Particle clustering emulsion phase

Besides emulsion polymerization with a separate monomer phase, emulsions consisting of diluted monomer droplets together with inorganic particles were also polymerized by the group of Ruckenstein [58]. They created emulsions of decane, a monomer and silica in an aqueous solution of surfactant, which were polymerized to latexes containing uniformly distributed inorganic particle clusters of submicrometer size. 

Flow nature Bubbles and emulsion phase Dispersed dilute phase and dense phase Core-annular flow, particle accumulation near wall and particle clustering Dispersed dilute flow with a thin particle layer on the wall... 

In Zerrouki s experiments, the preparation of aqueous phases of identical clusters is performed in six steps. First, colloidal particles of silica, 1.2 pm in diameter, are synthesized. Next, the surface of the particles is made hydrophobic by chemical grafting. Then, an oil-in-water premix emulsion is made by adding an octane suspension of the colloids in an aqueous solution. Controlled shear of the premix in a Couette-type apparatus is subsequently performed to obtain a quasi-monodisperse... 

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

A size distribution of particles is always desired rather than a single size in a fluidized bed. The two-phase theory of fluidized-bed operation is suspect when a bed contains appreciable lines, and models based on uniform particles should be used with caution. The dense phase in such cases should really be regarded as consisting of two phases emulsion and clusters of lines (d < 40 pm). Indeed, the results of Yadav et al. (1994) on commercial propylene ammoxidation catalyst clearly show that the lines agglomerate. A critical level of lines (30%) was found in terms of bed expansion, aeratability, and cluster size at which fluid-bed behavior is optimum. They proposed a model that takes the two dense phase components (emulsion and cluster) into account. Adding lines widens the limits of operable gas velocities and minimizes the segregation of particles. 

Bon and coworkers carried out a study on the fate of the nanoparticles throughout solids-stabilized emulsion polymerization [119], A quantitative method based on disk centrifugation was developed to monitor the amount of nanoparticles present in the water phase in solids-stabilized emulsion polymerizations of vinyl acetate, methyl methacrylate, and butyl acrylate. The concentration profile of nanoparticles in the water phase as a function of monomer conversion agreed with theoretical models developed for the packing densities in these systems [120]. Noteworthy was that in the case of silica-nanoparticle-stabilized emulsion polymerization of vinyl acetate, the event of late-stage limited coalescence, leading to small armored non-spherical clusters, could be predicted and explained on the basis of the concentration profiles and particle size measurements. Adjusting the amount of silica nanoparticles prevented this phenomenon. 

Microemulsions [512, 513] are special types of emulsions that form spontaneously and have very small particles. Microemulsions are optically clear, thermodynamically stable dispersions of two immiscible liquids obtained by the use of carefully adjusted surface-active molecules (surfactants). Both liquids in a microemulsion will be present in regions of the same order of magnitude, with the dispersed phase on the order of 10-100 nm. Aggregates of surface-active molecules, or micelles, form into colloidal-sized clusters in such a way that hydrophilic groups are directed toward the water. These definitions [514] are general in nature, but they suffice for the current purpose the interested reader is directed to texts on this... 

When emulsifiers, such as soaps, alkyl sulfates, and alkyl sulfonates, are present in water above a minimum concentration called the critical micelle concentration, the emulsifier molecules aggregate into micelles. In micelles, the hydrophobic ends of the emulsifier are clustered at its center, and the hydrophilic ends extend into the aqueous phase. Micelles are spherical or lamellar with dimensions on the order of 2.5 to 5 nm, depending on the emulsifier used. There may be 10 to 10 micelles/cm of water. When monomer initially is added, a small fraction of the molecules is absorbed into the micelles, swelling them slightly. Most of the monomer is dispersed as droplets 1000 to 3000 nm in diameter, stabilized with emulsifier molecules along their surface. They number about 10 ° to 10 particles/cm of water. These droplets serve as the monomer reservoir for the polymerization. A representation of emulsion polymerization early in the process is shown in Fig. 19.14. 

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