ABSTRACT
The vast majority of pharmaceutically active compounds exist in the solid state at standard temperatures and pressures. Inevitably, the solid drug will come into contact with a liquid phase, either during processing and/or in the formulation, and also ultimately during use. Consequently, the solid/liquid interface is of great importance. Surprisingly, such interfacial phenomena are given comparatively little coverage in many texts; because of the great importance of solid/liquid interactions, and the current deficiency of information, this area will be covered in some detail in this and the following sections.
2.1 The solid surface The discussion that follows is a necessary introduction to the solid state, which will include a consideration of the solid/vapour, the solid/solid, and the solid/liquid interface. This will form an introduction to this chapter, and to Chapters 3 and 4. 2.1.1 Solid surface tension and solid surface energy The ultimate characteristic of a solid is that it is a material which is rigid and resistant to stress. Just as with liquid surfaces, there is a net imbalance of forces in the surface of a solid, and so solids will have a surface energy. With liquids, the surface molecules are (reasonably) free to move, and consequently surface levelling is seen, resulting in a consistent surface energy over the entire surface.
