ABSTRACT
Microemulsions are thermodynamically stable and macroscopically isotropic mixtures of at least three components, namely water, oil, and surfactant, where the surfactant forms an extended lm separating water and oil on a nanoscale. The curvature of this surfactant lm depends on various parameters such as the
temperature, the salt, or the cosurfactant concentration. With these parameters a phase inversion can be induced, i.e. that an oil-in-water (o/w)-droplet microemulsion can be inverted into a water-in-oil (w/o)-droplet microemulsion via a bicontinuous microemulsion. In the special case of ternary water-oil-surfactant systems where the surfactant is a nonionic n-alkyl polyglycol ether (CiEj) the phase inversion can be induced simply by changing the temperature as is illustrated by the test tubes in Figure 12.1 (reviewed in Ref. [1]). Figure 12.1 represents a schematic T(g)-section through the phase prism of such a ternary system. Starting with equal volumes of water and oil and measuring the phase diagram as a function of the temperature T and the surfactant mass fraction g, one obtains the well-known “ sh”-shape phase diagram [2]. At g < g0 no microemulsion is formed as the surfactant is needed to saturate the solvents [3]. At g > g0 and low T an o/w-microemulsion coexists with an excess oil phase (2), while at high T a w/omicroemulsion coexists with an excess water phase (2
- ). At intermediate T and g < g~
a microemulsion coexists with an excess water and an excess oil phase (3). The microstructure of this middle-phase microemulsion is bicontinuous around the mean temperature T
~ , while it consists of a w/o network structure next to the 3-2
- ;
and of an o/w network structure next to the 2-3 phase boundary. With increasing g, more water and oil are solubilized until a 1-phase microemulsion appears (1).
