Supercritical CO2 Microemulsions with Mixed Surfactant Systems

Publication Date (Web): August 21, 2008. Copyright © 2008 American Chemical Society. * To whom all correspondence should be addressed. E-mail: sagisa...
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10116

Langmuir 2008, 24, 10116-10122

Water/Supercritical CO2 Microemulsions with Mixed Surfactant Systems Masanobu Sagisaka,*,† Daisuke Koike,‡ Yasuaki Mashimo,‡ Satoshi Yoda,§ Yoshihiro Takebayashi,§ Takeshi Furuya,§ Atsushi Yoshizawa,† Hideki Sakai,‡,| Masahiko Abe,‡,| and Katsuto Otake⊥ Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki UniVersity, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan, Department of Science and Engineering, Faculty of Science and Technology, Tokyo UniVersity of Science, Yamazaki 2641, Noda, Chiba 278-8510, Japan, Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology, Tsukuba Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan, Institute of Colloid and Interface Science, Tokyo UniVersity of Science, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-0825, Japan, and Department of Industrial Chemistry, Faculty of Engineering, Tokyo UniVersity of Science, Ichigaya Funakawaramachi 12-1, Shinjuku-ku, Tokyo 162-0826, Japan ReceiVed May 7, 2008. ReVised Manuscript ReceiVed July 12, 2008 Phase behavior was investigated for water/supercritical CO2 (W/scCO2) microemulsions stabilized with sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2) mixed with various guest surfactants. Only for the mixtures with fluorocarbon-hydrocarbon hybrid anionic surfactants (FC6-HCn), the maximum water-to-surfactant molar ratio (W0c) was larger than that estimated from linear interpolation of the W0c values for pure 8FS(EO)2 and pure guest surfactant. Fourier transform infrared (FT-IR) measurement for the microemulsion revealed that the mixing of 8FS(EO)2 with FC6-HCn can prevent a phase transition from the microemulsion to the liquid crystal even in the presence of excess water. It was also found from the measurement of water/scCO2 interfacial tension that the area occupied per surfactant molecule was markedly increased by the mixing with FC6-HCn. The loose molecular packing, probably due to a microsegregation of 8FS(EO)2 and FC6-HCn, is consistent with the enhanced stability of the microemulsion upon surfactant mixing.

1. Introduction A microemulsion of water and supercritical CO2 (scCO2), that is, a reverse micelle encapsulating an aqueous core in scCO2, is a promising “universal solvent” with advantages of both water and scCO2.1-8 A number of studies have been directed toward the development of a surfactant system that can stabilize the formation of a water/scCO2 microemulsion.1-41 We have recently shown that a fluorinated analogue of aerosol-OT * To whom all correspondence should be addressed. E-mail: sagisaka@ cc.hirosaki-u.ac.jp. Telephone and fax: +81-172-39-3569. † Hirosaki University. ‡ Department of Science and Engineering, Tokyo University of Science. § National Institute of Advanced Industrial Science and Technology. | Institute of Colloid and Interface Science, Tokyo University of Science. ⊥ Department of Industrial Chemistry, Tokyo University of Science. (1) Sagisaka, M.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kitiyanan, B.; Kondo, Y.; Yoshino, N.; Takebayashi, K.; Sakai, H.; Abe, M. Langmuir 2003, 19, 220. (2) Sagisaka, M.; Ozaki, Y.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M. Mater. Technol. 2003, 21, 36. (3) Sagisaka, M.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M. Langmuir 2003, 19, 8161. (4) Sagisaka, M.; Fujii, T.; Ozaki, Y.; Yoda, S.; Takebayashi, Y.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M.; Otake, K. Langmuir 2004, 20, 2560. (5) Sagisaka, M.; Koike, D.; Yoda, S.; Takebayashi, Y.; Furuya, T.; Yoshizawa, A.; Sakai, H.; Abe, M.; Otake, K. Langmuir 2007, 23, 8747. (6) Sagisaka, M.; Fujii, T.; Koike, D.; Yoda, S.; Takebayashi, Y.; Furuya, T.; Yoshizawa, A.; Sakai, H.; Abe, M.; Otake, K. Langmuir 2007, 23, 2369. (7) Eastoe, J.; Bayazit, Z.; Martel, S.; Steytler, D. C.; Heenan, R. K. Langmuir 1996, 12, 1423. (8) Eastoe, J.; Cazelles, B. M. H.; Steytler, D. C.; Holmes, J. D.; Pitt, A. R.; Wear, T. J.; Heenan, R. K. Langmuir 1997, 13, 6980. (9) Ritter, J. M.; Paulaitis, M. E. Langmuir 1990, 6, 935. (10) Yee, G. G.; Fulton, J. L.; Smith, R. D. Langmuir 1992, 8, 377. (11) Hutton, B. H.; Perera, J. M.; Grieser, F.; Stevens, G. W. Colloids Surf., A 1999, 146, 227. (12) Hutton, B. H.; Perera, J. M.; Grieser, F.; Stevens, G. W. Colloids Surf., A 2001, 189, 177. (13) Eastoe, J.; Paul, A.; Nave, S.; Steytler, D. C.; Robinson, B. H.; Rumsey, E.; Thorpe, M.; Heenan, R. K. J. Am. Chem. Soc. 2001, 123, 988.

