Reduced-charge Montmorillonites Research Articles

Functionalization of reduced montmorillonite as an adsorbent toward aqueous anion: role of layer charge density

Montmorillonite (Mt) is widely used in various fields due to its abundant resources and excellent adsorption performance, while the hydration and adsorption properties of raw or functionalized Mt are affected by its intrinsic property, layer charge density (LCD). Herein, a series of reduced-charge montmorillonites (RC-Mt) with high, medium, and low layer charges were prepared by the method of lithiation and microwaving Ca-Mt. Meanwhile, the effects of artificially regulated layer charge density on Mt’s structure, hydration properties, and the removal ability of aqueous anions by the poly (diallyl dimethylammonium chloride) functionalized Mt (PDDA-Mt) were also investigated. Results showed that the LCD of Mt decreased with the increasing microwave time or power. In addition, the physicochemical properties of RC-Mt which included cation exchange capacity (CEC), colloidal index, and swelling properties decreased as the LCD decreased. Furthermore, the adsorption capacity of Mt for NO3- was significantly improved following the PDDA functionalization. The kinetic modeling suggests that the Pseudo-first-order model better characterized the adsorption of Ca-Mt and Li-Mt than the Pseudo-second-order model. On the other hand, the good fit of the Pseudo-second-order model implied that the adsorption process of nitrate on RC-Mt and PDDA-Mt is mainly driven by chemisorption control. The adsorption thermodynamics indicated that the adsorption process was spontaneous. Furthermore, the FTIR and XPS results suggested that in addition to diffusion and electrostatic interactions, the adsorption mechanism of PDDA-Mt was mainly attributed to the ion exchange between chloride and nitrate on the quaternary ammonium group of PDDA.

Spectral behaviour of TMPyP/layered silicate hybrid nanomaterials in aqueous dispersions of reduced-charge montmorillonites

Spectral properties of tetracationic organic dye, meso-tetrakis(N-methylpyridinium-4-yl) porphyrin adsorbed on layered silicate templates of various layer charge were investigated. A series of reduced charge Nanocor montmorillonites was used as host matrices. The study primarily focuses on the influence of the layer charge on the emission characteristics and photoactivity of prepared materials. Usually, adsorption of dyes such as porphyrins leads to significant changes in their spectral characteristics with the main goal being the preservation of their photoactivity. The spectral changes can be explained in terms of e.g. structural changes (flattening, protonation), as well as molecular aggregation. Hence, the aim of this study is to clarify the way to influence the spectral properties and the subsequent photoactivity of incorporated molecules via a strategy of appropriate combination of a photoactive component with specific inorganic templates.

Measuring the Layer Charge of Dioctahedral Smectite by O—D Vibrational Spectroscopy

AbstractLayer charge (LC) is a fundamental property of smectite but its measurement remains challenging and tedious to apply on a high-throughput basis. The present study demonstrates that the position of a sharp, high-energy O—D stretching band of adsorbed D2O (νO—D, at ~2686–2700 cm−1), determined by infrared spectroscopy, correlates with LC and provides a simple method for its measurement. Twenty nine natural dioctahedral smectites and 14 reduced-charge montmorillonites with LC determined previously by different methodologies were saturated with D2O and examined by attenuated total reflectance infrared spectroscopy (ATR-IR). The samples included smectites in Mg, Ca, Na, Li, K, and Cs forms and covered the full range of the smectite LC (0.2 to 0.6 e per formula unit). Statistically significant correlations were found between νO—D and LC values determined with each of the two main methods of LC determination: the structural formula method (R2 = 0.96, σ = 0.02, ~0.2 < LC < 0.6) and the alkylammonium method (R2 = 0.92, σ = 0.01, 0.27 < LC < 0.37). These correlations were based on Li- and Na-saturated smectites, respectively, but other cationic forms can be employed provided that the exchangeable cations are of sufficiently high hydration enthalpy (e.g. Mg2+ or Ca2+, but not K+ or Cs+). The new method is fast, low-cost, implemented easily in laboratories equipped with ATR-FTIR, and applicable to samples as small as ~5 mg.

