Particle formation/structure

The Smith-Ewart theory has been modified by several researchers [13,20-24]. These researchers argued against the Smith-Ewart theory that (1) the particle formation also occurs in the absence of micellar structure, (2) the predictions on particle number with the Smith-Ewart theory are higher relative to actual case. 

Besides electronic effects, structure sensitivity phenomena can be understood on the basis of geometric effects. The shape of (metal) nanoparticles is determined by the minimization of the particles free surface energy. According to Wulffs law, this requirement is met if (on condition of thermodynamic equilibrium) for all surfaces that delimit the (crystalline) particle, the ratio between their corresponding energies cr, and their distance to the particle center hi is constant [153]. In (non-model) catalysts, the particles real structure however is furthermore determined by the interaction with the support [154] and by the formation of defects for which Figure 14 shows an example. 

Figure 2 schematically presents a synthetic strategy for the preparation of the structured catalyst with ME-derived palladium nanoparticles. After the particles formation in a reverse ME [23], the hydrocarbon is evaporated and methanol is added to dissolve a surfactant and flocculate nanoparticles, which are subsequently isolated by centrifugation. Flocculated nanoparticles are redispersed in water by ultrasound giving macroscopically homogeneous solution. This can be used for the incipient wetness impregnation of the support. By varying a water-to-surfactant ratio in the initial ME, catalysts with size-controlled monodispersed nanoparticles may be obtained. 

For the production of CLPs and VLPs with baculovirus infected insect cells the specific proteins that are required for particle formation should be chosen at an early step. The specific particle composition, along with the expression of other non-structural (NS) proteins often has a great impact on particle stability [11] or on particle localisation [12], i.e. - cell-associated or secreted to the supernatant. 

Since the properties of these particulate materials are basically determined by their mean size, size distribution, external shape, internal structure, and chemical composition, the science in the mechanistic study of particle formation and the fundamental technology in their synthesis and characteristic control may constitute the background for the essential development of colloid science and pertinent industries. Scientists have now learned how to form monodispersed fine particles of different shapes of simple or mixed chemical compositions, and, as a result, it is now possible to design many powders of exact and reproducible characteristics for a variety of uses. These achievements are especially important in the manufacture of high-quality products requiring stringent specification of properties. 

Three different ways have been developed to produce nanoparticle of PE-surfs. The most simple one is the mixing of polyelectrolytes and surfactants in non-stoichiometric quantities. An example for this is the complexation of poly(ethylene imine) with dodecanoic acid (PEI-C12). It forms a solid-state complex that is water-insoluble when the number of complexable amino functions is equal to the number of carboxylic acid groups [128]. Its structure is smectic A-like. The same complex forms nanoparticles when the polymer is used in an excess of 50% [129]. The particles exhibit hydrodynamic diameters in the range of 80-150 nm, which depend on the preparation conditions, i.e., the particle formation is kinetically controlled. Each particle consists of a relatively compact core surrounded by a diffuse corona. PEI-C12 forms the core, while non-complexed PEI acts as a cationic-active dispersing agent. It was found that the nanoparticles show high zeta potentials (approximate to +40 mV) and are stable in NaCl solutions at concentrations of up to 0.3 mol l-1. The stabilization of the nanoparticles results from a combination of ionic and steric contributions. A variation of the pH value was used to activate the dissolution of the particles. 

The kinetics and mechanisms of particle growth and polymer structure development are comparatively well understood compared to those of particle nucleation. Therefore, the rate of polymerization and the properties of the polymer produced can be (roughly) estimated as long as the number of polymer particles produced is known (for example, in seeded emulsion polymerization). However, the prediction of the number of polymer particles produced is still far from being an estabUshed technique. Therefore, further efforts are needed to qualitatively and quantitatively clarify the effects of numerous factors that affect the process of particle formation in order to gain a more quantitative understanding of emulsion polymerization. 

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