Phase Behavior and Microemulsion Formation in Compressible

University of Delaware, Newark, Delaware 19716, and DuPont Central Research ... Development, Experimental Station, P.O. Box 80356, Wilmington, Delawar...
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Langmuir 1999, 15, 4480-4485

Phase Behavior and Microemulsion Formation in Compressible Perfluorinated Monomer Oil and Water Mixtures Anka N. Dobreva-Veleva,† Eric W. Kaler,*,† Kai-Volker Schubert,‡ Andrew E. Feiring,‡ and William B. Farnham‡ Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, and DuPont Central Research and Development, Experimental Station, P.O. Box 80356, Wilmington, Delaware 19880-0356 Received November 10, 1998. In Final Form: April 9, 1999 The phase behavior of mixtures of water, hexafluoropropylene (HFP), ammonium perfluorooctanoate (C8), fluorinated alcohol, and ammonium chloride is reported as a function of temperature, pressure, electrolyte concentration, and hydrophobicity of the surfactant blend. The addition of a short-chain alcohol, hexafluoro-2-methyl-2-propanol, to the water-HFP-C8 mixture promotes formation of microemulsion phases. Replacing the weak amphiphile with a medium-chain alcohol, for example, 2-perfluorobutyl-2propanol or 2-perfluorohexyl-2-propanol, produces large liquid crystalline regions. Microemulsion formulations containing the fluorinated olefin in a near-critical state along with water, fluorinated surfactant, fluorinated alcohol, and salt follow the generic patterns of phase behavior common for conventional liquid mixtures as a function of experimental variables. Pressure has a strong effect on the phase behavior when one of the components is compressible.

Introduction Microemulsion polymerization allows the preparation of stable, monodisperse microlatices containing particles smaller than those produced with classical emulsion polymerization and provides a potential route for production of polymers with novel physical properties. Successful development of a microemulsion polymerization process requires an understanding of the complex phase behavior of mixtures containing the target monomer. Polymerization of or in organized media at atmospheric pressure has been studied extensively. The work done in this field is well reviewed by Candau1 and by Antonietti et al.2 A summary of the most recent results in polymerized microemulsion investigations is given by Desai et al.3 and by Eastoe and Warne.4 Microemulsions made at elevated pressures and containing supercritical fluids could offer an interesting media in which to conduct polymerization. Supercritical fluids, that is, materials above their critical temperature (Tc) and critical pressure (Pc), possess intriguing physical properties that can be readily varied over a wide range by adjusting pressure. There is little known about polymerizable microemulsion formulations containing components in a near-critical or supercritical state.5-10 Most reports describe the potential of supercritical fluids as a solvent in polymerization processes, with only a few9,10 considering microemulsion polymerization of monomers in a near-critical or supercritical state. It has been demonstrated previously11,12 that, by proper choice of surfactant, temperature, * To whom correspondence should be addressed. E-mail: kaler@ che.udel.edu. Fax: (302) 831-4466. Phone: (302) 831-3553. † University of Delaware. ‡ DuPont Central Research and Development. (1) Candau, F. In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach Science Publishers: Philadelphia, PA, 1992; p 215. (2) Antonietti, M.; Basten, R.; Lohmann, S. Macromol. Chem. Phys. 1995, 196, 441. (3) Desai, S. D.; Gordon, R. D.; Gronda, A. M.; Cussler, E. L. Curr. Opin. Colloid Interface Sci. 1996, 1, 519. (4) Eastoe, J.; Warne, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 800.

