Mesoscale structures in microemulsions

Abstract

Due to the existence of intermediate mesoscopic internal structures, soft matter exhibitsvarious fascinating non-linear and non-equilibrium phenomena. In this review article, wefocus on microemulsions consisting of water, oil, and surfactant from the viewpoint of softmatter physics. Microemulsions exhibit a rich phase behavior as the composition and/orthe temperature is varied. In the middle phase, oil and water mix in the presence ofsurfactant molecules to form a mesoscopic bicontinuous structure. To explain thecomplex behavior of microemulsions, it is useful to employ phenomenologicalapproaches such as the Ginzburg–Landau theory or the membrane theory. Wediscuss the Ginzburg–Landau theory and also review the Teubner–Strey model, theGompper–Schick model, and the two-order-parameter model. Based on these models, wediscuss the structure of the middle phase and its wetting transition. The membranetheory proposed by Helfrich is also useful for describing the physical properties ofmicroemulsions. Various structures in microemulsions, such as droplets, bicontinuousand network structures, are properly accounted for by the curvature elasticitymodel. We focus on the Exxon model which clarifies the physical origin of themiddle phase. Within the phenomenological level of description, we review thedynamical aspects of droplet and bicontinuous microemulsions. We also give anoverview of microemulsions found in multicomponent polymeric systems (polymericmicroemulsions). A discussion on recent applications of microemulsions completes thereview.