Molecular structure, configurational entropy and viscosity of silicate melts: Link through the Adam and Gibbs theory of viscous flow

The Adam and Gibbs theory depicts the viscous flow of silicate melts as governed by the cooperative re-arrangement of molecular sub-systems. Considering that such subsystems involve the silicate Q(n) units (II = number of bridging oxygens), this study presents a model that links the Q(n) unit fractions to the melt configurational entropy at the glass transition temperature T-g, S-conf(T-g), and finally, to its viscosity eta. With 13 adjustable parameters, the model reproduces ri and T-g of melts in the Na2O-K2O-SiO2 system (60 <= [SiO2] <= 100 mol%) with 1 sigma standard deviations of 0.18 log unit and 10.6 degrees, respectively. The model helps understanding the links between the melt chemical composition, structure, S-conf and eta. For instance, small compositional changes in highly polymerized melts generate important changes in their S-conf (T-g) because of an excess of entropy generated by mixing Si between Q(4) and Q(3) units. Changing the melt silica concentration affects the Q(n) unit distribution, this resulting in non-linear changes in the topological contribution to S-conf (T-g). The model also indicates that, at [SiO2] >= 60 mol%, the mixed alkali effect has negligible impact on the silicate glass Q(n) unit distribution, as corroborated by Raman spectroscopy data on mixed Na-K tri- and tetrasilicate glasses. Such model may be critical to link the melt structure to its physical and thermodynamic properties, but its refinement requires further high-quality quantitative structural data on-silicate and aluminosilicate melts. (C) 2017 Elsevier B.V. All rights reserved.