Microstructure and transport in midrange microemulsions

In microemulsions containing small amounts of oil in water (or water in oil), the fluid microstructure is usually that of swollen micelles (or swollen inverted micelles), i.e. the solution consists of small globules of oil (or water) coated with a surfactant layer and dispersed in a continuous water (or oil) rich medium. When the volume fractions of both oil and water are appreciable, microemulsions can be bicontinuous, i.e., they have both oil-continuous and water-continuous domains separated by surfactant-rich regions. The transition has been identified as a percolation process since it occurs when water and oil relative proportions reach a percolation threshold. Recently, however a transition between globular and bicontinuous microstructure has been found in microemulsions of nearly equal and fixed volumes of water and oil. In this paper we discuss these apparently different transitions and report the results of new viscosity studies in the equal oil to water volume microemulsions.