Quantitative determination of water speciation in aluminosilicate glasses: a comparative NMR and IR spectroscopic study

Abstract

Water speciation in hydrous aluminosilicate glasses (with NaAlSi 3O 8, KAlSi 3O 8, LiAlSi 3O 8 and LiAlSi 4O 10 compositions) was studied with infrared (IR) and static 1H nuclear magnetic resonance (NMR) spectroscopy. Using IR spectroscopy, the water speciation was determined from the peak intensities (linear absorbance as well as integrated intensity) of the near-infrared (NIR) absorption bands at 4500 and 5200 cm −1 assigned to structurally bonded hydroxyl groups and molecular H 2O, respectively. For LiAlSi 4O 10 glasses, a new calibration of the linear and integral molar absorption coefficients of the IR absorption bands at 4500 and 5200 cm −1 is presented. Using NMR spectroscopy, the water speciation was determined from the static 1H NMR spectra acquired at temperatures between 170 and 130 K, where the 1H NMR signal consists of a well-defined Pake doublet due to rigid water molecules and an overlapping, but narrower, central peak due to structurally bonded OH groups. The distinct nature of these two signals (doublet and Gaussian) enables a reliable deconvolution of the 1H NMR spectra and quantitative determination of water speciation in the glasses. For a series of hydrous NaAlSi 3O 8 glasses containing 1.5–10 wt.% water, we found a very good agreement of water speciation determined by NMR and IR spectroscopy (about 4% standard deviation in OH concentration), demonstrating the reliability of both methods. Total water contents determined with 1H NMR spectroscopy are in excellent agreement with results from Karl–Fischer titration (KFT, <2% standard deviation). Depending on whether peak heights or peak areas are used for the evaluation of NIR spectra, relatively large deviations in water speciation (12–24% in OH content) are observed for KAlSi 3O 8, LiAlSi 3O 8 and LiAlSi 4O 10 glasses. Static 1H NMR spectroscopy on KAlSi 3O 8 and LiAlSi 4O 10 glasses containing 2.80–4.25 wt.% water tends to support the use of peak areas instead of peak heights for calculation of water speciation from NIR spectra. However, in the case of the LiAlSi 3O 8 glass (4.04 wt.% water), no clear conclusion can be drawn from the NMR data and other effects such as the choice of the baseline for the NIR peaks or water-dependent molar absorption coefficients may have to be taken into account. In order to resolve these questions, more systematic NMR and IR spectroscopic studies on glass series covering a large range of water concentrations are required.