ADSORPTION OF TRIETHYLAMINE AND WATER-VAPOR AND THE MODIFICATION OF SILICA SURFACE BY GASEOUS TRIMETHYLCHLOROSILANE

Vapor-phase modification of silica surfaces with different surface concentrations of hydroxy groups, αOH was carried out with trimethylchlorosilane (TMCS) at 400°C and heats of adsorption of TMCS, triethylamine (Et3N), and water vapor on the silicas were investigated. The αOH values of an initial (unmodified) silica sample treated in vacuo at 500° and 1000°C were about 3.2 and 0.8 μmole/m2, respectively. In the latter case, by replacing all the remaining surface OH groups with trimethylsiloxy (-OSiMe3) groups it was possible to obtain a completely hydroxy-free silica surface with αOSiMe ≈ 0.8 μmole/m2. The silica with αOH ≈ 3.2 μmole/m2, thrice-treated with TMCS at 400°C, had αOSiMe ≈ 2.7 μmole/m2 and contained a minor amount of remaining OH groups (αOH ≈ 0.5 μmole/m2). Rehydroxylation of both these OSiMe3-modified silica samples gave the same results: αOH between 0.6 and 0.8 μmole/m2. The sample with αOSiMe ≈ 2.7 μmole/m2 exhibited the lowest adsorption capacity in respect of Et3N and water. A procedure is proposed for determining αOH from heats of Et3N adsorption dependance on surface coverage and from isotherms of Et3N adsorption. This procedure enabled the degrees of hydroxylation and modification with OSiMe3 groups to be quantitatively characterized for the silica samples in question. The IR spectra were in good agreement with these results. © 1979.