310 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (A) (B) ,•, 500 / "- ANS treated 4oo .,,• m 300 untreated ß p- 7.oX • 200 • 100 • o 310 360 410 460 310 360 410 460 Wavelength (nm) Wavelength (nm) • 500 '=• ANS tred 4OO 2• [ untreated • 300 • 200 ß • 100 Figure 5. Spectra of ANS-treated isolated human stratum corneum soaked in water. (A) Distilled water. (B) Distilled water buffered to pH 9.5 with TEA. Fluorescence studies showed that rinsing with SLS or TEA-laurate reduced the ANS emission more than rinsing with SLI and water. This indicates that the removal or desorption of ANS by SLS or TEA-laurate is higher than that by SLI or water. While the removal of ANS by water rinse is due to the reequilibrium and desorption of bound ANS, the enhanced removal of in-surfactant solutions is attributed to displacement of ANS by the bound surfactant molecules. Accordingly, the fluorescence results after rinsing with SLS, TEA-laurate, and SLI are given in Figure 6A in terms of fractional displacement of ANS. The fractional displacements were calculated from the intensity ratios of ANS emission maxima (=465 nm) before and after treatment. For example, relative to a water-treated control, TEA-laurate displaced the ANS by about 37% more than did SLI. The differences between treatments are also significant at the shorter treatment time of one minute. In this case, a 40 mM solution of TEA-laurate displaced about 86% of the ANS, whereas the corresponding figures for SLS and SLI were 51% and 41%, respectively (Figure 6B). As mentioned above, the enhanced removal of ANS by various surfactant solutions is attributed to its displacement by bound surfactant molecules. Although the higher pH of the TEA-laurate solution contributes to greater ANS displacement, the increase is significantly higher than that due to pH alone. Thus the change of pH as the primary removal mechanism can be ruled out. Other possible mechanisms of ANS removal are discounted for the following reasons: For example, enhanced desorption of ANS because of its solubilization in surfactant micelles can be ruled out because, as mentioned earlier, ANS has very low affinity for anionic surfactant micelies. The possibility that changes in ionic strength during rinsing with surfactants resulted in ANS displacement also can be ruled out since changes in ionic strength were negligible and neutral salts are known to have only a minor effect on the binding of ANS.

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.