Penetration of mixed micelles into the epidermis: effect of mixing sodium dodecyl sulfate with dodecyl hexa(ethylene oxide).

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) (C(12)E(6)) was measured for three SDS concentrations (25 mM, 50 mM, and 100 mM) and three SDS solution compositions (1, 0.83, and 0.50). The addition of C(12)E(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 C(12)E(6) decreased the SDS monomer concentration, thus reducing the driving force for the penetration of monomeric SDS into the epidermis, and (ii) the addition of C(12)E(6) reduced, or prevented, the penetration of micellar SDS into the epidermis. Using dynamic light scattering, the hydrodynamic radii of the SDS/C(12)E(6) micelles were determined to be 20 A, for the alpha(m) = 1 micelles, 24 A for the alpha(m) = 0.83 micelles, and 27 A for the alpha(m) = 0.50 micelles (where alpha(m) denotes the SDS micelle composition). A comparison with typical stratum corneum aqueous pore radii reported in the literature (10-28 A) suggests that the alpha(m) = 1 (pure SDS) micelles are able to penetrate into the epidermis, while the alpha(m) = 0.83 and the alpha(m) = 0.50 SDS/C(12)E(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 C(12)E(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.