Kinetics of ion-exchange on montmorillonite clays

The kinetics for the exchange of Li⁺, K⁺, Rb⁺, and Cs⁺ for Na⁺ as the exchangeable cation on bentonite and montmorillonite K10 and KSF have been studied using conductimetric stoppedflow. Dilute aqueous suspensions of the clays, of particle sizes of a few micrometers, were used, so that diffusion was fast and the rate-determining step was the substitution of one cation by another on the lattice surface. The kinetics were treated in terms of relaxation from equilibrium. Relaxation times ranged from 100 to 250 ms, and forward rate constants from 30 to 500 M^?1 s^?1. The reactions had very low activation enthalpies (7–25 kJ mol^?1) and were only slow enough to be studied by the stopped-flow technique because of the large negative entropies of activation (?120 to ?170 J K^?1 mol^?1). © 1993 John Wiley & Sons, Inc.     

蒙脱土催化的有机化学反应

本文从合成有机化学角度, 按反应类型综述了蒙脱土类催化剂近年来在有机化学反应中的应用。这些结果表明这类催化剂比传统的催化剂在产率、选择性等多方面有更高的效率。     

On the interaction of anionic detergents and montmorillonite clays

Summary The interaction of sodium decane sulfonate and sodium dodecyl sulfate with sodium and calcium montmorillonite was investigated byx-ray diffraction. Sorption of detergent by clay was calculated from the decrease in detergent concentration and from carbon analysis of the solids.The two detergents did not interact with sodium montmorillonite either below or above their critical micelle concentrations. Calcium montmorillonite sorbed both detergents, but metathesis, leading to the precipitation of some calcium alkyl sulfate and sulfonate, and hydrolysis of dodecyl sulfate complicated the situation.Thex-ray patterns of calcium montmorillonite treated with detergent solutions and dried contained two sets of (00l) reflections. One, rather broad, belonged to unmodified clay. Like untreated dry clay, its basal spacing increased by 6 Å on exposure to 85% relative humidity, owing to insertion of two monolayers of water between adjacent clay lamellae. The other set consisted of narrow reflections; their basal spacing, which did not increase on exposure to 85% relative humidity, exceeded that of unmodified clay by a distance equal to the extended length of the C₁₀H₂₁SO₃- and C₁₂H₂₅OSO₃-anions, respectively. Therefore, the sorbed, intercalated detergent molecules were fully extended, with their chain axis perpendicular to the plane of the clay lamellae.Sorption may have occurred in two steps: (1) binding of alkyl sulfonate and sulfate anions by calcium counterions of the clay, followed by (2) sorption of sodium alkyl sulfonate and sulfate molecules by association between their hydrocarbon tails and those of previously sorbed detergent anions throughvan der Waals forces.

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Zusammenfassung Die Aufnahme von Natriumdecylsulfonat und Natriumdodecylsulfat in Natrium- und Calcium-Montmorillonit wurde durch Analyse der Gleichgewichtslösungen und der Bodenkörper untersucht. Aus röntgenographisch bestimmten Schichtabständen wurde auf die Anordnung der Detergentienmoleküle im Schichtzwischenraum geschlossen.

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With 5 tables     

Adsorption of polyethylene glycol from aqueous solution on montmorillonite clays

Adsorption rates and capacities of polyethylene glycol (PEG) were investigated for five montmorillonite clays. The adsorption of PEG for all the montmorillonite clays was rapid, and equilibrium was attained within 30 min. The adsorption isotherms of PEG for all the montmorillonites conformed to the Freundlich equation. The adsorption heats were 7.3 and 11.6 kJ · mol^–1(mw.:2000), and 8.7 and 14.2 kJ · mol^–1(mw.:20000) for the montmorillonite and the bentonite II-Ca, respectively. Adsorption capacities for all the clay samples approached constants for the molecular weight of PEG over 2000, though they increased with the increase of molecular weight under 2000. The adsorption capacities were slightly influenced by a nearly neutral pH. The montmorillonite clays which had different interlayer cations showed quite different adsorption capacities. The bentonite II-Ca, the acid clay, and the activated clay showed large adsorption capacities that were 30–50 % of that of an activated carbon.     