surfactant, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2sulfosuccinate (8FS(EO)2), is the most effective formulation for stabilizing a transparent single phase Winsor-IV water/scCO2 microemulsion (IVµE), where the maximum water-surfactant ratio W0c is 32.1-6 However, it has also been found that 8FS(EO)2 (14) Liu, J.; Han, B.; Li, G.; Zhang, X.; He, J.; Liu, Z. Langmuir 2001, 17, 8040. (15) Liu, J.; Han, B.; Wang, Z.; Zhang, J.; Li, G.; Yang, G. Langmuir 2002, 18, 3086. (16) Liu, J.; Han, B.; Zhang, J.; Li, G.; Zhang, X.; Wang, J.; Dong, B. Chem.sEur. J. 2002, 8, 1356. (17) Liu, J.; Zhang, J.; Mu, T.; Han, B.; Li, G.; Wang, J.; Dong, B. J. Supercrit. Fluids 2003, 26, 275. (18) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51. (19) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127. (20) Johnston, K. P.; Harrison, K. L.; Klarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624. (21) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. Langmuir 1997, 13, 3934. (22) 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, 6707. (23) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 119, 6399. (24) Niemeyer, E. D.; Bright, F. V. J. Phys. Chem. B 1998, 102, 1474. (25) daRocha, S. R. P.; Harrison, K. L.; Johnston, K. P. Langmuir 1999, 15, 419. (26) daRocha, S. R. P.; Johnston, K. P. Langmuir 2000, 16, 3690. (27) Pandey, S.; Baker, G. A.; Kane, M. A.; Bonzagni, N. J.; Bright, F. V. Langmuir 2000, 16, 5593. (28) Ohde, H.; Hunt, F.; Kihara, S.; Wai, C. M. Anal. Chem. 2000, 72, 4738. (29) Lee, C T., Jr.; Bhargava, P.; Johnston, K. P. J. Phys. Chem. B 2000, 104, 4448. (30) Lee, D.; Hutchison, J. C.; DeSimone, J. M.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 8406. (31) Fremgen, D. E.; Smotkin, E. S.; Gerald, R. E.; Klingler, R. J.; Rathke, J. W. J. Supercrit. Fluids 2001, 19, 287. (32) Meziani, M. J.; Sun, Y.-P. Langmuir 2002, 18, 3787. (33) Loeker, F.; Marr, P. C.; Howdle, S. M. Colloids Surf., A 2003, 214, 143. (34) Eastoe, J.; Downer, A. M.; Paul, A.; Steytler, D. C.; Rumsey, E. Prog. Colloid Polym. Sci. 2000, 115, 214.

10.1021/la8014145 CCC: $40.75  2008 American Chemical Society Published on Web 08/21/2008

Water/Supercritical CO2 Microemulsions Table 1. Molecular Structures of Surfactants

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2. Experimental Section 2.1. Materials. Sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)2-sulfosuccinate, 8FS(EO)2, and sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-alkanesulfonates, FC6-HCn (HC chain length n ) 4 and 6), were synthesized in our laboratory and purified to >99% as described previously.43-50 Perfluoropolyether ammonium carboxylate, PFPECOONH4, and N-[3-dodecyloxy-2-hydroxypropyl]L-arginine L-gultamate, C12HEAGlu, were generously provided by Daikin Industries Ltd. and Ajinomoto Co., Inc., respectively.1,12 They were used as received. Adeka Pluronic L31 (Asahi Denka) and decyltrimethylammonium chloride, DeTAC (Tokyo Kasei), were also used as received. Tergitol TMN-6 (Sanyo Trading) was used after drying in a vacuum. Table 1 shows the molecular structures of these surfactants. Distilled water (Otsuka Pharmaceutical, injection grade, pH ) 6.5) and pure CO2 (Tomoe Shokai, 99.99%) were used without purification. 2.2. Phase Behavior Measurement. The phase behavior of the water/surfactant/scCO2 system was measured at temperatures ranging from 35 to 75 °C, CO2 densities ranging from 0.70 to 0.85 g/cm3, and W0c values ranging from 0 to 35. The W0c parameter is generally used to express a water-to-surfactant molar ratio in a reverse micelle in scCO2, where the solubility of water in scCO2 is taken into account as1-6

