Microemulsions in Supercritical Hydrochlorofluorocarbons - American

isooctane, difluoromethane, trifluoromethane, hexafluoroethane, sulfur hexafluoride, xenon, and carbon dioxide) at conditions just above or below the ...
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Langmuir 1996, 12, 5289-5295

5289

Microemulsions in Supercritical Hydrochlorofluorocarbons Kevin Jackson and John L. Fulton* Materials and Chemical Sciences Department, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 Received March 7, 1996. In Final Form: July 25, 1996X We report the properties of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) microemulsions formed in supercritical hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons, and fluorocarbons. The fluids used in this study include compounds that are of low toxicity and flammability and that are expected to remain environmentally acceptable well into the next century (e.g., 1,1,1,2-tetrafluoroethane (R134a) and chlorodifluoromethane (R22)). We show that it is possible to form a water-in-oil type of microemulsion in a low molecular weight HCFC (R22). In addition to these HCFCs, we also review the ability to form microemulsions in 14 other fluids (ethane, propene, propane, n-butane, n-pentane, n-hexane, isobutane, isooctane, difluoromethane, trifluoromethane, hexafluoroethane, sulfur hexafluoride, xenon, and carbon dioxide) at conditions just above or below the critical point (0.75 < T/Tc < 1.1) of the solvent. Due to the proximity of these liquids to the critical point, it is possible to make substantial changes in the densities of these solvents with modest changes in pressure. These 16 fluids have contrasting physical and chemical properties. We find that the parameter which universally predicts the ability of these solvents to form a microemulsion is the high-frequency dielectric constant (UV-vis light frequencies). This solvent dielectric constant is the parameter that governs the magnitude of the intermicellar van der Waals attractive forces but may also be relevant to the short-range attractive forces (surfactant tail to surfactant tail) that possibly control the phase behavior of these systems. We report extensively the phase behavior of AOT and didodecyldimethylammonium bromide microemulsions formed in a supercritical HCFC, R22. Microemulsions formed in supercritical R22 were demonstrated to have strongly density-dependent maximum molar water-to-surfactant ratios, Wo. When the pressure is increased from 100 to 400 bar, Wo increases from 5 to 50, making the solvency of the polar or ionic species in these systems highly pressure tunable. It was also shown that HCFC-based microemulsions are capable of solubilizing high molecular weight proteins, such as cytochrome c, which demonstrates their usefulness for separations from aqueous solutions. We show that microemulsions in HCFCs are practical alternatives to other fluids, such as supercritical carbon dioxide.

Introduction Reverse micelles and microemulsions have been extensively studied in liquid phase systems for decades. This early work included studies of the phase behavior of common surfactants, such as sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and didodecyldimethylammonium bromide (DDAB), in hydrocarbon solvents.1,2 However, studies of reverse micelle and microemulsion phases in near-critical (Tc >T > 0.75 Tc) or supercritical fluids (T > Tc) are a relatively recent development3-8 since their discovery by Gale et al. in 1987.9 When the continuous phase solvent is a low-polarity or nonpolar near-critical or supercritical fluid, pressure or density manipulation become important factors governing the properties of the microemulsion. In addition, the lower diffusion rates associated with the macromolecular microemulsion droplets are mostly offset by the high diffusion rates inherent with supercritical fluids. In effect, we have a pressure* To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Frank, S. G.; Zografi, G. J. Colloid Interface Sci. 1969, 29, 27-35. (2) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J. Phys. Chem. 1988, 92, 774-783. (3) Fulton, J. L.; Blitz, J. P.; Tingey, J. M.; Smith, R. D. J. Phys. Chem. 1989, 93, 4198-4204. (4) Beckman, E. J.; Fulton, J. L.; Smith, R. D. In Supercritical Fluid Technology: Reviews in Modern Theory and Applications; Bruno, T. J., Ely, J. F., Eds.; CRC Press: Boston, 1991; pp 405-449. (5) McFann, G. J.; Johnston, K. P. Langmuir 1993, 9, 2942-2948. (6) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Science 1994, 265, 356-359. (7) Darab, J. G.; Pfund, D. M.; Fulton, J. L.; Linehan, J. C. Langmuir 1994, 10, 135-141. (8) Harrison, K.; Goveas, J.; Johnston, K. P. Langmuir 1994, 10, 3536-3541. (9) Gale, R. W.; Fulton, J. L.; Smith, R. D. J. Am. Chem. Soc. 1987, 109, 920-921.

