ABSTRACT
This chapter will deal with fundamental aspects of interfacial phenomena which do not fit readily into the subjects addressed so far, or which merit some attention over and above that which has been given under a specific heading in an earlier chapter.When the subject of interfacial phenomena is raised, it is most usual to think first of liquid based systems. This may be in terms of emulsion systems, adsorption concepts, wetting, detergency and many other similar situations. However, equally common are the occurrence of interfacial interactions between two solids. The real difficulty is to what extent solid/solid interactions are related to true concepts of interfacial phenomena. This question will not be explored here except to say that interfacial considerations relating to solid/solid interfaces are limited by the real difficulties in obtaining true solid/solid interfacial contact. It has already been stated (Chapter 2) that solids are rough, and as a direct consequence of this roughness, they have surface energies which vary over their surfaces. A further consequence of surface roughness is that the surface properties of solids vary as a consequence of physical manipulation, as well as the chemical structure. Practical attempts to model solid/solid interfacial behaviour will be addressed in later chapters (e.g. Chapter 6).The existence of surface roughness and adsorbed vapour at the solid interface means that solid/solid interfaces are comparatively hard to produce and their properties are hard to quantify. As such solid/solid interactions to some extent fall outside of the scope of this book on surface phenomena, they will be covered in much less detail than the interfacial phenomena covered in Chapters 1 to 3. It is, however, important to introduce this subject as it is so important in many areas of pharmaceutical science, and indeed in life in general. There are many pharmaceutical examples where solid/solid interfacial phenomena are vital, and these can be described predominantly under the headings of friction, lubrication, and adhesion. 115
Other issues which will be addressed here are aerosols, and relationships between interfacial phenomena and other physical properties.
