Phase Behavior of CO2-Expanded Fluorinated Microemulsions

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Phase Behavior of CO2-Expanded Fluorinated Microemulsions Yeh Wei Kho, Daniel C. Conrad,† and Barbara L. Knutson* Department of Chemical & Materials Engineering, University of Kentucky, Lexington, Kentucky 40506-0046 Received August 19, 2003. In Final Form: January 15, 2004 The formation of CO2-expanded, fluorinated reverse microemulsions is demonstrated for the system of perfluoropolyether (PFPE) surfactant (ClPFPE-NH4, MW ) 632) and PFPE oil (PFPE, MW ) 580). The phase behavior of this system is examined as a function of temperature (25-45 °C), pressure, CO2 concentration, and water to surfactant molar ratios (Wo ) 10 and 20). Visual observations of one-phase behavior consistent with reverse microemulsion formation are further supported by spectroscopic measurements that establish the existence of a bulk water environment within the aqueous core. Microemulsion formation is not observed in the absence of CO2 for this PFPE surfactant/PFPE oil system, and a CO2 content greater than 70 mol % is required to induce microemulsion formation. Over the range of water loadings and temperatures investigated, the lowest cloud point pressure is observed at 46 bar (5 wt % ClPFPE-NH4 in PFPE oil, Wo ) 20, xCO2 ) 0.7, T ) 25 °C). In the regions where one-phase behavior is observed, the cloud point pressures increase with temperature, water loadings, and CO2 content. The driving forces of microemulsion formation in the CO2-expanded fluorinated solvent are discussed relative to traditional reverse microemulsions and CO2-continuous microemulsions.

Introduction The recent interest in CO2-expanded solvents for chemical synthesis1,2 and separation3 is based on their pressure-tunable solvent properties, their enhanced mass transfer characteristics (relative to organic solvents), and the reduced operating pressures relative to supercritical fluid processes. The use of CO2-expanded media addresses the limited ability of pure CO2 to solvate a variety of polar species. An alternative approach to increase the solvent power of compressed CO2 is the formation of reverse microemulsions using CO2-philic surfactants.4-6 The aqueous core of these reverse microemulsions provides a polar environment for organic and enzymatic reactions,7-9 extraction,10 and nanoparticle syntheses.11-13 Similar to compressed CO2, numerous fluorinated or partially fluorinated solvents are neither hydrophilic nor * Corresponding author: [email protected]. † Whirlpool Corporation, Benton Harbor, MI. (1) Musie, G.; Wei, M.; Subramaniam, B.; Busch, D. H. Coord. Chem. Rev. 2001, 219-221, 789-820. (2) Wei, M.; Musie, G. T.; Busch, D. H.; Subramaniam, B. J. Am. Chem. Soc. 2002, 124, 2513-2517. (3) Jessop, P. G.; Olmstead, M. M.; Ablan, C. D.; Grabenauer, M.; Sheppard, D.; Eckert, C. A.; Liotta, C. L. Inorg. Chem. 2002, 41, 34633468. (4) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127-7129. (5) Harrison, K.; Goveas, J.; Johnston, K. P.; O’Rear, E. A., III Langmuir 1994, 10, 3536-3541. (6) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624-626. (7) Jacobson, G. B.; Lee, C. T. J.; Johnston, K. P. J. Org. Chem. 1999, 64, 1201-1206. (8) Jacobson, G. B.; Lee, C. T. J.; da Rocha, S. R. P.; Johnston, K. P. J. Org. Chem. 1999, 64, 1207-1210. (9) Holmes, J. D.; Steytler, D. C.; Rees, G. D.; Robinson, B. H. Langmuir 1998, 14, 6371-6376. (10) Yazdi, A. V.; Beckman, E. J. Mater. Res. 1995, 10, 530-537. (11) Ji, M.; Chen, X.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631-2632. (12) Dong, X.; Potter, D.; Erkey, C. Ind. Eng. Chem. Res. 2002, 41, 4489-4493. (13) McLeod, M. C.; McHenry, R. S.; Beckman, E. J.; Roberts, C. B. J. Phys. Chem. B 2003, 107, 2693-2700.

