GEL SYNTHESIS OF AN ALBITE (NAALSI3O8) GLASS - AN NMR ANALYSIS

We have extended the range and complexity of glasses formed by the combination of metal-alkoxides by synthesizing a clear, homogeneous gel of albite (NaAlSi3O8) composition and heat-treating it to make a densified glass. Systematic changes in Si-29, Al-27, and Na-23 NMR spectra of gels with heat treatment are similar to differences in published spectra of hydrated albite glasses with varying water contents. The Si-29 spectra showed one peak; if Q3(OH) species are present in the samples, it is not obvious in the spectra. Analysis of the rate of spin transfer in CPMAS experiments suggests a Si-H distance approximately 15% shorter than that seen in Q3(OH) species in silica gel. Both the Al-27 and Na-23 peaks show an increase in quadrupolar broadening with heat treatment of the gel. Two peaks are present in the Na-23 spectra for gels heat-treated to 300 and 500-degrees-C, possibly due to sites with different sodium coordination. It is proposed that the differences in spectra can be explained by the differences in volatile content of the samples (including water as either H2O or OH-, and organic species such as ethanol). In the fresh gels with high contents of these polar species, the sodium is fully coordinated by polar molecules, with hydrogens bonding to the bridging oxygens. The volatiles are removed with heating, allowing the structure to relax to a point closer to that of an anhydrous albite glass made from a quenched melt.