Anisotropy of Water Dynamics in Clays: Insights from Molecular Simulations for Experimental QENS Analysis

We measure H2O dynamics in a well-defined synthetic hectorite clay by the neutron spin echo technique, in the temperature range from 240 to 347 K. The interlayer spaces of this anisotropic material contain two layers of confined water, corresponding to the so-called bihydrated state. We analyze the experimental data in light of parallel molecular dynamics simulations. Simulations demonstrate that H2O diffusion in the direction perpendicular to the clay layers is not negligible and has to be taken into account in the experimental data analysis. A diffusive model with only two fitting parameters D-perpendicular to and D-parallel to is well adapted for such analysis. A clear physical meaning for the two fitting parameters exists, in view of the geometry of the system. Experimentally, the diffusion coefficients parallel to the clay layers D-parallel to were estimated to be slowed down by a factor of 5 compared to bulk water. Further, the activation energy of the diffusion process is higher than in bulk water especially toward the lower temperatures within the range studied (20.3 kJ/mol above 300 K increasing to 28.4 kJ/mol below 300 K). Simulations suggest that this is connected to the presence of the cations (1 cation per every 8 water molecules) rather than to the formation of hydrogen bonds between H2O molecules and the clay layers. However, improvements of microscopic force fields are necessary to achieve a full quantitative interpretation of the experimental water diffusion coefficients. We suggest the importance of polarizability in such endeavors.