Microemulsions as microreactor

Garti, N., Microemulsions as microreactors for food applications, Curr. Opin. Colloid Int., 8, 197, 2003. 

Recently, the possibility of using microemulsions as microreactors for the control of polymorphism of organic compounds, speciflcally amino acids and peptides, was demonstrated by Furedi-Milhofer et al. [107,108]. In these experiments, the organic molecules were solubilized in hot microemulsions and crystallized by slow cooling. In some cases, in contrast to the precipitation of ionic clusters (see above), the procedure was most effective when the solute mole-... 

Enzymes when hosted in reverse micelles can catalyze reactions that are not favored in aqueous media. Products of high-added value can be thus produced in these media. The potential technical and commercial applications of enzyme-containing microemulsions as microreactors are mainly linked to their unique physicochemical properties. The potential biotechnological applications of microemulsions with immobilized biocatalysts such as enzymes are described in Chapter 12 by Kunz and coworkers and in Chapter 13 by Xenakis and coworkers. 

One of the powerful techniques for obtaining the ultrafine particles is based on the use of microemulsions as microreactors in order to control the growth of the particles [80, 81]. For the purpose of the method described for obtaining ultrafine particles, water-in-oil (w/o) microemulsions used are formed by nanodroplets of water dispersed in oil. The size of the microemulsion droplets can be modified in the range 5-50 nm by varying the relation of the components of the microemulsion (e.g., changing W = [water]/[stabilizer] in the recipe) or by varying the microemulsion... 

Fanun M, Leser M, Aserin A and Garti N. 2(X)la. Sucrose ester microemulsions as microreactors for model Maillard reaction. Colloids and Surfaces A - Physicochemical and Engineering Aspects 175-187. 

Attempts have been made to enzymatically hydrolyze lecithin with phospholipase A2 to form lysolecithin. The product is a good imitation of what nature offers in very small quantities. The partially hydrolyzed lecithins, or, the fully converted lysolecithin, are the subject of recent work conducted to carry out the reactions in microemulsions as microreactor for the PLAj enzymatic process (Fig. 28) [79,80]. 

As microemulsions (or reverse micelles) can act as microreactors to confine the growth of nanocrystallites in cores, and micelles can also serve as templates to direct the formation of mesoporous silica [11]. Besides mesoporous silica oxide, a variety of non-silica oxides have been prepared [11-13]. Here we use neutral amine as surfactant to prepare mesoporous Ti02-... 

The microemulsion method utilizes a water/oil/surfactant system to construct a micro reactor, in which NCs could be s)mthesized. The microemulsions have a wide range of applications from oil recovery fo fhe s)mfhesis of nanoparticles. Microemulsion is a system of water, oil, and surfactant, and it is an optically isotropic and thermod3mamically stable solution. At molecular scale, the microemulsion is heterogeneous with an internal structure either of nanospherical monosized droplefs (micelles or reverse micelles) or a bicontinuous phase, depending on the given temperature as well as the ratio of its constituents (Eriksson et al., 2004). The small droplets could be utilized as microreactors in order to s)mthesize the fine NCs in a controllable way. 

Reverse micelle systems (or water-in-oil microemulsions) are used as microreactors to synthesize ultrafine particles with a narrow particle size distribution by controlling the growth process [90]. Reverse micelles are nanometer-scale surfactant associated in colloid shaped structures formed in a nonpolar organic solvent. Polar solvents such as water are easily soluble inside reverse micelle because the inside of the reverse micelles is quite hydrophihc. Reverse micelle systems are thermodynamically stable, isotropic, transparent mixtures of oil and water separated by a thin... 

This chapter is devoted to chemical reactivity, so structures of association colloids are taken as givens. The properties of solvent, surfactant, and cosurfactant that control aggregate structure are discussed in terms of theoretical models in Refs. 1--6, 18, and 23-27. Current interpretations of the effects of association colloids on chemical reactivity view aggregates as microreactors, i.e., reaction regions distinct from bulk solvent, but distributed throughout the solution (Fig. 1). In normal and cosurfactant-modified micelles and in O/W microemulsions, ionic or polar groups that are in contact with the bulk aqueous medium... 

