Adsorption of the phosphate groups on silica hydroxyls: An ab initio study

Adsorption of the phosphate groups on a silica surface is investigated by ab initio calculations of hydrogen-bond complexes of dihydrogen and dimethyl phosphate anions with orthosilicic acid. It is found that the phosphate groups can form strong hydrogen-bonded complexes with the silanols of the silica surface with stabilization energies of ca. -14 kcal/mol per hydrogen bond. The effect of the coordination of the phosphate species by water, orthosilicic acid, and a sodium cation on the geometry and stability of the -sc/-sc conformation is investigated through consideration of 2-fold axial symmetry complexes. Calculations suggest the reduction of the COPO torsion angle of the -sc/-sc conformation of the phosphodiester backbones upon adsorption of molecules containing these moieties on silica compared to the value of this angle found for phosphates in vacuo and in hydrated forms. Both the polarizing and electron density abstracting abilities of silanols cause electron density redistribution, giving rise to strengthening of the phosphoester P-O bonding, weakening of the phosphinyl P=O and ester C-O bonding in adsorbed phosphate moieties. A very strong band at 3300 cm(-1) appearing during the phosphate adsorption on the silanol hydroxyls is suggested for use as an indication of this phenomenon.