Structure vs. composition: A solid-state 1H and 29Si NMR study of quenched glasses along the Na 2O-SiO 2-H 2O join

Abstract

A suite of six hydrous (7 wt.% H 2O) sodium silicate glasses spanning sodium octasilicate to sodium disilicate in composition were analyzed using 29Si single pulse (SP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, 1H- 29Si cross polarization (CP) MAS NMR, and fast MAS 1H-NMR. From the 29Si SPMAS data it is observed that at low sodium compositions dissolved water significantly depolymerizes the silicate network. At higher sodium contents, however, dissolved H 2O does not affect a significant increase in depolymerization over that predicted based on the Na/Si ratio alone. The fast MAS 1H-NMR data reveal considerable complexity in proton environments in each of the glasses studied. The fast MAS 1H-NMR spectra of the highest sodium concentration glasses do not exhibit evidence of signficantly greater fractions of dissolved water as molecular H 2O than the lower sodium concentration glasses requiring that the decrease in polymerization at high sodium contents involves a change in sodium solution mechanism. Variable contact time 1H- 29Si cross polarization (CP) MAS NMR data reveal an increase in the rotating frame spin lattice relaxation rate constant ( T 1ρ*) for various Q n species with increasing sodium content that correlates with a reduction in the average 1H- 29Si coupling strength. At the highest sodium concentration, however, T 1ρ* drops significantly, consistent with a change in the Na 2O solution mechanism.