Instead of continuing the previous series of review papers [3,4], I place emphasis in this contribution on the fundamental aspects of monodisperse nanoparticle formation. We shall see how the inner water cores of the microemulsion systems work as microreactors. Moreover, the nucleation process is approached, and a minimum number of atoms forming the nuclei is proposed. The role of the surfactant and the cosurfactant are analyzed in the light of the formation of the first nuclei. Finally, the role of the adsorbed molecules in the monodisperse nature of the particles is examined. The different parts are illustrated taking into account the available literature. 

Furthermore, the temporal evolution of a train of drops in the presence of surfactant will be influenced by Marangoni effects. Drops may catch up with their neighbors due to Marangoni retardation effects discussed above, and this may lead to destabilization of a microemulsion or variations of timing in situations where the droplets are used as microreactors in series. 

Microemulsions may have advantages over conventional solvents for synthesizing conducting polymers. W/o microemulsion droplets can serve as microreactors to control polymer growth kinetics and particle size [47]. 

Sucrose esters have two attractive properties biocompatibility and temperature insensitivity [68-77]. We are investigating microemulsions based on sucrose esters in order to use them as microreactors for enzymatic and chemical reactions [42,78,79]. SZT-DSC has been applied to model microemulsion systems based on sucrose monostearate (HLB 15, also designated as S-1570). 

In recent years, W/O microemulsions have found numerous applications as microreactors for specific reactions (for comprehensive reviews, see Refs. 94 and 95). Thus, it has been shown that hydrophilic enzymes can be solubilized without loss of enzymatic activity and used to catalyze various chemical and photochemical reactions [96,97]. Other interesting applications involve the polymerization of solubilizates in microemulsions [98] and the preparation of micro-porous polymeric materials by polymerization of single-phase microemulsions [99]. Furthermore, microemulsions have been used as microreactors for the synthesis of nanosized particles for various applications [93,95] such as metal clusters (Pt, Pd, Rh, Au) for catalysis [100,101], semiconductor clusters [102-104] (ZnS, CdS, etc.), silver halides [105], calcium carbonates, and calcium fiuoride [106]. Recently it was shown [107,108] that it is possible to use W/O microemulsions for the control of polymorphism of water-soluble organic compounds. In most of these appUcations, one or more reactants are solubilized within a microemulsion and then a reaction is initiated. Depending on its molecular structure. 

Dispersions (mostly emulsions and microemulsions) are used as microreactors or nanoreactors for important organic and enzymatic processes, and serve as reservoirs for the solubilization of materials. The behavior of the solubilized matter is also dramatically affected by thermal fluctuations. Hydration or solvation, as well as other interactions of cosolvents, are also studied through thermal treatment. It is therefore important to bring to the reader s attention the options and the scope, as well as the limitations, of the thermal behavior of dispersed systems. 

Microemulsions have been used as confined reaction media during the past two decades, since, due to the very small size of the droplets, they can act as microreactors capable to control the size of the particles and at the same time to inhibit the aggregation by adsorption of the surfactants on the particle surface when the particle size approaches that of the microreactor droplet. The synthesis of nanoparticles using reactions in microemulsions was first described by Boutonnet and cowoikers They synthesized monodispersed metal particles of Pt, Pd, Rh and Ir by reduction of metal salts with hydrogen or hydrazine in water in oil (w/o) microemulsions. Since then, many different types of materials have been prepared using microemulsions, including metal carbonates, metal oxides, " metal chalcogenides, "" polymers," etc. 

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... 

