Aging of TiO2 nanocomposites used in sunscreen. Dispersion and fate of the degradation products in aqueous environment

Aging in water of a TiO2-based nanocomposite used in sunscreen cosmetics has been studied as a function of light and time. It consisted initially in a TiO2 core, coated with Al(OH)3 and polydimethylsiloxane (PDMS) layers. Size measurement, coating alteration, and surface charge were followed by laser diffraction, TEM/EDS, ICP-AES and electrophoretic mobility measurement. The nanocomposite rapidly underwent progressive dispersion in the aqueous phase, enabled by the dissolution of the PDMS layer. A stable suspension of colloidal byproducts from 50 to 700nm in size was formed. Their positively charged Al(OH)3 surface was evidenced with an isoelectric point around 7-8, controlling the dispersion stability. The critical coagulation concentrations measured with NaCl and CaCl2 was 2 OE 10 super(-2) and 8 OE 10 super(-3) M respectively. The presence of natural organic matter affected the colloidal stability according to the NOM/byproduct ratio. A 2 wt% ratio favored bridging flocculation, whereas a 20 wt% ratio induced sterical stabilization. Aging in aqueous conditions of TiO2 nanocomposite used in sunscreens induces rapid generation of nanometric byproducts remaining stable in suspension.