Thermodynamic analysis of water interaction with excipient films

The interaction of water with excipients that can form moisture-protective coatings was examined earlier by the application of theoretical models. In this study, thermodynamic analysis of water-excipient film systems has been performed do elucidate the mechanistic details of the water-excipient inte r-action. Partial molal free energies, enthalpies, and entropies were compu ted for films of lipidic (glyceryl behenate, GB) and polymeric (polyvinyl a lcohol, PVA) coating excipients using the temperature dependence of the ads orption process. The analysis of free energy changes showed that excipient films were not inert participants in the water sorption process. The isoter ic heats of adsorption confirmed that water formed hydrogen bonds with the excipient films and allowed estimation of number of hydrogen bonds per wate r molecule. This result also provided the reason for hysteresis during dryi ng. A comparative evaluation of the application of theoretical models and t hermodynamic analysis revealed that results obtained from both approaches w ere not always complementary. An exponential relationship was found to exis t between sorption microrate constants and water activity for the PVA films at all temperatures.