Diffusion of Interlayer Cations in Swelling Clays as a Function of Water Content: Case of Montmorillonites Saturated with Alkali Cations

摘要

Swelling 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.