Preformed microemulsions

A different example of triphasic catalysis for the Heck, Stille and Suzuki reactions relied on a three-phase microemulsion/sol-gel transport system. Gelation of an z-octyl(triethoxy)silane, tetramethoxysilane and Pd(OAc)2 mixture in a H2O/CH2CI2 system led to a hydrophobicitized sol-gel matrix that entrapped a phosphine-free Pd(ii) precatalyst. The immobilized precatalyst was added to a preformed microemulsion obtained by mixing the hydrophobic components of a cross coupling reaction with water, sodium dodecyl sulfate and a co-surfactant, typically zz-propanol or butanol. This immobilized palladium catalyst was leach proof and easily recyclable. 

The choice of suitable surfactants and additional chemicals for the decontamination of source zones largely depends on the type of pollutant and the structure of the soil (mainly on adsorption behaviour and hydraulic conductivity). Adsorbed and solid pollutants or very viscous liquid phases cannot be mobilised. Preformed microemulsions, co-solvents or co-surfactants can be favourably used for such contaminations in order to enhance the solubilisation capacity of surfactants. NAPL with low viscosity can easily be mobilised and also effectively solubilised by microemulsion-forming surfactant systems. Mobilisation is usually much more efficient. It is achieved by reducing the interfacial tension between NAPL and water. Droplets of organic liquids, which are trapped in the pore bodies, can more easily be transported through the pore necks at lower interfacial tension (see Fig. 10.2). The onset of mobilisation is determined by the trapping number, which is dependent on... 

A complex system containing a branched anionic surfactant, non-ionic surfactants, rapeseed oil methyl ester and an aqueous calcium chloride solution was found to form bicontinuous microemulsions even at low temperatures [46, 90]. This type of microemulsion has been studied for DNAPL extraction on a large scale in an artificial aquifer and later in a joint project with different partners financed by the German Federal Ministry of Education and Research (BMBF) [91 ]. The project network applied an integrated concept regarding aspects of hydraulics, reuse and biodegradation [92]. Three large-scale experiments each with some hundreds of litres of preformed microemulsion were performed. Whereas extraction of perchloroethylene in the field-scale experiment was not successful... 

Preformed microemulsions containing co-solvents and co-surfactants have been used for laboratory experiments [94] and a field test [55] in Canada. The systems were developed for the extraction of a viscous oil containing up to 16% of chlorinated solvents from a site at Ville Mercier. The contaminant is a DNAPL with a density of 1.05 g/cm3 and thus exhibits only a small density difference compared to chlorinated solvents [94]. It could not be extracted effectively by the usual Winsor I systems containing n-butanol as a co-surfactant. The addition of solvents was necessary for effective solubilisation of the contaminant [94]. A preformed microemulsion containing D-limonene, toluene, n-butanol, Hostapur SAS (secondary alkane sulphonate sodium salt) and water was injected into a field test site at Thouin Sand Pit near Montreal. In previous column experiments, a composition of 13.16% D-limonene, 13.16% toluene, 9.21% n-butanol, 9.21% Hostapur SAS and 0.3% of sodium ortho-silicate in water was used as a preformed microemulsion for the extraction of DNAPL from Ville Mercier. 

Preformed micro emulsions can also be used for soil decontamination. The application of bioremediation with microemulsions containing nutrients for oil spills is already a well-known technology [84, 85] and is also proposed for in situ treatment of DNAPL sites [86]. Studies on contaminant extraction, however, are less frequent. In most cases, these systems have been discussed and investigated for adsorbed or highly viscous contaminants which can only be solubilised. Enhancement of solubilisation in micro emulsions compared with surfactant solutions was found for pyrene [87] and patented for ex situ treatment of contaminated soil [88]. An interesting cost-effective variation uses partially sulphated castor oil [89]. 

Metallie nanoeatalysts ean be incorporated into the mesoporous structure by a variety of methods. Prefabricated nanopartieles can be incorporated into mesoporous solids by adding the partieles into the sol-gel mixture or, if the particles are formed by mieroemulsions (see Section 9.2.5), the microemulsion can be incorporated into the preformed mesoporous structure. Alternatively, metal salts can be added during gel formation or after the mesoporous structure has formed [12]. An example of the former is the synthesis of WO, and WO Pt films that were made by synthesizing W(OC2H5)e and H2PtCl6 sol-gel solutions, followed by aging and calcination [13]. A significant drawback associated with this method is that the catalytic nanoparticles may be buried within the structure rather than near the pores. If the partieles are not located near the pores they will not be accessible to reactants and therefore will not be efficient catalysts. 

Figure 7.1 Synthetic strategies to obtain tnetal ceria core-shell structures, (a) Co-precipitation of either preformed metal particles or metal particle precursors and the metal-oxide precursor, (b) Microemulsion, (c) Direct functionalization of preformed metal particles.

In the first section, various kinds of functional polymer, in particular the most used conductive polymer, conjugated polymer (CP), redox polymer, metallopolymer. Selection of the correct functional polymer depends on the desired properties of the resulting nanocomposites. The second part of the chapter focuses on the basic approaches used in the preparation of polymeric nanoparticles. As mentioned earlier, there are two basic approaches in the recent literature to synthesize the polymeric nanoparticles. In this section, we focus on the discussion of the common and widely used preparation methods for various kinds of polymeric nanoparticles. The polymerization method is based on the encapsulation of nanoparticles through heterogeneous polymerization in dispersion media. This method can be further classified into emulsion, microemulsion and miniemulsion. Polymer encapsulated nanoparticles can also be prepared directly from preformed polymer, where this approach is based on the specific interactions between nanoparticles and the preformed polymer, such as electrostatic interactions, hydrophobic interactions and secondary molecular interactions or self-assembly method. 

An ab initio emulsion polymerisation involves the emulsification of one or more monomers in a continuous aqueous phase and stabilisation of the droplets by a surfactant. In a seeded emulsion polymerisation, one starts instead with a preformed seed latex. Usually, a water-soluble initiator is used to start the free-radical polymerisation. The locus of polymerisation is within the submicron polymer particles (either formed during the process or added at the start), which are swollen with monomer during the polymerisation process, and dispersed in the aqueous phase. The final product is a latex comprising a colloidal dispersion of polymer particles in water. Ab initio emulsion polymerisation differs from suspension, mini- and microemulsion polymerisations in that the particles form as a separate phase during the polymerisation process. The particle size is much smaller than those formed in a suspension polymerisation. 

Nanocapsules act like a reservoir, which are called vesicular systems. They carry the active substance entrapped in the solid polymeric membrane or on their surfaces. The cavily inside contains either oil or water. A schematic diagram of Polymer Nanocapsules is shown in Fig. 9.2 [5], There are different methods that are used nowadays to prepare polymeric nanoparticles, such as nanoprecipitation (also termed as the solvent diffusion and solvent displacement method), solvent evaporation, dialysis, microemulsion, surfactant-free emulsion, salling-out, supercritical fluid technology, and interfacial polymerization [2]. Among these methods, nanoprecipitation is a fast and simple process, which does not require a pre-prepared polymer emulsion before the nanoparticle preparation. It produces a dispersion of nanoparticles by precipitation of preformed hydrophobic polymer solution. Under... 

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