Interaction of TiO 2 nano-particles with organic UV absorbers

Abstract

The yellowing caused by the addition of butyl methoxy dibenzoylmethane (BMDBM), or benzophenone, to dispersions of titania has been measured quantitatively for a range of uncoated and surface-treated nano-particulate TiO 2 materials. FTIR spectra of BMDBM adsorbed on the uncoated titania were interpreted with the aid of corresponding spectra of adsorbed acetyl acetonate and supporting DFT calculations. The IR spectra suggest that chemisorption of BMDBM by bidentate chelation of the diketone anion to surface titaniums is responsible for the observed yellowing. Silica coatings, deposited on the TiO 2 surface led to a large reduction in yellowing with both BMDBM and benzophenone-3. This was consistent with the coating uniformity and coherence inferred from reduction in the rates of propan-2-ol photo-oxidation. It is proposed that silica coating reduces interactions of butyl methoxy dibenzoylmethane (BMDBM) and benzophenone-3 with the highly polarizing Ti surface cations. A beneficial practical consequence is that aesthetically undesirable discolouration of cosmetic formulations containing combinations of these active ingredients is reduced. Propan-2-ol oxidation suggested that, on the same TiO 2, alumina coatings were less uniform. Patchier coverage led to more yellowing of the titania because it allowed greater access of the organic to the underlying TiO 2. In addition, at low levels of alumina the coating induced more yellowing than on the uncoated metal oxide surface. This was attributed to coordination to coordinatively unsaturated aluminiums. The presence of an organic coating, such as surface bound stearate, further reduced yellowing interactions with organic UV absorbers probably because the stearate competes effectively for (i.e. blocks) surface sites.