Peculiar Structure of Water in Slightly Superhydrated Vermiculite Clay Studied by Car-Parrinello Molecular Dynamics Simulations

An unusual water structure was found using Car-Parrinello Molecular Dynamics simulations of a slightly superhydrated Na-vermiculite clay, containing six water molecules per sodium cation, corresponding to that of recent experiments performed to detect dynamical crossovers in the adsorbed supercooled water. In our simulations we observe that water forms two closed packed hexagonal plane layers parallel to aluminosilicate sheets and linked to their surface oxygens by hydrogen bonds. The presence of these hydrogen bonds and the coordination to Na cations lying midway between the clay layers, lowers the number of water-water hydrogen bonds, thus making the closed-packed structure thermodynamically accessible. Remarkably, we have found that this structure is stable across a wide range of temperatures. At the best of our knowledge, this kind of water structure was never reported before, except for a similar hexatic phase (liquid-like, but with a local hexagonal compact symmetry) resulting from a two-dimensional model of water proposed by Vilanova and Franzese.