Synthetic Smectites Research Articles

Controls on tetrahedral Fe(III) abundance in 2:1 phyllosilicates—Reply

AbstractThe model of Fe3+ distribution between octahedra and tetrahedra in dioctahedral smectites by Decarreau and Petit (2014) used data from infrared analysis. From their own and other general evidence, the resulting data are likely to be affected by significant uncertainty. This aside, their model has limited application because it is based on synthetic smectites containing only Si, Al, and Fe3+.

Revisiting the cation mass-valence sum approach to assigning infrared OH-bands in dioctahedral smectites in the light of new data from synthetic Ga-Fe3+ smectites

A series of pure Ga-Fe3+ smectites were synthesized at 180°C in order to complete IR data in the Al-Ga-Fe3+ system. MIR and NIR data from natural and synthetic smectites in the Al-Fe3+, Al-Ga and Fe3+-Ga chemical series exemplified a case where some ν OH, δ OH and (ν+δ) OH band positions did not follow the relation: ν OH (cm−1)=(1/2π c)*(kOH/μOH)1/2. This relation is often used to approximate the position of the OH vibration bands from the reduced masses of octahedral cations linked to hydroxyls in clay minerals. In this study, better relations were established between the wavenumbers of the OH absorption band of smectites and the average ionic radius of neighboring octahedral cations. Similar trends were established previously for M-(OH)2 hydroxides (G.W. Brindley, C. Kao, Phys. Chem. Miner. 10 (1984) 187-191) suggesting a possible generalization of a link between OH frequencies in IR spectra and the average ionic radii of neighboring octahedral cations for hydroxylated minerals.

Fe3+/Al3+ partitioning between tetrahedral and octahedral sites in dioctahedral smectites

AbstractThe distribution of Al3+ and Fe3+ between octahedral and tetrahedral sites of dioctahedral smectites was shown to be controlled by a partition coefficient Kd(4/6) = [(Fe3+)4×(Al3+)6]/[(Fe3+)6×(Al3+)4]. The Kd(4/6) value is near 0.006 for natural dioctahedral smectites, formed between 2 and ~100°C, and near 0.0174 for smectites synthesized at 200°C. These Kd(4/6) values, obtained from both chemical and spectroscopic data, were consistent with those calculated using the ionic radii of cations and Brice’s model (Brice, 1975). The partition coefficient approach explained well why for natural and synthetic dioctahedral smectites no tetrahedral Fe3+ is detected when the total Fe3+ content is below 3 atoms per unit cell (24 oxygen atoms).

Hydrothermal synthesis of dioctahedral smectites: The Al–Fe3 + chemical series. Part II: Crystal-chemistry

The crystal-chemistry of a series of synthetic Al–Fe3+ smectites was studied in detail using near and mid infrared spectroscopy. Chemical and NIR data indicated a quite complete range of octahedral Al for Fe3+ substitution, and therefore, the solid-solution between beidellite and nontronite end-members was continuous and complete. The wavenumbers of several infrared absorption bands were correlated with the chemistry of the synthetic smectites, providing a useful tool to constrain their structural formulae and also for assisting in assignments of similar bands in natural smectites. The Al and Fe3+ cations were shown to be randomly distributed in the octahedral sheet of synthesized smectites. Despite the high availability of iron during synthesis, generally only a small amount of tetrahedral Fe3+ was observed.

Preparation of a series of group XIII metal–quinolate complexes in natural and synthetic smectites

The preparation of metal–quinolate complexes, Mq 3 (M = Al(III), Ga(III) and In(III); q = 8-hydroxyquinoline) in the interlayer spaces of smectites (natural montmorillonite and synthetic saponite), as well as their optical properties were investigated. The basal spacings of the products increased through solid–solid reactions between M(III)-smectites and 8-hydroxyquinoline, indicating the intercalation of 8-hydroxyquinoline. The photoluminescence maxima and intensities of the intercalation compounds varied depending on the metal ions and nature of clay minerals. The PL intensities of Mq 3 complexes in synthetic saponite were higher than those of the complexes in montmorillonite because of the difference in molecular structure and/or molecular packing, as well as the presence of the quenching impurities in montmorillonite.

