Water Nanoconfined in Clays: The Structure of Na Vermiculite Revisited by Ab Initio Simulations

The results of Car-Parrinello molecular dynamics simulations of vermiculite clay at the hydration level found in the natural mineral (4 H2O per Na+) are reported. The structure and the vibrational spectrum of the aluminosilicate layer is well reproduced by the simulations. As for the structure of the Na+ cations and of water molecules adsorbed in the interlayer, the ones proposed in X-ray diffraction experimental papers are not in full agreement with the results of neutron diffraction experiments found in literature, yielding only the density profile perpendicular to the aluminosilicate layers, but including hydrogens. Our calculations result in a structure of the interlayer content, which is in agreement with the neutron-diffraction density profile, and yet compatible with X-ray diffraction data. Some features of the adsorbed water resulting from the simulation are an example of incomplete cation hydration and can be of general interest.