Steady electro-optic characteristics of noninteracting colloidal particles

In previous papers on the electro-optic effects of beta-FeOOH particles we proposed a new procedure for analysis of the low frequency behavior of charged particles. The procedure is based on comparison of characteristic field intensity curves on an appropriate scale and helps to test the dependence of the slow effects on particle surface electric polarizability (relaxing in the kilohertz domain). The results stimulated us to test the applicability of the method to other samples and to reconsider the literature data on the electro-optic behavior of charged colloids in the hydrodynamic domain. The aim of the present paper is to demonstrate on a series of samples similar features of the electro-optic responses of charged particles in the relaxation interval of particle rotation. The analysis leads to a new hypothesis for explanation of the complicated low frequency behavior of charged particles. The superposition of two slow effects (linear and quadratic with field intensity), relaxing in the relaxation interval of particle rotation, can explain the complicated frequency curves in this domain. One of the slow effects is observed for all polarizable particles. It is of negative sign and displays the features of an induced dipole effect dependent on the "kilohertz" induced moment. It corresponds to a slow stage of the surface polarization process related to electrokinetic charge. The linear slow effect shows permanent dipole like behavior and appears only at certain ionic content of the medium. It shows no direct dependence on counterion mobility and on the "kilohertz" induced moment and is probably due to surface charge nonuniformity.     

Amine groups functionalized gel-type resin supported Pd catalysts: Physicochemical and catalytic properties in hydrogenation of alkynes

Palladium catalysts (0.125–0.5 wt.% Pd) supported by amine groups—functionalized gel-type resin (FCN) were studied in the hydrogenation of alkynes reagents, 2-butyne-1,4-diol and phenylacetylene. The catalysts were prepared by two routes. The first, “OAc” is based on the immobilization of Pd-precursor in the pre-swollen resin from THF solution of Pd(OAc)₂, followed by chemical reduction of the Pd-centers. This method produces Pd particles of size in nano-scale. The second procedure, “aq” implies the deposition of Pd-species on dry resin beads using aqueous solution of PdCl₂. Reduction of these Pd-species gives relatively large Pd particles, dominating are 30–50 nm in size. The SEM studies performed over the cross-section of catalysts grains showed location of Pd in outer shell of polymer beads in both “OAc” and “aq” catalysts; however, thinner layer of Pd appears in “aq” series catalysts. In the presence of all catalysts, prepared by “OAc” and “aq” methods the selectivity towards alkenes is high, above 90%. The catalysts of “aq’ series are much more active and more selective than “OAc” analogues giving selectivity to alkene ca. 94% at almost complete conversion of alkynes. Moreover, catalytic performance of “aq’ series catalyst is unchanged under recycling use. The catalyst was recovered and reused 4 times, maintaining its catalytic efficiency.     

Effect of spatial inhomogeneities on the rate of homogeneous nucleation in systems with aerosol particles

The presence of growing particles in a system leads to spatial inhomogencities in the vapor concentration. The effect of these spatial variations on the rate of formation of new particles by homogeneous nucleation is examined theoretically using a cell model. Results indicate that the presence of these inhomogeneities in systems both with and without initial aerosol has generally little effect on the final number concentration of particles following a nucleation “event.”     

Solid-stabilized emulsions

The comprehension of bulk properties of solid-stabilized emulsions (stability, compressibility, elasticity) in relation with interfacial properties has progressed. The association of oil, water and particles allows a large set of materials to be obtained, where emulsions are used either as intermediate or end products. The efficiency of some stimulus-responsive particles to stabilize or destabilize emulsions “on demand” has been experimentally evidenced.     

Dynamic electro-optic properties of macromolecules and nanoparticles in solution: a review of computational and simulation methodologies

This paper reviews some theories, and computational and simulation procedures available for the calculation of the time-course of electro-optic properties of particles in solution. For rigid particles, the time evolution of the properties is directly related to their rotational diffusion; therefore, the computational procedures for the calculation of hydrodynamic properties find a direct application in electro-optics. Several of such computational procedures, based on bead models, are reviewed. For flexible particles, the simultaneous effects the external field and the flexibility can be treated with Brownian dynamics simulation. We illustrate the various procedures, with applications to rigid bent rods and flexible, wormlike or hinged rods, trying to show how the absence or presence of flexibility, and its kind, influences the dynamic electro-optic properties, which are therefore valuable sources of information about the conformation of macromolecules and nanoparticles.     

