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

Clay minerals are of remarkable interest for understanding various natural phenomena as well as for applications such as the treatment of toxic and radioactive waste materials and catalysis. Water confined in clays has been recently addressed both theoretically and experimentally for the study of fundamental water properties. Using Car-Parrinello molecular dynamics simulations at different temperatures of slightly superhydrated Na-vermiculite clay we found an unusual water structure. 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 and makes the closed-packed structure possible. The considered water content corresponds to that of interesting experiments performed to detect dynamical crossovers in the adsorbed supercooled water.