Inverse-microemulsion

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]. 

One of disadvantages of the microemukion method lies in its expensiveness due to the large amounfc of surfactant (as much as 20-30%) added to the system. Another drawback is that the surfactant ensuring colloidal stability is adsorbed on the surface of the nanoparticles, thereby decreasing their usability. The disadvantages may be circumvented by the application of micellar synthesis, in the course of which the 

Problem concerning of removal of some (highly-boiling point) solvenk from products can be solved by synthesis of particles in the solvents with low-boiling temperature, such as CO2 solution. Silver and copper nanoparticles were synthesized in AOT reverse micelles in compressed propane and supercritical- CO2 solutions 

A reverse microemulsion method is applied for the preparation of composite nanoparticles [96]. By controlling the amount of surfactant and water, fabrication of particles in water-in-oil microemulsions (reverse micelles) affords great control over the size and shape of the particles [15]. This procedure takes advantage of two selforganizing processes. First, the reverse micelles are used to synthesize metallic 

A reverse-micelle technique k considered an efficient route to produce high quality, monodisperse magnetic and superparamagnetic nanoparticles. It has been shown that the basic magnetic properties such as coercivity (He), saturation magnetization (Ms), Curie temperature (Jc) as well as lattice constank can be tuned by varying the cation stoichiometry [109]. In thk method, cation occupancy, elemental composition. 

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Barton J 1996 Free-radical polymerization in inverse microemulsions Prog. Polym. Sc/. 21 399-438... 

Manufacturing processes have been improved by use of on-line computer control and statistical process control leading to more uniform final products. Production methods now include inverse (water-in-oil) suspension polymerization, inverse emulsion polymerization, and continuous aqueous solution polymerization on moving belts. Conventional azo, peroxy, redox, and gamma-ray initiators are used in batch and continuous processes. Recent patents describe processes for preparing transparent and stable microlatexes by inverse microemulsion polymerization. New methods have also been described for reducing residual acrylamide monomer in finished products. 

Over the past few years, a large number of experimental approaches have been successfully used as routes to synthesize nanorods or nanowires based on titania, such as combining sol-gel processing with electrophoretic deposition,152 spin-on process,153 sol-gel template method,154-157 metalorganic chemical vapor deposition,158-159 anodic oxidative hydrolysis,160 sonochemical synthesis,161 inverse microemulsion method,162 molten salt-assisted and pyrolysis routes163 and hydrothermal synthesis.163-171 We will discuss more in detail the latter preparation, because the advantage of this technique is that nanorods can be obtained in relatively large amounts. 

Inverse least squares, 539-41 Inverse micelles, 25487 Inverse microemulsion polymerization, 20 461... 

Barnickel P, Wokaun A (1990) Synthesis of Metal Colloids in Inverse Microemulsions. Mol Phys 69 1-9... 

Finally, very recently Shea et al. successfully employed inverse microemulsion polymerisation for the preparation of MIP beads in the tens of nanometers range using hydrophilic peptides as template molecules. In this case it was the template molecule which was prefunctionahsed with a hydrophobic chain to orient it towards the surface of the growing bead during polymerisation. The rebinding efficiency of the resulting nanoparticles was however found to depend markedly on the nature of the employed template and to be lower than that recorded with beads of similar... 

Figure 3.10 (a) Schematic of an inverse microemulsion in which two chemicals (A and B) dissolved in aqueous droplets react to produce a solid (C) whose size is controlled by the size of the droplet in which... 

All these techniques can be combined to obtain more precise control over several scales, as done by Stucky et al. to create hollow microcavities in mesoporous silica. In order to do that, the inorganic precursor (silicon alkoxide) was dissolved in the oil droplets of a regular (not inverse) microemulsion, while the surfactants were in the aqueous phase. Both the mesopore (produced by the surfactant) and microcavity (formed by the oil droplets) sizes could be controlled by varying the... 

Corpart and Candau [68, 69] described the formulation of polyampholytes containing both positive and negative charges in inverse microemulsions. The copolymers can show very different behaviors in the aqueous solution, ranging from insoluble, water-swollen hydrogels to water-soluble compounds, depending on the monomer composition. For polyampholytes with balanced stoichiometry, the polymer behavior is controlled by attractive electrostatic forces. The compound is usually insoluble in water, but becomes soluble upon the addition of salt. 

Vaskova V, Hlouskova Z, Barton J, Juranicova V (1992) Polymerization in inverse microemulsions, 4 locus of initiation by ammonium peroxodisulfate and 2,2-azoisobutyronitrile. Macromol Chem 193(3) 627-637... 