and pressure, reverse micelles can be formed in supercritical fluids, and as in other supercritical fluid systems, density (and so pressure) plays a major role in setting phase behavior. However, the focus has been primarily on the behavior of microemulsions formed in supercritical fluids in which the water-oil ratio is relatively low, resulting in a single-phase system consisting of waterswollen reverse micelles dispersed in the fluid,13-28 with (5) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945. (6) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Science 1994, 265, 356. (7) Adamsky, F. A.; Beckman, E. J. Macromolecules 1994, 27, 312. (8) Beckman, E. J.; Smith, R. D. J. Phys. Chem. 1990, 94, 345. (9) Wu, H. S.; Hegenbarth, J.; Xin-Kang, C.; Jian-Guo, C. PCT Int. Appl. WO 9622313, 1996; Chem. Abstr. 1996, 125, 196730. (10) Giannetti, E.; Visca, M. Eur. Pat. Appl. EP 250767, 1988; Chem. Abstr. 1988, 108, 222277. (11) Gale, R. W.; Fulton, J. L.; Smith, R. D. J. Am. Chem. Soc. 1987, 109, 920. (12) Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1988, 92, 2903. (13) Fulton J. L.; Blitz, J. P.; Tingey, J. M.; Smith, R. D. J. Phys. Chem. 1989, 93, 4198. (14) Smith, R. D.; Fulton, J. L.; Blitz, J. P.; Tingey, J. M. J. Phys. Chem. 1990, 94, 781. (15) Tingey, J. M.; Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1990, 94, 1997. (16) Kaler, E. W.; Billman, J. F.; Smith, R. D. J. Phys. Chem. 1991, 95, 458. (17) Tingey, J. M.; Fulton, J. L.; Matson, D. W.; Smith, R. D. J. Phys. Chem. 1991, 95, 1445. (18) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127. (19) Zhang, J.; White, G. L.; Fulton, G. L. J. Phys. Chem. 1995, 99, 5540. (20) Jackson, K.; Fulton, J. L. Langmuir 1996, 12, 5289. (21) Lemert, R. M.; Fuller, R. A.; Johnston, K. P. J. Phys. Chem. 1990, 94, 6021. (22) Peck, D. J.; Schechter, R. S.; Johnston, K. P. J. Phys. Chem. 1991, 95, 9541. (23) Peck, D. G.; Johnston, K. P. J. Phys. Chem. 1993, 97, 5661. (24) McFann, G. J.; Johnston, K. P.; Howdle, S. M. AIChE J. 1994, 40, 543. (25) Harrison, K.; Goveas, G.; Johnston, K. P.; O’rear, E. A., III. Langmuir 1994, 10, 3536. (26) Eastoe, J.; Robinson, B. H.; Steytler, D. C. J. Chem. Soc., Faraday Trans. 1990, 86, 511. Eastoe, J.; Young, W. K.; Robinson, B. H.; Steytler, D. C. J. Chem. Soc., Faraday Trans. 1990, 86, 2883.

