SURFACTANT-SKIN INTERACTIONS 311 (A) (B) z 1.o • 0.8 • 0.6 0.4. = 0.2 .,,., "-' 0.0 I hr @37q2 Water $LI SLS Laurate • 1.0 Z • 0.8 • 0.6 •,, 0.4 r,• '• 0.2 • o.o• I min. @37øC water SLI SLS Laurate Figure 6. Fractional displacement of ANS from human stratum corneum treated with water and with aqueous solutions of pure anionic surfactants. Fractional displacement is defined as l - the ratio of ANS emission maxima (•465 nm) before and after treatment. (A) 1-hr treatment • 37øC. (B) l-rain treatment 6' 37øC. Clearly, the above results show that TEA-laurate and SLS displaced more ANS than did SLI. This implies that TEA-laurate and SLS bind much more to the corneum than SLI. Direct binding results, determined using radiolabeled surfactants, support these con- siderations. The binding isotherms in Figure 7 indicate a significantly greater binding of laurate compared to SLI at surfactant concentrations of 20 mM and above, i.e., at characteristic in-use concentrations. Interestingly, the binding of SI. under these con- ditions became similar to that of the harsh surfactant SLS. Thus the binding values are consistent with the ANS displacement results given above and the known harshness of soaps and SLS toward skin. The correlation of surfactant binding with ANS displace- ment can be seen clearly from Figure 8. Note that the extent of binding at short contact times also correlates with the surfactant binding (see inset at Figure 8, where the one-minute ANS displacement is plotted against the corresponding surfactant binding). Kinetic details of surfactant binding given in Figure 9 show that the binding of SLI in one hour is only about the same as that of SLS in two minutes. Clearly, the results presented here show a correlation between the extent of surfactant binding with harsh- ness toward skin as well as the ability to displace ANS. INTERACTIONS OF CORNEUM WITH CLEANSING PRODUCTS The ANS displacement technique was used to compare the relative deposition of anionic surfactants from the three formulated cleansing bars to human stratum corneum. The ANS-treated corneum was contacted with a 10% dispersion of the test product for one minute and then rinsed. The ANS intensities before and after the treatments were measured. The experiments were carried out at 37øC, so as to be above the Krafft point of all the tested compositions. The experimental parameters reflect the typical usage conditions during cleansing.

312 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 3 0.3 0.1 0.03 0.01 0.003 0.001 HMSC, 1 Hr. @ 37• Na Laurate 0 20 40 60 80 100 Free Surfactant Concentration, mM Figure 7. Binding of surfactants on isolated human stratum corneum measured with radiolabeled surfac- rants equilibrated 1 hr @ 37øC. The fractional decrease in the ANS intensities following water and product treatments are shown in Figure 10. Compared to water, all three products reduced the ANS fluorescence intensity. Importantly, even at the short contact time of one minute, significant differences in the ability of the three surfactants to reduce the ANS intensity can be noted. The reduction in the intensity of the bound ANS upon treatment of the corneum with the various cleansing agents, similar to the treatment with pure surfac- tants, is attributed to ANS displacement by the surfactants in the cleansing formulation (see above and reference 13). This conclusion is further supported by the observation that as with the case of treatment with pure surfactants, the reduction in ANS intensity is actually accompanied by an increase in tryptophan fluorescence intensity (data not shown). The contribution to the ANS displacement resulting from enhanced solubility of ANS due to preferential solubilization in surfactant micelies, as well as pH and ionic strength- related effects, are ruled out for reasons stated earlier. Furthermore, direct fluorescence measurements of the soak solution that had been in contact with ANS-treated corneum exhibited an intensity comparable to what would have been measured if the amount of ANS removed from the corneum had been dissolved in water. A comparison of the extent of ANS displacement by the various cleansers shows that the isethionate-based composition, Bar A, displaced significantly less protein-bound ANS from the corneum. The TEA soap, Bar B, displaced about 60% more ANS than Bar A, while the pure soap, Bar C, displaced nearly twice the amount of ANS than did Bar A. The most likely mechanism for the probe displacement is that the anionic surfactants in