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tunable solvent in contrast to a conventional liquid solvent where pressure has little effect. Supercritical fluids having moderate critical temperatures lack the ability to solvate high molecular weight, highly polar, or ionic species. This problem is currently addressed by two different approaches, i.e., (1) by adding a simple entrainer or modifier or (2) forming a reverse microemulsion phase by adding a nonionic or ionic surfactant of low volatility, such as the anionic surfactant AOT or the cationic surfactant DDAB. The use of entrainers is the currently accepted technique but produces formidable environmental problems compared to using pure supercritical fluids. Microemulsion phases, however, are capable of solubilizing highly polar and even ionic species, such as biomolecules10-12 and dyes,3,13 in supercritical fluids with significantly reduced environmental impacts. An example of a supercritical fluid microemulsion is given in Figure 1. In the depicted case, the twin hydrocarbon-tail molecule is the commonly used reverse microemulsion surfactant AOT. The range of potential applications of these microemulsion systems is extensive, including general separations, enhanced oil recovery,14 reactions,15 chromatographic separations,9,16 cleaning (e.g., dry cleaning, soils, activated carbons, (10) Smith, R. D.; Fulton, J. L.; Blitz, J. P.; Tingey, J. M. J. Phys. Chem. 1990, 94, 781-787. (11) Lemert, R. M.; Fuller, R. A.; Johnston, K. P. J. Phys. Chem. 1990, 94, 6021-6028. (12) Beckman, E. J.; Smith, R. D. J. Phys. Chem. 1990, 94, 345-350. (13) McFann, G. J.; Johnston, K. P.; Howdle, S. M. AIChE J. 1994, 40, 543-555. (14) Carnahan, N. F.; Quintero, L.; Pfund, D. M.; Fulton, J. L.; Smith, R. D.; Capel, M.; Leontaritis, K. Langmuir 1993, 9, 2035-2044. (15) Beckman, E. J.; Fulton, J. L.; Matson, D. W.; Smith, R. D. In Supercritical Fluid Technology; Johnston, K. P., Penninger, J. L., Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989; p 184.

© 1996 American Chemical Society

5290 Langmuir, Vol. 12, No. 22, 1996

Figure 1. Structure of a microemulsion in a supercritical hydrochlorofluorocarbon solvent.

organic sulfur from coal, ceramics, catalysts, etc.),17 and polymerizations.18 In this paper we explore the low molecular weight hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and fluorocarbons as alternatives to fluids, such as supercritical carbon dioxide19-23 and the supercritical alkanes (e.g., ethane, propane). In selecting these halocarbons, we have considered not only their physical properties, such as critical points and compressibility, but also their toxicity, flammability, and environmental impact. Often, the nature of the halogenated compounds, such as their higher dipole moment and polarizability, means that, even as pure fluids, they show a greater affinity for more polar species than does carbon dioxide. An attractive feature of the halocarbons used in this study is their low toxicity, and indeed some are used in medical applications such as for blood substitutes and as anesthetics. As is commonly known, most chlorofluorocarbons (CFCs) have a very high ozone depletion level. However, if one or more chlorine or fluorine atoms are substituted with a hydrogen atom, then they become much less problematic. These compounds are known as hydrochlorofluorocarbons, or in the case of fluorocarbons, they are referred to as hydrofluorocarbons. Due to their oxidation in the troposphere, HCFCs and HFCs have a significantly lower half-life than CFCs and therefore a significantly lower ozone depletion potential.24 HCFCs and HFCs also have a significantly lower global warming potential (greenhouse effect) than their ozone depleting counterparts.25 An example of a HCFC is the refrigerant chlorodifluoromethane (R22). Unlike most CFCs, the gradual phasing out of compounds such as R22, as dictated by the Montreal Protocol, will not occur until well into the next century.26 The surfactant AOT forms small reverse micelles in low-polarity solvents even in the absence of water. (16) Tingey, J. M.; Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1990, 94, 1997-2004. (17) Jackson, K.; Fulton, J. L. In Cleaning with Supercritical Fluids; Narita, G.; McHardy, J., Eds.; Noyes Publishing Co.: Park Ridge, NJ, in press. (18) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945-947. (19) Fulton, J. L.; Pfund, D. M.; McClain, J. B.; Romack, T. J.; Maury, E. E.; Combes, J. R.; Samulski, E. T.; DeSimone, J. M.; Capel, M. Langmuir 1995, 11, 4241-4249. (20) Yee, G. G.; Fulton, J. L.; Smith, R. D. Langmuir 1992, 8, 377384. (21) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51-65. (22) 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. (23) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127-7129. (24) Ravishankara, A. R.; Turnipseed, A. A.; Jensen, N. R.; Barone, S.; Mills, M.; Howard, C. J.; Solomon, S. Science 1994, 263, 71-75. (25) Scientific Assessment of Stratospheric Ozone: 1989. Report No. 20; World Meteorolical Organization, Global Warming Research and Monitoring Project: Geneva, Switzerland, 1990; p 458. (26) First Review Meeting of Parties to the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, London, June 1990, and Copenhagen 1992.

