Diffusion of water in a synthetic clay with tetrahedral charges by combined neutron time-of-flight measurements and molecular dynamics simulations

The dynamics of water molecules confined in the interlayer space of a synthetic Na saponite clay with a tetrahedral layer charge of 0.7 per half unit cell were studied by combining time-of-flight quasi-elastic neutron experiments (QENS) and molecular dynamics simulation along the water adsorption isotherms. In the monolayer regime, three different adsorbed amounts corresponding to increasing fillings of the interlayer space were investigated whereas two situations (adsorption and desorption) were investigated in the bilayer region. In a first step, molecular dynamics were used to reexamine some of the approximations classically used in the analysis of water motion by QENS (i.e., separation between rotational and translational diffusion, preferred orientation of molecules, and contribution to the elastic incoherent structure factor). A careful analysis of experimental data obtained at two wavelengths and for two orientations of the clay platelets and validated by simulations yielded rotational and translational movements of water molecules in the clay interlayer. Two rotational movements at two different time scales are revealed. The faster one is rather independent of the adsorbed water amount whereas the slower one assigned to the rotation of hydrated cations is significantly slowed down in the monolayer regime, especially upon completion of the first water layer. Translational movements are limited in the longitudinal direction and are significantly slower than in bulk water in the radial direction, even in the bilayer regime.