Substrate size effects in the modeling of molecular grafting: Case of organo-silane chains on silica

Ab initio density functional theory (DFT) calculations have been carried out on the grafting of two different chains organo-silane compounds, (A)3SiCH2CH2OCOCH3 (A = OH, Cl) on SiO2 hydroxylated solid surfaces. The silica surface is modeled with various cluster models by considering sizes ranging from Si2O3H4 to Si12O14H24. The chemical reactions due to the interaction between the organosilane molecules and the surface give rise to two final products corresponding to the dissociative adsorption of these organosilane molecules on the surface. The chemical nature of these complexes exhibits similar characteristics. They are stabilized by two different interactions: a covalent bond SiOSi on one side of the chain, i.e. the dissociative grafting of the organosilane chains on the silica surface, and an hydrogen bond HO…O on the other side of the chain. The size effect of the cluster models on the electronic and geometrical properties of the grafting complexes is investigated. This paper illustrates the richness of information that can be obtained from localized phenomena taking place at the organic/inorganic interface via molecular models studied with DFT/B3LYP/6-31+G** techniques.