Quantum-chemical simulation of the adsorption-induced reduction of strength of siloxane bonds

Abstract

Mechanical strength of silicate glasses is known to decrease markedly due to the adsorption of molecules from the environment, especially in aqueous alkali solutions. This effect, known as the adsorption-induced reduction of strength (AIRS), has not yet been fully understood. Here, the dependence on the chemical nature and electronic properties of adsorbates of the AIRS of siloxane bonds in silica was studied by means of quantum-chemical calculations at the wB97X-D3/def2-TZVP level of theory. A siloxane bond was modelled by H₃Si-O-SiH₃ and (HO)₃Si-O-Si(OH)₃ clusters, and the AIRS was simulated by a linear tensile deformation of the siloxane bond in the presence of the following adsorbates: OH^-, Cl^-, H₂O, H⁺ and H₃O⁺. Potential energy profiles and derivative force curves of the siloxane bond rupture were obtained. The varying effect of the adsorbates on the energy-force characteristics of the AIRS can be explained by changes in the bond lengths and electron occupancy. It is shown that the AIRS of the siloxane bonds increases with an increase in the nucleophilicity of the adsorbates, and correlates with an adsorbate-induced redistribution of electron density.

Keywords: Adsorption induced reduction of strength; Bond rupture; First principles; Siloxane bond; Tensile strain.