Microemulsion (ME) systems have attracted commercial interest for many years. Long before Hoar et al. described MEs for the fi rst time in 1943, women in Australia used a transparent dispersion of eucalyptus oil, water, soap fl akes, and spirit to wash wool. Indeed Hoar and Schulman were the fi rst to introduce the term “microemulsion” to describe transparent, fl uid systems obtained by titration of a conventional emulsion with medium chain alcohol such as pentanol or hexanol. Since then, the term ME has been used to describe multicomponent systems comprising a nonpolar, an aqueous, surfactant, and cosurfactant components. ME systems can be formulated without a cosurfactant, i.e., using a single surfactant, although cosurfactants or an auxiliary surfactant is widely used. It is critical to point out that the term ME was (and occasionally is) used in the literature to describe various surfactant association systems (micelles and reverse micelles), mesophases and liquid crystalline systems (lamellar, hexagonal, and cubic), and even coarse emulsions that are micronized using external energy (submicron emulsions). To avoid such confusion, Danielsson and Lindmann [1] introduced the following defi nition: “Microemulsion is a system of water, oil and amphiphile which is optically isotropic and a thermodynamically stable liquid solution.” By this defi nition, the following systems were excluded:

Systems that are surfactant-free• Liquid crystalline systems (mesophases)• Dilute surfactant systems with water or oil alone (micellar and nonmicellar)• Kinetically stabilized conventional emulsions including micronized coarse • emulsions

TABLE 9.1 Comparison of Coarse Emulsions and MEs

The term microemulsion is often incorrectly used in the literature to describe oil and water dispersions of small droplet size produced by prolonged ultrasound mixing, high-shear homogenization, and microfl uidization, i.e., submicron emulsions. The major differences between a micro-and coarse emulsion are shown in Table 9.1 [2].