192 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.8 0.4 0.2 ø 0 0.18 pH 5.5, 1 hr SDS 37C ß 20 40 60 mM Surfactant Figure 7. Effect of temperature on binding of SDS and SLI to human SC. SLI 37C SLI RT 80 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 rain 0 • I I , [ • [ t 2 4 6 8 10 12 pH Figure 8. Binding of SLI to human stratum corneum as a function of pH. DISCUSSION The binding behavior of SDS, SLI, and soap molecules presented here clearly correlates with their expected harshness toward skin as determined by the zein dissolution tech- nique (see inset in Figure 9). These findings are also consistent with the higher harshness of soaps compared to syndet bars containing cocoyl isethionate as the main active (14). The binding of SLI at all the surfactant concentrations is lower than that of SDS. The higher binding of oleate than that of other surfactants at low concentrations is consistent with the higher surface activity of oleate because of its longer hydrocarbon chain length.

BINDING OF SURFACTANTS 193 0.5 -,- 0.4 0.3 0.2 0 Surfactant % Zein (40 mM) Dissolved SL! 62% SDS 80% Na/TEA Laurate 86% Na/TEA Oleate 92% SDS Na/TEA Laurate Na/TEA Oleate SLI 0 10 20 30 40 50 Surfactant Concentration, mM Figure 9. Binding of surfactants to HSC, one-hour treatment, 37C. Zein solubilization values at 40 mM surfactants are also included. Interestingly, laurate and SDS bind almost to the same extent at higher concentrations. All these observations are in agreement with the accepted notion that surfactants with compact head groups interact much more strongly with the stratum corneum proteins than those with large head groups. The observation that in most cases the surfactant binding increases above the CMC suggests that micelies may have a role in controlling the binding behavior of surfactants. For example, Figures 2-7 show that both SDS and SLI binding exhibit a near plateau or a slowly rising region just above the CMC. However, in the case of SDS at concen- trations above 60 mM, a sharply rising region is observed. This is especially visible in the case of the GPSC. A plateau in the binding isotherm above the CMC would indicate that only the surfactant monomer is involved in this phase of the binding process. From a practical point of view, these results suggest that the initial "plateau binding" can be lowered by lowering the surfactant CMC, since monomer activity does not increase appreciably above the surfactant CMC. Thus, if micellization is made more favorable by lowering the CMC, then binding of the surfactant to the corneum at any given con- centration should be effectively reduced. This is consistent with the conventional wis- dom of increasing the mildness of anionic surfactants by using nonionic surfactant coactives to lower their CMC. ROLE OF MICELLES If we accept the above hypothesis that monomer activity controls the surfactant binding