lipophilic. Fluorinated surfactants are used to enhance the poor solvation characteristics of fluorous solvents for water, organic solvents, and polar compounds. For example, fluorinated ligands have been incorporated in catalysts to improve solubility in fluorous media14 and employed in fluorinated amphiphiles for drug delivery.15,16 An alternative approach to overcome the feeble solvent power of fluorinated solvents is of the formation of fluorinated reverse microemulsions. One of the most studied fluorinated reverse microemulsion systems is based on perfluoropolyether (PFPE) species.17,18 Other examples of fluorinated solvents used in forming reverse microemulsions are perfluorinated and partially fluorinated solvents19,20 and hydrofluoroethers (HFEs).5,21,22 However, many fluorinated microemulsion formulations require the use of cosolvents (to improve surfactant solubility) and/or cosurfactants (to modify interfacial properties of surfactant monolayer). Heptafluoro-1-butanol19 and 1-butanol20 are among common examples of cosurfactants used to induce fluorinated solvent-continuous microemulsion formation. Other formulation variables used to drive reverse microemulsion formation include the hydrophilic-to-lipophilic balance of the surfactant (HLB), molecular volume of the oil, cosurfactant chain length, and salinity.23 (14) Barthel-Rosa, L. P.; Gladysz, J. A. Coord. Chem. Rev. 1999, 190192, 587-605. (15) Krafft, M. P.; Riess, J. G. Biochimie 1998, 80, 489-514. (16) Butz, N.; Porte´, C. H.; Krafft, M. P.; Vandamme, T. F. Int. J. Pharm. 2002, 238, 257-269. (17) Chittofrati, A.; Lenti, D.; Sanguineti, A.; Visca, M.; Gambi, C. M. C.; Senatra, D.; Zhen, Z. Colloids Surf. 1989, 41, 45-59. (18) Chittofrati, A.; Lenti, D. l. S. A.; Visca, M.; Gambi, C. M. C.; Senatra, D.; Zhou, Z. Prog. Colloid. Polym. Sci. 1989, 79, 218-225. (19) Oliveros, E.; Maurette, M.-T.; Braun, A. Helv. Chim. Acta 1983, 66, 1183-1188. (20) Ceschin, C.; Roques, J.; Malet-Martino, M. C.; Lattes, A. J. Chem. Technol. Biotechnol. 1985, 35A, 73-82. (21) Baran, J. R. J. J. Colloid Interface Sci. 2001, 234, 117-121. (22) Jariwala, C. P.; Chang, H. C.; Hill, D. S. Preparation of WaterRemoving Agent Comprising Fluorinated Solvent and a Fluoroalkyl Surfactant. Patent WO 2001027235 A1, 2001, 43 pp. (23) Binks, B. P.; Espert, A.; Fletcher, P. D. I.; Soubiran, L. Colloids Surf., A 2003, 212, 135-145.

10.1021/la035529z CCC: $27.50 © 2004 American Chemical Society Published on Web 02/26/2004

CO2-Expanded Fluorinated Microemulsions

Formation of reverse micelles in bulk CO2-expanded media combines the pressure-tunable properties of CO2expanded solvents and the favorable solvation characteristics of a reverse microemulsion. Recently, Zhang and co-workers24,25 successfully dispersed water into CO2 and ethylene-expanded organic solvent systems using a triblock copolymer (Pluronic P104; PEO27PPO61PEO27). The dissolution of the compressed fluid into the organic phase created a solvent blend which preferentially solvated the hydrophobic moiety (poly(phenylene oxide), PPO) of the copolymer.25 Zhang and colleagues suggested that the copolymer aggregated into a reverse micelle structure to minimize the unfavorable interactions between the hydrophilic moiety (poly(ethylene oxide), PEO) and the expanded solvent.25 The formation of reverse micelle aggregates25 was induced at significantly lower pressures (400 nm), were recorded and averaged.

