Sol emulsification

In the last review, several variants of the sol-gel process for the preparation of oxide powders were discussed, such as thermal crystallization of gels, mechanical dispersion of the sols, gel microspheres precipitated through pH control, and sol emulsification processes. 

Sol emulsification processes are also used for the synthesis of oxide particles of predetermined size range. The method involves aqueous emulsion droplets in immiscible organic continuous liquid phases as isolated compartments for reactions. Two main varieties of dispersions are involved in these phenomena, namely, macroemulsions in which the droplet sizes are generally several tens of micrometers and microemulsions with droplet diameters generally up to 20-30 nm. 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

Control of the particle size while retaining precise control over the release rate is enabled by compartmentalization of the sol-gel solution into droplets of definite size. This can be achieved by emulsification of the sol-gel solution by mixing it with a solution composed of a surfactant and a non-polar solvent (Figure 2.13). When an active molecule is located in the aqueous droplet of a W/O emulsion, encapsulation occurs as the silicon precursors polymerize to build an oxide cage around the active species. By changing the solvent-surfactant combination, the particle size can be varied from 10 nm to 100 pm as the size of the particles is controlled by the size of the emulsion droplet, which acts as a nano-reactor for the sol-gel reaction (Figure 2.13). 

Biocompatible silica nanoparticles prepared by analogous sol formation followed by emulsification and doped with the antibiotic... 

More complex geometries have been developed [40] and the influence of the geometrical structure has been examined. Although straight-through microchannel emulsification has been developed [39,41], the production rates are still low compared to those obtained with standard emulsification methods. However, the very high monodispersity makes this emulsification process very suitable for some specific fechnological applicafions such as polymeric microsphere synfhesis [42,43], microencapsulation [44], sol-gel chemistry, and electro-optical materials. 

The process has been extensively explored by Barbe and coworkers in Australia (where the same scientists established the spin-off company Ceramisphere), and can be viewed as an emulsification of a sol-gel solution in which gelation takes place concomitantly. Depending on the order of addition of the different chemicals, furthermore, the porous microparticles prepared from interfacial hydrolysis and condensation of TEOS in W/0 emulsion will be full porous matrix particles or core-shell capsules. Normally, if the emulsification of the sol-gel solution takes place concomitantly with gelation, full microparticles are formed with the dopant molecules homogeneously distributed within the inner huge porosity of the particles (Figure 18.3). 

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). 

Chem. Descrip. Aromatic acid phosphate ester Uses Surfactant lubricant antirust antistat good emulsification detergency pigment dispersant polymerization emulsifier Features Acid and alkali resist. sol. to high cone, electrolyte Iiq. Properties Transparent Iiq. sp.gr. 1.11 pH 2.0 (3% aq.) anionic 100% solids... 

Finally, under well-defined conditions, it is possible to polymerize performed emulsion droplets. This is especially true for emulsions prepared by condensation methods where the conditions can be controlled in such a way that both secondary nucleation can be avoided and droplet or particle stability can be maintained during the entire polymerization. In the case of emulsions prepared by comminution techniques, suspension polymerization is a good example of a system where the (conditions) properties of emulsions can be converted into the corresponding properties of sols/suspensions. For smaller drop sizes, the solubility of the monomer in water is crucial, but unfortunately, very hydrophobic monomers are technically unimportant, at least nowadays. The addition of hydrophobic molecules needs tailored emulsification procedures regarding and DSD, and a certain maturation time to result in stable emulsions. Miniemulsion polymerization is a promising way, although the question as to what extent a 1 1 copy of an emulsion is possible is still waiting for an answer. 

Some important processes for the formation of sols involve first the formation of an emulsion or a liquid aerosol. In suspension or dispersion polymerization, a monomer or monomer mixture is emulsified to a drop size approximately the same as that of the final desired particle. Polymerization is then initiated using an initiator soluble in the monomer, so that chain growth occurs within each individual drop. The result (with luck) is a dispersion of polymer particles with the same average size as the original monomer emulsion. Normally, some type of stabilizer system is employed in the emulsification stage—a surfactant or very small particles of some material such as silica. 

