ABSTRACT
Compounds in which there is a clear separation between lyophilic (solvent-liking) and
lyophobic (solvent-hating) regions of the molecule are referred to as amphiphilic or
amphipathic compounds or amphiphiles. When, as with most pharmaceutical systems, the
solvent is water, we refer to hydrophilic and hydrophobic regions, respectively. Note, of
course, that a polar group will act as a lyophilic group in a polar solvent such as water, but
as a lyophobic group in a nonpolar solvent such as heptane. The dual nature of these
compounds leads to their accumulation at interfaces in such a way as to remove the
hydrophobic group from the aqueous environment, with a consequent reduction of surface
or interfacial tension, hence their alternative description as surface-active agents or
simply as surfactants. The adsorption at the various interfaces between solids, liquids,
and gases results in changes in the nature of the interface, which are of considerable
importance in pharmacy. In this chapter, the various characteristic properties of
surfactants, which arise as a consequence of their amphiphilic nature, will be examined,
with particular emphasis on the role of surfactants in reducing interfacial tension, so
facilitating the formation of emulsions and microemulsions; adsorbing onto solid surfaces
with implications for the wetting of the particle, its dissolution in solution and
enhancement of the stability of suspensions; and the formation of micelles capable of
solubilizing water-insoluble drugs.
