Preparation of nanostructured materials by heterocoagulation-interaction of montmorillonite with synthetic hematite particles

Hybrid colloidal gels with tunable elasticity formed by charge-driven assembly between spherical soft nanoparticles and discotic nanosilicates

Colloidal gels based on electrostatic interparticle attractions hold unexploited potential for tailoring their microstructure and properties. Here, we demonstrate that hetero-aggregation between oppositely charged particles with different geometries is a viable strategy for controlling their properties. Specifically, we studied hybrid colloidal gels prepared by the charge-driven assembly of oppositely charged spherical gelatin nanoparticles and two-dimensional (2D) nanosilicates. We show that the asymmetry between the building blocks and the resulting anisotropic interparticle interactions produces a variety of nanostructures and hybrid colloidal gels that exhibit high elasticity at low colloidal volume fractions. Tuning the competition between different attractive interactions in the system by varying the spatial charge heterogeneity on the 2D nanosheets, composition, and ionic strength was found to alter the mechanism of gel formation and their rheological properties. Remarkably, increasing the mass ratio of 2D nanosheets to spherical nanoparticles at a constant total mass fraction affords hybrid gels that exhibit an inverse relationship between elasticity and volume fraction. However, these hybrid gels are easily fluidized and exhibit rapid structural recovery once the stress is removed. These features allow for the engineering of versatile 3D-printable hybrid colloidal gels, whose structure and viscoelastic response are governed by parameters that have not been explored before.

Composite materials based on heteroaggregated particles: Fundamentals and applications

Homoaggregation of dispersed particles, i.e., aggregation of particles of the same shape, charge, size, and composition, is a well-studied field and various theoretical and experimental approaches exist to understand the major phenomena involved in such processes. Besides, heteroaggregation of particles, i.e., aggregation of particles of different shape, charge, size, or composition, has attracted widespread interest due to its relevance in various biomedical, industrial, and environmental systems. For instance, heteroaggregation of plastic contaminant particles with naturally occurring solid materials in waters (e.g., clays, silica and organic polymers) plays an important role in the decontamination technologies. Moreover, nanofabrication processes involving heteroaggregation of particles to prepare novel composite materials are widely implemented in fundamental science and in more applied disciplines. In such procedures, stable particle dispersions are mixed and the desired structure forms owing to the presence of interparticle forces of various origins, which can be tuned by performing appropriate surface functionalization as well as altering the experimental conditions. These composites are widely used in different fields from sensing through catalysis to biomedical delivery. The present review summarizes the recent progresses in the field including new findings regarding the basic principles in particle heteroaggregation, preparation strategies of heteroaggregated structures of different morphology, and the application of the obtained hybrid composites. Such information will be very helpful to those involved in the design of novel composites consisting of different nano or colloidal particles.

Akaganeite nanorices deposited muscovite mica surfaces as sunlight active green photocatalyst

Thin films of akaganeite [FeO(OH)] nanorices deposited muscovite mica (ANPM) surfaces are synthesized using the facile urea assisted controlled self-assembly technique. The synthesized materials are characterized using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy, atomic force microscopy, X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and thermogravimetric analysis (TGA). The prepared nanorices on mica surfaces show average particle length and width of 200 and 50 nm, respectively. Synthesized material acts as an efficient photocatalyst under UV and sunlight conditions as demonstrated by the degradation of standard methylene blue (MB) solution. The MB degradation efficiencies of the catalyst under exposure to 180 min sunlight and UV are 89% and 87.5%, respectively, which shows that the catalyst is more highly active under sunlight than under UV light. Therefore, the synthesized material is a potential green photocatalyst in efficient treatment of industrial dye effluents under direct sunlight.

Formation of environmentally persistent free radical (EPFR) in iron(III) cation-exchanged smectite clay

Environmentally persistent free radicals (EPFRs) have been found at a number of Superfund sites, with EPFRs being formed via a proposed redox process at ambient environmental conditions. The possibility of such a redox process taking place at ambient environmental conditions is studied utilizing a surrogate soil system of phenol and iron(III)-exchanged calcium montmorillonite clay, Fe(III)CaM. Sorption of phenol by the Fe(III)CaM is demonstrated by Fourier-transformed infra-red (FT-IR) spectroscopy, as evidenced by the peaks between 1345 cm(-1) and 1595 cm(-1), and at lower frequencies between 694 cm(-1) and 806 cm(-1), as well as X-ray diffraction (XRD) spectroscopy, as shown by an increase in interlayer spacing within Fe(III)CaM. The formation and characterization of the EPFRs is determined by electron paramagnetic resonance (EPR) spectroscopy, showing phenoxyl-type radical with a g-factor of 2.0034 and ΔHP-P of 6.1 G at an average concentration of 7.5 × 10(17) spins per g. EPFRs lifetime data are indicative of oxygen and water molecules being responsible for EPFR decay. The change in the oxidation state of the iron redox center is studied by X-ray absorption near-edge structure (XANES) spectroscopy, showing that 23% of the Fe(III) is reduced to Fe(II). X-ray photoemission spectroscopy (XPS) results confirm the XANES results. These findings, when combined with the EPFR concentration data, demonstrate that the stoichiometry of the EPFR formation under the conditions of this study is 1.5 × 10(-2) spins per Fe(II) atom.

