An overlap of bands produced by the O−H stretching vibrations of H₂O (O–H_w) and structural OH (O−H_s) 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−D_w signature at 2700–2200 cm⁻¹ as a proxy for the O−H_w signature in the 3700–3000 cm⁻¹ region. In addition to the expected liquid-like bands of D₂O in the interlayer, these O−D_w 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 Na-smectites 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 the range 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−H_w analog at ~3630 cm⁻¹ overlapping with typical OH_s bands (e.g. Al₂OH). As a result of this overlap, the sharp 3630 cm⁻¹ O−H_w contribution was often missed or attributed solely to O−H_s.

Organo-clays represent a special challenge for molecular simulations because they require accurate representation of the clay and the organic/aqueous sections of the model system and accurate representation of the interactions between them. Due to the broad range of force-field models available, an important question to ask is which sets of parameters will best suit the molecular modeling of the organo-intercalated smectites? To answer this question, the structure of the ethylene glycol (EG)-smectite complex is used here as a testing model because the intercalation of EG in smectites provides a stable interlayer complex with relatively constant basal spacing.Three smectite samples with substantially different layer charge and charge localization were selected for X-ray diffraction (XRD) measurements. Their molecular models were built and molecular-dynamics simulations performed using various combinations of the organic force fields (CGenFF, GAFF, CVFF, and OPLS-aa) with ClayFF and INTERFACE force fields used to describe smectites. The simulations covered a range of different EG and water contents. For every structure, the density distribution of interlayer species along the direction perpendicular to the layer plane was calculated and then used to optimize the XRD patterns for these simulated models.A comparison of these results with experimental XRD patterns shows very large discrepancies in the structures and basal spacings obtained for different layer charges as well as for different force fields and their combinations. The most significant factor affecting the accuracy of the calculated XRD patterns was the selection of the clay-mineral force-field parameters. The second important conclusion is that a slight modification of the basal oxygen parameters for non-electrostatic interactions (increase of their effective atomic diameters) may be a simple and straightforward way to improve significantly the agreement between the modeled XRD patterns with experiments, especially for high-charge smectites. Generally, among organic force fields, the least accurate results were obtained with CGenFF. For unmodified ClayFF, its combination with GAFF gave the best results, while the two other sets (OPLS-aa and CVFF) gave the best results in combination with ClayFFmod. The INTERFACE and INTERFACEmod produced much better results for low-charge montmorillonite than for high-charge smectites.