ABSTRACT
When considering the solid/vapour interface, it is necessary to understand processes of adsorption and absorption. Adsorption has already been defined (Chapter 2) as the presence of greater concentrations of a material at the surface than is present in the bulk. Absorption is usually defined as the passage of a molecule across a barrier membrane, and is the essential requirement for enteral drug delivery routes to the systemic circulation. However, a gas or vapour can also pass into the structure of amorphous material, such that the uptake onto/into the solid is a sum of adsorption (to the surface) and absorption (into the bulk). If the uptake is thought to consist of both adsorption and absorption processes, it is often referred to by the general term of sorption.There are many processes at the solid/vapour interface which are of pharmaceutical interest, but two of the most important are water/solid interactions, and surface area determination using nitrogen (or similar inert gas)/solid interactions. This chapter will consist of two areas of interest, firstly dealing with general concepts of adsorption at the solid/vapour interface, and then considering the specific case of water/solid interactions.
3.1 The basis o f adsorption The nature of the solid state was described in Chapter 2, where it was reported that the surface energy of a solid (unlike a liquid) is a function of its chemical nature, and its previous treatment history. Consequently, solids will adsorb vapour in different manners depending upon their history (surface energy). Furthermore, as previous treatment causes the transition of crystalline materials into amorphous materials, solids will also absorb vapours to different extents following changes in processing history.As has already repeatedly been emphasised, interfacial behaviour is always related to a “desire” to reduce the total interfacial free energy of any system. As most
