J.M. Cases

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.

Mechanism of formation of aluminum trihydroxide from keggin Al13 polymers

The mechanism of formation of layered aluminum hydroxides from aluminum chloride solution progressively neutralized by caustic soda has been studied by infrared spectroscopy, small-angle X-ray scattering, and 27Al high-resolution solid-state nuclear magnetic resonance. In the starting solution most of the cationic species belong to the so-called Al13 polymer (Keggin structure) made from 12 octahedra surrounding an aluminum tetrahedron. This polymer is stable in solution for hydrolysis ratio R = (OH)(Alt) 2.8, the hydrolysis mechanism is driven faster, leading to the formation of highly polymerized octahedral layers and to the disappearance of tetrahedral aluminum. For R = 3, the short-range order characteristic of bayerite is obtained very rapidly (⋍24 h). Thus crystalline Al trihydroxides are not formed by progressive condensation of flat hexamers made from Al octahedra but by solid-state structural rearrangement, without redissolution steps.

Thermodynamic and microdynamic behavior of water in clay suspensions and gels

The behavior of water in suspensions of swelling smectites (hectorite and laponite) or of a non-swelling phyllosilicate (kaolinite) was studied over a large range of concentrations of the solid (C), namely, between 0.1 and 20% for the former, or between 5 and 60%, by weight, for the latter. This covers different states: the dilute opalescent suspension, the thixotropic gel, and the stiff paste. The spin-lattice relaxation rates T1−1 of the proton or of the deuteron were measured at different temperatures as a function of the concentration C. Linear relationships were obtained when T1−1 was plotted against the C. The linear relationship T1−1 vs C implies that the average size of the tactoids in the suspensions of smectites is practically independent of the concentration of solid. It results from this observation that the external surface area available to water does not change with concentration. The heats of immersion being independent of C in the range 0.5% < C < 2%, it follows that from the thermodynamic data the upper limit of the number of layers submitted to the surface force field is smaller than or of the order of three. A theory derived for NMR studies of electrolyte solutions permitted physical interpretation of the experimental slopes and intercepts of the T1−1 vs C plots. It follows that the water molecules in these suspensions are distributed among two phases. Phase a contains the so-called free water between the tactoids resulting from the aggregation of individual sheets of the smectites or between the kaolinite individual crystals. In that phase and irrespective of C, the diffusion coefficient of water is the same at the microscale of the NMR techniques as that of pure water. Phase b contains the water molecules influenced by the surface force fields. Their rotational correlation time is about two orders of magnitude (∼10−10 sec) lower than that of liquid water and they are characterized by a lifetime in phase b which is about 10−9 sec at 298°K. The average number of water layers in phase b is about 3. This number is in the same range as the statistical degree of coverage observed when the heat of immersion becomes equal to the surface energy of pure liquid water. Therefore, the external surface area determined by using the Harkins and Jura method represents very closely that of phase b. For smectites phase b is identified with the water layers on the external surface of tactoids (phase “be”). In conclusion neither at the thermodynamical scale nor at the microdynamic range is a long-range force acting on the water molecules in phase b detectable with NMR or heats of immersion techniques used here.

FTIR reflectance vs. EPR studies of structural iron in kaolinites

The substitution of Fe3+ in the kaolinite structure is studied by EPR spectrometry and by FTIR spectrometry on a large set of kaolins from different origins (sedimentary and primary ores, soil kaolins). The IR bands at 3598 and 875 cm−1, observed in the literature only in the case of disordered kaolins or in Fe-rich environments (synthetic, lateritic), are revealed by high-resolution IR analysis, whatever the origin and the total Fe content of the samples. The EPR bands corresponding to Fe3+ substituted in sites II of the octahedral sheet increase when the IR absorbance near 3600 cm−1 increases. Two IR absorption bands near 4465 cm−1 and 7025 cm−1 are observed for the first time, both in transmission and diffuse reflectance on all samples. These bands are assigned to the combination of the 3598 and 875 cm−1 bands and to the first harmonic of the band at 3598 cm−1, respectively. The area of the band at 4465 cm−1 in diffuse reflectance is quantitatively correlated to the abundance of Fe3+ located in centers II as measured by ESR. This directly confirms the assignment of the two IR bands at 3598 and 875 cm−1 to OH stretching and deformation vibration bands in octahedral FE3+ environment in the kaolinite structure, respectively. Effects due to the size of particles and to the main kaolins impurities on the near infrared spectra, are also discussed.

