Quantitative Characterization of sorption through isothermal microcalorimetry

inTroDuCTion When a problem is encountered in the production or storage of solid dosage forms, invariably the problem is somehow connected to the sorption of water. In the presence of water, the physical properties of solids may be altered causing changes in flow properties, free volume, and compressibility; or there may be an accelerated interconversion between polymorphic forms seriously affecting bioavailability. Very often, shelf life or chemical stability is markedly compromised by high humidity1. Further, with regard to chemical stability, the reactivity of solids in their amorphous state is usually greater than in their crystalline state2. The availability of high sensitivity microcalorimeters and the computer software necessary to analyze the calorimetric data has resulted in the increased application of this technique in pharmaceutical systems3,4,5,6. This application note presents the results of a calorimetric investigation of the adsorption of water vapor on a crystalline solid and introduces a model to quantitate calorimetric adsorption isotherms. It further demonstrates how this model can be applied to characterize the interaction of water with an amorphous material: a freeze-dried decapeptide.