Abstract

The physical properties of surface active agent solutions differ from those of nonamphipathic molecule solutions (such as sugars) in one major aspect, namely the abrupt changes in their properties above a critical concentration [1]. This is illustrated in Fig. 3.1 which shows plots of several physical properties (osmotic pressure, surface tension, turbidity, solubilization, magnetic resonance, equivalent conductivity and self-diffusion) as a function of concentration for an anionic surfactant. At low concentrations, most properties are similar to those of a simple electrolyte. One notable exception is the surface tension, which decreases rapidly with increasing surfactant concentration. However, all the properties (interfacial and bulk) show an abrupt change at a particular concentration, which is consistent with the fact that at and above this concentration surface active molecules or ions associate to form larger units. These associated units are called micelles (self-assembled structures) and the first-formed aggregates are generally approximately spherical in shape. A schematic representation of a spherical micelle is given in Fig. 3.2.