Anisotropic Features of Water and Ion Dynamics in Synthetic Na- and Ca-Smectites with Tetrahedral Layer Charge. A Combined Quasi-elastic Neutron-Scattering and Molecular Dynamics Simulations Study

Abstract

The temperature- and orientation-dependent dynamics of water molecules and sodium or calcium cations confined in the interlayer space of synthetic saponite with contrasting layer charge were analyzed through the combination of three-axis neutron spectroscopy and molecular dynamics (MD) simulation. We first show that it is possible to generate MD simulated quasi-elastic spectra that are equivalent to the experimental ones. As a consequence, the analysis of spectra in terms of Lorentzian decomposition can be advantageously replaced by a direct exploitation of MD results. We show that such strategy provides classical information on the influence of clay crystal chemistry on water and ion dynamical features as well as reliable additional information on (i) dynamics associated to different types of water molecules (bonded or not to interlayer cations) and (ii) interlayer cation dynamics. The same strategy applied to data obtained at higher temperature provided further confirmation of the validity of the atomic potentials used in simulations while allowing the extraction of activation energies for water and cations translational motions.