Adsorption of phenols on reduced-charge montmorillonites modified by bispyridinium dibromides: Mechanism, kinetics and thermodynamics studies

Abstract A series of reduced-charge montmorillonites (RCMs) were modified by hexamethylene bispyridinium dibromides (HMBP), then used to remove phenols from aqueous solution. The effects of concentration of HMBP (C), clay layer charge, contact time (t), temperature (T) and pH were investigated using a batch technique. The results implied that the clay layer charge had significant influence on phenol adsorption and the optimum conditions were as follows: C of 1.0CEC, 120 min, 25 °C and pH of 6.0. The adsorption mechanism of phenol on the HMBP-Mt was studied by comparing the adsorption characters of substituted phenols, the results indicated that the uptake of phenols onto HMBP-Mt decreased in the order: phenol > p-chlorophenol > p-methylphenol > p-nitrophenol. This indicated that the π–π polar interaction existing between the pyridine ring and benzene ring in phenols played a dominated role in the adsorption of phenols. The adsorption kinetics demonstrated that the adsorption of phenol onto HMBP-Mt followed the pseudo-second-order model. The adsorption isotherms at the temperatures of 25, 40 and 55 °C were determined and modeled using four different models. The best-fitted adsorption isotherm models were found to be in the order: Langmiur ≈ Redlich-Peterson > Temkin > Freundlich. The thermodynamic study of adsorption process showed that the adsorption of phenol with HMBP-Mt was carried out spontaneously, and the process was randomly increasing and exothermic in nature.

Effect of Layer Charge on the Formation of Polymer/Layered Silicate Nanocomposites: Intercalation of Polystyrene

Reduced-charge montmorillonites (RCMs), layered silicates of variable charge density, were modified with alkylammonium (AA) cations to prepare organophilic materials with variable surface activities. The intercalation of polystyrene (PS) from melts was investigated by X-ray diffraction. Slight differences in layer charge density significantly affected the activity of organoclays. The specimens based on high charge RCMs, which had been premodified with long-chain AA cations, exhibited the highest activities to intercalate PS. Expansion of some organically modified silicates based on small AA cations in PS melts led to higher d001 values. On the other hand, the presence of the largest AA cations significantly reduced free space for polymer intercalation. This study may help with designing strategies for choosing appropriate inorganic fillers for polymer nanocomposites to control their structure and properties. Understanding of the layer charge effect can help to explain some controversial results in the literature, when seemingly similar materials of organoclays exhibited different properties and interactions with polymers.

Modification of reduced-charge montmorillonites by a series of Gemini surfactants: Characterization and application in methyl orange removal

The influences that the spacer chain length of Gemini surfactants and clay layer charge have on the structures and sorption characteristics of organoclays have been investigated. Organoclays were obtained by modifying a series of reduced charge montmorillonites (RCMs) using three Gemini surfactants with different spacer length. And their structures and sorption characteristics for methyl orange (MO) were examined. It was suggested that the amount, spacer length of Gemini surfactant and clay layer charge had significant effects on the microstructure of the organoclays. The adsorption experiments results claimed that the uptake of MO onto organoclays was in the order: 16-4-16-Mt>16-8-16-Mt>16-6-16-Mt, while it increased with increasing clay layer charge. The adsorption isotherms of MO onto the organoclays could be best described by Langmuir equation, and the adsorption kinetic was in good agreement with the pseudo-second-order model. Thermodynamic parameters demonstrated that the sorption process was spontaneous and endothermic in nature. This work will provide a deep insight into the interaction of Gemini-modified clays and MO, which pave the way for their practical applications in anionic dye adsorption.

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.

