Reverse micelle in supercritical fluids

Recently, the first observation of reverse micelles in supercritical fluid (dense gas) solvents has been reported (2) for the surfactant sodium bis(2-ethyhexyl) sulfosuccinate (AOT) in fluids such as ethane and propane. The properties of these systems have several attributes which are relevant to secondary oil recovery. In the supercritical fluid region, where the fluid temperature and pressure are above those of the critical point, the properties of the fluid are uniquely different from either the gas... 

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)... 

Olesik, S.V. Hedrich, D. Reverse Micelles in Supercritical Fluids, ARO/ONR Workshops on Supercritical Fluid Technologies, Seattle, WA, May 6-8, 1987. 

The spectroscopic probe pyridine-N-oxide was used to characterize polar microdomains in reverse micelles in supercritical ethane from 50 to 300 bar. For both anionic and nonionic surfactants, the polarities of these microdomains were adjusted continuously over a wide range using modest pressure changes. The solubilization of water in the micelles increases significantly with the addition of the cosolvent octane or the co-surfactant octanol. Quantitative solubilities are reported for the first time for hydrophiles in reverse micelles in supercritical fluids. The amino acid tryptophan has been solubilized in ethane at the 0.1 wt.% level with the use of an anionic surfactant, sodium di-2-ethylhexyl sulfosuccinate (AOT). The existence of polar microdomains in aggregates in supercritical fluids at relatively low pressures, along with the adjustability of these domains with pressure, presents new possibilities for separation and reaction processes involving hydrophilic substances. 

Let us now discuss the structure of a reverse micelle. As the name suggests it has a structural arrangement exactly opposite to that of a micelle. Reverse micelles generally refer to aggregates of surficants (e.g., dioctyl sulfosuccinate, AOT) formed in a non-polar solvent. In this situation, the polar headgroups of the surficants point inward (core) and the hydrocarbon chains project outward into the non-polar solvent [3]. The solvent one uses for reverse micelle formation is usually liquid hydrocarbons. Recently the formation of reverse micelles in supercritical fluids such as ethane, propane, and carbon dioxide has been observed. 

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 ... 

Most recently, Fox, Johnston, and co-workers (65) showed how an environmentally-sensitive fluorescent species could be used to probe the water pool of AOT reverse micelles in supercritical alkanes. The interesting observation was that the emission spectra were not shifting with fluid density. These results were consistent with the postulate of the micelle interior not changing with the supercritical fluid density paralleling conclusions reached using other spectroscopic techniques (61-64). 

Figure 2. Apparent hydrodynamic diameters of AOT reverse micelles In supercritical xenon as a function of pressure and density (of the pure fluid) at 25 C, with (a) W - 1 and (b) W - 5. (AOT) - 150 mM.

Studies of reversed micelles dispersed in supercritical fluids have shown their ability to solubihze hydrophihc substances, including biomolecules and dyes, opening the door to many new applications [60,61]. In particular, solutions of reversed micelles in liquid and supercritical carbon dioxide have been suggested as novel media for processes generating a minimum amount of waste and with a low energy requirement [62]. 

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... 

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... 

In the studies described here, we examine in more detail the properties of these surfactant aggregates solubilized in supercritical ethane and propane. We present the results of solubility measurements of AOT in pure ethane and propane and of conductance and density measurements of supercritical fluid reverse micelle solutions. The effect of temperature and pressure on phase behavior of ternary mixtures consisting of AOT/water/supercritical ethane or propane are also examined. We report that the phase behavior of these systems is dependent on fluid pressure in contrast to liquid systems where similar changes in pressure have little or no effect. We have focused our attention on the reverse micelle region where mixtures containing 80 to 100% by weight alkane were examined. The new evidence supports and extends our initial findings related to reverse micelle structures in supercritical fluids. We report properties of these systems which may be important in the field of enhanced oil recovery. 

The existence of a reverse micelle phase in supercritical fluids has been confirmed from solubility, conductivity and density measurements. The picture of the aggregate structure in fluids is one of a typical reverse micelle structure surrounded by a shell of liquid-like ethane, with this larger aggregate structure dispersed in a supercritical fluid continuous phase. 

The reverse micelle phase behavior in supercritical fluids is markedly different than in liquids. By increasing fluid pressure, the maximum amount of solubilized water increases, indicating that these higher molecular weight structures are better solvated by the denser fluid phase. The phase behavior of these systems is in part due to packing constraints of the surfactant molecules and the solubility of large micellar aggregates in the supercritical fluid phase. 

Pressure Tuning of Reverse Micelles for Adjustable Solvation of Hydrophiles in Supercritical Fluids... 

Shield et al. (22) have demonstrated that reverse micelles can be used in organic solvents to recover proteins selectively from aqueous solutions. Protein denaturation can occur, however, during recovery from the organic phase, which requires changes in pH or ionic strength. Supercritical fluid solvents offer the potential advantage that proteins could be recovered simply by changing the pressure. Additional potential applications of surfactants in supercritical fluids... 

Highly polar microdomains exist in reverse micelles of AOT and nonionic polyethylene oxide surfactants in ethane, even below 100 bar, both with and without cosolvents. Without cosolvents these domains are likely very small since values of Wo are small. The addition of the cosolvent octane provides a means to take up large amounts of water over a wide pressure range. The polarities in the interior of the micelles approach that of bulk water. The existence of polar microdomains in supercritical fluids at relatively low pressures presents an opportunity for new separation and reaction processes involving hydrophilic substances. 

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"... 

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. 

Reverse micelles also form in supercritical fluids, as evidenced by changes in the fluorescence and absorption spectra of probes, in the two phase region -... 

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... 

The same team has also described the selective hydrogenation of cis-2-pentenenitrile with surfactant-stabiUzed ammonium perfluorotetradecanoate bimetallic Pd-Ru nanopartides prepared via in situ reduction of their simple salts in reverse micelles in SCCO2 [22]. The optimized ratio Pd Ru nanopartide (1 1) shows the highest activity for the hydrogenation of functionalised alkene under mild conditions. No hydrogenation of the terminal nitrile of the molecule in amine was observed and, finally, this fluorinated micelle-hosted bimetallic catalyst gives relevant activity and selectivity in the supercritical fluid without deactivation for at least three catalytic cycles. 

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. 

The strueture of reverse micelle and microemulsion phases in near critical and supercritical fluid as determined from dynamic light scattering studies has been published by Johnston and Penninger in Supercritical Fluid Science and Technology, ACS Symp. Ser. Washington DC, 1989. 

There are several appealing factors for the use of micellar supercritical phases in chromatography. The peak efficiencies obtained in SFC are higher than in LC because the solute diffusion coefficients are higher in supercritical fluids than in liquids. In SFC, mass transfers are enhanced by the combination of high diffiision coefficients and low viscosities. This could compensate for the low efficiency induced by micelles. The polar aqueous core of the reverse micelles should allow the separation of hydrophilic or even ionic solutes with supercritical fluids. These polar compounds are difficult to analyze in SFC [10]. 

Fulton, J. L. and Smith, R. D. (1988) Reverse Micelle and Microemulsion Phases in Supercritical Fluids, J, Phys. Ghent. 92, 2903-2907. 

---