The determination of equilibrium constants, Δ G, Δ H and Δ S for vapour interaction with a pharmaceutical drug, using gravimetric vapour sorption

Abstract

The application of gravimetric vapour sorption (GVS) to the characterisation of pharmaceutical drugs is often restricted to the study of gross behaviour such as a measure of hygroscopicity. Although useful in early development of a drug substance, for example, in salt selection screening exercises, such types of analysis may not contribute to a fundamental understanding of the properties of the material. This paper reports a new methodology for GVS experimentation that will allow specific sorption parameters to be calculated; equilibrium constant ( K), van’t Hoff enthalpy change (Δ H v), Gibbs free energy for sorption (Δ G) and the entropy change for sorption (Δ S). Unlike other reports of such type of analysis that require the application of a specific model, this method is model free. The analysis does require that over the narrow temperature range of the study Δ H v is constant and there is no change in interaction mechanism.