ИСТИНА

It is well known that the strength of silicates in contact with water solutions often appears to be significantly less than that in inactive environments, and the reason of this is the phenomenon of the adsorption induced reduction of strength (AIRS). It is established that the strength of silicates drops as a result of an auto-catalytic reaction between the siloxane bonds and hydroxyl groups adsorbed on the surface of silica [1,2]. However, it is not clear yet why hydroxyl groups induce the greatest effect of strength reduction compared to other components of water solutions, e.g. hydronium ions. In order to answer this question, we performed a quantum-chemical simulation of tensile deformation of the siloxane bond in the presence of various adsorbates. The simulations were carried out using the ORCA 4.0 code [3] within the wB97XD3/def2-TZVP level of theory. The rupture of the siloxane bond was modelled using the simplest model cluster H3Si-O-SiH3, and the effect of the AIRS was simulated by adding the following species to the model cluster: OH-, Cl-, H+, H3O+ and H2O. Table 1 presents some of the calculated characteristics of the siloxane bond rupture. The calculated activation energy of breaking the siloxane bond in the absence of adsorbates is in a reasonable agreement with the experimental value of 418 kJ/mol [1]. The calculated activation energies also indicate that the mechanism of nucleophilic substitution is more kinetically favourable that the electrophilic substitution, and the strength of the siloxane bond decreases with increasing nucleophilicity of the adsorbates. It is also found that the activation energy of rupture correlates with electron density, changing in the presence of the adsorbates, rather than with the heat of adsorption.