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The osmotic character of long-range interlamellar swelling in smectite clays is widely accepted and has been evidenced in the interlayer space by X-ray diffraction. Such a behavior in mesopores was not experimentally confirmed until the determination of the mesopore size distribution in Na-montmorillonite prepared from MX80 bentonite using thermoporometry experiments. This is confirmed here for other montmorillonite samples where the interlayer cations are alkaline and Ca(2+) cations. The nature of the interlayer cation is found as strongly influencing the behavior of the size and the swelling of mesopores. These results are supported by the BJH (Barrett, Joyner and Halenda) pore radius values issued from the nitrogen adsorption-desorption isotherms at the dry state. Thermoporometry results as a function of relative humidity ranging from 11% to 97% have shown an evolution of the mesopore sizes for a purified Na-montmorillonite. New thermoporometry data are presented in this article and confirm that the interparticle spaces in K-, Cs-, or Ca-montmorillonites are not strongly modified for all the range of relative humidity: the swelling is not observed or is strongly limited. It appears in contrast that only Li- and Na-montmorillonites undergo a mesopore swelling, distinct from the interlayer swelling. More generally, our results confirm the possibility to use thermoporometry or differential scanning calorimetry to study the structure and the evolution of swelling materials in wetting conditions such as natural clays or biological cells. In this paper, we describe the different key steps of the hydration of swelling clays such as montmorillonites saturated with alkaline cations. Using thermoporometry results combined with X-ray diffraction data, we distinguish the evolution of the porosity at the two different scales and propose a sequence of hydration dependent on the interlayer cation. From this study, it is shown that the interlayer spaces are not completely filled when the mesopores start to fill up. This implies that the swelling observed in the mesopores for Li and Na samples is due to an osmotic swelling. For the other samples, it is difficult to conclude definitively. Furthermore, we determine the different proportion of water (interlayer water and mesopore water) present in our samples by the original combination of (1) X-ray diffraction data, (2) the pore size distribution obtained by thermoporometry, and (3) recent adsorption isotherm results. It is found that the interlayer space is never completely filled by water at the studied relative humidity values for all samples except for the Cs sample.

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International audienceThe key feature of swelling clays such as montmorillonite, in contrast with the nonswelling clays, is their ability to adsorb water in the interlayer space. This interlayer water interacts with the interlayer cations or with the silicate layer surface inside the interlayer space, or with both. However, no direct experimental technique offers the possibility to determine separately these two contributions. In order to determine the hydration energy for interlayer alkali cations, we use a combination of electrostatic calculations of the surface energy and measurements of immersion heats in clays. The results show that Li+ and Na+ cations are characterized by a strongly exothermic hydration energy in the interlayer space, in contrast with K+, Rb+, and Cs+ which have a much lower hydration energy in the interlayer space. The extreme situation is that of Cs+, for which an endothermic hydration energy value is obtained. These trends are in good agreement with results from molecular modeling calculations and consistent with the evolution observed in the water adsorption isotherms. The hydration energy of the silicate layers was also calculated, and the total driving force for hydration in swelling clays could therefore be determined. For Li+- and Na+-montmorillonite, the hydration of cations is clearly the main contribution to the overall driving force for the hydration of clay. On the contrary, hydration of the silicate layers plays the most important role in the hydration of montmorillonite exchanged with the larger cations such as K+, Rb+, and Cs+. These results provide a physical basis for the differences observed in macroscopic swelling behavior between Li+- and Na+-montmorillonite, on one hand, and K+-, Rb+- and Cs+-montmorillonite, on the other hand

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.

International audienceSwelling clays act as ion-exchange membranes owing to the presence of extra-framework ions surrounded by water molecules. A better understanding of the motion of the ions present in the pore space is required to predict the diffusion properties in these solids. Since water molecules tend to adsorb both on ions and clay surfaces, they moderate the interactions between the ions and the framework. The hydration of ions therefore has an impact on their diffusion properties. In this paper, the MX-80 montmorillonite considered for nuclear waste disposal applications has been selected and studied using an approach combining measurements of complex impedance spectroscopy and water adsorption isotherms. The number of charge carriers has been estimated, and the diffusion coefficient for interlayer cations at various hydration states was determined. The evolution of the diffusion coefficient is subsequently correlated with the effect of the opening of the interlayer space and its hydration state. The resulting picture sheds light on the reversible or irreversible character of ion exchange. Finally, our results obtained from the difference between the interlayer diffusion coefficients extracted from conductivity measurements and those obtained at the macroscopic scale is discussed in terms of the textural parameters

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