Results and Discussion Phase Behavior of CO2-Expanded Fluorinated Reverse Microemulsion. The combined fluorophilicity and hydrophilicity of the ClPFPE-NH4 surfactant should be conducive to fluorinated-continuous microemulsion formation. The fluorinated oil (PFPE, MW ) 580 g/mol) used in this study is nearly immiscible with water (14 ppm water solubility; Ausimont product literature) and has very similar chemical structure and molecular weight to the surfactant tail of ClPFPE-NH4. Therefore, the oil/ tail interactions are assumed favorable, producing a wellsolvated surfactant tail region at the water/oil interface. The ammonium carboxylate ionic headgroup is highly hydrophilic,28 as evidenced by ClPFPE-NH4’s appreciable solubility in water (MW ) 504, 7 wt % at 25 °C).29 Water loadings as high as 27 wt %30 have been achieved in reverse microemulsions formed with PFPE-NH4 surfactants in various molecular weights of PFPE oils.17,18,31,32 However, for this system of ClPFPE-NH4 surfactant with PFPE oil (MW ) 580), the addition of a cosurfactant, such as heptafluorobutanol, is necessary to achieve fluorinated continuous microemulsion formation (data not shown). In addition, pressurizing the samples to 346 bar (without CO2 or cosurfactant addition) does not induce microemulsion formation. Many fluorinated surfactants are noted for their high CO2 solubility.33 Although significant solubility of fluorinated surfactants in CO2 in the absence of water is not required for reverse microemulsion formation,34 fluorinated surfactant-CO2 phase behavior is useful for the interpretation of reverse microemulsion phase behavior. (28) Heitz, M. P.; Carlier, C.; deGrazia, J.; Harrison, K. L.; Johnston, K. P.; Randolph, T. W.; Bright, F. V. J. Phys. Chem. B 1997, 101, 67076714. (29) Caboi, F.; Chittofrati, A.; Lazzari, P.; Monduzzi, M. Colloids Surf., A 1999, 160, 47-56. (30) Monduzzi, M.; Knackstedt, M. A.; Ninham, B. W. J. Phys. Chem. 1995, 99, 17772-17777. (31) Chittofrati, A.; Sanguineti, A.; Visca, M.; Kallay, N. Colloids Surf. 1992, 63, 219-233. (32) Chittofrati, A.; Visca, M.; Kallay, N. Colloids Surf., A 1993, 74, 251-258. (33) O’Neill, M. L.; Cao, Q.; Fang, M.; Johnston, K. P.; Wilkinson, S. P.; Smith, C. D.; Kerschner, J. L.; Jureller, S. H. Ind. Eng. Chem. Res. 1998, 37, 3067-3079. (34) Keiper, J. S.; Simhan, R.; DeSimone, J. M.; Wignall, G. D.; Melnichenko, Y. B.; Frielinghaus, H. J. Am. Chem. Soc. 2002, 124, 1834-1835.

Figure 3. Cloud point pressures of ClPFPE-NH4 in CO2 at 25 (]), 35 (0), 45 °C (4) as a function of surfactant concentration. Unfilled circles (O) indicate literature solubility data of PFPENH4 surfactant (MW 740; 0.008-4.6 wt %; 45 °C).35

The solubility of ClPFPE-NH4 in CO2 (as determined by cloud point pressures) was measured at 25, 35, and 45 °C (Figure 3). The surfactant solubility in CO2 decreases with increasing temperature. The solubility of ClPFPE-NH4 at 45 °C is slightly less than that of PFPE-NH4 surfactant35 at the same temperature, despite the higher molecular weight of PFPE-NH4 (740 g/mol vs 632 g/mol) (Figure 3). The terminal chlorine in the surfactant molecule appears to decrease the effective CO2-philicity of the surfactant tail, reducing its solubility in CO2 relative to PFPE-NH4. The PFPE surfactants, ClPFPE-NH4 and PFPE-NH4, have also been used successfully in forming CO2-continuous microemulsions.6,36,37 Subsequent investigations using spectroscopic28,38 and light scattering techniques39,40 confirm the existence of spherical water droplets with radii ranging from 16 to 36 Å dispersed in CO2. The extension of ClPFPE-NH4 surfactant to reverse microemulsion formation in CO2-expanded PFPE is examined over a range of CO2 concentrations. The significant solubility of CO2 in the PFPE oil, despite its high molecular weight, is confirmed by its volume expansion behavior (data not shown). At 25 °C, the volume of the PFPE oil/CO2 system increases exponentially with CO2 dissolution as the saturation pressure of CO2 (64.3 bar) is approached. This expansion behavior is typical for CO2-expanded organic41 and fluorinated solvents.27 All experiments were conducted at sufficient pressures to keep the CO2 dissolved in the fluorinated solvent (i.e., in the absence of an excess CO2 phase). (35) Harrison, K. L.; Johnston;, K. P.; Sanchez, I. C. Langmuir 1996, 12, 2637-2644. (36) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. Langmuir 1997, 13, 3934-3937. (37) Blattner, C.; Bittner, J.; Schmeer, G.; Kunz, W. Phys. Chem. Chem. Phys. 2002, 4, 1921-1927. (38) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 119, 6399-6406. (39) Lee, C. T. J.; Psathas, P. A.; Ziegler, K. J.; Johnston, K. P.; Dai, H. J.; Cochran, H. D.; Melnichenko, Y. B.; Wignall, G. D. J. Phys. Chem. B 2000, 104, 11094-11102. (40) Lee, C. T. J.; Johnston, K. P.; Dai, H. J.; Cochran, H. D.; Melnichenko, Y. B.; Wignall, G. D. J. Phys. Chem. B 2001, 105, 35403548. (41) Kordikowski, A.; Schenk, A. P.; Van Nielen, R. M.; Peters, C. J. J. Supercrit. Fluids 1995, 8, 205-216.