Preparation of Cu nanoparticles with controlled particle size and distribution via reaction temperature and sonication

This paper presents metallic copper nanoparticles (CuNPs) that differ according to the process parameters used, such as bath temperature and sonication. The effect of different reactions on the size and distribution of CuNPs that had been formed in ethylene glycol solvent were characterized by the X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses after extraction from the solvent. The optimal dimensional properties, including size, distribution, and agglomeration, of CuNPs were determined by controlling the reaction temperature. On the other hand, the mechanically induced sonication process enhances the formation of the selective CuNPs because of the many homogeneous interactions among precursors, reducing agents, and capping agents related to the nucleation and growth of CuNPs. The mechanics of the origin of the diverse CuNPs of different size and distribution are discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermal characterization of montmorillonite clays saturated with various cations

Emanation thermal analysis (ETA), thermogravimetry and high temperature XRD were used to characterize the thermal behavior during dehydration of natural Na montmorillonite (Upton Wyoming, USA) and homoionic montmorillonite (MMT) samples saturated with different cations, i.e. Li⁺, Cs⁺, NH₄⁺, Mg²⁺ and Al³⁺. ETA results characterized radon mobility and microstructure changes that accompanied the mass loss of the samples due to dehydration on heating in air. A collapse of interlayer space between the silicate sheets after water release from the MMT samples was characterized by a decrease of the radon release rate, ΔE. Decreases in c-axis basal spacing (d ₀₀₁) values determined from XRD patterns for the different montmorillonite samples follow the sequence:

Effect of particle size on the oxidizability of platinum clusters

The catalytic properties of transition metal particles often depend crucially on their chemical environment, but so far, little is known about how the effects of the environment vary with particle size, especially for clusters consisting of only a few atoms. To gain insight into this topic, we have studied the oxygen affinity of free Pt(x) clusters as a function of cluster size (x = 1, 2, 3, 4, 5, and 10) using density functional theory (DFT) calculations (GGA-PW91). DFT-based Nosé-Hoover molecular dynamics has been used to explore the configuration space of the Pt(x)O(x) and Pt(x)O(2x) clusters, leading to the discovery of several novel Pt-oxide structures. The formation of small Pt-oxide clusters by oxidizing the corresponding Pt(x) clusters is found to be significantly more exothermic than the formation of bulk Pt-oxides from Pt metal. The exothermicity generally increases as cluster size decreases but exhibits strongly nonlinear dependence on the cluster size. The nanoclusters are also structurally distinct from the bulk oxides and prefer one- and two-dimensional chain and ringlike shapes. These findings help elucidate the oxidation behavior of Pt nanoclusters and lay the foundation for understanding the reactivity of Pt nanoclusters in oxidizing chemical environments.     

Effect of sonication and freezing-thawing on the aggregate size and dynamic surface tension of aqueous DPPC dispersions

The effect of sonication and freezing-thawing on the aggregate size and dynamic surface tension of aqueous dipalmitoylphosphatidylcholine (DPPC) dispersions was studied by cryogenic-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), UV-vis spectroturbidimetry, and surface tensiometry. When 1000 ppm (0.1 wt%) DPPC dispersions were prepared with a certain protocol, including extensive sonication, they contained mostly frozen vesicles and were quite clear, transparent, and stable for at least 30 days. The average dispersed vesicles diameter was 80 nm in water and 90 nm in standard phosphate saline buffer. After a freeze-thaw cycle, this dispersion became turbid, and precipitates of coagulated vesicles were observed with large particles of average size of 1.5x10(3) nm. The vesicle coagulation is due to the local salt concentration increase during the freezing of water. This dispersion has much higher equilibrium and dynamic surface tension than those before freezing. When this freeze-thawed dispersion was subjected to a resonication at 55 degrees C, smaller vesicles with sizes of ca. 70 nm were produced, and a lower surface tension behavior was restored as before freezing. Similar behavior was observed at 30 ppm DPPC. These results indicate that the freeze-thaw cycle causes substantial aggregation and precipitation of the vesicles. These results have implications for designing efficient protocols of lipid dispersion preparation and lung surfactant replacement formulations in treating respiratory disease and for effective administration.     

Effect of sonication on polymeric membranes

This article studies the effect of 47 kHz ultrasonic (US) waves on polymeric membranes immersed in an aqueous bath. The membranes under study are made from three different polymers: polyethersulfone (PES), polyvinylidenefluoride (PVDF) and polyacrylonitrile (PAN) and present various molecular weight cut-off (MWCO). The evolution of the polymeric structure exposed to US was followed by the measurement of the water permeability and the A_k/Δx parameter which represents the ratio of surface porosity to thickness. Results showed that important variations occurred on certain membranes after irradiation. In addition, microscopic imaging using field emission electron scanning microscopy (FESEM) was performed on irradiated membranes in order to visualize the nature of the degradation. An image analysis method gives the evolution of the pore density, porosity and pore size distribution of a homogeneous area of this membrane before and after irradiation.It has been shown that, over the three materials tested, only the PES is affected by the ultrasonic treatment over all its surface, whereas the others present no significant change in the measured parameters except the PAN (50 kDa) and PVDF (40 kDa) membranes whose edges are affected. In conclusion, in spite of their great efficiency in enhancing filtration processes, ultrasonic waves have to be used with care as the polymeric material itself is sensitive to the ultrasonic waves at the chosen frequency.     

