Scattering particles increase absorbance of dyes--a model study with relevance for sunscreens

Sunscreens used for the protection of human skin against the harmful effects of solar radiation contain UV absorbers as key ingredients, which are either dissolved in one of the phases of the preparation or, when insoluble, suspended as particles. Although the UV protective effect of particulate UV filters, inorganic and organic, is mainly due to absorption, they scatter UV and visible light. The scattering can have an additional attenuating effect on the incoming radiation by increasing the pathlength of the photons, especially when soluble filters are also present. This is investigated with model systems of dyes and absorbing and non-absorbing particles. The presence of particles causes an increase of the dye absorbance without changing dye concentration or cuvette thickness. It is possible to relate this amplification of dye absorbance to the turbidity of the system. Plots are constructed which allow for a given particle type the representation of all data on one single curve, though measured at different turbidity and cuvette thickness. With that, extrapolations to practical applications of sunscreens are possible.