Pickering emulsions in cosmetic formulation

Cosmetic products are formulations containing active substances intended to come into contact mainly with the skin, nails, or mucous membranes. Hydrophilic active ingredients can simply be dissolved in water, while hydrophobic active ingredients are generally dissolved in oils. Moderately polar solvents are rarely used because they are poorly tolerated by living tissue such as skin. Cosmetic products therefore often combine water and oils in the form of emulsions. Emulsions are droplet dispersions consisting of two immiscible liquid phases, usually an aqueous phase and an oily phase. The droplets are usually stabilized by the adsorption on their surface of surfactant molecules whose amphiphilic molecular structure comprises a polar hydrophilic head and a hydrophobic hydrocarbon tail. The formulation of emulsions is highly versatile in terms of the choice and concentrations of the ingredients used in their composition and their manufacturing processes. In fact, their oil-in-water (O/W) or water-in-oil (W/O) orientation can be chosen, as can their physicochemical parameters (pH, viscosity, etc.) and their particle size (droplet size) on several scales ranging from nano- to micrometers. Emulsions are the galenic forms of choice in cosmetology because they offer the technical and sensory benefits that consumers demand for skin care and beautification. They are therefore the basis for many cosmetic products, including lotions, skin care creams, sunscreens, mascaras, foundations, shaving creams, and more.

The range of possibilities has been expanded by the recent introduction of Pickering emulsions in cosmetic formulations. These are stabilized by solid particles adsorbed onto the surface of the droplets instead of conventional surfactants. The partial wetting of solid particles by water and oil is the physicochemical property that allows their adsorption at the water/oil interface and the stabilization of emulsions. These emulsions are highly resistant to coalescence*, which makes them very stable.

In this article, we discuss the physicochemistry of Pickering emulsions, the nature of particles of interest for cosmetic formulations, and describe the advantages of Pickering emulsions over conventional emulsions for skin applications in cosmetic products. The skin is a unique organ that acts as a barrier to the entry of exogenous molecules into the body. The cutaneous administration of active substances therefore requires the appropriate design of emulsion formulations whose ingredients contribute to the cutaneous absorption of the active substances.