Mechanism of Adsorption and Desorption of Water Vapor by Homoionic Montmorillonites: 2. The Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺-Exchanged Forms

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.

Thermodynamics of Hydration in Minerals: How to Predict These Entities

“…In each figure, the data of the free energy of hydration (Eq. 50) from the experimental isotherms [15,16] [63] (blue and red lines, respectively) agrees well with the experimental data acquired by [15,16] in high relative humidity.…”

“…It appears that integral enthalpy of hydration modelled by Vieillard et al [63] (black line) based on experimental measurements of heat of adsorption from [15][16][17] encompasses nearly all experimental data. As the standard entropy of hydration is assumed constant in [66] and [68] models, the integral hydration enthalpies provided by these two previous models are nearly linear.…”

“…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.…”

Influence of pH on the interlayer cationic composition and hydration state of Ca-montmorillonite: Analytical chemistry, chemical modelling and XRD profile modelling study

“…For this purpose, the combination of chemical modelling and XRD profile 46 modelling were used to follow the exchange of Ca 2+ cations by protons under acidic 47 conditions. Specific X-ray diffraction modelling techniques based on a trial-and-error 48 approach were used to refine the early descriptions of smectite hydration by taking into 49 account the possible coexistence in the smectite structure of different layer types, each 50 exhibiting a specific hydration state (Bérend et al, 1995;Cases et al, 1997;Cuadros, 1997; 51 Ferrage et al, 2005aFerrage et al, , 2005b. In particular, the likely mutual exclusion of calcium and protons 52 in smectite interlayers and the expected hydration contrast between such calcium-and proton-53 saturated smectite layers was used to follow the H 3 O + -for-Ca 2+ exchange supposed to occur at 54 low pH.…”