Infrared Spectroscopic Analyses on the Nature of Water in Montmorillonite

Abstract Interlayer cations and moisture content greatly influence the molecular vibrations of H 2 O in montmorillonite as shown through reflectance spectroscopy in the infrared. The absorptions due to H 2 O have been studied in montmorillonites exchanged with H, Na, Ca, Mg and Fe 3+ interlayer cations under variable moisture environments. Band assignments have been made for absorptions in the 3 µ m region due to structural OH vibrations, symmetric and asymmetric H 2 O stretching vibrations and the H 2 O bending overtone. Changes in the energies of the absorptions due to H 2 O stretching vibrations were observed as the samples were dehydrated by reducing the atmospheric pressure. Absorptions near 3620 cm −1 and 3550 cm −1 have been assigned to water bound directly to cations (inner sphere) and surface-bonded H 2 O and absorptions near 3450 cm −1 and 3350 cm −1 have been assigned to additional adsorbed water molecules. Band assignments have been made for combination bands in the near-infrared as well. Absorptions near 1.41 μ m and 1.91 μ m are assigned to bound H 2 O combination bands, while the shoulders near 1.46 μ m and 1.97 μ m are assigned to combinations of additional H 2 O molecules adsorbed in the interlayer regions and along grain surfaces.