CO2-Expanded Fluorinated Microemulsions

Figure 4. Cloud point pressures of CO2-expanded fluorinated microemulsions as a function of CO2 content: (a) Wo ) 10 at 25 (]), 35 (0), and 45 °C (4); (b) Wo ) 20 at 25 (]), 35 (0), and 45 °C (4).

The presence of CO2 at low concentrations (xCO2 < 0.5) in the PFPE oil/ClPFPE-NH4/water system results in a stirred mixture with an orange tint (transmitted light). A similar tint has been observed below the CPP in AOT/ water/oil systems in the presence of supercritical solvents42 and PFPE-NH4/water systems in the presence of CO2.43 The tint is attributed to the presence of large droplet clusters. As the CO2 concentration in our study is increased beyond 50 mol %, the orange-tinted mixture becomes an opaque emulsion that phase separates in the absence of stirring. At CO2 compositions of 70 mol % or greater, increasing the pressure (at constant temperature and water loading) acts as a “switch” to induce the one-phase behavior associated with microemulsion formation. The CPPs for 5 wt % ClPFPE-NH4 surfactant (with respect to PFPE oil) in CO2 expanded PFPE oil are determined as a function of CO2 content (xCO2) at several temperatures (25, 35, 45 °C) and water to surfactant molar ratios (Wo) of 10 (Figure 4a) and 20 (Figure 4b). At Wo ) 10 (Figure 4a), CO2 compositions greater than 80 mol % are required to form microemulsions at 25 and 35 °C. However, at 45 °C, microemulsion formation (Wo ) 10) is observed at a CO2 composition as low as 70 mol %. At Wo ) 20, microemulsion formation is observed at xCO2 > 0.7 at 25 °C and at xCO2 > 0.8 at 35 and 45 °C. The pressure required to induce microemulsion phase behavior increases with temperature and water content, as observed previously for CO2-continuous microemulsions.6,36,37 For example, at 80 mol % CO2 and 25 °C, the CPPs are 49 and 52.8 bar at Wo values of 10 and 20, respectively. The pressure required to stabilize the droplets also increases with increasing amounts of dissolved CO2. The lowest CPPs observed at 25 °C (49 bar at Wo ) 10 and xCO2 ) 0.8; 46 bar at Wo ) 20 and xCO2 ) 0.7), are well below the CO2’s liquefying pressure of 64.3 bar. These (42) Tingey, J. M.; Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1990, 94, 1997-2004. (43) Lee, C. T. J.; Bhargava, P.; Johnston, K. P. J. Phys. Chem. B 2000, 104, 4448-4456.

