Competition between ligands for Al2O3 in aqueous solution

The adsorption of two classes of carboxylic ligands (i.e., aliphatic and aromatic small molecules), onto α-alumina nanoparticles was investigated. A new methodology was used whereby two molecules were simultaneously equilibrated with the inorganic material. A two-dimensional representation of the adsorption of the two complexing molecules enables us to differentiate between pairs of ligands with (i) independent adsorption on different sites of the alumina particles, (ii) competing adsorption on the same sites, or (iii) a mix thereof. Both the highest affinity ligands (tetracarboxylic acid, citric acid, and tiron), and the way they compete with lower affinity ligands have been identified. The combination of carbon skeleton and complexing groups required to produce the ligand of highest affinity at pH 5 has been recognized. In particular, the role of the OH in the α position of a carboxylic group and the role of the distance between two carboxylic groups are emphasized.