Surface modification of nanoclays by catalytically active transition metal ions

A unique class of nanoclays was prepared by modification of pristine clays or organoclays (Cloisite C20A) with transition metal ions (TMIs). The composition, structure, morphology and thermal properties of TMI-modified nanoclays were investigated by atomic absorption spectroscopy (AAS), elemental analysis (EA), scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray absorption near-edge structure (XANES) spectroscopy. The content of TMIs in modified clays was found to be close to the limiting value of ion exchange capacity. SEM and X-ray results confirmed that TMIs were located between the mineral layers instead of being adsorbed on the surface of clay particles. TGA results indicated that the TMI treatment of organoclays could significantly increase the thermal stability, which was more pronounced in air than in nitrogen. Temperature-resolved SAXS measurements revealed that the presence of TMIs increased the onset temperature of structural degradation. The higher thermal stability of TMI-modified organoclays can be attributed to the change in the thermal degradation mechanism, resulting in a decrease in the yield of volatile products and the formation of char facilitated by the presence of catalytically active TMIs.     

Sorption of Bisphenol A as Model for Sorption Ability of Organoclays

The arrangement of bisphenol A molecules into organoclays and their interactions with the intercalated surfactant were studied. The organoclays were prepared via solid-state intercalation of four cationic surfactants, such as dodecyltrimethyl-, tetradecyltrimethyl-, hexadecyltrimethyl-, and didodecyldimethyl-ammonium, as bromide salts, at different loading levels into the interlayers of two natural clays. The natural clays, the prepared organoclays, and the spent organoclays were characterized by X-ray powder diffraction, infrared spectroscopy, and scanning electron microscopy. X-ray powder diffraction measurements showed successive interlayer expansions of the d₀₀₁ basal spacing due to the intercalation of the cationic surfactants and the bisphenol A sorption. The increased d₀₀₁ basal spacing of the organoclays after bisphenol A sorption indicates that the molecules are integrated between the alkyl chains of the surfactant in the organoclays interlayers. Infrared spectroscopy was employed to probe the intercalation of the cationic surfactants and the sorbed bisphenol A. New characteristic bands attributed to the bisphenol A phenol rings appear in the range 1518–1613 cm⁻¹ on the infrared spectra of the spent organoclays, proving the presence of bisphenol A in the hydrophobic interlayers. Scanning electron microscopy of the organoclays before and after BPA sorption shows that their morphology becomes fluffy and that the presence of the organic molecules expands the clay structure.     

Intercalation of a nonionic surfactant (C10E3) bilayer into a Na-montmorillonite clay

A nonionic surfactant, triethylene glycol mono-n-decyl ether (C(10)E(3)), characterized by its lamellar phase state, was introduced in the interlayer of a Na-montmorillonite clay at several concentrations. The synthesized organoclays were characterized by small-angle X-ray scattering in conjunction with Fourier transform infrared spectroscopy and adsorption isotherms. Experiments showed that a bilayer of C(10)E(3) was intercalated into the interlayer space of the naturally exchanged Na-montmorillonite, resulting in the aggregation of the lyotropic liquid crystal state in the lamellar phase. This behavior strongly differs from previous observations of confinement of nonionic surfactants in clays where the expansion of the interlayer space was limited to two monolayers parallel to the silicate surface and cationic surfactants in clays where the intercalation of organic compounds is introduced into the clay galleries through ion exchange. The confinement of a bilayer of C(10)E(3) nonionic surfactant in clays offers new perspectives for the realization of hybrid nanomaterials, since the synthesized organoclays preserve the electrostatic characteristics of the clays, thus allowing further ion exchange while presenting at the same time a hydrophobic surface and a maximum opening of the interlayer space for the adsorption of neutral organic molecules of important size with functional properties.     

Thermal stability and mechanical properties of sisal in cycle process

Differential thermal analysis (DTA) was the first thermal analysis technique used to qualitatively characterize natural clays and respective curves has been used since more than 60 years as their ‘fingerprint’. With the development of microprocessed equipments in the last decades, derivative thermogravimetric (DTG) curves also may be used for this purpose in some cases, which also may allow a quantitative characterization of clay components. TG and DTG curves are more indicated than DTA or DSC curves to identify and to better analyze the several decomposition steps of natural or synthetic organoclays. These questions are discussed in applications developed to characterize Brazilian kaolinitic clays, bentonites and organophilic clays.     