W0c )

forms a liquid-crystal (LC)-like precipitate at values of W0c larger than 32.5 In our previous study,6 aimed at the stabilization of the 8FS(EO)2 microemulsion, the effects of surfactant mixing on the interfacial properties of water/CO2 were examined for several quaternary systems of 8FS(EO)2/hydrocarbon (HC) guest surfactant/water/CO2. One of the mixtures of a guest surfactant with 8FS(EO)2 had positive synergistic effects on not only interfacial activities but also the microemulsifying power. Unfortunately, in the mixed surfactant system, the maximum W0c value of IVµE was less than 32, and it eventually formed a LC with an increase in W0c.6 To examine a more effective guest surfactant structure for stabilizing the 8FS(EO)2 microemulsion, we studied the phase behavior of 8FS(EO)2/water/CO2 mixtures including several types of guest surfactants; this study focused primarily on the formation of the microemulsion and LC. The surfactants listed in Table 1 were employed as guest surfactants and mixed with 8FS(EO)2. TMN-6 and L31 are nonionic HC surfactants, DeTAC is a cationic HC-type surfactant, C12HEAGlu42 is an amphoteric HC-type surfactant, PFPECOONH41 is an anionic fluorocarbon (FC)-type surfactant, and FC6-HCn is an anionic FC-HC hybridtype surfactant.1,4 They were used for examining the effects of the hydrophilic group and FC/HC balance; the FC chain is both hydrophobic and CO2-philic, whereas the HC chain is hydrophobic but not CO2-philic.1-41 (35) Eastoe, J.; Downer, A.; Paul, A.; Steytler, D. C.; Rumsey, E.; Penfold, J.; Heenan, R. K. Phys. Chem. Chem. Phys. 2000, 2, 5235. (36) Liu, Z.-T.; Erkey, C. Langmuir 2001, 17, 274. (37) Steytler, D. C.; Rumsey, E.; Thorpe, M.; Eastoe, J.; Paul, A.; Heenan, R. K. Langmuir 2001, 17, 7948. (38) Eastoe, J.; Paul, A.; Downer, A.; Steytler, D. C.; Rumsey, E. Langmuir 2002, 18, 3014. (39) Dong, X.; Erkey, C.; Dai, H.-J.; Li, H.-C.; Cochran, H. D.; Lin, J. S. Ind. Eng. Chem. Res. 2002, 41, 1038. (40) Senapati, S.; Keiper, J. S.; DeSimone, J. M.; Wignall, G. D.; Melnichenko, Y. B.; Frielinghaus, H.; Berkowitz, M. L. Langmuir 2002, 18, 7371. (41) Harrison, K.; Goveas, J.; Johnston, K. P.; O’Rear, E. A. Langmuir 1994, 10, 3536. (42) Tabohashi, T.; Tobita, K.; Sakamoto, K; Kouchi, T.; Yokoyama, S.; Sakai, H.; Abe, M. Colloids Surf., B 2001, 20, 79.

[water]0-[water]S [surfactant]0

(1)

where [water]0 is the number of moles of water in the system, [water]S is the number of moles of water soluble in the given amount of pure CO2, and [surfactant]0 is the number of moles of the surfactant in the system. Predetermined amounts of the surfactant and CO2, where the molar ratio of the surfactant to CO2 was fixed at 0.08 mol %, were loaded in a variable-volume high-pressure optical cell. Water was then added into the optical cell with a six-port valve, until a clear Winsor-IV microemulsion (IVµE) solution became a turbid macroemulsion (E) solution, a “Winsor-I or Winsor-II microemulsion”-like two-phase (2Ø) composed of CO2 and separated excess water, or a LC-like precipitated hydrated surfactant (P). The W0c value at the phase transition was determined for each temperature and CO2 density. The [water]S value was obtained from the literature data.51,52 The densities of CO2 were calculated using Span-Wagner EOS.53 In the case of equimolar surfactant mixtures, equal moles (molar surfactant-to-CO2 ratios of 0.04 mol % each) of both surfactants were added directly into the cell. The formation of a microemulsion, namely, the existence of a core of bulk water in scCO2, was examined with a high-pressure Fourier transform infrared (FT-IR) photometer (JASCO Co. Ltd., FT/IR 620) connected to the experimental apparatus. In FT-IR measurements, the mixtures were circulated between the optical vessel and the FT-IR cell until the absorbance of H2O became constant. The valves between the vessel and the FT-IR cell were then closed, and the FT-IR spectrum was measured. Detailed descriptions of the experimental apparatus and procedures of the measurements are given in our previous reports.1-6,52 (43) Yoshino, N.; Komine, N.; Suzuki, J.; Arima, Y.; Hirai, H. Bull. Chem. Soc. Jpn. 1991, 64, 3262. (44) Yoshino, N.; Morita, M.; Ito, A.; Abe, M. J. Fluorine Chem. 1995, 70, 187. (45) Abe, M.; Kondo, Y.; Sagisaka, M.; Hideki, S.; Morita, Y.; Kaise, T.; Yoshino, N. J. Jpn. Soc. Colour Mater. 2000, 73, 53. (46) Sagisaka, M.; Ito, A.; Kondo, Y.; Yoshino, N.; Kwon, K. O.; Sakai, H.; Abe, M. Colloids Surf., A 2001, 185, 749. (47) Ito, A.; Kamogawa, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. J. Jpn. Oil Chem. Soc. 1996, 45, 479. (48) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1996, 12, 5768. (49) Kondo, Y.; Yokochi, E.; Mizumura, S.; Yoshino, N. J. Fluorine Chem. 1998, 91, 147. (50) Yoshino, N.; Hamano, K.; Omiya, T.; Kondo, Y.; Ito, A.; Abe, M. Langmuir 1995, 11, 466.