Increasing particle size of a synthetic smectite for polymer nanocomposites using a supercritical hydrothermal treatment

We have studied the effect of a supercritical hydrothermal process on the structural and surface morphological properties of synthetic smectite clay, stevensite (ST), in terms of the particle size, in order to enhance the functionality of the synthetic smectites as an inorganic filler for transparent clay/polymer nanocomposites. The ST aqueous suspensions were treated in a flow reactor system at 673 K and 25 MPa. The structural characterizations revealed that the ST retained a layered structure composed of polymeric sheets of SiO 4 tetrahedra after the treatment. The treated ST possessed a particle size of 71 nm, approximately twice that of the original ST (36 nm) for the 0.1 wt.% suspension using an operation condition at a flow rate of 0.085 g s −1. SEM observation revealed that an enlarged particle was formed from cohesive aggregates, suggesting that the increase in size of the particles was caused by the cohesion of the microcrystallites or primary particles of ST. The treated ST was subsequently used to prepare nanocomposites with carboxymethylcellulose sodium salt (CMC Na) to evaluate the effect of the supercritical treatment. The treated ST nanocomposite films retained their transparency which is very similar to the original ST nanocomposite films. Furthermore, the nanocomposite films, which had a high CMC Na ratio ranging from 40 to 90 wt.%, showed improved oxygen barrier properties when compared with those of original ST. The tortuous model revealed that this improvement was mainly due to the increase of the particle size. Consequently, the supercritical treatment successfully brought about the growth of the ST particles, leading to the development of functional synthetic clays for clay/polymer nanocomposites.

Thermodynamic assessment of the variation of the surface areas of two synthetic swelling clays during adsorption of water

Two synthetic smectites (montmorillonite and beidellite) are studied by a water adsorption technique in order to assess their specific surface areas under atmospheric conditions. A route recently proposed for extracting the thermodynamic data from experimental adsorption isotherms is used. The variation of the specific surface area during water adsorption is obtained, which can be linked to the enlargement of the interlayer space determined using X-ray diffraction. This variation is compared to an idealized specific surface area obtained from TEM and X-ray measurements in agreement with crystallographic models. All these results are also compared with those obtained previously for a natural montmorillonite. A simple view of swelling is proposed.

Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series

The aim of this study is to synthesize and finely characterize montmorillonite samples, dioctahedral smectites without tetrahedral charges (structural formulae Nax(Al(2−x)Mgx)Si4O10(OH)2), to allow their use as reference samples in clay science. The montmorillonites synthesis under hydrothermal conditions at different pressures and with various layer charge deficit has been attempted. The temperature was fixed at 320 °C, the pressure parameter values were 20 MPa, 80 MPa, 120 MPa and 200 MPa. The Mg content varied from 0.25 to 0.60 per half unit cell. The reaction products have been characterized with multi-technique analyses (ICP-AES, EMP, CEC, XRD, FTIR, NMR and TGA).Montmorillonite phase was only produced at 120 and 200 MPa.At 20 and 80 MPa, the results suggest that a 0.33 and 0.16-tetrahedral charge deficit exist in the formed samples. Moreover, the octahedral occupancies are higher than two (2.15 and 2.07 at 20 and 80 MPa respectively). In these experimental conditions, the synthetic smectites are mixtures between montmorillonite, beidellite and saponite.At 120 MPa and for a Mg content of 0.25 or higher than 0.33, the synthetic products were also mixtures of smectites. Tetrahedral charge deficits of 0.11, 0.11 and 0.15 were found for Mg contents of 0.25, 0.50 and 0.60 respectively. The octahedral occupancy was also higher than 2.00.A montmorillonite phase with only octahedral charges and an octahedral occupancy near 2.00 was synthesized for a Mg content of 0.33 and at pressures equal to or higher than 120 MPa. This low charge reference smectite shows a very low amount of accessory minerals and an octahedral charge deficit only created by the presence of magnesium in the structure. This montmorillonite can be compared structurally to the most studied natural one: the montmorillonite SWy-2 from Wyoming.

Solid-state NMR study of modified clays and polymer/clay nanocomposites

Intercalation of surfactant and polymer chains between the clay platelets gives rise to molecular ordering that changes both the chain conformation and mobility with respect to the bulk phase. As a local probe, nuclear magnetic resonance is particularly suited for such investigations, and this review reports the main results obtained in the solid state. The properties of the modified clays are studied as a function of the surfactant loading, the nature of the head group, and the length of the hydrocarbon chain(s). The structure and charge of the mineral also influence the behaviour of molecules in the gallery space. Among papers on polymer/clay nanocomposites, those dealing with poly(ethylene oxide) have been studied in particular, using a multinuclear approach. Natural clays often contain paramagnetic species such as Fe(III) that perturb the nuclear magnetic resonance (NMR) relaxation processes and can prevent observation of appropriate NMR data. Accordingly, most organic/clay hybrids are studied with hectorite or synthetic smectites. However, the paramagnetic effect has also been found useful in characterizing clay dispersion within the polymer matrix of the nanocomposites.

