Repartition of oil miscible and water soluble UV filters in an applied sunscreen film determined by confocal Raman microspectroscopy

Abstract

Photoprotection provided by topical sunscreens is expressed by the sun protection factor (SPF) which depends primarily on the UV filters contained in the product and the applied sunscreen amount. Recently, the vehicle was shown to significantly impact film thickness distribution of an applied sunscreen and sunscreen efficacy. In the present work, repartition of the UV filters within the sunscreen film upon application is investigated for its role to affect sun protection efficacy. The spatial repartition of an oil-miscible and a water-soluble UV filter within the sunscreen film was studied using confocal Raman microspectroscopy. Epidermis of pig ear skin was used as substrate for application of three different sunscreen formulations, an oil-in-water emulsion, a water-in-oil emulsion, and a clear lipo-alcoholic spray (CAS) and SPF in vitro was measured. Considerable differences in the repartition of the UV filters upon application and evaporation of volatile ingredients were found between the tested formulations. A nearly continuous phase of lipid-miscible UV filter was formed only for the WO formulation with dispersed aggregates of water-soluble UV filter. OW emulsion and CAS exhibited interspersed patches of the two UV filters, whereas the segregated UV filter domains of the latter formulation were by comparison of a much larger scale and spanned the entire thickness of the sunscreen film. CAS therefore differed markedly from the other two formulations with respect to filter repartition. This difference should be reflected in SPF when the absorption spectra of the employed UV filters are not the same. Confocal Raman microspectroscopy was shown to be a powerful technique for studying this mechanism of sun protection performance of sunscreens.