10.1021/la9815805 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/05/1999

Compressible Perfluorinated Monomer Oil and Water Mixtures

fewer examinations of the phase behavior of mixtures containing equal amounts of water and compressible oil.29-31 Here we explore the phase behavior at elevated pressures of fluorinated materials and show that homogeneous microemulsions can form in mixtures of water-hexafluoropropylene (HFP)-ammonium perfluorooctanoate (C8)fluorinated alcohol. Further we explore the effect of the amphiphilic strength of the fluorinated alcohol on the phase behavior by changing the chain length of the hydrophobic part of the amphiphile as well as the influence of electrolyte. View cell observations were performed to examine the influence of temperature, pressure, and composition on phase behavior and to identify conditions (concentrations and ranges of temperature and pressure) where single microemulsion phases form. Experimental Section Materials. Hexafluoropropylene (HFP) (Tc ) 85 °C, Pc ) 32.5 bar) (>99.8%) was a DuPont product. Ammonium chloride (>99%) was purchased from Fischer Scientific. Ammonium perfluorooctanoate (C8) (>97%) was obtained from the 3M Company. Hexafluoro-2-methyl-2-propanol (>97%) was prepared as described in ref 32 from reaction of hexafluoroacetone with methyllithium. 2-Perfluorobutyl-2-propanol33 (>97%) and 2-perfluorohexyl-2-propanol34 (>97%) were prepared from perfluorobutyl iodide and perfluorohexyl iodide, respectively, by reaction with methyllithium and acetone.35 All materials were used as received, without further purification. Water was twice distilled and deionized so the electrical conductivity was 1.8 × 107 Ω‚cm. Procedure. Phase behavior observations at elevated pressures were performed using a variable-volume sapphire tube equilibrium view cell. The cell design and experimental technique are described in detail elsewhere.36 The view cell was a sapphire tube (Insaco, Quakertown, PA) 17.78 cm long with an i.d. of 1.113 ( 0.001 cm and an o.d. of 2.083 ( 0.001 cm. It was sealed internally on each end with Viton O-rings and Polymite backing rings mounted on stainless steel backing nuts and was held in a C-clamp that also provided support for the sapphire tube. An important feature of the cell design was the movable piston, which allowed the internal volume of the equilibrium chamber to be varied. This variable-volume design allowed pressure to be set independently of the sample composition and temperature. Pressure in the cell was generated using a water-filled syringe pump (High Pressure Equipment, Erie, PA, No 62-6-10) that moved the piston and compressed the sample. Sample pressure was determined indirectly by measuring the pressure of the pressurizing fluid to within (0.5 bar with a Bourdon tube gauge (Dresser Industries, Newton Square, CT). The pressure drop across the piston was determined with two pressure gauges to be less than 1-2 bar at 300 bar. The entire view cell assembly was immersed in a highprecision, custom-designed water bath (Hart Scientific, Pleasant Grove, UT). The custom-designed temperature bath has a window for phase behavior observations. Mixing of the sample was accomplished with a Teflon-coated magnetic stir bar that was coupled with a ring magnet. The magnet was connected by a mechanical arm to a gear motor (McMaster-Carr, New Brunswick, NJ). A sample was prepared by loading a known amount of surfactant and alcohol into the cell and evacuating the air space above the sample. The HFP was added under its own pressure (27) Bartscherer, K. A.; Minier, M.; Renon, H. Fluid Phase Equilib. 1995, 107, 93. (28) Steytler, D. C. Curr. Opin. Colloid Interface Sci. 1996, 1, 236. (29) McFann, G. J.; Johnston, K. P. J. Phys. Chem. 1991, 95, 4889. (30) McFann, G. J.; Johnston, K. P. Lamgmuir 1993, 9, 2942. (31) Beckman, E. J.; Smith, R. D. J. Phys. Chem. 1991, 95, 3253. (32) Krespan, C. G. J. Org. Chem. 1969, 34, 1278. (33) Van der Puy, M.; Poss, A. J.; Persichini, P. J.; Ellis, L. J. Fluorine Chem. 1994, 67, 215. (34) Santini, G.; LeBlanc, M.; Reiss, J. G. J. Chem. Soc., Chem. Commun. 1975, 16, 678. (35) Gassman, P. G.; O’Reilly, N. J. Tetrahedron Lett. 1985, 26, 5243. (36) Zielinski, R. Ph.D. Dissertation, University of Delaware, 1997.