Effect of the platinum content on the microstructure and micropore size distribution of Pt/alumina-pillared clays

The aim of this work is to study the effect of the platinum content (0-1.8 wt % Pt) on the microstructure of an alumina-pillared clay. For this purpose, the nitrogen physisorption data at -196 degrees C, the micropore size distributions of the supported platinum catalysts, and the hydrogen chemisorption results at 30 degrees C have been analyzed and compared. The preparation of the catalysts has modified the textural properties of the Al-pillared clay support, giving rise to a loss of surface area and micropore volume. After reduction at 420 degrees C, the presence of dispersed metallic platinum with mean crystallite size in the 22-55 A range has been found by hydrogen adsorption. Comparison of all results reveals that the platinum species block the micropore entrances by steric hindrance to nitrogen access as the platinum content increases.     

Effect of particle size on pyrolysis characteristics of Elbistan lignite

In this study, the relationship between particle size and pyrolysis characteristics of Elbistan lignite was examined by using the thermogravimetric (TG/DTG) and differential thermal analysis (DTA) techniques. Lignite samples were separated into different size fractions. Experiments were conducted at non-isothermal conditions with a heating rate of 10°C min⁻¹ under nitrogen atmosphere up to 900°C. Pyrolysis regions, maximum pyrolysis rates and characteristic peak temperatures were determined from TG/DTG curves. Thermogravimetric data were analyzed by a reaction rate model assuming first-order kinetics. Apparent activation energy (E) and Arrhenius constant (A _r) of pyrolysis reaction of each particle size fraction were evaluated by applying Arrhenius kinetic model. The apparent activation energies in the essential pyrolysis region were calculated as 27.36 and 28.81 kJ mol⁻¹ for the largest (−2360+2000 μm) and finest (−38 μm) particle sizes, respectively.     

Effect of particle size distribution on the collection efficiency of Brownian particles

The main purpose of the present paper is to investigate the effect of the normal Gaussian size distribution on the deposition of Brownian particles onto a spherical collector, by applying the Brownian dynamic simulation method and the Kuwabara flow field model with different types of DLVO interaction energy curves and the shadow effect. The simulation results show that the collection efficiency of Brownian particles always increases with a wider particle size distribution region. The same increased tendencies are also observed for the case of increasing Reynolds number and for the case of increasing the particle size to the collector size ratio. When compared to the available experimental data, the present simulation method fits well with the experimental data when the specific deposit per collector is not large.     

Acid activated montmorillonite and vermiculite clays as dehydration and cracking catalysts

Treatment of montmorillonite and vermiculite with hydrochloric acid results in an overall increase in acidity and acidic sites. IR and DSC studies of base adsorbed catalysts have revealed the heterogeneity of surface acidity on the activated catalysts. Clay catalysts have been found to dehydrate 1-butanol to 1-butene, which directly isomerizes tocis- andtrans-2-butenes, confirming the presence of Lewis and Brönsted acid sites. Cumene cracking studies using the clay catalyst also confirm the presence of Brönsted acid sites. The temperature and concentration of acid have been found to have a significant influence on the catalytic activity.     

粒径分布对水焦浆性质的影响

分析研究了不同粒径分布的石油焦成浆性及制备水焦浆的流变性和稳定性。结果表明,石油焦的成浆性较好,成浆浓度近70%,浆体的表观黏度均随浓度的增大而增大。粒径分布越宽,越有利于堆积,堆积效率越高,可制浆浓度越高,制备水焦浆的最佳药剂量越低,采用萘系分散剂制备的水焦浆呈胀塑性流型,粒径分布越宽,胀塑性越弱,利用静置观察法与Turbiscan Lab稳定性分析仪测定相结合评价水焦浆的稳定性,水焦浆的稳定性差,且粒径越大,析水率越低,沉降区的焦粉颗粒越易发生聚结,底部越易产生硬沉淀,稳定性越差。颗粒聚结是水焦浆稳定性的主要影响因素。     

Effect of polyaniline particle size on the properties of a polyaniline-nylon 6 composite

A method is proposed for the preparation of a conducting polyaniline-nylon 6 composite with controlled morphology of its surface. The method consists of the ammonium persulfate-induced oxidation polymerization of aniline in the presence of nylon 6. As has been established with the use of atomic force microscopy, spherical polyaniline particles are formed on the surface of nylon 6 and their sizes decrease with an increase in the duration of nylon 6 hydrothermal modification at 100° C. It is shown that a decline in polyaniline particle size in the polyaniline-nylon 6 composite enhances its reactivity with respect to KI.     

