168 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Acid High pH: Aqueous Soap Solution Water Soap Increasing Soap Figure 12. The free fatty acid/soap lamellar liquid crystal as base for the stratum corneum lipids requires a minimum water content for stability. Below that water concentration lipid crystals form. content was reduced. This is a new role for a "moisturizer," and it is likely that this new concept, which is easily quantified, will bring new development in the area of cosmetic additives. The layered liquid crystalline structure per se is also the main factor in the barrier to water transport. It has recently been demonstrated that the specific lipid in the struc- ture is not important for the water barrier (26) a layered structure of only the partly saponified free fatty acids forms a satisfactory barrier (27). In this context, it should be noticed that a stratum corneum lipid composition of only saturated chains leads to a crystalline structure only. A model structure with crystalline lipids showed extremely fast water evaporation (28), a result in excellent agreement with the in vivo experiences under the essential fatty acid deficiency syndrome (29-30). This liquid crystalline structure of the stratum corneum lipids makes it possible to outline the interaction between the association structures in different ointments (earlier sections) and the stratum corneum.

AMPHIPHILIC ASSOCIATION STRUCTURES 169 Figure 13. An isotropic solution (left) is dark when viewed between crossed polarizers. A liquid crystal, on the other hand, is anisotropic (and hence birefringent) and radiant (right). The microemulsions _per se are in equilibrium with a liquid crystal (Figures 2 and 8). Hence, compositions inside the microemulsion region will dissolve a liquid crystal when brought in contact with it. The ramifications for the interaction between micro- emulsions and the stratum corneum are obvious a microemulsion in intact form will dissolve the ordered stratum lipid structure to form a liquid. With the lipids in a disordered liquid form, the barrier to water is lost and--what is more important--the barrier to transdermal transport inwards is also damaged. It is a well known fact that facilitated transdermal transport of foreign substances in the majority of cases leads to irritation. This kind of irritation may be called structure-induced irritation because its main cause is the enhanced permeability due to changed stratum corneum structure. However, the water in the microemulsion does evaporate in 10-20 minutes, and the actual influence by the microemulsion on the skin structure is determined by the struc- ture of the residue after the water is removed. If the remains on the skin are an oil solution of the surfactant (Figure 10), a disordering of the stratum corneum lipid structure with the consequence of irritation is a possi- bility. If, on the other hand, the final structure is a liquid crystal (Figure 10), the risk of the structure-induced irritation does not exist, but nonirritating surfactants for the microemulsions must be chosen. The emulsion system (Figure 5) has identical condi- tions. The vesicles or liposome solutions, on the other hand, automatically transform to a lameliar liquid crystal after evaporation of the water. Compositions of this kind should be considered safe from structural considerations, but the final lameliar structure is, of course, no protection against irritating action of individual components. It should be emphasized that the knowledge about the stratum corneum structure and about the association structures of ointment remainders makes it possible to give a precise evaluation of the influences on the stratum corneum structure. SUMMARY The amphiphilic association structures were shown to be involved in all parts of the cosmetic process, from the formulation efforts over the changes taking place when the