DFT periodic study of the adsorption of glycine on the anhydrous and hydroxylated (0001) surfaces of α-alumina

The adsorption of glycine (G) on the (0001) surfaces of alpha-alumina with terminations modeling increasing water activity, O3Al (anhydrous) and (Al3O-H, O3Al-OH) (oxy-hydroxide-like) were investigated at low coverage. Different possible glycine conformers have been considered (neutral (NG), zwitterion (ZG), anion (AG)), as well as different binding modes and orientations toward the surface. In all cases, glycine interacts preferentially with the surface through the carboxylic group. The binding with the O3Al termination of the anhydrous oxide surface may be unidentate Al-O(C) or bridging Al-OCO-Al. The anion perpendicular unidentate form is the most stable one with an energy of adsorption of Delta E-ads = -214 kJ/mol. The nature of the Al-O(C) bond is found to be iono-covalent. On the hydroxylated surface with (Al3O-H, O(3)AI-OH) terminations, two types of adsorption may occur: forming only H bonds with the surface (Delta E-ads = -70 kJ/mol) or by the combination of Al-OC and H bonds (Delta E-ads = -159 kJ/mol). An additional scenario was considered, where glycine substitutes a surface OH group and forms an Al-O-C bond, whereas the OH group combines with a proton from the surface to form a water molecule that coadsorbs with gycine. This reaction is thermodynamically favored (Delta E = -213 kJ/mol). The ZG and AG forms are isoenergetic in a unidentate parallel mode of adsorption. It is shown that coadsorbed water has a stabilizing effect and has an influence on the nature of the most stable G conformer.