Particle as stabilizer

Two Liquids Plus a Solid. SoHd particles may be used to stabilize an emulsion, avoiding the problem of simultaneous stabilization of both the oil drops of the emulsion and the soHd particles of the suspension. The key factor for the use of particles as stabilizers is their location. If they are located at the iaterface between the two Hquids, they will stabilize the emulsion, serving as a mechanical barrier to prevent the coalescence of the droplets (Fig. 17). 

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... 

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... 

Walther A, Hoffmann M, Mueller AHE (2008) Emulsion polymerization using Janus particles as stabilizers. Angew Chem Int Ed 47(4) 711-714... 

Nanoparticles in the GML/tetradecane/water system could also be formed in the presence of clay particles. The authors assumed a Pickering emulsion-like stabilization by the clay particles which are of disk-like shape with about Inm thickness and 30 nm diameter. Detailed information about the stabilization mechanism within these systems remains, however, still to be obtained. The use of clay particles as stabilizers holds some promise with regard to the preparation of "emulsifier-free" systems which are very interesting for pharmaceutical purposes. Hydrolysis of the monoglyceride was, however, pronounced in these dispersions due to the comparatively high pH introduced by the clay particle suspension. 

Duan et alP also reported the synthesis of poly(A -isopropylacrylamide)-silica composite microspheres by using inverse Pickering suspension polymerization with various sizes of silica particles as stabilizers. Figure 1.14 shows examples of such microgels stabilized by silica particles with mean diameters of 53, 301, 500 and 962 nm. To generate these nanocomposite structures, droplets of an aqueous solution of 7V-isopropylacrylamide were first dispersed in toluene and then stabilized by silica particles. The monomer was subsequently polymerized to obtain polymer silica composite microspheres. It was also observed that the thermo-responsive behavior of the polymer was not affected in the presence of silica, as a lower critical solution temperature of 32 °C for the poly(A -isopropylacrylamide) was also observed in the polymer-silica microspheres. 

There has been extensive interest in preparing polymer lay nanocomposites by using direct emulsion and miniemulsion polymerizations, which are covered in other chapters of this book. The focus of this section is on using nascent clay particles as stabilizing agents for direct emulsion polymerization. 

Atoms with the same number of protons but a different number of neutrons are called isotopes. To identify an isotope we use the symbol E, where E is the element s atomic symbol, Z is the element s atomic number (which is the number of protons), and A is the element s atomic mass number (which is the sum of the number of protons and neutrons). Although isotopes of a given element have the same chemical properties, their nuclear properties are different. The most important difference between isotopes is their stability. The nuclear configuration of a stable isotope remains constant with time. Unstable isotopes, however, spontaneously disintegrate, emitting radioactive particles as they transform into a more stable form. 

Aqueous dispersions are alternatives to solutions of Hquid and soHd resins. They are usuaUy offered in 50% soHds and may contain thickeners and cosolvents as stabilizers and to promote coalescence. Both heat-reactive (resole) and nonheat-reactive (novolak) systems exist that contain unsubstituted or substituted phenols or mixtures. A related technology produces large, stable particles that can be isolated as discrete particles (44). In aqueous dispersion, the resin stmcture is designed to produce a hydrophobic polymer, which is stabilized in water by an interfacial agent. 

The most commonly used emulsifiers are sodium, potassium, or ammonium salts of oleic acid, stearic acid, or rosin acids, or disproportionate rosin acids, either singly or in mixture. An aLkylsulfate or aLkylarenesulfonate can also be used or be present as a stabilizer. A useful stabilizer of this class is the condensation product of formaldehyde with the sodium salt of P-naphthalenesulfonic acid. AH these primary emulsifiers and stabilizers are anionic and on adsorption they confer a negative charge to the polymer particles. Latices stabilized with cationic or nonionic surfactants have been developed for special apphcations. Despite the high concentration of emulsifiers in most synthetic latices, only a small proportion is present in the aqueous phase nearly all of it is adsorbed on the polymer particles. 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

Fig. 17. SoHd particles adsorbed at the oil-water iaterface serve as stabilizers for an emulsion.

CuNPs) in Fig. 7 shows the monodisperse and uniformly distributed spherical particles of 10+5 nm diameter. The solution containing nanoparticles of silver was found to be transparent and stable for 6 months with no significant change in the surface plasmon and average particle size. However, in the absence of starch, the nanoparticles formed were observed to be immediately aggregated into black precipitate. The hydroxyl groups of the starch polymer act as passivation contacts for the stabilization of the metallic nanoparticles in the aqueous solution. The method can be extended for synthesis of various other metallic and bimetallic particles as well. 

The syntheses described in the preceding section can be performed using as stabilizers the classical ligands of organometalhc chemistry (e.g., amines, thiols, or phosphines) instead of polymers. The amount of ligand added allows control of the particle growth and therefore the size. 

The presence of shielding compounds interferes with subsequent processes, as the formation of metal-support interactions is able to stabilize supported particles. Moreover, the shielding effect of the colloid protectors prevents the contact of metal particles with the reacting molecules, thus avoiding the use of unsupported colloidal particles as a catalytic system [11]. 

Non-Aqueous Colloidal Metal Solutions. It has been difficult to prepare colloidal gold in non-aqueous media due to limitations in preparative methods (low salt solubilities, solvent reactivity, etc.), and the fact that the low dielectric constant of organic solvents has hindered stabilization of the particles. In aqueous solution the gold particles are stabilized by adsorption of innocent ions, such as chloride, and thus stabilized toward flocculation by the formation of a charged double layer, which is dependent on a solvent of high dielectric constant. Thus, it seemed that such electronic stabilization would be poor in organic media. 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

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