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Homogeneous Nucleation (homogeneous + nucleation)
Selected AbstractsNanoencapsulation of a hydrophobic compound by a miniemulsion polymerization processJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 9 2004Yingwu Luo Abstract The nanoencapsulation of hydrophobic compounds by miniemulsion polymerization, a convenient one-step encapsulation technique for nanocapsules, was investigated in terms of the thermodynamics and kinetics. The encapsulation was achieved by polymerization inducing phase separation within minidroplets dispersed in an aqueous phase. Thermodynamic factors (the level and type of surfactant, the level of the hydrophilic comonomer, and the monomer/paraffin ratio), kinetic factors (the level of the crosslinking agent or chain-transfer agent), and nucleation modes were all found to have a great influence on the latex morphology. Specifically, for a styrene/paraffin system, there were optimum levels of sodium dodecyl sulfate (1.0 wt %), the hydrophilic comonomer (1.0 wt % methyl acrylate acid), and the chain-transfer agent (0.2 wt % n -dodecanethiol) for obtaining well-defined nanocapsules of paraffin with a styrene/paraffin ratio of 1:1. When the styrene/paraffin ratio was reduced, however, it was more difficult to achieve a fully encapsulated particle morphology. Homogeneous nucleation could compete with encapsulation, and this resulted in a pure polymer particle and a half-moon morphology. Conditions were also found under which complete encapsulation could be observed with a water-soluble initiator (potassium persulfate), contrary to certain reports. Replacing potassium persulfate with an oil-soluble initiator (2,2-azobisisobutyronitrile) had little influence on the morphology under those conditions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2145,2154, 2004 [source] Determination of nucleation parameters and the solid liquid interfacial energy of the KCl-ethanol-water systemCRYSTAL RESEARCH AND TECHNOLOGY, Issue 5 2007Waid Omar Abstract The kinetic parameters of homogeneous nucleation of KCl in different ethanol-water solvent mixtures were determined at 25°C from the experimental measurements of the width of the metastable zone at different cooling rates. The ethanol mass ratio in the ethanol water solvent mixture was varied from 0-0.9 and the metastable zone width for each solvent mixture was measured under the cooling rates of 10, 20 and 30 K/h. The influence of ethanol ratio on the activity coefficient was calculated. It was found that increasing the ethanol ratio in the solvent mixture leads to an increase in the mean molal activity coefficient. The experimental results obtained showed that the increase in the ethanol ratio in the solvent widens the metastable zone for the crystallization of KCl. Also it has inferred from the calculations based on the classical nucleation theory that increasing of the ethanol ratio in the solvent mixture resulted in an increase of the nucleation rate order, increase of the critical size of nuclei and increase of the solid liquid interfacial energy. It has been found that the solid liquid interfacial energy can be good correlated with inversely proportionality to the solubility. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Equilibrium studies of protein aggregates and homogeneous nucleation in protein formulationJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 2 2010Sylvia Kiese Abstract Shaking or heat stress may induce protein aggregates. Aggregation behavior of an IgG1 stressed by shaking or heat following static storage at 5 and 25°C was investigated to determine whether protein aggregates exist in equilibrium. Aggregates were detected using different analytical methods including visual inspection, turbidity, light obscuration, size exclusion chromatography, and dynamic light scattering. Significant differences were evident between shaken and heated samples upon storage. Visible and subvisible particles (insoluble aggregates), turbidity and z -average diameter decreased whilst soluble aggregate content increased in shaken samples over time. Insoluble aggregates were considered to be reversible and dissociate into soluble aggregates and both aggregate types existed in equilibrium. Heat-induced aggregates had a denatured protein structure and upon static storage, no significant change in insoluble aggregates content was shown, whilst changes in soluble aggregates content occurred. This suggested that heat-induced insoluble aggregates were irreversible and not in equilibrium with soluble aggregates. Additionally, the aggregation behavior of unstressed IgG1 after spiking with heavily aggregated material (shaken or heat stressed) was studied. The aggregation behavior was not significantly altered, independent of the spiking concentration over time. Thus, neither mechanically stressed native nor temperature-induced denatured aggregates were involved in nucleating or propagating aggregation. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:632,644, 2010 [source] Synthesis of oily core-hybrid shell nanocapsules through interfacial free radical copolymerization in miniemulsion: Droplet formation and nucleationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2010Z. H. Cao Abstract Nanocapsules with an oily core and an organic/inorganic hybrid shell were elaborated by miniemulsion (co)polymerization of styrene, divinylbenzene, ,-methacryloyloxy propyl trimethoxysilane, and N -isopropyl acrylamide. The hybrid copolymer shell membrane was formed by polymerization-induced phase separation at the interface of the oily nanodroplets with water. It was shown that the size, size distribution, and colloidal stability of the miniemulsion droplets were extremely dependent on the nature of the oil phase, the monomer content and the surfactant concentration. The less water-soluble the hydrocarbon template and the higher the monomer content, the better the droplet stability. The successful formation of nanocapsules with the targeted core-shell morphology (i.e., a liquid core surrounded by a solid shell) was evidenced by cryogenic transmission electron microscopy. Both nanocapsules and nanoparticles were produced by polymerization of the miniemulsion droplets. The proportion of nanoparticles increased with increasing monomer concentration in the oil phase. These undesirable nanoparticles were presumably formed by homogeneous nucleation as we showed that micellar nucleation could be neglected under our experimental conditions even for high surfactant concentrations. The introduction of ,-methacryloyloxy propyl