Surfactant Penetration into Human Skin from Sodium Dodecyl Sulfate and Lauramidopropyl Betaine Mixtures

Surfactant mixtures are used in a variety of personal care and cosmetic applications but are known to be harsh on the skin. The purpose of this study was to examine anionic surfactant penetration into human skin from nonideal surfactant mixtures under short-time exposure conditions that are relevant to realistic exposure scenarios. This was done by measuring the penetration of a radiolabeled probe (¹⁴C-SDS) into human cadaver skin in Franz diffusion cells in vitro from the mixtures of sodium dodecyl sulfate (SDS) and lauramidopropyl betaine (LAPB). Monomer and micelle concentrations in the SDS/LAPB/¹⁴C-SDS mixtures were predicted using a regular solution theory approximation. We confirmed that the mixtures of SDS and LAPB exhibit nonideal behavior with a net attraction between the two surfactants. Penetration of ¹⁴C-SDS into excised human skin from the mixtures of SDS and LAPB was found to decrease in a log-linear manner with increasing mole fraction of LAPB in the bulk solution (R² = 0.97, p < 0.001). Additionally, the penetration of ¹⁴C-SDS into excised human skin from the mixtures of SDS and LAPB was found to correlate linearly and strongly with the predicted values of ¹⁴C-SDS monomer concentration in SDS/LAPB/¹⁴C-SDS mixtures (R² = 0.95, p < 0.01). ¹⁴C-SDS penetration from the mixed surfactant composition could be quantitatively reconciled with that from an SDS-only composition by postulating a secondary, positive contribution from LAPB related to its own penetration and binding to skin components that increased SDS penetration at low concentrations. This research therefore supports a monomer penetration theory of surfactant penetration into the skin, combined with a measurable impact of favorable surfactant interactions within the tissue.