Structures and Wettability Alterations of a Series of Bispyridinium Dibromides Exchanged with Reduced-Charge Montmorillonites

Capillary rise tests were performed to investigate the influences that the structure of the modifier and layer charge of clay had on wetting properties of organoclays, which were prepared by ion exchange using bispyridinium dibromides (BPs) with different spacer length and the reduced-charge montmorillonites (RCMs). Their structures were examined by Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), and nitrogen adsorption–desorption isotherms. The results indicated that BPs had been successfully intercalated into interlayers and lay in the monolayer. The d001 basal spacing of organo-RCMs increased with the spacer length of BPs increasing, whereas it decreased gradually as the layer charge of the RCMs was decreased, independent of the type of BPs. Whether the organic modification made the Brunauer–Emmett–Teller (BET) surface area increase or decrease depended upon the size of the organic cations and the layer charge of the clays. The wettability alterations of the organo-RCMs for deionized water and cyclohexane were also compared. Both the spacer length of BPs and the layer charge of RCMs had important effects on the relative wettability of organo-RCMs. The hydrophilicity of organo-RCMs was increased with the spacer length of BPs increasing, namely, in the order as follows: C2-2Py-RCMs < C6-2Py-RCMs < C10-2Py-RCMs. In addition, the hydrophobicity was increased with a decrease in the layer charge. The results of this work were supposed to provide some reference information for regulating the wettability of the organo-RCMs by simultaneously controlling the type of modifiers along with the layer charge characteristics, to provide theoretical guidance for the favorable change in reservoir wettability.

Wettability studies of reduced-charge montmorillonites modified by quaternary ammonium salts using capillary rise test

A series of montmorillonites with systematically reduced layer charge were prepared by a standard method of different fractions of Li+-fixation in montmorillonite, which could be used to mimic an oil reservoir. Three kinds of quaternary ammonium salts (QASs) with different cationic charges and structure sizes 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 and were compared using the Lipophilic to Hydrophilic Ratio (LHR). The results indicate that RCMs singularly modified by three kinds of QASs show different wettability characteristics, whereas compound modified by QASs have a synergistic effect on the wettability. Moreover, the compound type of the QASs plays a more important role on LHR values than the mix proportion. The results of this work also indicate that it is possible to regulate the wettability of the organo-RCMs by simultaneous control of the type, the compound ratio of QASs and the layer charge characteristics, which is helpful in providing theoretical guidance for the oil production.

Restricting layer collapse enhances the adsorption capacity of reduced-charge organoclays

By restricting layer collapse, and increasing the exposure of siloxane surfaces, in the interlayer space of organoclays, their capacity for adsorbing hydrophobic organic contaminants has been enhanced. The organoclays were prepared by replacing a proportion (20–80%) of the Li+ ions, occupying interlayer sites in montmorillonite, with tetramethylammonium (TMA) ions. The TMA-exchanged samples were then heated at 200°C for 12h to induce migration of most interlayer Li+ ions into the silicate layers, and as a result of which the layer charge was reduced. Finally, the remaining Li+ cations in the reduced-charge montmorillonite were exchanged with TMA. The structural and adsorptive characteristics of the novel reduced-charge organoclays (N-TMA-Mt) are compared with those of their traditional counterparts (T-TMA-Mt), obtained by direct intercalation of TMA into reduced-charge montmorillonites. As layer charge decreased, both the specific surface area and adsorption capacity (for nitrobenzene) of T-TMA-Mt increased to a maximum, and then declined. In the case of N-TMA-Mt, however, both parameters increased as layer charge decreased. When the layer charge of N-TMA-Mt decreased to approximately 60% of the value for the original montmorillonite, the adsorption capacity of the sample was greater than that of T-TMA-Mt. XRD analysis indicates that the layer structure of N-TMA-Mt is more rigid than that of T-TMA-Mt. The above results indicate that the pre-exchanged TMA cations in the interlayer space of N-TMA-Mt act as pillars, restricting layer collapse (during thermal treatment), and increasing the exposure of siloxane surfaces. As a result, the capacity of N-TMA-Mt for adsorbing hydrophobic organic contaminants is superior to that of traditional organoclays.

