Optimum Tail Length of Fluorinated Double-Tail Anionic Surfactant for

Jul 18, 2007 - Self-organization Behavior of Surfactants in Supercritical Carbon Dioxide. Masanobu SAGISAKA , Atsushi YOSHIZAWA , Katsuto OTAKE...
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Langmuir 2007, 23, 8784-8788

Optimum Tail Length of Fluorinated Double-Tail Anionic Surfactant for Water/Supercritical CO2 Microemulsion Formation Masanobu Sagisaka,*,† Daisuke Koike,‡ Satoshi Yoda,§ Yoshihiro Takebayashi,§ Takeshi Furuya,§ Atsushi Yoshizawa,† Hideki Sakai,‡,| Masahiko Abe,‡,| and Katsuto Otake⊥ Department of Materials Science and Technology, Faculty of Science and Technology, Hirosaki UniVersity, Bunkyo-cho 3, 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 February 26, 2007. In Final Form: May 16, 2007 The effect of surfactant tail structure on the stability of a water/supercritical CO2 microemulsion (W/scCO2 µE) was examined for various fluorinated double-tail anionic surfactants of different fluorocarbon chain lengths, F(CF2)n (n ) 4, 6, 8, and 10), and oxyethylene spacer lengths, (CH2CH2O)m/2 (m ) 2 and 4). The phase behavior of the water/surfactant/CO2 systems was studied over a wide range of CO2 densities from 0.70 to 0.85 g/cm3 (temperatures from 35 to 75 °C and pressures up to 500 bar) and corrected water-to-surfactant molar ratios (W0c). All of the surfactants yielded a W/scCO2 µE phase, that is, a transparent homogeneous phase with a water content larger than that permitted by the solubility of water in pure CO2. With increasing W0c, a phase transition occurred from the µE phase to a macroemulsion or a lamella-like liquid crystal phase. The maximum W0c value was obtained at a tail length of 1214 Å, indicating the presence of an optimum surfactant tail length for W/scCO2 µE formation.

1. Introduction Carbon dioxide is a nontoxic, inflammable, inexpensive, environmentally benign, and abundant solvent. Under the supercritical condition, these advantages are combined with the unique properties of a supercritical fluid such as adjustable solvent power, enhanced mass transfer characteristics, and low surface tension.1 Supercritical carbon dioxide (scCO2) therefore has received much attention as an alternative to toxic organic solvents, prompting extensive research into the development of scCO2based processes.2-6 Unfortunately, because scCO2 is nonpolar and has weak van der Waals forces,7 it is not suitable for dissolving polar substances. This disadvantage has limited the application of scCO2 to chemical processes such as separation, reaction, and material formation.7-9 One of the most promising approaches for enhancing the solubility is to use reversed micelles with high-density aqueous cores in the continuous scCO2 phase, in other words, a water-in-scCO2 microemulsion (W/scCO2 µE). Since such an organized fluid has the attractive characteristics of scCO2 as well as the solvating * To whom all correspondence should be addressed. Telephone and fax: +81-172-39-3569. E-mail: [email protected]. † 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) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. AIChE J. 1995, 41, 1723. (2) Desimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945. (3) Dixon, D. J.; Bodmeier, R. A.; Johnston, K. P. AIChE J. 1993, 39, 127. (4) Ikushima, Y. AdV. Colloid Interface Sci. 1994, 265, 356. (5) Adamsky, F. A.; Beckman, E. J. Macromolecules 1994, 27, 312. (6) Jessop, P. G.; Ikariya, T.; Noyori, R. Nature 1994, 368, 231. (7) O’ Shea, K. E.; Kirmse, K. M.; Fox, M. A.; Johnston, K. P. J. Phys. Chem. 1991, 95, 7863. (8) Beckman, E. J. Science 1996, 271, 613. (9) Leitner, W. Nature 2000, 405, 129.