Stabilization of hydroxyl-group-terminated SERS-marker molecules on microAg particles by silanization

Surfactant modified montmorillonitic clays synthesized by ion exchange using the hydrothermal reaction method have been compared using XRD and thermal analysis. X-ray diffraction (XRD) shows the changes in the surface properties of organoclays through expansion with surfactant loading. A polynomial relationship exists between the basal spacing and the CEC loading described by the equation y=0.3232x(2) + 0.2052x+1.2834 with R(2)=0.9955. Different arrangements of the surfactant molecules in the organoclays are inferred from the changes in basal spacings. para-Nitrophenol also causes the expansion of the montmorillonite clay and affects the arrangements of the surfactant molecules within the clay layers. Changes in the surfactant molecular arrangements were analyzed by thermogravimetry. Additional thermal decomposition steps were observed when para-nitrophenol is adsorbed on the organoclay.     

A X-ray photoelectron spectroscopy study of HDTMAB distribution within organoclays

X-ray photoelectron spectroscopy (XPS) in combination with X-ray diffraction (XRD) and high-resolution thermogravimetric analysis (HRTG) has been used to investigate the surfactant distribution within the organoclays prepared at different surfactant concentrations. This study demonstrates that the surfactant distribution within the organoclays depends strongly on the surfactant loadings. In the organoclays prepared at relative low surfactant concentrations, the surfactant cations mainly locate in the clay interlayer, whereas the surfactants occupy both the clay interlayer space and the interparticle pores in the organoclays prepared at high surfactant concentrations. This is in accordance with the dramatic pore volume decrease of organoclays compared to those of starting clays. XPS survey scans show that, at low surfactant concentration (<1.0 CEC), the ion exchange between Na⁺ and HDTMA⁺ is dominant, whereas both cations and ion pairs occur in the organoclays prepared at high concentrations (>1.0 CEC). High-resolution XPS spectra show that the modification of clay with surfactants has prominent influences on the binding energies of the atoms in both clays and surfactants, and nitrogen is the most sensitive to the surfactant distribution within the organoclays.     

Effect of polymerically‐modified clay structure on the morphology and properties of UV‐cured polyurethane acrylate/clay nanocomposites

Polyurethane acrylate (PUA)/clay nanocomposites were prepared by UV‐curing from a series of styrene‐based polymerically‐modified clays and PUA resin. Effect of the chemical structure of the polymeric surfactants on the morphology and tensile properties of nanocomposites has been explored. X‐ray diffraction (XRD) and transmission electron microscopy (TEM) experimental results indicated that surfactants having hydroxyl or amino groups show better dispersion and some of the clay platelets were fully exfoliated. However, the composites formed from pristine clay and other polymerically‐modified clays without hydroxyl or amino groups typically contained both tactoids and intercalated structure. The mechanical properties of PUA composites were greatly improved where the organoclays dispersed well. Thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) were carried out to examine the thermal properties of the composites. The results showed that the loading of polymerically‐modified clays do not effect the thermal stability, but increased the Tgs of PUA/clay composites. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal analyis of hexadecyltrimethylammonium–montmorillonites

Na-montmorillonite (Na-MONT) was loaded with hexadecyltrimethylammonium cations (HDTMA) by replacing 41 and 90% of the exchangeable Na with HDTMA, labeled OC-41 and OC-90, respectively. Na-MONT, OC-41, and OC-90 were heated in air up to 900 °C. Unheated and thermally treated organoclays heated at 150, 250, 360, and 420 °C are used in our laboratory as sorbents of different hazardous organic compounds from waste water. In order to get a better knowledge about the composition and nature of the thermally treated organoclays Na-MONT and the two organo-clays were studied by thermogravimetry (TG) in air and under nitrogen. Carbon and hydrogen contents in each of the thermal treated sample were determined and their infrared spectra were recorded. The present results showed that at 150 °C both organoclays lost water but not intercalated HDTMA cations. At 250 °C, many HDTMA cations persisted in OC-41, but in OC-90 significant part of the cations were air-oxidized into H₂O and CO₂ and the residual carbon formed charcoal. After heating both samples at 360 °C charcoal was present in both organo clays. This charcoal persisted at 420 °C but was gradually oxidized by air with further rise in temperature. TG runs under nitrogen showed stepwise degradation corresponding to interlayer water desorption followed by decomposition of the organic compound, volatilization of small fragments and condensation of non-volatile fragments into quasi-charcoal. After dehydroxylation of the clay the last stages of organic matter pyrolysis and volatilization occurred.     

