Pickering stabilizers

Fig. 20. a Scheme for using silica particles as stabilizer for monomer droplets of miniemul-sions(Pickering stabilization) b Represents a latex with a monomer to silica ratio of 1 0.32... 

FIGURE 13.18 Pickering stabilization. Small solid particles, preferably wetted by the continuous phase, adsorbed onto emulsion drops. 

For coalescence of emulsion droplets, an important variable is whether a flattened film between the droplets is formed. This is governed by the ratio of the external stress over the Laplace pressure. The external stress can be due to colloidal attraction (e.g., van der Waals forces), a shear stress, or gravitational forces in a sediment layer. Small protein-stabilized droplets will not deform, except in a sediment layer in a centrifuge, and they are very stable to coalescence. If the drops are large, the interfacial tension is low, and the external stress is high, droplets will deform and coalescence can readily occur. Water-in-oil emulsions cannot be made with protein as the surfactant, and it is often difficult to stabilize them against coalescence, except by a layer of small hydrophobic particles (Pickering stabilization). 

Silica nanoparticles are also hydrophilic and have therefore to be functionalized prior to encapsulation. Without functionalization, the negatively charged silica particles can be used as Pickering stabilizers, leading to hybrid nanoparticles with silica located on the surface of the nanoparticles (see Fig. 13a) [100]. 

Keywords CoUoids Heterocoagulation Nanocomposites Pickering stabilization - Polymer latex Self-assembly Supracolloidal structures... 

Pickering Stabilization Adhesion of Particles to Soft Interfaces. 34... 

We have seen from the above discussion that solid particles can adhere to a soft interface, and thus to monomer droplets. The effect of Pickering stabilization protects the droplets from coalescence. The use of solid particles as stabilizers in emulsion-based polymerization techniques was first described in open literature by... 

Muller and coworkers prepared disc-like polymer Janus particles from assembled films of the triblock copolymer SBM and, after hydrolysis of the ester groups into methacrylic acid units, used these as Pickering stabilizer in the soap-free emulsion polymerization of styrene and butyl acrylate [111]. Armes and coworkers described the synthesis of PMMA/siUca nanocomposite particles in aqueous alcoholic media using silica nanoparticles as stabilizer [112], extending this method to operate in water with a glycerol-modified silica sol [113, 114]. Sacanna showed that methacryloxypropyltrimethoxysilane [115] in the presence of nanosized silica led to spontaneous emulsification in water, which upon a two-step polymerization procedure afforded armored particles with an outer shell of PMMA [116]. Bon and coworkers demonstrated the preparation of armored hybrid polymer latex particles via emulsion polymerization of methyl methacrylate and ethyl methacrylate stabilized by unmodified silica nanoparticles (Ludox TM O) [117]. Performance of an additional conventional seeded emulsion polymerization step provided a straightforward route to more complex multilayered nanocomposite polymer colloids (see Fig. 14). 

Bon Stefan AF, Chen T (2007) Pickering stabilization as a tool in the fabrication of complex nanopattemed silica microcapsules. Langmuir 23(19) 9527-9530... 

As mentioned above, almost all polymerizations performed in the presence of an auxiliary comonomer were conducted without surfactant [47-55, 60]. Since these early works, several groups have demonstrated that silica particles can stabilize polymer latexes provided that wetting of the nanoparticles with the polymer chains is favorable [64-68]. The overall process, often referred to as Pickering stabilization by reference to the stabilization of two immiscible liquids by solid particles [69], has... 

Recently, a number of groups have also reported the Pickering stabilization of monomer-containing lipid droplets by clays, and their subsequent free radical polymerization [291]. As these articles fall outside the scope of this review, they will not be discussed further. Although not strictly speaking in the scope of the present review, it is also worth mentioning the recent work of Voorn et al. on the first surfactant-free inverse emulsion polymerization stabilized with hydrophobic MMT platelets [292],... 

Figure 8.5. Principle of the action of Pickering stabilizers (not to scale). The horizontal line indicates the oil-water interface, with the oil phase being assumed to be located above the latter os, contact angle between a Pickering stabilizer particle and the oil phase ws, contact angle between a Pickering stabilizer particle and the water phase...

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]. 

In the second part of this article, polymer latexes surrounded by anisotropic Laponite platelets have been successfully obtained by the two routes. It was demonstrated that the clay particles play the role of a pickering stabilizer and are capable to stabilize the composite particles whose diameter depends on the amount of Laponite initially introduced into the reactor. The higher the clay concentration, the larger the composite particle number and, therefore, the higher the polymerization rate as predicted from the emulsion polymerization theory. 

Pickering stabilization is applied, for example, in emulsions and foams in food stuffs, pharmaceutical products, and cosmetics. It may also play an adverse role in biological waste water purification where bacteria stabilize undesired foaming. 

FIGURE 8.13 Pickering stabilization of a liquid/liquid interface. 

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