Structural Characterization of Reduced-Charge Montmorillonites. Evidence Based on FTIR Spectroscopy, Thermal Behavior, and Layer-Charge Systematics

AbstractIn the present study, the gradual layer-charge reduction of two Li-saturated smectites, SAz-1 from Arizona, USA, and FEO-G from Troodos, Cyprus, with octahedral charge of 0.54 electrons per half unit cell (e/huc) and 0.39 e/huc, respectively, was monitored by X-ray diffraction of K-saturated, ethylene glycol-solvated samples, by thermogravimetry-differential thermogravimetry, and by mid- and near-Fourier transform infrared spectroscopy after heating at 80–300ºC. With increasing heating temperature, the layer charge and cation exchange capacity (CEC) of both smectites decreased gradually due to Li fixation. At temperatures &gt;200ºC, ~25% residual CEC was observed, suggesting incomplete Li fixation due to kinetic constraints. Dehydration of the original Li-smectites occurred in two steps, one peaking at ~100ºC and another at 175–180ºC. The latter decreased upon progressive Li fixation and vanished from smectites treated above ~125ºC. Dehydroxylation occurred at 635–640ºC in both smectites and was not affected by Li fixation. The second derivative analysis of the infrared spectra showed that Li fixation was manifested in both smectites by the growth of two new sharp OH-stretching fundamentals at ~3640 and 3670 cm−1 and their overtones at ~7115 and 7170 cm−1. The new bands constitute pairs of fixed energy and relative intensity which grow simultaneously at the expense of the broad OH-stretching and overtone features of the original smectites. Based on this result, Li fixation is suggested to be accompanied by the simultaneous formation of two distinct trioctahedral-like structural OH species, which is compatible with Li+ occupying trans-octahedral vacancies in both smectites.

Fluorescence of reduced charge montmorillonite complexes with methylene blue: Experiments and molecular modeling

The intensity of fluorescence of montmorillonites fully saturated by methylene blue (MB) is very poor due to energy dissipation in MB aggregates. A series of reduced charge montmorillonites (RCM) were prepared from Na-homoionic SWy and Ca homoionic SAz with the aim to decrease the MB aggregation. Fine tuning MB adsorption degree by charge reduction and MB concentration enabled controlled production of different dye species from aggregates via dimers to monomers. It was shown that the intensity of the fluorescence of low-loaded MB–RCM complexes is enhanced by several orders of magnitude with respect to dye-saturated original montmorillonites. XRD analyses, molecular modeling, and diffuse reflectance spectroscopy revealed that low MB-loaded RCMs are very probably adsorbed mainly on the external montmorillonite surface as isolated dye molecules. Such a state cannot be achieved in the solid state without very careful tailoring of the host–guest interaction.

Interaction of montmorillonite with phenothiazine dyes and pyronin in aqueous dispersions: A visible spectroscopy study

Abstract Layer charge is one of the key parameters used for the characterisation of expandable clay minerals, smectites. It determines most significant properties of the material which are important from the industrial application point of view. This work is related to a novel method introduced to characterize the layer charge of smectites, based on using cationic organic dyes as molecular sensors. One xanthene and four phenothiazine cationic dyes were tested using reduced charge montmorillonites (RCMs) and compared with methylene blue, which has been used most frequently. The characterization of the charge was based on the formation of molecular assemblies (H- and J-aggregates) composed by dye cations, which were easily detectable using absorption spectroscopy in the UV/VIS spectrum. More detailed characterization of the spectra required calculations of second-derivative curves. For all of the reaction systems tested in this work, the molecular aggregation increased with the layer charge of RCMs. Slight to moderate differences in the formation of dye assemblies related to the differences in the molecular structures of the individual dye cations. For example, the molecular asymmetry of azure A brought about the formation of coexistent species of similar structures. The structure of the heteroaromatic skeleton affected the extent of the aggregation and spectral changes with time. The presence of reactive, non-substituted amino groups in thionine cations probably partially decomposed in the clay mineral colloids based on high-charge RCMs. Any of the tested dyes could be used as molecular sensors for empirical characterization of the layer charge of clays taking into account the differences mentioned above.

