346 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 10% and also for a number of simple electrolytes such as NaCI, RbCI and CaCI2, all at 1 to 2 M. In each case the lag or "breakthrough" times for these materials ranged from two to six days, which indicates that the stratum corneum provides a fairly good barrier even against concentrated salts. This is also confirmed by previous studies (18). SODIUM LAURYL SULFATE However, the stratum corneum is not such an effective barrier against the anionic surfactant sodium lauryl sulfate. At the higher concentrations a steady attack on the membrane appears to take place as evidenced by permeation curves with continually increasing slope see for example the data of Figure 3 and Figure 4. But at the very lowest concentrations the permeability curves begin to resemble the idealized curve of Figure 2, especially for the initial stages. Furthermore, approximately the same lag time is obtained for different concentrations, so that an attempt can be made to calculate P and D. Figure 6 shows data for runs at 0.01 and 0.001% SLS which give a P of 1.7 to 2.3 x 10 -8 cm/sec and aD of 1.0 x 10 -•ø cm2/sec. The only study in the literature with which a meaningful comparison can be made seems to be the work of Howes (10) he 1.6 0.01% 0.001% I I I 0 1 2 3 4 5 6 DAYS Figure 6. Permeability of 0.01 and 0.001% sodium lauryl sulfate

PERMEATION OF KERATINOUS SUBSTRATES 347 8 7 0 I I 0 1 2 3 4 5 6 7 HOURS Figure 7. Normalized permeability curves for sodium lauryl sulfate permitting data at different concentra- tions to be shown on the same plot. Data at 10% were divided by 10, data at 0.1% divided by 0.1, etc. obtained permeability constants for SLS through human epidermis of 0.17 to 3.0 x 10 -8 cm/sec, the lower value being at 6 hr of contact and the higher at 24 hr. His concentration was about 0.7% SLS, which is somewhat higher than those in Figure 6. As the concentration increases, still greater changes occur in the membranes. This can be conveniently shown by a series of "normalized" permeation curves in Figure 7. In this plot the flux has been divided by the solution concentration. Thus the slopes of the various curves are directly proportional to permeability. There is a continual increase in slope with concentration, although the effect is not large for the first 8 hr. Figure 8 shows the same type of behavior over a wide range of concentration and time. This is perhaps the most impressive example from this study of the vigorous attack by SLS on skin. The data of Figures 7 and 8 raise an interesting question about the mechanism of penetration of SLS through the skin. Above the critical micelle concentration (CMC),