Control of human skin wettability using the pH of anionic surfactant solution treatments

The outermost layer of human skin, or stratum corneum, acts as a protective barrier between underlying living tissue and the external environment. The wettability of this tissue layer can influence spreading of chemicals and the adhesion of pathogenic microorganisms. We show in this article that the wettability of isolated human stratum corneum can be controlled through treatment with solutions of the anionic surfactant, sodium lauryl sulfate, buffered to different pH values. Relative to control treatments with the buffer solution alone, surfactant solution treatments under acidic conditions cause delipidated stratum corneum to become more hydrophobic. In contrast, alkaline conditions cause the stratum corneum to become more hydrophilic; irrespective of lipid composition. This transition is consistent with a reorientation of bound surfactants at the tissue interface. Under acidic conditions, electrostatic binding of negatively charged surfactant head groups with positively charged keratin in the stratum corneum would increase tissue hydrophobicity due to the exposed hydrophobic tails. However, a hydrophobic based attraction of the apolar surfactant tails to the stratum corneum surface under alkaline conditions would leave the hydrophilic surfactant head groups exposed, causing increased tissue hydrophilicity. Changes in wettability with pH become diminished when lipids ordinarily found in stratum corneum are present, suggesting the lipids partially inhibit surfactant binding. Profilometry studies of the tissue topography highlight that surfactant induced changes in stratum corneum surface roughness cannot account for the observed changes in wettability.