A novel “green” synthesis of starch-capped CdSe nanostructures

This paper reports a “green” facile, room temperature, one-pot synthesis of starch-capped CdSe nanostructures with an obvious quantum confinement effect via a novel non-organometallic method. It is found that by simply tuning the Cd:Se molar ratio, dots and elongated particles of high aspect ratio could be prepared selectively in the presence of the same ligand concentration without any post-treatment. Spherical particles were produced at 1:1 ratio, while elongated particles were produced at 0.5:1 Cd:Se ratio. The X-ray diffraction (XRD) analysis showed that the particles were predominantly of wurtzite structure, with sharp diffraction patterns regardless of their size and shapes. We inferred that the elongated particles are formed by self-reorganisation occurring via adhesion between the spherical nanoparticles as a result of dipole–dipole interactions.     

A new technique for metropolis Monte Carlo simulation to capture aggregate structures of fine particles: Cluster-moving Monte Carlo algorithm

We present a new Monte Carlo simulation procedure which is capable of capturing aggregate structures in a suspension where fine particles are dispersed. The algorithm we call the “cluster-moving” Monte Carlo algorithm involves moving aggregates (clusters) as unitary particles at every certain Monte Carlo step. We discuss here the theoretical background of the cluster-moving Monte Carlo algorithm and the availability of the algorithm for simulations of systems where fine particles aggregate. The results of simulations for two model systems, magnetic fluids and colloidal dispersions, have shown that the new algorithm produces much more rapid convergence than the conventional one for unstable dispersion systems and reproduces physically reasonable aggregate structures of fine particles.     

Phase-death mode in two-coupled chemical oscillators studied with reactors of different volume and by simulation

The behavior of the phase-death mode (cessation of oscillations and transition to a steady state) in the two-coupled Belousov–Zhabotinsky reactions is studied in the asymmetric coupling. In an experiment, the types of synchronized modes and their regions were investigated as an extension of our previous study. This experiment is compared to a simulation using the two-coupled three-variable Oregonator models. The results confirm that the phase-death mode changes from “bistable” to “monostable” and reveal the stable region of the latter.     

Structural Peculiarities of Water Solutions in <Emphasis Type="Italic">n</Emphasis>-Alkanols Derived from the Study of Bulk Properties at Different Temperatures

The structural behavior of water microimpurities in n-alkanols (C₁-C₇) is considered by using our and literature data on the bulk properties of infinitely dilute solutions of H₂O in the above organic solvents at 278.15 K, 298.15 K, and 318.15 K. Volume effects of water solution in hypothetical alkanols HOH (pseudowater) and H(CH₂)OH (pseudomethanol) with molar volumes corresponding to volume effects in water and methanol in pure liquid states have been evaluated. The behavior of water in methanol, which is distinct from the behavior of other H₂O—n-alkanol systems, is of configurational nature and is associated with the unique molecular structure of this alcohol (i.e., with the absence of hydrocarbon chains in its molecules) and also with steric peculiarities of the arrangement of solute molecules in the structural matrix of the solvent (so-called “edge effect”).Original Russian Text Copyright © 2004 by E. V. Ivanov, V. K. Abrosimov, and E. Yu. Lebedeva__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 862–869, September–October, 2004.     

Luminescence spectra and Raman excitation profiles in small CdS particles

The “edge”-luminescence spectra and the Raman excitation profiles of small CdS particles chemically deposited on transparent substrates show size effects. The peaks are shifted to higher frequencies as the size of particles decreases. Aggregated samples show maxima at lower frequencies than those of the bulk material and behave as amorphous films. After the samples are warmed, a new peak appears in the luminescence spectra whose position depends on the size and aggregation of the particles. An explanation of the shifts based on the size-quantization of the band width or of the localized electronic levels is given.     

