Microemulsions: Structures, surfactant layer properties and wetting transitions

We review briefly the basic known features of microemulsion structures, emphasizing the importance of the surfactant layer bending elasticity. The results for water-alkane-nonionic-surfactant systems, confirming the close relationship between the maximum characteristic size in the microemulsion and the persistence length of the surfactant layer, are presented. We show that microemulsions are formed when the surfactant layer bending moduli are in a well defined range: if the bending modulus is too large, ordered lamellar phases are obtained, while if it is too small, the surfactant film cannot form, and the medium is a structureless molecular mixture. The evolution between microemulsions and molecular mixtures is continuous; its relationship with the wetting transition between the microemulsion and the two excess phases is discussed.