Formation Of Primary Particles Research Articles

Thermophoretic Collection and Analysis of Submicrometer Ag Particles Emitted from a Graphite Tube-Type Electrothermal Vaporizer

Using thermophoretic collection with a cooled Cu probe, particles generated in an electrothermal vaporizer (ETV) from a AgNO(3) solution sample have been collected and analyzed using transmission electron microscopy and high-energy electron diffraction (HEED). Ag particles are spherical and, interestingly, have diameters falling into one of four size regimes, <5, 15, 25, and >40 nm. An extremely large number (estimated at >10(8)) of particles with diameters of <100 nm are collected. For an aqueous AgNO(3) solution deposited in the ETV, the HEED pattern of the collected aerosol particles exiting the pulse heated ETV matched body centered cubic Ag((s)), and the large number of diffraction spots suggests that particles are composed of microcrystalline domains. The features of the particles confirm earlier predictions of homonucleation as the primary particle formation mechanism. "Groupings" of apparently disconnected particles were a unique feature seen from the pulse-heated vaporization of the dried sample. This morphology is unique and does not appear like any other clustering or aggregation of particles reported elsewhere. It is not clear what causes this particle grouping, although the absence of these groupings when a 700 °C thermal pretreatment step was introduced suggests that the AgNO(3) decomposition plays a role in their formation. It is suggested that the particles formed from homonucleation are created very near the graphite surface and are cooled quite rapidly to form the solid silver particles. A mechanism is presented to explain the appearance of silver in the gas phase at temperatures below the vaporization temperature for silver metal.

Preparation of Rare-Earth-Metal Oxalate Spherical Particles in Emulsion Liquid Membrane System Using Alkylphosphinic Acid as Cation Carrier

Oxalate particles of rare-earth-metal elements were prepared by using an emulsion liquid membrane (ELM, water-in-oil-in-water (W/O/W) emulsion) system, consisting of Span 83 (sorbitan sesquioleate) as a surfactant and bis(1,1,3,3-tetramethylbutyl)phosphinic acid as an extractant (cation carrier). Rare-earth-metal ions were extracted from the external water phase and stripped into the internal water phase to make oxalate particles. Well-defined spherical oxalate particles, smaller than 0.5 μm in diameter, were obtained for lighter and heavier rare-earth-metals such as Pr, Sm, Eu, Gd, Dy, Y, and Yb, following the formation of primary particles of about 20 nm in size. The present result differs from that obtained using EHPNA (2-ethylhexylphosphonic acid mono-2-ethylhexyl ester) as an extractant, where no submicrometer-sized spherical particles are obtained for the heavier rare-earth metals such as Dy, Y, and Yb. The smaller extracting ability of alkylphosphinic acid eases the stripping of rare-earth-metal ions by oxalic acid into the internal phase. Effects of the internal water droplet size, the rare-earth-metal ion concentration in the external water phase, and the volume ratio of organic membrane phase to internal water phase of the W/O emulsion (O/A ratio), on the size of spherical particles, were investigated. The control of the particle size was found to be feasible by control of the feed rare-earth-metal concentration and the size of the internal water droplets.

Effect of morphology on the brittle ductile transition of polymer blends: 4. Influence of the rubber particle spatial distribution in poly(vinyl chloride)/nitrile rubber blends

We have derived a new equation for correlating the average matrix ligament thickness with other morphological parameters of a polymer blend with the pseudonetwork morphology in order to evaluate the effect of rubber particle spatial distribution on the brittle ductile transition of poly(vinyl chloride) (PVC)/nitrile rubber (NBR) blends. Two types of rubber particle spatial distributions, i.e. well-dispersed particles and pseudonetwork, are investigated. The formation of PVC primary particles favours the toughening of PVC. An approximate master curve of brittle ductile transition for the blends with the pseudonetwork morphology is obtained by plotting the impact strength versus the average matrix ligament thickness. The critical matrix ligament thickness for the blends with the pseudonetwork morphology is 0.11 μm, which is much larger than that (0.059 μm) for the blends with the morphology of well-dispersed rubber particles. Therefore, pseudonetwork morphology is much more effective in toughening PVC than the morphology of well-dispersed rubber particles. Moreover, the critical matrix ligament thickness depends on the rubber particle spatial distribution, and is thus not a characteristic of the PVC matrix. The effects of morphological parameters and PVC molecular weight on the toughness of PVC/NBR blends are discussed.

