Influence Of Layer Charge Research Articles

Influence of layer charge on hydration properties of synthetic octahedrally-charged Na-saturated trioctahedral swelling phyllosilicates

Smectite hydration impacts dynamical properties of interlayer cations and thus the transfer and fate of H2O, contaminants, and nutriments in surficial environments where this ubiquitous clay mineral is often one of the main mineral components. The influence of key crystal-chemical parameters, such as the amount of charge or the presence of fluorine, rather than hydroxyl groups, in smectite anionic framework, on hydration, organization of interlayer species, and related properties has been described for tetrahedrally substituted trioctahedral smectites (saponites). Despite the ubiquitous character of octahedrally substituted smectites, that make most of the world bentonite deposits, the influence of charge location on smectite hydration properties has not received similar attention. A set of octahedrally substituted trioctahedral smectites (hectorites) with a common structural formula NaxMg6-xLixSi8.0O20(OH)4 and a layer charge (x) varying from 0.8 to 1.6 was thus synthesized hydrothermally. The distribution of charge-compensating Na+ cations and of associated H2O molecules was determined experimentally from the modeling of X-ray diffraction data obtained along water vapor desorption isotherms. Consistent distributions of charge-compensating cations and of associated H2O molecules were also computed from GCMC simulations as a function of layer charge. Interlayer H2O contents [2.5–5.5 and 8.0–10.0 H2O molecules per O20(OH)4 for 1W and 2W hydrates, respectively] are similar in all Na-saturated smectite samples, independent of the location and amount of their layer charge. In contrast to synthetic saponite, for which stability of most hydrated layers was increased by increasing layer charge, the stability of synthetic hectorite hydrates is only marginally affected by layer charge. Consistently, the layer-to-layer distance of Na-saturated hectorite 2W (and 1W) layers is independent of layer charge (15.10–15.65 Å and 12.0–12.6 Å, respectively). The contrasting hydration behavior of synthetic Na-saturated saponite and hectorite is likely due to different electrostatic attraction between the 2:1 layer and interlayer cation, the charge undersaturation of O atoms at the surface of hectorite 2:1 layer being more diffuse compared to saponite. Combined with previous results on saponites, the present data and sample set provides key constraints to assess the validity of force fields simulating clay-water interactions for an unmatched variety of smectite with contrasting locations and amounts of layer charge deficits.

Influence of layer charge and charge location on the swelling pressure of dioctahedral smectites

Swelling pressure of dioctahedral smectites in the montmorillonite – beidellite series was investigated by molecular dynamics simulations. The pressure was found to correlate inversely with the magnitude of the layer charge in the range of −0.5 to −1.0 per unit cell. The beidellite type smectites were found to have lower swelling pressure than the montmorillonite type smectites. A clear effect of the type of interlayer cations on the swelling pressure was found. The sodium smectites sustained significant pressure even at longer interlayer distances, while in calcium smectites the pressure decreased soon after the initial swelling. The simulation results are in good agreement with experimental observations and provide a tool for predicting macroscopic swelling behavior in smectites.

Influence of layer charge, hydration state and cation nature on the collective dynamics of interlayer water in synthetic swelling clay minerals

The dynamical and structural properties of water in clay interlayers can be considered as those of a bidimensional confined fluid with significant anisotropy as revealed by measurements and simulations of water diffusion at the molecular scale. The present paper investigates water collective motions by using coherent inelastic neutron scattering (INS) on deuterated samples of synthetic saponite samples with low (0.7 per unit cell) and high (1.4 per unit cell) charge, Na+ and Ca2+ interlayer cations, various hydration states and at different temperatures. In all cases, two excitations are needed to fit the inelastic signals. The low-energy excitation of the collective dynamics of interlayer water evidences a clear anisotropy and displays a behavior significantly different from that of bulk water, this difference being stronger in the direction perpendicular to the clay layer. In the parallel direction, experimental results and molecular dynamics simulations show that the features of the low-energy excitation and in particular, the energy corresponding to the nondispersive component are linked to the structuration and rigidity of the interlayer water network. Indeed, this position displays an upward energy shift with increasing confinement and decreasing temperature. In contrast, data obtained in the perpendicular direction, display a dispersive behavior for Q values ≥0.5Å−1 in contrast with what is observed for bulk water and in the parallel direction. Such a behavior must somehow be linked to a correlation between the motions of water molecules located in adjacent interlayer space that is likely transmitted through the solid clay layer.