10118 Langmuir, Vol. 24, No. 18, 2008 2.3. Interfacial Tension Measurement at Water/scCO2 Interface. The interfacial tension (IFT) at a water/scCO2 interface in the presence of a surfactant was measured with a high-pressure pendant-drop tensiometer developed in our laboratory. The tensiometer consists of two high-pressure cells, a reservoir cell containing water saturated with CO2 and a variable-volume cell that has optical windows for visual observations of the droplet shape, and contains a CO2-surfactant mixture. The two cells were separated by a twoway valve. A stainless steel tube (o.d. 0.16 mm and i.d. 0.10 mm) and a glass capillary (o.d. 0.03 mm and i.d. 0.01 mm) were used to form drops in the variable-volume cell. The following procedure was carried out for the measurements. A given amount of surfactant/acetone solution was added in the optical cell. For the measurement of a surfactant mixture system, the solution was prepared by mixing equal volumes of equimolar surfactant solutions. After evaporating acetone in a vacuum, CO2 was supplied to the optical cell. Through vigorous stirring of the surfactant/CO2 mixture, a transparent single phase appeared, and it was identified as a homogeneous mixture. The pressure of the reservoir cell was increased so that it was higher than that of the variable-volume cell. The two-way valve was then opened slowly, and a drop was formed at the tip of the stainless steel tube. When the drop was first formed in the view cell, water began to dissolve in the surfactant/CO2 solution and the size of the drop reduced. Thus, by adding water from the reservoir cell, the size of the drop was kept almost constant. Once a suitable droplet was formed, the two-way valve was closed. When the IFT was lower than ∼10 mN/m, a glass capillary with a smaller diameter than that of the stainless steel tube was used. Detailed descriptions of the experimental apparatus and procedures are given in our previous reports.2,4,6 The IFT was determined from the shape of the droplet using axisymmetric drop shape analysis (ADSA) based on the Laplace equation.54-57 In the analysis, the densities of the CO2-rich and water-rich phases were assumed to be equal to those of their respective pure constituents. The densities of CO2 and water were calculated using Span-Wagner EOS.53,58 Several images of the droplet were taken at certain time intervals to attain thermodynamic equilibrium, until the variation in the observed IFT became smaller than 0.1 mN/m per 20 min. At least five drops were formed under each experimental condition to obtain an average value. For examining surfactant packing at the water/scCO2 interface, a steric model of one surfactant molecule without other molecules was calculated by MM2 simulation (Chem 3D; CambridgeSoft Corp., Cambridge, MA).

3. Results and Discussion 3.1. Maximum W0c for IVµE of Pure Surfactant. The dissolution behavior of 0.08 mol % surfactant in scCO2 was examined before measuring the maximum W0c value of IVµE. Under dry conditions, 0.08 mol % TMN-6, PFPECOONH4, and FC6-HCn were completely soluble in CO2, but the other surfactants were not so. When water was added into the surfactant/ CO2 mixtures, 8FS(EO)2 dissolved considerably in CO2 with a reverse micelle formation, although DeTAC, L31, and C12HEAGlu remained at the bottom of the cell. To evaluate the effect of surfactant mixing on IVµE, the maximum W0c values of IVµE were measured for pure and mixed (51) Wiebe, R. Chem. ReV. 1941, 29, 475. (52) Takebayashi, Y.; Mashimo, Y.; Koike, D.; Yoda, S.; Furuya, T.; Sagisaka, M.; Otake, K.; Sakai, H.; Abe, M J. Phys. Chem. B 2008, 112, 8943. (53) Span, R.; Wagner, W. J. Phys. Chem. Ref. Data 1996, 25, 1509. (54) Jennings, J. W.; Pallas, N. R. Langmuir 1988, 4, 959. (55) Rotenberg, Y.; Boruvka, L.; Neumann, A. W. J. Colloid Interface Sci. 1983, 93, 169. (56) Li, D.; Cheng, P.; Neumann, A. W. AdV. Colloid Interface Sci. 1992, 39, 347. (57) del Rı´o, O. I.; Neumann, A. W. J. Colloid Interface Sci. 1997, 196, 136. (58) Haar, L.; Gallagher, J. S.; Kell, S. S. S. NBS/NRC Steam Tables: Thermodynamic and Transport Properties and Computer Programs for Vapor and Liquid States of Water in SI Units; Taylor & Francis Inc.: New York, 1984.

