MICROEMULSIONS IN SUPERCRITICAL FLUIDS

Most of the early work involving microemulsions in supercritical fluids utilized the supercritical alkanes, ethane and propane, with the surfactant AOT. Table 1 gives a summary of the surfactant systems that have been studied in supercritical hydrocarbon solvents. More recently, there has been some success with the formation of 

The properties of the supercritical alkane microemulsions given in Table 1 can be summarized as follows. For methane and ethane, 

66 66 Sodium poly(hexafluoropropylene oxide) carboxylate/water/COj (4) 

Tiyptophan (Amino acid) AOT/octanol/water/ethane (33) 

Pressure-dependent effects can be exploited to significant advantage in a supercritical microemulsion-based cleaning operation. Pressure will have a strong influence on the microstructure of microemulsion phases in compressible fluids as well as on their phase behavior, Microstructure includes the size, shape, and spatial 

Ammonium poly(hexafluoropropylene oxide) carboxylic acid 120 (48) 

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High-pressure FT-IR spectroscopy has been used to clarify (1) the rotational isomerism of molecules, (2) characteristics of water and the water-head group, and (3) RSO3 Na4- interactions in reverse micellar aggregates in supercritical ethane. This work demonstrates interesting pressure, temperature, and salt effects on an enzyme-catalyzed esterification and/or maintenance of a one-phase microemulsion in supercritical fluids from practical and theoretical points of view (Ikushima, 1997). 

In a previous paper we reported our initial observations of reverse micelles and microemulsions in supercritical fluids ( ). We reported that reverse micelles in a supercritical alkane systems can solubilize a highly polar dye (Malachite Green) and that a high molecular weight protein (Cytochrome C, MW = 12,384) can be... 

Several years ago we reported initial observations of reverse micelles and microemulsions in supercritical fluid solvents (JL) These studies suggested the possibility of creating a previously unsuspected broad range of organized molecular assemblies in dense gas solvents. Such systems are of interest due to potential applications which exploit the readily variable properties of supercritical fluids as well as the unique solvent environments of reverse micelles and microemulsions. These initial studies showed that even gram quantities of proteins, such as Cytochrome-c (Mwt. 12,842 dalton) could be solvated in a liter of supercritical ethane or propane due to the microemulsion solvent environment, something which is not achievable with "conventional"... 

Gale RW, Fulton JL, Smith RD. Organized molecular assemblies in the gas phase reverse micelles and microemulsions in supercritical fluids. J. Am Chem Soc 1987 109 920-1. Leitner W, Jessop PG. Green solvents, supercritical solvents. In Anastas PT, editor. Handbook of green chemistry, vol 4. Weinhem Wiley-VCH 2013. 

RW Gale, JL Fulton, RD Smith. Organized molecular assemblies in the gas phase reverse micelles and microemulsions in supercritical fluids. J Am Chem Soc 109 920-921, 1987. 

Surfactants and Colloids in Supercritical Fluids Because very few nonvolatile molecules are soluble in CO2, many types of hydrophilic or lipophilic species may be dispersed in the form of polymer latexes (e.g., polystyrene), microemulsions, macroemulsions, and inorganic suspensions of metals and metal oxides (Shah et al., op. cit.). The environmentally benign, nontoxic, and nonflammable fluids water and CO2 are the two most abundant and inexpensive solvents on earth. Fluorocarbon and hydrocarbon-based surfactants have been used to form reverse micelles, water-in-C02... 

Microemulsions. Systems comprising microwater droplets suspended in an scCO T oil phase can be achieved with the use of appropriate surfactants, of which the best appear to be fluorinated. Microemulsions in supercritical hydrofluoro carbons are also possible. Potential may also exist for speciality coatings via low concentration solutions of fluorinated products in supercritical fluid for, e.g., thin-fitm deposition, conformal coatings, and release coatings. Supercritical CO2 will dissolve in formulated systems to improve flow and plasticize melt-processable materials to improve melt-flow characteristics and lower the glass transition temperature. 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

The question arises as to whether the microemulsion phase retains the high transport rates for which supercritical fluids are well know.f lf Various spectroscopic studies that are described in the following section have been used to directly measure the transport properties in supercritical fluid or near-critical liquid microemulsions. 

Reverse Mieelles. Reverse Micelles in supercritical fluids are currently being studied for several distinct applications (15-18). Normal micelles and microemulsions in aqueous solutions are known to be capable of increasing solution viscosity in several applications including the surfactant flooding of petroleum reservoirs.(19) If reverse micelles or microemulsions can be formed in C02> an increase in solution viscosity could possibly occur. The surfactants chosen as candidates for CO2 flooding application should be characterized by low water solubility and a strong CO2 solubilityi minimal adsorption onto the porous media and stability at reservoir conditions. (20)... 

