Formulation of Pickering emulsions for the development of surfactant-free sunscreen creams

Pickering emulsions are increasingly used in the pharmaceutical and cosmetic fields, especially for topical applications, since these systems require solid particles as emulsifiers instead of surfactants which are known to cause skin irritation. The solid inorganic nanoparticles (TiO₂ and ZnO) used as UV filters in sunscreen formulations may also stabilize emulsion droplets, so that the utility of surfactants may be questioned. Surfactant-free sunscreen emulsions solely stabilized by such nanoparticles (NPs) have been studied.

The ability of these NPs to stabilize o/w emulsions containing a 'model' oil phase, the C₁₂ -C₁₅ alkylbenzoate, has been assessed. ZnO and hydrophilic silica-coated TiO₂ NPs widely used in sunscreen products were used together with their mixtures. The emulsification efficiency, the control of droplet size and the stability of o/w Pickering emulsions solely stabilized by NPs were investigated. A ZnO/TiO₂ NPs mixture characterized by a theoretical SPF of 45 was finally used as unique emulsifiers to develop a surfactant-free sunscreen emulsion.

Stable Pickering emulsions containing 10 up to 60 wt% of C₁₂ -C₁₅ alkyl benzoate were formulated with 2 wt% ZnO in the aqueous phase. The droplet size was controlled by the solid NPs content with respect to oil and the emulsification process. Hydrophilic TiO₂ NPs did not allow the stabilization of emulsions. The substitution of TiO₂ for ZnO up to 60-70 wt% in a 20/80 o/w emulsion was successfully performed. Finally, a ZnO/TiO₂ NP mixture was tested as unique emulsifier system for the formulation of a sunscreen cream. Despite a lower viscosity, the obtained Pickering emulsion was stable and exhibited a photoprotective effect similar to the corresponding surfactant-based sunscreen cream with an in vitro SPF of about 45.

Surfactant-free Pickering emulsions can be stabilized by the UV-filter nanoparticles for the manufacture of sunscreen products.