Adsorption characteristics of metal-EDTA complexes onto hydrous oxides

The adsorption characteristics of a variety of metal-EDTA complexes onto hydrous oxides, principally aluminum oxide (γ-Al₂O₃), were examined in aqueous solution. Adsorption of these complexes increased with increasing proton concentration due to the formation of surface complexes between EDTA and the surface hydroxo groups, specifically the AlOH₂⁺ surface groups. The pH-dependent adsorptive behavior and the magnitude of adsorption of the “free” EDTA species were similar to those of the metal complexes. The results also showed that the adsorption of “free” EDTA was exothermic, while the adsorption of Ni(II)-EDTA complexes was endothermic in the lower pH region (3.5) and exothermic at higher pH values (6.0). This implied that the surface preferred the NiHEDTA⁻¹ species rather than the NiEDTA⁻² species. Specific adsorption of the metal complexes was evidenced by the charge reversal exhibited by the γ-Al₂O₃ particles at the highest surface loadings. A quantitative model was formulated based on the pH-dependent speciation of the oxide surface, speciation of the metal complexes in solution, and ζ potential measurements. This model proved valid over a wide range of pH (3–10) and for both high (>50% coverage) and low (<10% coverage) surface loadings.     

Scaling procedures in colloidal electro-optics

In a series of recent papers, we demonstrate the possibility to use the complete electro-optic saturation curves (or arbitrary part of them) to analyse the polarization mechanisms of charged colloidal particles. Scaling of the electro-optic saturation curves by field intensity permits to find the frequency range in which the functional dependence of the effects, hence the orientation mechanism remains unchanged. Similar procedure can be applied to verify the changes of this dependence with variation of particle electric parameters. When different polarization mechanisms are involved the new scale helps their analysis.

The aim of the present paper is to summarize the basic applications and advantages of the scaling method in the electro-optic investigation of charged colloids.     

Controlled production of emulsions and particles by milli- and microfluidic techniques

The recent developments of soft lithography and microfluidic techniques now permit the manipulation of small quantities of fluids with very good control and reproducibility. These advances open a new “bottom-up” route to emulsification that paves the way to the fabrication of calibrated hierarchically organized emulsions and particles. In this article, we describe the microfluidic techniques elaborated for engineering emulsions and new dispersed materials and discuss their advantages over “top-down” approaches. We review and comment the high potentialities these techniques offer to emulsion and colloid science, to the development of high-throughput set-ups for chemistry, physics and biology. We illustrate them through a few examples taken from the current literature.     

Iron-modified mesoporous SBA-15 silica: Preparation and characterization studies

Fe-SBA-15 materials with different Si/Fe ratios (Si/Fe = 100, 60, 15) have been synthesized by hydrothermal method and characterized by several spectroscopic techniques. Electron spin resonance and Mössbauer spectroscopy, along with electron microscopy and X-ray diffraction, allowed differentiation of several iron species. These species correspond to hematite particles, very small “isolated” or oligomeric Fe^III species possibly incorporated in the mesoporous silica wall, and Fe^III oxide clusters either isolated or agglomerated, forming “rafts” at the surface of the silica and exhibiting ferromagnetic ordering. Because of their agglomeration, these clusters appear with a two-peak size distribution, with one peak corresponding to the isolated clusters formed in the mesopores and still embedded in them and the other corresponding to the agglomerates spread on the surface of the mesoporous silica particles.     

Electro-optic characteristics of aqueous beta-FeOOH particles

This paper presents a detailed analysis of the electro-optic behavior of suspensions of noninteracting monodisperse beta-FeOOH particles. The electro-optic parameters are determined for aqueous suspensions of the oxide particles and the influences of surface charge and Debye layer thickness are verified. Since the conventional method of frequency analysis is inconsistent in the low-frequency range, new electro-optic parameters are introduced to define the frequency variation of the effects. Electric polarizability is determined with precision to a constant, and its relative variations are followed. As reported for other oxides, electric polarizability correlates with charge variations in the diffuse part of the particle surface electric layer, and its relaxation frequency increases with surface charge density, indicating a Maxwell-Wagner type of surface polarization. The alternating component of the responses yields particle relaxation frequency and the phase shift of the responses at this frequency. For all studied samples the phase shift at particle relaxation frequency is 45 degrees. The relative changes in the steady component of the responses in the low-frequency range are followed by field intensity curves at characteristic frequencies of the samples. Electrophoretic rotation is the process consistent with our data for the low-frequency effect. The results show that it is enhanced by the combined actions of low or slowly relaxing polarizability and significant electrophoretic mobility.     

