Surfactants

Surface active agents, or surfactants, are molecules which are characterised by having two regions in their chemical structure, one hydrophobic, and the other hydrophilic1 . The hydrophobic region would consist of a hydrocarbon chain, and the hydrophilic region can be an ionisable, polar, or water soluble group. Consequently, surfactants are often defined as being amphiphilic in that they have an attraction to both aqueous and oil phases. For many systems the polar region will be a small part of the overall molecule, but that is not always the case, as will be seen below.The amphiphilic nature of these molecules is such that part of the molecule will tend to be expelled from the bulk of a liquid in which they are dissolved (or dispersed), whilst the other part of the molecule will freely interact with the liquid. This leads to a tendency for the molecules to perform two functions; firsdy, they will be forced to liquid interfaces, and secondly, they will tend to align in an ordered fashion at those interfaces. The “adsorption” of surfactant molecules at the surface of a liquid will result in a fall in surface tension (see Chapter 1), and the adsorption to the interface between two liquids will reduce the interfacial tension and aid the formation of emulsions (see Chapter 1 and below). A further property of surfactants is that, as the concentration in solution rises, they will tend to self­associate into structures known as micelles (see Section 5.2). The fundamental aspects of surfactant behaviour will be addressed in this chapter, but the role of surfactants has already been discussed on many occasions in the preceding chapters, and will be the subject of much of Part B of this book. The reason for surfactants being covered at all stages throughout this book is that they can influence most interfacial phenomena, and form a part of the everyday life of each and every one of us. This may be in a scientific context, or as a household aid, or as an environmental 1 Whilst it is most common to talk in terms of water, it is appropriate to note that the surfactant can function in non-aqueous systems, where there is a need for a part of the molecule to have an attraction for the solvent (lyophilic), and another part to have no attraction for the solvent (lyophobic).