Résumé
Microemulsions are thermodynamically stable phases of typically oil and water, and a surfactant covering the interface between them. In this chapter, we review their basic properties, starting with molecular properties and curvature, and relating them to Winsor phases and the Kahlweit-Strey fish phase diagram. The underlying physics based on surfactant properties allows understanding of, e.g., counter-intuitive evolutions of conductivity caused by transitions from a water-continuous phase to disconnected water droplets by adding water. Small-angle scattering has been a method of choice for quantitative investigations of the structure of microemulsions. Some guidelines for analysis based on absolute units, specific surface, curvature, power laws, characteristic sizes, and dilution laws will be given before focusing on detailed modeling. Phases of droplets in a continuous matrix can be understood as swollen (possibly inverse) micelles or colloids, and examples using contrast variation and an analysis based on colloidal form and structure factors will be discussed in this chapter. The existence of bicontinuous phases has triggered considerable interest in the past, and only the combination of self-diffusion NMR, electron microscopy, and small-angle scattering provided a complete picture. Small-angle neutron scattering with contrast variation has provided both pure interfacial film scattering, and the structure of the interwingled bulk phases. While the quantity of the surfactant film surface is accessible in the high-q Porod domain, curvature corrections allow determining its bending properties. The description of the bulk phases by various geometrical or thermodynamical models is also reviewed. Starting with the Debye-Büche expression for inhomogeneous solids, which has only one characteristic length, the well-known Teubner-Strey expression with two length scales will be discussed. Geometrical models based on tessellation of space, starting with the cubic cell model, and extending to DOC-models based on disordered Voronoi polyhedra can be used to distribute the available volumes and surfaces, for different connected geometries based on lamellae or cylinders. The generation of random volumes using level cut plane waves or wavelets will close the discussion of available families of models. This introductory chapter is rounded up with an outlook to current research in the field, from nano- to macroemulsions. Depending on the bending stiffness of the surfactant film, new structures and concepts have emerged.