Microemulsions Droplet phase reactions

If the objective is to keep the enzyme active and stable in an aqueous phase but otherwise to use as much organic phase as possible, microemulsions are an option as a reaction medium. In contrast to ordinary emulsions they are thermodynamically stable and, at a particle diameter of 1-20 nm, accommodate most often only one enzyme molecule (Figure 12.5). The microemulsion droplets communicate rapidly and exchange their contents through elastic collisions. The boundary between microemulsions and reversed micelles is not clearly delineated, and the two notions are often used interchangeably. Enzyme of almost all classes and structures have been solubilized in microemulsion systems and used for reactions (Shield, 1986). 

Activity and stability are often comparable to values in aqueous media. Many substrates which cannot be made to react in water or in pure organic solvents such as hexane owing to lack of solubility can be brought to reaction in microemulsions. Whereas enzyme structure and mechanism do not seem to change upon transition from water to the microemulsion phase (Bommarius, 1995), partitioning effects often are very important. Besides an enhanced or diminished concentration of substrates in the vidnity of microemulsion droplets and thus of enzyme molecules, the effective pH values in the water pool of the droplets can be shifted in the presence of charged surfactants. Frequently, observed acceleration or deceleration effects on enzyme reactions can be explained with such partitioning effects (Jobe, 1989). 

In Section III, we have analyzed a simple model for discerning the impact of the fluctuations of the droplet phase of the microemulsion on the reaction kinetics in such media. The results of this analysis are consistent with those outlined in Section IL We have dehneated the different temporal regimes to identify the explicit effect of the dynamical fluctuations of the sink. Finally,... 

Microemulsion polymerisation has shown a great advantage over conventional polymerisation strategies such as emulsion polymerisation with respect to the end particle size, polydispersity and reproducibility of the product characteristics. Although we still face severe problems regarding the polymerisation of microemulsions (see Section 11.2 in Chapter 11), it has been employed for the synthesis of polymeric nanoparticles of pharmaceutical interest. Microemulsion polymerisation involves free-radical polymerisation in a large number of monomer-swollen microemulsion droplets and represents a thermodynamically stable, transparent one-phase reaction system. Generally, the microemulsion droplet is considered as initiation locus for the polymerisation. The type of microemulsion used for the polymerisation depends on the monomer properties [148]. 

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. 

The distribution coefficients of PAN and CAS indicators between microemulsion droplets and the water-continuous phase in O/W microemulsions constituted of anionic, cationic, and nonionic surfactants have been measured in order to investigate the mechanism of enhanced sensitivity of reactions in O/W microemulsion [31]. From Table 2 one can see that the distribution coefficients of PAN and CAS indicators in all O/W microemulsions are larger than those in micelles with the same surfactants. Thus, we can conclude from these results that the reason for higher sensitivity in microemulsions is that a microemulsion has greater solubilization capacity for indicators or complexes. 

This book is divided into five parts as follows Part I Historieal Perspeetive Part II Structural Aspects and Characterization of Microemulsions Part III Reactions in Microemulsions Part IV Applications of Microemulsions and Part V Future Prospects. The book opens with the chapter on the historical development of microemulsion systems by two leading authorities (Lindman and Friberg) who have significantly contributed to the field of microemulsions. In the next two chapters J. Th. G. Overbeek (the doyen of colloid science) and coworkers and E. Ruckenstein advance different approaches to describe the thermodynamics of microemulsion systems. While a full description of microemulsion thermodynamics is far from complete, the droplet type model predicts the experimental observations quite well. A theory that predicts the global phase behavior and the detailed properties of the phases as a function of experimentally adjustable parameters is still under development. 

The technique of pyrene fluorescence intensity measurements was proposed to study the particle nucleation mechanisms involved in the 0/W microemulsion polymerization (31). The experimental data showed that microemulsion droplets are the major particle nucleation loci for the polymerization system with the more hydrophobic ST as the monomer. This is followed by the flocculation of latex particles with the remaining droplets. In contrast, the polymer reactions taking place initially in the continuous aqueous phase (homogeneous nucleation) plays an important role in the MMA microemulsion polymerization. 

It is well documented that the pH inside microemulsion droplets in SCCO2 is typically 3. In order to determine whether the unusual selectivity is due to the low pH inside microemulsion droplets in SCCO2, a hydrogenation reaction was examined by addition of a phosphate bnffer to raise the aqueous pH phase in waster-in-scC02 microemulsions to 7.5. Based on the obtained results, an increase in pH was found ineffective in changing the conversion and selectivity [68]. 

Emulsion-related processes are currently in use for the synthesis of oxide particles of predetermined size ranges. These involve the use of aqueous emulsion droplets dispersed in immiscible organic continuous liquid phases as isolated compartments for reaction the particles formed in the process are expected to follow the sizes of the droplets within which they were generated, though the case does not always follow this logic. The two main varieties of dispersion that are involved in these phenomena are macroemulsions(sAso called ordinary emulsions or only emulsions ), in which the droplet sizes are generally several tens of micrometers, and microemulsions (droplet diameter generally up to 20-30 nm). A brief discussion follows on the basic principles in the formation of these emulsion systems, how these systems are utilized in particle preparation specifically in sol-gel systems and the nature of the products in different chemical systems. 

W/o microemulsion solutions are mostly transparent, isotropic liquid media with nanosized water droplets that are dispersed in the continuous oil phase and stabilized by surfactant molecules at the water/oil interface. These surfactant-covered water pools offer a unique microenvironment for the formation of nanoparticles. They not only act as microreactors for processing reactions but also exhibit the process aggregation of particles because the surfactants could adsorb on the particle surface when the particle size approaches to that of the water pool. As a result, the particles obtained in such a medium are generally very fine [76]. Inverse microemulsion droplets, however, are slightly polydisperse due to less strict transformation of... 

For the W/O microemulsion polymerization of acrylamide stabilized by sodium bis(2-ethylhexyl)sulfosuccinate and initiated by 2,2 -azobisisobutyro-nitrile, the initiation reactions take place predominantly in the acrylamide/ water-toluene interfacial layer, in which the encounter of initiator radicals with monomer molecules is facilitated [70-74]. On the other hand, as would be expected, free radical polymerization is initiated primarily within the acryl-amide/water cores of the microemulsion droplets when the water-soluble persulfate initiator is used. The technique of steady-state fluorescence of indoUc probes quenched by acrylamide and selectively located in different phases (the continuous toluene phase, the acrylamide/water-oil interface and the acryl-amide/water phase) of the W/O microemulsion system stabilized by sodium bis(2-ethylhexyl)sulfosuccinate was adopted to study the consumption of monomer during polymerization [79]. The experimental results show that acrylamide is consumed evenly from all parts of the microemulsion polymerization system, regardless of the initial microemulsion composition and the nature of initiator. 

Microemulsions or reverse micelles are composed of enzyme-containing, surfactant-stabilized aqueous micro droplets in a continuous oiganic phase. Such systems may be considered as a kind of immobilization in enzymatic synthesis reactions. 

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