Jackson and Fulton

Therefore, phase behavior measurements of binary AOT/ solvent systems lead to an understanding of the factors that promote micelle formation. Indeed, it is the micellemicelle interactions which govern the phase behavior of certain systems at relatively low surfactant and water concentrations where the micelle phase equilibria is analogous to the normal vapor/liquid equilibria.16,27 In these systems, phase separation occurs when, at some arbitrary water content, the micelles become large enough such that the attractive interactions cause them to coalesce to form a microemulsion-droplet rich phase (“liquid”) and a microemulsion-droplet depleted phase (“gas”). Three possible mechanisms of attractive micelle-micelle interactions governing this type of phase behavior have been proposed for supercritical fluid solutions including (1) interdroplet van der Waals interactions between the hydrocarbon tails and the aqueous cores of the droplets.16 These interdroplet interactions are dependent upon the temperature, droplet radius, hydrocarbon tail length, and the nature of the continuous phase solvent. Another proposed attractive force (2) is due to the tail-tail overlap and entanglement between two micelles which leads to strong, very short-range interactions as suggested by Bothorel et al. for liquid systems.28 Finally (3), a solvent exclusion effect is proposed where small solvent molecules penetrate the micelle tails leading to a higher density layer of solvent on the surface of the micelle (a clustering effect).28-30 This mechanism involves two micelles moving together, displacing solvent molecules from the tail region into the bulk solvent, leading to an increase in their molecular volume and a net effective attractive force between the two micelles. Various methods have been utilized to explore these short-range micelle-micelle interactive forces and aggregation.31,32 In 1995 Koper et al.32 concluded from their studies that further work is required to define the exact nature of the attractive interactions between the microemulsion droplets in low-polarity solvents. They concluded that the attractive terms found were too large to be attributed solely to van der Waals interactions. The present study was designed, in part, to further explore the nature of these short-range interactions in unique, pressure-tunable solventssthe near-critical and supercritical fluids. We examine how the simple properties (e.g., the high-frequency or UV-vis dielectric constant, vis) of the solvents can be used as predictors of their ability to form a microemulsion in a range of different fluids. Furthermore, we relate these properties to the interdroplet attractive forces that govern the phase behavior of the systems. We also present extensive phase diagrams for several new microemulsions formed in near-critical and supercritical HCFC solvents. Experimental Section Carbon dioxide (SFC grade), propene (>99.0% purity), sulfur hexafluoride (>99.8% purity), and the refrigerants R22 (chlorodifluoromethane, >99.9% purity), R32 (difluoromethane, >99.9% purity), R23 (trifluoromethane, >99.9% purity), and R116 (hexafluoroethane, >99.6% purity) were obtained from Scott Specialty Gases. “Suva” refrigerant R134a (1,1,1,2-tetrafluoroethane, >99.9% purity) was obtained from DuPont Chemical (27) Fulton, J. L.; Yee, G. G.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 169-174. (28) Lemaire, B.; Bothorel, P.; Roux, D. J. Phys. Chem. 1983, 87, 1023-1028. (29) Peck, D. G.; Johnston, K. P. J. Phys. Chem. 1993, 97, 56615667. (30) Peck, D. G.; Schechter, R. S.; Johnston, K. P. J. Phys. Chem. 1991, 95, 9541-9549. (31) Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 226-234. (32) Koper, G. J. M.; Sager, W. F. C.; Smeets, J.; Bedeaux, D. J. Phys. Chem. 1995, 99, 13291-13300.