The effect of layer charge and exchangeable cations on sorption of biphenyl on montmorillonites

Abstract Montmorillonies separated from the bentonites SAz-1 (Cheto, AZ, USA), and Cressfield (New South Wales, Australia) were used as starting materials. Reduced charge montmorillonites (RCMs) were prepared from these chemically different and Li-saturated montmorillonites via heating at temperatures in the range of 120–300°C. The residual exchangeable Li+ cations were then replaced with tetramethylammonium (TMA+) or hexadecyltrimethylammonium (HDTMA+) cations and the ability of the modified montmorillonites to adsorb biphenyl was investigated. Lower adsorption was observed for Li-montmorillonites than for the organoclays. The extent of adsorption was dependent on both the layer charge of montmorillonite and the size of alkylammonium cations. HDTMA-forms prepared from unheated Li-montmorillonites adsorbed biphenyl better than the organoclays prepared from RCMs. In contrast, the TMA-samples prepared from the Li-montmorillonites that were not heated showed low uptake of biphenyl probably due to high content of TMA+ cations. Reduction of the layer charge, resulting in lower content of TMA+ cations, increased sorption efficiency of both TMA-montmorillonites. The best adsorbents of biphenyl were HDTMA-SAz-1 prepared from the unheated Li-SAz-1 and TMA-Cressfield prepared from the Li-form heated at 180°C. These samples removed about 80% of biphenyl from its aqueous solutions

Structure of cationic dyes assemblies intercalated in the films of montmorillonite

Hybrid materials based on cationic organic dyes intercalated in oriented clay mineral films were prepared via dye adsorption from aqueous solutions on the films having been prepared by a spin-coating method. The films with four thiazine dyes thionine, azure A, azure B and methylene blue and one representative xanthene dye, pyronin Y (Py), were investigated. Four reduced charge montmorillonites (RCM) were used as clay mineral templates, representing a semi-synthetic series of materials of almost the same composition, but different in the layer charge and related properties. The interlayer structure was investigated using X-ray diffraction and linearly-polarized absorption spectroscopy in ultraviolet–visible region. The effects of the molecular structure of the dyes and the different layer charge densities of the clays mineral on the molecular orientation were studied. The structure of the assemblies was very similar for thiazine dyes, but different for the case of Py. In the latter case, the formation of bimolecular layers of Py cations led to a significantly higher interlayer expansion. Unusual structural properties of Py/RCM films were interpreted in terms of the presence of polar/non-polar zones in heteroaromatic skeleton of Py cation to influence dye cation aggregation to specific molecular assemblies.

Fluorescence resonance energy transfer between two cationic laser dyes in presence of the series of reduced-charge montmorillonites: Effect of the layer charge

Series of montmorillonites with systematically reduced layer charges represent a suitable model for studying various properties and interactions of layered inorganic compounds. The reduced-charge montmorillonites (RCMs) used in this study were prepared by a standard method of Li +-fixation in Nanocor montmorillonite at 100–300 °C. The layer charge gradually decreased with increasing temperature of RCM preparation. Li +-fixation led in some cases to the loss of expandability due to the formation of mixed swelling/nonswelling and homogeneous nonswelling phases. The interaction of two cationic dyes—rhodamine 3B (R3B) and oxazine 4 (Ox4)—with reduced-charge montmorillonites in dispersions was studied by means of UV/vis absorption and fluorescence spectroscopy. Montmorillonite with the highest charge density induced the formation of H-aggregates of the dye cations characterized by a sandwich-type structure. As the layer charge decreased, the amount of the H-aggregates was reduced in favor of H-dimers and monomers. RCMs with low charge density suppressed dye cation aggregation and mainly monomeric forms were detected. The process of energy transfer from R3B to Ox4 was detected as decreasing the emission from the energy donor and increasing the emission from the energy acceptor. The energy transfer was clearly influenced by the properties of RCM templates. The dye cations adsorbed at the surface of the highest-charge specimen formed H-aggregates, which were efficient luminescence quenchers. Fluorescence resonance energy transfer (FRET) gradually increased with the charge reduction to be optimal at the templates with medium layer charge. Substantial decrease of the layer charge and reduction of clay mineral swelling led to the decrease of both the luminescence and the efficiency of FRET. The relations of energy transfer processes to the layer charges and swelling properties of montmorillonite are analyzed in detail.

