342 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Z SLOPE • - P, cm/sec D(cm2/sec) = h 2 6T o T 0 TIME Figure 2. Idealized permeation curve P = (KD)/h [3] where K is a unitless partition coefficient, being the ratio at equilibrium of the concentration of solute in the membrane to the concentration of solute in solution (15). This allows determination of K, or of P by two different methods ifK is known. RESULTS AND DISCUSSION In view of the fact that each permeation run uses a different individual skin, some general comments on reproducibility are appropriate. Generally speaking, precision was found to be good. For example, Figure 3 shows three separate stratum corneum samples treated for permeability to 10% SLS, a very aggressive environment. Better results could be obtained at lower concentration: Figure 4 contains data for two dif- ferent runs at 0.5% SLS. However, it was not always possible to obtain such good agreement, particularly at the higher concentrations where the membranes are substantially dissolved with time. Occasional skins showed immediate passage of

PERMEATION OF KERATINOUS SUBSTRATES 343 ß I I I I I I I I I I I I I f I 0 10 20 30 40 50 60 HOURS Figure 3. Three separate runs at 10% sodium lauryl sulfate surfactant, presumably because of a pinhole or quick breakdown. Such membranes were discarded and the data not used. BARRIER PROPERTIES Extensive studies of moisture vapor transmission have shown that the barrier function is well developed at birth for young rats (16) and we have confirmed these findings (17). In this sense the stratum comeurn of such animals can be regarded as a reasonable model for human skin. An examination was also made of the permeability of liquid water (tagged by tritium oxide) through these membranes. The permeability was found to be 5.5 x 10 -7 cm/sec. This is in satisfactory agreement with the value of 2.8 x 10 -7 cm/sec determined for human stratum comeurn by Scheuplein (15). It should be noted that this relatively low value of permeability means that there is rather slow, back diffusion of water from the lower reservoir of the permeability cell