REACTION OF METHYLSILANOLS WITH HYDRATED SILICA SURFACES - THE HYDROLYSIS OF TRICHLOROMETHYLSILANES, DICHLOROMETHYLSILANES, AND MONOCHLOROMETHYLSILANES AND THE EFFECTS OF CURING

It is well-established that the tendency of alkylsilanols to condense increases with the number of hydroxyl groups of the silicon atom. We have used infrared spectroscopy to provide direct evidence that shows that at the solid/gas interface methylchlorosilanes are fully hydrolyzed to methylsilanols by surface water on a hydrated silica and the percentage of the adsorbed methylsilanols that condense with the surface hydroxyl groups proceeds in the order (CH3)(3)SiOH > (CH3)(2)Si(OH)(2) > CH3Si(OH)(3). This is opposite to the expected trend and the discrepancy is explained in terms of a difference between the kinetics of self-condensation and reaction with the surface hydroxyl groups. Although the tendency of self-condensation increases with the number of hydroxyl groups attached to the silicon atom the tendency for reaction the surface hydroxyl groups follows the reverse order. The transformations in the adsorbed species upon curing have also been investigated. At room temperature an adsorbed methyltrisilanol does not condense with the surface hydroxyl groups and is partially cross-linked via intermolecular Si-O-Si bonds. Curing completes the crosslinking process and also results in a few attachments to the surface through a Si-O-Si bond. Both the dimethyldisilanol and the trimethylsilanol condense fully at room temperature to form surface Si-O-Si bonds. No change in these adsorbed species occurs with curing.