properties of bulk water, it is expected to behave as a “universal solvent”.10-16 A number of studies have been directed toward the development of a surfactant system that stabilizes W/scCO2 µE formation.10-51 We have recently shown that a fluorinated analogue of aerosol(10) Sagisaka, M.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kitiyanan, B.; Kondo, Y.; Yoshino, N.; Takebayashi, K.; Sakai, H.; Abe, M. Langmuir 2003, 19, 220. (11) Sagisaka, M.; Ozaki, Y.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M. Mater. Technol. 2003, 21, 36. (12) Sagisaka, M.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M. Langmuir 2003, 19, 8161. (13) Sagisaka, M.; Fujii, T.; Ozaki, Y.; Yoda, S.; Takebayashi, Y.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M.; Otake, K. Langmuir 2004, 20, 2560. (14) Sagisaka, M.; Fujii, T.; Koike, D.; Yoda, S.; Takebayashi, Y.; Furuya, T.; Yoshizawa, A.; Sakai, H.; Abe, M.; Otake, K. Langmuir 2007, 23, 2369. (15) Eastoe, J.; Bayazit, Z.; Martel, S.; Steytler, D. C.; Heenan, R. K. Langmuir 1996, 12, 1423. (16) 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. (17) Ritter, J. M.; Paulaitis, M. E. Langmuir 1990, 6, 935. (18) Yee, G. G.; Fulton, J. L.; Smith, R. D. Langmuir 1992, 8, 377. (19) Hutton, B. H.; Perera, J. M.; Grieser, F.; Stevens, G. W. Colloids Surf., A 1999, 146, 227. (20) Hutton, B. H.; Perera, J. M.; Grieser, F.; Stevens, G. W. Colloids Surf., A 2001, 189, 177. (21) 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. (22) Liu, J.; Han, B.; Li, G.; Zhang, X.; He, J.; Liu, Z. Langmuir 2001, 17, 8040. (23) Liu, J.; Han, B.; Wang, Z.; Zhang, J.; Li, G.; Yang, G. Langmuir 2002, 18, 3086. (24) Liu, J.; Han, B.; Zhang, J.; Li, G.; Zhang, X.; Wang, J.; Dong, B. Chem.s Eur. J. 2002, 8, 1356. (25) Liu, J.; Zhang, J.; Mu, T.; Han, B.; Li, G.; Wang, J.; Dong, B. J. Supercrit. Fluids 2003, 26, 275. (26) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51. (27) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127. (28) 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. (29) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. Langmuir 1997, 13, 3934. (30) 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.

10.1021/la700564z CCC: $37.00 © 2007 American Chemical Society Published on Web 07/18/2007

Efficient Surfactant for W/scCO2 µE Formation

OT surfactant, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)2-sulfosuccinate (8FS(EO)2), is the most effective formulation for stabilizing a W/scCO2 µE, where the molar water-to-surfactant ratio W0c reaches a maximum value of 32.10-14 It was also found, however, that 8FS(EO)2 forms a liquid crystal (LC)-like precipitate at values of W0c larger than 32. A promising strategy to prevent LC formation is by adding structural disorder into the system,52,53 for example, introducing flexible spacers as well as branched structures into the surfactant tail. As pointed out in our earlier paper,12 increasing fluorocarbon chain length enhances the CO-philicity and the hydrophobicity. The enhancement of these characteristics often results in the surfactant’s microemulsifying power in scCO2 being increased.12 In the present work, with the aim of examining a suitable tail structure for µE formation, we synthesized a number of 8FS(EO)2-analogue surfactants with different fluorocarbon chain lengths and/or flexible oxyethylene spacers, and examined the phase behavior in W/scCO2 mixtures with the surfactants through visual and microscopic observation. The microemulsifying power of surfactants in CO2 was evaluated with the maximum W0c of the W/scCO2 µE.

Langmuir, Vol. 23, No. 17, 2007 8785 Table 1. Molecular Structures of nFS(EO)m and Their Properties in Water54-57

a Critical micelle concentration (CMC) in water at 30 °C. b Surface tension at CMC. c At 75 °C.