Vaskova V, Juranicova V, Barton J (1990) Polymerization in inverse microemulsions, 1. Homopolymerizations of water- and oil-soluble monomers in inverse microemulsions. Macromol Chem 191(3) 717-723... 

The polymerization of a water-soluble monomer such as AM, acrylic acid (AA), sodium acrylate (NaA), or 2-hydroxyethylmethacrylate (HEMA), can be carried out easily in inverse microemulsion or/and bicontinuous microemulsion. These water-soluble monomers also act as cosurfactants, increasing the flexibility and the fluidity of the interfaces, which enhances the solubilization of the monomer. A cosurfactant effect during the polymerization of vinyl acetate in anionic microemulsions has also been reported [12]. 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

The polymerization of acrylamide (AM) and the copolymerization of acrylamide-sodium acrylate in inverse microemulsions have been studied extensively by Candau [10,11,13-15], Barton [16, 17], and Capek [18-20]. One of the major uses for these inverse microlatexes is in enhanced oil recovery processes [21]. Water-soluble polymers for high molecular weights are also used as flocculants in water treatments, as thickeners in paints, and retention aids in papermaking. 

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. 

Inverse microemulsion plus a second reactant, as illustrated in Fig. 13... 

Since numerous papers have been published on the preparation of inorganic nanomaterials by micro emulsions, we do not intend to review all of them here. As examples, we will describe how inverse microemulsions can be used to pre-... 

Fig. 13 Synthesis of nanoparticles in microemulsions using a double inverse microemulsions b inverse microemulsion plus a trigger c inverse microemulsion plus reactant [122]...

Table 2 Inorganic materials synthesized using inverse microemulsion...

Material Inverse microemulsion (surfactant-oil) Investigation/results Reference... 

BaFei20i9 NP-5-I-NP-9 and cyclohexane Ultrafine, high coercivity BaFei20i9 was prepared via inverse microemulsion and compared to that prepared using the conventional method [145]... 

Semiconductor nanoparticles have been extensively studied in recent years owing to their strongly size-dependent optical properties. Among these nanomaterials, CdS and PbS are particularly attractive due to their nonlinear optical behavior and unusual fluorescence or photoluminescence properties [ 136,137]. A number of studies have been published recently regarding the preparation of CdS, PbS and ZnS nanoparticles in inverse microemulsion systems [138-143]. In these works, NP-5/NP-9 was the most commonly used surfactant and petroleum ether the most commonly used oil. The aqueous phase for each inverse microemulsion consisted of cadmium nitrate (0.1 M) and ammonia sulfide (0.1 M) respectively. CdS was recovered from the mixture of double microemulsions [141]. Electron microscopy revealed that the spherical particles were aroimd 10-20 nm in diameter, as seen in Fig. 14. 

The synthesis of PbS-coated CdS was conducted in a microemulsion system similar to that mentioned above, except that a third inverse microemulsion. 

In our studies, the nano-sized sihca was synthesized using cheap sodium orthosilicate and sodiirm metasihcate rather than expensive alkoxysilanes. An inverse microemulsion containing NP-5/NP-9 as surfactant and cylcohexane or petroleum ether as the oil was used to carry out the hydrolysis and condensation of sodium orthosilicate or metasilicate using an acidic medium [152, 153]. The spherical silica particles of size 10-20 nm were obtained in a system using cyclohexane as the oil and with sodium orthosihcate of 0.01-0.1 M [152]. The particle size increased as the concentration of sodium orthosihcate and the pH were increased. Sihca particles prepared in basic conditions were more uniform in size than those prepared in an acidic medium. But calcined sihca powders with larger specific areas (350-400 mVg) were obtained for those prepared in an acidic medium. 

The double inverse microemulsion method was also used to synthesize per-ovskite-type mixed metal oxides [ 155]. One microemulsion solution contained nitrate salts of either Ba(N03)2/Pb(N03)2, La(N03)3/Cu(N03)2 or La(N03)3/ Ni(N03)2, and the other microemulsion contained ammonium oxalate or oxalic acid as the precipitant. These metal oxalate particles of about 20 nm were readily calcined into single phase perovskite-type BaPb03, La2Cu04 and LaNi03. The calcinations required for the microemulsion-derived mixed oxalates were 100-250 °C below the temperatures used for the metal oxalates prepared by a conventional aqueous solution precipitation method. 

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