Of the possible emulsifiers, most are what are considered true surfactants, in that they are effective at lowering significantly the interfacial tension between the two hquid phases. Other additives such as polymers and sols function primarily as stabihzers, rather than emulsifiers. Most polymers are not sufficiently effective at lowering interfacial tensions to act in that regard. In addition, because of their molecular size, the adsorption process for polymers is generally very slow relative to the timescale of the emulsification process. The same applies to stabilizing colloids, in which their action requires the wetting of the particles by the two hquid phases to facihtate their location at the interface. The primary function of polymers and sols in emulsions is in the retardation of droplet flocculation and coalescence. 

Fuchigami T, Toki M, Nakanishi K. 2000. Membrane emulsification using sol-gel derived macroporous silica glass. / Sol-Gel Sci Techn 19 337-341. 

Uses Emulsifer base for metalworking fluids low use level base for naphthenic oils rust protectant Features Easily waste-treatable Properties Liq. oil-sol. 

Features Exo, detergency, emulsification Properties Wh, sol, cloud pt, > 100 C HLB 16,0 Adekatol NP-650 [Adeka Fine Chem,]... 

Uses Surfactant, wetting agent, emulsifer, stabilizer, antlstat, foaming agent for detergents, shampoos, cosmetics, textiles, metal plating, petrol, additives, paper, plastics, rubber gel sensitizer for latex foam hair conditioner Properties Gardner 4 max. cl. Ilq. sol. In water sp.gr. 0.997 vise. 2097 cp HLB 18,4 pour pt, 35 F flash pt. nonflamm. surf, tens. 30.8 dynes/cm (0.1%) 40% act. In water... 

Chem. Descrip. Nonionic surfactants with coupling agents Uses Agric, surfactant, emulsifer for pesticides adjuvant for methane arsenates and other water-sol, herbicides... 

Uses Emulsifer in microemulsions stabilizer, foam booster Properties Amber liq. sol. in water, xylene disp. in ethanol insol. in soya bean oil dens. 1123 kgirr vise. 4200 cSt pour pt. 5 C flash pt. 30 C pH (1% in water) 7.0-8.5 surface tens. 33 mN/m wetting power 18 sec. 85-90% act. 

Uses Polymerization emulsif er for vinyl acetate and acrylic emulsions stabilizer for syn. latexes wetting agent in electrolyte sol ns. surfactant for cosmetics, paints, other industries... 

Uses Emulsif er for cosmetics solubilizer for essential and fragrance oils Properties Gardner color < 2 vise. si. cloudy liq. very faint char, odor/taste sol. in water, ethanol, isopropanol, ethereal oils and fragrances vise, (hoeppler) < 1,850 mPa S HLB 14-16 acid no. < 1 mg/g iodine no. < 1 g/IOOg sapon. no. 45-55 mg/g hyd. no. 60-75 mg/g... 

Uses Emulsifer, solubilizer, solvent for essential oils, steroids and fat-sol. vitamins in pharmaceuticals (oral sol ns, topicals, parenterals), cosmetics, veterinary medicine emulsifier for topical creams and parenterally administered cyclosporine dispersant, bioavailability enhancer for vitamins in feed and veterinary medicines Features Stable towards electrolytes (except mercury II chloride)... 

Uses W/o emulsifer for nonpolar oils, o/w emulsifier for cosmetics Properties Wh, to si, yish, powd, disp, in water sol. in wheat germ oil, avocado oil, paraffin oil insol. in propylene glycol HLB9 1 iodine no. 1.0 max. sapon. no. 140-1870 100% cone. 

Uses Detergent, emulsifer for personal care prods. softener for leather Properties Gardner 2 max. Ilq. sol. In water, IPA HLB 13,4 acid no. 2 max. sapon, no, 60-70... 

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