2. Experimental Section 2.1. Materials. The surfactants used in this work were sodium bis(1H,1H,2H,2H-tridecafluorodecyl)-2-sulfosuccinate (6FS(EO)2), sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2), sodium bis(1H,1H,2H,2H-henicosafluorododecyl)-2sulfosuccinate (10FS(EO)2), sodium bis((1H,1H,2H,2H-nonafluorohexyl)-(oxyethylene))-2-sulfosuccinate (4FS(EO)4), sodium bis((1H,1H,2H,2H-tridecafluorooctyl)-(oxyethylene))-2sulfosuccinate (6FS(EO)4), and sodium bis((1H,1H,2H,2Hheptadecafluorodecyl)-(oxyethylene))-2-sulfosuccinate (8FS(EO)4). These surfactants were synthesized in our laboratory and purified to >99%, as described previously.54-57 Different n and m series of (31) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 119, 6399. (32) Niemeyer, E. D.; Bright, F. V. J. Phys. Chem. B 1998, 102, 1474. (33) daRocha, S. R. P.; Harrison, K. L.; Johnston, K. P. Langmuir 1999, 15, 419. (34) daRocha, S. R. P.; Johnston, K. P. Langmuir 2000, 16, 3690. (35) Pandey, S.; Baker, G. A.; Kane, M. A.; Bonzagni, N. J.; Bright, F. V. Langmuir 2000, 16, 5593. (36) Ohde, H.; Hunt, F.; Kihara, S.; Wai, C. M. Anal. Chem. 2000, 72, 4738. (37) Lee, C. T., Jr.; Bhargava, P.; Johnston, K. P. J. Phys. Chem. B 2000, 104, 4448. (38) Lee, D.; Hutchison, J. C.; DeSimone, J. M.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 8406. (39) Fremgen, D. E.; Smotkin, E. S.; Gerald, R. E.; Klingler, R. J.; Rathke, J. W. J. Supercrit. Fluids 2001, 19, 287. (40) Meziani, M. J.; Sun, Y.-P. Langmuir 2002, 18, 3787. (41) Loeker, F.; Marr, P. C.; Howdle, S. M. Colloids Surf., A 2003, 214, 143. (42) Eastoe, J.; Downer, A. M.; Paul, A.; Steytler, D. C.; Rumsey, E. Prog. Colloid Polym. Sci. 2000, 115, 214. (43) Eastoe, J.; Downer, A.; Paul, A.; Steytler, D. C.; Rumsey, E.; Penfold, J.; Heenan, R. K. Phys. Chem. Chem. Phys. 2000, 2, 5235. (44) Liu, Z.-T.; Erkey, C. Langmuir 2001, 17, 274. (45) Steytler, D. C.; Rumsey, E.; Thorpe, M.; Eastoe, J.; Paul, A.; Heenan, R. K. Langmuir 2001, 17, 7948. (46) Eastoe, J.; Paul, A.; Downer, A.; Steytler, D. C.; Rumsey, E. Langmuir 2002, 18, 3014. (47) Dong, X.; Erkey, C.; Dai, H.-J.; Li, H.-C.; Cochran, H. D.; Lin, J. S. Ind. Eng. Chem. Res. 2002, 41, 1038. (48) Senapati, S.; Keiper, J. S.; DeSimone, J. M.; Wignall, G. D.; Melnichenko, Y. B.; Frielinghaus, H.; Berkowitz, M. L. Langmuir 2002, 18, 7371. (49) Harrison, K.; Goveas, J.; Johnston, K. P.; O’ Rear, E. A. Langmuir 1994, 10, 3536. (50) O’ Neill, M. L.; Cao, Q.; Fang, M.; Johnston, K. P.; Wilkinson, S. P.; Smith, C. D.; Kerschner, J. L.; Jureller, S. H. Ind. Eng. Chem. Res. 1998, 37, 3067. (51) Ryoo, W.; Webber, S. E.; Johnston, K. P. Ind Eng. Chem. Res. 2003, 42, 6348. (52) Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2000, 16, 8741. (53) Eastoe, J.; Hetherington, K. J.; Sharpe, D.; Dong, J.; Heenan, R. K.; Steytler, D. C. Colloids Surf., A 1997, 128, 209. (54) Yoshino, N.; Komine, N.; Suzuki, J.; Arima, Y.; Hirai, H. Bull. Chem. Soc. Jpn. 1991, 64, 3262.