Langmuir, Vol. 15, No. 13, 1999 4481 from a 75 cm3 high-pressure cylinder (Whitey, distributed by Wilmington Valve and Fitting, Wilmington, DE). The amount of HFP loaded into the cell was determined by weighting the cylinder before and after loading. The desired composition was adjusted by injecting water (or brine) with a calibrated hand-operated syringe pump (High Pressure Equipment, Erie, PA, No 37-6-30). Keeping the water (or brine) in the syringe pump at a constant pressure above the view cell pressure allowed the amount of the injected liquid to be determined from the vernier scale on the screw on the pump. The average experimental uncertainties in loading were (0.15%. Phase boundaries at constant temperature were determined by observing the pressure where the sample became turbid. This was essentially accomplished by adjusting the amount of the pressurizing fluid in the cell. Care was taken to allow the sample to equilibrate thermally at its new pressure before a final observation for that pressure was taken. The phase boundary was bracketed from above and from below by determining the onset of cloudiness and the limit of the one-phase region. Phase boundaries at constant temperature and composition were determined to within 2 bar, while the phase boundaries of the temperature-composition sections through the phase prism were determined within 0.5 °C. In the course of the study, liquid crystals were observed. Because the sapphire tube is intrinsically birefringent, additional experiments were performed in a heavy-wall Pyrex view cell. Anisotropy of the liquid crystalline phases was observed by viewing the sample held between polarizing filters. Phase Diagram Determination. Kahlweit and co-workers37,38 have developed a formalism for studying phase behavior in multicomponent systems. Using a mixture of water (A), oil (B), surfactant 1 (C), surfactant 2 (D), and electrolyte (E), the phase space is defined by six independent variablesstemperature, pressure, and four mass fractions. The most convenient variables for our use are the temperature T, the pressure P, the mass ratio of oil to water R ) (B/(A + B)) × 100, the mass fraction of surfactant 1 in the mixture, γ ) (C/(A + B + C + D + E)) × 100, the mass ratio of the surfactants in the mixture, δ ) (D/(C + D)) × 100, and the mass ratio of the salt in the brine,  ) (E/(A + E)) × 100. We consider the mixture containing water-oil-surfactant 1-surfactant 2-electrolyte as a pseudoternary by making the assumption that the water-electrolyte mixture and the surfactant 1-surfactant 2 mixture act as pseudocomponents. Sections of equal masses of oil and water (R ) 50) as a function of temperature and surfactant concentration typically show a onephase microemulsion region at relatively high surfactant concentrations (a so-called “fish tail”) in contact with a three-phase body at lower surfactant concentrations (“fish body”). These sections through the phase prism are useful for determining the least amount of surfactant necessary to completely solubilize equal masses of oil and water (or the efficiency denoted by γ˜ ) and the extent and the average temperature T ˜ of the three-phase body for a given oil. The three-phase body, consisting of a middlephase microemulsion in equilibrium with an excess oil and water phase, is surrounded by two-phase regions. At low temperatures, mixtures containing ionic surfactants form a surfactant-rich oil phase that is in equilibrium with an excess water phase. Because oil is usually lighter than water, this case is denoted as 2 h , where the bar indicates the phase (the upper oil-rich one) in which the amphiphile is mainly dissolved. At high temperatures the twophase region consists of a surfactant-rich water phase that is in equilibrium with an excess oil phase (denoted by 2). For fluorocarbons, however, the oil density is greater than the density of water. Thus, the abbreviation 2h for a surfactant-rich oil phase in equilibrium with an excess water phase does not correspond to the situation in the view cell, as in fact the larger phase containing the surfactant is on the bottom and the small excess water phase is on the top. Nonetheless, we use the traditional notation and translate the experimental results.

Results Phase Behavior of Water (A)-Hexafluoropropylene (B)-Ammonium Perfluorooctanoate (C). Ter(37) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654. (38) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881.

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Figure 1. (a) Vertical sections through the pseudoternaryphase prism for water-hexafluoropropylene-ammonium perfluorooctanoate-hexafluoro-2-methyl-2-propanol mixtures as a function of temperature and surfactant concentration at constant elevated pressure at equal mass fractions of water and hexafluoropropylene (R ) 50) for δ ) 25. (b) Temperaturepressure projection of the phase space at constant composition for R ) 50, δ ) 25, and γ ) 28 (upper curve), γ ) 27 (middle curve), and γ ) 24 (lower curve).

nary mixtures of water-HFP-C8 alone produce stable emulsions throughout the experimental temperature and pressure range (1-80 °C and 1-280 bar), so an additive must be used to find microemulsion phase behavior in a practical temperature range. Throughout the experimental temperature window the surfactant is mainly dissolved in water. Thus, we have added a hydrophobic amphiphile, a fluorinated alcohol, to the water-HFP-C8 mixture in order to tune the phase behavior by increasing the solubility of the surfactant in the oil. Effect of Short-Chain Alcohol (r ) 50, δ ) 25). Figure 1a shows the temperature-composition phase diagram for mixtures of water-HFP-C8-hexafluoro-2-methyl-2propanol at constant elevated pressures and equal mass fractions of water and HFP (R ) 50) for δ ) 25. At low temperatures and high surfactant concentrations a single homogeneous microemulsion phase forms (denoted by 1). As temperature increases, the surfactant partitions mainly in the water and the homogeneous mixture separates into a surfactant-rich water phase in equilibrium with an excess oil phase (denoted by 2). Note again that the abbreviation 2 has the opposite of its usual meaning because of the high density of the oil. At lower surfactant concentrations the system is in two-phase equilibrium. The 1 f 2 phase boundary moves to higher temperatures as pressure increases (Figure 1a) while the region of existence of the single microemulsion phase widens and the 2 coexistence region shrinks. Figure 1b shows the temperature-pressure projections through the phase space at constant composition (i.e., R ) 50, δ ) 25, and γ ) 28 (upper curve), γ ) 27 (middle curve), and γ ) 24 (lower curve)). The temperature of the 1 f 2 phase transition rises with pressure. An increase

