Water-in-SCF microemulsions

One problem of using water-in-oil microemulsions for nanoparticle synthesis or chemical reaction is the separation and removal of solvent from products. A unique feature of the water-in-SCF microemulsions is that SCF reverse micelle phase stability is strongly dependent on the fluid pressure (density), at higher temperatures it is necessary to use higher pressures, or the density will be insufficient, the microemulsion will break, and phase separation can be accomplished [16]. So the reaction product can be obtained by a reduction in pressure without the need for extraction from reverse micelles as in the case of organic solvents [23]. 

A wide variety of reactions has been performed in water-in-scC02 microemulsions. The following examples show how the properties of a fluid at conditions higher than critical (where, in particular, the specific properties are prevailing) can describe the changes observed in the catalytic reactivity or selectivity in water-in-SCF microemulsions. 

SCFs are an environmentally friendly alternative to organic solvents as media for biocatalysis. A key feature of biocatalysis in SCFs is the tunability of the medium [75]. Enzymatic activity in SCFs has been proven and well documented [76]. Limiting factors, which may affect enzymatic activity in supercritical solvent systems, have been identified and are well characterized. A major limitation to the broader use of SCFs is their inability to dissolve a wide range of hydrophilic and ionic compounds, which greatly impedes their ability to carry out biolransformation with polar substrates. The interest in water-in-SCF microemulsion as reaction media stems from the fact that in such systems high concentrations of both polar and apolar molecules can be dissolved within the dispersed aqueous and continuous SCF phases, respectively. 

Despite the great potential of such methods for the preparation of polymers with specific properties, up to now very few studies have been carried out on polymerization in water-in-SCF microemulsions. 

Thermodynamically stable microemulsions and kinetically stable emulsions may be utilized to bring water and nonvolatile hydrophilic substances, such as proteins, ions, and catalysts, into contact with a SCF-continuous phase (e.g. CO2) for separation, reaction and materials formation processes. Reactions between hydrophilic and hydrophobic substrates may be accomplished in these colloids without requiring toxic organic solvents or phase transfer catalysts. CO2 and aqueous phases may be mixed together over a wide range in composition in w/c and c/w emulsions. The emulsion is easily broken by decreasing the pressure to separate the water and CO2 phases, facilitating product recovery and CO2 recycle. Reaction rates can be enhanced due to the considerably lower microviscosity in a w/c as compared to a water-in-alkane microemulsion or emulsion. 

The combination of water-in-oil microemulsion and SCFs is a promising topic and may find more applications with some interesting advantages by utilizing the unique properties of SCFs. These include pressure-dependent variables such as viscosity, density, and diffusion rate, as well as the ability to readily manipulate the P-T phase behavior in the multicomponent micelle systems [13]. Much of the current research efforts in this area have been directed toward the SCF-continuous microemulsions. 

The first observations of the micelles and microemulsions formation in SCFs (supercritical ethane and propane) were given in the pioneering wo by Gale et al. [ 15]. As multicomponent reaction mixtures in the vicinity of a critical point can exhibit phase behavior phenomena not occurring in ordinary gas or liquid mixtures, the primary focus on these novel microemulsion systems has been on the phase behavior of water-in-SCF miCToemulsions. A detailed coverage of the phase behavior of this system is outside the scope of this chapter. 

The water-in-C02 microemulsion described in this section behaves like a nanoreactor, allowing ionic reactions to take place in the SCF phase. In principle, this system can be used to smdy chemical reactions and to synthesize nanoparticles involving any aqueous ionic species that are normally not soluble in supercritical CO2. 

Since CO2 is widely considered to be the desirable SCF because of its environmentally benign characteristics and ambient critical temperature, the use of supercritical CO2 for the preparation and processing of nanomaterials has naturally received considerable attention. However, as discussed in previous sections, the poor solubility of most solutes in supercritical CO2 represents a major limitation. Surfactants containing both C02-soluble and hydrophilic moieties are often added to CO2 to form reverse micelles. Such water-in-C02 microemulsions offer a convenient means of dissolving hydrophilic compounds, as demonstrated by the in situ methods for the preparation of nanoparticles (240,247-249). For the production of nanoscale metals and semiconductors via RESOLV, on the other... 

The use of water-in-C02 microemulsions with RESOLV for nanoparticle production serves as an alternative to the in situ reaction method reported by Wai, Fulton, and coworkers (247,248) and by Johnston and coworkers (240,249). However, a significant feature of the RESOLV method is that the formation of nanoparticles occurs under ambient conditions outside the SCF chamber, avoiding difficulties associated with the in situ reaction method regarding collection of the as-prepared nanoparticles. In addition, as discussed in previous sections. 

It is quite common to add small amounts of cosolvents to SCFs (especially CO2) to increase the solubilities of heavy organic solutes. Unfortunately, not much seems to dissolve in CO2 (Gwynne, 1996). In order to extend its use in drug research and in the food and paper industries, microemulsions are formed in CO2. As the amount of water in the system increases, the microemulsions, called micelles, swell up to the point that the water they contain effectively resembles bulk water. 

The first generation of research involving surfactants in SCFs addressed water/oil (w/o) microemulsions (Fulton and Smith, 1988 Johnston et al., 1989) and polymer latexes (Everett and Stageman, 1978) in ethane and propane (Bartscherer et al., 1995 Fulton, 1999 McFann and Johnston, 1999). This work provided a foundation for studies in C02, which has modestly weaker van der Waals forces (polarizability per volume) than ethane. Consequently, polymers with low cohesive energy densities and thus low surface tensions are the most soluble in C02 for example, fluor-oacrylates (DeSimone et al., 1992), fluorocarbons, fluoroethers (Singley et al., 1997), siloxanes, and to a lesser extent propylene oxide. Since C02 is... 

There has been much interest in recent years in exploiting the properties of microemulsion phases in SCFs (48-52). A reverse micelle or microemulsion system of particular interest is one based on CO2 because of its minimal environmental impact in chemical applications. Since water and CO2 are the two most abundant, inexpensive, and environmentally compatible solvents, the ap-... 

Harrison et al. reported the first w/c microemulsion in 1994 (20). A hybrid surfactant, namely F7H7, made of respectively one hydrocarbon and one fluorocarbon chain attached onto the same sulfate head group, was able to stabilize a w/c microemulsion at 35 and 262 bar. For a surfactant concentration of 1.9 wt %, water up to a w = 32 value ([water]/surfactant]) could be dispersed. A spherical micellar structure was confirmed by small-angle neutron scattering (SANS) experiments (21). This surfactant was later the subject of dynamic molecular simulations (22, 23). The calculations were consistent with the SANS data and high diffusivity was predicted, highlighting this important feature of low-density and low-viscosity supercritical fluids (SCF). 

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