Preparation and properties of transparent poly(methyl methacrylate) nanocomposite films

PMMA nanocomposites based on the synthetic smectite were prepared and characterized. The synthetic smectites, STN and SPN, were effectively hybridized in the PMMA matrix, as confirmed by X-ray diffraction patterns and by transmission electron microscopy, and optical and thermal properties were measured. The morphologies of PMMA nanocomposite films, containing 1–9 wt% with respect to synthetic smectite, were defined as exfoliated-immiscible and exfoliated-intercalated nanocompostites for CHT and CHP series, respectively. The optical properties, such as haze and UV transmittance, were maintained for organoclay loading of 9 wt% for CHP series. Thermal stability evaluated by TGA showed a small increase in the maximum decomposition temperature (T d); however, the coefficient of thermal expansion (CTE) dramatically decreased with the amount of organoclay content. The storage modulus and loss modulus increased with the weight fraction of organoclay.

Direct Surface Force Measurement for Synthetic Smectites Using the Atomic Force Microscope

An atomic force microscope with a colloid probe technique was used to measure forces interacting between smectites, that is, saponite and hectorite. The measured forces fit well to the forces calculated by the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory at separations larger than 4 nm over concentrations of 10-5−10-2 M NaCl and the pH range of 4−10. There is also a good agreement between zeta potential and the electrical double-layer (EDL) potential extracted from the best fit to the DLVO force curve. According to the analysis on the pH dependence of the EDL potential using the Gouy−Chapman−Stern−Graham model, the inner Helmholtz layer capacitance for the good fit of the outer Helmholtz plane potential to the EDL potential required a much lower value for hectorite (∼5 μF/cm2) than that for saponite (∼500 μF/cm2), reflecting the difference in the location of the lattice charge between saponite and hectorite. This provided evidence for the fact that the EDL force for smectites is dominated by the location of the charges as well as the density of the charges. Extra short-range repulsion was observed at separations below ∼3 nm with increasing NaCl concentration and pH and then disappeared with decreasing pH or the NaCl concentration. The short-range repulsion was extracted as a double-exponential function, that is, F/R = A1 exp(−D/D1) + A2 exp(−D/D2), by subtracting the DLVO forces from the measured forces in the same way as the study with a surface force apparatus (Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531). The values of two decay lengths (D1 and D2) for smectites showed a very similar result to those for muscovite mica rather than silica. It was also found that the sum of the force constants (A1 + A2) was closely related to the density and structure of the lattice charge.

Electrokinetc Study of Synthetic Smectites by Flat Plate Streaming Potential Technique

ζ potentials of synthetic smectites with different locations of lattice charges, i.e., saponite and hectorite, were investigated as functions of pH and NaCl concentration by a flat plate streaming potential technique where two smectite-coated plates were separated by a Teflon gasket to form a slit channel. The lattice charge of saponite originates from the isomorphous substitution of Si4+ by Al3+ ions in tetrahedral silica sheets, whereas the lattice charge of hectorite is caused by the isomorphous substitution of Mg2+ by Li+ ions in octahedral magnesia sheets. The ζ potential for saponite was more sensitive to the pH of electrolyte solution than that for hectorite in acidic solution. Such a difference in pH dependence of the ζ potential could be explained by the difference in the location of the lattice charges between saponite and hectorite as a result of an analysis of the inner Helmholtz layer capacitance by using the Gouy−Chapman−Stern−Graham (GCSG) model with a single-site dissociation of surface groups. On the other hand, the dependence of the ζ potential on NaCl concentrations could not be fully interpreted by the GCSG model with a single value of the outer Helmholtz layer capacitance, which suggests a shift of the shear plane and/or a change in the dielectric constant of the outer Helmholtz layer with changing NaCl concentration.

Stabilisation d'ions Fe 2+ dans le réseau de smectites ferrifères de synthèse

Smectites were synthesized from gels containing the cations: Si 4+, Al 3+, Fe 2+, Mg 2+, Na + or Si 4+, Al 3+, Fe 2+, Na +. Gels were prepared at 25°C in reducing conditions (glove box with nitrogen circulation, oxygen partial pressure = 10 −6 atm). During synthesis, performed at 200°C for 41 days in teflon coated steel bombs put in a furnace, gas exchanges occur between bombs and laboratory atmospheres. The synthetic smectites are ferric beidellites with a Fe 2+/Fe total ratio ranging from 0 to 0.2. The octahedral Fe 2+ ions in smectites appear stable over several months.

Ga/Al Substitutions in Synthetic Kaolinites and Smectites

The Ga for A1 substitution in kaolinites and smectites was studied using clay synthesis from initial gels having an (AI + Ga)/Si ratio of kaolinites and MGa = Ga/(Ga + A1) ratio ranging from 0 to 1. Only kaolinite was obtained for MGa in the range 0-0.10. For higher MGa, synthesized clays were both kaolinite and beidellite, or pure beidellites. The evolution of b cell parameters, and the appearance of a vAI-OH-Ga absorption band at 3600 cm -1 in FFIR spectra, proves the progressive substitution of AI by Ga in kaolinite. The low substitution of AI by Ga in kaolinite can be correlated with the large difference in ionic radii between AI 3+ and Ga 3§ which causes a large increase in cell dimensions. By contrast, the incorporation of Ga in smectites is easy and can be complete.

