Pickering stabilization, inorganic

Pickering stabilizers, commonly used in styrene suspension polymerization, are inorganic solids, insoluble in the aqueous phase. Their main advantage is that they can be removed easily from the final particulate product (e.g., by dilute acid), which improves the clarity and transparency of the polymer. Also, the amount of polymer deposited on the wall and on other parts of the reactor decreases, which considerably improves the heat transfer rate from the reaction medium to the coolant. Finally, it should be mentioned that inorganic powders are usually cheaper [5]. 

Inverted raspberry-like morphologies (the mineral particles being located at the surface of the latex spheres) have also been discussed in Sections 4.4.2.2 and 4.4.2.4 about colloidal silica and layered silicates, respectively. These are mainly a consequence of the surfactant-like behavior of the inorganic particles in specific situations. This was clearly illustrated in a recent report by Landfester, who showed that silica or clays can be used as pickering stabilizers of miniemulsion jxjlymer-izations, resulting therefore in the formation of armored latexes, the surface of which was recovered by the small inorganic particles [99,131]. 

It was discussed that the structure created by the ternary system oil/water/ nanoparticle follows the laws of spreading thermodynamics, as they hold for ternary immiscible emulsions (oil 1 /oil 2/water) [114,116,117]. The only difference is that the interfacial area and the curvature of the solid nanoparticle has to stay constant, i.e., an additional boundary condition is added. When the inorganic nanoparticles possess, beside charges, also a certain hydrophobic character, they become enriched at the oil-water interface, which is the physical base of the stabilizing power of special inorganic nanostructures, the so-called Picker-... 

Emulsified droplets and micelles/vesicles formed by surfactant and amphiphilic block copolymers are often used as soft templates (Figure 11.1a). An inorganic shell can be formed on these templates if certain attractive interactions exist between the inorganic species and template surfeces. Pickering emulsions -emulsions stabilized by inorganic nanoparticles instead of amphiphilic molecules -can be converted to hollow particles by connecting the nanoparticles to each other. The templates are finally removed by solvent extraction, calcination, or simple... 

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