Depletion of the protective aluminum hydroxide coating in TiO2-based sunscreens by swimming pool water ingredients

In sunscreen lotion (SSL) formulations, titanium dioxide (nTiO2) nanoparticles are coated with an Al(OH)3 layer to shield against the harmful effects of hydroxyl radicals (OH), superoxide anion radicals (O2−), and other reactive oxygen species (ROS) (e.g. H2O2) generated when TiO2 nanoparticles are exposed to UV radiation. Therefore, it is crucial to ensure their structural stability in the environment where the protective layer may be compromised and adverse health and environmental effects can occur. The main focal point of our work was to research the stability of the Al(OH)3 layer in swimming pool water. Thus 2 g L−1 of SSL was subjected to treatment with swimming pool water (SPW) containing from 0.2 to 7 ppm chlorine (HOCl/OCl−) concentrations. The changes in the protective coating of TiO2 nanoparticles were analyzed using several X-ray based microscopic techniques in addition to Fourier transform infrared spectroscopy (FTIR) and Zeta potential measurements. Results indicated that an increase in chlorine concentration in SPW significantly affected the integrity of the Al(OH)3 protective layer and increased zeta potential from −64 mV to nearly −8 mV, rendering rather unstable TiO2 nanoparticles. The highest redistribution of Al (At%) from ∼4 to as high as 15.6 was achieved when SSL was subjected to 3.5 and 7 ppm of chlorine in SPW. Results strongly suggest that water chemistry influences the characteristics of TiO2 in sunscreen environment.

Swimming pool water (SPW) ingredients such as OCl− and CaSO4·2H2O significantly affected the integrity of a protective Al(OH)3 layer on TiO2 nanoparticles found in sunscreen lotion formulations.Figure optionsDownload as PowerPoint slideHighlights
► Al(OH)3 coating on TiO2 protects SSL nanoparticle from hydroxyl radical formation.
► Swimming pool water constituents affect the integrity of Al(OH)3 coating.
► Swimming pool water constituents induce the formation of free OH radicals.