Thermal stability of octadecyltrimethylammonium bromide modified montmorillonite organoclay

Organoclays are significant for providing a mechanism for the adsorption of organic molecules from potable water. As such their thermal stability is important. A combination of thermogravimetric analysis and infrared emission spectroscopy was used to determine this stability. Infrared emission spectroscopy (IES) was used to investigate the changes in the structure and surface characteristics of water and surfactant molecules in montmorillonite, octadecyltrimethylammonium bromide and organoclays prepared with the surfactant octadecyltrimethylammonium bromide with different surfactant loadings. These spectra collected at different temperatures give support to the results obtained from the thermal analysis and also provide additional evidence for the dehydration which is difficult to obtain by normal thermoanalytical techniques. The spectra provide information on the conformation of the surfactant molecules in the clay layers and the thermal decomposition of the organoclays. Infrared emission spectroscopy proved to be a useful tool for the study of the thermal stability of the organoclays.     

Novel architectures in porous materials based on clays

Modification of layered clays in view to develop porous materials, mainly for catalytic applications, has been afforded in the past via intercalation reaction of aluminum and other polyoxycations or through generation of mesoporous silica between the layers of the silicate. In this paper it is introduced examples of an alternative route for the preparation of porous nanoarchitectures based on the sol–gel method that profits from the swelling ability of organoclays in organic solvents to incorporate silicon and/or other metal (e.g., Ti, Al,…) alkoxides in the interlayer region of the silicates where they are hydrolyzed in a controlled manner. Their further polycondensation originates the formation of an oxide matrix and after a thermal treatment is possible the consolidation of oxide nanoparticles between delaminated smectites and vermiculites. It is also showed how this colloidal route can be applied to the generation of oxide nanoparticles bonded to the external surface of fibrous clays, such as sepiolite. Finally, it is also summarized with various examples the potential interest of the resulting porous clay nanoarchitecture materials in applications as acid catalysts, photocatalysts or nanofillers in polymer–clay nanocomposites.     

Study on intercalation and exfoliation behavior of organoclays in epoxy resin

Intercalation and exfoliation behavior of organoclays in epoxy resin has been studied through XRD and DSC. It was found that the organoclays were easily intercalated by epoxy oligomer to form a stable epoxy/clay intercalated hybrid. Under appropriate conditions the clays were able to be further exfoliated as the epoxy resin was cured; thus, a nanocomposite was obtained. It was also found that the exfoliating ability of the organoclays was basically determined by the nature of the clays and the curing agent used. The exfoliation mechanism is discussed in this paper. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 115–120, 2001     

Insertion of fluorophore dyes between Cloisite Na+ layered for preparation of novel organoclays

The aim of this study was to obtain novel photo-functional organomontmorillonites from the intercalation reaction of Cloisite Na⁺ and fluorescent dyes of auramine O, and safranin T in an aqueous solution. The insertion of surfactants in the interlamellar space of nanoclay was followed by Fourier transform infrared spectroscopy and X-ray measurements. An X-ray diffraction analysis established that incorporation of the organic dye cations into the Cloisite Na⁺ expands remarkably the mineral interplanar distances from 1.17 to 1.83–1.97 nm. Field emission scanning electron microscopy was used to study the morphology of the synthesized organoclays. The thermal behavior of the novel hybrid materials was investigated by thermogravimetric analysis and the results show that the organo modified clays verify stepwise decomposition corresponding to initial weight loss from residual water desorption, followed by decomposition of the fluorescent dyes and the dehydroxylation of structural water of the clay layers. Fluorescence properties of the cationic dyes, auramine O, and safranin T incorporated in the clay have significant differences from their behavior in organic solvents and water and the results show that both dyes exhibit a significant fluorescence emission at room temperature when adsorbed in clay.     