Influence of Layer Charge and Modifier on Wettability of Organo-Montmorillonite

To study the influence of layer charge and modifier on wettability plays an important role on the diagnosis of oil reservoir. In this paper, two quaternary ammonium salts (QASs) with different chain-length were used singularly or in compound form to modify the reduced-charge montmorillonites (RCMs). The wettability alterations of the organo-RCMs for deionized water and cyclohexane were studied based on capillary rise method. The results showed that RCMs singularly modified by QASs show different wettability characteristics. Moreover, there is a synergistic effect on the wettability when the RCMs are compound modified by two QASs and the organo-RCMs turn to more hydrophilic. The results of this work also indicated that it is possible to regulate the wettability of the RCMs by choosing different recipe of QASs, which is helpful to provide theoretical guidance for the oil production.

Hydration Properties and Interlayer Organization of Water and Ions in Synthetic Na-Smectite with Tetrahedral Layer Charge. Part 2. Toward a Precise Coupling between Molecular Simulations and Diffraction Data

A specific methodology was developed to collate the interlayer configurations resulting from Grand-Canonical Monte Carlo (GCMC) simulations with experimental X-ray and neutron diffraction data for two synthetic Na-saturated saponites having contrasting layer charge. Numerical simulations were performed assuming different existing sets of atomic partial charge and Lennard-Jones parameters for clay and water. For each parameter set and for the two samples in both the mono- and bihydrated states, the water contents resulting from GCMC simulations were first compared to water vapor desorption gravimetry data. The density distributions of interlayer species were then used to generate 00l intensities that were compared to X-ray and neutron diffraction data, the latter being recorded on both hydrogenated and deuterated specimens. The CLAYFF model [Cygan et al. J. Phys. Chem. B2004, 108, 1255] is shown to better account for water content and organization compared to the model developed by Skipper et al. (Clays Clay Miner.1995, 43, 285) and modified by Smith (Langmuir1998, 14, 5959). However, diffraction patterns calculated for bihydrated samples from CLAYFF simulations did not match satisfactorily the diffraction data. Lennard-Jones parameters were thus modified for oxygen atoms from the clay layer. When combined with the SPC/E water model, this modified version of CLAYFF allows matching experimental water contents and fitting the complete set of diffraction data. Relevant information may thus be derived on the influence of layer charge on the orientational properties of interlayer water molecules which differs for the different clay models. Finally, the approach used in the present study proved powerful for assessing atomic interaction parameters considered for computational simulations.

Hydration Properties and Interlayer Organization of Water and Ions in Synthetic Na-Smectite with Tetrahedral Layer Charge. Part 1. Results from X-ray Diffraction Profile Modeling

The dehydration of two Na-saturated synthetic saponites with contrasting layer charge was studied by modeling the X-ray diffraction (XRD) patterns recorded along a water vapor desorption isotherm. The interlayer configurations used to reproduce the XRD data over a large angular range include Na+ cations located in the interlayer midplane and H2O molecules normally distributed about one or two main positions for mono- and bihydrated layers, respectively. Although strongly reduced in comparison to natural smectites, hydration heterogeneity was systematically observed for these synthetic saponites, especially along the transition between two hydration states. Using improved models for the description of the interlayer organization, the influence of layer charge on the structure of interlayer water can be precisely assessed. In addition, the comparison with water contents obtained from water vapor gravimetry experiments allows discriminating the relative contributions of H2O molecules from 1W and 2W interlayers (crystalline water) and from the pore space network.

Investigation of dioctahedral smectite hydration properties by modeling of X-ray diffraction profiles: Influence of layer charge and charge location

Hydration of the <1 μm size fraction of a high-charge montmorillonite (Clay Minerals Society Source Clay SAz-1), and of low- and high-charge beidellites (Source Clays SbId-1 and SbCa-1, respectively) was studied by modeling of X-ray diffraction patterns recorded under controlled relative humidity (RH) for Sr- and/or Ca-saturated specimens. The influence of layer charge and charge location on smectite hydration was studied. Distribution of layers with different hydration states (dehydrated-0W, monohydrated-1W, bihydrated-2W, or tri-hydrated-3W) within smectite crystals often leads to two distinct contributions to the X-ray diffraction pattern, each contribution having different layer types randomly interstratified. Structure models are more heterogeneous for beidellite than for montmorillonite. For beidellite, two distinct populations of particles with different coherent scattering domain sizes account for the heterogeneity. Increased hydration heterogeneity in beidellite originates also from the presence of 0W (non-expandable) and of 1W layers under high relative humidity (RH) conditions. Similarly, after ethylene-glycol (EG) solvation, some beidellite layers incorporate only one plane of EG molecules whereas homogeneous swelling was observed for montmorillonite with the systematic presence of two planes of EG molecules. For montmorillonite and beidellite, the increase of layer charge shifts the 2W-to-1W and the 1Wto- 0W transitions toward lower RH values. For all samples, layer thickness of 0, 1, and 2W layer types was similar to that determined for low-charge SWy-1 montmorillonite (Source Clay SWy-1), and no change of layer thickness was observed as a function of the amount or of the location of layer charge. However, layer thickness increased with increasing RH conditions.

