Chemical reactions, microemulsion

In 1959, J. H. Schulman introduced the term microemulsion for transparent-solutions of a model four-component system [126]. Basically, microemulsions consist of water, an oily component, surfactant, and co-surfactant. A three phase diagram illustrating the area of existence of microemulsions is presented in Fig. 6 [24]. The phase equilibria, structures, applications, and chemical reactions of microemulsion have been reviewed by Sjoblom et al. [127]. In contrast to macroemulsions, microemulsions are optically transparent, isotropic, and thermodynamically stable [128, 129]. Microemulsions have been subject of various... 

J Sjoblom, R Lindberg, SE Friberg. Microemulsions-phase equilibria characterization, structures, applications and chemical reactions, Adv Colloid Interf Sci 65 125-287, 1996. 

These microdroplets can act as a reaction medium, as do micelles or vesicles. They affect indicator equilibria and can change overall rates of chemical reactions, and the cosurfactant may react nucleophilically with substrate in a microemulsion droplet. Mixtures of surfactants and cosurfactants, e.g. medium chain length alcohols or amines, are similar to o/w microemulsions in that they have ionic head groups and cosurfactant at their surface in contact with water. They are probably best described as swollen micelles, but it is convenient to consider their effects upon reaction rates as being similar to those of microemulsions (Athanassakis et al., 1982). 

Conventional extraction with chemical reaction is used for solutes that are insoluble in the organic phase unless they react with a reagent present in that phase. An example of this is the extraction of metal ions described by Eq. (15.8). In this case, if the organic phase is replaced by a W/O microemulsion containing the reactant, there is usually extraction enhancement due to the solubilization of the metal complex in the microphase. There are two possible ways of forming a W/O microemulsion in the solvent phase ... 

When we consider the metals of nanoscopic size, fine metal particles from micrometer to nanometer size can be synthesized by both physical and chemical methods. The former method provides the fine metal particles by decreasing the size by addition of energy to the bulk metal, while in the latter methods, fine particles can be produced by increasing the size from metal atoms obtained by reduction of metal ions in solution. Since chemical reactions usually take place in homogeneous solution in any case, this chapter includes most of the cases of synthesis and growth of fine metal particles. However, the polyol process, reaction in microemulsions, and formation in the gas phase are omitted, since they are described in later chapters by specialists in those fields. 

The nano-sized particles of calcium carbonate and barium carbonate have specific characteristics. They are important materials for the industry. The main object of this investigation is to obtain nanoparticles of calcium carbonate and barium carbonate by chemical reaction carried out in microemulsion of water in oil. The nanoparticles obtained are spherical. Their sizes vary from 20 to 30 nm. The shape and size of particles are determinated by electron microscopy. 

Nano-sized particles of barium and calcium carbonate were obtained by a chemical reaction in a microemulsion. The particles were studied by electron microscopy and were found to possess spherical shape and diameters from 20 to 30 nm. 

Chemical reactions conducted within microemulsions occur with the same modes of control afforded by micelles. Thus, photoactivity of semiconductors [75] and metal colloids [76] formed within microemulsions are maintained, as is the capacity for initiation of photooxidative polymerization [77],... 

Microemulsions are used as reaction media for a variety of chemical reactions. The aqueous droplets of water-in-oil micro emulsions can be regarded as minireactors for the preparation of nanoparticles of metals and metal salts and particles of the same size as the starting microemulsion droplets can be obtained [1-3]. Polymerisation in micro emulsions is an efficient way to prepare nanolatexes and also to make polymers of very high molecular weight. Both discontinuous and bicontinuous micro emulsions have been used for the purpose [4]. Microemulsions are also of interest as media for enzymatic reactions. Much work has been done with lipase-catalysed reactions and water-in-oil microemulsions have been found suitable for ester synthesis and hydrolysis, as well as for transesterification [5,6]. 

