Structure and physicochemistry of surfactants, their impact on skin toxicity, barrier function and skin moisturization

Etudiant :Emmanuelle LEMERY

Directeur ou Directrice :Bolzinger M.A., Briançon Stéphanie

Financement :CDD URGO

Date de la soutenance :23/03/2015

Creams, shampoo, soaps, shower gel, all these cosmetics products, daily used; have in common the presence of an essential raw material for their formulation: the surfactant or emulsifier. Those molecules are often in contact with the skin. Indeed, thanks to its special amphiphilic structure, surfactant helps the stabilization of emulsion, the foam production and brings detergent properties to cosmetic cleansers, solubilizing fat and dirt present at the skin surface. Those molecules can also interact with skin components.

One of the first evidence of the interaction between the surfactant and the skin is the clinical signs observed following a chronical or a long exposition to formula containing a high rate of surfactants. These molecules are now well-known to initiate irritant contact dermatitis and are real public health matters concerning professional diseases to detergents. However, considering the multitude of surfactants on the market, action mechanisms of surfactants on the skin are still not well known, especially for nonionic surfactants, widely used in skin care products and considered as very mild. Sodium lauryl sulfate, an anionic surfactant, is nowadays a model molecule widely studied.

Skin toxicity is often linked to the interaction of the surfactant with the skin proteins, charged molecules. According to that, ionic surfactants are considered to be the more toxic for the skin. Furthermore, the monomeric form of the surfactant is described to be the entity responsible for skin toxicity because it can easily insert into the lipid bilayers of the stratum corneum and reach the deeper layers of the skin compare to the micelles. The surfactant’s toxicity is also linked to its critical micellar concentration.

The aim of this research project was to improve the knowledge on several physicochemical properties of numerous surfactants classes to better understand their interactions and effects on the skin. Several levels of study have been developed. After a detailed physicochemical analysis of surfactants, in vitro experiments helped to evaluate the surfactant’s effect on skin toxicity. The barrier function of the skin was study (surface/detergent properties, lipid matrix organization and the evaluation of the lipid extraction by surfactants) through ex vivo experiments. Finally, skin hydration measurements were made in vivo.

Our studies showed a noteworthy toxicity of some nonionic surfactants, on the opposite, some ionic surfactants were very mild. Parameters known in literature such as CMC or the charges of the surfactants were questioned. Several explanations were highlighted considering the surfactant organization in water and its behavior on the skin barrier, bringing new areas for future research to have a better understanding of the effect of surfactant on the skin.

Moreover, skin toxicity could be linked to one of the three levels studied on skin barrier: the lipid matrix disorganization.