Supercritical fluid microemulsions

Because of the obvious importance of a COj-based system, there has been much work in the area of developing this type of supercritical fluid microemulsion. On the surface, the strategy is simple and interesting parallels can be made to the first soap or surfactants used in water thousands of years ago. For instance, with... 

As this chaper goes to press, there is still more activity in the supercritical microemulsion area. A revieW emphasizing experimental techniques and theoretical explanations has appeared. A recent paper by Johnston et,al. l reports some solubility of a protein in a COj microemulsion. Another paper by Jackson et al. describes the formation of supercritical fluid microemulsions in environmentally friendly CFC s. We expect many more developments in these areas over the next few years. 

Figure 2.4-6 High pressure closed-loop recirculating cell used for EPR studies of supercritical fluid microemulsions and emulsions RD = rupture disk P = pressure transducer T = thermocouple.

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... 

Liu JC, Ikushima Y, Shervani Z (2004) Investigation on the solubilization of organic dyes and micro-polarity in AOT water-in-C02 microemulsions with fluorinated cosurfactant by using UV-Vis spectroscopy. J Supercrit Fluids 32 97-103... 

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). 

Biocatalysis and Microemulsions in Near-critical and Supercritical Fluids... 

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 use of inverse micelles and microemulsions of AOT in supercritical or near supercritical fluids as extractants for valuable hydrophilic substances such as proteins continues to develop. FT-IR studies of the pressure dependence of the water core structure in various parts of the phase diagrams of such systems have been described (89). 

Since there are various types of fluids, there are different kinds of dispersions that might be encountered in EOR. Fluids may be liquid, gaseous, or in the supercritical state. In EOR, gases are sometimes further classified as condensible (i.e., steam) or as not condensible into a liquid state of essentially the same composition. Certain fluids that contain sufficiently large concentrations of surfactant are termed microemulsions. Hence, depending on the type of oil recovery process and the conditions employed, a dispersion might be a so-called "oil-in-water" emulsion, an emulsion in which one of the fluids is a microemulsion, a foam (i.e., a dispersion of gas in a liquid), or a dispersion in which one of the phases is a supercritical fluid. 

Reverse micelle and microemulsion solutions are mixtures of a surfactant, a nonpolar fluid and a polar solvent (typically water) which contain organized surfactant assemblies. The properties of a micelle phase in supercritical propane and ethane have been characterized by conductivity, density, and solubility measurements. The phase behavior of surfactant-supercritical fluid solutions is shown to be dependent on pressure, in contrast to liquid systems where pressure has little or no effect. Potential applications of this new class of solvents are discussed. 

For a pure supercritical fluid, the relationships between pressure, temperature and density are easily estimated (except very near the critical point) with reasonable precision from equations of state and conform quite closely to that given in Figure 1. The phase behavior of binary fluid systems is highly varied and much more complex than in single-component systems and has been well-described for selected binary systems (see, for example, reference 13 and references therein). A detailed discussion of the different types of binary fluid mixtures and the phase behavior of these systems can be found elsewhere (X2). Cubic ecjuations of state have been used successfully to describe the properties and phase behavior of multicomponent systems, particularly fot hydrocarbon mixtures (14.) The use of conventional ecjuations of state to describe properties of surfactant-supercritical fluid mixtures is not appropriate since they do not account for the formation of aggregates (the micellar pseudophase) or their solubilization in a supercritical fluid phase. A complete thermodynamic description of micelle and microemulsion formation in liquids remains a challenging problem, and no attempts have been made to extend these models to supercritical fluid phases. 

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... 

The application of supercritical microemulsions to cleaning is a new area and there have been no published results to date applying these systems to cleaning operations but the combination of surfactants with fluids would be a natural choice for many systems. The information in this chapter is to be used as a guide to the development of cleaning applications based upon supercritical fluid-based microemulsions. 

Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (7)j = r/7 > 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes. 

Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosuccinate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelleforming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/water systems is already well documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system. A The spherical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now well-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core. 

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... 

Table 3. Examples of Polar Solutes Solubilized in Supercritical and Near-critical Fluid Microemulsions...

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