Interfacial properties of amphiphilic systems

The standard Landau theory of interfacial properties is reviewed and seen to be in-adequate to describe the wetting properties of amphiphilic systems. A simple extension of it is introduced and shown to provide a good description of recent experimental results. The origin of ideal middle phase behavior is shown to be a complete unbinding transition of surfactant sheets. This middle phase is an excellent solubilizer, and produces a zero interfacial tension between coexisting oil and water phases. I shall discuss ternary mixtures of oil, water, and amphiphile, and how the observed behav-iors of the interfaces between the various phases are a direct consequence of the nature of the amphiphile itself. For this purposes, an amphiphile can be talien to be a molecule which, by construction, ha.s a part which likes water and another part which likes oil. Hence "amphiphile", from the Greek mea.ning Ioving both. These molecules try to make interfaces between the oil and water at which they can sit and lower their energy. In such ternary mixtures, one oberves several phases of which three liquid ones, which can all coexist, will be of interest to us here; a water-rich phase, an oil-rich phase, and a "middle " phase, which contains both water and oil, and more amphiphile than the other phases. An amphiphile is considered to be a good on