In situ spectroscopic study of water speciation in the depolymerized Na2Si2O5 melt

- Water speciation in Na2Si2O5 melts was studied in situ by NIR spectroscopy.
- No pressure dependence of water speciation equilibrium was found to 1.7 GPa at 900 °C.
- Water speciation at 900 °C is far less composition dependent than suggested before.

Water speciation in the sodium disilicate melts containing 1.3-8.1 wt.% of total water was studied by near-infrared spectroscopy at 900 °C and pressures from 0.2 to 1.7 GPa using externally heated diamond anvil cell. Hydrous sodium disilicate melt served both as a sample and as a pressure medium in the experiments, which allowed to keep the total water content of the melt constant and thereby to perform an internally consistent calibration of molar absorption coefficients for the near-infrared bands of water species. No pressure dependence of water speciation was observed in the studied pressure range, indicating that formation of structurally bound OH groups at the expense of H2O molecules in the melt is accompanied by a negligible volume change. At 900 °C, compositional dependence of water speciation was found to be much weaker than that expected from the previously reported data for low-temperature range near the glass transition. This can be explained by significant weakening of hydrogen bonding in the sodium silicate melts at high temperatures.