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J. Cosmet. Sci., 54, 143-159 (March/April 2003) Penetration of mixed micelies into the epidermis: Effect of mixing sodium dodecyl sulfate with dodecyl hexa(ethylene oxide) PETER N. MOORE, ANAT SHILOACH, SUDHAKAR PUVVADA, and DANIEL BLANKSCHTEIN, Department of Chemical Enginering, Massachusetts Institute of Technology, Cambridge, MA 02139 (P.N.M., D.B.) and Unilever Home and Personal Care NA, Trumbull, CT 06611 (A.S., S.P.). Accepted for publication November 15, 2002. Synopsis The penetration of the anionic surfactant sodium dodecyl sulfate (SDS) into the epidermis from contacting solutions of SDS and the nonionic surfactant dodecyl hexa(ethylene oxide) (C12E6) was measured for three SDS concentrations (25 raM, 50 raM, and 100 raM) and three SDS solution compositions (1, 0.83, and 0.50). The addition ofC•2E 6 to the SDS solutions was found to decrease the amount of SDS penetrating into the epidermis. The observed decrease occurred via two plausible mechanisms: (i) the addition of C12E 6 decreased the SDS monomer concentration, thus reducing the driving force for the penetration ofmonomeric SDS into the epidermis, and (ii) the addition of C12E 6 reduced, or prevented, the penetration of micellar SDS into the epidermis. Using dynamic light scattering, the hydrodynamic radii of the SDS/%2E6 micelies were determined to be 20 •, for the Otn• = 1 micelies, 24 • for the Otn• = 0.83 micelies, and 27 • for the Otn• = 0.50 micelles (where O•n• denotes the SDS micelle composition). A comparison with typical stratum comeurn aqueous pore radii reported in the literature (10-28 •) suggests that the O•n• = 1 (pure SDS) micelles are able to penetrate into the epidermis, while the o%• = 0.83 and the o• m = 0.50 SDS/C12E 6 mixed micelles are sterically hindered from doing so due to their larger sizes. The observed reduced penetration of SDS into the epidermis upon the addition of Ct2E 6 could lead to a reduction in the skin irritation potential of SDS, provided that there is a relationship between the concentration of SDS in the epidermis and the skin irritation induced by SDS. INTRODUCTION The study of why and how surfactants induce skin irritation and skin damage has broad implications, from the design of mild personal care products to assisting the transport of therapeutic drugs across the stratum corneum (SC) (1-12). Previous studies have compared the irritation potential of different surfactants (3,8,10,11,13-16), and have also determined how different surfactants can lead to changes in the permeability of the Address all correspondence to Daniel Blankschtein. 143