BINDING OF SURFACTANTS 191 21 SDS GPSC o SDS HSC LI GPSC 20 40 60 80 100 mM Surfactant Figure 6. SDS and SLI binding to human vs guinea pig stratum comeurn. tants. However, at higher concentrations, the binding of SDS and laurate is higher than that of oleate. SLI appears to bind the least under high-concentration conditions. In contrast to this, laurate binding was found to increase at about 20 mM and to become as much as that of SDS at high concentrations. The latter is particularly relevant since the concentrations of surfactants from cleansing bars under use conditions are no doubt much higher than 20 mM. Note that the extent of surfactant binding at a concentration of 40 mM and the tendency of the surfactant to irritate skin as measured by the zein dissolution test at the same surfactant concentration correlate well with each other (see the inset in Figure 9). As mentioned earlier, all the isotherms in Figure 9 correspond to one hour of contact of surfactant solutions with the stratum corneum. Since the practical times of interest from a personal cleansing point of view are of the order of a minute or less, binding isotherms were also determined after one minute of contact. The results given in Figure 10 show that the binding at one minute, as expected, is lower than that at one hour. The overall trend in the binding, however, appears to be similar to that at one hour. Thus, among the various surfactants tested, oleate binds the maximum at low concentrations. At higher levels, the laurate appears to bind much more than other surfactants.

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.