Silanol and water on silica studied by the CNDO MO method

Abstract

The CNDO/2 molecular-orbital method has been applied to silanol or water on silica. Silanols on silica bind strongly to the surface, though they are more acidic than alcohols. Hydrogen-bonding hydroxyls have a lower bond strength than the surface hydroxyls. On the specific adsorption of water or water dimers on silica, two types of electron flow are observed: H₂O (donor)→ SiOH (acceptor) and SiOH (donor)→ H₂O (acceptor); in the former the oxygen of the water interacts with the hydrogen of SiOH and in the latter the hydrogen of the water interacts with the oxygen of SiOH. The empirical shift of the hydroxyl absorption spectra of silanol and water on silica corresponds to the difference of calculated hydroxyl bond energies in the adsorption system and the isolated system. It is also theoretically explained that hydroxyls on silica tend to be centres for water adsorption and that it is difficult for the silica to release protons from its surface. On siloxane formation, the SiOSi bond angle may decrease in order to form a ring, which is the preferential shape for the Kiselev process.