Ethane/AOT

Figure 2. Ethane-rich corner of the ethane/AOT/water ternary phase diagram (weight %) at 37 c and at two pressures, 250 and 350 bar.

Equilibrium with Aqueous Phases. The formation and properties of reverse micelle and microemulsion phases in equilibrium with a second predominantly water continuous phase is of practical interest for extraction processes. Figure 7 compares apparent hydrodynamic diameters observed in the ethane/AOT/water system at 37 C for values of 1, 3 and 16. In single phase systems at W - 1 (a) and 3 (b) the apparent hydrodynamic diameter decreases with increased pressure due to decreased micelle-micelle interactions as the solvent power increases. In contrast for a system with an overall W - 16 (c), where a second aqueous phase exists, hydrodynamic diameter increases continuously with pressure. 

Figure 7. Hydrodynamic diameters for supercritical (37 C) ethane/AOT/water solutions as a function of pressure for W values of the overall system of 1 (e )f 3 (b - O) f 16 (c - ). ...

Most recently, Robinson and co-workers (72) used time-resolved decay of anisotropy experiments to probe the AOT reverse micelle system in ethane and propane. These authors conclude there was no local solvent density augmentation about the reverse micelle. In addition, the rotational dynamics of their probe (perylene tetracarboxylate) was independent of fluid density. This observation was consistent with results from Johnston and co-workers and Smith and co-workers (61-65,72). 

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... 

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. 

A high pressure vibrating tube densimeter (Mettler-Paar DMA 512) was used to measure the density of the AOT/water/supercritical ethane solutions. By recirculating water from a thermostated water bath through the water-jacketed measuring cell, the temperature of the cell could be controlled to 0.01 C. The micelle solutions were prepared by loading measured amounts of AOT and water into a... 

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. 

Measurements of supercritical ethane density versus the AOT concentration shown in Figure 4 (T = 37 C, P = 250 bar) indicate that the properties of the supercritical continuous phase resemble those of the pure fluid. The dispersed micelle phase does not appear to increase the critical temperature or critical pressure of the binary solution to the point of inducing a phase change in the system. There is a small increase in density as surfactant is added to the system which confirms the visual observation that a second liquid phase of much higher density is not formed. 

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... 

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

Johnston et al. l also examined the solvatochromic shift of pyridine N-oxide in an ethane/CjEj (C = 10-13 E = 5) water-in-oil microemuision, also in equilibrium with a lower liquid phase. Contrary to the behavior exhibited by the AOT system, the nonionic microemulsions display a polar environment at low pressures, which becomes progressively less polar as pressure increases. At a pressure of only 50 bar, they reported that the probe s environment resembles that observed in bulk hexane. Added water increases the polarity somewhat, yet a cosurfactant (octanol) is required to produce an environment similar to that in bulk water. The polarity of the ethane/ water/surfactant/cosurfactant system remains essentially constant as pressure increases up to 350 bar. 

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. 

Anionic Surfactant AOT has been studied extensively since it forms reverse micelles readily in a variety of organic solvents, even without a co-surfactantf28 >. Figure 6 shows the solvatochromic shifts of 0.0002 M pyridine-N-oxide in solutions of AOT in SCF ethane at 345 bar. No water was added to the system, but it is likely that Wo was 1 given the difficulty of completely dehydrating AOT(22). The pressure was fixed at 345 bar so that all of the AOT solutions would be in the one-phase region(16). Notice how closely the results for ethane match those for... 

Figure 6. Xmax for pyridine-N-oxide as an indicator of aggregation for AOT in ethane without added water...

Again, an examination of water solubilization into micelles in various solvents helps explain the differences for this surfactant and AOT. The uptake of water into 0.1 M Cn-14 EO5 was found to increase as ACN decreased from 10 (decane) to 5 (pentane). We do not expect that this increase would continue all the way to ethane. However, we do find that the optimal ACN for water uptake into this surfactant is less than 8, the value for AOT. The lower ACN for Ci 1-14 EO5 versus AOT could be a factor in the explanation of their different behavior in ethane. 

Table HI. Addition of octane to SCF ethane to swell reverse micelles of AOT with water at modest pressures at 35 C...

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