CHEMICAL-REACTIONS AT SILICA SURFACES

Chemical modification of the silica surface can provide a powerful method for probing the nature of the surface hydroxyl groups and how these might be modified via thermal activation. In the present paper, it is shown how vibrational spectroscopic methods (infrared and Raman) can be used to study silanol groups which have been modified after reaction with a variety of very reactive hydrogen sequestering agents (D2O, ZnMe2, BCl3, TiCl4, AlMe3, GaMe3, BEt3 and (SiMe3)2NH), having differing steric dimensions and it is demonstrated that a nearly complete vibrational spectrum for some of the corresponding new surface species can be obtained. With the use of fast scanning FTIR spectroscopy, spectra were able to be recorded in less than a second and the application of these time-resolved methods has been used to probe differences in the reactivity of isolated and H-bonded silanol groups. The spectroscopic evidence suggests that the ability of a reactant to react bifunctionally, i.e. to react with more than one SiOH group, is important in determining the initial reactivity of the H-bonded silanols relative to those that are isolated or non-H-bonded. Further, the number of inaccessible and H-bonded silanols that do not react increases as the apparent size of the reactant molecule increases, regardless of whether the reactant can react bifunctionally. Finally, for the larger probe molecules used ((SiMe3)2NH and BEt3), the new chemisorbed surface species block other H-bonded silanols and prevent them from reacting with either of these probes. © 1990.