Phase of microemulsions

For this problem already the simple mean field approximation becomes rather involved [197,213]. Therefore, we describe here only an approach, which is even more simplified, appropriate for wavenumbers q near the characteristic wavenumber q, but strictly correct neither for q—>0 nor for large q the spirit of our approach is similar to the long wavelength approximation encountered in the mean field theory of blends, Eq. (7). That is, we write the effective free energy functional as an expansion in powers of t t and include terms (Vv /)2 as well as (V2 /)2, as in the related problem of lamellar phases of microemulsions [232,233],namely [234]... 

At higher concentrations of sinks, one might also expect shape transitions to ordered surfactant phases of microemulsion. 

The variety of structures encountered in microemulsions offers great versatility for choosing the locus of polymerization. Besides polymerization in globular microemulsions, several studies have dealt with polymerization of monomers in the other phases of microemulsions. One of the main goals underlying these studies was to use the microstructure of microemulsions as a template to produce solid polymers with similar characteristics. For example, incorporation of large amount of hydrophobic monomers in the continuous phase of W/O microemulsions should yield solid polymers with a Swiss cheese-like structure capable of encapsulating the disperse phase (water). This would allow the inclusion of materials (metallic colloidal particles as catalysts, photochromic compounds, etc.) in the disperse phase that would otherwise be insoluble in the polymer. 

Porous solids were obtained by copolymerization of styrene-divinylbenzene [40,42] and cyclohexyl methacrylate-allyl methacrylate [41] in the continuous phase of microemulsions. Menger et al. [40] showed that the pore size of the material, always larger than the initial droplet size, is highly dependent on the water/surfactant ratio in the... 

As for polymerization of hydrophobic monomers in the bicontinuous phase of microemulsions, the initial structure is not preserved upon polymerization. However, a notable difference from the former systems is that the final system is a microlatex that is remarkably transparent (100% optical transmission), fluid, and stable, with a particle size remaining unchanged over years even at high volume fractions ( 60%) [20]. The microlatex consists of water-swollen spherical polymer particles with a narrow size distribution according to QELS and TEM experiments. This result is of major importance with regard to inverse emulsion polymerization, which is known to produce unstable latices with a broad particle size distribution [23]. 

It should be amply evident from the accounts of Chapters 1-5 that assisted by well-established basic observations, various investigators have utilized both macro-and microemulsion processes for the synthesis of a large variety of inorganic substances in particulate form. The activity is more pronounced in case of microemulsion-mediated synthesis for the obvious reasons that (i) the method yields nanoparticles of many useful substances for special applications and (ii) in addition to the spherical shape that comes generally from the spherical droplet phase of microemulsions, other shapes like nanorods and nanowires can be produced by this method in specific cases. Both the methods, however, have their own advantages and limitations, and products of both the processes have been demonstrated to have potentials for application. The following text discusses some of these points. 

Polymerization in the Continuous or Bicontinuous Phases of Microemulsion. In an attempt to prepare hydrophobic polymers used to... 

Bimetallic Palladium/Gold Nanoparticles Zhang and coworkers [57] synthesized bimetallic palladium/gold nanoparticles in the IL-based microemulsion. The palladium and gold precursors were dissolved in dispersive and continuous phase of microemulsion (H2O/TX-100/[bmim][PFJ), respectively. [PdCy ions were reduced in situ by TX-lOO in dispersive water phase to prepare Pd nanoparticles and then [AuClJ crossed through the interface film and reacted with the as-prepared Pd nanoparticles to form Pd Au nanoparticles. 

The three-phase systems are the most interesting ones for applications. Indeed, the interfacial tension between the middle phase of microemulsion and the upper or lower phase is then extremely low. ° Besides, the middle-phase microemulsion corresponds to maximum amounts of oil and water that can be mixed and still form a one-phase system. The middle-phase microemulsion has a bicontinuous structure where the oil and water domains are intimately mixed and extend over microscopic distances (see Figure 1.7 bottom). As in droplet systems, a mixed film of surfactant and cosurfactant separates water and oil domains. This type of structure undergoes large and spontaneous fluctuations. The approach of the compositions associated with the Winsor I Winsor III and Winsor II —> Winsor III phase transitions is critical, with a divergence of many properties of the system such as the intensity of scattered light or the interfacial tension. Also the interactions between droplets in the microemulsion phase of Winsor I or Winsor II systems become increasingly attractive at the approach of the transition. ... 

Polymerization reactions have been carried out in microemulsions of all types of stmctures. As we have noted earlier, microemulsions can be of the droplet type, either with isolated water droplets dispersed in a continuous oil phase (w/o microemulsion) that usually occur in systems with high oil content or with isolated oil droplets dispersed in a continuous water phase (o/w microemulsion), typically occurring in water-rich region. Nondroplet-type microemulsions, on the other hand, feature continuous oil and water phases intertwined in dynamic extended networks and are called bicontinuous microemulsions. A monomer can be incorporated in any of the water and oil phases of microemulsions and polymerized by normal... 

Solid porous materials can be prepared by polymerization of all three types (o/w, w/o, and bicontinuous middle phase) of microemulsions [71]. Gupta and Singh [34] obtained porous polymers by polymerization of styrene/divinylbenzene as the continuous phase in an oil-continuous (w/o) microemulsion. Which was prepared using AOT as surfactant. The polymerization was carried out thermally at 70 °C using benzoyl peroxide as initiator. The porous materials may eventually be transformed into porous membranes. Such membranes have many applications in the field of separation science, composites, medicines, and biotechnology. Depending on the pore size, the porous polymers could be used for the separation of dust as also microparticles such as virus, bacteria, pigments, colloidal particles, etc. 

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