Figure 1. Schematic representation of the experimental apparatus. nFS(EO)m (n ) 4, 6, 8, or 10 and m ) 2 or 4) were used to examine the effects of CO2-philic fluorocarbon chain lengths and oxyethylene spacers situated between the hydrophilic head group and the CO2philic fluorocarbon chains, respectively. The molecular structures of nFS(EO)m and their properties in water are summarized in Table 1.54-57 Ultrapure water of 18.2 MΩ cm resistivity was taken from a Millipore Milli-Q Plus system and used for the experiments. CO2 of 99.99% purity (Ekika Carbon Dioxide Co. Ltd.) and methyl orange (Acros Organics) were used without further treatment. 2.2. Phase Behavior Measurement. The phase behavior of W/scCO2 mixtures each containing one of the surfactants was examined by visual and optical microscopic observations. The experimental apparatus is shown in Figure 1. A high-pressure vessel with an optical window and a moving piston inside was used to observe phase changes in the mixture at varying pressures and temperatures without changing the composition of the mixture. A six-port valve (Valco Instruments Co. Ltd.) with a sample loop (25 µL, Valco Instruments Co. Ltd.) and a circulation pump (Nihon Seimitsu Kagaku, NP-S-321) was attached to the optical cell to enable a certain amount of water to be introduced into the system. The phase behavior observations were conducted at densities ranging from 0.70 to 0.85 g/cm3 and W0c values ranging from 0 to (55) Yoshino, N.; Morita, M.; Ito, A.; Abe, M. J. Fluorine Chem. 1995, 70, 187. (56) Abe, M.; Kondo, Y.; Sagisaka, M.; Hideki, S.; Morita, Y.; Kaise, T.; Yoshino, N. J. Jpn. Soc. Colour Mater. 2000, 73, 53. (57) Sagisaka, M.; Ito, A.; Kondo, Y.; Yoshino, N.; Kwon, K. O.; Sakai, H.; Abe, M. Colloids Surf., A 2001, 185, 749.

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35. The W0c parameter is generally used to express a molar waterto-surfactant ratio in a reversed micelle in scCO2, where the solubility of water in scCO2 is taken into account as1-5 W0c )

[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 surfactant in the system. Predetermined amounts of 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 microemulsion (µE) solution became a turbid macroemulsion (E) solution or a precipitated hydrated surfactant (P). Aqueous 0.1 wt % methyl orange solution was often used as loaded water for confirming µE formation. The W0c value at the phase transition was determined at each temperature and CO2 density. The densities of CO2 were calculated using an accurate correlation equation.58 The [water]S value was obtained from the literature.59 Detailed descriptions of the experimental apparatus and procedures of the measurements are given in our previous reports.10-14 To examine what the P-phase is, microscopic observation for the 10FS(EO)2/water/CO2 mixture was conducted with an optical microscope (Olympus, BX-51) and a sapphire window cell (Kouatsukiki; thickness: 13 mm) connected to the experimental apparatus. The sapphire window cell was made in stainless steel (SUS316) with two 5 mm thick sapphire windows. Each window was 3 mm (inside diameter), and they were positioned to provide a perpendicular 3 mm optical path. Each window was sealed against the stainless steel body of the cell by using an O-ring. Tight screwing of windows compressed the O-rings between the stainless parts and the sapphire window and provided an excellent seal. This seal was tested up to 400 bar. The sapphire window cell was temperature-controlled with cartridge heaters. By polarized microscopic observation, textures obtained from the sapphire window cell were compared with those from a standard glass cell using an aqueous lamella liquid crystal solution at ambient pressure, and a good agreement was confirmed between them. For microscopic observations, the mixture was circulated between the optical vessel and the sapphire window cell until the surfactant precipitates of the P-phase were observed via microscopy. The circulation was then stopped, and microscopic observation was carried out.