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Figure 1. Maximum water-to-surfactant molar ratio, W0c, of Winsor-IV W/scCO2 microemulsion (VIµE) with pure surfactant systems at 75 °C and 400 bar. The molar ratio of the surfactant to CO2 was fixed at 0.08 mol %.

surfactant systems at 75 °C and 400 bar. Figure 1 shows the maximum W0c values for pure surfactant systems. Except for TMN-6, the HC surfactants could not microemulsify with water/ supercritical CO2 when W0c > 0. With regard to the CO2 properties, CO2-philicity has been ascribed to a weak van der Waals force (low polarity), to unpaired electrons that act as a Lewis base, or to the large free volume.1-41 Compared with typical CO2-philic compounds,1-41 L31, DeTAC, and C12HEAGlu would have stronger van der Waals forces, which arise from a larger HC part with a linear shape and/or a larger hydrophilic group, resulting in less CO2-philicity and microemulsifying power. In the case of TMN-6, the IVµE phase was formed with W0c up to 16 under the current measurement condition. As pointed out by Johnston et al.,59 the highly methylated HC with a large free volume could enhance solvation by CO2 and serve as a CO2-philic group. PFPECOONH4 and FC6-HCn yielded IVµEs, although their W0c values were lower than that of 8FS(EO)2.1-4 The FC chains of these surfactants, which have a CO2-like weak van der Waals force and no dipole moment, exhibit strong CO2 philicity1-41 and few mutual or other interactions. They increase the microemulsifying power of the surfactants by enabling them to effectively decrease the water/CO2 IFT and the repulsive interactions between their reverse micelles. For pure surfactant systems, detailed examinations have been carried out in previous studies.1-6 3.2. Maximum W0c for IVµE of Mixed Surfactant. Figure 2 shows the maximum W0c values that were estimated from the results in Figure 1 and measured for the mixed surfactant systems. In Figure 2, the estimated and measured values are indicated by open and filled bars, respectively. Rs represents the molar ratio of the guest surfactant per total moles of the surfactant. For mixed 8FS(EO)2/nonionic HC surfactant systems, transparent one-phase IVµE appeared, although L31 had low solubility and no microemulsifying power in CO2. This suggests that these surfactants and 8FS(EO)2 were miscible and formed mixed reversed micelles yielding IVµE. Unfortunately, the maximum W0c values of these mixed surfactant systems were not larger than the estimated values; therefore, a LC was eventually formed at W0c above the maximum values. Mixtures of DeTAC with Rs ) 0.3-0.5 and C12HEAGlu with Rs ) 0.5 remained in the form of a two-phase having separated (59) Ryoo, W.; Webber, S. E.; Johnston, K. P. Ind. Eng. Chem. Res. 2003, 42, 6348.

Water/Supercritical CO2 Microemulsions

Figure 2. Maximum water-to-surfactant molar ratio, W0c, of WinsorIV W/scCO2 microemulsion (VIµE) with mixed surfactant systems at 75 °C and 400 bar. Filled and open bars indicate the values obtained by actual measurements and those estimated from the results of pure surfactant systems, respectively. On the basis of the assumption that there is no mixed micelle formation, the estimated value, W0cest, was calculated by the following equation: W0cest ) RsW0cguest + (1 Rs)W0chost, where Rs is the molar ratio of the guest surfactant to the total surfactant amount, W0cguest is the maximum W0c for the guest surfactant shown in Figure 1, and W0chost is the maximum W0c for 8FS(EO)2, that is, W0chost ) 32. The molar ratio of the surfactant to CO2 was fixed at 0.08 mol %.

