Reactions in Emulsions

In emulsions the interface becomes very important relative to the bulk of the liquids. Not only, therefore, are the characteristics of interfacial reactions conveniently studied with these systems, but also the amounts of the reaction products are generally sufficient for ordinary chemical analyses to be carried out. 

The large areas of the oil-water interfaces in an emulsion have proved important in several chemical operations—emulsion polymerizations, saponifications, and the condensation of peptides. The particular influence of the interface depends on many different factors, including the increased opportunity offered for reaction between oil-soluble and water-soluble components, the possibility that the surface may bear an electrical charge, and the uniform orientation of the superficial reactant molecules under the action of the directive forces of the two different liquids. 

Emulsifiers commonly used commercially include the soaps of fatty acids, sulfates of fatty alcohols, and sulfonates of aliphatic and alkylated aromatic compounds. All of these compounds form solubilizing micelles. (See Figs. 7 and 8.) The Cig soap is among the best additives (61a). 

The advantage of the emulsion method of polymerization lies in the wide choice of catalysts that may be used. In mass reactions such agents as oil-soluble peroxides, diazoamine derivatives, and diazothioethers must be used. Emulsification permits the use of water-soluble catalysts, as well as mixtures of these with oil-soluble materials. The reaction then occurs at the monomer-water interface, with a high probability that the free radicals formed will immediately react with the adjacent monomer. Use of quarternary emulsifiers such as CTAB or Emulsol 607 in such 

In the polymerization of styrene or the copolymerization of isoprene and styrene, a soap such as potassium laurate has been found to be a powerful catalyst. The reaction, starting from an emulsion of monomer drops, ends with polymer particles, and is characterized both by the gradual disappearance of the soap from the aqueous phase during the reaction, and by the fact that the polymer particles, much smaller than in an uncatalyzed reaction, increase in size as the yield of polymer is increased. 

---

RV Kucher., VI Karban. Chemical Reactions in Emulsion. Kiev Naukova Dumka, 1973 [in Russian]. 

The rapidly increasing practical importance of reactions at liquid interfaces is seen from the now common industrial use of reactions in emulsions, including the processes of soap manufacture and polymerization of synthetic rubber. 

Ultrasound-assisted electrolytic reduction of emulsions of activated unsaturated systems provides a method for hydrogenation of water-insoluble materials in an aqueous environment [284]. The effect of ultrasound on electrochemical reactions in emulsions may vary depending on the reaction in some cases solubilization of an insoluble reaction product is furthered, whereas in other cases the heterogeneous rate constant is influenced [284]. 

Studies on the formation of polymer latex particles have provided an alternative mechanism whereby uniform particles can result from a homogeneous nucleation-precipitation reaction. In emulsion polymerization, an insoluble monomer is mixed with water and a water-soluble free radical initiator is added. Final particle size depends on... 

Although already in the initial patent from CSIRO the application of RAFT in emulsion is reported (38), it appeared to be quite troublesome to cany out a well-controlled reaction in emulsion. 

We have recently shown that lipoxygenases from plants can act on substrate lipids emulsified by proteins or bile salts [42], This means that the interfacial activation is not essential for lipoxygenase reaction in emulsion systems. The data of the three-dimensional structure of lipoxygenases have shown the absence of a lid but the presence of the cavity functioning as a path for a substrate lipid from the exterior of the enzyme molecule to the catalytic site [43]. It is thought that lipoxygenase uses this cavity for the uptake of substrate lipid from the oil droplet surface into the interior catalytic site without substantial conformational change, i.e., interfacial activation. 

Transfer to cloud and wake) = (Reaction in cloud cloud and wake) -l- (Transfer to emulsion) (Transfer to emulsion) = (Reaction in emulsion)... 

Dual function filtrodynamics, to both characterize the presence, onset, and evolution of particulates that occur in polymerization processes and protect instrumentation through which diluted polymer reactor solution flows (e.g., as in ACOMP), is currently under development. Challenges include delineating which types of filters work best with given particulate systems (e.g., microgels from natural product solutions, or from microgels occurring in polymerization reactions in emulsions and inverse emulsions, etc.)... 

Diene Polymerization. Simultaneously with the resolution of the question of the crosslinking reaction in emulsion polymerization of dienes, it became possible to evaluate the rate constants of the polymerization reaction itself. This was, of course, primarily due to the definitive analysis of the kinetics of emulsion polymerization by Ewart and Smith. By a careful study of various "recipes" used in the emulsion copolymerization of butadiene-styrene, it was found that certain low-temperature systems obeyed Case II of the Ewart-Smith treatment, i. e.,... 

Reed and coworkers developed a strategy for automatic continuous online monitoring of polymerization (ACOMP). The method may be used during the initial development of the polymerization process, its optimization and monitoring of the continuous reaction. ACOMP automatically dilutes samples from the reactor and measures its properties, e.g., refractive index, near infrared (NIR) spectra, LSc, [ ]], etc., from which it computes evolution of MW, MWD, degree of conversion, copolymer composition (in copolymerization) and others. The method has been applied to a variety of the free radical homo- and co-polymerizations, including the reactions in emulsion or suspension. ... 

---