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 Qⁿ units (n = number of bridging oxygens), this study presents a model that links the Qⁿ unit fractions to the melt configurational entropy at the glass transition temperature T_g, S^conf(T_g), and finally, to its viscosity η. With 13 adjustable parameters, the model reproduces η and T_g of melts in the Na₂O-K₂O-SiO₂ system (60 ≤ [SiO₂] ≤ 100 mol%) with 1σ standard deviations of 0.18 log unit and 10.6°, respectively. The model helps understanding the links between the melt chemical composition, structure, S^conf and η. 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 Q⁴ and Q³ units. Changing the melt silica concentration affects the Qⁿ 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 Qⁿ 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.