Quantitative links between microscopic properties and viscosity of liquids in the system SiO2–Na2O

The Adam–Gibbs theory of structural relaxation is presented in terms of the microscopic parameters controlling viscous flow; Δμ, the energy barrier hindering rearrangement of a single silicate monomer, and z* the number of monomers that must simultaneously overcome their barrier in order for a change in configuration to occur. Independent viscosity, calorimetric and spectroscopic data for glasses and melts in the system SiO2–Na2O are used to show that z* at the glass transition has a value of approximately 10 for all of the compositions considered and that microscopic energy barriers are on the order of several tens of kJ per mole of silicon. Furthermore, the variation of configurational entropy of glasses in the system SiO2–Na2O has been determined from the viscosity and calorimetric data. It is concluded that a stable melt complex with a stoichiometry close to Na2Si3O7 exists and that medium range order plays an important role in determining the configurational entropies, although the details are unclear. Overall, a self-consistent picture of the quantitative links between microscopic and macroscopic properties is determined for this simple system, providing an encouraging first step towards a fully generalisable predictive model for silicate melt viscosity.