Figure 2. Vertical sections through the pseudoternary-phase prism for water-hexafluoropropylene-ammonium perfluorooctanoate-hexafluoro-2-methyl-2-propanol-ammonium chloride mixtures as a function of temperature and surfactant concentration at equal mass fractions of water and hexafluoropropylene (R ) 50) for δ ) 25 and  ) 2 at 50 bar (top), 100 bar (center), and 150 bar (bottom). The insets give the position of the three-phase body (3). Note that the temperature scale is two times larger than that in Figure 1a. The abbreviations 2 h and 2 in this figure and throughout the whole paper have the opposite of their usual meanings because of the high density of the fluorinated oil.

in pressure at constant temperature causes solubilization of the excess compressible oil phase into a microemulsion. Effect of Electrolyte (E ) 2, r ) 50, δ ) 25). Figure 2 shows temperature-composition vertical sections through the phase space for mixtures of water-HFPC8-hexafluoro-2-methyl-2- propanol-NH4Cl at constant elevated pressures P ) 50 bar (top), P ) 100 bar (center), and P ) 150 bar (bottom) for R ) 50, δ ) 25, and  ) 2. At high surfactant concentrations and low temperatures, the mixture forms a water-in-oil microemulsion in equilibrium with an excess water phase (2h ). At the same concentrations but at moderate temperatures, a single homogeneous microemulsion phase forms. Finally, at high temperatures, two phases are again present but the phases are now a surfactant-rich aqueous phase in equilibrium with a small excess oil phase (2). At lower surfactant concentrations, a three-phase body appears consisting of a middle-phase microemulsion in equilibrium with excess

Compressible Perfluorinated Monomer Oil and Water Mixtures

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Figure 3. Temperature-pressure projections of the phase space at constant composition for mixtures of water-hexafluoropropylene-ammonium perfluorooctanoate-hexafluoro-2methyl-2-propanol-ammonium chloride at equal mass fractions of water and hexafluoropropylene (R ) 50) with δ ) 25 and  ) 2: (a) γ ) 27.7; (b) γ ) 25.1; (c) γ ) 19.3; (d) γ ) 14.9.

oil and water phases (3) (see the insets in Figure 2). For both  ) 0 and  ) 2, clear and nonopalescent microemulsions are produced, as seen in hydrogenated mixtures. The “fish tails” in Figure 2 are not symmetrical; symmetrical tails can be expected only when all the amphiphilic compounds have similar solubilities in both water and oil. Similar “tilted” fish have been seen on vertical sections through the phase space of other multicomponent systems at R ) 50.30,39 This behavior does not affect the qualitative features of the phase patterns. Comparing Figures 1a and 2 shows that adding ammonium chloride moves microemulsion phase behavior up dramatically on the temperature scale and widens the homogeneous microemulsion region. A three-phase body can now be observed in the experimental temperature window. Figure 2 clearly shows the effect of pressure on the temperature-driven phase transitions as well as on the extent and the position of the three-phase body. As pressure increases, the upper boundary of the “fish tail” corresponding to the 1 f 2 phase transition moves up the temperature scale while the lower phase boundary of the “fish tail” corresponding to the 2h f 1 phase transition moves down the temperature scale. The net effect is widening of the region of existence of the single microemulsion phase. The upper boundary of the “fish tail” moves on the temperature scale much faster than the lower boundary (Figure 2). Since the upper boundary is the one associated with the excess compressible oil phase, its movement with pressure is understandable. Pressure also influences the size and the position of the three-phase body (see the insets in Figure 2), and the three-phase body shrinks as pressure increases. At the highest pressure studied, the three-phase region disappears. The efficiency of the surfactant mixture γ˜ increases slightly and the temperature T ˜ decreases linearly with increasing pressure (i.e., for P ) 20 bar γ˜ ) 16.7 and T ˜ ) 12.6 °C whereas for P ) 120 bar γ˜ ) 15.1 and T ˜ ) 6.8 °C). Figure 3 shows temperature-pressure projections at constant composition for some of the mixtures studied. (39) Schubert, K.-V.; Kaler, E. W. Colloids Surf., A 1994, 84, 97.