excess water at any W0c value. This shows the negative effect of surfactant mixing on the microemulsifying power, since the maximum W0c < 0 was significantly lower than the estimated values. In general, cationic groups attract anionic groups by static electrointeraction and form an ion complex. Such complexation between 8FS(EO)2 and DeTAC (or C12HEAGlu) could drastically decrease hydrophilicities of their hydrophilic groups and get lost the excellent microemulsifying power of 8FS(EO)2 The microemulsifying power of 8FS(EO)2 was restored by decreasing Rs, as shown in the case of the C12HEAGlu mixture with Rs ) 0.2. On the other hand, DeTAC and C12HEAGlu were found to act as destabilizers for the LC of 8FS(EO)2, on the basis of the fact that two-phase mixtures were formed without LC. Their destabilizing ability could be due to an increased repulsive interaction between their HC and FC of 8FS(EO)2 via forming the ion complex, which results in a looser molecular packing than that of pure 8FS(EO)2. Therefore, if the molecular packing can be disordered by mixing a highly miscible surfactant without affecting the inherent properties of 8FS(EO)2 except LC formation, the resulting mixed surfactant system will allow the formation of IVµE, even though W0c becomes larger than the estimated W0c value for each pure surfactant system. A comparison between HC and FC surfactants reveals that the latter has a relatively higher microemulsifying power when mixed with 8FS(EO)2 in scCO2. The microemulsifying powers for the FC surfactant mixtures higher than those for the HC mixtures are mainly due to the differences in CO2-philicity, microemulsifying power, and miscibility of the guest surfactant with 8FS(EO)2. With an increase in W0c, a FC guest surfactant mixed with 8FS(EO)2 causes phase transition from IVµE to a twophase mixture and not to a LC. These results are similar to those obtained for the mixture of DeTAC or C12HEAGlu and suggest that the guest surfactants were highly miscible and formed mixed reverse micelles with 8FS(EO)2. Interestingly, the maximum W0c values of 8FS(EO)2/FC6-HC4 mixtures with Rs ) 0.8 and 0.5 were larger than the estimated values, which clearly displays

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the positive synergistic effect on the microemulsifying power. The other mixtures did not exhibit this effect. Another important point is that the positive synergistic effects gradually enhanced with an increase in Rs at Rs ) 0.2-0.8. Since Rs ) 1 is a pure FC6-HC4 system with maximum W0c ) 9, these results imply that the value of Rs at which the maximum W0c value of IVµE is obtained is between 0.5 and 1.0 for a mixed 8FS(EO)2/FC6HC4 system. 3.3. Phase Diagram for 8FS(EO)2/FC6-HCn Mixture. To study the positive synergistic effect in detail, the phase behavior of mixed 8FS(EO)2/FC6-HCn systems were examined as functions of W0c, temperature, and CO2 density. Their phase diagrams are shown in Figure 3. E, IVµE, 2Ø, and P indicate W/scCO2 macroemulsion phase, Winsor-IV W/scCO2 microemulsion phase, two-phase with a transparent CO2 phase and separated excess water, and precipitation of a hydrated surfactant (LC), respectively. The mixing of FC6-HCn with 8FS(EO)2 produced a drastic change in the phase diagram. For pure FC6-HCn (Figure 3a), no IVµE was observed at a CO2 density lower than 0.75 g/cm3; however, as shown in Figure 3b and c, the mixing of FC6-HCn with 8FS(EO)2 at Rs e 0.5 enabled the formation of IVµE even at the lowest CO2 density measured (0.70 g/cm3). When the CO2 density was increased, IVµE was more stabilized; that is, the phase boundary between E and IVµE shifted to a lower temperature. This was probably due to the enhanced solvation by CO2 around the hydrophobic tail of the surfactant.1-6 A comparison between the phase diagrams of FC6-HC4 and FC6-HC6 shows that both have similar darkened IVµE areas at the same CO2 density and Rs (e.g., Figure 3a and f), although the maximum W0c values of FC6-HC4 were slightly larger than those of FC6-HC6. The longer the HC chain length n of FC6HCn, the lower the CO2-philicity of the hydrophobic tail and consequently the microemulsifying power. As mentioned above, the maximum W0c values of IVµE reached 24 for Rs ) 0.5 and 27 for Rs ) 0.8 at a CO2 density of 0.85 g/cm3 and a temperature of 75 °C, thereby indicating a positive synergistic effect on IVµE formation. This positive synergistic effect was dependent on Rs; the maximum W0c value of IVµE for Rs ) 0.8 (Figure 3b) decreased with decreasing temperature, while that for Rs ) 0.5 (Figure 3c) was almost independent of temperature. The temperature dependence for Rs ) 0.8 will increase in the case of FC6-HC4-rich microemulsion surfaces that have mutual attractive interactions such as π-π stacking between benzene rings and induced-dipole interaction between HCs. Such attractive interactions promote the aggregation and fusion of reverse micelles and decrease stability. Since the speed of thermal motion of FC6-HC4 molecules increases with temperature, the effect of attractive interactions decreases at high temperature. In addition to the positive synergistic effect on the maximum W0c value of IVµE, no P-phase was observed for FC6-HCn/ 8FS(EO)2 mixtures. This suggests that FC6-HCn was highly miscible with 8FS(EO)2 and destabilized the formation of the LC. To examine 2Ø that appeared instead of the LC and the surfactant packing at the W/scCO2 interface for the mixed FC6HCn/8FS(EO)2 system, FT-IR spectra and water/scCO2 IFTs were measured. 3.4. FT-IR Spectrum for 8FS(EO)2/FC6-HCn Mixture. Figure 4 shows the FT-IR spectra of the water/8FS(EO)2/FC6HC4/CO2 system with Rs ) 0.5 and different values of W0c at 50 °C and 300 bar. For mixtures with W0c e 24, as shown in Figure 4a, large and broad bands were observed at 2800-3600 cm-1. These absorption bands were assigned to highly hydrogen

