Mesoscale structures in microemulsions

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