J. Phys. Chem. 1991, 95,7127-7129 R = l/211C(t)12 dt
For this case it is clear by inspection that R > ken. The plot of the entropy SB(t)and of P(t) SB(t)/ln NBverifies that the majority of molecules have dissociated prior to sampling any but a small fraction of the available phase space. Also shown in Figure 1 is lC(t)I2computed as if the molecule is stable (r 0). It is clear that due to healing the molecules can more rapidly sample the bound phase space. Figure 2 is the opposite extreme. Here, the quasibound states are just overlapping ( p ( I-) = 1.6). Here, only a finite minority of molecules have dissociated prior to widescale sampling of phase space. The results for a moderately selective preparation of the initial state are shown in Figure 3. Here, the initial density is a mixture
7127
of 5 states (out of the possible 60).The results shown are for p ( r ) = 8. Even for such a fairly selective preparation, the initial value of SBis high enough that significant sampling of phase space did occur while P(t) was still above l / e . Time-resolved experiment3' following optical excitation can provide a direct test of the competition between intramolecular energy redistribution and dissociation as discussed here. Acknowledgment. We thank Professors J. L. Kinsey and J. C. Lorquet for their comments. This work was supported by the US.-Israel Binational Science Foundation, BSF, Jerusalem, Israel. The Fritz Haber Research Center is supported by the Minerva Gesellschaft fur Gesellschaft fur die Forschung, mbH, Munich, BRD. (31) Zewail, A. H. Science 1988, 242, 1645.
Microemuisions in Near-Critical and Supercritical COP T.A. Hoefling, R. M. Enick, and E. J. Beckman* Chemical Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 (Received: June 26, 1991; In Final Form: August 2, 1991)
Previous work has shown that commercially available, alkyl-functionalizedamphiphiles are generally ineffective in producing a Winsor I1 (waterloil) microemulsion in C02. Consequently, a number of model surfactants have been synthesized which appear to preferentially dissolve in the C02-rich phase of a C02/water mixture, allowing solubilization of the hydrophilic dye thymol blue. Model surfactant design was based on the premise that the hydrophobic tails of surfactant intended for use in C02should contain functional groups with low solubility parameters (silicones and fluoro ethers) and low polarizability parameters (fluorinated alkanes), or which act as Lewis bases (tertiary amines), given that C 0 2 is a weak Lewis acid. The formation of stable reverse micelles in supercritical C02 would permit the use of such systems in selective extraction of polar compounds from aqueous solution, in emulsion polymerization schemes, or in the extraction of heavy metals from complex matrices.
Introduction Recent pioneering have shown that micelles can be formed from ionic and nonionic surfactants in near-critical and supercritical alkanes, alkenes, and noble gases. These studies have shown that fluid density plays an important role in regulating the phase behavior, solubilization capacity, and the microstructure of amphiphile/water/suprcriticalfluid (SCF) systems. Consequently, supercritical fluid based microemulsions can be used to perform selective extractions of polar materials from aqueous solution, and as novel vehicles for chemical reactions.&* Unfortunately, as shown by Consani and Smith? commercially available ionic and nonionic surfactants are almost entirely ineffective in generating Winsor I1 systems in carbon dioxide/water mixtures which are capable of solubilizing significant amounts of water or hydrophilic solutes. Given the advantageous properties of C02 (low toxicity, low cost, etc.), this deficiency could seriously limit the wider application of SCF-based microemulsion systems. Yet results of Randolph et al.1° and Ritter and Paulaitis" suggest (1) Fulton, J. L.; Smith, R. D. J . Phys. Chem. 1988, 92, 2903. (2) Blitz, J. P.; Fulton, J. L.; Smith, R. D. J . Phys. Chem. 1988,92,2707. (3) Yazdi, P.; McFann, G. J.; Johnston, K. P. J . Phys. Chem. 1990, 94,
7224. (4) Steytler, D. C.; Lovell, D. R.; Moulson, P. S.;Richmond, P.; Eastoe, J.; Robinson, B. H. Proc. I n f . Symp. Supercrit. Fluids 1988, 67.
(5) Johnston, K. P.;McFann, G. J.; Lemert, R. M. In Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J. M. L., Eds.;ACS Symp. Ser. No.406; American Chemical Society: Washington, DC. (6) Tingey, J. M.; Fulton, J. L.; Smith, R. D. J . Phys. Chem. 1990. 94,
1997. (7) Beckman, E. J.; Smith, R. D. J . Supercrir. Fluids 1990, 3, 205. (8) Lemert, R. M.; Fuller, R. A.; Johnston, K.P. J . Phys. Chem. 1990, 94, 602 1. (9) Consani, K. A.; Smith, R. D. J . Supercrit. Fluids 1990, 3, 51.