New synthesis strategies for effective functionalization of kaolinite and saponite with silylating agents

Functionalization of Brazilian São Simão kaolinite and Spanish Yunclillos saponite with the alkoxysilanes 3-aminopropyltriethoxysilane and 3-mercaptopropyltrimethoxysilane is reported. The resulting hybrids were characterized by X-ray diffraction, thermal analysis, infrared absorption spectroscopy, and scanning electron microscopy, which demonstrated the effectiveness of the interlamellar grafting process. The X-ray diffractograms revealed incorporation of the alkoxide molecules into the interlayer space of the clays. The displacement of the stretching bands of interlayer hydroxyls in the infrared spectra of the modified kaolinites and the increased intensity of the Mg–OH vibrations in the spectra of the modified saponites confirmed the functionalization of the clays. The thermal behavior of the organoclays confirmed the stability of the hybrids, which was dependent on the clay used for preparation of the materials.     

Adsorption of p-nitrophenol on organoclays

In this study, organoclays were prepared through ion exchange of a single cationic surfactant, hexadecyltrimethylammonium bromide and characterised by a range of methods including X-ray diffraction (XRD) and thermogravimetric analysis. Changes in the surface properties of montmorillonite and the organoclays were observed and the basal spacings of organoclays with and without p-nitrophenol were determined using XRD. The thermal stability of both organoclays were measured using thermogravimetry. As the surfactant loading increased, the expanded basal spacings were observed, and different molecular configurations of surfactant were identified. When the surfactant loading exceeded 1.0 CEC, surfactant molecules tend to adsorb strongly on the clay surface and this resulted in increased affinity to organic compounds. The adsorbed p-nitrophenol and the surfactant decomposed simultaneously. Hence, the surfactant molecules and adsorbed p-nitrophenol are important in determining the thermal stabilities of organoclays. This study enhances the understanding of the structure and adsorption properties of organoclays and has further implications for the application of organoclays as filter materials for the removal of organic pollutants in aqueous solutions.     

Organoclay-enzyme film electrodes

This paper aims at showing the interest of organoclays (clay minerals containing organic groups covalently attached to the inorganic particles) as suitable host matrices likely to immobilize enzymes onto electrode surfaces for biosensing applications. The organoclays used in this work were natural Cameroonian smectites grafted with either aminopropyl (AP) or trimethylpropylammonium (TMPA) groups. The first ones were exploited for their ability to anchor biomolecules by covalent bonding while the second category exhibited favorable electrostatic interactions with negatively charged enzymes due to ion exchange properties that were pointed out here by means of multisweep cyclic voltammetry. AP-clay materials were applied to the immobilization of glucose oxidase (GOD) and TMPA-clays for polyphenol oxidase (PPO) anchoring. When deposited onto the surface of platinum or glassy carbon electrodes as enzyme/organoclay films, these systems were evaluated as biosensing electrochemical devices for detection of glucose and catechol chosen as model analytes. The advantageous features of these organoclays were discussed by comparison to the performance of related film electrodes made of non-functionalized clays. It appeared that organoclays provide a favorable environment to enzymes activity, as highlighted from the biosensors characteristics and determination of Michaelis-Menten constants.     

Influence of cationic surfactant removal on the thermal stability of organoclays

The microstructure, thermal stability, surface energy, and swelling characteristics of two kinds of commercial organoclays, before and after washing treatment with a mixture of H2O/ethanol, were investigated using X-ray diffraction (XRD), thermogravimetric analysis (TG/DTG), wettability measurement, and swelling measurement. This study demonstrates that the external-surface physically adsorbed surfactant can be removed after washing treatment, resulting in an increase in thermal stability and a decrease in surface energy of the resultant organoclays. Organoclays are difficult to be introduced into a solvent when their surface energies are lower than that of the solvent. On the other hand, the organoclay with gamma(organoclay) < gamma(solvent) is easier to be swollen and expandable by the solvent. The swelling and basal spacing measurements of the organoclays introduced into organic media indicate that the swelling factor and the interlayer swelling are two independent parameters. Both the polar character of the solvent and the swelling capacity of clay have a prominent effect on the interlayer swelling of the organoclays.     