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CPPs coincide with the bubble point pressure of the binary CO2-expanded PFPE oil at the same composition. These pressures required to induce CO2-expanded fluorinated microemulsion formation are comparable to the lowest reported CPPs for fluorosurfactants in water-in-CO2 microemulsions at similar water loadings (but slightly lower temperatures).44,45 Eastoe and co-workers demonstrated that a fluorinated, dichained sodium sulfosuccinate (di-CF4) could stabilize a water-in-CO2 microemulsion ([surf.] ) 0.05 mol dm-3; Wo ) 10; T ) 20 °C) above a cloud point pressure of ∼50 bar.44 Furthermore, a phosphate analogue of the fluorinated dichained sulfosuccinate surfactant (di-HCF6-P) was also shown to stabilize a water-in-CO2 microemulsion ([surf.] ) 0.05 mol dm-3; Wo ) 10; T ) 15 °C) above similar cloud point pressure of 50 bar.45 Similarly, the lowest pressures required to stabilize the CO2-expanded reverse microemulsions at 35 °C (76.1 bar at Wo ) 10 and 95 bar at Wo ) 20) are considerably less than some reported CPPs for PFPE-NH4 stabilized waterin-CO2 microemulsion (156 and 176 bar, Wo ) 11 and 21, respectively (2.1 wt % surfactant, T ) 35 °C).36 In contrast, the CPPs at Wo ) 20, T ) 45 °C (188-189 bar) are higher than the reported CPP (158.1 bar) for the non-chlorineterminated PFPE-NH4 water-in-CO2 microemulsion at similar water loading and temperature (Wo ) 20.7, T ) 45 °C).28 Several experimental observations in Figure 4 are initially unexpected. Specifically, at Wo ) 10, microemulsions form at the highest temperature (45 °C; xCO2 > 0.7) with less CO2 than at the lower temperatures (25 and 35 °C; xCO2 > 0.8) (Figure 4a). A possible explanation for this observation at Wo ) 10 is based on the expected increased solubility of the surfactant, ClPFPE-NH4, in PFPE oil with increasing temperature, as observed for PFPE-NH4 in PFPE oil.18 In contrast, the solubility of ClPFPE-NH4 in CO2 decreases with increasing temperature (Figure 3). Therefore, more dissolved CO2 is needed at low temperatures to create favorable solvation of the surfactant in the continuous, CO2-expanded phase. In contrast, at Wo ) 20 the microemulsions form at the lowest temperature with less CO2 (Figure 4b). At higher water loading, due to the higher solubility of ClPFPENH4 in water, the relative amount of ClPFPE-NH4 that partitions between the CO2-expanded phase and water plays an important role in driving reverse microemulsion formation. At constant mole fraction of CO2, lower temperature favors the solvation of the surfactant in the CO2-expanded phase, and thus microemulsion formation. However, the authors could not account for the CPP behavior at Wo ) 20 and 45 °C, which lacks pressure dependence and results in a higher CPP than water-inCO2 microemulsion formed at similar conditions. Further discussions of factors driving microemulsion formation are presented below. Water Core Characterization Using a Spectroscopic Probe. The nature of the water droplets dispersed in the CO2-expanded fluorinated solvent was explored using a solvatochromic probe, methyl orange (MO). MO has been used previously to probe the aqueous environment in reverse micelles46 and CO2-continuous microemulsions.6,28 The absorption spectra of MO in deionized water, a polar solvent (methanol), in CO2 saturated (44) Eastoe, J.; Downer, A.; Paul, A.; Steytler, D. C.; Rumsey, E.; Penfold, J.; Heenan, R. K. Phys. Chem. Chem. Phys. 2000, 2, 52355242. (45) Steytler, D. C.; Rumsey, E.; Thorpe, M.; Eastoe, J.; Paul, A.; Heenan, R. K. Langmuir 2001, 17, 7948-7950. (46) Qi, L.; Ma, J. J. Colloid Interface Sci. 1998, 197, 36-42.

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Table 1. Maximum Absorption Wavelengths (λmax, nm) of Methyl Orange solvent environment

λmax, nm

water methanol CO2-saturated aqueous solution (125 bar, T ) 35 °C) CO2-expanded surfactant/PFPE oil solution (no added water), xCO2 ) 0.8, T ) 35 °C, P ) 125 bar CO2-expanded PFPE microemulsion, Wo ) 10, xCO2 ) 0.80; T ) 35 °C, P ) 125 bar

464.5 nm 422.8 nm 501.3 nm; shoulder 540 nm 421 nm 457 nm; shoulder 540 nm

Figure 5. Spectra of methyl orange in CO2-expanded PFPE microemulsion at Wo ) 20 and 25 °C, as a function of CO2 addition: dark solid line, spectra at xCO2 ) 0.7; light line, spectra at xCO2 ) 0.8; (×) spectra at xCO2 ) 0.9.