Supercritical Hydrochlorofluorocarbons

Figure 2. Schematic of the high-pressure view cell and supporting apparatus. Co. The surfactants, Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate, Fisher Scientific, >98.0% purity) and DDAB (didodecyldimethylammonium bromide, Kodak Chemical Co., >99.0% purity) were used as received. The insulin (Calbiochem, >98.0% purity) and horse heart cytochrome c (Sigma, >95.0% purity) were also used as received. Water was distilled and deionized prior to system use. The refrigerant R22 was predried by passing the liquid through a large molecular sieve during the transfer from the cylinder to the pump. A schematic of the apparatus used in these studies is shown in Figure 2. The design and construction of the pressure vessel has been described earlier.33 Briefly, the vessel consisted of a 316 stainless steel monoblock with two optical ports containing opposing 2.5-cm-diameter sapphire windows. The windows were sealed to the metal block using 2.54-cm-O.D. gold-plated metal V-ring seals (Parker, no. 8812-2002-0100). The vessel had a maximum working pressure rating of 420 bar. Indirect viewing of the vessel contents was achieved by monitoring the vessel with a miniature CCD color camera (Toshiba, model no. IKM41A) with a telephoto lens. A 3 mm × 10 mm PTFE-coated magnetic stir bar was placed into the vessel to facilitate stirring. The total internal volume of the vessel with the stir bar in place was 15.9 mL. The temperature was controlled to within (0.2 °C using a three-mode controller with a platinum resistance probe (Omega, no. N2001). The water was directly injected into the microemulsion solution using a precalibrated incremental hydraulic hand pump (High Pressure Equipment. model no. 626-10). Both the fluid pressure and the pump pressure were monitored to (0.1 bar with electronic pressure transducers, P1 and P2, respectively (Precise Sensors, Inc., no. C451). A syringe pump (Isco, model no. 260D) with a capacity of 250 mL and a maximum pressure rating of 690 bar was used to deliver the solvent. A wet-test meter (Petroleum Instruments) was used to quantify the number of moles of gas in the microemulsion solution. Initially, the solubility of AOT in each near-critical solvent was determined at 25 °C and at the corresponding vapor pressure. In the case of R116 and xenon, where the critical temperatures are below 25 °C, the equivalent experiment was performed at 15 and 13.5 °C, respectively. The solubility criterion, for the purpose of this study, was defined as the condition where a 10% (v/v) AOT mixture was a single-phase solution. For those systems reported to have no AOT solubility, we observed a solid (AOT)fluid equilibrium. In this case, the majority of the introduced AOT remains as an insoluble solid indicating solubility levels ,10% (v/v). Although our system contained a binary mixture of 98% pure AOT and solvent, the surfactant retained a small amount of residual water which corresponded to a Wo of about 0.5. For the studies near the critical point of R22, we report liquidliquid equilibria in the ternary phase diagrams. For these detailed phase behavior experiments involving supercritical R22, water was added to the system to form larger microemulsion droplets. The amount of water that is added to the system is generally defined by the molar water-to-surfactant ratio, W ≡ (33) Blitz, J. P.; Fulton, J. L.; Smith, R. D. Appl. Spectrosc. 1989, 43, 812-816.

Langmuir, Vol. 12, No. 22, 1996 5291 [H2O]/[surfactant]. Furthermore, in this work, Wo is defined as the maximum molar water-to-surfactant ratio at the “cloud point” for a given set of conditions. The solubility of the continuousphase solvent in the aqueous core is assumed to be negligible. The procedure for determining the Wo values was as follows. A quantity of Aerosol-OT or DDAB was placed into the vessel via one of the two connection ports. The vessel was then heated to the desired temperature. After the cell and transfer lines were purged with the vapor phase, the vessel was then filled to the desired pressure and was allowed to equilibrate for 20 min. Using the hand-operated pump, water was added in microliter quantities until the phase boundary (liquid-liquid) was reached, corresponding to the maximum at Wo. The resultant liquidliquid phase boundary corresponds to a cloud point or “demixing pressure” as opposed to a true equilibrium point. In conventional liquid systems, one has the advantage that microemulsion stability at multiple conditions can be observed over a period of several months. Unfortunately, from a practical point of view, this is generally not possible with a pressurized system. However, we believe that the kinetics of these systems is relatively fast, so that we are in fact probably observing true thermodynamic equilibria. It must also be noted that the critical point of the mixture is different from that of a pure fluid. Therefore, once the water and surfactant are added, the critical properties of the solution may be significantly different from that of pure R22. Since the concentrations for this study were in the range of 50200 mM, the effects due to the critical micelle concentration on the phase behavior were considered negligible. The ternary diagrams shown in Figures 4 to 7 are reported in weight percent. The mass of R22 in the microemulsion was estimated using the densities produced from the Peng-Robinson equation of state. These values were checked using the wet-test meter at several different temperatures and pressures and gave errors of less than 10%. The reported Wo values were all corrected by taking into account water solubility in the R22. Water solubility was measured according to the method described by Jackson et al.34 The solubility of water in R22 at 25 °C was 0.14 wt % for all of the pressures and at 102.3 °C was 0.15 wt % at 100 bar, 0.30 wt % at 200 bar, 0.53 wt % at 300 bar, and 1.03 wt % at 400 bar. To access the magnitude of the interdroplet van der Waals forces, one needs the complete frequency spectrum of the dielectric constant for the fluid and for the droplets (or reverse micelles). However, to a large extent, the high-frequency (UV-vis region) dielectric constant, vis, determines the magnitude of the van der Waals interactions.16 Data for the dielectric constants at visible frequencies are currently not available for many of these fluids of interest, so approximate values are given in Table 1. These data are thus derived from the literature values of the gas phase polarizability3,35-37 and do not include the higher-order terms in the viral expansion.16 In order to calculate vis we also need the near-critical liquid densities.35 Liquid densities at 25 and 15 °C for R116 were calculated from a modified form of the Rackett equation.38 The density of xenon at 13.5 °C was obtained from Michels et al.39 The density of R134a at 25 °C was obtained from Morrison et al.40 The density of SF6 at 25 °C was extrapolated from various sources.41-44 Data for the low-frequency, static dielectric constants were extracted from various sources.35,42 (34) Jackson, K.; Fulton, J. L. Anal. Chem. 1995, 67, 2368-2372. (35) Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1995. (36) Meyer, C. W.; Morrison, G. J. Phys. Chem. 1991, 95, 38603866. (37) Miller, C. K.; Orr, B. J.; Ward, J. F. J. Chem. Phys. 1981, 74, 4858-4871. (38) Yaws, C. L.; Yang, H-C. In Thermodynamic and Physical Property Data; Yaws, C. L., Ed.; Gulf Publishing Co.: Houston, TX, 1992, p 92. (39) Michels, A.; Wassenaar, T.; Louwerse, P. Physica 1954, 20, 99106. (40) Morrison, G.; Ward, D. K. Fluid Phase Equilib. 1991, 62, 6586. (41) Encyclopedie Des Gaz; Elsevier Scientific Publishing Co.: Amsterdam, 1976. (42) Lange’s Handbook of Chemistry, 14th ed.; Dean, J. A., Ed.; McGraw-Hill Inc., New York, 1992. (43) Matheson Gas Data Book, 6th ed.; Braker, W; Mossman, A. L., Eds.; Matheson Co.: Lyndhurst, NJ, 1980. (44) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The properties of Gases and Liquids, 3rd ed.; McGraw-Hill: New York, 1977.

