2024-03-18 23:00:00
Cosmetic products are formulations containing active substances intended to be mainly in contact with the skin, nails or mucous membranes. Hydrophilic active ingredients can simply be dissolved in water and hydrophobic active ingredients are generally dissolved in oils. Medium polar solvents are rarely used because they are poorly tolerated by the living tissue of the skin. Cosmetic products therefore often combine water and oils in the form of emulsions. Emulsions are droplet dispersions consisting of two immiscible liquid phases, generally an aqueous phase and an oil phase. Droplets are usually stabilized by the adsorption on their surface of surfactant molecules whose amphiphilic molecular structure includes a hydrophilic polar head and a hydrophobic hydrocarbon tail. The formulation of emulsions is very versatile by the choice and concentrations of the ingredients which enter into their composition as well as by their manufacturing processes. Indeed, their oil-in-water (O/W) or water-in-oil (W/O) direction can be chosen, as can their physicochemical parameters (pH, viscosity, etc.) and their particle size (droplet size) on several scales ranging from nano- to micrometer. Emulsions are dosage forms of choice in cosmetology because they allow us to respond from a technical and sensory point of view to the benefits of use requested by consumers for the care or beautification of their skin. They are thus the basis of many cosmetic products: milks, treatment creams, sunscreens, mascaras, foundations, shaving creams, etc.
The range of possibilities is expanded by the recent introduction of Pickering emulsions in cosmetic formulation. The latter are stabilized by solid particles adsorbed on the surface of the droplets instead of conventional surfactants. The partial wetting of solid particles by water and oil is the physicochemical property which allows their adsorption at the water/oil interface and the stabilization of emulsions. These are very resistant to coalescence*, which makes them very stable.
In this article, we address the physicochemistry of Pickering emulsions, the nature of particles of interest for cosmetic formulations, and we describe the advantages of Pickering emulsions compared to conventional emulsions for skin applications of cosmetic products. The skin is a particular organ which acts as a barrier to the entry of exogenous molecules into the body. The cutaneous administration of active substances therefore requires an adapted design of the formulation of emulsions whose ingredients contribute to the cutaneous absorption of the active substances.
* a glossary is presented at the end of the article.
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