Heteroaggregation of nanoparticles with biocolloids and geocolloids

The application of nanoparticles has raised concern over the safety of these materials to human health and the ecosystem. After release into an aquatic environment, nanoparticles are likely to experience heteroaggregation with biocolloids, geocolloids, natural organic matter (NOM) and other types of nanoparticles. Heteroaggregation is of vital importance for determining the fate and transport of nanoparticles in aqueous phase and sediments. In this article, we review the typical cases of heteroaggregation between nanoparticles and biocolloids and/or geocolloids, mechanisms, modeling, and important indicators used to determine heteroaggregation in aqueous phase. The major mechanisms of heteroaggregation include electric force, bridging, hydrogen bonding, and chemical bonding. The modeling of heteroaggregation typically considers DLVO, X-DLVO, and fractal dimension. The major indicators for studying heteroaggregation of nanoparticles include surface charge measurements, size measurements, observation of morphology of particles and aggregates, and heteroaggregation rate determination. In the end, we summarize the research challenges and perspective for the heteroaggregation of nanoparticles, such as the determination of αhetero values and heteroaggregation rates; more accurate analytical methods instead of DLS for heteroaggregation measurements; sensitive analytical techniques to measure low concentrations of nanoparticles in heteroaggregation systems; appropriate characterization of NOM at the molecular level to understand the structures and fractionation of NOM; effects of different types, concentrations, and fractions of NOM on the heteroaggregation of nanoparticles; the quantitative adsorption and desorption of NOM onto the surface of nanoparticles and heteroaggregates; and a better understanding of the fundamental mechanisms and modeling of heteroaggregation in natural water which is a complex system containing NOM, nanoparticles, biocolloids and geocolloids.

ATR-FTIR studies on the nature of surface complexes and desorption efficiency of p-arsanilic acid on iron (oxyhydr)oxides

The fate of organoarsenicals introduced to the environment through the application of arsenic-contaminated manure has attracted considerable attention after the recent implementation of the latest maximum contaminant level (MCL) of total arsenic in drinking water by the U.S. Environmental Protection Agency (EPA). We report herein detailed spectroscopic analysis of the surface structure of p-arsanilic acid (p-AsA) adsorbed on Fe-(oxyhydr)oxides using attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Spectra of p-AsA(ads) were collected in situ as a function of pH and ionic strength and using D20 at 298 K in flow mode. Results indicate the formation of inner-sphere complexes, which are likely monodentate and become protonated under acidic pH(D). We also examined the desorption efficiency of p-AsA(ads) due to flowing electrolyte and phosphate solutions as low as 0.1 mol/m3 (3 ppm P) by collecting ATR-FTIR spectra as a function of time. Our results suggest that aqueous phosphate is an efficient desorbing anion of p-AsA(ads), which has implications on its bioavailability and mobility in geochemical environments.

Dry grinding of Ca and octadecyltrimethylammonium montmorillonite

Structural changes in Ca and octadecyltrimethylammonium (ODTMA) montmorillonite induced by dry grinding in a high-energy planetary ball mill were investigated by X-ray powder diffraction (XRD) analysis, infrared (IR) spectroscopy, and scanning electron microscopy (SEM). The organomontmorillonite is more resistant toward mechanical destruction than the Ca form. Both XRD and IR analyses indicate a complete breakdown of the mineral layers after 5 min of grinding of Ca-JP. The XRD pattern of the ODTMA clay shows disappearance of the layer ordering along the c axis after 40 min of grinding, while persistence of the layered structure for all tested samples is proven by the IR spectroscopy. The grinding process affects chemical bonds in the OH, SiOAl, and SiOSi groups of montmorillonite; however, no changes are observed for CH bonds of the organocations. In contrast to the Ca form, all absorption bands typical for montmorillonite appear in the IR spectrum of the ODTMA montmorillonite even after 40 min of grinding. The majority of the bands are undetectable in the spectrum of Ca montmorillonite ground for 5 min. Amorphization of the montmorillonite caused by an intense grinding process is markedly slower after replacing the inorganic cations with the long-chain alkylammonium cations.

Synthesis and characterization of ZnS-montmorillonite nanocomposites and their application for degrading eosin B

Nanocomposites of zinc sulfide (ZnS) and montmorillonite (MMT) were prepared via a hydrothermal route. In this method, the MMT treated with hexadecyltrimethyl ammonium bromide (HTAB) aqueous solution was dispersed in the aqueous solution of thiourea and Zn(OOCHCH(3))(2)2H(2)O, and heated at 170 degrees C for about 4 h, resulting in ZnS-MMT composites. The as-prepared nanocomposites were characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen sorption analysis. It was demonstrated that the interlayer space of MMT was enlarged from 0.98 to 3.77 nm after the treatment with HTAB aqueous solution, and the ZnS nanoparticles were deposited on the layers of MMT. Nitrogen sorption analysis demonstrated that the specific surface area of the samples decreased from 39.2 m(2)/g of the pristine MMT to 5.9 m(2)/g of the final ZnS-MMT composites. The resulting ZnS-MMT nanocomposites (50.0 mg) could degrade eosin B completely in aqueous solution (75 ml, 3.2x10(-5) M) within 20 min under UV irradiation.

Heteroaggregation in binary mixtures of oppositely charged colloidal particles

Heteroaggregation (or heterocoagulation) rate constants have been measured in mixtures of well-characterized colloidal particles of opposite charge with multiangle static and dynamic light scattering. This technique permits routine measurements of absolute heteroaggregation rate constants, also in the presence of homoaggregation. Particularly with multiangle dynamic light scattering, one is able to estimate absolute heteroaggregation rate constants accurately in the fast aggregation regime for the first time. Heteroaggregation rate constants have also been measured over a wide range of parameters, for example, ionic strength and different surface charge densities. Amidine latex particles, sulfate latex particles, and silica particles have been used for these experiments, and they were well characterized with respect to their charging and homoaggregation behavior. It was shown that heteroaggregation rate constants of oppositely charged particles increase slowly with decreasing ionic strength, and provided the surface charge is sufficiently large, the rate constant is largely independent of the surface charge. These trends can be well described with DLVO theory without adjustable parameters.