Particles in microemulsions

The results sustained the opinion that the synthesis of nano-particles in microemulsion (w/o) is to be preferred. In common emulsions, both phases spontaneously separate from each other while microemulsions are thermodynamically stable, do not segregate and appear to be transparent. This can be explained with the size of the water droplets [14,11]. Microemulsions are also characterized by the so called dynamic exchange process . The emulsion droplets in such emulsions constantly integrate and disintegrate, thus exchanging substance between each other. 

Tojo, C., Blanco, M.C., Rivadulla, F. and Lopez-Quintela, M.A. (1997) Kinetics ofthe formation of particles in microemulsions. Langmuir, 13, 1970-1977. 

Iron diffraction data that the oxide phase was a-FeOOH. The observation of aggregated nanosize primary particles in microemulsion-mediated synthesis is reminiscent of the previous findings that in homogeneous solution spherical amorphous particles aggregate to form rods and rafts [35,36]. 

Figure 10.3 Schematic diagram showing the preparation of Ti02 particles in microemulsions.

Typically, droplets in emulsions contain dispersed particles in the size range 0.1-10 p.m, which covers the wavelengths of visible light. Thus emulsions can often appear cloudy because they scatter light. In contrast, the particles in microemulsions are smaller (1-100 nm) and the sample appears clear and is optically isotropic. 

Although the synthesis of inorganic particles in microemulsions is already widespread, only polymer nanoparticles have been synthesized in microemulsion media as far as the organic particles are concerned. In this chapter, it will be shown that it is also possible to synthesize organic particles by a direct precipitation reaction in the microemulsions. 

By performing in situ the polymerization of acrylamide in water/AOT/toluene microemulsions, clear and stable inverse latexes of water-swollen polyacrylamide particles stabilized by AOT and dispersed in toluene have been found [192-194], It was shown that the final dispersions consist of two species of particles in equilibrium, surfactant-coated polymer particles (size about 400 A) with narrow size distribution and small AOT micelles (size about 30 A). 

Kim D, Oh S, Lee J (1999) Preparation of ultrafine monodispersed Indium-Tin Oxide particles in AOT-Based reverse microemulsions as nanoreactors. Langmuir 15(5) 1599-1603... 

Polymerization in microemulsions allows the synthesis of ultrafine latex particles in the size range of 5 to 50 nm with a narrow size distribution [33], The deposition of an ordered monolayer of such spheres is known to be increasingly difficult as the diameter of such particles decreases [34], Vigorous Brownian motion and capillary effects create a state of disorder in the system that is difficult... 

Yener DO, Giesche H (2001) Synthesis or pure and manganese-, nickel- and zinc-doped ferrite particles in water-in-oil microemulsions. J Am Ceram Soc 84 1987-1995... 

Silica particles synthesized in nonionic w/o microemulsions (e.g., poly-oxythylene alkyl phenyl ether/alkane/water) typically have a narrow size distribution with the average value between 25 and 75 nm [54,55]. Both water and surfactant are necessary components for the formation of stable silica suspensions in microemulsions. The amounts of each phase present in the micro emulsion system has an influence on the resulting size of the silica nanoparticle. The role of residual water (that is the water that is present in the interface between the silica particle and the surfactant) is considered important in providing stability to the silica nanoparticle in the oil... 

Towey TF, Khanlodhi A, Robinson BH (1990) Kinetics and Mechanism of Formation of Quantiun-Sized Cadmium-Sulfide Particles in Water Aerosol-Ot Oil Microemulsions. J Chem Soc Faraday Trans 86 3757-3762... 

ForCaCO, particles in W/O microemulsions of hexaethylene glycol dodecyl ether, however, the values of Nm/Np lie between 0.044 and 0.28, indicating that the particles form by the destruction of micelle-solubilizing aqueous Ca(OH)2 rather than by the intermicellar exchange process (5). In the case of the formation of CaCO-) particles in the Ca salt of Aerosol OT-cyclohexane system, as seen in Table 7.2.2, the particles... 

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. 

Unilamellar vesicles have been used as a reactor for the synthesis of nanos-meter-scale magnetic particles (13,14). By adding alkaline solution to vesicles containing intravesicular solutions of Fe2+ and Fe3+, the Fe /Fe resulted in the formation of membrane-bound discrete particles of different ion oxide particles. These results together with the particle formation in microemulsion are not only of interest in colloid chemistry but also have significance in mineralization in biosystems, such as magnetotactic bacteria, where particles are formed within enclosed organic compartments. 

Even the traditional methods discussed in this chapter can be used for concentrated dispersions through contrast matching. For example, silica particles coated with silane coupling agents in a refractive index-matched mixture of ethanol and toluene can be used in combination with visible probe particles to study the dynamics of particles in dense systems. In the case of microemulsions (Chapter 8), selective deuteration of a component (oil, water, or surfactant) can be used in neutron scattering experiments even to measure the curvature of the oil-water interface. 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

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