REACTIONS OF CHLOROSILANES WITH SILICA SURFACES

Reaction curves have been obtained spectroscopically for the reactions of (CH3)3SiCl, (CH3)2SiCl2, (CH3)SiCl3, and SiCl4 with the free hydroxyl groups on silica. Kinetic analysis of these curves shows that the monofunctional chlorosilane follows 1.0-order kinetics with respect to the number of surface bonding sites; the multifunctional silanes all follow 1.5 ± 0.2-order kinetics with respect to the number of surface bonding sites. The 1.5 overall reaction order is attributed to the presence of about 50% single OH groups on the surface and 50% geminal OH groups. The four chlorosilanes have an experimental activation energy of 22 ± 2 kcal. A fast initial reaction, to the extent of 10-15%, takes place and is believed to be due in part to direct replacement of OH by Cl. The validity of the kinetic analysis and confirmation of the replacement of OH by Cl reaction are shown by chlorine analyses of the treated silica. In the temperature range studied (200-400°) the reaction rates are independent of the pressure of chlorosilane. From high resolution spectra it is postulated that a band at 3747 cm-1 is due to single OH groups, and that bands at 3751 and 3743 cm-1 are due to geminal OH groups. The kinetics of the chlorosilane reactions are compared and contrasted to the analogous methoxysilane reactions with silica.