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Figure 3. W0c-temperature-CO2 density phase diagram of water/surfactant/scCO2 mixtures for pure and mixed surfactant systems of 8FS(EO)2 and FC6-HCn. IVµE, E, 2Ø, and P denote Winsor-IV W/scCO2 microemulsion phase, macroemulsion phase, two-phase with a transparent CO2 phase and separated excess water, and precipitation of the hydrated surfactant, respectively. Rs is the molar ratio of the guest surfactant to the total surfactant amount. The molar ratio of the surfactant to CO2 was fixed at 0.08 mol %.

Figure 4. FT-IR spectra of H2O in scCO2 with 0.08 mol % equimolar 8FS(EO)2/FC6-HC4 mixture at 50 °C and 300 bar: (a) µE and (b) 2Ø. IR bands of CO2 and 8FS(EO)2 have been removed by computer subtraction.

bonded H2O in reverse micelles, as it is known that the O-H stretching frequencies decrease in proportion to the hydrogen bond energy.23,33 These results demonstrate that the transparent single phases with W0c e 24 are Winsor-VI W/scCO2 microemulsion phases. In addition, this broad absorption band increased significantly with an increase in W0c, reflecting the swelling of

the reverse micelles. Figure 4b shows the spectra for the transparent scCO2 phase in 2Ø with W0c g 24. These spectra also indicate large and broad bands similar to those at W0c e 24 and reveal the existence of reversed micelles having an aqueous core in 2Ø, that is, a Winsor-II W/scCO2 microemulsion (W/scCO2 microemulsion having separated excess water). Furthermore, the absorbencies of Winsor-II W/scCO2 microemulsions were almost the same, in spite of different W0c values. This suggests that although W0c increased in this system, (i) the Winsor-II microemulsion continued to exist without the formation of the LC, (ii) the water-to-surfactant molar ratio of reversed micelles encapsulating an aqueous core was constant at 24, and (iii) separated excess water increased by an amount equal to (W0c 24). The formation of the Winsor-II microemulsion phase for a water/scCO2 system is quite rare, whereas for a water/liquid CO2 system, Eastoe et al. have proved the existence of the Winsor-II microemulsion by using a high-pressure small-angle neurtron scattering (SANS) technique.8,35 The appearance of the WinsorII microemulsion instead of the LC as well as the positive synergistic effect on the microemulsifying power indicates that our objectivesstabilization of the 8FS(EO)2 microemulsion by surfactant mixingswas achieved.

Water/Supercritical CO2 Microemulsions

Langmuir, Vol. 24, No. 18, 2008 10121 Table 2. Water/scCO2 Interfacial Properties for 8FS(EO)2, FC6HC4, and Their Equimolar Mixture at 40 °C and 250 bar surfactant system

105 cµc (mol %)a

105Γm (mol/m2)b

Aav (Γ2/molecule)c

8FS(EO)2 FC6-HC4 8FS(EO)2/FC6-HC4 (Rs ) 0.5)

1.9d 7.0d 1.0

1.5d 1.6d 0.5

114d 106d 307

a Critical microemulsion concentration. b Γm, maximum saturation of the surfactant monolayer. c Average area occupied per one surfactant molecule at the cµc with uncertainties (20Γ2. d Given by refs 2 and 4.

Figure 5. Water/scCO2 interfacial tension versus total surfactant mole fraction for pure surfactants of 8FS(EO)2 and FC6-HC4, and their equimolar mixture at 40 °C and 250 bar. The arrows indicate critical microemulsion concentrations (cµc’s).

3.5. Interfacial Tension for 8FS(EO)2/FC6-HCn Mixture. To elucidate the effect of FC6-HC4 mixing on the molecular packing of 8FS(EO)2 at the water/CO2 interface, water/scCO2 IFTs were measured for the mixed surfactant systems. Figure 5 shows the IFT for the equimolar 8FS(EO)2/FC6-HC4 mixture, as compared to the IFTs for pure constituents reported previously.2,4 The IFT is plotted against the total mole fraction of the surfactants (i.e., the molar surfactant-to-CO2 ratio) on a logarithmic scale. The IFT was a decreasing function of the surfactant mole fraction. A comparison between FC6-HC4, 8FS(EO)2, and their equimolar mixture clearly revealed that the interfacial activity was enhanced by surfactant mixing; for example, the surfactant concentrations at which IFT became 6 mN/m were in the order of FC6-HC4 > 8FS(EO)2 > equimolar mixture. Such a positive synergistic effect of surfactant mixing on the interfacial activity was observed by using not only the FC guest surfactant but also the HC guest surfactant in our previous study.6 Each interfacial tension curve exhibits a kink. The arrows in Figure 5 indicate the presence of a kink where the concentration dependence of the IFT changes drastically. The kink represents the critical microemulsion concentration (cµc), where the surfactant starts to form a reverse micelle with an encapsulated aqueous core.1-6 We further calculated the average area, Aav, occupied by one surfactant molecule at the cµc. The Aav value is given by

