Surfactants in systems with oil and water, including microemulsions

The solubilization of small amounts of nonpolar oil by surfactant micelles was described in Chapter 9. In the present chapter surfactant behaviour in systems (such as emulsions) that contain comparable amounts of water and oil are considered. Above the critical aggregation concentration surfactant distribution between the oil and water is partly determined by the preferred curvature of the aggregates present, although surfactant ‘monomer’ distribution is unrelated to where the aggregates reside. The formation of microemulsion droplets in so-called Winsor systems is described, as are the concomitant changes in oil/water interfacial tension, which become ultralow and pass through a minimum. Microemulsion droplets, like micelles, are dynamic structures, and aspects of drop dynamics are introduced. In the Appendix some methods for the determination of the size of microemulsion droplets, including conventional and dynamic light scattering and small angle neutron scattering, are described.