Surface complexes of acetate on edge surfaces of 2:1 type phyllosilicate: Insights from density functional theory calculation

To explore the complexation mechanisms of carboxylate on phyllosilicate edge surfaces, we simulate acetate complexes on the (0 1 0) type edge of pyrophyllite by using density functional theory method. We take into account the intrinsic long-range order and all the possible complex sets under common environments. This study discloses that H-bonding interactions occur widely and play important roles in both inner-sphere and outer-sphere fashions. In inner-sphere complexes, one acetate C-O bond elongates to form a covalent bond with surface Al atom; the other C-O either forms a covalent bond with Al or interacts with surface hydroxyls via H-bonds. In outer-sphere complexes, the acetate can capture a proton from the surface groups to form an acid molecule. For the groups of both substrate and ligand, the variations in geometrical parameters caused by H-bonding interactions depend on the role it plays (i.e., proton donor or acceptor). By comparing the edge structures before and after interaction, we found that the carboxylate binding can modify the surface structures. In the inner-sphere complexes, the exposed Al atom can be stabilized by a single acetate ion through either monodentate or bidentate schemes, whereas the Al atoms complexing both an acetate and a hydroxyl may significantly deviate outwards from the bulk equilibrium positions. In the outer-sphere complexes, some H-bondings are strong enough to polarize the metal-oxygen bonds and therefore distort the local coordination structure of metal in the substrate, which may make the metal susceptible to release.