Aav )

1 NAΓm

(2)

where NA is the Avogadro constant and Γm is the surface excess per unit area of the interface separating water and scCO2 phases. If the interface and bulk phases are in (thermodynamic) equilibrium and the thermodynamic properties of the dilute solutions are assumed to be ideal, Γm can be obtained by the Gibbs adsorption equation

Γm ) -

dγ 1 2.303RT d log c

(

)

(3)

where R is the gas constant, T is the temperature, and c is the concentration of the surfactant. The value of dγ/d log c at the cµc was obtained from the slope of the plot in Figure 5 on the low-

Figure 6. Schematic representation of steric models of nFS(EO)2 and FC6-HC4 and their packing models at water/scCO2 interface.

concentration side. In the calculation, the dissociation of the counterions of the surfactant was assumed to be negligible due to the low dielectric constant of scCO2. Using eqs 2 and 3, Γm and Aav were calculated. These values are listed in Table 2, along with cµc’s. For pure surfactant systems, the values measured in our earlier study2,4 are also shown in the table. The cµc value for the surfactant mixture was lower than that for pure surfactants. This indicates low 8FS(EO)2-FC6-HC4 segregation at the interface for the diluted surfactant solution of 0, although the mixtures did not form a LC-like precipitate. The maximum W0c values of most of the mixed FC guest surfactant/8FS(EO)2 systems were larger than those of mixed HC guest surfactant/8FS(EO)2 systems and did not form LC-like precipitates. By mixing the anionic hybrid surfactant FC6-HC4 with 8FS(EO)2, interestingly, the maximum W0c value exceeded (60) Kunitake, T.; Higashi, N. J. Am. Chem. Soc. 1985, 107, 692. (61) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Biochim. Biophys. Acta 1977, 470, 185. (62) Cullis, P. R.; Kruijff, B. D. Biochim. Biophys. Acta 1979, 559, 399. (63) Israelachvili, J. N. Chem. Scr. 1985, 25, 7.

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the value estimated from the results of pure systems, thereby clearly indicating the positive synergistic effect of surfactant mixing on the microemulsifying power. Further investigation of the mixed FC6-HC4/8FS(EO)2 system was performed with FT-IR spectroscopic and IFT measurements. FT-IR spectroscopic measurements revealed that the mixed reverse micelles in VIµE were swollen by water with an increase in W0c, and when W0c approached the maximum value, VIµE was transformed into the Winsor-II microemulsion without forming a LC. This indicates that the mixing of FC6-HC4 with 8FS(EO)2 induced a phase transition from the LC to the WinsorII microemulsion. From IFT measurements, the mixing of 8FS(EO)2 and FC6-HC4 was found to increase Aav and the interfacial activity compared with their pure surfactant systems. In particular, the effect of surfactant mixing on Aav was clear; Aav for the surfactant mixture was 3 times larger than that for pure surfactant systems. By examining the surfactant steric structures and their molecular packing at the W/scCO2 interface, the drastic increase in Aav due to surfactant mixing was attributed to high microsegregation with FC6-HC4 molecules, which caused an increase in the critical packing parameter and loosening of the molecular packing. The increased critical packing parameter and loose molecular packing can play an important role in stabilizing the microemulsion rather than the LC. By mixing the guest surfactant, FC6-HC4, this study successfully transformed the VIµE to a Winsor-II microemulsion without forming a LC for the mixed 8FS(EO)2/water/CO2 system. The positive synergistic effect on the microemulsifying power was also revealed. Unfortunately, the maximum W0c value of VIµE in this study was less than the largest value ever reported.1-6 For green technologies used in dry cleaning, nanomaterial synthesis, dyeing, biomaterial extraction, and specialized reactions, it is necessary to produce a good microemulsifer for the W/scCO2 system. As shown in this study, surfactant mixing could be one method to produce a highly efficient microemulsifer, while proper molecular design of a novel surfactant could be another method. Further studies of surfactant mixing and surfactant molecular design could help in the preparation of novel assemblies in scCO2. Acknowledgment. This work was supported by the TEPCO Research Foundation, the SEPT Research Foundation, the Foundation of the Promotion of International Scientific Research, a Grant for Priority Research Designated by the President of Hirosaki University, and the Foundation of the Association for the Progress of New Chemistry. LA8014145