0022-3654/91/2095-7127$02.50/0
that molecular aggregates can form in C02 given the proper conditions. Thus, in a departure from previous investigations, we are synthesizing model surfactants specifically for use in C02. Our goal is to optimize the design of surfactants which preferentially partition to the C02-rich phase in water/C02 mixtures, forming reverse micelles which can then be used in extractions and polymerizations. Because the curvature of a micellar interface, and thus the phase behavior of a water/oil/amphiphile mixture, depends on a balance of specific interactions between oil, water, hydrophobe, and hydrophile,I2 the negative results obtained by Consani could be attributed to poor C02-hydrophobe interaction or overwhelmingly favorable water-hydrophile and hydrophobe-hydrophobe associations. The fact that the majority of the amphiphiles evaluated by Consani were found to exhibit minimal solubility in C02 suggests that hydrophobic tail-hydrophobic tail interactions outweigh C02-hydrophobic tail interactions in these systems.12 Although the solvent power of C02 is often compared to that exhibited by hexane, recent work suggests that ether, perfluorohexane, ethyl acetate, or even acetone might be a better model on the molecular level where specific interactions dominate.13J4 Thus, the strong contrast in the performance of commercial surfactants in C 0 2and alkanes is not entirely surprising in that (10) Randolph, T. W.; Clark, D. S.;Blanch, H. W.; Prausnitz, J. M. Science 1988, 238, 387. (1 1) Ritter, J. M.; Paulaitis, M. E. In Supercritical Nuid Science and Technology; Johnston, K.P., Penninger. J. M. L., Eds.;ACS Symp. Ser. No. 406; American Chemical Society: Washington, DC. (12) Bourrel, M.;Schcchter, R. S.Microemulsions and Relafed Sysfems; Surfactant Science Series, Marcel Dekker: New York, 1988; Vol. 30. (1 3) Walsh, J. M.; Ikonomou, G. D.; Donohue, M. D. Fluid Phase Equflib. 1987, 33, 295. (14) Hyatt, J. A. J . Org. Chem. 1984, 49, 5097.
0 1991 American Chemical Society
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7128 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
these amphiphiles are intended for use in conventional liquid alkanes and aromatics. Given this background, our program focuses on synthesis of model surfactants which possess a hydrophobic tail (or tails) containing functional groups which interact favorably (in a thermodynamic) sense with CO,. The large body of information concerning the phase behavior of C02-based mixtures15-23suggests investigation of compounds with low solubility parameters (below 6.5 ( c a l / ~ m ~ )and ~ . ~low ) polarizability parameters, as well as Lewis bases, given that carbon dioxide is a weak Lewis acid. Functional groups with these characteristics include siloxanes, perfluorinated ethers and alkanes, tertiary amines, aliphatic ethers, esters, and carbonates. Increasing the number of such groups should increase the likelihood of forming a Winsor I1 system (reverse micelles in equilibrium with excess water) in CO, by increasing the relative importance of the tail-CO, interactions. Because the direction of curvature of the micellar interface is a function of the balance of interactions between hydrophile, hydrophobe, water, and C02,the system can also be propelled toward Winsor I1 formation via reduction of the polarity of the polar head group, although this will likely decrease the solubilizing power of the microemulsion as well. In this brief communication,we report the initial results of this program, which suggests that our methodology for producing highly C 0 2 soluble surfactants is indeed valid. Experimental Section Materials. All solvents used during the synthesis of the model surfactants were HPLC grade from Aldrich Chemical and were dried by using 4A molecular sieves. Fumaryl chloride (Aldrich), fluorinated alcohols (Strem Chemical), thymol blue (Aldrich), sodium hydrogen sulfite (Aldrich), and carboxylated fluoro ether oligomers (DuPont) were used as received. Carbon dioxide (bone dry) was received from Linde Division of Union Carbide Corp. and used without further purification. Water was doubly distilled. Fluorinated AOT Analogues. Fluorinated analogues to the commonly used surfactant AOT (bis(2-ethylhexyl) sodium sulfosuccinate) have been prepared by generating a fluorinated fumaric diester, followed by sulfonation. To prepare the diester, fumaryl chloride is added dropwise to a solution of fluorinated alcohol in trichlorotrifluoroethaneat approximately 10 OC (1 :2 molar ratio) under nitrogen. Cross-linked poly(viny1pyridine) (Reilly Industries) is added to scavenge the HCl produced during the reaction. After 1-2 h, the solution is decanted from the flask and the solvent removed under vacuum. The generation of the ester is confirmed via IR spectroscopy by following the disappearance of the alcohol hydroxyl peak (3200 cm-') and the appearance