Adsorption of Catechols, 2-Aminophenols, and 1,2-Phenylenediamines at the Metal (Hydr)Oxide/Water Interface: Effect of Ring Substituents on the Adsorption onto TiO₂

Abstract

The extent of adsorption and the value of the adsorption equilibrium constant (Ksintr) for simple organic ligands is influenced by the identity of the ligand donor groups and other substituents on the aromatic ring. Catechols adsorb onto TiO2 to a significantly greater extent than 2-aminophenols; the adsorption of 1,2-phenylenediamines is negligible. The TiO2 surface has a high ionic contribution to bonding; ligands possessing donor groups with the highest ionic contribution to bonding adsorb to the greatest extent. A covalent contribution to bonding can increase binding to Ti(IV)-containing surfaces, but only when the ionic contribution is already strong. Within each ligand class, substituents alter the competition between protons and surface sites for binding the deprotonated ligand. For this reason, pKa1, pKa2, and log Ksintr are all important in determining the extent of adsorption. Additionally, substituents that impart hydrophobicity also raise the extent of adsorption and the value of log Ksintr.