Adsorption of triethylamine and water vapor and the modification of silica surface by gaseous trimethylchlorosilane

Abstract

Vapor-phase modification of silica surfaces with different surface concentrations of hydroxy groups, α OH was carried out with trimethylchlorosilane (TMCS) at 400°C and heats of adsorption of TMCS, triethylamine (Et 3N), and water vapor on the silicas were investigated. The α OH values of an initial (unmodified) silica sample treated in vacuo at 500° and 1000°C were about 3.2 and 0.8 μmole/m 2, respectively. In the latter case, by replacing all the remaining surface OH groups with trimethylsiloxy (−OSiMe 3) groups it was possible to obtain a completely hydroxy-free silica surface with α OSiMe 3 ≈ 0.8 μmole/m 2. The silica with α OH ≈ 3.2 μmole/m 2, thrice-treated with TMCS at 400°C, had α OSiMe 3 ≈ 2.7 μmole/m 2 and contained a minor amount of remaining OH groups ( α OH ≈ 0.5 μmole/m 2). Rehydroxylation of both these OSiMe 3-modified silica samples gave the same results: α OH between 0.6 and 0.8 μmole/m 2. The sample with α OSiMe 3 ≈ 2.7 μmole/m 2 exhibited the lowest adsorption capacity in respect of Et 3N and water. A procedure is proposed for determining α OH from heats of Et 3N adsorption dependance on surface coverage and from isotherms of Et 3N adsorption. This procedure enabled the degrees of hydroxylation and modification with OSiMe 3 groups to be quantitatively characterized for the silica samples in question. The IR spectra were in good agreement with these results.