Stabilization by particles

Since most colloidal dispersions are stabilized by particle interactions, the use of equation (10.51) may lead to biased estimates of particle size that are often concentration dependent. The effect may be taken into account by expanding the diffusion coefficient to a concentration power series that, at low concentrations, gives ... 

The effects of particles on foam stability are usually discussed in terms of individual particles because a single particle can rupture a thin liquid film. Emulsion stabilization by particles alone presumably involves close-packed monolayers aroimd the emulsion drops. However, less than close-packed layers can, in principle, have important effects on emulsion stability in systems stabilized by smfactants, and this area warrants further study. 

Among the many applications of LB films, the creation or arrangement of colloidal particles in these films is a unique one. On one hand, colloidal particles such as 10-nm silver sols stabilized by oleic acid can be spread at the air-water interface and LB deposited to create unique optical and electrooptical properties for devices [185]. 

Figure C2.3.11 Key surfactant stmctures (not to scale) in emulsion polymerization micelles containing monomer and oligomer, growing polymer particle stabilized by surfactant and an emulsion droplet of monomer (reservoir) also coated with surfactant. Adapted from figure 4-1 in [67],...

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

The inverse emulsion form is made by emulsifying an aqueous monomer solution in a light hydrocarbon oil to form an oil-continuous emulsion stabilized by a surfactant system (21). This is polymerized to form an emulsion of aqueous polymer particle ranging in size from 1.0 to about 10 pm dispersed in oil. By addition of appropriate surfactants, the emulsion is made self-inverting, which means that when it is added to water with agitation, the oil is emulsified and the polymer goes into solution in a few minutes. Alternatively, a surfactant can be added to the water before addition of the inverse polymer emulsion (see Emulsions). 

Fig. 4. (a) Polymer bridging between particles and (b), particle stabilization by adsorbed polymer (32). 

In a suspension polymerization, monomer is suspended ia water as 0.1—5 mm droplets, stabilized by protective coUoids or suspending agents. Polymerization is initiated by a monomer-soluble initiator and takes place within the monomer droplets. The water serves as both the dispersion medium and a heat-transfer agent. Particle size is controlled primarily by the rate of agitation and the concentration and type of suspending aids. The polymer is obtained as small beads of about 0.1—5 mm in diameter, which are isolated by filtration or centrifugation. 

In the post-dispersion process, the soHd phenoHc resin is added to a mixture of water, cosolvent, and dispersant at high shear mixing, possibly with heating. The cosolvent, frequently an alcohol or glycol ether, and heat soften the resin and permit small particles to form. On cooling, the resin particles, stabilized by dispersant and perhaps thickener, harden and resist settling and agglomeration. Both resole and novolak resins have been made by this process (25). 

The product of an emulsion polymerization is a latex ie, polymer particles on the order of 0.5—0.15 p.m stabilized by the soap. These form the basis for the popular latex paints. SoHd mbber is recovered by coagulating the latex with ionic salts and acids (see Latex technology). 

Fig. 2. Problems in wetting A, Hquids that wet the exterior before displacing gas from pores leave gas trapped in the submerged clump B, fully wetted clumps of buoyant particles do not sink C, nonwetting Hquids do not penetrate and displace gas from pores, so clump remains buoyant and caimot submerge and D, foam produced from air is drawn under the surface, sheared into small bubbles, and stabilized by the wetting agent.

Water and Waste Water Treatment. PAG products are used in water treatment for removal of suspended soHds (turbidity) and other contaminants such as natural organic matter from surface waters. Microorganisms and colloidal particles of silt and clay are stabilized by surface electrostatic charges preventing the particles from coalescing. Historically, alum (aluminum sulfate hydrate) was used to neutralize these charges by surface adsorption of Al cations formed upon hydrolysis of the alum. Since 1983 PAG has been sold as an alum replacement in the treatment of natural water for U.S. municipal and industrial use. 

In the absence of a suitable soHd phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous siUca to quartz at high temperatures. Particle size influences the stabiUty of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkaU (1,33). 

Copper Hydroxide. Copper(II) hydroxide [20427-59-2] Cu(OH)2, produced by reaction of a copper salt solution and sodium hydroxide, is a blue, gelatinous, voluminous precipitate of limited stabiUty. The thermodynamically unstable copper hydroxide can be kiaetically stabilized by a suitable production method. Usually ammonia or phosphates ate iacorporated iato the hydroxide to produce a color-stable product. The ammonia processed copper hydroxide (16—19) is almost stoichiometric and copper content as high as 64% is not uncommon. The phosphate produced material (20,21) is lower ia copper (57—59%) and has a finer particle size and higher surface area than the ammonia processed hydroxide. Other methods of production generally rely on the formation of an iasoluble copper precursor prior to the formation of the hydroxide (22—26). 

Because enzymes can be intraceUularly associated with cell membranes, whole microbial cells, viable or nonviable, can be used to exploit the activity of one or more types of enzyme and cofactor regeneration, eg, alcohol production from sugar with yeast cells. Viable cells may be further stabilized by entrapment in aqueous gel beads or attached to the surface of spherical particles. Otherwise cells are usually homogenized and cross-linked with glutaraldehyde [111-30-8] to form an insoluble yet penetrable matrix. This is the method upon which the principal industrial appHcations of immobilized enzymes is based. 

The remarkable stability of onion-like particles[15] suggests that single-shell graphitic molecules (giant fullerenes) containing thousands of atoms are unstable and would collapse to form multi-layer particles in this way the system is stabilized by the energy gain from the van der Waals interaction between shells [15,26,27],... 

Suppose we have a physical system with small rigid particles immersed in an atomic solvent. We assume that the densities of the solvent and the colloid material are roughly equal. Then the particles will not settle to the bottom of their container due to gravity. As theorists, we have to model the interactions present in the system. The obvious interaction is the excluded-volume effect caused by the finite volume of the particles. Experimental realizations are suspensions of sterically stabilized PMMA particles, (Fig. 4). Formally, the interaction potential can be written as... 

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

Many precipitates, such as Fe(OH)3, form initially as colloidal suspensions. The tiny particles are kept from settling out by Brownian motion, the motion of small particles resulting from constant bombardment by solvent molecules. The sol is further stabilized by the adsorption of ions on the surfaces of the particles. The ions attract a layer of water molecules that prevents the particles from adhering to one another. 

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