MECHANISM OF ADSORPTION AND DESORPTION OF WATER-VAPOR BY HOMOIONIC MONTMORILLONITE .1. THE SODIUM-EXCHANGED FORM

The structural changes which occur during the intracrystalline swelling of a well-characterized Wyoming sodium montmorillonite have been investigated using controlled-rate thermal analysis, nitrogen adsorption volumetry, water adsorption and desorption gravimetry, immersion microcalorimetry in water, and X-ray diffraction under controlled humidity conditions. The experimental X-ray powder patterns of the 001 reflections have been compared with the theoretical simulations to determine the structural change of the montmorillonite during hydration and dehydration. At relative water vapor pressures lower than 0.16, water absorbs only on the external surfaces of the tactoids. During this stage, the size of the tactoids decreases in order to reach a state identical to that determined after immersion m water (six clay layers thick) and the specific surface area increases from 43 to 105 m2/g. Between 0.16 and 0.50 relative H2O pressure, after the monolayer capacity has been reached on external surfaces, a first-layer hydrate is formed on about 40% of the interlamellar space. For water pressures higher than 0.5 and up to 0.93, a two- and a three-layer hydrate are formed after the bilayer capacity is obtained on the external surface and the effective internal specific surface area reaches a value of about 710 m2/g. The osmotic swelling of the montmorillonite from the two-layer hydration state is an isoenthalpic process. The different hydrates are never homogeneous, and never do H2O molecules completely fill the interlamellar spaces. Heat of hydration of exchangeable cations and the surface pressure on the external surface of tactoids are the driving forces of intracrystalline swelling of sodium montmorillonite. During desorption, the external surfaces mainly lose water down to a relative vapor pressure of 0.72. About 70% of the interlamellar space is occupied with a monolayer hydrate between 0.5 and 0.25 relative pressure of H2O. At a relative vapor pressure of 0.05, the initial dry state is reached again. It was abo possible to distinguish different stages of hydration corresponding to the solvation of exchangeable cations and the accumulation of water molecules adsorbed on the silicate surface in the interlamellar space.