156 JOURNAL OF COSMETIC SCIENCE but the surface charge density of the SDS/C•2E 6 mixed micelies decreases as o• m de- creases. This reduction in surface charge density should make it easier for the less negatively charged mixed micelies to access the negatively charged skin pores. However, the addition of C12E6 also causes the micelies to grow and sterically hinders their access to the skin pores, thereby counteracting this surface charge reduction effect. Future work aimed at studying the effect of electrostatics on permeant penetration into the epidermis should examine the penetration of fixed-size charged species at different ionic strengths. CONCLUSIONS It is well known that mixing surfactants can lead to a reduction in the skin irritation potential of a surfactant system (6,24,26). Based on the premise that the irritating surfactant must penetrate into the skin to induce skin irritation, we tested whether mixing the irritating surfactant SDS with C12E6 affected the amount of SDS penetrating into the epidermis (Cski•). We found that increasing the concentration of C12E6 in the contacting solution, while maintaining a fixed concentration of SDS, led to a decrease in Cski• ,. Provided that the skin irritation induced by SDS is related to C,.•i ,, these findings are consistent with the expectation of reducing skin irritation by mixing surfactants. In our recent paper (28), we found that both monomeric and miceliar SDS are able to penetrate into the epidermis. An important consideration in the case of SDS/C•2E 6 surfactant mixtures was whether the reduction in the amount of SDS penetrating into the epidermis was due to the reduced SDS monomer concentration and/or due to a reduction in the skin penetration ability of miceliar SDS. A regression analysis, based on our experimental results, demonstrated that only pure SDS micelies (O•n• = 1) contrib- uted to C,/•i,, at a level comparable to the contribution of the SDS monomers, particularly at the highest surfactant concentrations examined (see Figure 3a). For the SDS/C•2E 6 surfactant mixtures, corresponding to mixed micelies having compositions of o• m = 0.83 and 0.50, the monomeric SDS contributed significantly more to skin penetration than the miceliar SDS, which essentially did not contribute to C,/•i , (see Figures 3b and 3c). Consequently, mixing SDS with C•2E 6 reduced Cs•i, both by reducing the concentration of monomeric SDS and by almost entirely preventing miceliar SDS from penetrating into the epidermis. Using DLS measurements, we demonstrated that the average hydrodynamic radii of the SDS/C•2E 6 mixed micelies increased as the solution composition of SDS decreased. This corresponded to the observed decreased ability of the SDS/C•2E 6 mixed micelies to penetrate into the SC. Comparing the hydrodynamic radii of the SDS/C•2E 6 mixed micelies examined (24 • for gm = 0.83 and 27 • for gm = 0.50) with the hydrodynamic radii of the PEO-bound SDS micelies in our previous paper (25 •, in reference (28)), the steric hindrance model for the prevention of micelie penetration into the skin remains consistent with our experimental findings in this paper, with SDS in the larger mixed micelies not contributing to From our results, one can understand how the monomer penetration model was derived from mixed-surfactant skin irritation data. Mixing surfactants often leads to growth in micelie size (30,31). When the mixed micelies cannot penetrate into the skin, then the surfactant penetration mechanism reduces to the monomer penetration model. In that case, since the CMC is comparable to the surfactant monomer concentration, there

PENETRATION OF MIXED MICELLES INTO THE EPIDERMIS 157 is a direct correlation between the CMC and the observed skin irritation. However, preventing the micellar SDS, or for that matter any miceliar surfactant, from penetrating into the skin has a pronounced effect on skin irritation, because it should eliminate the dose-dependent behavior commonly observed for pure surfactant systems (2,3,8,13,16,18). Once the micelies are prevented from penetrating into the skin, the only other mechanism to reduce C•i, involves a reduction in the surfactant toohomer concentration. ACKNOWLEDGMENTS We thank Hua Tang for numerous useful discussions about the preparation of skin samples and other experimental details, as well as on hindered-transport theories. We also thank Allison Fielder for her assistance in developing the surfactant skin penetration technique. We are grateful to Unilever Home and Personal Care NA for financial support for this work. Peter Moore thanks the Natural Sciences and Engineering Re- search Council of Canada for the award of a PGS-B scholarship. REFERENCES (lO) (11) (12) (13) (14) (15) (1) W. Abraham, "Surfactant Effects on Skin Barrier," in Surfactants in Cosmetk•, M. M. Rieger and L. D. Rhein, Eds. (Marcel Dekker, New York, 1997), pp. 437-487. (2) K. I. Cumming and A.J. Winfield, In vitro evaluation of a series of sodium carboxylates as dermal penetration enhancers, Int. J. Pharm., 108, 141-148 (1994). (3) J. A. Faucher and E. D. Goddard, Interaction of keratinous substrates with sodium lauryl sulfate II. Permeation through stratum corneum, J. Soc. Cosmet. Ch•z., 29, 339-352 (1978). (4) C.L. Froebe, F.A. Simion, L.D. Rhein, R.H. Cagan, and A. Kligman, Stratum corneum lipid removal by surfactants: Relation to in vivo irritation, Dermatologica, 181, 277-283 (1990). (5) S. E. Friberg, L. Goldsmith, H. Suhaimi, and L. D. Rhein, Surfactants and stratum corneum lipids, Colloids Surf, 30, 1-12 (1988). (6) T.J. Hall-Manning, G. H. Holland, G. Rennie, P. Revell, J. Hines, M.D. Barratt, and D. A. Bas- ketter, Skin irritation potential of mixed surfactant systems, Fd. Chem. Toxic., 36, 233-238 (1998). (7) I. Hama, H. Sasamoto, T. Tamura, T. Nakamura, and K. Miura, Skin compatibility and ecotoxicity of ethoxylated fatty methyl ester nonionics, J. Surf Det., 1, 93-97 (1998). (8) C. H. Lee and H. I. Maibach, Study of cumulative irritant contact dermatitis in man utilizing open application on subclinically irritated skin, Contact Dermatitis, 30, 271-275 (1994). (9) K. D. Peck, J. Hsu, S. K. Li, A.-H. Ghanem, and W. I. Higuchi, Flux enhancement effects of ionic surfactants upon passive and electroosmotic transdermal transport, J. Pharm. Sci., 87, 1161-1169 (1998). L.D. Rhein, C. R. Robbins, K. Fernee, and R. Cantore, Surfactant structure effects on swelling of isolated human stratum corneum, J. Soc. Cosmet. Chem., 37, 125-139 (1986). K.-P. Wilhelm, G. Freitag, and H. H. Wolff, Surfactant-induced skin irritation and skin repair,J. Am. Acad. Dermatol., 30, 944-949 (1994). H. Tang, S. Mitragotri, D. Blankschtein, and R. Langer, Theoretical description of transdermal transport of hydrophilic permeants: Application to low-frequency sonophoresis, J. Pharm. Sci., 90, 545-568 (2001). T. Agner and J. Serup, Sodium lauryl sulphate for irritant patch testing•A dose-response study using bioengineering methods for determination of skin irritation, J. Invest. Dermatol., 95, 543-547 (1990). J. Vilaplana, J. M. Mascaro, C. Trullas, J. Coll, C. Romaguera, C. Zemba, and C. Pelejero, Human irritant response to different qualities and concentrations of cocoamidopropylbetaines: A possible model of paradoxical irritant response, Contact Dermatitis, 26, 289-294 (1992). K.-P. Wilhelm, M. Samblebe, and C.-P. Siegers, Quantitative in vitro assessment of N-alkyl sulphate-