Dehydration enthalpy of alkali-cations-exchanged montmorillonite from thermogravimetric analysis.

Dehydration of a series of homoionic alkali-exchanged montmorillonites is studied at different treatment temperatures by means of thermogravimetric analysis. More specifically, we investigate the last stages of dehydration when the number of adsorbed water molecules corresponds, at maximum, to a monolayer. Weight losses are measured at several constant temperatures as a function of time. Application of Van't Hoff's law yields the dehydration enthalpy. Trends and data similar to those reported from other experimental conditions are found. Comparison with X-ray data and with the dissociation enthalpy of alkali cation/water complexes shows that dehydration of weakly hydrated homoionic alkali montmorillonites results from the competition between opposite energy contributions due to (i) the cation solvation, (ii) the hydration of the silicate interlayer surface, and (iii) the structural swelling. So, depending on the balance between these various energy contributions, different behaviors are observed according to the nature of the alkali cations.

Hydration of a Na(+)-montmorillonite studied by thermally stimulated depolarization current.

Thermally stimulated depolarization current (TSDC) technique is a powerful tool for probing dipole re-orientational motions in condensed matter. In the case of cation-exchangeable aluminosilicates, it allows the assessment of the potential barrier related to the hopping mechanism of cations and, consequently, the measurement of its evolution when molecules, i.e. water, are adsorbed and interact with the cations embedded in the solid framework. Then, using suitable models based on thermodynamics, the analysis of TSDC signals obtained at various hydration states provides insights about the surface properties of the studied solid and the mechanism of adsorption at the cationic site. In this work, TSDC is used to study the first stage, i.e. when the number of adsorbed molecules is below the occurrence of the water monolayer, of water adsorption in a Na(+)-montmorillonite from Mostaganem (Algeria). It is shown that the hydration process follows two stages. Using the "chemical force" concept it can then be concluded that when the number of adsorbed water molecules per cation is lower than 2, cation-water interaction dominates the energetics of adsorption, whereas at higher water loading the water "chemical force" is also involved into water-water and/or water-clay framework interactions. The number of water molecules for the monohydrated state is found to be about 7.

Analysis of Thermally Stimulated Depolarization Currents (TSDC) Measured on Exchanged Clays.

This work shows that hydration of clays can be studied by means of a new interpretation of thermally stimulated depolarization currents technique. These measurements have been performed on four exchanged natural clays: Na-montmorillonite, Ca-montmorillonite, kaolinite, and illite. The results are analyzed using both the recently developed analysis of relaxation times distribution and the electronegativity equalization method. They provide evidence of the surface heterogeneity of clays. It is established that sites, identical from a crystallographic point of view, are different when the energy of the phenomenon is considered. The main interest of this work is to give for the first time a value of the hydration energy of cation onto clay surfaces. Copyright 2000 Academic Press.