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 to elucidate the mechanistic details of the water-excipient interaction. Partial molal free energies, enthalpies, and entropies were computed for films of lipidic (glyceryl behenate, GB) and polymeric (polyvinyl alcohol, PVA) coating excipients using the temperature dependence of the adsorption process. The analysis of free energy changes showed that excipient films were not inert participants in the water sorption process. The isoteric heats of adsorption confirmed that water formed hydrogen bonds with the excipient films and allowed estimation of number of hydrogen bonds per water molecule. This result also provided the reason for hysteresis during drying. A comparative evaluation of the application of theoretical models and thermodynamic analysis revealed that results obtained from both approaches were not always complementary. An exponential relationship was found to exist between sorption microrate constants and water activity for the PVA films at all temperatures.

Hot-melt coating: water sorption behavior of excipient films.

Hot-melt coating allows encapsulation of water-labile, drug-laden substrates to form a barrier that resists moisture ingress. To understand the interaction of water with excipients that can form moisture-protective coatings, sorption behavior of films of lipidic (glyceryl behenate) and polymeric (polyvinyl alcohol) coating excipients was investigated. A simple and rapid method using a new, fully automated instrumental technique to investigate the sorption/desorption behavior of excipient films is reported. Further, the influence of temperature and film thickness on the sorption behavior of films is examined. Both excipient films displayed sorption isotherms that were classified as type III and demonstrated hysteresis during desorption. The sorption data for both films did not follow the Langmuir model, and the BET model could only be used restrictively. The GAB model fitted the sorption data at all conditions and over the entire range of water activity studied. The ability of the Young and Nelson model to explain the hysteresis behavior, from analytical and mechanistic perspectives, is evaluated. Temperature and film thickness were found to profoundly influence the nature of moisture interaction and distribution of moisture in the excipient films. An Arrhenius-type relationship was observed between equilibrium moisture content of excipient films and temperature at constant water activity.