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A quasi-elastic neutron scattering study of water dynamics confined in a model clay system (a synthetic hectorite with Na+ compensating counterions) is presented. The neutron spin echo (NSE) data obtained at ambient temperature for the monohydrated and bihydrated hectorite powder are analysed with the help of models taking into account the anisotropy of the system. A powder-averaged two-dimensional (2D) diffusion model applied to the monohydrated state yields a 2D diffusion coefficient in the plane of the clay layers () of 2.8 × 10−10 m2 s−1, where an isotropic analysis of the data would have underestimated the diffusion coefficient by approximately 25%. In the case of the bihydrated state, the analysis by a model including a possible diffusion perpendicular to the clay layers () leads to a range of values for between 3.5 and 8.7 × 10−10 m2 s−1, depending on the ratio . We conclude that simple isotropic analysis can only give a rough estimate (order of magnitude) of the water dynamics in the medium.

A simple two-state model is proposed to explicitly derive the ionic contribution to the frequency-dependent dielectric permittivity of clay. This model is based on a separation of time scales and accounts for two possible solvation modes (inner/outer-sphere complexes) for ions in the interlayer spacing and a possible chemical exchange between both forms. The influence on the permittivity of thermodynamic (distribution constant K(d)) and dynamic (diffusion coefficient, chemical relaxation rate) parameters is discussed. In turn, this model is used to analyze experimental data obtained with Na-montmorillonite for two relative humidities. The values of the parameters extracted from these measurements, and their variation with water content, show that the proposed model is at least reasonable.

The microdynamics of water and ions in two types of clays (montmorillonite and hectorite) was investigated with Broadband Dielectric Spectroscopy (40 Hz to 5 GHz). Four to five relaxations were observed depending on the relative humidity, the clay type and the nature of the compensating counterion (Na + or Cs + ). A tentative assignment of the relaxations to physical mechanisms is proposed.

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