Influence of layer charge on hydration properties of synthetic octahedrally-charged Na-saturated trioctahedral swelling phyllosilicates

Abstract

Smectite hydration impacts dynamical properties of interlayer cations and thus the transfer and fate of H2O, contaminants, and nutriments in surficial environments where this ubiquitous clay mineral is often one of the main mineral components. The influence of key crystal-chemical parameters, such as the amount of charge or the presence of fluorine, rather than hydroxyl groups, in smectite anionic framework, on hydration, organization of interlayer species, and related properties has been described for tetrahedrally substituted trioctahedral smectites (saponites). Despite the ubiquitous character of octahedrally substituted smectites, that make most of the world bentonite deposits, the influence of charge location on smectite hydration properties has not received similar attention. A set of octahedrally substituted trioctahedral smectites (hectorites) with a common structural formula NaxMg6-xLixSi8.0O20(OH)4 and a layer charge (x) varying from 0.8 to 1.6 was thus synthesized hydrothermally. The distribution of charge-compensating Na+ cations and of associated H2O molecules was determined experimentally from the modeling of X-ray diffraction data obtained along water vapor desorption isotherms. Consistent distributions of charge-compensating cations and of associated H2O molecules were also computed from GCMC simulations as a function of layer charge. Interlayer H2O contents [2.5–5.5 and 8.0–10.0 H2O molecules per O20(OH)4 for 1W and 2W hydrates, respectively] are similar in all Na-saturated smectite samples, independent of the location and amount of their layer charge. In contrast to synthetic saponite, for which stability of most hydrated layers was increased by increasing layer charge, the stability of synthetic hectorite hydrates is only marginally affected by layer charge. Consistently, the layer-to-layer distance of Na-saturated hectorite 2W (and 1W) layers is independent of layer charge (15.10–15.65 Å and 12.0–12.6 Å, respectively). The contrasting hydration behavior of synthetic Na-saturated saponite and hectorite is likely due to different electrostatic attraction between the 2:1 layer and interlayer cation, the charge undersaturation of O atoms at the surface of hectorite 2:1 layer being more diffuse compared to saponite. Combined with previous results on saponites, the present data and sample set provides key constraints to assess the validity of force fields simulating clay-water interactions for an unmatched variety of smectite with contrasting locations and amounts of layer charge deficits.