Surfactant-water-supercritical fluid systems

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 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). 

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. 

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 phase boundary lines for supercritical ethane at 250 and 350 bar are shown in Figure 2. The surfactant was found to be only slightly soluble in ethane below 200 bar at 37 C, so that the ternary phase behavior was studied at higher pressures where the AOT/ethane binary system is a single phase. As pressure is increased, more water is solubilized in the micelle core and larger micelles can exist in the supercritical fluid continuous phase. The maximum amount of water solubilized in the supercritical ethane-reverse micelle phase is relatively low, reaching a W value of 4 at 350 bar. 

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. 

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... 

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. 

The effect of surfactants on the interfacial tension between water and supercritical fluids is a key property for describing emulsions and microemulsions (8), as shown in Figure 2. The v axis may be any formulation variable that influences surfactant partitioning between the phases such as the pressure or temperature. A minimum in y is observed at the phase inversion point, where the system is balanced with respect to the partitioning of the surfactant... 

The potential of microemulsions for organometaUic-catalyzed hydrogenations in water/scC02 biphasic systems has been assessed using the rhodium-catalyzed hydrogenation of styrene as a common test reaction [Eq. (7)] [31]. The water-soluble Wilkinson complex [RhCl(TPPTS)3] was applied as catalyst precursor together with anionic perfluoropolyether carboxylates, cationic Lodyne A, or nonionic poly-(butene oxide)-b-poly(ethylene oxide) surfactants. The interfacial tension is small in the presence of the supercritical fluid and small amounts of surfactant (0.1-2.0 wt.%) suffice to form stable microemulsions. The droplet diameter of the microemulsions varied between 0.5 and 15 pm and a surface area of up to 10 m was obtained. 

Figure 1 shows the aggregation behavior of AOT in liquid cyclohexane and supercritical fluid ethane. The systems are one-phase without added water. Surfactant aggregation is indicated by the solvatochromic probe pyridine A -oxide. Pyridine A -oxide was used because of its small size and large dipole moment (/x = 4.3 D), which allow it to partition to the center of reverse micelles instead of being trapped at the surfactant interface. This molecule is a blue shift indicator in that its U V absorption maximum shifts to lower... 

A ternary system composed of oil, water, and surfactant can form a wide variety of aggregated structures. Two characteristic compositions are frequently studied reverse micelle systems in which the amount of oil greatly exceeds the amount of water, and systems in which oil and water are present in relatively equal amounts (Winsor systems). Reverse micelle systems were discussed in the previous section this section is devoted to Winsor systems having an oil phase composed of a supercritical fluid or compressed liquid alkane. It should be noted, however, that these two types of systems merely represent two specific regions in the space of ternary oil-water-surfactant compositions, and both are subject to the same thermodynamic considerations. 

Most highly polar and ionic species are not amenable to processing with desirable solvents such as carbon dioxide or any other solvent such as water that has a higher critical temperature well above the decomposition temperature of many solutes. In such instances, the combination of the unique properties of supercritical fluids with those of micro-emulsions can be used to increase the range of applications of supercritical fluids. The resulting thermodynamically stable systems generally contain water, a surfactant and a supercritical fluid (as opposed to a non-polar liquid in liquid micro-emulsions). The possible supercritical fluids that could be used in these systems include carbon dioxide, ethylene, ethane, propane, propylene, n-butane, and n-pentane while many ionic and non-ionic surfactants can be used. The major difference between the liquid based emulsions and the supercritical ones is the effect of pressure. The pressure affects the miscibility gaps as well as the microstracture of the micro-emulsion phase. 

HAR Haruki, M., Matsuura, K., Kaida, Y., Kihara, S.-L, and Takishima, S., Microscopic phase behavior of supercritical carbon dioxide + non-ionic surfactant + water systems at elevated pressures. Fluid Phase Equil, 289, 1, 2010. 

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... 

The general observation from DLS studies is that the apparent hydrodynamic diameter increases as the pressure is decreased towards a phase boundary (where surfactant and water will precipitate to form a second phase). Figures 2 and 3 show DLS results for AOT/water micelles in supercritical xenon (at 25 C) and ethane (at 37 C), respectively. Results are presented for [H20]/[A0T] molar ratios (W) of 1 (a) and 5 (b). All measurements were obtained In single-phase systems at constant W. The apparent hydrodynamic micelle diameter decreases with increasing pressure or density of the continuous phase in both fluids. The second cumulant in Equation 1, which is a qualitative measure of the polydispersity of the system, is very close to zero for all conditions of this study. There is no statistically... 

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