Interactions of limonene and carvone on titanium dioxide surfaces

Limonene, a monoterpene, found in cleaning products and air fresheners can interact with a variety of surfaces in indoor environments. An oxidation product of limonene, carvone, has been reported to cause contact allergens. In this study, we have investigated the interactions of limonene and carvone with TiO 2 , a component of paint and self-cleaning surfaces, at 297 ± 1 K with FTIR spectroscopy and force field-based molecular dynamics and ab initio simulations. The IR absorption spectra and computational methods show that limonene forms π-hydrogen bonds with the surface O-H groups on the TiO 2 surface and that carvone adsorbs on the TiO 2 surface through a variety of molecular interactions including through carbonyl oxygen atoms with Ti 4+ surface atoms, O-H hydrogen bonding (carbonyl O HO) and π-hydrogen bonds with surface O-H groups. Furthermore, we investigated the effects of relative humidity (RH) on the adsorption of limonene and carvone on the TiO 2 surface. The spectroscopic results show that the adsorbed limonene can be completely displaced by water at a relative humidity of ca. 50% RH (∼2 MLs of water) and that 25% of carvone is displaced at ca. 67% RH, which agrees with the calculated free energies of adsorption which show carvone more strongly adsorbs on the surface relative to limonene and thus would be harder to displace from the surface. Overall, this study shows how a monoterpene and its oxidation product interact with TiO 2 and the impact of relative humidity on these interactions. Limonene, a monoterpene, found in cleaning products and air fresheners, and carvone, one of its oxidation products, interact with titanium dioxide surfaces found in indoor environments.