Time resolved small angle X-ray scattering of inorganic–organic gel formation kinetics

Formation of hybrid inorganic–organic gels prepared from triethoxysilyl terminated poly(tetramethylene oxide) (ET-PTMO) and tetraethoxysilane (TEOS) by a two-step sol–gel process was investigated using time-resolved small-angle X-ray scattering (SAXS). Reaction conditions used in the first step of the process (high excess of water, acid catalyst) allowed nearly complete hydrolysis of ethoxysilyl groups to silanols. As revealed by SAXS, an excess of base catalyst in the second step promotes formation of primary silica particles and their aggregation in the environment of the swollen polymer network formed simultaneously. The size of the particles and their aggregates sizes were studied as a function of reaction conditions (molar ratios of TEOS/polymer and acid/base catalyst). Higher TEOS content resulted in bigger primary particles initially formed in the second step of the process. The `fractal' dimension of aggregates was found to be about 2.2–2.4 corresponding to more compact aggregates.

Preparation of spherical oxalate particles of rare earths in emulsion liquid membrane system

AbstractSubmicron‐sized spherical oxalate particles of rare earths were prepared by using an emulsion liquid membrane (ELM, water‐in‐oil‐in‐water (W/O/W) emulsion) system, consisting of Span 83 (sorbitan sesquioleate) as a surfactant and EHPNA (2‐ethylhexylphosphonic acid mono‐2‐ethylhexyl ester) as an extractant (cation carrier). Rare‐earth ions were extracted from the external water phase and stripped into the internal water phase to make submicron‐sized oxalate particles. In the case of Ce, Pr, Nd, Sm, and Gd, well‐defined spherical oxalate particles (0.2 – 0.6 μm dia.) were obtained, following the formation of primary particles of about 20 nm. In contrast, less spherical or rather tabular particles were formed for La, Dy, and Y, possibly owing to their formation in the external phase following some breaking of the water‐in‐oil (W/O) emulsion drop. Oxalate particles of Yb were not obtained in this system. Effects of the internal water droplet size, the rare‐earth ion concentration in the external water phase, and the volume ratio of organic membrane phase to internal water phase of the W/O emulsion (O/A ratio), on the size of spherical particles, were investigated. A simulation study based on the model of transport mechanism of rare‐earth ions through the organic membrane phase revealed that the particle size was determined by the distribution of rare‐earth ions into the internal water droplets, as confirmed by the experimental results. The control of the particle size was found to be feasible by control of the feed rare‐earth concentration and size of the internal water droplets.

Comparison of the structural information on sol-gel systems provided by pyrene labeled fluorescent probes

The influence of the acid catalyst concentration on the structural evolution of a sol-gel system was studied by doping TEOS based starting solutions with two fluorescent probes: a polystyrene chain (M n = 1700) and a much shorter alkane chain (M = 172), both of them labeled at both ends with 1-pyrenyl. For this purpose, each probe was incorporated in two TEOS : H2O : C2H5OH mixtures (molar ratios 1 : 4 : 1), one at pH 1.2 and the other at pH 2.5 (respectively below and above the isoelectric point of silica). Very low concentrations of the probes were used (≤10-6M), so the pyrene dimmers and excimers were formed only intramolecularly. The ratios of excimer to monomer fluorescence intensities at excitation wavelength of 360 nm (where mainly the ground state pyrene dimmers are excited) were studied as a function of time. Different evolutions of these ratios were observed, which allowed us to predict that the silica structure develops via the formation of primary particles, even at pH values below the isoelectric point of silica, where it is not possible to directly detect their formation.

