328 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS • 6- 5- 4- 3- lO% 1% 0.1% 0 I I I I 0 1 2 3 4 5 6 7 8 HOURS Figure 3. Sorption of sodium lauryl sulfate by bleached hair RESULTS AND DISCUSSION SODIUM LAURYL SULFATE Typical sorption curves at various concentrations are shown in Figure 3 for bleached hair and in Figure 4 for stratum corneum. Similar curves were obtained for undamaged hair and the uptakes in that case were approximately an order of magnitude less than found for bleached hair. For all these substrates the course of sorption follows closely a linear dependence on the square root of time, consistent with a diffusion process. In Figure 5 the data are plotted in this manner for both bleached and undamaged hair. A linear dependence is observed except for the first 15 to 30 min, where a sort of "lag time" is observed. Analogous behavior has been noted before in the dyeing of wool, a physically similar type of process (10,11). The initial lag is characteristic of the presence of a surface barrier, which in this case is postulated to be the so-called epicuticle (12). The data for stratum corneum (Figure 6) also shows good linearity in •/t but there is no evidence of a surface barrier. The slopes of the uptake - •/t lines can be regarded as a measure of the rate of sorp- tion and it is evident that these rates continually increase with concentration. Thus Figure 7 shows the uptakes at 1 hr (which are proportional to the slopes) plotted against the total surfactant concentration for stratum corneum and bleached hair. An interesting feature occurs in both cases: the rate function closely approximates two straight lines which intersect at the critical micelie concentration, CMC, i.e., the point

SORPTION OF KERATINOUS SUBSTRATES 329 70 •_ 60 z 15% lO% 5% 20 10 ---1% 0.1% 0 1 2 3 4 5 6 HOURS Figure 4. Sorption of sodium lauryl sulfate by stratum corneum where micelies begin to form. (For SLS this concentration is 0.24%.) Undamaged hair also shows this phenomenon, but the rates are considerably lower than for bleached hair. It is not surprising that the CMC is important in terms of sorption rate. The diffusion mechanism of sorption strongly suggests that it is the monomer species which enters the substrate. Above the CMC most of the added surfactant is known to exist in the so- lution as micelies, i.e., roughly spherical aggregates of 50 to 100 surfactant molecules. These entities do not seem to penetrate the keratin, probably because of the combina- tion of size and negative charge. Little is known about the precise monomer concentra- tion above the CMC, although it is often stated that it remains constant. There is evi- dence, however, that it may increase slowly. For example, osmotic pressure data (13) show a pattern of two intersecting lines similar to Figure 7, but the measurements were not extended very far above the CMC. Mysels (14) in some ingenious dialysis experi- ments has shown that the rate of passage of SLS through a membrane impermeable to micelies continues to increase above the CMC and he cites this as evidence for increas-