TiO2-promoted mineralization of organic sunscreens in water suspension and sodium dodecyl sulfate micelles

Abstract

The photostability of the widely used UVB sunscreen agents 2′-ethylhexyl-2-cyano-3-phenylcinnamate (1), 2-hydroxy-4-methoxybenzophenone (2), octyl salicilate (3), and 2′-ethylhexyl-4-methoxycinnamate (4) has been investigated under UVA irradiation in the absence and presence of TiO₂, an inorganic filter commonly employed in combination with organic filters in sunscreen preparations. In the absence of TiO₂, 1–3 are photostable and 4 undergoes the expected E–Z isomerization; the presence of TiO₂ caused mineralization of the organic filters and, surprisingly, the process is noticeably faster in the presence of surfactant than in sunscreen and water suspensions. The results indicate that in water suspension, mineralization is likely to occur on or near the TiO₂ particle surface; when the organic sunscreens are segregated in the micelle core, reactive radicals, produced during TiO₂-promoted degradation of the micellar system, may participate in sunscreen degradation. In addition, a pre-fluorescent probe for carbon-centered radical detection, 4-(3-hydroxy-2-methyl-4-quinolineoxy)-2,2,6,6,-tetramethylpiperidine-1-oxyl free radical or QT (5), was employed to demonstrate that carbon-centered radicals are evolved during micelle degradation and may participate in the mineralization of sunscreens.