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.

164 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Another way of relaxing the restraint from a low R value is to form vesicles by dis- persing the lamellar liquid crystalline phase as vesicles (or liposomes) in excess solvent, usually water (Figure 9). The liquid crystal is dispersed in the liquid by vigorous me- chanical treatment. The parallel lamellae now form closed rings separating inner spheres of water from the continuous part. Vehicles such as these are useful for cosmetic preparations because they offer a conve- nient method to solubilize active substances into an aqueous-based ointment without the structure-based cause for subsequent irritation that is a potential problem with microemulsions. The microemulsions dissolve a liquid crystal (Microemulsions section) and will, hence, disorder the lipid structure of the stratum corneum (vide infra). A disordered stratum lipid structure facilitates transdermal transport, resulting in an en- hanced potential for irritation. The vesicles, on the other hand, will always form a lamellar liquid crystal on the skin with water evaporation. Hence, no disordering takes place, there is no facilitated transdermal transport, and any irritation is specifically due to the chemical compounds present. Water o------ ------o Water Figure 9. Vesicles or liposomes consist of one or more molecular bilayers separating an inner water droplet from the continuous water.