Figure 4. Vertical sections through the pseudoternary-phase prism for water-hexafluoropropylene-ammonium perfluorooctanoate-2-perfluorobutyl-2-propanol-ammonium chloride mixtures (solid lines) and for water-hexafluoropropyleneammonium perfluorooctanoate-2-perfluorohexyl-2-propanolammonium chloride mixtures (dotted lines) as a function of temperature and surfactant concentration at equal mass fractions of water and hexafluoropropylene (R ) 50) for δ ) 25 and  ) 2 at 100 bar (top), 150 bar (center), and 200 bar (bottom). Note that the temperature scale is different from that in Figures 1a and 2.

The temperature of the 1 f 2 phase transition rises while the temperature of the 2 h f 1 phase transition decreases with pressure. T-P dependencies for the 1 f 2 phase transitions are steeper than those corresponding to the 2 h f 1 phase boundary. However, for both types of transitions, increasing pressure causes solubilization of the excess phases. An increase in pressure results in shrinking of the three-phase body and widening of the one-phase region. At constant temperature the three-phase mixture becomes a single phase as pressure is applied and passes through either the 2 or the 2 h region. Effect of Medium-Chain Alcohol. (A) 2-Perfluorobutyl-2-propanol. To study the influence of the hydrophobicity of the fluorinated alcohol on the microemulsion phase behavior, an amphiphile with a longer hydrophobic chain, C4F9C(CH3)2OH, was introduced into the mixture. Note that this alcohol has a longer chain than hexafluoro-2methyl-2-propanol but also that a CH3 group is substituted for a CF3 group at the tertiary carbon. Figure 4 shows the

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Figure 5. Temperature-pressure projections of the phase space at constant composition for mixtures of water-hexafluoropropylene-ammonium perfluorooctanoate-2-perfluorobutyl2-propanol-ammonium chloride at equal mass fractions of water and hexafluoropropylene (R ) 50) with δ ) 25 and  ) 2: (a) γ ) 17.7; (b) γ ) 16.3; (c) γ ) 12.4; (d) γ ) 10.0.

temperature-composition phase diagram for mixtures of water-HFP-C8-2-perfluorobutyl-2-propanol-NH4Cl at constant elevated pressures P ) 100 bar (top), P ) 150 bar (center), and P ) 200 bar (bottom) for R ) 50, δ ) 25, and  ) 2 (solid lines). At very low temperatures and relatively high surfactant concentrations, a water-in-oil microemulsion exists in equilibrium with an excess water phase (2 h ). As temperature increases, a single microemulsion phase forms. At moderate temperatures and the same surfactant concentration, there appears a coexistence region of liquid crystalline and microemulsion phases (marked as LR; the region of existence of the embedded pure liquid crystalline phase was not determined). As temperature increases again, the microemulsion phase exists alone (1). Finally, the homogeneous mixture separates into an oil-in-water microemulsion in equilibrium with an excess oil phase (2). The microemulsion phases produced in water-HFPC8-2-perfluorobutyl-2-propanol-NH4Cl mixtures are bluish and highly opalescent, and the longer chain alcohol 2-perfluorobutyl-2-propanol promotes the formation of a large liquid crystalline region. The existence of liquid crystalline phases is a signature of the action of a strong amphiphile. The more hydrophobic alcohol moves the microemulsion phase behavior up on the temperature scale, makes the surfactant mixture more efficient, and moves the three-phase body outside the experimental temperature and pressure window (compare Figures 2 and 4). The effect of pressure on the temperature-driven phase transitions is less pronounced than that observed in water-HFP-C8-hexafluoro-2-methyl-2-propanolNH4Cl mixtures. The temperatures of the LR f 1 and the 1 f 2 phase transitions increase with pressure while the phase boundaries corresponding to the 2 h f 1 and the 1 f LR phase transitions move down in temperature. As a net result the extent of the LR coexistence region increases, the 2 and the 2 h regions shrink, and the extent of the single microemulsion phase remains the same. Figure 5 shows temperature-pressure projections at constant composition for some water-HFP-C8-2-perfluorobutyl-2-propanol-NH4Cl mixtures. For the LR f 1and the 1 f 2 phase transitions, the slope is positive while the 2h f 1