3. Results and Discussion 3.1. Phase Behavior of Water/nFS(EO)m/ScCO2 Mixtures. The phase behavior of ternary mixtures of water/nFS(EO)m/ scCO2 was monitored by visual observation, and representative images of the phases observed in this study are shown in Figure 2. These photographs show (i) a transparent W/scCO2 microemulsion (µE) with W0c > 0, (ii) a turbid W/scCO2 macroemulsion (E), which was made to suddenly appear by lowering the pressure from that of the µE phase to that of the E phase across the µE-E phase boundary, (iii) two phases, that is, a separated water phase and a neat scCO2 phase, which appeared after the E phase was maintained for several minutes or hours without stirring, and (iv) a precipitated phase (P) containing hydrated nFS(EO)m particles, which was often observed at water loading larger than nFS(EO)m can microemulsify. For an additional demonstration of the formation of a µE, the existence of aqueous domains in the µE phase was revealed by the use of a dye, methyl orange (MO). MO does not dissolve in pure CO2, but it dissolves in water and is incorporated within (58) Span, R.; Wagner, W. J. Phys. Chem. Ref. Data 1996, 25, 1509. (59) Wiebe, R. Chem. ReV. 1941, 29, 475.

Figure 2. Photographs showing the appearance of typical phases of ternary water/nFS(EO)m /scCO2 mixtures.

Figure 3. Photographs of scCO2 mixtures including (a) a separated aqueous MO solution, (b) 6FS(EO)2, (c) 6FS(EO)2 + aqueous MO solution with W0c ) -6, and (d) 6FS(EO)2 + aqueous MO solution with W0c ) 15. The photographs were taken at 300 bar and 75 °C. The MO concentration in water was 0.1 wt %. The mole fraction of 6FS(EO)2-to-CO2 was 0.08 mol % for (b-d).

the water-rich pockets of the µE while dying the solution red.28 Figure 3a shows a photograph of a separated aqueous MO solution in scCO2 without any surfactants. The upper scCO2 phase was colorless, and it demonstrated that MO did not disperse in pure scCO2. Figure 3b shows a photograph of a colorless 6FS(EO)2/ scCO2 solution with MO under dry conditions. Figure 3c and d show photographs of 6FS(EO)2/scCO2 solutions with aqueous MO solutions of W0c ) -6 and 15, respectively. A negative W0c value means that the added amount of water was lower than that required to reach the saturation solubility of water in CO2 under the experimental conditions. The photographs shown in Figure 3c and d show the dyed scCO2 phases with MO, which confirm the formation of the reversed micelle solubilizing aqueous MO solution. The existence of a µE at a water concentration lower than that corresponding to the saturation solubility of water in pure CO2 is considered to be the result of partition equilibrium of water molecules between the CO2 phase and the hydrophilic µE core. The µE became reddish with an increasing W0c of added aqueous MO solution, representing the swelling of the µE with the solution as reported in our earlier paper.10 To confirm whether LCs exist in the P-phase, optical microscopic observation with two polarizers was carried out for the precipitate in the water/scCO2/10FS(EO)2 mixture. Figure 4 shows a typical micrograph of the 10FS(EO)2 precipitates under

Efficient Surfactant for W/scCO2 µE Formation

Figure 4. Polarized micrograph of precipitates in the 10FS(EO)2/ water/scCO2 mixture.

crossed nicols. As shown in Figure 4, anisotropic particles with sizes of several micrometers were observed, and they were identified as LC particles. The LC particles were also found to have a large dark cross part in the interiors thereof by microscopic observation, representing a kind of lamella LC.60,61 3.2. Effects of Fluorocarbon Chain Length and Flexible Oxyethylene Spacers on W/scCO2 µE Formation. Figure 5 shows W0c-temperature phase diagrams for the W/scCO2 mixtures with nFS(EO)m at various CO2 densities. As mentioned above, E, µE, and P indicate the macroemulsion phase, the µE phase, and the precipitation of hydrated surfactant, respectively.