aqueous solution (PCO2 ) 125 bar, T ) 35 °C), and in a CO2 (T ) 35 °C, P ) 125 bar) expanded surfactant/PFPE oil solution (no added water) were measured to provide comparison to the microemulsion system (Table 1). The maximum absorption wavelengths of water (465 nm), methanol (423 nm), and of CO2-saturated aqueous solution (501 nm, shoulder at 540 nm) agree with literature values.6,38 MO in pure methanol and ClPFPE-NH4 surfactant/PFPE oil/CO2 mixture (125 bar, 35 °C, xCO2 ) 0.8; no added water) exhibits similar maximum absorption wavelengths (423 and 421 nm, respectively). This observation suggests that, in the absence of added water, MO is solvated in a mixture environment whose polarity is similar to methanol.13,47 When water is added to the mixture (Wo ) 10; xCO2 ) 0.80; T ) 35 °C, P ) 125 bar), an absorption maximum at 457 nm (compared to pure water λmax ) 465 nm) as well as a distinct shoulder at 540 nm are observed in the CO2-expanded fluorinated microemulsion. This shoulder in the absorption spectrum, which is also observed in the CO2-saturated aqueous solution (PCO2 ) 125 bar, T ) 35 °C), has been observed previously and attributed to the presence of carbonic acid in the water cores of CO2-continuous microemulsions.38 At Wo ) 20 and 25 °C, the relative size of the shoulder increases with increasing CO2 content and the apparent maximum absorption wavelength of MO decreases from 473 nm (xCO2 ) 0.7) to 465 nm (xCO2 ) 0.9) (Figure 5). The location of the absorption maxima for the CO2-expanded microemulsion samples suggests that the dye exists in an acidified, bulk-water environment. Interpretation of Microemulsion Formation. The ability of pressure to act as a “phase switch” for CO2expanded fluorinated microemulsion formation suggests significant changes in the solvation characteristics of the ClPFPE-NH4/PFPE oil/CO2 system with moderate pressure. The effect of pressure on the phase behavior of CO2continuous microemulsions is frequently described as a (47) Roberts, C. B.; Thompson, J. B. J. Phys. Chem. B 1998, 102, 9074-9080.

Figure 6. Densities of CO2-expanded PFPE oil at 45 °C as a function of pressure. Unfilled triangles represent densities of CO2-expanded PFPE oil mixtures at their bubble point (saturation conditions) for a range of CO2 contents (xCO2 ) 0.1-0.95). The remaining symbols represent the densities at fixed CO2 mole fractions in the single-phase region.

density effect. At constant temperature, increased density (pressure) leads to more favorable surfactant tail solvation, as interpreted by the increased Hildebrand solubility parameter of bulk CO2 (δCO2).43 A simple analysis of the Hildebrand solubility parameters of the CO2-expanded system using ideal mixing rules suggests that increasing pressure at a constant CO2 concentration and temperature should also favor solvation of the surfactant tails by increasing δ of the CO2-expanded PFPE oil. The difference between the solubility parameters of ClPFPE-NH4 surfactant tail (δtail )13.5 MPa0.5)48 and CO2-expanded PFPE oil mixture (for example, δCO2/PFPEoil ) 12.3 MPa0.5 at xCO2 ) 0.8 and 45 °C) would thereby be reduced. However, this interpretation of the pressure effect on microemulsion formation in CO2-expanded PFPE oil implies large increases in solvent density with pressure, since δ is proportional to solvent density.49 In contrast, the measured densities of CO2-expanded PFPE oil are a weak function of pressure over the temperature (25-45 °C) and CO2 concentrations investigated (as shown in Figure 6 at 45 °C). The maximum change in the densities of CO2-expanded PFPE oil is observed at high concentrations of CO2 (xCO2 ) 0.95) and was less than 17, 23, and 31% at 25, 35, and 45 °C, respectively, over a pressure range of 59-277 bar. Unlike CO2-continuous microemulsion formation, changes in bulk solvent strength do not adequately describe the effect of pressure on the formation of CO2-expanded fluorinated microemulsion (at a given temperature, water loading, and CO2 content). Water-in-oil microemulsion formation requires a balance between inducing favorable (negative) curvature at (48) da Rocha, S. R. P.; Johnston, K. P. Langmuir 2000, 16, 36903695. (49) Johnston, K. P. Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J. M. L., Eds.; American Chemical Society: Washington, DC, 1989, pp 1-13.

