Kinetically stable emulsion

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

Phase separation into water-in-water emulsions Spheres, teardrops, fibres, trapped in kinetically stable state by changing solution or environmental conditions (temperature, pH, ionic composition, solvent quality) so that one or both phases thickens or gels Norton and Frith, 2001 McClements, 2006... 

Microemulsions are fluid, transparent, thermodynamically stable oil and water systems, stabilized by a surfactant usually in conjunction with a cosurfactant that may be a short-chain alcohol, amine, or other weakly amphiphilic molecule. An interesting characteristic of microemulsions is that the diameter of the droplets is in the range of 100-1000 A, whereas the diameter of droplets in a kinetically stable macroemulsion is 5000 A. The small droplet size allows the microemulsion to act as carriers for drugs that are poorly soluble in water. The suggested method of preparation of microemulsions is as follows the surfactant, oil, and water are mixed to form a milky emulsion and titrated with a fourth component, the cosurfactant,... 

These are transparent or translucent systems covering the size range from 5 to 50nm. Unlike emulsions and nanoemulsions (which are only kinetically stable), microemulsions are thermodynamically stable as the free energy of their formation is either zero or negative. Microemulsions are better considered as swollen micelles normal micelles can be swollen by some oil in the core of the micelle to form O/W microemulsions. Reverse micelles can be swollen by water in the core to form W/O microemulsions. 

A mixture comprising three or four components (water, oil, emulsifier and comulsifier) can form both the kinetically stable (mini-)emulsion and thermodynamically stable mini- or micro-emulsion. 

As compressed carbon dioxide is a nonpolar molecule with weak van der Waals forces (low polarizability per volume), it is a relatively weak solvent [1], Thus, many interesting separations and chemical reactions involving insoluble substances in CO2 can be expected to take place in heterogeneous systems, for example, microemulsions, emulsions, latexes and suspensions. Microemulsion droplets 2-10 nm in diameter are optically transparent and thermodynamically stable, whereas kinetically stable emulsions and latexes in the range from 200 nm to 10 pm are opaque and thermodynamically unstable. 

The next section describes measurements of interfacial tension and surfactant adsorption. The sections on w/c and o/c microemulsions discuss phase behavior, spectroscopic and scattering studies of polarity, pH, aggregation, droplet size, and protein solubilization. The formation of w/c microemulsions, which has been achieved only recently [19, 20], offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science for waste minimization and treatment, and materials science. Recently, kinetically stable w/c emulsions have been formed for water volume percentages from 10 to 75, as described below. Stabilization and flocculation of w/c and o/c emulsions are characterized as a function of the surfactant adsorption and the solvation of the C02-philic group of the surfactant. The last two sections describe phase transfer reactions between lipophiles and hydrophiles in w/c microemulsions and emulsions and in situ mechanistic studies of dispersion polymerization. 

Many volatile low-molecular-weight organics are completely miscible with carbon dioxide at relatively modest temperatures and pressures. However, nonvolatile compounds or those with higher molecular weights, especially polymers, are often insoluble. Insoluble liquid compounds may be dispersed into CO2 with the aid of appropriate surfactants to form a kinetically stable o/c emulsion [10,11]. Stable emulsions are important in separation processes, heterogeneous reactions and materials formation processes, such as precipitation with a compressed fluid antisolvent [40]. These emulsions are the precursors to solid latex particles in dispersion polymerization. Stabilization of o/c emulsions has been studied in-situ to understand surfactant design for polymerization [10,11]. 

Thermodynamically stable microemulsions and kinetically stable emulsions may be utilized to bring water and nonvolatile hydrophilic substances, such as proteins, ions, and catalysts, into contact with a SCF-continuous phase (e.g. CO2) for separation, reaction and materials formation processes. Reactions between hydrophilic and hydrophobic substrates may be accomplished in these colloids without requiring toxic organic solvents or phase transfer catalysts. CO2 and aqueous phases may be mixed together over a wide range in composition in w/c and c/w emulsions. The emulsion is easily broken by decreasing the pressure to separate the water and CO2 phases, facilitating product recovery and CO2 recycle. Reaction rates can be enhanced due to the considerably lower microviscosity in a w/c as compared to a water-in-alkane microemulsion or emulsion. 

The main difference between emulsions and microemulsions lies in the size and shape of the droplets of dispersed phase, which causes the differences in the thermodynamic stability of the two systems. Emulsions allow the drug to be administered as a dispersed oil solution and thus are kinetically stable but thermodynamically unstable. After storage or aging, droplets will coalesce and the two phases separate. Unlike emulsions, microemulsions are thermodynamically stable and phases do not separate on storage. Another important difference between the two systems is their appearance emulsions have a cloudy appearance, while microemulsions are transparent because of the lower dispersed phase size than macroemulsions. 

While inverse (mini)emulsion polymerization forms kinetically stable macro-emulsions at, below, or around the CMC, inverse microemulsion polymerization produces thermodynamically stable microemulsions upon further addition of emulsifier above the critical threshold. This process also involves aqueous droplets, stably dispersed with the aid of a large amount of oil-soluble surfactants... 

Styrene in water without surfactant. Although the polydispersity indexes for the synthesis of the polyNaSS macroalkoxyamine were rather low (1.2 < PDI < 1.3), no data were provided concerning the polymerization. The same observation can be made for the kinetic and macromolecular characteristics of the emulsion polymerization. Stable dispersions were obtained, but the solid content did not exceed 5wt%. The particle size distributions were quite broad and within the 50-200nm range. 

Microemulsions were initially considered to be a special case of emulsions, i.e., kinetically stable dispersions of two mutually immiscible solvents, although of an unusually high stability They are clear, seemingly one-phase systems and can be prepared without considerable input of mechanical energy. As they are thermodynamically stable, however, it is more plausible to conceive of microemulsions as being aggregated (micellar or swollen micellar) solutions [5],... 

Emulsions formed in crude oil and bitumen during extraction operations are usually water-in-oil (W/O) macroemulsions (>0.1 to 100 om in diameter). Macroemulsions are kinetically stable, unlike microe-mulsions, which are thermodynamically stable. In conventional oil recovery (high-energy process), the crude is often in contact with formation water or injection water, as in secondary recovery. In tertiary or enhanced oil recovery, surfactants are used purposely in water floods to make microemulsions for enhancing the flowability of the crude. Crude-oil macroemulsions are produced when two immiscible liquid phases such as oil and water are mixed via the input of mechanical or thermal energy into the processes. Conventional crudes held under high pressures and temperatures amidst... 

---