Reactivity of a silica network of glass Molecular mechanism of the dissolution of a silica network in aqueous HF-HCI solutions

Abstract

The molecular model of the dissolution mechanism of a silica network of glass in aqueous HF-HCl solutions is proposed. This model is based on two main assumptions: Firstly, the silicon atoms of the silica network are not saturated coordinatively and link with oxygen and fluorine atoms of the surface having a double-bond character. The dπ-pπ bonds are responsible for the change of polarity of the bonds Siδ+ - Oδ- and Siδ+ - Fδ- to Siδ- - Oδ+ and Siδ- - Fδ+. Secondly, there are two types of adsorption sites an a silica surface: the sites which act as acceptors of protons and the sites which act as donors of protons. The kinetic equations for the dissolution rate of a silica network of glass in aqueous HF-HCl solutions derived from the model are described. They assume a first-order dependence on HF2 - ions concentration in the solution. The concentration dependence on H+ and HF expressed by Langmuir's isotherms are described. The experimental results confirm the proposed model. It is shown that the increase in the dissolution rate of a silica network of glass in aqueous HF-HCl solutions is due to the increase in electronic density on the glass surface. HF molecules and HF2 - ions adsorbed on this surface were responsible for the increase in electronic density. The interactions of a silica network of glass with the HF-HCl solutions are highly autocatalytic as assumed in the model. Protons play a catalytic role both in the reaction of HF molecules and HF2 - ions with the surface while the increase of the kinetic reaction constant for the reaction of protons with the surface is due to the HF molecules. The double role of the protons in the solution process is explained.