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In recent works,10-51 the fluorocarbon chain length of surfactants that form the W/scCO2 type microemulsion has been shown to play an important role in the lowering of the molecular hydrophilic/CO2-philic balance (HCB)51 for stabilizing a µE. In our earlier work with an nFS(EO)m analogue surfactant, di-HCFn (Rf ) H(CF2)nCH2-), the maximum W0c of the µE increased with increasing n, and this trend was found to hold for fluorocarbon chain length n in the range from 4 to 8. In this study, we synthesized 6FS(EO)2 and 10FS(EO)2 and examined the relationship between the chain length n of nFS(EO)2 and the maximum W0c of the µE. Although 8FS(EO)2 has a larger maximum W0c than 6FS(EO)2, that of 10FS(EO)2 was lower than those of 8FS(EO)2 and 6FS(EO)2. The suitable fluorocarbon chain length for a µE was n ) 8 in the nFS(EO)2 series. On the other hand, n ) 6 was the optimum fluorocarbon length in nFS(EO)4 of n ) 4, 6, and 8. To examine the effect of surfactant tail length n on the microemulsion formation capability of nFS(EO)m, the values of maximum W0c are shown as a function of tail (Rf) length in Figure 6. The Rf lengths of the surfactants were obtained by MM2 (Molecular Mechanics program 2) calculation, and they are listed in Table 1. In each series of nFS(EO)2 and nFS(EO)4, the maximum W0c values peaked at an Rf length of 12-14 Å,

Figure 5. W0c-temperature-CO2 density phase diagram of water/nFS(EO)m/scCO2 systems. E, µE, and P denote the macroemulsion phase, the µE phase, and the precipitation of hydrated surfactant, respectively. The molar ratio of the surfactant to CO2 was fixed at 0.08 mol %.

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In the majority of the nFS(EO)m series, LCs formed with too much loading of water to allow formation of the µE phase. LC formation, however, did not occur in 4FS(EO)4, according to the phase diagram thereof. The flexible oxyethylene spacers in nFS(EO)4 were expected to destabilize the formation of LCs by increasing the molecular motion, and actually they succeeded in preventing the formation of LCs in 4FS(EO)4. On the other hand, if “linear” and “rigid” fluorocarbon chains of nFS(EO)4 become longer than n ) 4, the increased L/B ratio and molecular rigidity would diminish the effect of oxyethylene spacers on LC destabilization and allow LCs to form in 6FS(EO)4 and 8FS(EO)4.

4. Conclusions

Figure 6. Relationship between tail (Rf) length and maximum W0c for W/scCO2 µEs at 75 °C for the nFS(EO)2 and nFS(EO)4 series.