Spectral Properties of Rhodamine 3B Adsorbed on the Surface of Montmorillonites with Variable Layer Charge

Montmorillonite was thermally treated at several temperatures to reduce the charge density of its layer surface. Absorption and fluorescence (steady-state and time-resolved) spectroscopies are now applied to study the adsorption of rhodamine 3B (R3B) laser dye in reduced charge montmorillonites (RCMs) in aqueous suspensions. The decrease in the charge density increases the intermolecular distance between adsorbed R3B molecules, reducing the tendency of the dye to self-associate. H-type and J-type aggregates of R3B in RCMs are spectroscopically characterized, the fluorescent J-aggregates being more extensively formed by decreasing the charge density. Both the reduction in the dye aggregation and the formation of J-type aggregates enhance the fluorescence efficiency of R3B dye adsorbed in montmorillonite particles. Absorption with linearly polarized light reveals that the H-aggregates are more disposed toward the perpendicular of the clay surface than the monomer and J-aggregates species.

Preparation and Properties of Reduced-Charge Smectites — A Review

AbstractLayer charge reduction of selected homoionic swelling clay minerals upon heating is reviewed. This phenomenon is known for Li+-montmorillonites as the Hofmann-Klemen effect. Aspects covered in the review include: mechanism of the charge reduction caused by the irreversible migration of small cations into the mineral layers; final sites of the fixed cations in the octahedral and/or tetrahedral sheets, as deduced on the basis of structural and spectroscopic data obtained in several studies; effects of octahedral and tetrahedral components of the layer charge; properties of the exchangeable cations; and the chemistry and structure of the mineral on charge reduction. Current knowledge has been summarized on the changes of various mineral properties connected with charge reduction, including the loss of swelling and the decrease in the cation exchange capacity, the most important change upon cation fixation. Also discussed are the preparation, properties, and advantages of uses of series of reduced-charge montmorillonites (RCMs) in research; interactions of RCMs with alkylammonium cations and organic cationic dyes, with some examples on the interactions with organic polymers and other organic compounds. Properties of organo-complexes and composite materials prepared from RCMs are also included in this review.

Molecular arrangement of rhodamine 6G cations in the films of layered silicates: the effect of the layer charge

Montmorillonite nanocor saturated with Li+ cations was used for the synthesis of reduced charge montmorillonites (RCMs). The films of RCMs with intercalated rhodamine 6G (R6G) were prepared by adsorption of the dye on montmorillonite film deposited on a quartz slide. Penetration of the dye into the interlayer spaces of silicates was monitored by elemental analysis and X-ray diffraction measurements of the prepared R6G/silicate films. Polarized UV-Vis spectroscopy was used for the characterization of the molecular orientation of dye cations on silicate surface. There were several species of R6G cations in the interlayer spaces of the inorganic host, which was easily resolvable by the polarized spectra. The cations forming H-aggregates and absorbing light at low wavelengths were oriented in nearly perpendicular fashion, which was observed as a strongly positive dichroism, i.e., the increase of the dichroic ratio with film tilting. Adsorbed isolated dye cations were inclined at low angles with respect to the plane of the silicate surface. The layer charge of the inorganic host affected only partially the orientation of dye cations in the H-aggregates, but rather it controlled the extent of aggregation and in this way the number of nearly perpendicularly oriented cations.

Hydration of reduced-charge montmorillonite

AbstractA series of reduced-charge montmorillonites with cation exchange capacities of 89, 73, 49 and 29% of the starting mineral was prepared from a Li-saturated smectite (Kriva Palanka, Republic of Macedonia) by heating at 110, 130, 160 and 300°C for 24 h (samples KP110 – KP300, respectively). Hydration properties of this series were investigated gravimetrically and by in situ XRD at different relative humidities (RHs). In the gravimetric experiments, higher water contents were observed for desorption than for sorption and hysteresis was present over the whole range of RHs for all the samples. The parent montmorillonite and the samples KP110 and KP130 retained similar amounts of water under the same conditions, thus showing that the decreased negative charge on the layers had minor effect on the water uptake at all investigated RHs. Significantly decreased water content was retained by KP160 while KP300 contained only 8% water at 100% RH. The d001 values of the parent montmorillonite and the samples KP110 and KP130 increased with RH, while those of KP160 and KP300 were independent of RH and remained at ∼10.4 and ∼9.6 Å, respectively.