Michèle François

Mechanism of adsorption and desorption of water vapor by homoionic montmorillonites; 2, The Li (super +) , Na (super +) , K (super +) , Rb (super +) and Cs (super +) -exchanged forms

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.

Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite; 3, The Mg (super 2+) , Ca (super 2+) , and Ba (super 3+) exchanged forms

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.

Surface heterogeneity of minerals

Abstract The precise study of adsorption mechanisms at solid–liquid interfaces requires a good analysis of the surface heterogeneity of the studied solids. For that purpose, molecular probe technique is one of the most powerful, especially at solid–gas interfaces. Indeed, low-pressure gas adsorption coupled to modelling of derivative adsorption isotherms as a function of logarithm of pressure allows to study qualitatively and quantitatively the effect of surface heterogeneity on the energy distribution of adsorption centres. The present review points out the interests of that approach to determine the shape of particles, the presence of high-energy adsorption sites and the surface polarity. Results comparing adsorption at solid–gas and solid–liquid interfaces are also mentioned. To cite this article: F. Villieras et al., C. R. Geoscience 334 (2002) 597–609.

EVOLUTION OF THE POROUS STRUCTURE AND SURFACE AREA OF PALYGORSKITE UNDER VACUUM THERMAL TREATMENT

The modification of the external surface area and the two types of microporosity of palygorskite (structural and interfiber porosity) were examined as a function of the temperature of a vacuum thermal treatment to 500°C. The methods used included: controlled-transformation-rate thermal analysis, N2 and Ar low-temperature adsorption microcalorimetry, conventional and continuous gas-adsorption volumetry (for N2 and Ar) at 77 K and CO2 at 273 and 293 K, water vapor adsorption gravimetry, and immersion microcalorimetry in water. At temperatures < 100°C only 18% of the structural microporosity was available to N2, 13% to Ar, and 100% to CO2 at 273 K. In both experiments the channels filled at very low relative pressures. At temperatures between 70° and 130°C the structure folded, and the mineral transformed to anhydrous palygorskite, which showed no structural microporosity. The interfiber microporosity was found to be independent of the temperature treatment, and the external surface area decreased slightly from 65 to 54 m2/g. The water adsorption isotherms showed that the folding of the structure was reversible up to final outgassing temperatures >225°C.

Chapter 11. Static and dynamic studies of the energetic surface heterogeneity of clay minerals

Publisher Summary Clay minerals are typical examples of heterogeneous adsorbents, showing both surface geometric and energy distributions. The heterogeneity of clays is governed by the geochemical crystallisation conditions, generating a strong relationship between the structure, shape and chemical surface properties of these solids. Such complexity obliges to develop the modern experimental techniques and modeling methods for studying solid surfaces. In this way, the high resolution low pressure quasiequilibrium adsorption technique, first developed to characterize geometrical heterogeneity of clay minerals, looks very powerful and promising for studying their energetic heterogeneity. Furthermore, the possibility to detect very high energy surface sites opens new investigation fields as those sites are always involved in interfacial interactions.

Calorimetric studies of simple ion adsorption at oxide/electrolyte interface titration experiments and their theoretical analysis based on 2-pK charging mechanism and on the triple layer model

Abstract The expressions developed recently by us to describe the temperature dependence of the titration curves are used to describe the calorimetric effects accompanying ion adsorption at the oxide/electrolyte interface. For the purpose of illustration the data obtained by “calorimetric titration” of TiO 2 suspension in NaCl were subjected to a quantitative analysis, based on the developed equations. A proper choice of adsorption parameters leads to a quantitative agreement between both the experimental and theoretical titration curves, and between the experimental and theoretical heats of proton adsorption at different pH values. This quantitative analysis shows the risk of a simple interpretation on a qualitative level of the enthalpic effects accompanying ion adsorption. This is because the recorded enthalpic effects carry complicated information about the enthalpic effects of various surface (complexation) reactions occurring simultaneously.

Micropore formation due to thermal decomposition of hydroxide layer of Mg-chlorites: interactions with water

Abstract The first stage of dehydroxylation of magnesian chlorites involves the dehydroxylation of the brucite-like layer which removed water from the structure. This reaction provokes the modification of basal reflection intensities and the development of long basal spacings. Infrared spectroscopy as well as thermogravimetry and water vapour adsorption reveal the formation of structural micropores filled with molecular atmospheric water once the samples are cooled down. A high temperature treatment is needed to release the different phases condenced in these micropores. A heterogeneous dehydroxylation mechanism is proposed involving magnesium and oxygen concentration in acceptor regions and micropores in donor regions. This leads to a structure where micropores and enriched oxide interlayers alternate along the z-axis of the mineral which generates long-basal spacings. According to this model theoretical calculation shows that only a part of the microporous volume is accessible to water vapour.

Modification of calcium carbonate surface properties: macroscopic and microscopic investigations

Understanding the wettability of mineral powders and its modification by the addition of various surface active agents is crucial for many industrial applications. In many cases, wettability is investigated by macroscopic characterization techniques. In this framework, we decided to study non-porous calcium carbonate powders coated by known amounts of water-repellent molecules. A detailed characterization of the interface, focusing on the analysis of surface heterogeneity, was carried out using water vapor, nitrogen, and argon adsorption. It clearly reveals specific adsorption sites for water-repellent molecules. Wettability and immersion enthalpy measurements show that saturation of the carbonate surface by water-repellent molecules is not necessary for obtaining maximum hydrophobicity. It is reached for approximately one-third of surface saturation; at that point, some high-energy surface sites are still available for water and nitrogen adsorption. This suggests that wettability is not only linked to th...

Vacuum Thermal Stability and Textural Properties of Attapulgite

Summary Evolution of the external surface area and the two types of microporosity of attapulgite (structural and inter-fiber) were examined as a function of a vacuum thermal treatment upt to 500°C. The methods used include: controlled transformation rate thermal analysis, N 2 and Ar low temperature adsorption calorimetry, water vapor adsorption gravimetry and quasi equilibrium gas adsorption procedure of N 2 at 77K and CO 2 at 273 and 293K. Depending on the outgassing conditions, i.e. the residual pressure, the structure folds 150 to 70°C. For lower temperature, only a part (18%) of the structural microporosity is available to N 2 , 13% to argon and 100% to CO 2 . With water, the structure can rehydrate after the structure is folded up to an outgassing temperature of 225°C.