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Figure 6. Temperature-pressure projections of the phase space at constant composition for mixtures of water-hexafluoropropylene-ammonium perfluorooctanoate-2-perfluorohexyl2-propanol-ammonium chloride at equal mass fractions of water and hexafluoropropylene (R ) 50) with δ ) 25 and  ) 2: (a) γ ) 19.9; (b) γ ) 17.3; (c) γ ) 15.1; (d) γ ) 11.9.

and the 1 f LR phase boundaries are insensitive to pressure. Increasing pressure at constant temperature causes solubilization of the excess phases, and microemulsions form. Increasing pressure further may result in liquid crystalline formation. (B) 2-Perfluorohexyl-2-propanol. Finally, the effect of the most hydrophobic alcohol, C6F13C(CH3)2OH, on microemulsion phase behavior was studied. Figure 4 shows the temperature-composition phase diagram for mixtures of water-HFP-C8-2-perfluorohexyl-2-propanol-NH4Cl at constant elevated pressures P ) 100 bar (top), P ) 150 bar (center), and P ) 200 bar (bottom) for R ) 50, δ ) 25, and  ) 2 (dotted lines). The same phase sequence is observed as temperature increases as that for waterHFP-C8-2-perfluorobutyl-2-propanol-NH4Cl mixtures, that is, 2 h f 1 f LR f 1 f 2. Microemulsion phases formed in this quinary mixture are not opalescent. There is no difference in the efficiencies of the C8/2-perfluorobutyl2-propanol and C8/2-perfluorohexyl-2-propanol surfactant mixtures, although the temperatures of the LR f 1 and the 1 f 2 phase transitions move slightly downward for the longer chain alcohol (dotted lines). Figure 6 shows the temperature-pressure projections at constant composition for mixtures of water-HFP-C8-2-perfluorohexyl2-propanol-NH4Cl. The T-P dependencies given in Figure 6 are similar to those for mixtures containing 2-perfluorobutyl-2-propanol and show that the 2 h (or 2) f 1 and the 1 f LR phase transitions occur as pressure increases at constant temperature and composition. Discussion According to the thermodynamic model developed by Kahlweit and co-workers,37,38 the complex phase behavior of multicomponent mixtures is understood to arise through the interplay of features of the corresponding binary systems: (1) the location of the miscibility gap and its critical point (cpδ at Tδ) in the binary water-surfactant mixture; (2) the location of the critical point (cpR at TR) in the binary oil-surfactant mixture; (3) the location of the critical point in the water-oil mixture. The mutual solubility of water and HFP is very low, so the upper critical