Organic reactions in micro emulsions need not be performed in one-phase systems. It has been found that most reactions work well also in two-phase Winsor I or Winsor II systems, i.e. an oil-in-water microemulsion coexisting with excess oil or a water-in-oil microemulsion coexisting with excess water, respectively [7, 8]. A Winsor III system, i.e. a three-phase system in which a middle phase microemulsion coexists with both oil and water, has also been successfully used as reaction medium [9]. The transport of reactants from the excess oil or water phase to the microemulsion phase, where the reaction takes place, is evidently fast compared to the rate of the reaction. This is a practically important aspect on the use of micro emulsions as media for chemical reactions because it simplifies the formulation work. Formulating a Winsor I or Winsor II system is usually much easier than formulating a one-phase microemulsion of the whole reaction mixture. Winsor systems can also be of value to simplify the work-up process, in particular to separate the product from the surfactant, as will be discussed below in Sect. 2.4 (see also [6]). 

Micro emulsion droplets and micellar aggregates can catalyse or inhibit chemical reactions by compartmentalization and by concentration of reactants and products. The catalytic effect in micelles has been widely studied, a typical reaction being base catalysed hydrolysis of lipophilic esters. This rate enhancement is normally referred to as micellar catalysis. The analogous effect occurring in microemulsions may be called microemulsion catalysis. 

A great variety of chemical reactions can be advantageously carried out in microemulsions [860-862]. In one of the first papers in this field, Menger et al. described the imidazole-catalyzed hydrolysis of 4-nitrophenyl acetate in water/octane microemulsions with AOT as an anionic surfactant [=sodium bis(2-ethyl-l-hexyl)-sulfosuccinate] [864]. The solubilized water, containing the imidazole eatalyst, is confined in spherical pools encased by surfactant molecules, which have only their anionic head groups (-SOb ) immersed in the aqueous droplets. When the ester, dissolved in water-insoluble organic solvents, is added to this water/octane/AOT/imidazole system, it readily undergoes the catalysed hydrolysis under mild reaction conditions (25 °C). 

Abstract This review describes how the unique nanostructures of water-in-oU (W/0), oil-in-water (0/W) and bicontinuous microemulsions have been used for the syntheses of some organic and inorganic nanomaterials. Polymer nanoparticles of diameter approximately 10-50 nm can easily be obtained, not only from the polymerization of monomers in all three types of microemulsions, but also from aWinsor l-like system. A Winsor 1-like system with a semi-continuous process can be used to produce microlatexes with high weight ratios of polymer to surfactant (up to 25). On the other hand, to form inorganic nanoparticles, it is best to carry out the appropriate chemical reactions in W/0- and bicontinuous microemulsions. 

For reactions in inverse microemulsions that involve the total confinement of the reactant species within the dispersed water droplets, the exchange of reactants by the coalescence of the two droplets take place prior to their chemical reaction. The chemical reaction produces an (almost) insoluble product. The reaction medium is first saturated with this product. When the saturation exceeds a critical limit, nucleation occurs. Then the nuclei start to grow rapidly and consume the reaction product leading to a decline in the supersaturation. As soon as the supersaturation falls below the critical level, no further nucleation occurs, so only the existing particles grow beyond this point. If the time period of nucleation is short in comparision to the growth period, rather monodisperse particles are obtained. 

The small droplet size in microemulsions also leads to a large surface-to-volume ratio in an oil-water system. This is important for chemical reactions in which the rate of reaction depends on the interfacial area. The microemulsion can also be classified as W/O or 0/W similar to macroemulsion systems. 

Figure 11 The preparation of nanoparticles by the inverse micelle process in which a chemical reaction between microemulsion or inverse micelles after collision and perhaps fusion converts the soluble salt into an insoluble metal or metal oxide as shown. Source From Ref. 75.

Sjoblom, J., Lindberg, R., and Eriberg, S. E., Microemulsions—phase equibbria characterization, structures, appbcations and chemical reactions, Adv. Coll. Interf. ScL, 95, 125-287, 1996. 

Kahlweit, M., Strey, R. andSchomacker, R. (1989) Microemulsions as liquid media for chemical reactions. In W. Knoche and R. Schomacker (eds), Reactions in Compartmentalized Liquids. Springer, Berlin, Heidelberg, pp. 1-10. 

In recent years there is also a great interest to the investigation of chemical reactions taking place in microemulsion systems. This is one of the methods used for the preparation of the nanometer-sized particles with narrow size distribution [21,25]. 

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. 

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