Are formic and acetic acid covered TiO2 surfaces hydrophobic?

Experimental studies under ultrahigh vacuum have reported the spontaneous formation of mixed formate/acetate monolayers on rutile titanium dioxide (TiO2) surfaces previously exposed to ambient air or water. Using deep learning accelerated molecular dynamics combined with enhanced sampling, we show that this is not the case in aqueous environments, where formic and acetic acid localize in the interfacial water layers, but do not displace adsorbed water on the TiO2 surface. This preferred localization is attributed to the strong interaction between these carboxylic acids and the surface oxygens, which is often accompanied by the transfer of the acid proton to the latter. While these localized acids make the anatase TiO2 surface hydrophobic, the higher fraction of adsorbed water dissociation can reverse their effect on rutile – a finding relevant to the UV light-induced hydrophilicity of TiO2. Importantly, the localized acids do not block the undercoordinated surface Ti sites and should not decrease the photocatalytic activity.