5292 Langmuir, Vol. 12, No. 22, 1996

Jackson and Fulton Table 1. Properties of the Fluids

solventd

dipole moment/D

R23 R116 (15.0 °C)a CO2 ethane xenon (13.5 °C)a R32 SF6 R134a propaneb propeneb isobutaneb n-butaneb R22b n-pentaneb n-hexaneb isooctaneb

1.65 0 0 0 0 1.98 0 2.06 0 0.37 0.132 0 1.42 0 0 0

static dielectric constant of liquid, o at 20-25 °C

UV-vis frequency dielectric constant, vis at 25 °C, vp

solubility parameter, δ at 25 °C, vp/ (J/cm3)1/2

critical temperature, Tc/K

critical pressure, Pc/bar

5.20

1.295 1.398 1.405 1.409 1.503 1.542 1.568 1.580 1.654 1.705 1.726 1.765 1.789 1.831 1.885 1.917

1.651 4.230 7.049 5.237 4.47 15.154 7.928 13.610 11.654 11.743 12.547 13.480 13.538 14.359 14.850 14.113

299.3 293.2 304.2 305.4 289.8 351.7 318.7 374.3 370.1 365.0 408.2 425.2 369.3 469.2 507.1 544.0

48.2 31.4 73.8 48.8 58.4 58.2 37.6 40.7 42.5 46.1 36.8 38.0 49.7 33.7 30.1 25.8

1.45 53.74 c 1.67 1.88 1.75 1.77 6.11 1.84 1.89 1.94

a Data calculated at the corresponding temperature due to critical temperatures being below 25 °C. b AOT solubilized in the fluid (10% v/v) at 25 °C and at the corresponding vapor pressure. c Value at 152 K. d Key: R22, chlorodifluoromethane; R23, trifluoromethane (fluoroform); R32, difluoromethane (methylene fluoride); R116, hexafluoroethane (perfluoroethane); R134a, 1,1,1,2-tetrafluoroethane.

Figure 3. AOT solubility (10% v/v) in the fluids studied at 25 °C as a function of the solvent dielectric constant at UV-vis frequencies, vis.

