Effect of Solvents on the Sun Protection Factor and Broad-Spectrum Protection of Three Organic UV Filters.

Solvents play an essential role in the performance of ultraviolet (UV) filters. The goal of this study was to understand how the sun protection factor (SPF) and broad-spectrum protection of three organic UV filters (homosalate, ethylhexyl salicylate, and butyl methoxydibenzoylmethane) and a combination of these are influenced by solvents. Twenty-four solvents were selected based on the ingredient active gap for testing. Mixtures of UV filters and solvents were formulated, and SPF, wavelength of maximum absorbance, broad-spectrum protection, and spreadability were evaluated. Results indicate that SPF of organic sunscreens can be significantly enhanced by solvents. Relying on solubility data only was not found to be a good approach in this study. The most efficient solvents shared multiple similar structural characteristics, including ester bonds, conjugated structure, aromatic rings, and -CN groups; however, the absence of some of these structural elements did not necessarily prevent a solvent from being a booster. The wavelength of maximum absorbance was significantly shifted in the UVA range by most solvents, whereas minimal or no shift was observed in the UVB range. Results of this study provide practical information that can guide sunscreen formulators in selecting solvents for UV filters and making more effective sunscreens.