Influence of layer charge and charge distribution of smectites on the flow behaviour and swelling of bentonites

The influence of layer charge and charge distribution of dioctahedral smectites on the rheological and swelling properties of bentonites is examined. Layer charge and charge distribution were determined by XRD using the LayerCharge program [Christidis, G.E., Eberl, D.D., 2003. Determination of layer charge characteristics of smectites. Clays Clay Miner. 51, 644–655.]. The rheological properties were determined, after sodium exchange using the optimum amount of Na 2CO 3, from free swelling tests. Rheological properties were determined using 6.42% suspensions according to industrial practice. In smectites with layer charges of − 0.425 to − 0.470 per half formula unit (phfu), layer charge is inversely correlated with free swelling, viscosity, gel strength, yield strength and thixotropic behaviour. In these smectites, the rheological properties are directly associated with the proportion of low charge layers. By contrast, in low charge and high charge smectites there is no systematic relation between layer charge or the proportion of low charge layers and rheological properties. However, low charge smectites yield more viscous suspensions and swell more than high charge smectites. The rheological properties of bentonites also are affected by the proportion of tetrahedral charge (i.e. beidellitic charge), by the existence of fine-grained minerals having clay size, such as opal-CT and to a lesser degree by the ionic strength and the pH of the suspension. A new method for classification of smectites according to the layer charge based on the XRD characteristics of smecites is proposed, that also is consistent with variations in rheological properties. In this classification scheme the term smectites with intermediate layer charge is proposed.

Influence of layer charge on the hydroxyl stretching of trioctahedral clay minerals: A vibrational study of synthetic Na- and K-saponites

To understand the behavior of octahedral hydroxyl-stretching modes (νOH) as a function of increasing tetrahedral layer charge, infrared and Raman spectroscopic studies were carried out on a series of synthetic saponites. The nature of the monovalent interlayer cation (Na or K) and the hydration of the samples were shown to strongly influence spectral features. In the case of hydrated Na-samples, the νOH position shifts from 3678 to 3686 cm−1 with increasing layer charge. For hydrated K-saponites, two components are observed around 3720 and 3680 cm−1, assigned to OH perturbed by compensating cation and free OH groups, respectively. In the case of dehydrated samples both K and Na-saponites exhibit two components at 3677 and 3720 cm−1. A linear relationship between the relative area of these two νOH components and the layer charge was then evidenced. To assign the features obtained for hydrated Na-samples in the OH-stretching modes domain, the Raman spectra were decomposed using four νOH components around 3675, 3680, 3685, and 3690 cm−1. The evolution with layer charge of the relative proportions of these four components can be described by taking into account the location of hydrated cations, related to the distribution of tetrahedral substitutions. Such an assumption was confirmed using a simple statistical model that yields four populations of OH groups in the octahedral layer of hydrated Na-saponites.

Influence of Layer Charge on the Catalytic Activity of Mildly Acid-Activated Tetramethylammonium-Exchanged Bentonites

SWy-2 (Wyoming), STx-1 (Texas), and SAz-1 (Cheto, Arizona) montmorillonite and Sap-Ca saponite (California) were treated with different amounts of 12 M HCl and 1 M tetramethylammonium (TMA(+)) chloride solution at room temperature. The catalytic activity of these hybrid acid-activated organoclays (AAOCs) was directly measured using the isomerization of alpha-pinene at 120 degrees C for 1 h to yield camphene, limonene, and other minor products and compared to clays acid-treated in the absence of TMA(+) cations. The total conversion for the isomerization of cc-pinene was excellent for AAOCs formed from SWy-2 (88%) and STx-1 (73%), moderate for Sap-Ca (50%), and low for SAz-1 (20%). Samples treated with TMA(+) alone did not exhibit any marked catalytic activity. Acid-treated Sap-Ca and STx-1, which contained no TMA(+) cations were also effective catalysts for the isomerization process. TMA(+) cations were unexpectedly resistant to exchange by protons.