W/o microemulsions have been used for many years as microreactors for the synthesis of ultrafine metallic particles [78, 79]. Since the pioneer works of Boutonnet et al. [80], who studied the production of colloidal Pt, Pd, Rh, and Ir particles by hydrogen or hydrazine (N2H4) reduction in w/o microemulsions, many studies have been made on the synthesis of this type of material. A reverse micelle (microemulsion) method, as a kind of soft technique, is a suitable way for obtaining the uniform and size controllable nanoparticles. The droplet dimension was modulated by various parameters, in particular W [81]. Some studies indicated that with the assistant of cosurfactant, the size of nanoparticles prepared in quaternary reverse micelle system is more controllable [82]. For example, compared with the anionic (AOT) ternary reverse micelle system, the droplet dimension of the quaternary cationic (cetyltrimethyl-ammonium bromide, CTAB) reverse micelles can be elaborately adjusted by changing W with the additional modulation of cosurfactant at the interface of water and oil. The microstmcture and djmamic exchange process are dominated by the influence of cosuifactant on the curvature radius and interface rigidity of the droplets in the quaternary reverse miceUe [82]. 

The book first discusses. self-assembling processes taking place in aqueous surfactant solutions and the dynamic character of surfactant self-assemblies. The next chapter reviews methods that permit the. study of the dynamics of self-assemblies. The dynamics of micelles of surfactants and block copolymers,. solubilized systems, microemulsions, vesicles, and lyotropic liquid crystals/mesophases are reviewed. successively. The authors point out the similarities and differences in the behavior of the.se different self-as.semblies. Much emphasis is put on the processes of surfactant exchange and of micelle formation/breakdown that determine the surfactant residence time in micelles, and the micelle lifetime. The la.st three chapters cover topics for which the dynamics of. surfactant self-assemblies can be important for a better understanding of observed behaviors dynamics of surfactant adsorption on surfaces, rheology of viscoelastic surfactant solutions, and kinetics of chemical reactions performed in surfactant self-assemblies used as microreactors. 

The purpose of this book is to present an up-to-date picture of the dynamics aspects of self-assemblies of surfactants and amphiphilic block copolymers, from micelles to solubilized systems, microemulsions, vesicles, and lyotropic mesophases. It is organized as follows. The first chapter introduces amphiphiles, surfactants, and self-assembhes of surfactants and examines the importance of dynamics of self-assembhes in surfactant science. Chapter 2 briefly reviews the main techniques that have been used to study the dynamics of self- assembhes. Chapters 3 and 4 deal with the dynamics of micelles of surfactants and of amphiphilic block copolymers, respectively. The dynamics of microemulsions comes next, in Chapter 5. Chapters 6 and 7 review the dynamics of vesicles and of transitions between mesophases. The last three chapters deal with topics for which the dynamics of self-assembhes is important for the understanding of the observed behaviors. The dynamics of surfactant adsorption on surfaces are considered in Chapter 8. The rheology of viscoelastic surfactant solutions and its relation to micelle dynamics are reviewed in Chapter 9. The last chapter deals with the kinetics of chemical reactions performed in surfactant self-assembhes used as microreactors. 

Lysolecithins, which are more hydrophilic, show stronger oil-in-water emulsifying properties. Stable microemulsions have been prepared with various fractionated lecithins. These mlcroemulsions are used in direct applications as reservoirs for certain active materials (flavors, antioxidants, nutraceuticals, etc.) or as microreactors for enzymatic reactions. Several of these applications were... 

Being thermodynamically stable nano-dispersions of water-in-oil or oil-in-water, microemulsions can be considered as microreactors to carry out chemical reactions and, in particular, to synthesize nanomaterials. Microemulsions thus have received much recent attention as media for synthesis of nanoparticles Pt, Pd, Rh, and Ir by reducing corresponding salts in the water micropools of w/o microemulsions with hydrazine or hydrogen gas and as media for polymerization to produce thermodynamically stable latexes in the nano-size range (<50 nm) not... 

Microemulsion is used as a special microreactor to limit the nano-sized particles growth. The shape of the microreactor depends on reaction conditions [9]. This method increases the homogeneity of the chemical composition at nano-level and facilitates the preparation of nano-particles with comparatively equal sizes [11]. The specific properties of the nano-particles make them suitable for microelectronics, ceramics, catalysis, medicine, cosmetics, as piezoelectric materials, conductors, etc. 

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