Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite. 1. The sodium-exchanged form

Abstract

Abstract--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.

Structural Breathing of Graphite Oxide Pressurized in Basic and Acidic Solutions.

“…ref 38 ) . In a simple model, graphite oxide could be considered as a semipenetrative membrane: the interlayer space of the GO structure has a higher concentration of ions compared to the surrounding liquid medium and this causes an osmotic pressure and osmotic flow of liquid inside interlayer space.…”

Thermodynamics of Hydration in Minerals: How to Predict These Entities

“…Unlike zeolites, smectites are clearly not inert phases, as the particles size increases with the relative humidity [15]. Adsorbed water is distributed throughout the interlayer space, the outer surfaces of particles and the open pores space in the sample.…”

“…Adsorbed water is distributed throughout the interlayer space, the outer surfaces of particles and the open pores space in the sample. [15][16][17] provided measured values of the surface covering waters for both the hydration and dehydration reactions of a set of eight homo-ionic SWy-1 montmorillonites saturated by alkali and alkaline-earth cations from their BET specific surface area (Table 1) and their basal spacing variations with relative humidity. To quantify the effective amount of water involved in the hydration reaction, the amount of poral H2O must be quantified and subtracted from the total amount of H2O taken up by the clay sample.…”

“…Some of the difficult parameters to control include the amount of H2O adsorbed by the sample (requires knowledge of the amount of H2O in the sample while immersed in water), the accurate measurement of the initial state of dehydration and the potential for irreversible change in the zeolite structure during dehydration such that the sample does not re-adsorb its full capacity of H2O during immersion [14]. For clays, the measurements of heat of adsorption were performed on eight homoionic SWy-1 montmorillonites [15][16][17] [18] [19] [20] and on vermiculites [21]. One advantage of the method is that it is a direct measurement of the enthalpy of hydration.…”

Kimberlite degradation: The role of cation type

“…Adsorption and desorption studies on Na + , Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ -containing smectites showed that these cations have a swelling effect on smectites ( [Cases et al, 1997], [Rytwo et al, 1996], [Badreddine et al, 2002] and [Prost, 1981]). Ferrage et al (2005) showed that, at room temperature and a relative humidity of 35%, interlayer spacings corresponding to mainly two water layers for smectites saturated with Mg 2+ and Ca 2+ .…”