Chem. Pharm. Bull. 53(6) 662—665 (2005)

factors that affect stability. The interaction of pharmaceutical solids with water can affect their chemical stability as well as their physical and mechanical properties. The influence of moisture on solid-state stability has therefore been extensively documented. It is important now to understand and to anticipate the behaviour of a product exposed to a variety of environmental conditions (e.g. relative humidity) in order to recommend appropriate storage conditions for maintaining the quality of a drug throughout its proposed shelf life. Determination of a moisture sorption isotherm is the general approach for characterizing the relationship between water and solids. The Brunauer–Emmett–Teller (BET) model is the most widely used method for predicting moisture sorption by solids, especially for systems exhibiting type II adsorption behaviour. In some cases it is also suitable for describing isotherms type I, III and IV. An important application of the BET isotherm is the evaluation of solid material surface area. In general, the BET model describes the isotherms well up to a relative humidity of 50%, depending on the material and the type of sorption isotherm. The range is limited because the model cannot describe properly the water sorption in multilayers. In order to analyze the sorption isotherm over a wider range of relative humidities, a Guggenheim–Anderson–de Boer (GAB) model was developed by Guggenheim, Anderson and de Boer based on some modified assumptions of the BET model. These assumptions are: the presence of an intermediate adsorbed layer having different adsorption and liquefaction heats and also the presence of a finite number of adsorption layers. However, the GAB equation also provides the monolayer sorption values and could be used for solid surface area determinations, at the same time covering a broader range of humidity conditions. Both methods have become very popular in food technology where the theory of mono and multilayer adsorption is applied to the sorption of water by a wide variety of dehydrated foods. Here the monolayer value is considered to represent the optimal moisture content for the preservation of these food products. Due to the above mentioned reasons the GAB isotherm has been recommended as the fundamental equation for water sorption by foods. With regard to pharmaceutical additives, there is not much information in the literature except for cellulose, starch and their derivatives, so the main objective of our work was to evaluate the sorption behaviour of several pharmaceutically important excipients. Both approaches to the study of moisture sorption behaviour are presented, with the aim of finding the most suitable model for forecasting the sorption behaviour of studied compound. Additionally, isothermal microcalorimetry was used to evaluate the interaction of water with the excipients, based on the ability of this technique to detect energy changes accompanying sorption processes.