trimethoxysilane was considered to be the main reason for homogeneous nucleation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 593,603, 2010 [source] A New Strategy for the Synthesis of Polyaniline Nanostructures: From Nanofibers to NanowiresMACROMOLECULAR RAPID COMMUNICATIONS, Issue 6 2007Jing Li Abstract A new approach for the synthesis of polyaniline (PANI) nanostructures under UV light illumination has been developed, which is the first report of a templateless chemical process for preparing pure PANI nanowires. The acceleration effect of photo-assistance on the polymerization can promote the homogeneous nucleation and elongation of the nanofibers and nanowires, leading to easy preparation of tunable diameters of the nanowires and nanofibers of PANI. [source] Condensation and aggregation of solar corundum and corundum-hibonite grainsMETEORITICS & PLANETARY SCIENCE, Issue 7-8 2007T. M. Nakamura Some of the corundum and corundum-hibonite grains occur as aggregates of 2 to 6 grains having similar sizes. The oxygen isotopic compositions of some of the corundum-bearing grains suggest their solar nebula origin. 26Al- 26Mg systematics of one corundum grain showed the canonical initial 26Al/27Al ratio, also suggesting a solar nebula origin. Quantitative evaluation of condensation and accretion processes made based on the homogeneous nucleation of corundum, diffusion-controlled hibonite formation, collisions of grains in the nebula, and critical velocity for sticking, indicates that, in contrast to the hibonite-bearing aggregates of corundum grains, the hibonite-free corundum aggregates could not have formed in the slowly cooling nebular region with solar composition. We suggest instead that such aggregates formed near the protosun, either in a region that stayed above the condensation temperature of hibonite for a long time or in a chemically fractionated, Ca-depleted region, and were subsequently physically removed from this hot region, e.g., by disk wind. [source] Snow metamorphism as revealed by scanning electron microscopyMICROSCOPY RESEARCH AND TECHNIQUE, Issue 1 2003Florent Dominé Abstract Current theories of snow metamorphism indicate that sublimating snow crystals have rounded shapes, while growing crystals have shapes that depend on growth rates. At slow growth rates, crystals are rounded. At moderate rates, they have flat faces with rounded edges. At fast growth rates, crystals have flat faces with sharp edges, and they have hollow faces at very fast growth rates. The main growth/sublimation mechanism is thought to be by the homogeneous nucleation of new layers at or near crystal edges. It was also suggested that the equilibrium shape of snow crystals would be temperature dependent: rounded above ,10.5°C, and faceted below. To test these paradigms, we have performed SEM investigations of snow samples having undergone metamorphism under natural conditions, and of snow samples subjected to isothermal metamorphism at ,4° and ,15°C in the laboratory. In general, current theories predicting crystal shapes as a function of growth rates, and of whether crystals are growing or sublimating, are verified. However, the transition in equilibrium shapes from rounded to faceted at ,10.5°C is not observed in our isothermal experiments that reveal a predominance of rounded shapes after more than a month of metamorphism at ,4 and ,15°C. Some small crystals with flat faces that also have sharp angles at ,15°C, are observed in our isothermal experiments. These faces are newly formed, and contradict current theory. Several hypotheses are proposed to explain their occurrence. One is that they are due to sublimation at emerging dislocations. Microsc. Res. Tech. 62:33,48, 2003. © 2003 Wiley-Liss, Inc. [source] An overview of the microphysical structure of cirrus clouds observed during EMERALD-1THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 607 2005M. W. Gallagher Abstract High-resolution ice microphysical, turbulence, heat and water vapour flux data in cirrus clouds were collected by the Airborne Research Australia's (ARA) Egret Grob 520T research aircraft during the first Egret Microphysics with Extended Radiation and Lidar experiment (EMERALD-1). The in situ cirrus measurements were guided by simultaneous airborne lidar measurements collected by the ARA Super King Air research aircraft which flew below the cirrus and whose horizontal position was synchronized with the Egret. This allowed the microphysics and turbulence measurements to be interpreted and evaluated within the context of large-scale cirrus structure and its evolution. A significant feature of the clouds observed was the presence on occasion of active convective columns. Large variations in the cirrus dynamics were observed, with significant variations in the ice crystal habit from cloud top to cloud base and within the evaporating fall-streaks of precipitation. However, on average the picture presented is consistent with that shown by Heymsfield and Miloshevich, and by Kajikawa and Heymsfield, with the upper supersaturated region of the cloud acting as an active particle-generation zone where homogeneous nucleation proceeds apace; ice crystals there are initially dominated by small irregular or spheroidally shaped particles, some of which can be identified as proto or ,germ' rosettes. These are then observed to grow into more open bullet rosette and columnar types as they fall into the less supersaturated middle and lower layers of the cloud. The mean recognisable ice particle size fell within a very narrow size band, 70,90 µm, but the actual size distribution is thought to increase in a continuous manner to smaller sizes. However, there are currently instrument limitations that make it difficult to confirm this unambiguously. Unlike most previous studies, however, the cirrus clouds observed here were mostly devoid of pristine plate-like crystals, as nucleation and growth within the planar growth regime was rarely encountered. During some cases bullet rosettes, once formed, did undergo transition to the plate growth regime with complex crystal shapes resulting. The mean size of pristine bullet rosettes was again confined to a relatively narrow range. The likely nucleation processes dominating in cirrus clouds are discussed in the light of the observations. Very high concentrations of small ice crystals were sometimes detected, concentrations reaching a maximum of 10 000 L,1. There is strong evidence supporting these high concentrations which are probably produced by the homogeneous freezing of aerosol. Copyright © 2005 Royal Meteorological Society [source] |