mu m sieve fraction was consistent with a two-dimensional phase boundary mechanism. However, with a smaller sample weight (i.e., 6.0 mg), dehydration in both the unfractionated samples and the < 150 mu m fraction conformed to the same mechanism, the Avrami-Erofeev model (n = 1/4). The activation energy of dehydration, E(a), estimated from these mechanisms was 70-90 kJ mol(-1), depending on the mechanism of dehydration, sample weight and particle size. These analyses suggest that both the mechanism and the activation energy of dehydration of a hydrate could be significantly influenced by sample pre-history such as particle size, sample weight, crystal defects and surface characteristics. Therefore, meaningful comparisons of kinetic parameters should relate data acquired under similar experimental conditions.