Polycation-Exchanged Clays as Sorbents for Organic Pollutants: Influence of Layer Charge on Pollutant Sorption Capacity

The uptake curves for the adsorption of a polycation ([CH2CH(OH)CH2N(CH3)2]n+n(Cl)n−n) onto a low iron Texas bentonite, WL, have been determined and compared with those onto two source smectites, SWy-1 and SAz-1. The polycation exhibited a marked affinity for all the smectite surfaces but polycation uptake was maximized on the fine fractions of Na+–WL. Variable temperature X-ray diffraction of the clay polycation complexes showed that the polycation was segregated into different interlayers at loadings below 30% of the CEC. The adsorption of benzene onto TMA+-exchanged SWy-1, SAz-1, and WL showed that, despite having a comparable layer charge to SWy-1, WL was as poor a sorbent for benzene as SAz-1, which has a much higher layer charge. The adsorption ofp-nitrophenol (PNP) from aqueous solution onto the TMA+-exchanged sorbents exhibited the same trends, although a smaller amount was adsorbed due to the greater size of PNP. The amount of PNP adsorbed by polycation-treated SWy-1 depended upon the polycation loading, and PNP uptake maximized at loadings of 28 and 46 mg (g clay)−1. At higher loadings (80, 102, and 147 mg (g clay)−1) the uptake curves became linear and the PNP uptake decreased with increased polycation loading. No polycation-treated clay was as effective as TMA+-exchanged SWy-1 for the removal of PNP from water.

Synthesis and Properties of Terephthalate and Benzoate Intercalates of Mg−Al Layered Double Hydroxides Possessing Varying Layer Charge

The influence of layer charge on the intercalation of terephthalate and benzoate anions in MgAl layered double hydroxides has been studied. It is found that, under the synthesis conditions used here, incorporation of the anion is dependent upon the Mg:Al ratio of the host sheets. Pure terephthalate and benzoate intercalates are formed for a ratio of 2 or less from both chloride and nitrate solutions. At higher values of layer charge competitive incorporation of the counteranion present in the solution (i.e., chloride or nitrate) occurs with decreased incorporation of organic anions at ratio 3 or higher. Two extreme orientations are observed for the organic anion within the galleryeither vertical (i.e., perpendicular to the layers) or horizontal. In the case of terephthalate a monolayer arrangement exists, whereas for benzoate the vertical arrangement is a bilayer and the horizontal a monolayer. Whether a vertical or horizontal orientation is adopted depends upon the charge on the layers, the temperature, and the degree of hydration of the sample.

Calculations on a model intercalate containing a single layer of water molecules: a study of potassium vermiculite, K 2x Mg 6 (Si 4 _ x Al x ) 2 O 20 (OH) 4 .(H 2 O) 4 , for 1 ≤ x ≤ 0

The largest body of information concerning the swelling of clay minerals on exposure to moisture has been accumulated for the vermiculites and the smectites. The experimental findings for a range of phyllosilicates possessing various interlayer charges have shown that for silicates with zero charge on the interlayer (for example talc or pyrophyllite) there is no intercalation of water molecules within the sheets. Maximum water uptake, in this class of minerals, is found to take place in those having a fairly low interlayer charge (for example hectorites, montmorillonite). As the layer charge is increased further uptake falls to zero for fully saturated interlayers (for example beidellite and vermiculites). This paper represents a comprehensive study from a theoretical standpoint of some of these observations and enables comment to be made on the influence of layer charge on the intercalation process in addition to the examination of other features that have been held to be important (for example tetrahedral or octahedral site substitution) so enabling discussion of the work of the experimenters in this area. Calculations are reported for the electrostatic energy of a potassium vermiculite, K 2x Mg 6 (Si 4 _ x Al x ) 2 O 20 (OH) 4 .(H 2 O) 4 , which contains a single layer of water molecules intercalated into an expanded phlogopite. The crystal structure adopted is such that for x = 1, K 2 Mg 6 (Si 3 _ x Al) 2 O 20 (OH) 4 .(H 2 O) 4 , the oxygen atoms of the water molecules form a honeycomb-like network with the potassium ions in the holes. For x = 1, the calculation is straightforward and is based on our ‘generic’ approach reported previously. This approach also enabled us, in one single calculation, to study the influence of interlayer K + ions placed at various levels between the water layer and the silicate layer. It appears that the most favourable position depends on the charge distribution within the water molecule. The centrally positioned K + ion represents the most stable configuration when the charge on the hydrogen atom of the water q H " ≥ 0.6. For q H " below this value the most stable position for the K+ ions is between the water layer and the plane of the nearest oxygens in the silicate sheet. For x < 1 specific considerations had to be given to the likely arrangement of interlayer K + ions and water molecules (taken to be in the plane of the water layer), implying separate calculation of their interaction energies. The intercalation energy comprises the energy involved in the process of expanding a phlogopite from 10 A + to 12.5 A and arranging water molecules in the gap. It emerges that the intercalation is mainly determined by the expansion energy. Both the expansion energy and the arrangement energy depend on the silicate layer charge, which is determined by x. For large x no intercalation occurs owing to the large amount of energy required for expansion. For x very close to zero no intercalation occurs owing to the repulsive interaction of the water layer with the silicate layer. For intermediate values of x intercalation occurs owing to the large K -H 2 O interaction energy.