of the ester peak at 1745 cm-'. The fluorinated diesters are sulfonated by reaction with aqueous sodium hydrogen sulfite (Aldrich), 10% molar excess, at 100 OC for 4-6 h. The product is isolated from the excess salt by drying, Soxhlet extraction with acetone, and subsequent removal of residual acetone under vacuum. Fluoroalkyl and Fluoro Ether Carboxylate Salts. These materials are prepared simply by neutralizationof the precursor acids using aqueous ammonium hydroxide. The product is collected via vacuum filtration, washed with distilled water, and dried. (1 5) McHugh, M. A,; Krukonis, V. J. Supercritical Fluid Extraction; Butterworths: Stoneham, MA, 1986. (16) Harris, T. V.; Irani, C. A.; Pretzer, R. US.Patent No. 4,913,235, assigned to Chevron Research Co., April 3, 1990. (17) Dandge, D. K.; Heller, J. P.; Wilson, K. V. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 162. (18) Francis, A. W. J . Phys. Chem. 1954,58, 1099. (19) Fields, S. M.; Grolimond, K. J. High Res. Chromatogr. Chromatogr. Commun. 1988,II. 727. (20) Daneshvar, M.; Gulari, E. Proc. Inf.Symp. Supercrit. Fluids 1988,
51. (21) Sikorski, M. E.; Lundberg, J. L. Paper HS-22 presented at the American Physical Society Annual Meeting, Detroit, MI, 1984. (22) Bae, J. H.; Irani, C. A. Proc. 65th Annu. Tech. ConJ SPE 1990,73. (23) Iezzi, A.; Bendale, P.; Enick, R. M.; Turberg, M.; Brady, J. Fluid Phase Equilibr. 1989, 52, 307.
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1 -CO, CYLINDER 2 -COMPRESSOR 3 -HOUSE AIR 4 -DUAL POSITIVE DISPLACEMENT PUMP 5 -OIL RESERVOIR
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Figure 1. High-pressure, variable-volume view cell and supporting apparatus. PDMS LL Cloud Point
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Figure 2. Pressure-composition diagram, illustrating the liquid-liquid dew point locus (filled points) of poly(perfluoropropy1ene oxide), M, = 13 000, and poly(dimethylsiloxane), M, = 13 650 in carbon dioxide a t 22 "C.The tops of the three phase region (unfilled points) for the silicon and fluoro ether nearly overlap.
Hydroxyaluminum Surfactants. To prepare the hydroxyaluminum soaps, ammonium carboxylate and ammonium hydroxide are added to distilled water in a 2:l molar ratio. Aluminum sulfate hydrate (Aldrich) is then added at a molar ratio to the carboxylate of 1:4 while stirring, leading to the immediate formation of the difluoroalkyl (or difluoro ether)hydroxyaluminum precipitate. The product is collected via vacuum filtration, washed with distilled water to remove excess salt, and dried. Phase Behavior. Preliminary phase behavior studies are being conducted using a variable-volume high-pressure view cell (D.B. Robinson and Associates) shown in Figure 1. The material to be examined is loaded into the top of the quartz cylinder, which is itself encased in a stainless steel, windowed cell. The cell is sealed and the appropriate amount of the fluid of interest is added via one of the Ruska syringe pumps. Following fluid addition, pressure is regulated within the quartz tube by movement of the floating piston, the position of which is adjusted by the injection or withdrawal of silicone fluid using the second Ruska syringe pump. Seals on the piston and at the top of the quartz cylinder prevent mixing of the silicone fluid and the sample. Phase behavior can be observed visually, while phase volumes can be measured with a cathetometer. Good mixing of the contents can be achieved by adding a number of stainless steel ball bearings to the sample chamber and subsequently rocking the cell while under pressure. Results and Discussion As mentioned in the Introduction, the choice of fluoroalkyl, fluoro ether, and silicone moieties for construction of our model surfactants is based upon the assumption that these materials will exhibit a strong affinity for C02. Previous work has shown that this is indeed the case for fluorinated alkanes vs conventional alkanes.23 As shown in the pressure-composition diagram in Figure 2, this is also true for the silicones and fluoro ethers.
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7129
Letters 1 1.000,
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Figure 3. (a, top) Pressurecomposition diagram, illustrating the liquid-fluid dew point locus, of bis(poly(hexafluoropropy1ene oxide))hydroxyaluminum (molecular weight of fluoro ether chains is ca. 2500) in carbon dioxide at 40 'C. (b, middle) Pressurecomposition diagram, illustrating the liquid-fluid dew point locus, of ammonium poly(hexafluoropropylene oxide) carboxylate (molecular weight of fluoro ether is ca. 2500) in carbon dioxide at 40 OC. (c, bottom) Pressurecomposition diagram of bis(dodecafluorohepty1)sodium sulfofumarate in carbon dioxide at 40 OC. Dashed line indicates three-phase pressure; data at weight fractions of 0.03 and higher are dew points, those at 0.02 and lower, bubble points.
