162 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Nonionic Surfactants for Microemulsions Hydrocarbon/ microemulsion W/O O/W Aliphatic C 12EO 3 C 12EO3 Aromatic C 12EO3 C 12EO 10-12 special purposes. The nonionic surfactants for microemulsions depend on the kind of hydrocarbon used Table III gives an indication of suitable structures. The combination of an ionic surfactant and cosurfactant is used for microemulsions with the phase diagram in Figure 2 as a basis. W/O microemulsions are formulated by addition of hydrocarbon to the alcohol solution and constitute a continuous region from that area (Figure 6) (18). The O/W microemulsion regions are extensions from the miceliar solutions in the water corners, according to Figure 7 (20). The oil is solubilized to some extent into the lameIlar liquid crystal, and the final diagram looks approximately as in Figure 8. VESICLES The lameIlar liquid crystal would require an R value of 1.0, but the lameIlar structure is the only one geometrically possible for R values in the whole range 1.0-0.5 (Table I). Hence, compositions with R values close to 0.5 do not have a structure at the lowest possible free energy, and modifications to enhance the volume of the hydrocarbon chain (Equation 1) would be favored energetically. This fact has been used to enhance the W/O MICROEM ULS!ONS C=OH + x % CeH• C•OH HtO HtO + x%CeHe KOL+ x%CeH e Figure 6. The W/O microemulsions emanate in a continuous region from the alcohol solution in the system without hydrocarbon.

AMPHIPHILIC ASSOCIATION STRUCTURES 163 C6C 2 H H 20 C12S04 Figure 7. The O/W microemulsion forms an extension from the aqueous miceliar solutions. solubility (21) of certain substances that are only poorly soluble in isotropic liquids. It should be emphasized that the increase in solubility may be pronounced for substances that are very sparingly soluble in solvents. HYDROCARBON • COSURFACMICROEMULSIONSO/•,,•W , Lameliar liquid crystal .. WATER SURFACTANT O/W MICROEMULSIONS Figure 8. In a typical microemulsion system stabilized by an ionic surfactant and a cosurfactant, a lamellar liquid crystal separates the W/O and O/W microemulsions.