Surface active substances, also known as surfactants, are those substances which preferentially adsorb at the air-liquid, liquid-liquid or liquid-solid interfaces. The surface activity of a solute refers to a particular solvent. Molecules of surface active substances contain at least two distinct parts, a moiety which interacts strongly with the solvent, the lyophilic part, and another moiety the lyophobic part, whose interaction with the solvent is less than its interaction with molecules of a structure similar to its own. The lyophilic moiety may interact with the solvent through solvation, hydrogen; bonding or acid-base interactions in addition to van der Waals forces. This part of the molecule confers solubility on the molecule while the lyophobic part restricts or even prevents solution. In oil-water systems the hydrophobic group is usually a single or branched chain hydrocarbon containing 8-18 carbon atoms, the solubility of the molecule decreasing as the length of the chain increases. The balance between the two parts of the surface active molecule the hydrophile—lyophile balance, the H.L.B., is critical for the performance of the substance in any particular application. The H.L.B. number originally proposed by Davies (1961), is an empirical and convenient measure in common use for the composition.
Anionic surfactants are by far the commonest and used predominantly as detergents. Cationics being positively charged have a greater bacterial affinity and are used in medical applications and cosmetics. They now adsorb readily to negatively charged textile fibers and are efficient softeners and conditioners. The nonionic surfactants are finding increasing application as dispersing agents in the water borne paints industry, in emulsion technology and in the rheological behavior of pastes, slurries and drilling muds. Details of individual surfactants are given in Rosen (1972).
Davies, J. T. and Rideal, E. K. (196.1) Interfacial Phenomena, Academic Press, New York.
Rosen, M. J. and Goldsmith, H. A. (1972) Systematic Analysis of Surface-Active Agents, Chemical Analysis, Vol. 12, 2nd edn., John Wiley and Sons.