Particles, formation from sols

Adsorption of nitric and sulfuric acids on ice particles provides the sol of the nitrating mixture. An important catalyst of aromatic nitration, nitrous acid, is typical for polluted atmospheres. Combustion sources contribute to air pollution via soot and NO emissions. The observed formation of HNO2 results from the reduction of nitrogen oxides in the presence of water by C—O and C—H groups in soot (Ammann et al. 1998). As seen, gas-phase nitration is important ecologically. 

Most film and particle formation techniques can be divided into gas-phase and liquid-phase deposition processes, which are briefly discussed in this section. Deposition of metal and metal oxides from metal enolate sources results from application of CVD, ALD, spin-coating, electrochemical and sol-gel methods, which are discussed in detail... 

There is no principal difference in particle packing from suspensions and particulate sols. In both cases the densification of the dispersion may be accompanied by agglomeration of the particles cind the formation of a continuous... 

Results of XAFS measurements are compiled in Table 1 with the results of the estimates of the TEM measurements (for details on TEM results see ref. [ 13 ]). The catalysts are named according to the stabilizing polymer and particle size determined by TEM. Except for the PVP stabilized catalyst reduced with methanol, an excellent agreement exists between the size of the metal particles estimated from TEM and EXAFS. The methanol reduced sample shows the smallest particle size when estimated from EXAFS, while it appears to be much larger in TEM. At present we would like to speculate that this is due to the formation of agglomerates (diameter 3.5 nm) formed from very small particles (diameter 1.2 nm). Consequendy, the eatalyst is named Rh/PVP/3.5-a to indicate the difference with the sol containing primary particles of 3.5 nm in diameter. 

The hydrolysis of J-diketonate-modified cerium(IV) isopropoxide leads to the formation of colloidal solutions or gels. The complexation ratio (x = acac/Ce) appears to be the key parameter to tailor the size of cerium oxide particles. Precipitation is observed when x < 0.1, whereas sols are obtained when 0.1 < X < 1. The mean hydrodynamic diameter of these particles decreases from 450 to 15 A when x goes from 0.1 to 1. Only solute molecular clusters are formed when the complexing ratio becomes larger than 1 (1 < x < 2). 

In order to further understand the mechanism in this low-pressure environment, a sol precursor containing colloidal nanoparticles was also studied [22]. From the experimental results, dispersed nanoparticles could be obtained at a relatively low pressure (20 Torr). A possible mechanism for the sol-to-dry particle formation in the spray-drying process under low-pressure conditions was proposed based on the experimental results and available theories. Droplet breakup due to competition between osmotic and Laplace pressure, and particle fragmentation due to fast drying, were considered to be two major factors. 

For sample BTH, the procedure is the same as that of BTX except that the surfiictant was dissolved in nitrate solution instead of ammonia solution. For BHX, the phase transfer process was separated from sol formation and surfactant adsorption, i.e. the organic solvent was added after rather than simultaneously. Sample BHH was prepared as follows the sol particles were prepared by reverse-sequence, namely first co-precipitation then phase transfer by adding the emulsion of surfactant, benzene and water (because the surfactant cannot be dissolved in benzene), which means that the sol... 

TEM images of particles deposited from the standard sol yielded an (number) average diameter of 1.5 nm (mass-weighted mean 2.2 nm), and a coefficient of variation (o/) of 38 %, but some cluster coalescence within the spedmen was observed. This caused bands of much coarser particles (<10 nm) at locally high concentrations to be present in the specimen. Only the well separated clusters were measured for the distribution presented here. The addition of the colloid-stabilising agent gelatin to the hydrosol prevented the formation of these bands of presumably... 

The basic theory of gel formation from colloidal particles has been formulated by Thomas and McCorkle (228), who show that the Verwey-Overbeek theory for the interaction of two spherical double layers around adjacent spherical colloidal particles leads to isotropic flocculation. New particles can be attached more readily to the ends of a chainlike floe where the repulsion energy barrier is at a minimum. It is this type of aggregation that converts a sol to a gel at a certain point by forming an infinite network of chains of particles throughout the sol volume. (See also Chapter 3.)... 

The theory of the formation and structure of silica gels formed by the polymeriza tion of silicic acids hu been dbcussed in Chapter 3 and similar aspects of gels made from sols of discrete colloidal particles dealt with in Chapter 4. Characterization of gels has been described in the foregoing section. Described here are methods of mak ing gels and the factors and process variables that affect and control gel properties and uses., ... 

Gel formation from colloidal silica has been discussed in Chapter 4. Colloidal silica now available as concentrated sols of various particle sizes will no doubt play an increasingly important role in gel manufacture. ... 

Extremely small particles are not that difficult to prepare, and have been known for a very long time, usually prepared in the forms of sols or smokes (fumes). The challenge is to isolate them in a form that is easily handled and readily dispersible. Some type of agglomeration/aggregation is generally used to isolate the particles, but this is usually partly irreversible, resulting in products that cannot be redispersed into a polymer in their nanoparticle form. Carbon blacks and most fumed and precipitated silicas are examples of such products. One way around this is to form the particles directly in the polymer, or in a liquid monomeg but this has restricted applicability, partly because there are usually coproducts from the particle formation process to deal with. It is only recently that sufficient commercial incentive has been identified to focus real attention on this problem. 

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