A photochemical investigation of two suncreen absorbers in a polar and a non-polar medium.

Abstract

Protection against the harmful effects of ultraviolet radiation is of increasing importance due to the depletion of stratospheric ozone, which shields the earth from harmful UVC rays (in the range 200-280 nm) and some UVB rays (in the range 280-290 nm). In addition, as the skin is repeatedly exposed to solar radiation, the possibility exists that the incidence of skin cancer is enhanced. This has led to the increased use of commercial sunscreens, which apart from their benefits, also have undesirable effects such as photodegradation and skin penetration. We therefore studied the photodegradation of two sunscreen absorbers that are used in most sunscreen formulations. The combination of the UVB filter, 2-ethylhexyl-p-methoxycinnamate (EHMC), and the UVA filter, avobenzone (AVO), are commonly used in sunscreen products. These two filters are known to exhibit differing photostabilities in different media. The aim of this project was to investigate their photochemical behaviour in a polar and a non-polar solvent and to identify the UV-induced breakdown products. Methanol was chosen as the polar medium and cyclohexane as the non-polar medium. The sunscreen filters were irradiated either singly or in combination in the two solvents with wavelengths greater than 300 nm. The irradiated samples were analysed by UV-spectrophotometry, high performance liquid chromatography (HPLC) and gas chromatography (GC). The effects of direct irradiation with UVB and UVA light, quenchers and photosensitisers were also examined. EHMC is supplied commercially as the trans-isomer and upon irradiation photoisomerises, in both methanol and cyclohexane, to its cis-isomer. AVO is photostable in methanol but photodegrades in cyclohexane. This behaviour is also evident when mixtures of the two filters are irradiated. The loss in absorbance of both EHMC and AVO was monitored by UV-spectrophotometric analysis. Since EHMC does not absorb UV light at the wavelength of maximum absorbance of AVO, a method to quantify the amount of EHMC and AVO present in the mixture was devised. In order to identify the degradation products, HPLC and GC techniques were implemented. The photoproducts formed in the polar methanolic medium were separated and quantified by HPLC analysis. Gas chromatography with flame ionisation detection (GC-FID) was used to separate the photoproducts formed in the non-polar cyclohexane medium. Gas chromatography with mass-spectral detection (GC-MS) was used to identify the photoproducts formed upon irradiation of AVO and to show that UVA irradiation of AVO photosensitises the isomerisation of EHMC. The rate of a photochemical reaction depends upon a number of factors including the number of photons absorbed by sunscreen absorbers. Chemical actinometry was used to determine the number of photons absorbed by EHMC, AVO and the mixture of the two in methanol and cyclohexane. The number of photons absorbed by AVO in cyclohexane was used to determine the quantum yield for the photodegradation of AVO. We also determined the rate constants for EHMC photoisomerisation and AVO photodegradation. Finally, we investigated the effect of sunlight on commercial sunscreens containing EHMC and AVO.