248 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1/2 (designated as (t/lZ)•/2 or simply half-time) is de, termined experimentally according to the relationship • _ 0.04919 (4) (t/l 2) More accurate values for D are obtained when the average of sorption Ds and desorp- tion Dd is taken. Our findings indicate that the diffusion properties of the water vapor- stratum comeurn system can be characterized by a concentration-dependent diffusion coefficient. A detailed analysis of the behavior of the diffusion coefficient as a function of the water content in stratum comeurn shows that in highly swelling samples, a maximum is observed in the relationship between the mean diffusion coefficient (•) and water content. If the mean diffusion coefficient is corrected for swelling (intrinsic diffusion coefficient [Di]) D, - (5) (t -- V)' lOO BENZENE _ ATER 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 RELATIVE VAPOR PRESSURE Figure 2. Sotpriori isotherms of benzene and water vapor on female stratum corneum, age 20, at 23øC

150 THE STRATUM CORNEUM 249 lOO 5o _.- TOLUENE 0.1 0.2 0.3 0.4 0.5 0.6 0.7 O.S 0.9 i.0 RELA•,'IVE VAPOR PRESSURE Figure 3. Sorption isotherm of toluene vapor on female stratum comeurn, age 20, at 23øC where V is the volume fraction of water, it was found that the diffusion coefficient usually shows a continuous increase with increase in water content. It was thought of interest to extend our investigations to some organic vapor systems to improve our understanding of the physical and chemical factors associated with sorptive and barrier properties of the stratum comeurn. The benzene and toluene vapor sorption isotherms of female stratum corneum (age 20), are shown in Figs. 2 and 3. The uncertainty in the uptake values at the high end of the isotherms is quite high (+15 per cent at the highest relative vapor pressure) nevertheless, it is clear that the sorption isotherms are of type II according to the BET classification. Similar sorption isotherms were obtained for other human comeurn sam- ples and the results generally indicate a higher sorption capacity for the organic vapor. However, if the uptake values are expressed in moles rather than the conventional weight increase in grams per 100 gm comeurn, it was found that the organic vapor (benzene, toluene) sorption values were generally higher than those obtained for water up to about 0.5 relative vapor pressure then progressively fell below the water uptake values. These preliminary findings seem to indicate that there are more binding sites available for benzene or toulene in the comeurn samples examined in the vapor range where the sorption process is presumed to follow a Langmuir model. At higher relative vapor pressure, formation of multilayers and clusters of sorbate molecules may occur, hence no binding between the sorbate molecules and the substrate. These data seem to