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A microscopic description of the effect of hydration on the behavior of extraframework cations in zeolite Na + -mordenite is reported. Energy minimization techniques, combined with appropriate interatomic potentials to describe the potential energy surface of this complex system, have been used to determine the site selectivity of both cations and water molecules as a function of the hydration level. We have thus shown that the positions of the cations in the main channels are substantially perturbed upon the sorption of water molecules whereas those of the cations located in the small side channels are only slightly shifted. This modeling has been successfully compared with experimental data obtained by dielectric relaxation spectroscopy.

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The structure of swelling clays consists of a framework composed of negatively charged clay platelets and interlayer cations. Swelling clays present different behaviors as a function of the nature of the interlayer cation: the swelling capacity and the adsorption ability are strongly modified. Therefore the determination of the properties of these interlayer cations is prerequisite to understand the features of the clay swelling and the exchange properties. In this paper, we propose to study the interactions between the interlayer cations and the clay framework by determining the activation energy for the cation motion. This cation displacement is observable under the action of an external electrical field during conductivity measurements. Indeed, it has already been shown that the complex impedance spectroscopy (CIS) is a well-adapted tool to study the energy related to the motion of an electric charge. We propose to determine energy values for the cation motions by electrical conductivity measurements performed at both dry and fully hydrated states for montmorillonites clays to study the influence of the nature of the cation on these energies. Then the so-obtained values in montmorillonites, linked to the hydration energy for the cation, are compared with the clay hydration energy obtained from Van Damme method using gravimetric measurements. We find that the hydration energy decreases from Li+ to Cs+ following the same trend than the weight of the cation (as well as the polarizability) in montmorillonites. These results are also in agreement with the hydration energy of the interlayer cations and layer surfaces from calculations which compare surface energy and immersion data, published recently.