• A. Kuligiewicz
  • A. Derkowski
  • M. Szczerba
  • V. Gionis
  • G.~D. Chryssikos
Revisiting the Infrared Spectrum of the Water-Smectite Interface]
Abstract—An overlap of bands produced by the O-H stretching vibrations of H2O (O-Hw) and structural OH (O-Hs) in smectite hampers the study by infrared spectroscopy (IR) of both their layer and interlayer structure. The present study re-evaluated the D2O saturation of smectite as a tool to enable separation of the overlapping bands at ambient conditions. Real-time monitoring by Attenuated Total Reflectance infrared spectroscopy (ATR-IR) was employed during in situ sample drying and H2O or D2O saturation at ambient temperature. Six dioctahedral and one trioctahedral pure smectites in Ca2+-, Na+-, and Cs+-cationic forms were studied to explore variability in total layer charge, charge location, and interlayer cation. The IR data showed the interlayer O-Dw signature at 2700-2200 cm-1 as a proxy for the O-Hw signature in the 3700-3000 cm-1 region. In addition to the expected liquid-like bands of D2O in the interlayer, these O-Dw spectra exhibited an additional sharp stretching feature in the 2695-2680 cm-1 range. No significant cation dependence of the sharp band position was observed between pairs of Ca- and Nasmectites for relative humidity (RH) between 60 and 80%, despite the large difference in the ionic potential between these interlayer cations. The intensity of the sharp band was found to be almost insensitive to changes in water content within 60-80% RH. The sharp band frequency decreased linearly with increasing total charge of the 2:1 layer (and can be used as a proxy for it), but no effect of charge location could be discerned. In agreement with early studies, this band was attributed to D2O located on the surface of the interlayer, pointing one O–D group toward the siloxane surface. Based on its high frequency, this band was indicative of free O-D oscillators, with very little or no involvement in hydrogen bonding (‘‘dangling OD’’). By analogy to the spectra of D2O-smectites, the spectrum of H2O-smectites also involves a sharp O-Hw analog at ~3630 cm-1 overlapping with typical OHs bands (e.g. Al2OH). As a result of this overlap, the sharp, 3630 cm-1 O-Hw contribution was often missed or attributed solely to O-Hs.
sample, spectroscopy
instrument-technique, sample, spectral data, earth sciences
Clays and Clay Minerals
15 - 29
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