ASSESSING REDUCTION OF SLS IRRITATION 327 irritation response as measured by the different instruments was observed between panelists, the rank order of irritancy of the various molar ratios was similar in all of the participants. EFFECT OF THE DEGREE OF ETHOXYLATION OF AN ETHER CARBOXYLATE ON THE SKIN IRRITATION RESPONSE SLEC-5 and -13 differ only by their degree of ethoxylation. SLEC-5 contains in its chemical structure an average of 4 moles of ethylene oxide, while SLEC-13 contains an average of 11.4 moles (9). The superiority of SLEC-13 over SLEC-5 at reducing SLS irritancy potential was clearly demonstrated by all of the evaluation methods (cf. Figures 2A-E vs 1A-E). Although both exerted beneficial effects in reducing SLS irritancy potential, less SLEC-13 was needed to modify the SLS effect after an equivalent number of patch applications. These effects are summarized in Table V, which shows the mean molar ratios of 1.0% SLS:SLEC-5 or -13 that induced •50% of the SLS value. These results support the claims for validity and sensitivity of this test approach. RECOVERY OF THE SKIN AFTER DISCONTINUANCE OF PATCH APPLICATIONS Approximately 72 hours separated the Day 8 evaluations from the removal of the last patches on Day 5. In the case of the test sites where patching was discontinued after Day 3 or 4 (i.e., sites patched with 1.0% SLS or molar ratios of 1:0.08, 1:0.10, or 1:0.13), the recovery period ranged up to five days. The Day 8 evaluations showed recovery of the skin by varying degrees. Three of five evaluation methods (visual grading, laser Doppler flowmetry, and skin color reflectance) demonstrated that sites treated with mixtures of SLS and SLEC-13 had recovered to a greater extent than those treated with mixtures of SLS and SLEC-5 (Table V). These results further confirm the earlier obser- vations that mixtures containing SLEC-13 resulted in less damage to the skin than those containing SLEC-5. COMPARISON BETWEEN DIFFERENT EVALUATION METHODS The skin irritation potential of surfactants is the expression of very complex interactions occurring in the various regions of the skin following exposure. A multiple instrumental approach is therefore valuable toward understanding the mechanisms of surfactant- induced changes. In this study, stratum corneum barrier impairment and increased cutaneous blood flow were assessed by various noninvasive methods. Skin barrier func- tion was evaluated by TEWL and electrical conductance measurements, while vascular events were quantified by visual grading of erythema, laser Doppler flowmetry, and skin color reflectance, Thus, results from the different methods reflect different aspects of the irritation process. Also, the various evaluation methods vary considerably in their range of possible changes. Whereas differences between baseline and upper limit values for TEWL, electrical conductance, and laser Doppler flowmetry may be twenty-fold or greater, redness (a* values from the L*a*b* system) as measured by skin color reflectance can only range between approximately 5 and 15. The methods monitored increasingly damaging effects at the patched sites throughout the test week and subsequent recovery of the skin after a rest period. The damaging

328 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS effects were inversely related to the molar percentage of SLEC-5 or -13 within the various mixtures, and were generally more pronounced in the panelists treated with SLS and SLEC-5. In the group of panelists treated with mixtures of SLS and SLEC-5, half of the correlation coefficients between evaluation methods were significant, whereas they were all significant in the panelists treated with SLS and SLEC-13 (Table VI). COMPARISON WITH STUDIES FROM THE LITERATURE The ability of ethoxylated anionic surfactants to modify the irritancy potential of SLS has been investigated by Rhein and co-workers (5,7) using ether sulfates with varying degrees of ethoxylation. However, no comparable report was found involving ether carboxylates. The series of (C •2--C 14) alkyl ether carboxylates to which SLEC-5 and -13 belong can be compared in terms of its mollifying effect on SLS to that of the (C z2--C z4) alkyl ether sulfates studied by Rhein et alo (5,7). Surfactants from both series are anionics, ethoxylated, and possess on average the same carbon chain length they differ only by the nature of their polar head group: a carboxylate vs a sulfate. In the present study, the intensity of the irritation response was inversely related to the molar percentage of ether carboxylate within the different mixtures of 1.0% SLS:SLEC-5 or -13 even though the total surfactant concentration was increased by the added cosurfactant. Also, SLEC- 13, the higher ethoxylated surfactant, was more effective in modifying SLS irritancy than SLEC-5. Rhein and co-workers (7) performed a 21-day cumulative irritation patch test with various molar ratios of SLS (at 0.25 and 1.0%) and a (C12--C14) alkyl ether sulfate containing 7 moles of ethylene oxide (AEOS-7EO). They demonstrated that addition of AEOS-7EO to a constant concentration of SLS resulted in a significant reduction in erythema. Rhein et al. (5) also reported in vitro findings using a stratum corneum denaturation model measuring membrane swelling induced by mixtures of 1.0% SLS and 1.0% of a (C12--C14) alkyl ether sulfate containing 3 or 6 moles of ethylene oxide (AEOS-3EO or -6EO). Mixtures of the ether sulfates with SLS caused significantly less swelling than 1.0% SLS alone. AEOS-6EO (i.e., the higher ethoxylated surfactant) was more effective than AEOS-3EO at inhibiting swelling. MECHANISMS FOR REDUCTION OF SLS IRRITATION POTENTIAL The reduced skin response observed with mixtures of SLS and milder surfactants may be explained by several possible mechanisms (1,2). Most likely, formation of mixed mi- celies occurs with the addition of a milder cosurfactant (i.e., SLEC-5 or -13), resulting in a reduction in the critical micelle concentration (CMC) of SLS. This brings about a lowering of the irritating SLS monomer concentration, hence producing a milder system (3,4,7,21). It has been proposed and demonstrated that the surfactant monomers rather than the micelles interact with the skin to elicit irritation (3,4,7,21). Micelies are unlikely candidates to penetrate the keratin because of the combination of size and energy limitations (7). The greater effectiveness of SLEC-13 in relationship to SLEC-5 at reducing SLS irritation potential may result from several phenomena. In anionic surfactants, ethoxylation in- creases the hydrophilicity that counterbalances the hydrophobic effect of the alkyl