252 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 3o I o o x 20 o r• 10 40 50 I lOO CONCENTRATION (%) Figure 6. Mean desorption diffusion coefficient (Dd) of toluene vapor in stratum corneum (female, age 20) as a function of concentration, at 23øC

THE STRATUM CORNEUM 253 150 100 ,0 50 100 RELATIVE HUMIDITY (%) Figure 7. Sorption isotherm of water vapor in untreated (curve A) and organic solvent-water treated (curve B) stratum comeurn (guinea pig), at 32øC confirm our earlier conclusions, based on the wetting behavior of human stratum cor- neum and hoof keratin, that these keratinous materials are, on balance, hydrophobic in nature (15). The diffusion process of toluene vapor in the stratum corneum is also quite interesting. The graphs of the intrinsic and mean diffusion coefficients as a function of concentra- tion are shown in Fig. 4. It can be seen that, as the concentration of toluene in the -- stratum comeurn increases, the diffusion coefficient (D, or D) increases by three orders of magnitude in the range of relative vapor pressure examined. In the case of water vapor sorption, the observed increase old with C was one order of magnitude at most, as shown in Fig. 5. Starting from the high concentration end, the desorption process is marked by a constant mean diffusion coefficient •d as shown in Fig. 6 this is followed -- by a sharp decrease in the value of Pd. The constancy in the Dd values can be in- terpreted as reflecting the evaporation process of the "free" toluene in the co• neum, and the sharp decrease in the diffusion coefficient marks the start of desorption from high energy binding sites.