Photochemical and evaporation behaviour of sunscreen formulations

Sunscreens ensure protection of the human skin against the harmful effects of the UV radiation in the UVB (290 – 320 nm) and UVA (320 – 400 nm) wavelength ranges. They commonly contain one or more organic UV absorbers that absorb light in the wavelength range of interest and/or inorganic particles, such as metal oxide semiconductors that absorb, scatter and reflect light. The efficiency of these formulations is measured by introducing the Sun Protection Factor (SPF). Currently, SPF is estimated from in vivo measurements, which is a time-consuming and expensive process. For a validated method, however, SPF can also be determined in vitro by measuring the diffuse optical transmittance as a function of wavelength. In this study, we show how in vitro SPF changes as a function of time upon i) evaporation of volatile components and ii) exposure under UV irradiation.We start by investigating the evaporation of solution films containing a selection of UV absorbers. We show that during solvent evaporation, an initial film, which is deposited on a smooth quartz plate, progressively dewets. The solvent loss causes an increase in the solute concentration until its solubility limit is reached. From that point onwards, solute precipitation is expected to occur. Hence, the combined effect of dewetting and solute precipitation during evaporation leads to a decrease in the absorbance and thus to the in vitro SPF values. We model the evolution of film spectra and we compare it with experiment. Results confirm that both are in reasonable agreement.The addition of either inorganic particulates in the nm size or polymer was also pursued. Our data confirm that their use aid film pinning at its base edge and suppressed dewetting. However, it did not tackle the problem of precipitation during solvent evaporation. Throughout our experimental procedure, the solvent of choice was the slow evaporating propane-1,2-diol (PG). We show that the same behaviour is observed when a fast- evaporating solvent, such as n-decane is used. Dewetting is also suppressed when alternative substrates such as Vitro Skin and keratin-lipid film are used.We have extended our investigation to the evaporation of particle-stabilised emulsion films, consisting of equal volumes of involatile squalane (SQ) and slow-evaporating PG and stabilised by partially hydrophobised silica particles. We show for these emulsions that the loss in the absorbance is due to the loss of light scattering. The loss of the latter results from the collapse of the emulsion structure during evaporation.We also examine how the in vitro SPF of solutions containing UV absorbers varies with standard solar irradiation. With the use of chemical actinometry, we have determined the overall quantum yields of the photochemical processes of photolabile UV absorbers. Using the obtained parameters, we have developed models to calculate the evolution of the film spectra and derived SPF values for both non-scattering films consisting of solutions of multiple UV absorbers and for highly scattering emulsion films. Finally, we are able to predict the SPF changes as a result of the photochemical processes upon “standard” sunlight exposure.