Influence of tetrahedral layer charge on the organization of interlayer water and ions in synthetic Na-saturated smectites

Smectite hydration strongly influences dynamical properties of interlayer cations and thus the fate of H₂O and pollutants in surficial systems where smectite-based materials are often used as a major barrier component. Smectite crystal chemistry is known to rule its hydration, although the influence of specific parameters such as the amount and location of layer charge deficit remains poorly understood. A set of tetrahedrally charged trioctahedral smectites, with a common structural formula ^inter[Na_x]^oct[Mg₆]^tet[Si_8.0-xAl_x]O₂₀(OH,F)₄ and a layer charge (x) varying from 0.8 to 3.0, were thus synthesized to assess the influence of layer charge on smectite hydration and interlayer structure. Both hydroxylated and fluorinated samples were synthesized because of the increasing use of the latter varieties in recent spectroscopic studies aiming at the determination of interlayer H₂O dynamical properties. The distribution of charge-compensating cations and of associated H₂O molecules was determined both experimentally from the modeling of X-ray diffraction patterns and numerically from Monte Carlo molecular simulations performed in the grand canonical ensemble. The consistency of both approaches for hydroxylated samples allowed gaining insights into the specific influence of smectite crystal chemistry. For a given hydration state, H₂O content is about constant in hydroxylated saponites, independent of layer charge, whereas smectite layer-to-layer distance decreases with increasing layer charge because of the enhanced cation-layer electrostatic attraction. As a result, positional disorder of interlayer H₂O molecules is reduced because of stronger steric constraints and of the increased density of electronegative sites at the surface of the clay layer. Fluorine-for-hydroxyl isomorphic substitutions likely increase further electronegativity of the clay layer surface leading to further reduction of the interlayer H₂O content and to the formation of Na⁺ inner sphere complexes at the clay layer surface. When normalized to the number of interlayer cations, the number of interlayer H₂O molecules decreases with increasing layer charge, and the proportion of these H₂O molecules hydrating interlayer cations increases, thus increasing the stability of most hydrated states toward lower relative humidity conditions. Smectite hydration evolution appears as a steady process with no tendency to interlayer cation ordering at the smectite-to-vermiculite limit of ∼1.3 charge per O₂₀(OH,F)₄.