Results and Discussion AOT Solubility and Correlations to Solvent Parameters. Initially we explored the solubility of AOT in the liquid phase (25 °C) at the vapor pressure for a number of different solvents including carbon dioxide, propene, sulfur hexafluoride, chlorodifluoromethane (R22), difluoromethane (R32), trifluoromethane (R23), hexafluoroethane (R116) and 1,1,1,2-tetrafluoroethane (R134a). Previous studies16,45 have reported the AOT solubility in other solvents at 25 °C, including ethane, propane, n-butane, n-pentane, n-hexane, xenon, 2,2,4-trimethylpentane (isooctane), and 2-methylpropane (isobutane). The critical data for these solvents are given in Table 1. Of these 16 chemically-diverse liquid phases, AOT was found to be soluble (10% v/v) in propane, n-butane, n-pentane, n-hexane, propene, isooctane, isobutane, and chlorodifluoromethane (R22). As shown in Table 1 and Figure 3, the solubility of AOT (10% v/v) is solely dependent upon the value of the dielectric constant of the solvent, vis, in the UV-vis frequency range. It should be noted that vis for the polar fluids is significantly different from the more commonly cited static or low-frequency dielectric constant, o (see (45) Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1988, 92, 2903-2907.

Table 1). When vis is greater than 1.61, AOT was found to be soluble (10% v/v). This rule applies to a number of both polar and nonpolar liquids at a temperature of 25 °C. As vis is density dependent, we can usually solvate the AOT at some arbitrary high pressure. For instance, if we pressurize xenon to 200 bar (dielectric constant ) 1.61), AOT becomes soluble. vis is the single solvent parameter that can best be related to the magnitude of the interdroplet van der Waals forces. Indeed, a correlation between Wo values and vis has been previously shown to exist by Tingey et al. for near-critical and liquid alkanes.16 Some recent modeling29 and experimental results46 suggest that the magnitude of the attractive interactions is less than that which was previously believed47 and is more consistent with the magnitude of the van der Waals interactions. However, in the recent review by Koper et al.,32 there is a substantial amount of evidence indicating the existence of high droplet-droplet attractive interactions. It may be possible that vis is important in this purported short-range attractive interaction. Distinction between these two mechanisms of interaction is not possible in this analysis; thus, this issue still remains unresolved. We also explored the solvent’s solubility parameter as a predictor of AOT solubility. Researchers have attempted previously to use this cohesive energy or solubility parameter concept to describe the attractive forces between micelles.48-50 Little et al.48 suggested that the Hildebrand solubility parameter is the property of the solvent which can predict the solubility of sulfonate surfactants. Furthermore, they suggest that for a Hildebrand solvent solubility parameter of less than 13.3 (J/ cm3)1/2, sulfonate surfactants exhibit limited solubility in liquid solvents. Little and co-workers have also investigated the effect of the solvent solubility parameter and static dielectric constant, o, on the solubility of a range of different surfactants.48,49 Other studies have also involved relating micelle properties to the static or low(46) Pfund, D. M.; Fulton, J. L. In Proceedings of the 3rd International Symposium on Supercritical Fluids; Brunner, G., Perrut, M., Eds.; Strasbourg, 1994; pp 235-240. (47) Kaler, E. W.; Billman, J. F.; Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1991, 95, 458-462. (48) Little, R. C.; Singleterry, C. R. J. Phys. Chem. 1964, 68, 34533465. (49) Little, R. C. J. Colloid Interface Sci. 1978, 65, 587-588. (50) Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1971, 35, 636642.

Supercritical Hydrochlorofluorocarbons

Langmuir, Vol. 12, No. 22, 1996 5293

Figure 5. Ternary phase diagram of an AOT/water/liquid R22 system in the oil-rich corner of the ternary phase diagram at 25 °C. Phase boundary lines for various pressures up to 400 bar are indicated.

Figure 4. Ternary phase diagrams of AOT/water/supercritical propane systems in the oil-rich corner of the ternary phase diagram. The temperatures of the systems are 25 and 103 °C, respectively, with pressures up to 300 bar. (Compositions are given in weight percent.) The regions to the right of the solid lines are the one-phase, clear microemulsions. The various W value lines ([H2O]/[AOT]) are also indicated.

constant51,52

frequency dielectric and the dielectric constant in the midrange (microwave) frequencies.53 In this work the solubility parameters were calculated using the Hildebrand method54 for the fluids at 25 °C and at the corresponding vapor pressure. The solubility parameter for pure AOT is given as 25 (J/cc)1/2.54 In this study, it was also found that a solvent with a solubility parameter greater than approximately 11.7 (J/cc)1/2 solubilized AOT (10% v/v) in the near-critical liquid. However, there are two clear exceptions to this trend, R32 and R134a. It should be noted that these two solvents had dipole moments greater than that of water (1.85 D). Thus, the solubility parameter is not a universal predictor of AOT solubility as was found for vis for these near-critical solvents. Ternary Phase Behavior of R22. R22 was the only refrigerant studied that dissolved AOT at ambient conditions, and hence R22 was chosen as a solvent for further studies. The AOT/water/R22 system was compared to the data of Fulton et al.10 for the AOT/water/propane system (Tc ) 96.7 °C) in both the liquid and supercritical states. The data for the AOT/water/propane system is reproduced in Figure 4. When comparing the properties of the different fluids, it is most useful to make these comparisons at the same reduced parameters (i.e., Tr ) T/Tc). Therefore, the R22 (Tc ) 96.2 °C) experiments were (51) D’Angelo, M.; Fioretto, D.; Onori, G.; Palmieri, L.; Santucci, A. Phys. Rev. E 1995, 52, R4620-R4623. (52) van Dijk, M. A.; Joosten, J. G. H.; Levine, Y. K. J. Phys. Chem. 1989, 93, 2506-2512. (53) Marquardt, P.; Nimtz, G. Phys. Rev. Lett. 1986, 57, 1036-1039. (54) Handbook of Solubility Parameters and Other Cohesion Parameters; Barton, A. F. M., Ed.; CRC Press: Boca Raton, FL, 1983.

