Methods previously used to distinguish between water adsorbed on external surfaces and in the interlamellar space of Na-montmorillonite during adsorption and desorption of water vapor have been extended to a set of homoionic Li-, Na-, K-, Rb- and Cs-montmorillonite. The textural and structural features have been investigated at different stages of hydration and dehydration using controlled-rate thermal analysis, nitrogen adsorption volumetry, water adsorption gravimetry, immersion microcalorimetry and X-ray powder diffraction under controlled humidity conditions. During hydration, the size of the quasi-crystals decreases from 33 layers to 8 layers for Na-montmorillonite and from 25 layers to 10 layers for K-montmorillonite, but remains stable around 8–11 layers for Cs-montmorillonite. Each homoionic species leads to a one-layer hydrate, which starts forming at specific values of water vapor relative pressure. Li-, Na- and K-montmorillonite can form a two-layer hydrate. By comparing experimental X-ray diffraction patterns with theoretically simulated ones, the evolution of structural characteristics of montmorillonites during hydration or desorption can be described. Using structural and textural data, it is shown that during adsorption: (1) the rate of filling of interlamellar space of the one layer hydrate increases with the relative pressure but decreases with the size of the cations; and (2) the different hydrated states are never homogeneous.

The swelling of some well-defined Mg-, Ca-, Sr- and Ba- homoionic montmorillonites was studied in the domain of water relative pressures lower than 0.95. This involves the expansion of the crystal lattice itself, commonly known as the “interlamellar expansion” or “inner crystalline swelling”. The initial freeze-dried clays were characterized by nitrogen adsorption-desorption volumetry and controlled transformation rate thermal analysis. The evolution of the structural and textural properties of these different clays at different stages of hydration and dehydration was investigated using water adsorption gravimetry, immersion microcalorimetry at different precoverage water vapor relative pressures and X-raydiffraction (XRD) under controlled humidity conditions. Large textural variations are observed in the dry state depending on the exchangeable cations. The 2-layer hydrate exhibits the most ordered layer stacking. Water is mainly adsorbed in the interlamellar space. With increasing water pressure, each homoionic species leads to a 1-layer hydrate and, with the exception of Ba-montmorillonite, to a predominant 2-layer hydrate. The relative pressure corresponding to the formation of the 2-layer hydrate decreases with increasing hydration energy of the interlayer cation. For Ca-, Sr- or Mg-montmoriHonites, simulation of XRD patterns leads to the definition of successive homogeneous states corresponding to the 2-layer hydrate. Furthermore, it yields the water filling ratio corresponding to the different hydration states during adsorption and desorption of water vapor.