Effect of preparation conditions on the formation of primary ZnO particles in filter expansion aerosol generator

Effect of preparation conditions on the formation of primary ZnO particles in filter expansion aerosol generator

Modeling Small Cluster Deposition on the Primary Particles of Aerosol Agglomerates

ABSTRACT Dense particles, such as the primary particles in aerosol agglomerates, are formed by coagulation with rapid coalescence, or by condensation-like growth. This latter mechanism is explored in a new model. It uses the discrete-sectional model to describe the evolution of the particle mass distribution. In addition, new equations are used to describe the formation of primary particles. Primary particles are “born” when a new particle is formed above a critical size D melt. Particles smaller than D melt are assumed to be liquid-like, sintering rapidly in the available time. Changes in the final primary particle size Dp can be attributed to growth from small particle deposition. This growth increases as the monomer source rate decreases. For conditions representative of TiO2 formation from chloride in a tube reactor, Dp /D melt ranges from 1.5 to 3. When D meit decreases, as would be expected if temperature decreases, Dp /D melt increases. The result is that Dp is much less sensitive to temperature than would be expected from simple sintering models.

The mechanism of flocculation of a Saccharomyces cerevisiae cell homogenate using polyethyleneimine

The effects of polyethyleneimine, electrolyte concentration and pH on the electrophoretic mobility values for whole yeast, yeast homogenate and borax clarified homogenate were observed. The polyethyleneimine concentration at which there is optimum clarification of the borax-clarified homogenate corresponded to a mobility value of zero. Increased floc size up to the point of zero mobility was obtained by use of polyethyleneimine of larger molecular weight and also higher concentration. The effect of ageing under shear on the size of the flocs was examined and indicated reversibility of break-up at low shear rates while at higher shear rates the flocs were irreversibly broken. Such growth and ageing observations suggest floc formation comprises a two-stage process; the initial formation of primary particles by polymer bridging, and the subsequent formation of larger flocs as these primary particles are brought together by charge neutralisation.

MODELLING AND ANALYSIS OF PARTICLE FORMATION DURING AGGLOMERATIVE CRYSTAL PRECIPITATION PROCESSES

Abstract While nucleation and crystal growth are normally considered to be the primary particle formation processes during industrial crystallization, secondary processes including crystal agglomeration and breakage can have a determining effect on product form, especially of precipitates. In addition to the usual overall size distribution, primary and secondary particle size, particle structure and morphology then become important characteristics affecting both product quality and downstream processing performanc In this paper, the kinetics of processes which determine the formation of both primary particles and crystal agglomerates are briefly reviewed together with established and recent population balance and simulation techniques for analysing and predicting primary and secondary particle size distributions and total specific surface area. These approaches are then illustrated by some experimental and modelling studies of agglomerative precipitation reported recently in the literature.

Role of homogeneous nucleation in the formation of primary zeolite particles

Different mechanisms of nucleation in the liquid phase, such as homogeneous, heterogeneous, secondary and autocatalytic nucleation, were assumed in the literature as the dominant processes of the formation of primary zeolite particles. Since just homogeneous nucleation was very frequently pointed out as possible process of the formation of primary zeolite particles, the possibility of the occurrence of homogeneous nucleation during zeolite crystallization from aluminosilicate gels was analyzed in accordance with the classical theory of homogeneous nucleation. Analysis of three different crystallizing systems show that the rate of homogeneous nucleation is extremely low under the given synthesis conditions, so the other nucleation processes must be considered relevant for the formation of primary zeolite particles.

Studies on microgels, 1. Microgel formation studied by gel‐permeation chromatography with an on‐line light scattering detector

AbstractThe previously described microgel synthesis in which living poly(4‐tert‐butylstyrene) was reacted with 1,4‐divinylbenzene was studied. Samples were taken after the addition of the crosslinker and characterized by gel‐permeation chromatography (GPC) with an on‐line multi angle laser light scattering (MALLS)‐ and a differential refractive index (DRI) detector. Primary particle formation was found early in the reaction followed by interparticular crosslinking. The GPC calibration curves allowed the derivation of a branching plot. The branching plots showed, that in addition to interparticular crosslink reactions also intraparticular crosslink reactions are occurring. The plots also showed that the primary particles scale like linear polymers, which led to the conclusion that they consist of comb‐like structures with poly(4‐vinylstyrene) as the backbone and poly(4‐tert‐butylstyrene) as branches.

