A DFT Study of Catechol Polymer Adhesion onto γ-Alumina (110) Surfaces

The adhesion mechanism of catechol, a key functional group in marine bioadhesives, to dehydroxylated γ-alumina (γ-Al₂O₃^D) (110) and hydroxylated (γ-Al₂O₃^H) (110) surfaces was investigated using periodic density functional theory (DFT) calculations. Adhesion interface models were constructed by integrating catechol, modeled after the experimentally synthesized poly(3,4-dihydroxystyrene), with γ-Al₂O₃^D (110) and γ-Al₂O₃^H (110) surfaces, referred to as the catechol/γ-Al₂O₃^D (110) and catechol/γ-Al₂O₃^H (110) interfaces. Stable catechol-surface complexes, adhesion energies, and interaction sites were identified through DFT-based geometric optimizations of selected models. DFT calculations revealed that hydroxy groups of catechol interact with both γ-Al₂O₃^D (110) and γ-Al₂O₃^H (110) surfaces, with charge density difference (CDD) and crystal orbital Hamilton population (COHP) analysis highlighting hydrogen bonding as the predominant and robust interaction at both interfaces. Adhesive forces were computed by differentiating energy curves obtained through the vertical displacement of the catechol from the adherend surfaces. The results revealed that the adhesive force on the catechol/γ-Al₂O₃^D (110) interface is more substantial than on the catechol/γ-Al₂O₃^H (110) interface. The adhesive force, decomposed into DFT and dispersion components, showed a pronounced contribution from the DFT component at both interfaces. Furthermore, the catechol structures, with truncated hydroxy hydrogen atoms achieved by stretching O-H bonds, adsorbed on the γ-Al₂O₃^D (110) and γ-Al₂O₃^H (110) surfaces were obtained as the models of chemisorption and physisorption, respectively. On the γ-Al₂O₃^D (110) surface, catechol is converted into semiquinone in correspondence with a chemical reaction involving chemisorption, proton transfer, and the associated electron transfer. As a result of these interaction modes, the adhesive force is significantly higher compared to the structure, where catechol's hydroxy groups remain intact, with an energy of -233.5 kJ/mol per surface model. In contrast, catechol adsorption on the γ-Al₂O₃^H (110) surface occurs mainly by physisorption via hydrogen bonding.