Dehydroxylation and silanization processes on the silica surface are studied by ab initio molecular dynamics. The (100) and (111) surfaces of beta -cristobalite are used as two possible models of the hydroxylated amorphous surface. The activation energy and latent heat for the dehydroxylation reactions of the (100) surface computed by constrained ab initio molecular dynamics are in reasonable agreement with experimental data on the amorphous surface. Adhesion reactions of silanes are simulated aiming at elucidating the binding mechanism of organosilanes used for instance as silica-polymer coupling agents. The simulation have provided insights on the occurrence of multiple silica-silane bonds and on the role of hydrolization of silane by physisorbed water in the adhesion on the wet surface.