Influence of the layer charge and clay particle size on the interactions between the cationic dye methylene blue and clays in an aqueous suspension

The spectroscopic behavior of the dye MB in suspensions of different clays have been used for evaluating layer charge density influence on the adsorption properties of the particles. The clays with higher charge density, like SAz-1 and SCa-3, promote a higher aggregation and do not show deaggregation at longer times, so that practically only the aggregate peak at approximately 570 nm is observed, without any change with time. This is due to, on one side, the larger particle size that decreases the surface area available for adsorption. Additionally, the clay layers will be held together more tightly, avoiding the migration of the dye to the interlamellar region. On the other hand, SWy-1, having a lower charge density, shows a completely different behavior. The dye molecules, initially adsorbed as aggregates on the outer surface of the clay, deaggregate to form monomers that migrate to the interlamellar spaces, giving rise to absorption bands at 670 and 760 nm. Experiments using Ca-exchanged SWy-1, variation of the ionic strength by addition of salt, and the use of different size fractions of the clays confirm the finding that the main factor ruling the adsorption behavior of the probe is the size of the clay particles.     

悬浮液进样原子吸收光谱分析中样品的粒径效应

研究了悬浮液进样 FAAS和悬浮液进样 GFAAS中样品粒径效应。实验表明 ,悬浮液颗粒粒径 <30 μm能满足石墨炉原子化测定需要 ,而火焰原子化要求在粒径≤ 5 μm的前提下 ,再根据待测元素的性质选择合适的火焰类型 ,方能达到较满意的测定效果。     

Effect of colloidal particle size on adsorbed monodisperse and bidisperse monolayers

Coating hydrogel films or microspheres by an adsorbed colloidal shell is one synthesis method for forming colloidosomes. The colloidal shell allows control of the release rate of encapsulated materials, as well as selective transport. Previous studies found that the packing density of self-assembled, adsorbed colloidal monolayers is independent of the colloidal particle size. In this paper we develop an equilibrium model that correlates the packing density of charged colloidal particles in an adsorbed shell to the particle dimensions in monodisperse and bidisperse systems. In systems where the molar concentration in solution is fixed, the increase in adsorption energy with increasing particle size leads to a monotonic increase in the monolayer packing density with particle radius. However, in systems where the mass fraction of the particles in the adsorbing solutions is fixed, increasing particle size also reduces the molar concentration of particles in solution, thereby reducing the probability of adsorption. The result is a nonmonotonic dependence of the packing density in the adsorbed layer on the particle radius. In bidisperse monolayers composed of two particle sizes, the packing density in the layer increases significantly with size asymmetry. These results may be utilized to design the properties of colloidal shells and coatings to achieve specific properties such as transport rate and selectivity.     

Morphological and thermal properties of ABS/montmorillonite nanocomposites using two different ABS polymers and four different montmorillonite clays

ABS/Clay nanocomposites were prepared using two ABS with different Acrylonitrile (AN) contents and four montmorillonite clays; a natural clay (CNa+) and three modified clays, Cloisites 10A, 20A, and 30B. The composites were prepared in a twin‐screw extruder. Results were analyzed considering the effect of clay and ABS type, on the clay dispersion, intercalation and exfoliation, as well as on the storage modulus and thermal stability of the nanocomposites. XRD and TEM confirm that when using an ABS with higher AN content (ABS2), a better dispersion and intercalation–exfoliation can be obtained. Cloisites 20A and 30B, respectively the one with greater initial intergallery spacing, but lower polarity and with smaller inter‐gallery spacing but greater polarity, produce the ABS nanocomposites with the greater intergallery spacing. Both ABS polymers have similar storage modulus and T_g and in both cases, the modulus increases with the 4 wt % clay. This increase is greater with the modified clays and slightly greater with the ABS2. T_g, from tan δ, increases very little with the 4 wt % clay, but again, this is slightly greater with ABS2. TGA and flammability tests show that the dispersed clay enhances the thermal stability and that the ABS with higher AN content produces a greater increase in fire retardancy. Tests also show that the better thermal stability and fire retardancy is obtained with the Cloisites 20A or 30B. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 190–200, 2008