Simulation of the Molecular Mass Distributions of Si Anions on the Liquidus of the Na2O—SiO2 System with Regard for the Disproportionation of Q Structons

摘要

A new version of the STRUCTON (2009) computer model is proposed for the simulation of the molecular mass distributions (MMD) characterizing the diversity of anions in silicate melts depending on their polymerization and temperature. In contrast to earlier versions, the new version of the model accounts for disproportionation reactions of Q~n species and makes use of their proportions in the statistical simulations of the origin of real Si—O complexes. The new potentialities of the STRUCTON program package are illustrated by its application to studying the structural—chemical characteristics of melts in the Na2O—SiO2 system along its liquidus line, including the points of eutectics and phase transitions at 0.333 ≤ N_(SiO2) ≤ 0.500. This problem is solved with the use of a temperature-composition dependence of polymerization constants K_p~(Na) Na in the Toop—Samis approximation. The variations in K_p~(Na) were proved to be as large as three orders of magnitude due to both the temperature effect at a constant composition and the composition effect at a constant temperature. The results of the MMD simulations on the liquidus show that the concentration of the SiO4 ion strongly decreases, and the proportion of chain species increases compared to those at a stochastic distribution. The concentration of the Si2O_7~(6-) anion reaches its maximum (～42%) at 40 mol % in the liquid, i.e., the composition of Na6Si2O7. At N_(SiO2) > 0.40, this ion dominates over the SiO_4~(4-) . monomer. More silicic melts with N_(SiO2) ≥ 0.45, are dominated by (Si_nO_(3n))~(3n-) ring species, and the concentrations of these species are related as (Si3O9)~(6-) > (Si4O_(12))~(8-) > (Si5O_(15))~(10-). The maximum concentration of these flat rings also occurs near the composition of stoichiometric metasilicate with Si/O = 0.333. The comparison of the dependence of the average size of anions i_(av) and the average number of their species on depolymerization indicates that a change in the proportion of Q~n species in melt at decreasing temperature results in structural restyling and an increase in the average size of Si—O complexes. The average number of anion species thereby decreases compared to that in a stochastic MMD. The results presented in this publication direct the progress in the thermodynamic theory of silicate melts to a new avenue that makes use of the capabilities and advantages of the ion—polymer model, the theory of associated solutions, spectroscopic data, and the experimental study of variations in oxide activities depending on composition and temperature.