Revisiting water speciation in hydrous alumino-silicate glasses: A discrepancy between solid-state 1H NMR and NIR spectroscopy in the determination of X-OH and H2O.

The speciation of water dissolved into and reacted with hydrous alumino-silicate glasses (of NaAlSi 3 O 8 and "rhyolitic" compositions) quenched from high temperature is re-investigated where the predominant species are expected to be "X(Si and Al)-OH" and "H 2 O". Only, two analytical methods are capable of assessing such speciation: Near InfraRed (NIR) spectroscopy and solid-state 1H Nuclear Magnetic Resonance (NMR) spectroscopy. It is observed that the apparent water speciation, as a function of total water content, as determined by NIR spectroscopy is nearly the opposite from what the 1H NMR data reveal. Deuterium (2H) NMR and silicon (29Si) NMR report consistent trends in apparent speciation (depolymerization) with those indicated by the 1H NMR data. Compared with four previous NMR studies of hydrous NaAlSi 3 O 8 glasses it is shown that whereas NIR data always report the same apparent systematic variation in the intensity of the 4500 ("X-OH") and 5200 ("H 2 O") cm−1 bands with total water content, multiple 1H NMR studies of hydrous NaAlSi 3 O 8 report a wide range in OH/H 2 O. The discrepancy between the various NMR studies likely reflects differences in how the various glasses were made. Specifically, quench rate (fast or slow) and synthesis pressure (higher or lower), might impose a strong effect on observed water speciation in glasses via 1H NMR. It is concluded that the application of NIR spectroscopy, specifically the use of the intensities of the 4500 ("X-OH") and 5200 ("H 2 O") cm−1 NIR bands, does not provide an accurate assessment of water speciation in hydrous alumino-silicate glasses. NIR spectroscopy does remain a very valuable analytical tool for determination of total water content. [ABSTRACT FROM AUTHOR]