Montmorillonite was equilibrated with high normality Cl solutions to assess the possible presence of MeCl+ ion pairs in smectite interlayers suggested by chemical modeling of cation exchange experimental studies. Structural modifications induced by the presence of such ion pairs, and more especially those related to smectite hydration properties, were characterized from the modeling of experimental X-ray diffraction (XRD) profiles. As compared to those obtained from samples prepared at low ionic strength, XRD patterns from samples equilibrated in high ionic strength CaCl2 solutions exhibited a small positional shift of 00l basal reflections indicating a greater layer thickness. The rationality of basal reflection positions is also improved and the width of these reflections is decreased. These qualitative modifications are related to the existence of a more homogeneous hydration state with the sole presence at 40% relative humidity (RH) of bi-hydrated smectite layers (2W layers) in high ionic strength samples. By contrast, layers with contrasting hydration states coexist in samples prepared at low ionic strength. The stability of this homogeneous 2W hydration state is also extended towards low RH values in the sample prepared at high ionic strength.

In addition, the intensity distribution is modified in samples prepared at high ionic strength as compared to those obtained at low ionic strength. In particular the relative intensity of the 002 reflection is strongly enhanced in the former samples. This modification arises from an increased electron density in the interlayer mid-plane of 2W layers which is best explained by the presence of cation-chloride ion pairs replacing the divalent cations occupying this structural position in low ionic strength samples. The increased amounts of interlayer species (ion pairs and H2O molecules), which are confirmed by near-infrared diffuse reflectance spectroscopy results, and the larger size of CaCl+ pairs as compared to Ca2+ cations lead to a more stable layer thickness, probably as a result of decreased layer corrugation. Consistent results were obtained for Sr and Mg cations.

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