INTERACTION OF SURFACTANTS AND KERATINS 51 1000 ß Qx10 9 g/cm 2 0 ' 5 10 15 Number of strips from the solution interface Figure 11. Sodium dodecyl sulfate (SDS) content of strips of isolated stratum comeurn after limited exposure to SDS solution. (Reproduced with permission from reference 10.) rate of migration of the material in question. Unlike the equilibrium uptakes, the diffusion coefficients of charged and uncharged materials are very similar in magni- tudes and are about four order of magnitude lower than those of the same compounds in water (3). The relatively low values of the diffusion coefficients cause highly asymmetrical distributions of the penetrating materials in the keratinous tissues during short exposure times. Poret (10) has studied the distributions of sodium lauryl sulfate and of capryl monoglyceride in vitro, using isolated stratum corneum and, in vivo, using guinea pigs, during relatively short exposure times. Exposing stratum corneum samples to C •4 labeled compounds and stripping layer after layer, he found that the concentration of both these penetrants decreased rapidly with the distance from the solution skin interface (Figure 11). It is interesting to note that the normalized concentrations of both these substances, i.e., the actual concentrations divided by the respective contents of the second strips, were identical with experimental error (Figure 12), confirming again the previous findings that the diffusion coefficients of the various substances in keratins are, tO a good approximation, independent of the

52 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.75 0.25- T I T T r T T I [I • 2 4 6 8 10 12 Number of strips from the solution interface Figure 12. The distribution of sodium dodecyl sulfate (SDS) and of monocapry! glyceride (MCG) in stratum corneum in normalised units. Qn and Q• denote the quantities in the n-th and 2nd strip respectively. Solid lines-SDS broken lines-MCG. (Reproduced with permission from reference 10.) presence or absence of charges on the molecule (3). An important conclusion that emerged from these studies is that, if the physical/chemical properties of keratins are affected by the presence of surfactants in the tissues, asymmetrical distributions of the surfactant molecules induce high degrees of anisotropies in the properties affected. This aspect of the surfactant keratin interaction has important practical implications, and will be discussed further in Section VI. V. THE EFFECTS OF DETERGENTS ON THE STRUCTURE AND THE PROPERTIES OF KERATINS Having discussed the uptake of surfactants from a thermodynamic and kinetic point of view, the questions of the actual location of the surfactant molecules within the keratin structure and the effects on the cosmetically important attributes of the presence of these molecules on properties of hair, skin and nails have to be considered. In a series