BINDING OF SURFACTANTS 189 CMC Hairless Guinea Pig Stratum Corneum pH 5.5, RT 6 hr 1 hr 24 hr 0 20 40 60 80 100 120 SDS (mM) Figure 3. Influence of equilibration time on SDS binding to GPSC. Hairless Guinea Pig Stratum Corneum CMC pH 5.5, 1 hr treatment, RT / Untreated C •pidized i ,• GPSC I 0 0 ' 20 40 60 80 100 120 SDS (mM) Figure 4. Effect of delipidizarion of GPSC by chloroform-methanol on SDS binding. one-hour treatment, and room temperature, SDS binds more than SLI to human stratum corneum (HSC). Importantly, SLI binding, unlike that of SDS, does not increase significantly above its CMC. Interestingly, both SDS and SLI show significantly lower binding to HSC than to GPSC (Figure 6). For example, SDS binding to HSC is only about 0.4 mg/mg of the corneum compared to the 2 mg/mg in the case of GPSC. The reason for this large difference between GPSC and HSC is not clear at present and requires further study. One likely hypothesis is that the corneocyte envelope and the consequent resistance to swelling are different for GPSC and HSC. Ef•ct of temperature. The effect of temperature on surfactant binding to HSC was also

190 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.8 O.6 0.4 13.2 HSC, pH 5.5, 1 Hr treatment, RT 00 100 SDS • ,•_3 SLI 20 40 60 80 mM Surfactant Figure 5. Binding of SDS and SLI to human stratum corneum. investigated. Figure 7 (one-hour treatment, pH 5.5) shows an increase in binding for SDS by an increase from room temperature to 37øC. The increase in temperature has a much less pronounced effect on the binding of SLI. Ef •ct ofpH. The effect of pH on the binding of SLI was investigated. Initial attempts to alter the pH of treatment solutions using NaOH showed that the pH of the system during contact with the HSC changed because of the inherent buffering ability of the corneum. Such pH changes due to the buffering ability of skin have been reported in the literature (31). Experiments were therefore conducted in the present study by main- taining the pH of the system constant by using a 0.04 M buffer solution. The binding was determined at a constant initial SLI concentration of 20 mM. The results given in Figure 8 show that the binding of SLI to HSC exhibits a minimum around pH 7-9, with higher binding both at lower and higher pH values. The pH dependence results ob- tained under constant pH conditions clearly indicate that surfactants can interact with the skin under both low and high pH values and that their interaction is the lowest in the pH 7-9 region. BINDING OF SOAP MOLECULES TO SKIN Binding isotherms for soap molecules such as TEA-laurate and TEA-oleate at 37øC and at a pH of 9.5 after one hour of contact are given in Figure 9. Since these experiments were done using stratum corneum samples from sources different from those used for the results reported in Figures 2 to 8, SDS and SLI binding were repeated using the new stratum corneum samples. A comparison of the data in Figure 9 for SDS and SLI with those in Figure 5 shows differences in the absolute magnitude of binding of SDS and SLI to different samples. The trends in surfactant binding and the relative extent are, however, essentially the same for both the samples. As can be seen, at low surfactant levels, oleate binds much more than all other surfac-