Polymerization of silicate and aluminate tetrahedra in glasses, melts and aqueous solutions—II. The network modifying effects of Mg 2+ , K +, Na +, Li +, H +, OH −, F −, Cl −, H 2O, CO 2 and H 3O + on silicate polymers

Abstract

The effect of the group IA and VIIA ions, as well as Mg 2+, and the molecules H 2O, CO 2, H 3O + and OH − on the energy of the Si-O bond in a H 6Si 2O 7 cluster has been calculated using semiempirical molecular orbital calculations (CNDO/2). Three types of elementary processes, i.e. substitution, addition, and polymerization reactions have been used to interpret data on the dynamic viscosity, surface tension and surface charge, hydrolytic weakening, diffusivity, conductivity, freezing point depression, and degree of polymerization of silicates in melts, glasses, and aqueous solutions. As a test of our calculational procedure, observed X-ray emission spectra of binary alkali silicate glasses were compared with calculated electronic spectra. The well known bondlength variations between the bridging bond [Si-O(br)] and the non-bridging bond [Si-O(nbr)] in alkali silicates are shown to be due to the propagation of oscillating bond-energy patterns through the silica framework. A kinetic interpretation of some results of our calculations is given in terms of the Bell-Evans-Polanyi reaction principle.