Figure 6. Ternary phase diagram of an AOT/water/supercritical R22 system in the oil-rich corner of the ternary phase diagram at 102.3 °C. Phase boundary lines for various pressures up to 400 bar are indicated.

performed at 102.3 °C, corresponding to the same reduced temperature of 1.015 used by Fulton et al. for propane at 103.0 °C. Wo values were measured at temperatures of 25 and 102.3 °C, at pressures of 100, 200, 300, and 400 bar, and at surfactant concentrations of 50, 100, 150, and 200 mM. The results for the AOT/water/R22 system at 25 and 102.3 °C are plotted in the ternary diagrams shown in Figures 5 and 6, respectively. Only the solvent (oil) rich portion of the diagram has been plotted. In Figures 5 and 6, positions corresponding to the various W values are indicated by the lighter-shaded lines. The bold solid lines represent the Wo values found for the R22 system. In the regions to the right of these lines, the system is a single-phase, clear solution, whereas in the regions to the left of these lines, two phases are observed. For the AOT/water/R22 system at 102.3 °C, the system showed a highly pressure dependent Wo increasing from 5.1 at 100 bar to 50.3 at 400 bar. The density of R22 increased only moderately over this range, from 0.848 to 1.186 g/mL. For R22, Wo increased from 5.1 at 100 bar to 32.9 at 300 bar compared to the respective values of 4.5 to 12 for propane showing that R22 is a much more densitydependent solvent and that it is a much better solvent for the larger micelles produced at higher Wo values. Higher Wo values in R22 versus propane are expected since the dielectric constant of R22, vis ) 1.79 in the near-critical liquid phase (see Figure 3), is close to the optimum value of 1.81 reported by Tingey et al.16 for the maximum possible solubilization of water. It was found that Wo was somewhat dependent upon the concentration of the

5294 Langmuir, Vol. 12, No. 22, 1996

Figure 7. Ternary phase diagram of a DDAB/water/liquid and supercritical R22 system in the oil-rich corner of the ternary phase diagram at 25 and 102.3 °C, respectively. Phase boundary lines for various pressures up to 400 bar are indicated.

surfactant at 102.3 °C for lower concentrations of surfactant. Thus, as the concentration of AOT is reduced below 100 mM, Wo becomes dependent upon the concentration of AOT. In this region, Wo increases somewhat with decreasing AOT concentration, probably reflecting an entropy of mixing contribution. This deviation at lower concentrations and pressures has been demonstrated earlier by Gale et al.9 for the AOT/water/ethane systems. Generally, above a 100 mM concentration, Wo is constant with AOT concentration. For the AOT/water/R22 system at 25 °C, Wo was found to be independent of pressure. This is in contrast to the liquid propane system where values of Wo increase significantly with pressure (7.0 at 10 bar to 36 at 250 bar). On the basis of what was reported by Tingey et al.,16 one would predict that Wo should also continue to increase with increasing pressure (increasing vis) for the AOT/water/liquid R22 system. However, at 25 °C a semisolid liquid crystal phase is formed whose properties are mostly independent of the pressure up to 400 bar. The R22 evidently stabilizes a liquid crystal phase that has a lower free energy than the micelle liquid phase corresponding to a micelle-micelle equilibrium. Thus, for R22 at 25 °C, this liquid crystal phase prevents us from drawing conclusions about the micelle attractive interactions that dominate the phase behavior of the other systems showing micelle-micelle equilibria. Tingey et al.16 found similar liquid crystal formation for n-butane and n-pentane with AOT/water systems. We also explore the solubility of the cationic surfactant didodecyldimethylammonium bromide in HCFCs. DDAB was soluble in pure R22 at 25 and 102.3 °C. To dissolve DDAB in most of the other solvents requires the addition of large amounts of water.55 DDAB/R22 at 25 °C showed only a very slight water solubility over that of the pure R22, producing Wo values of less than 1 (Figure 7). These low Wo values suggest that small, premicellar aggregates are preferred over micelles and also that the micellemicelle equilibria do not govern the phase behavior under these conditions. This trend agrees with the findings of Tingey et al.55 for the DDAB/water/propane system at 100 °C and 350 bar, where they found that water was only about 1% soluble in the solution. They also reported that upon increasing the water content to 15% (w/w), a second single-phase region was found to exist. The existence of a single-phase region near the center of the ternary phase diagrams for the DDAB/water/alkane systems has already been well documented.2,56 No single-phase region was (55) Tingey, J. M.; Fulton, J. L.; Matson, D. W.; Smith, R. D. J. Phys. Chem. 1991, 95, 1445-1448.