Effect of self-degradation products on crystallization of protease thermolysin

The effect of self-degradation products of protease thermolysin on the crystallization of thermolysin was investigated. Crystallizations were carried out at the concentration of the self-degradation products of 0 to 0.622 mg/ml, 5°C and pH 7.0. The initial concentration of thermolysin was constant (1.70 ± 0.01 mg/ml). Crystallizations were monitored by dynamic light scattering and photomicroscopy. The crystallization of thermolysin in the presence of the self-degradation products proceeded through two successive steps: the formation of primary particles and the formation of large crystals by the aggregation of the primary particles. Low concentration of the self-degradation products (0.212 mg/ml) accelerated the formation of the primary particles and also the formation of the large crystals. High concentration of the self-degradation products, however, inhibited the formation of the primary particles and their aggregation to the large crystals. As the result, a large number of small aggregates which had not grown to the large crystals were observed by photomicroscopy. An analysis of the crystals and the primary particles formed in the presence of the self-degradation products by gel filtration high performance liquid chromatography revealed that the self-degradation products are not incorporated in the primary particles, but are incorporated probably in the openings between the primary particles during the crystallization.

Effect of supersaturation ratio on the growth rate and number of protease thermolysin crystals

The effect of initial supersaturation ratio on the growth rate and number of protease thermolysin crystals was investigated at the initial supersaturation ratio of 8.9 to 17.9 and pH 7.0. The average diameter and number of thermolysin crystals were measured by dynamic light scattering during crystallization. The crystallization of thermolysin proceeded through two successive steps: the formation of primary particles and the formation of large crystals from the primary particles. The size of the primary particles was not affected by the initial supersaturation ratio and was about 60 nm in diameter. The growth rate of thermolysin crystals had a maximum at the initial supersaturation ratio of 9.5. On the other hand, the number of thermolysin crystals decreased with an increase of the initial supersaturation ratio, and turned to increase through a minimum at the initial supersaturation ratio of 13.4. The dependencies of the size and number of thermolysin crystals on the initial supersaturation ratio were explained by the successive two step mechanism of crystallization.

Electrostatic and steric stabilization of PVC primary particles

AbstractProblems related to the formation and stability of primary particles in VCM suspension and bulk polymerization were theoretically analyzed, based on the DLVO (Deryaguin‐Landau‐Verway‐Overbeek) and HVO (Hesselink‐Vrij‐Overbeek) theories of stability of colloidal systems. In the absence of secondary stabilizers, PVC primary particles were solely stabilized by negative electric charges, as a result of the presence of HCl formed by a chain transfer reaction of chloride redicals to PVC polymer chains. The dependence of the stability of primary particles on the ionic strength of the medium, temperature, and total particle charge was quantitatively investigated in terms of the stability ratio and the particle coalescence rate constant. The steric stabilization of primary particles, arising from the addition of surfactants or polymers to the monomer phase, was examined, using the HVO theory. It was shown that PMMA improved the stability of primary particles and, thus, could alter the particle agglomeration kinetics. The effects of molar mass and the amount of the absorbed PMMA, as well as the influence of solvent quality on the steric stabilization of primary particles, were also investigated. The experimental results, published on the electrostatic and steric stabilization of PVC primary particles, were in good agreement with the theoretical predictions obtained in this study. © 1993 John Wiley &amp; Sons, Inc.