MODIFICATION OF THE POROUS STRUCTURE AND SURFACE AREA OF SEPIOLITE UNDER VACUUM THERMAL TREATMENT

Modifications of the external surface area and the two types of microporosity of sepiolite (structural microporosity and inter-fiber porosity) were examined as a function of the temperature of a vacuum thermal treatment to 500°C. The methods used included: reciprocal thermal analysis, N2 and Ar low-temperature adsorption microcalorimetry, gas adsorption volumetry (for N2, Ar, and Kr at 77 K and CO2 at 273 and 293 K), water-vapor adsorption gravimetry, and immersion microcalorimetry into liquid water at 303 K. If the sample was not heated >100°C, only 20% of the structural microporosity was available to N2, whereas 52% was available to CO2 at 293 K. In both experiments, the channels filled at very low relative pressures. At >350°C, the structure transformed to anhydrous sepiolite, which showed no structural microporosity. The inter-fiber microporosity decreased from 0.031 to 0.025 cmVg (as seen with N2), and the external specific surface area decreased from 120 to 48 m2/g. The water adsorption isotherms showed a lower and lower affinity of the external surface of fibers for water as the temperature of thermal treatment increased. The thickness of the bound water on the external surface was estimated to be ≤ 3.5 monolayers, i.e., less than 10 Å.

Zeta potential of magnesian carbonates in inorganic electrolytes

The streaming potential method allows the localization of the p.z.c. of magnesite at pH < 6.5 and that of dolomite at pH < 7. The curves of variation of the electrokinetic potential as a function of pH show that H+ and OH− are the potential determining ions of carbonates. After immersion in an aqueous solution, these surfaces show the presence of holes saturated with water molecules. The sites thus formed dissociate to create a net charge on the surface which is positive or negative according to the pH of the solution. The ions of the lattice do not absorb specifically onto the interface and thus do not change the sign of the electrokinetic potential. The pH value of the p.z.c. for each carbonate is lower than that of the corresponding i.e.p. calculated by thermodynamics, due to the pK of the dissociation reactions of surface acids, whose values are lower than those of the same reactions in solution.

Adsorption of nonionic polyacrylamide on sodium montmorillonite: Relation between adsorption, ξ potential, turbidity, enthalpy of adsorption data and 13C-NMR in aqueous solution

Abstract The adsorption of nonionic polyacrylamide on sodium nontomorillonite is studied using different molecular weight macromolecules (mol wt = 4.40 × 104, 1.20 × 105, 3.70 × 105, 3 × 106), with the monomeric unit represented by the isobutyramide molecule. All of the adsorption runs are completed by turbidimetric, electrokinetic, X-ray scattering, and calorimetric data. The adsorption of isobutyramide reveals the nature of the different adsorption sites (broked edge faces and external and internal basal faces). At the plateau of the isotherm, the amount adsorbed is approximately equal to (Qa) 23.5 g per 100 g of solid, i.e., one isobutyramide molecule for two hexagonal cavities. This is the maximum number of acrylamide units in contact with the solid. Our results on macromolecule adsorption seem to show that the QB ∼ 20 ga · 100 gs−1, the macromolecules have statistically a flat conformation. For higher Qa values, the macromolecules adsorb, making larger and larger loops into the bulk solution. This process is achieved along a destruction of the turbostratic gathering of the tactoids. The 13C-nuclear magnetic resonance of the CO groups is the adsorbed phase and in aqueous soluton corroborate the conformation variations along the isotherms. It is demonstrated that line broadening is due to a chemical shift dispersion arising from the surface site heterogeneity or the rotational isomerism hindrance accompanying a change of macromolecular structure near the surface. If Qa is lower than 20 ga · 100 gs−1, linewidth remains constant and equal to 420 Hz (± 15 Hz). For Qa high than 20 ga · 100 gs−1, linewidth decreases to 106 Hz (mol wt = 3 × 106 and Qa = 68 ga · 100 gs−1, indicating a higher mobility of the chains when Qa increases.

Mechanism of oleate interaction on salt-type minerals. V : Adsorption and precipitation reactions in relation to the solid/liquid ratio in the synthetic fluorite-sodium oleate system

The mechanism of adsorption of oleate onto a synthetic fluorite sample at different solid/liquid ratios in alkaline solutions is elucidated through adsorption isotherms in conjunction with ζ-potential and diffuse reflectance Fourier transform-infrared spectroscopic studies. These studies show that the chemisorption of oleate on surface calcium species followed by the physical adsorption of hydrocarbon chains (bilayer formation) precede the precipitation of calcium oleate in bulk solution. With an increase in the solid/liquid ratio, the bulk precipitation of calcium oleate after bilayer formation is found to be reduced, and at a high solid concentration, the isotherm is seen to level off with bilayer formation. As the solid concentration increases, the concentration of calcium in bulk solution decreases because of its adsorption on the head group of the hydrophilic layer of oleate (bilayer), and hence the extent of bulk precipitation decreases. The adsorption density for a monolayer coverage corresponds to the condensed state of an alkyl chain with a cross-sectional area of 33A˚2 (liquid-crystal state). Application of the two-dimensional condensation model is considered and no evidence for the surface precipitation process is observed.

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.