Compressible Perfluorinated Monomer Oil and Water Mixtures

solution temperature (UCST) of the water-HFP miscibility gap is located well above the experimental window and plays no role in the further discussion. Water and ammonium perfluorooctanoate are completely miscible throughout the experimental temperature range (0-80 °C) at low surfactant concentrations, and liquid crystals form at higher surfactant concentrations.40 Thus the UCST, Tδ, of the water-ammonium perfluorooctanoate miscibility gap is located below 0 °C, as is typical for mixtures containing ionic surfactants. Ammonium perfluorooctanoate is nearly insoluble in HFP, even at elevated temperatures. Consequently, TR is located above the experimental window. Ternary mixtures of water-HFP-C8 form stable emulsions, so an additive is required to tune the phase behavior by moving the critical points of the watersurfactant and/or the oil-surfactant miscibility gaps along the temperature scale. The UCST of the HFP-C8 miscibility gap was lowered by adding a hydrophobic amphiphile, hexafluoro-2-methyl-2-propanol, to the waterHFP-C8 mixture. Thus, microemulsion phase behavior was brought in the experimental temperature window, as seen from Figure 1a. Further, adding electrolyte decreases the solubility of the amphiphile in water and thus moves the UCST of the water-surfactant binary up on the temperature scale. This tuning of the water-C8 and oil-C8 miscibility gaps brings the three-phase body in the experimental temperature window, as shown in Figure 2. The longer the hydrophobic chain of the alcohol, the stronger its influence on the UCST of the oilsurfactant mixture and on the phase behavior. 2-Perfluorobutyl-2-propanol and 2-perfluorohexyl-2-propanol move the 1 f 2 phase boundary up on the temperature scale and reduce the minimum amount of ammonium perfluorooctanoate required to completely homogenize equal amounts of oil and water. Additional experiments have shown that fluorinated alcohols have considerable solubility in HFP throughout the whole experimental window. Once fluorinated alcohols are partitioned in the oil, they are best considered as a “co-oil” rather than a “cosurfactant”. Thus, the alcohols increase the effective hydrophilicity of the HFP. As the surfactant concentration decreases, less alcohol is available to act as a co-oil and the oil behaves more like HFP. This continuous change in the hydrophilicity of the oil blend as surfactant concentration decreases produces the tilted three-phase body.39 Increasing temperature enhances the miscibility of the surfactant in water while the addition of fluorinated (40) Tiddy, G. J. T.; Wheeler, B. A. J. Colloid Interface Sci. 1974, 47, 59.

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alcohol improves the solubility in the oil and limits the tendency of the surfactant to leave the oil for water. Addition of electrolyte pushes the surfactant out of the water phase. Thus, the effect of increasing electrolyte concentration is similar to the effect of adding a more hydrophobic amphiphile and to the effect of decreasing temperature. The influence of pressure on microemulsion phase behavior can be explained using similar arguments about surfactant solubility. Increasing pressure increases the solubility parameter41 of the compressible oil, so the surfactant and oil become more miscible. Thus, the UCST, TR, of the oil-surfactant binary is reduced as pressure rises. This causes the solubilization of the excess oil phase into a homogeneous microemulsion phase when pressure h f 1 phase is applied. Pressure also lowers Tδ, and the 2 transition occurs as pressure increases. The pressure effect is however overwhelmingly larger on the oil-surfactant binary because HFP is highly compressible. Nonetheless the effect of pressure on HFP mixtures with mediumchain alcohols is minor (Figures 5 and 6). This is likely due to the strong partitioning of the alcohol into the HFPrich phase and a concomitant decrease in its compressibility. Conclusions Hexafluoropropylene monomer can be solubilized in a microemulsion phase using a surfactant mixture containing ammonium perfluorooctanoate and fluorinated alcohol. Microemulsion phases can be made at different temperatures and pressures by changing surfactant concentration and/or by adding an appropriate electrolyte (NH4Cl). A short-chain fluorinated alcohol, hexafluoro-2-methyl-2propanol, promotes microemulsion formation over a wide temperature range. Medium-chain fluorinated alcohols, 2-perfluorobutyl-2-propanol and 2-perfluorohexyl-2-propanol, improve the efficiency of the surfactant mixture and generate large liquid crystalline regions. Microemulsion formulations containing the target monomer in a nearcritical state along with water, fluorinated surfactant, fluorinated alcohol, and salt follow the generic patterns of phase behavior common for conventional liquids as a function of temperature and salinity. Acknowledgment. This work was supported by DuPont Company and the State of Delaware Research Partnership. LA9815805 (41) McHugh, M.; Krukonis, V. Supercritical Fluid Extraction, 2nd ed.; Butterworth-Heinemann: Boston, 1994; Chapter 1.