In this article we describe the phase behavior of a microemulsion system chosen for the free radical polymerization of acrylamide within near-critical and supercritical alkane continuous phases. The effects of pressure, temperature, and composition on the phase behavior all influence the choice of operating parameters for the polymerization. These results not only provide a basis for subsequent polymerization studies, but also provide data on the properties of reverse micelles formed in supercritical fluids from nonionic surfactants. 

The reverse micelles refer to the aggregates of surfactants formed in nonpolar solvents, in which the polar head groups of the surfactants point inward while the hydrocarbon chains project outward into the nonpolar solvent (Fig. 7) [101-126], Their cmc depends on the nonpolar solvent used. The cmc of aerosol-OT (sodium dioctyl sulfosuccinate, AOT) in a hydrocarbon solvent is about 0.1 mM [102]. The AOT reverse micelle is fairly monodisperse with aggregation number around 20 and is spherical with a hydrodynamic radius of 1.5 nm. No salt effect is observed for NaCl concentration up to 0.4 M. Apart from liquid hydrocarbons, recently several microemulsions are reported in supercritical fluids such as ethane, propane, and carbon dioxide [111-113]. 

There has been much interest in recent years to exploit the properties of microemulsion phases in supercritical fluids (23-33). A reverse micelle or microemulsion system of particular interest is one based on CO2 because of its minimum environmental impact in chemical applications. Since water and CO2... 

Our initial studies have demonstrated that the Pd and Rh nanoparticles formed in the CO2 microemulsions are very effective catalysts for hydrogenation of olefins and arenes in supercritical CO2. Dispersing metal nanoparticles in supercritical CO2 utilizing the microemulsion is a new approach for homogenization of heterogeneous catalysis. This approach may have important applications for chemical synthesis in supercritical fluids. 

This brief survey begins in Sec. II with studies of the aggregation behavior of the anionic surfactant AOT (sodium bis-2-ethylhexyI sulfosuccinate) and of nonionic pol-y(ethylene oxide) alkyl ethers in supercritical fluid ethane and compressed liquid propane. One- and two-phase reverse micelle systems are formed in which the volume of the oil component greatly exceeds the volume of water. In Sec. Ill we continue with investigations into three-component systems of AOT, compressed liquid propane, and water. These microemulsion systems are of the classical Winsor type that contain water and oil in relatively equal amounts. We next examine the effect of the alkane carbon number of the oil on surfactant phase behavior in Sec. IV. Unusual reversals of phase behavior occur in alkanes lighter than hexane in both reverse micelle and Winsor systems. Unusual phase behavior, together with pressure-driven phase transitions, can be explained and modeled by a modest extension of existing theories of surfactant phase behavior. Finally, Sec. V describes efforts to create surfactants suitable for use in supercritical CO2, and applications of surfactants in supercritical fluids are covered in Sec. VI. 

Only limited work has been reported on microemulsion-mediated synthesis of aluminum hydroxide [44,45]. In the two publications available [44,45], AOT served as the surfactant. It is possible to form reverse micelles in supercritical fluid media [130], and Matson et al. [44] used such a medium and the microemulsion-plus-reactant technique to synthesize A1(0H)3 particles at 110°C. With supercritical propane as the continuous phase, anhydrous ammonia was injected into the reversed micellar solution containing solubilized Al + [as an aqueous A1(N03)3 solution]. Referring to Fig. 1 and Table 2, the resulting precipitation process followed reaction path AP3 the added ammonia reacted with water molecules in the aqueous pseudophase of the microemulsion to generate hydroxide ions ... 

Since the discovery of microemulsion phases in supercritical fluids in the mid-1980s [1] and their subsequent characterization [2-16], there has been much interest in exploiting the unusual properties of the supercritical fluid phase in applications of these systems. One such application is as a new type of solvent for chemical reactions. In the following sections, I discuss the properties of these systems for reactions, review the progress so far, and analyze the future potential. As a prelude to these discussions, I begin with a brief overview of what is known about the molecular structure of microemulsions in near-critical and supercritical fluids. The details of the primary and secondary molecular structures of various types of microemulsion phases can dramatically affect the reactivity in these systems. 

Several classes of chemical reactions are possible in microemulsions formed in supercritical fluids. Catalytic hydrogenation or oxidation reactions using molecular hydrogen and oxygen as reactants are particularly well suited for these studies as both are very soluble in supercritical fluid solvents. A potentially useful role for these oxidation reactions is the destruction of hazardous chemical wastes or contaminated materials. 

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