Hydration Water and Swelling Behavior of Magadiite. The H+, Na+, K+, Mg2+, and Ca2+ Exchanged Forms

Abstract

The hydration behavior of synthetic magadiite exchanged with H+, Na+, K+, Mg2+, and Ca2+ was investigated by combining thermal analyses, water adsorption gravimetry, X-ray diffraction, and spectroscopic techniques, including 29Si NMR, infrared, and Raman under various pressures. Thermal analyses reveal distinct water populations, the number of which depends on the valency of the cation. Except for H-magadiite, which does not swell, water adsorption isotherms exhibit two steps, corresponding to two increases of the interlayer distance. These phenomena are shifted toward higher relative pressures in the case of divalent exchanged samples. For Na- and K-magadiite, the adsorption of the first water molecules generates a water molecule in a C1 symmetry, interacting simultaneously with both the cation and the surface. For higher relative pressure, two new populations of water molecules appear: (i) water molecules linked to the cation and (ii) doubly hydrogen-bonded molecules. In the case of Mg2+, the first hydr...