Challenging the surfactant monomer skin penetration model: penetration of sodium dodecyl sulfate micelles into the epidermis.

The penetration of sodium dodecyl sulfate (SDS) into the epidermis was measured using (14)C-radiolabeled SDS. It was found that, at surfactant concentrations that exceed the critical micelle concentration (CMC) of SDS, the concentration of SDS measured in the epidermis increased as the total SDS concentration in the solution contacting the skin increased, thus demonstrating that micellar SDS contributes to the penetration of SDS into the epidermis. The observed SDS dose-dependent response contradicts the widely accepted view that only surfactant monomers penetrate into the skin, while surfactant in micellar form does not contribute to surfactant penetration into the skin. Nevertheless, this finding is consistent with previously unexplained observations of a dose-dependent damage to the skin induced by SDS at concentrations above the CMC. When poly(ethylene oxide) (PEO) was mixed with SDS, SDS micelles bound to PEO did not contribute to the concentration of SDS in the epidermis, while SDS in free SDS micelles did. Dynamic light-scattering measurements revealed an average hydrodynamic radius of 20 A for the SDS micelles, and a larger radius of 25 A for the PEO-bound SDS micelles. A comparison with typical aqueous pore radii in the stratum corneum measured in the literature (10-28 A) suggests that the SDS micelles may be able to penetrate into the skin, while the PEO-bound SDS micelles may be sterically hindered from penetrating into the skin.