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-

330 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Z "' 6- o 5- z 4- 3- 2- 1- BLEACHED VIRGIN BROWN ß I 2 3 HOURS Figure 5. Sorption of 10% sodium lauryl sulfate by bleached and virgin brown hair ing activity of the monomer in this region. Unfortunately it is very difficult to make direct measurements that are unambiguously related to the SLS monomer concentra- tion in the region well above the CMC. Thus, while it is not clear that monomer concentration in fact does increase there, the sotpriori data shown here are consistent with such an interpretation and they show the same kind of phenomenon found in the dialysis experiments of Mysels. THE DIFFUSION PROCESS From the uptake vs. •/t curves a rough estimate can be made of the diffusion constant of SLS in the keratinous medium, either hair or skin. For hair, the formulation com- monly employed is that which represents diffusion into an infinitely long cylinder at short times (9): Q (t) _ 4 / Dt -- -- •(•) r w