Chemisorption of HSi(OH)3 on silica surfaces. An ab initio periodic study.
Résumé
The study of the grafting of trialkoxysilane R'Si(OR)3 mols. on amorphous SiO2 was undertaken at the Hartree-Fock level using a biperiodic model for the surface. Different types of slab cut out from the model system Edingtonite (a tetragonal SiO2 structure with five SiO2 groups per unit cell) were used to simulate isolated and interacting silanol sites at the amorphous silica surface, while only the simple case of HSi(OH)3 was considered for the interacting mol. In a first step, for each type of surface the geometrical parameters were optimized and the surface formation energy detd. The geometrical structure of the grafted mol. is compared with that of the isolated one. The geometrical strains of the surfaces with either isolated or interacting silanols are also compared before and after the grafting reactions. The calcd. values of the chemisorption energies show that the grafting process is favored on isolated silanols only if correlation effects are included.