Mechanisms of anionic surfactant penetration into human skin: Investigating monomer, micelle and submicellar aggregate penetration theories

Abstract

OBJECTIVE: Once penetrated into the stratum corneum, anionic surfactants bind to and denature stratum corneum proteins as well as intercalate into and extract intercellular lipids. With repeated exposures, this leads to skin dryness and irritation, compromising barrier function and skin health. The mechanisms of anionic surfactant penetration into the skin, however, are still widely debated. The objective of this study was to evaluate current theories of surfactant penetration into human skin. METHODS: A test set comprising 15 anionic surfactant systems and one non-ionic surfactant, all having either dodecyl or lauryl alkyl chains, was tested for surfactant penetration into split-thickness human cadaver skin in vitro using radiolabelled sodium dodecyl sulphate ( 14 C-SDS). Select physical properties of these formulations thought to be associated with skin penetration including critical micelle concentration, micelle diameter, filtrate concentration and zeta potential were also measured. RESULTS: 14 C-SDS penetration into human cadaver skin from surfactant systems in vitro was found to correlate well with CMC (R 2 = 0.34, P < 0.05), filtrate concentration (R 2 = 0.36, P < 0.05) and zeta potential (R 2 = 0.76, P < 0.001), but poorly with micelle diameter (R 2 = 0.12). Furthermore, the latter measure correlated inversely with penetration compared to what would be expected based on the micelle penetration theory. CONCLUSION: Neither monomer nor micelle penetration theories are sufficient to explain anionic surfactant penetration into human skin. Submicellar (or premicellar) aggregate penetration theory is difficult to defend at relevant surfactant concentrations. We propose a new hypothesis for this mechanism in which short-term penetration is based on monomer concentration and longer term penetration is based on surfactant-induced damage to the skin barrier.

Microbial Biosurfactants in Cosmetic and Personal Skincare Pharmaceutical Formulations

“…Micelles are formed from the aggregates of surfactant monomers in solution at a specific concentration called the critical micelle concentration (CMC) and temperature [ 46 ]. Some proposed theories on monomer, micelle and sub-micelle mechanisms of skin penetration remain debatable [ 47 ]. While some researchers claim that there is a reduction in rate of surfactant penetration through skin layers when the CMC is reached, owing to the relatively large size of micelles and their surface activities, others report that both micelles and monomers of chemical surfactants have the tendency to penetrate through skin layers and associated cells; and given that micelles are unstable, they may disintegrate into monomers after coming into contact with the skin.…”

“…Additionally, micelles smaller than aqueous pore of stratum corneum could penetrate through the skin. Moreover, other small-sized micelles (sub-micelles) emerging during the continuous micelle formation and disintegration also have the potential to penetrate through the skin [ 18 , 47 ].…”

“…Personal cleansing products such as shampoos are designed to be in contact with the skin and hair, but for a short while. Nevertheless, during this period, their interaction with the skin and its associated cells could affect the structural integrity of the stratum corneum, denature proteins, and solubilize intracellular lipids [ 47 ]. Microbial biosurfactants proposed as substitutes for chemical surfactants have been demonstrated to be compatible with the human skin and provide excellent skin surface moisturization [ 22 , 31 , 58 , 117 ].…”

Synthesis and Characterization of Fluorescent Surfactants for Studying the Penetration of Cosmetic Surfactants on the Skin

“…As a possible skin irritant, surfactants will have a significant impact on the safety of cosmetics if they can enter the dermis and cells (Nizioł-Łukaszewska et al, 2017;Morris et al, 2019). Therefore, better understanding of the penetration behavior of surfactants would help guide their safer application in cosmetics.…”

“…Therefore, better understanding of the penetration behavior of surfactants would help guide their safer application in cosmetics. Due to the fact that surfactants are difficult to be observed in skin or cells by microscopy, the radioisotope labeling method was developed to locate surfactants during skin penetration (Blank and Gould, 1961;Loden, 1990;Morris et al, 2019). Coherent anti-Stokes Raman scattering microspectroscopy revealed the accumulation of isotopically labeled surfactants inside cells Supporting information Additional supporting information may be found online in the Supporting Information section at the end of the article.…”