H2O/OH ratio determination in hydrous aluminosilicate glasses by static proton NMR and the effect of chemical shift anisotropy

Abstract

Static 1H NMR spectra of hydrous NaAlSi3O8 glasses have been acquired at low temperature (140 K) in order to quantitatively determine OH and H2O concentrations. Since both components overlap in the spectra, an unambiguous determination of the line shapes is required. The structurally bonded hydroxyl groups are well described by a Gaussian line and the water molecules exhibit a Pake doublet-like line shape due to the strong proton–proton dipolar interaction. However, at proton resonance frequencies used in this study (360 MHz), the Pake doublet has an asymmetric line shape due to chemical shift anisotropy (CSA), which is significant and must be included in any simulation in order to reproduce the experimental line shape successfully. The simulations for rigid water molecules dissolved in our hydrous aluminosilicate glasses result in a CSA of 30±5 ppm and a dipolar interaction constant of 63.8±2.5 kHz (i.e., dipolar coupling constant (DCC) of 42.6±1.7 kHz), corresponding to a proton–proton distance of rij=154±2 pm. In contrast to earlier work, water speciation obtained from the simulations of our 1H NMR spectra are in excellent agreement with those obtained from infrared (IR) spectroscopy.