Jackson and Fulton

found for our DDAB/water/R22 system at 25 °C up to 750 mM surfactant concentration and 65% (w/w) water. However, at 102.3 °C, when the surfactant concentration was increased to 750 mM, a single-phase region was observed at water concentrations above 30% by weight (Figure 7). In addition, at 102.3 °C, we found Wo values from 6.7 to 14.4 for a concentration range of 50-200 mM DDAB, suggesting that small micelles may be forming in R22 at low water concentrations (Figure 7). No degradation of either AOT or DDAB was observed at the higher temperatures as the microemulsions were stable for several days under these conditions. Protein Solubility in R22 Microemulsions. To confirm the existence of a microemulsion phase in the AOT/R22 and DDAB/R22 systems, we determined the solubility of the high molecular weight metalloprotein cytochrome c. Cytochrome c has a molecular weight of 12 384 and has an orange-brown coloration in water which makes it ideal for these types of studies. Cytochrome c is insoluble in pure R22. The AOT/water/R22 system at 102.3 °C, at an intermediate pressure of 200 bar and a W value of 9.0, was able to dissolve at least a 50 µM concentration of cytochrome c very effectively. A single, clear phase was obtained which is a definitive indication of microemulsion formation. However, the DDAB/water/ R22 system at the same temperature did not dissolve any of the protein. To eliminate the possible charge effects of the protein within the micelles formed from this cationic surfactant DDAB, an anionic protein, insulin (MW ) 5749), was also tried. This too was insoluble. It is possible that other factors, such as the packing geometry and W values, have an important effect on the solubility of these proteins in a DDAB/water/R22 system, and thus it will require further study to verify microemulsion existence in this system. Conclusions We explore the solubility of AOT in 16 different liquid solvents in a region just below their critical points. We found that the solubility of AOT is well predicted by the value of the solvent dielectric constant, vis, in the UV-vis frequency range. When vis is greater than approximately 1.6, AOT was found to be soluble. This rule applies to a number of chemically-diverse polar and nonpolar nearcritical liquids at a temperature of 25 °C. This supports the previous results of Tingey and Fulton in 1990, where vis is related to the magnitude of the attractive interactions between the microemulsion droplets that control the phase separation of these systems. The phase behavior may be governed by the simple van der Waals interactions or by proposed additional short-range tail-tail interactions. The exact nature of these attractive forces remains unresolved. We show that the previously used correlations with the solubility parameter are not adequate to describe the observed AOT solubility nor is the polarity of the solvent within the limited range of this study. Microemulsion phases readily form in near-critical and supercritical chlorodifluoromethane (R22), a low molecular weight, environmentally acceptable HCFC solvent. Microemulsions formed in supercritical R22 were shown to have strongly density-dependent molar water-to-surfactant ratios, Wo, making the solvency of the system highly pressure tunable. Maximum molar water-to-surfactant ratios, Wo, values greater than 50, were observed for the AOT/water/R22 system. For the AOT/water/R22 system at 25 °C, it was found that an equilibrium semisolid liquid (56) Blum, F. D.; Evans, D. F.; Nanagara, B.; Warr, G. G. Langmuir 1988, 4, 1257-1261.

Supercritical Hydrochlorofluorocarbons

crystal phase is formed whose properties are mostly independent of the pressure, up to 400 bar. In the presence of this liquid crystal phase, the Wo of the microemulsion is much less pressure tunable than for the supercritical system. The existence of a single-phase region, at very high water concentrations, for the DDAB/water/R22 system was also found. It was also demonstrated that these environmentally-friendly, refrigerant-based supercritical fluid microemulsions are capable of solubilizing

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high molecular weight proteins that would normally be insoluble in the pure fluids. Acknowledgment. This research was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy, under Contract DE-AC06-76RLO 1830. LA960210I