Electrodes Modified with Synthetic Clay Minerals: Electron Transfer between Adsorbed Tris(2,2‘-bipyridyl) Metal Cations and Electroactive Cobalt Centers in Synthetic Smectites

Clay-modified electrodes (CMEs) were prepared with two synthetic cobalt smectites. Large increases in the peak currents were observed in the cyclic voltammograms of tris(2,2‘-bipyridyl) metal cations ([M(bpy)3]2+, M = Os, Fe, or Ru) adsorbed in these CMEs when the potential was scanned high enough to oxidize redox active cobalt centers in the lattices of the synthetic clays. The cathodic charges obtained in the cobalt clay were 18.6, 25.7, and 12.3 times larger than those obtained in electrodes modified with a natural montmorillonite for the osmium, ruthenium, and iron bipyridyl cations, respectively. These increases were attributed to electron transfer between the intercalated cations and the oxidized cobalt centers. The redox active cobalt centers in the clay lattices functioned as relays, transferring electrons between adsorbed cations that otherwise would be electrochemically inactive and the conductive substrate. In some cases, nearly 100% of the cations present in the cobalt CMEs participated in the...

Early-stage smectites from pyroclastic rocks of Almería (Spain)

In the volcanic region of Cabo de Gata (Almería, Spain) bentonite deposits were formed from pyroclastic rocks. There is evidence that the smectite composition changed from Fe-rich to Al-rich members as the degree of hydrothermal alteration increased. Because true initial stages of alteration do not occur in this region, artificial hydrolysis of rocks is needed to obtain early-stage smectites. Three typical volcanic rocks were hydrolyzed in Soxhlet extractors at 82°C for 1457 hr. with 243–302 l of percolating water. In these conditions, 11–12% of smectite was formed. Mineralogical and chemical data of synthetic smectites reveal that they are Fe-bearing beidellites. In fact, the mean value of octahedral Fe of synthetic smectite is 0.85 whereas for the natural one is 0.21. The tetrahedral charge is close to 0.6 and 0.2, respectively. The differences between the synthetic and the natural processes of formation are strictly kinetic and do not seem to affect either the mechanism of the hydrolytic reaction or the composition of neoformed smectites.

Synthesis of New Trioctahedral Mg-Smectite

Abstract New trioctahedral Mg-smectites were synthesized hydrothermally from homogeneous Mg–Si precipitates having a wide range of compositions, i.e., from Mg=3.2 to 6, vs. Si=8 in the atomic ratios. Dispersed solutions of the synthetic smectites formed transparent gels possessing good rheological properties.

Characterization of smectites synthesised from zeolites and mechanism of smectite synthesis

AbstractSmectites were synthesised hydrothermally from zeolites in an Mg-rich brine at 300°C under a confining pressure of 30 MPa and characterized by XRD using the Greene-Kelley Li-saturation test and the K+-saturation/glycolation test. The synthetic smectites were found to be dioctahedral with only tetrahedral negative layer charge; the layer charge was also deduced to be low and they are classified as beidellites. The mechanism of formation includes H+ saturation of zeolites by the hydrolysis of Mg2+ under hydrothermal conditions, followed by decomposition of the zeolites and recrystallization to beidellites. These results show that solution chemistry is an important factor affecting stability of zeolites under hydrothermal conditions.

Swelling Properties of Synthetic Smectites in Relation to Lattice Substitutions

Abstract Synthetic clays were prepared hydrothermally at 300° to 350°C and 15,000 psi from gels with compositions calculated to give a series of octahedrally and tetrahedrally substituted, 2:1-type layer silicates with charges of 0.5N, 1N, 1.5N, 2N, 2.5N, and 3N, where N equals the “normal” layer charge of montmorillonite. Besides montmorillonites and beidellites, the products also contained other components, kaolinite, saponite, paragonite and possibly amorphous materials, depending on the initial gel compositions. Cation exchange capacities of <2µ fractions treated for removal of amorphous constituents were within the narrow range of 0.95–1.35 meq per g rather than 0.45–2.70 meq per g as calculated from initial gel compositions. Comparisons between the expanding properties of these materials showed that the beidellites expanded less readily than the montmorillonites upon solvation with glycerol vapor and tended to give single interlayer complexes. These differences were most pronounced when Mg was the saturating ion, although they were also apparent with Ca. The K-saturated montmorillonites gave greater expansion than the beidellites on hydration at 100 per cent r. h. The montmorillonites also exhibited greater rehydration than the beidellites after heating at 400°–500°C. Beidellite exhibits properties of expansion that are intermediate between those of montmorillonite and vermiculite. The results of this investigation are consistent with the existence of somewhat stronger ionic attractions in the beidellites than in the montmorillonites. The source of the charge thus has an effect upon the properties of expansion.