Ravi S. Pappu, University of Rhode Island


It is a well-known fact that water can act as a detrimental variable in performance of pharmaceutical dosage forms. Derivatives of various starches and cellulose are widely used as excepients, generally in high proportions to the total amount making up the dosage form . A complete understanding of the uptake of moisture by cellulose polymers is important because moisture-polymer interaction affects various properties such as compressibility and stability. In this study I) hydroxypropyl methylcellulose (HPMC), 2) modified HPMC (SEPPIC®), 3) microcrystaline cellulose containing carrageenan (Lusterclear®) were used for studies of moisture polymer interaction. The interaction of moisture with these polymers was assessed using a simple and precise sorption system, which allows a rapid measure of uptake and loss of moisture. The effect of temperature on the sorption behavior of the film was alsoexamined. All the three excepient films displayed sorption isotherms that were classified as type II (Langmuir) and demonstrated hysteresis during desorption. The BET model could be used restrictively but the GAB model fitted the data over the entire range of water activity under study. The Young & Nelson model was successful in modeling hysteresis phenomenon but did not offer any mechanistic details. Thermodynamic analysis of water-excepient film system has also been performed to understand the mechanistic details of moisture-excepient interactions.Partial molal free energies, enthalpies and entropies were calculated for the three 11 (polymeric films. Results from the theoretical methods are useful for the predictive purpose where as thermodynamic studies offered mechanistic details of water-excepient interaction. A comparative study of the theoretical models and thermodynamic studies showed that the results from both the approaches were not always analogues to each other. In conclusion, I) dynamic vapor sorption is very useful in investigation of waterexcepient interaction 2) it is necessary to apply both theoretical models and thermodynamic concepts for complete understanding of water-excepient interactions.