Sorption interactions of volatile organic compounds with organoclays under different humidities by using linear solvation energy relationships

Organoclays are usually used as sorbents to reduce the spread of organic compounds and to remove them at contaminated sites. The sorption equilibrium and the mechanisms of volatile organic compounds (VOCs) on organoclays under different humidities are helpful for developing efficient organoclays and for predicting the fate of VOCs in the environment. In this study, the organoclay was synthesized through exchanging inorganic cations by hexadecyltrimethyl ammonium (HDTMA) into montmorillonite, resulting in 12?% of organic content. The surface area of organoclay was smaller than the unmodified clay due to the incorporation of organic cations into the interlayer. Both adsorption on organoclay surface and partition into the incorporated HDTMA in organoclay played roles on the sorption process. Compared the sorption coefficients in montmorillonite and different modified clays, the incorporated organic cations overcame the inhibition effect of hydrophilic surface of clay on the sorption process of hydrophobic organic compounds from water. The sorption coefficients of VOC vapors on organoclay were further characterized using a linear solvation energy relationship (LSER). The fitted LSER equations were obtained by a multiple regression of the sorption coefficients of 22 probe chemicals against their solvation parameters. The coefficients of the five-parameter LSER equations showed that high HDTMA-content montmorillonite interacts with VOC molecules mainly through dispersion, partly through dipolarity/polarizability and hydrogen-bonds as well as with negative π-/n-electron pair interaction. The interaction analysis by LSERs suggests that the potential predominant factors governing the sorption of VOCs are dispersion interactions under all tested humidity conditions, similar with the lower level modified clay. The derived LSER equations successfully fit the sorption coefficients of VOCs on organoclay under different humidity conditions. It is helpful to design better toxic vapor removal strategy and evaluate the fate of organic contaminants in the environment.     

Physicochemical study of novel organoclays as heavy metal ion adsorbents for environmental remediation

Four organic-modified clays based on a SWy-2 montmorillonite were prepared by embedding ammonium organic derivatives with different chelating functionalities (NH(2), COOH, SH or CS(2)) in the interlayer space of montmorillonite. Organic molecules such as (a) hexamethylenediamine, (b) 2-(dimethylamino)ethenethiol, (c) 5-aminovaleric acid and (d) hexamethylenediamine-dithiocarbamate were used for the clay modification in order to study the effect of the chelating functionality on heavy metal ions binding from aqueous solutions. The organoclays were characterized by powder X-ray diffraction (XRD), infrared (FTIR) and NMR spectroscopies. The experimental data showed that the organic molecules are intercalated into the interlamelar space with the long dimension parallel to the clay sheets. Their sorbing properties were evaluated for the removal of heavy metals, Pb, Cd and Zn, from aqueous solutions as a function of the pH. When compared with the unmodified SWy-2 montmorillonite, the modified clays show significant improvement in terms of sorbing selectivity as well as of metal loading capacity. The fit to adsorption data by a Surface Complexation Model shows that the intercalated molecules act as specific binding sites in the clay. These contribute additional sorption capacity which is additive to the variable charge edge-sites of the clay in competition with the permanent charge sites.     

Role of interfacial chemistry on the rheology and thermo-mechanical properties of clay-polymer nanocomposites for building applications

This study is directed towards investigating the role of the surface treatment of clay particles on the rheological and thermomechanical behaviour of clay-epoxy blends. Nanocomposites were prepared by mixing small amounts (5–10 mass %) of commercial organoclays or raw clays with an epoxy system commonly used in civil engineering. Rheological characterisations in the liquid state revealed a pronounced thixotropic character of the organoclay-based systems, which all exhibited a shear-thinning behaviour above a critical stress threshold (yield stress), depending on both the intensity of interfacial interactions and the degree of filler dispersion. On the other hand, systems based on raw clay particles behaved like Newtonian fluids, in the same way as the unreinforced polymer matrix. Complementary dynamic mechanical analyses (DMA) performed on the cured cross-linked nanocomposites also showed significant changes in the viscoelastic behaviour of the epoxy matrix due to the introduction of organoclays, whereas only minor variations were observed following the introduction of raw fillers. These results were consistent with nanoscale morphological characterisations performed by conventional X-ray diffraction (XRD) on the various hybrid systems. In this context, rheology and DMA appear as attractive alternative methods for assessing the filler dispersion at a macroscopic (and possibly more relevant) scale. This research is of practical interest for civil engineers, since clay reinforced-epoxies could in the future be used as coating materials with enhanced barrier performances, in order to protect infrastructures against environmental ageing or corrosion.