Résumé
Dehydration heat and ionic conductivity of a series of homoionic alkali (Li+, Na+, K+, Rb+, Cs+) and alkali rare-earth (Mg2+, Ca2+, Ba2+) exchanged montmorillonite (ref. CMS: SWy-2) are investigated at low water loading. Dehydration heat is obtained via Van't Hoff's law applied to weight losses measured at several constant temperatures, i.e. treatment temperature (TT), as a function of time by thermo-gravimetric analysis . Dc conductivity is measured at various temperature using Complex Impedance Spectroscopy on samples whose the water loading is fixed by TT . Comparison with the dissociation enthalpy of alkali and alkali rare-earth cation/water complexes shows that dehydration heat montmorillonites results from the competition between opposite energy contributions due to: i) cation solvation, ii) hydration of the silicate interlayer surface and iii) structural swelling. So, depending on the balance between these various energy contributions, different behaviors are observed accordingly to the nature of the extraframework cation (EFC) with a perculiar behavior for K+. Independently on EFC, dc conductivity follows an Arrhenius behavior when the water loading is closed to zero. In contrast, this behavior is no longer observed as long as water molecules are adsorbed and, hence, in interaction with EFC. In that case, the results are well described by the empirical Vogel-Tamman-Fulcher (VTF) form, i.e. , suggesting order/disorder transition in the water/cation sub-network even at low hydration state. This outcome depends on EFC in accordance with dehydration heat. It is then shown that dc conductivity evolution with both temperature and water loading is a significant source of information regarding adsorption in connection with EFC.