Mechanism of crystallization of enzyme protein thermolysin

The mechanism of crystallization of enzyme protein thermolysin was investigated. The size distribution of thermolysin precipitates was measured by dynamic light scattering during precipitation, and the surface and cross section of the finally obtained precipitate were observed by scanning electron microscopy. The thermolysin precipitates obtained at the initial supersaturation ratio of 8.9 to 164 and pH 7.0 were hexagonal rods having an average size of 9.2×1.5 μm, and were composed of a number of small particles of 15 to 200 nm in diameter. The average size of the small particles was 60 nm in diameter, and the formation of the particles was found to be completed in the early stage of precipitation. Observation of the finally recovered thermolysin precipitate by polarizing microscopy revealed that the precipitate is a crystalline solid. From these data, a possible mechanism of thermolysin crystallization was proposed. The crystallization proceeds through two steps: the first step is the formation of primary particles, and the second step is crystal growth by highly ordered aggregation of the primary particles.

Comparison of the Effects of Anionic and Cationic Surfactant on the Precipitation of Calcium Oxalates

AbstractInfluence of anionic, sodium dodecyl sulfate (SDS) and cationic, dodecylammonium chloride (DDACl) surfactant, on spontaneous batch precipitation of calcium oxalate is investigated. A combination of particle number and size analysis, thermogravimetric analysis, X‐ray diffraction, optical microscopy, electrophoretic and adsorption measurements is used to characterize the processes involved. SDS and DDACl affect the precipitation process at different stages. SDS acts as a nucleation modifier. The action of both surfactants at the stage of primary particle formation exhibits a complex dependence of the rates and mechanisms of crystal growth and aggregation on the structure of aqueous surfactant solutions. Both surfactants inhibit crystal growth and aggregation by adsorption due to the charge and hydrophobic effects. Preferential DDACl sorption on oxalic faces changes the habit of calcium oxalate monohydrate crystals.

Particle formation in emulsion polymerization, 2. Aggregation of primary particles

AbstractThe formation and aggregation of primary particles in the beginning of emulsion polymerization was monitored by surface tension measurements, viscometry, electron microscopy, and static and dynamic light scattering. The dimensions of the primary particles in the emulsion polymerizations of vinyl acetate and methyl methacrylate are of the order of 3 – 5 nm. Immediately after the formation of primary particles these begin to aggregate. The stability of these particles is discussed and conclusions are drawn concerning the formation of new particles in the course of polymerization or in seeded polymerization experiments.

Particle formation in emulsion polymerization, 1. Oligomers in emulsion polymerization of vinyl acetate

AbstractThe oligomers formed in the beginning of the emulsion polymerization of vinyl acetate were characterized by surface tension measurements, analysis of molecular weight distribution and light scattering. The results indicate that primary particle formation in the emulsion polymerization of vinyl acetate is due to an oligomeric micellization. Additionally, these oligomers are strongly adsorbed onto highly dispersed silica particles in the aqueous phase. This effect leads to a disaggregation of the filler particles during the beginning of the emulsion polymerization.

Crystallization of tetragonal (B 8) and cubic (B 1) modifications of zeolite NaP from freshly prepared gel: Part 2. Kinetics of crystallization

The influence of the composition of experimental systems on the kinetics of crystallization of tetragonal (B 8) and cubic (B 1) modifications of zeolite NaP from freshly prepared aluminosilicate gels was studied by various techniques (XRD, SEM, EDX, chemical analysis of the solid and the liquid phase, analysis of the polycondensation of silicon in the liquid phase). The formation of primary particles of zeolite P takes place by two nucleation processes: The first is the rapid heterogeneous nucleation in the liquid phase, and the second is the formation of particles of quasi-crystalline phase inside the gel matrix as the potential nuclei. The latter become the active nuclei after their release from the dissolved part of the gel and consequent full contact with the liquid phase. The kinetics of crystal growth of tetragonal modification of zeolite P is found to be expressed as dL dt c = k g[C Al ][C Si ] n , where dL dt c is the change in the size L of the discrete particles of tetragonal modification of zeolite P, t c is the time of crystallization, C Al and C Si are concentrations of silicon and aluminum in the liquid phase (which depend on the alkalinity of the system), and k g = 1.888 cm min −1 mol 5.567 dm −16.701 and n = 4.567 are constants.