The nature of hydroxyl groups in aluminosilicate glasses: Quantifying Si–OH and Al–OH abundances along the SiO2–NaAlSiO4 join by 1H, 27Al–1H and 29Si–1H NMR spectroscopy

The combined results of 27Al–1H and 1H–29Si–1H cross polarization NMR experiments for hydrous glasses (containing 0.5–2 wt% water) along the SiO2–NaAlSiO4 join confirm that the dissolution mechanism of water in aluminosilicate glasses is fundamentally the same as for Al-free systems, i.e. the dissolved water ruptures oxygen bridges and creates Si–OH and Al–OH groups, in addition to forming molecular water (H2Omol). The fraction of Al–OH increases non-linearly as the Al content increases with up to half of the OH groups as Al–OH for compositions close to NaAlSiO4. The relative abundances of the different species are controlled by the degree of Al-avoidance and the relative tendency of hydrolysis of the different types of oxygen bridges, Si–O–Si, Si–O–Al and Al–O–Al. A set of homogeneous reactions is derived to model the measured Al–OH/Si–OH speciation, and the obtained equilibrium constants are in agreement with literature data on the degree of Al-avoidance. With these equilibrium constants, the abundance of the different oxygen species, i.e. Si–O–Si, Si–O–Al, Al–O–Al, Si–OH, Al–OH and H2Omol, can be predicted for the entire range of water and Al contents.