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the oil/surfactant/water interface and minimizing the attractive interdroplet interactions, which leads to flocculation and phase separation.50 Factors potentially influencing this balance in CO2-expanded fluorinated solvents include surfactant tail solvation by CO2-expanded PFPE oil, pH of the water droplets in the microemulsion, CO2 penetration at the interface, and attractive droplet interactions across the bulk CO2-expanded media. The geometric balance of forces is described by the surfactant parameter (v/al), where v is the volume of the surfactant tail, l is the length of the surfactant tail, and a is the area of the surfactant headgroup.51 As a rule of thumb, a v/al > 1 is required for the negative interfacial curvature and formation of a reverse microemulsion. An analysis of the surfactant parameter (v/al) indicates the potential role of CO2 penetration in the surfactant tail to induce water-in-CO2 microemulsion formation using ClPFPE-NH4. The volume of the ClPFPE-NH4 tail (MW ) 632 g/mol), v, was estimated from Bondi volume52 to be 174 cm3/mol. The length of the surfactant tail, l, is ∼9 Å (assuming each PFPE segment length is ∼3 Å).48 A typical value for the area occupied by the surfactant carboxylic headgroup,53 60 Å2, was used in the calculation of the surfactant parameter for ClPFPE-NH4 in pure PFPE oil and water. However, recent surface tension measurements of Cl-terminated PFPE carboxylic acids (MW 504) with NH4+, K+, and Na+ counterions revealed that the headgroup area of the different surfactant salts may be slightly larger (71-77 Å2).29 The resulting surfactant parameter (0.53) suggests that a reverse microemulsion is not preferred (v/al < 1) in the pure PFPE solvent/water system at atmospheric conditions. A similar calculation at the water/ClPFPE-NH4/CO2 interface is based on the area occupied by PFPE-NH4 surfactant at the water/CO2 (MW 2500; 76-107 Å2, at 25-65 °C; FCO2 ) 0.842 g/cm3),48 which is larger than that of fluorinated amphiphiles at the oil-water interface. Similarly, Zielinski and co-workers had observed a headgroup area of ∼120 Å2 for the same surfactant (MW 695) at 35 °C.36 In contrast, significantly smaller surfactant headgroup areas (35-50 Å2) have been reported by Baglioni and co-workers in their small-angle neutron scattering study of microemulsions stabilized by PFPENH4 (700 MW) in PFPE oil (900 MW).54 Using the previously estimated tail volume for 632 MW ClPFPENH4 surfactant (174 cm3/mole), surfactant tail length (9 Å), and assuming a surfactant head area of 76 Å2, the surfactant parameter (v/al) was less than 1 (0.42), contrary to experimental observations of successful formation of water-in-CO2 microemulsions using ClPFPE-NH4.13,37 However, small molecules tend to penetrate further into the tail region of the surfactants in water-in-oil microemulsions.50 Significant compressed solvent penetration into the surfactant tail region has been established in reverse microemulsion systems containing compressible solvents such as propane55 and CO2.56 Molecular dynamics simulations of a PFPE-NH4 surfactant monolayer at the high-pressure CO2/water interface indicated that the (50) Hou, M.-J.; Shah, D. O. Langmuir 1987, 3, 1086-1096. (51) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525-1568. (52) Bondi, A. J. Phys. Chem. 1964, 68, 441-451. (53) Caboi, F.; Chittofrati, A.; Monduzzi, M.; Moriconi, C. Langmuir 1996, 12, 6022-6027. (54) Baglioni, P.; Gambi, C. M. C.; Giordano, R.; Senatra, D. J. Mol. Struct. 1996, 383, 165-169. (55) Peck, D. G.; Johnston, K. P. J. Phys. Chem. 1993, 97, 56615667. (56) da Rocha, S. P. R.; Johnston, K. P.; Rossky, P. J. J. Phys. Chem. B 2002, 106, 13250-13261.

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entire PFPE tail is well solvated by CO2, even up to the carbon atom next to the carboxylate headgroup.56 Further, recent molecular simulation revealed specific favorable interactions between CO2 and water at the interface.57 In the case of reverse microemulsion formation in CO2expanded PFPE oil, the increased tail volume due to strong CO2 penetration of the surfactant tails and favorable interactions between CO2/water binary system at the interface may favor a negative interfacial curvature (v/al > 1). This may explain the ability of CO2 to promote fluorinated continuous microemulsion formation. The importance of pH and electrolytes on CO2-continuous microemulsion formation has been previously demonstrated. The pH of the water core of PFPE-NH4 stabilized water-in-CO2 reverse microemulsions is reduced due to the presence of carbonic acid but remains essentially constant (between 3.1 and 3.5) over a broad range of pressures (108-193 bar, T ) 35 °C) and water loadings (Wo ) 10-20).58 A dramatic decrease in the pressure required to stabilize microemulsion formation is observed with increasing amount of buffering agent and increased pH.26 This was attributed to the increased efficiency of packing of the polar surfactant headgroups at the water/ CO2 interface.26 In addition, increased headgroup dissociation could further lower the pH of the insufficiently buffered (