Chemically modified silicon dioxide surfaces Reaction of n-alkyldimethylsilanols and n-oxaalkyldimethylsilanols with the hydrated surface of silicon dioxide — the question of the limiting surface concentration

Abstract

Firstly, a model is proposed for the surface of highly dispersed silicon dioxides (Aerosil, Cabosil). In the first part, the investigation of the uncatalysed reaction of the surface hydroxyls on Cabosil with alkyl and oxa-alkyldimethylsilanols is described. The reaction mechanism is discussed on the basis of kinetic measurements. The reaction attains a limiting surface concentration of 3.1 μmol m −2 of alkyldimethylsiloxy groups, a value which is independent of the chain length of the non-branched alkyl substituent, as well as of the reaction temperature between 200 and 340°C. By repeating the reaction, the surface concentration can be further increased. However, the limiting value attained after about three repeated treatments at 300°C depends on the chain length of the alkyl substituent, 4.0 μmol m −2 for trimethylsiloxy groups and 3.5 for the other alkyldimethylsiloxy groups with longer alkyl or oxa-alkyl substituents. An interpretation of the results is proposed in the light of the suggested model. In the second part, the investigation of the catalysed reaction is described. The reaction is catalysed by bases, the best catalyst having protons attached to the basic center. By repeating the reaction, a layer could be chemically bonded onto the surface, the density of which is determined only by the size of the substituent. The maximum attainable concentration depends also on the temperature. Such a dense layer could not be prepared with oxa-alkyldimethylsiloxy substituents.