Characterization and dynamic of water in alkali homoionic montmorillonites

The objective of this work is to study the effect of interlayer cations on the mechanism of adsorption-desorption of water in the case of a montmorillonite exchanged by alkali metals. This is done through thermodynamic measurements, i.e. isotherms of adsorption and desorption of water. The raw material is subjected to purification treatment by using the sedimentation followed by cationic exchange. IR and XRD analysis of the dry state confirm that the treatment used for obtaining homoionic clays does not deteriorate their structure. The value of the interlayer distance d₀₀₁obtained from XRD depends directly on the cation radius. IR spectra recorded on our montmorillonite reveals that the area ratio ν_OH/ν_SiO peaks which corresponds to the amount of adsorbed water under room atmosphere follows the sequence: K >Rb> Cs > Li > Na In accordance the evolution observed for the desorption enthalpies. The adsorption and desorption isotherms show that the nature of the compensating cation considerably changes the adsorbed / desorbed amount for a given water relative pressure. Thus, the total amount of water adsorbed at P/P₀ = 0.5 follows the same sequence.