Water structure and dynamics in a fully hydrated sodium vermiculite clay

We have investigated the structure and dynamics of confined water in a fully hydrated sodium vermiculite clay by neutron diffraction and neutron spin-echo (NSE) spectroscopy. All measurements were performed with the Q vector parallel to the clay layers, In the case of the diffraction experiments we used both protonated and deuterated water in order to elucidate the interlayer water structure. The structural results support a picture where the intercalated water is generally 'liquid like' with only a small degree of orientational ordering of the water molecules relative to the clay surfaces and other water molecules. The intermediate scattering function S(Q, t) measured by NSE can be described by the Kohlrausch-Williams-Watts stretched-exponential function, probably associated with a broad distribution of relaxation times owing to widely different local environments for the water molecules. Some water molecules are strongly interacting with the clay surfaces or the intercalated Na+ ions, whereas the remaining molecules are interacting only with other water molecules. The Q dependence of the average relaxation time is in good agreement with the Gaussian jump-length distribution model for translational diffusion, suggesting that most of the motions on the experimental time scale (3-3000 ps) have a translational character.