Certain compounds, because of their chemical structure, have a tendency to accumulate at the boundary between two phases. Such compounds are termed amphiphiles, surface-active agents or surfactants. Their adsorption at the various interfaces between solids, liquids and gases results in changes in the nature of the interface that are of considerable importance in pharmacy. For example, the lowering of the interfacial tension between oil and water phases facilitates emulsion formation; the adsorption of surfactants on insoluble particles enables these particles to be dispersed in the form of a suspension; and the incorporation of insoluble compounds within micelles of the surfactant can lead to the production of clear solutions.

In this chapter we will:

• see how the surface activity of a molecule is related to its molecular structure and look at the properties of some surfactants which are commonly used in pharmacy
  
• examine the nature and properties of films formed when water-soluble surfactants accumulate spontaneously at liquid–air interfaces and when insoluble surfactants are spread over the surface of a liquid to form a monolayer
  
• look at some of the factors that influence adsorption on to solid surfaces and how experimental data from adsorption experiments may be analysed to gain information on the process of adsorption
  
• examine why surfactants form aggregates or micelles in aqueous solutions when their concentration exceeds a critical concentration and some of the factors that influence micelle formation
  
• look at some of the properties of liquid crystals and surfactant vesicles and their potential as drug carriers
  
• see how micelles are able to solubilise water-insoluble drugs and discuss the pharmaceutical importance of this process.