Influence of Tetrahedral Layer Charge on the Organization of Interlayer Water and Ions in Synthetic Na-Saturated Smectites
Résumé
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[Nax]oct[Mg6]tet[Si8.0−xAlx]-
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. Fluorinefor-
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.