which suggests the existence of an optimum Rf length for the nFS(EO)m series. LC stability has been known to depend on the length-to-breadth ratio (L/B ratio) of molecules, which represents the easiness with which the molecules pack into an LC structure.62 Namely, the larger the L/B ratio is, the higher the stability of the LC structure is. If hydrophilic groups are the same like in an nFS(EO)m series, a longer tail length results in an increased L/B ratio and allows the molecules to pack easily into an LC structure. In studies63,64 of LCs, L/B ratio stabilizing LC formation was considered to be >3. For 8FS(EO)2 and 10FS(EO)2, their L/B ratios calculated by MM2 simulation were approximately 2.5 and 2.8, respectively, and the later was found to be more suitable for LC formation. The L/B ratio, therefore, is expected to be an important parameter which controls (or influences, decides) the stability of the microemulsion, in addition to the critical packing parameter (CPP)65-67 and HCB. In light of this information, an optimum CO2-philic chain length should exist for W/scCO2 µE formation. The optimum length, 12-14 Å, was expected to be the length at which the effect of Rf length on µE stability became equal to the effect of Rf length on LC stability. Thus, if the number of oxyethylene units of 8FS(EO)2 or the fluorocarbon chain length of 6FS(EO)4 were increased, the Rf length would be far from optimum. It results in LC formation stabilizing with a higher L/B ratio than that in the µE phase, and it also results in a lowered microemulsifying power through the µE phase being restricted in the W0c range by encroachment of the LC phase. On the other hand, if the Rf length was increased from being shorter to the optimum length, it would result in a more stabilized µE phase with the formation of LCs or W/scCO2 Es being suppressed by causing the formation of a thicker reversed micelle shell which is resistant to fusion. (60) Nakanishi, M.; Tsuchiya, K.; Ohkubo, T.; Sakai, H.; Abe, M. J. Oleo Sci. 2005, 54, 443. (61) Kaneko, T.; Yamaoka, Y.; Kaise, C.; Orita, M.; Sakai, H.; Abe, M. J. Oleo Sci. 2005, 54, 325. (62) Demus, H. Handbook of Liquid Crystal; Wiley-VCH: New York, 1998. (63) Longa, L.; Cholewiak, G.; Stelzer, J. Acta Phys. Pol., B 2000, 31, 801. (64) de Miguel, E.; Rull, L. F.; Chalam, M. K.; Gubbins, K. E. Mol. Phys. 1991, 74, 405. (65) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Biochim. Biophys. Acta 1977, 470, 185. (66) Cullis, P. R.; Kruijff, B. D. Biochim. Biophys. Acta 1979, 559, 399. (67) Israelachvili, J. N. Chem. Scr. 1985, 25, 7.

To develop an efficient surfactant for W/scCO2 µE formation, we synthesized a number of fluorinated double-tail anionic surfactants using 8FS(EO)2 and examined their microemulsifying power in a water/scCO2 system, that is, the maximum number of water molecules that can be microemulsified by one surfactant molecule, W0c. Special efforts were made to reveal the effects of surfactant tail structure, particularly those of fluorocarbon chain and oxyethylene spacer length, on W/scCO2 µE formation. Longer fluorocarbons and oxyethylene spacers were expected to stabilize the µE while lowering the hydrophilic/CO2-philic balance and increasing the molecular flexibility, respectively. The surfactants yielded W/scCO2 µEs with several different values of W0c but also resulted in the formation of lamella-like liquid crystals with increasing water content. The microemulsifyng power reached a maximum at a tail length of 12-14 Å, which was identified as the optimum chain length for µE formation. Increases in fluorocarbon chain length and oxyethylene spacers enhanced the microemulsifying power until the tail length became smaller than the optimum length. However, at lengths greater than the optimum length, LC formation dominated the phase due to an excessively high length-to-breadth ratio of the surfactant molecule. In our earlier papers, three important requirements were mentioned for designing a suitable surfactant for preparing W/scCO2 µEs: (1) the surfactant must be highly adsorbable onto the water/scCO2 interface; (2) there must be a weak attractive but strong steric repulsive interaction between the hydrophobic (or CO2-philic) groups which prevents reversed micelles from aggregating and fusing; and (3) the surfactant must be capable of markedly lowering water/scCO2 interfacial tension. Therefore, the CO2-philic tail should possess not only strong CO2-philicity to form a shell that is sufficiently thick to resist fusion and aggregation of droplets but also strong hydrophobicity to reduce the solubility of the surfactant in water. In this study, the optimum tail length for yielding a µE was confirmed and added to the requirements for designing the CO2-philic tail. 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, the Fund of the Promotion of International Scientific Research, and Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. LA700564Z