Effect of high pressure on the nematic–isotropic transition in aerosil–liquid crystal composites

We report the first high pressure investigations of the nematic–isotropic transition in the composites of a liquid crystal compound with hydrophilic aerosil particles. The low concentrations of the aerosil particles used create soft gels of the composites. As expected T_N–Iso, the nematic–isotropic transition at room pressure exhibits a non-monotonic variation with increasing aerosil concentration. This non-monotonic behaviour is seen in the isobaric scans over the wide range of pressures studied, and its “magnitude” is dependent on the pressure applied. The surprising result of the present investigations on these nanocolloidal systems is that the slope of the pressure–temperature boundary also exhibits a non-monotonic dependence with the aerosil concentration, which qualitatively is similar to that of the transition temperature variation. Employing the transition enthalpy values determined at room pressure using differential scanning calorimetric scans collected at low heating rates, we calculate the transition volume dependence on the aerosil concentration. The study adds a new dimension, namely, the influence of pressure on liquid crystalline transitions in restricted geometries.     

A note on electro-optic effects

Tinoco's second-order electro-optic effect in isotropic fluids is discussed for the case of very high electric fields. The possibility of observing a linear electro-optic effect in oriented molecules of certain symmetry types is also analyzed.     

Negative and linear positive magnetoresistance effect in silver-rich silver selenide

In the non-magnetic semiconductor silver selenide (Ag_2+δSe) with a minor heterogeneous silver excess (0.79 × 10⁻⁴ < δ ≤ 1 × 10⁻²) we measured either a saturating negative magnetoresistance (MR) effect, a linear positive MR effect or a superposition of both. This complex MR behavior depends on the amount of the silver metal excess, but is also strongly influenced by the thermal treatment of the samples. Excess silver that cannot be accommodated homogeneously in the silver selenide lattice creates structural heterogeneities and forms a microstructure which is controlled by the thermal annealing procedure. We suggest that small silver segregations at the grain boundaries of the silver selenide matrix (“nanofilms”) are responsible for the negative MR effect, whereas nanoscale silver particles within the silver selenide matrix (nanoparticles) cause a linear positive effect.     

Pore Structure Simulation of Gels with a Binary Monomer Size Distribution

The pore size distribution in silica gels can be tailored by the addition of silica soot particles during the gel formation. We introduce a numerical model in order to simulate the structure of this “composite gel”. The algorithm is based on Diffusion-Limited Cluster-Cluster Aggregation model with an initial binary distribution of monomer sizes. The textural properties of the simulated gels are calculated using a simple triangulation method. Nitrogen adsorption-desorption experiments show that with the powder addition the mean pore size is shifted towards larger pore size and the specific surface area decreases. Numerical results of the mean pore size, specific surface area, and particles are in good agreement with experimental data. Because of these textural properties this new type of gels and aerogels has larger permeability and interesting properties as host matrix. The composite gels and the numerical model could also be helpful to simulate the natural allophanic gel found in volcanic soils.     

Long range transport of biomass aerosol to Greenland: Multi-spectroscopic investigation of particles deposited in the snow

Summary Results are given from the NIST component of a pilot (“winter-over”) study of seasonal patterns of natural and anthropogenic species in air and snow transported to Summit, Greenland. Central to this research is the quantitative apportionment of fossil and biomass particulate carbon, based on advanced (micromolar) ¹⁴C accelerator mass spectrometry (AMS) applied to remote snow samples containing as little as 9 µg C/kg. The measurements were made practicable through stringent attention to the nature and sources of the isotopic-chemical blank, resulting in a blank reduction from »5 µg C to <0.5 µg C. An important result of this work is the first evidence of a seasonal pattern in biomass-C particles in Greenland snow. Although ¹⁴C AMS data serve to resolve fossil and biomass carbon quantitatively, a deeper understanding of the aerosol sources and character demands a multidisciplinary approach. This is illustrated with “multi-spectrometric” macro- and micro-analytical data for two cases involving substantial incursions of biomass aerosol to the Summit, Greenland snow.