Despite molecular weights of approximately 13000, these compounds display relatively high solubility in COz at moderate pressures. This figure shows pseudobinary representations of mixtures of carbon dioxide with oils which contain a distribution of molecular weights. Consequently, the critical points for both pseudobinary systems occur at concentrations greater than 10 wt %. Pressure-composition diagrams were also constructed for several of our model surfactants in pure C 0 2 at 40 OC (see Figure 3a-c). Unlike the systems in Figure 3a,b, which are again pseudobinaries (owing to the distribution of chain lengths in the precursor carboxylate), the data in Figure 3c show results from a true binary mixture. As in the case of the precursor materials, the model surfactants exhibit considerable solubility in supercritical carbon dioxide at moderate pressures. Not surprisingly, the more polar sulfonated material requires higher pressures to achieve a single-phase solution than either the hydroxyaluminum compound
or the carboxylate. In addition, three phase behavior was observed for the sulfonate. Finally, addition of each of these surfactants to COz allows the solubilization of the hydrophilic dye thymol blue, whereas this dye is completely insoluble in pure CO2 at pressures up to 10000 psi at 40 OC. A second series of experiments were conducted in COz/ water/thymol blue mixtures to evaluate the extractive power of the model surfactants. In each case, 0.5 g of surfactant and approximately 50 mg of thymol blue were added to 10 cm3 of distilled water in the Robinson high-pressure cell. The cell was sealed and 30 cm3 of COz added at 1000 psi. Following mixing, the contents were allowed to settle and the phase behavior o b served. The pressure was then increased by 500 psi, the contents were mixed, and the phase behavior was recorded. For the case of the fluorinated AOT analogues, a fluorinated tail chain length of seven, eight, or nine carbons produced a surfactant which apparently dissolved preferentially in the COz phase, and which extracted thymol blue from the aqueous phase above a particular threshold pressure (approximately 2500-3000 psi). The value of this threshold for solubilization did not vary significantly as tail length varies from seven to nine carbons, although increases in temperature increased the threshold pressure for extraction of the thymol blue. Although we could not measure phase compositions using our view cell, we have used the extraction of thymol blue into the COz phase, and, in a separate experiment, the lack of extraction of the COz-soluble dye Sudan Red into the aqueous phase, as an indirect confirmation of the location of the surfactant in the COz-rich,upper phase. If either the fluorinated tail length decreases (C4,for example) or the number of fluorines on the tails decreases significantly, the resulting surfactant appeared to dissolve in the aqueous phase preferentially (Sudan Red can be extracted by the aqueous phase), and given the resulting phase volumes, apparently absorbed a significant quantity of C02into the aqueous phase. These results are quite consistent with the previously described concept that the curvature of the micellar interface depends on the balance of interactions between hydrophile, hydrophobe, oil, and water. The fluoro ether hydroxyaluminum soaps, possessing both a less polar head group than the sulfonates and a tail which interacts extremely well with COz (see Figure 2), dissolve readily in the COz phase (they are insoluble in water) and extract thymol blue from the aqueous phase at the vapor pressure of COz at 23 OC. Increasing the temperature to 40 OC increases the threshold pressure to between 1200 and 1500 psi. Salts (Na, NH,,K) of the fluoro ether carboxylates also extract thymol blue from the aqueous phase at room temperature, yet preferentially dissolve in the aqueous phase at 40 OC. This is consistent with the o b servation that ionic surfactants display increased solubility in water as temperature increases.l2 Further, at constant total surfactant concentration, decreasing the C02/water ratio from 3:l to 2:l at room temperature also produces a system where little or no thymol blue is extracted into C 0 2 .
Conclusions We have found that synthesis of model surfactants where the hydrophobic tails are designed to provide a favorable thermodynamic interaction with carbon dioxide allows formation of C 0 2 solutions with high surfactant concentrations at moderate pressures. Further, COz solutions of several of the surfactants will extract thymol blue from aqueous solution. We are continuing to explore the effect of both head and tail structure on the phase behavior of C02/water/surfactant systems. Specifically, synthesis of fluoro- and fluoro ether phosphates and ammonium salts is presentlyunderway. In addition, we are generating a number of silicone functional analogues of the materials discussed in this communication. Future work will examine the distribution of surfactant between the phases, as well as the water content of the COz-rich phase. Collaboration with other research groups is planned in order to elucidate aggregate size and hydrophilic core environment to fully characterize the nature of these reverse micelles in C 0 2 .
