Influence of Tetrahedral Layer Charge on the Organization of Interlayer Water and Ions in Synthetic Na-Saturated Smectites

International audienceSmectite hydration strongly influences dynamical properties of interlayercations and thus the fate of H2O and pollutants in surficial systems where smectite-basedmaterials are often used as a major barrier component. Smectite crystal chemistry is knownto rule its hydration, although the influence of specific parameters such as the amount andlocation of layer charge deficit remains poorly understood. A set of tetrahedrally chargedtrioctahedral 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 assessthe influence of layer charge on smectite hydration and interlayer structure. Both hydroxylatedand fluorinated samples were synthesized because of the increasing use of the lattervarieties in recent spectroscopic studies aiming at the determination of interlayer H2Odynamical properties. The distribution of charge-compensating cations and of associatedH2O molecules was determined both experimentally from the modeling of X-ray diffractionpatterns and numerically from Monte Carlo molecular simulations performed in the grandcanonical ensemble. The consistency of both approaches for hydroxylated samples allowed gaining insights into the specificinfluence 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 theenhanced cation-layer electrostatic attraction. As a result, positional disorder of interlayer H2O molecules is reduced because ofstronger 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 reductionof the interlayer H2O content and to the formation of Na+ inner sphere complexes at the clay layer surface. When normalized tothe number of interlayer cations, the number of interlayer H2O molecules decreases with increasing layer charge, and theproportion of these H2O molecules hydrating interlayer cations increases, thus increasing the stability of most hydrated statestoward lower relative humidity conditions. Smectite hydration evolution appears as a steady process with no tendency tointerlayer cation ordering at the smectite-to-vermiculite limit of ∼1.3 charge per O20(OH,F)4