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Articles Preparation of a W/scCO2 Microemulsion Using Fluorinated Surfactants Masanobu Sagisaka,† Satoshi Yoda,‡ Yoshihiro Takebayashi,‡ Katsuto Otake,*,‡ Boonyarach Kitiyanan,§ Yukishige Kondo,|,⊥ Norio Yoshino,|,⊥ Kei Takebayashi,| Hideki Sakai,†,| and Masahiko Abe†,| Faculty of Science and Technology, Tokyo University of Science, Yamazaki 2641, Noda, Chiba 278-8510, Japan; National Institute of Advanced Industrial Science and Technology, Institute for Green Technology, Higashi 1-1, Tsukuba, Ibaragi 305-8565, Japan; The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Rd, Bangkok 10330, Thailand; Institute of Colloid and Interface Science, Tokyo University of Science, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-0825, Japan; and Faculty of Engineering, Tokyo University of Science, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-0825, Japan Received April 10, 2002. In Final Form: October 29, 2002 Formation of water (W) in supercritical carbon dioxide (scCO2) (W/scCO2) type microemulsions was examined using four hybrid surfactants, the sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-alkanesulfonates (FC6-HCn, n ) 2, 4, 6, and 8), which have a hydrocarbon chain of different length and a fluorocarbon chain in one molecule and an Aerosol-OT (AOT) analogue fluorinated twin tail type surfactant, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2). For comparison AOT was also used. The hybrid type surfactants (FC6-HCn) gave a transparent single phase, identified as a W/scCO2 microemulsion, with a water-to-surfactant molar ratio, W0c < 7, irrespective of hydrocarbon chain length. The fluorinated AOT analogue also yielded a transparent single phase, again identified as a W/scCO2 microemulsion, with a W0c value close to 32sone of the highest ever reported. The aqueous core in the 8FS(EO)2 reversed micelle was examined by FT-IR spectra using D2O. The spectra revealed that the aqueous core swells on addition of water and shrinks with increase in pressure. The remarkable ability of 8FS(EO)2 to form a W/scCO2 microemulsion would be brought about by its high adsorption capacity and its excellent facility to lower the water/scCO2 interfacial tension, in addition to a low interaction and strong steric repulsion between its CO2-philic groups.
1. Introduction As an alternative to toxic organic solvents, supercritical carbon dioxide (scCO2) has attracted much attention over the past decade, since it is nontoxic, inflammable, environmentally friendly, low cost, and readily available in large quantities.1,2 With these advantages, together with its unique properties as a supercritical fluid such as adjustable solvent power, enhanced mass transfer characteristics, and low surface tension, scCO2 has prompted extensive research to develop scCO2-based processes.3-7 Unfortunately, be* To whom all correspondence should be addressed. E-mail:
[email protected]. Phone and fax: +81-298-61-4567. † Faculty of Science and Technology, Tokyo University of Science. ‡ National Institute of Advanced Industrial Science and Technology, Institute for Green Technology. § Chulalongkorn University. | Institute of Colloid and Interface Science, Tokyo University of Science. ⊥ Faculty of Engineering, Tokyo University of Science. (1) McHugh, M. A.; Krukonis, V. J. Supercritical Fluids Extraction Principles and Practice, 2nd ed.; Butterworth-Heinemann: Stoneham, MA, 1993. (2) Shaffer, K. A.; DeSimone, J. M. Trends Polym. Sci. 1995, 3, 146. (3) Desimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945. (4) Dixon, D. J.; Bodmeier, R. A.; Johnston, K. P. AlChE J. 1993, 39, 127. (5) Ikushima, Y. Adv. Colloid Interface Sci. 1994, 265, 356.
cause CO2 is nonpolar and has weak van der Waals forces, it is not suitable for dissolving polar substances; this fact has limited its application to processes such as separation, reaction, and material formation. One approach to overcoming this limitation is to employ specialized scCO2 soluble surfactants that induce formation of reversed micelles with high-density aqueous cores in the continuous scCO2 phase, that is, water-in-scCO2 (W/scCO2) microemulsions. Since such systems would combine the attractive characteristics of scCO2 with the solvating properties of bulk water, they are expected to be a “universal solvent”.8 Several recent reports have aimed at identifying and/or designing CO2-soluble surfactants capable of forming W/scCO2 microemulsions.9-19 While, (6) Adamsky, F. A.; Beckman, E. J. Macromolecules 1994, 27, 312. (7) Jessop, P. G.; Ikariya, T.; Noyori, R. Nature 1994, 368, 231. (8) Goetheer, E. L. V.; Vortaman, M. A. G.; Keurentjes, J. T. F. Chem. Eng. Sci. 1999, 54, 1589. (9) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51. (10) Beckman, E. J.; Hoefling, T. A.; Enick, R. M. J. Phys. Chem. 1991, 95, 7127. (11) McFann, G.; Johnston, K. P.; Howdle, S. M. AlChE J. 1994, 40, 543. (12) Ritter, J. M.; Paulaitis, M. E. Langmuir 1990, 6, 935. (13) Harrison, K.; Goveas, J.; Johnston, K. P.; O’Rear, E. A. Langmuir 1994, 10, 3536. (14) 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.
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Table 1. Properties of Aqueous Solutions of FC6-HCn and 8FS(EO)2 at 25 °Ca
a
Data taken from refs 21-27, 31, and 32. b At air/aqueous solution interface at cmc. c At 50 °C.
in the early 1990s, more than 150 surfactants were systematically examined, none was able to take up more than a few molecules of water (∼3) per surfactant molecule, that is, a water-to-surfactant molar ratio (W0) > 3.9-12 Recently, it has been shown that several fluorinated surfactants dissolve in CO2 and have a high activity at the water/CO2 interface, suggesting the feasibility of forming W/scCO2 microemulsions.13-19 Among others, two surfactants described in this work are noteworthy for generating microemulsions having large amounts of water in their cores. One is a hybrid type surfactant, sodium 1-pentadecafluoroheptyl-1-octanesulfate (F7H7), that has both a hydrocarbon and a fluorocarbon chain in one molecule. It is able to microemulsify up to its own weight of water to form a stable W/scCO2 microemulsion.13 The other is a fluorinated Aerosol-OT (AOT) analogue surfactant, sodium bis(2,2,3,3,4,4,5,5-octafluoro-1-pentanol)sulfosuccinate (di-HCF4), that has two fluorocarbon chains. Eastoe et al. and Liu et al. demonstrated that di-HCF4 forms a W/CO2 microemulsion with a W0 value close to 30,18,19 which is extremely large compared with the values for perfluoropolyether (PFPE) surfactants and other fluorinated compounds reported previously.9-17 However, the water content in these systems is still low for industrial scale applications. At the same time, F7H7 has been found to have limited applicability, as it degrades easily at room temperature. In this study, the microemulsion forming abilities of four newly synthesized thermally stable hybrid type surfactants, sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-alkanesulfonates (FC6-HCn; n ) hydrocarbon chain length) and an AOT analogue type surfactant, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2), were examined in terms of the water uptake W0c.20 These substances are known to lower the air/water interfacial tension efficiently21-27 and possess (15) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 119, 6399. (16) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langmuir 1999, 15, 6613. (17) Kane, M. A.; Baker, G. A.; Pandney, S.; Bright, F. V. Langmuir 2000, 16, 4901. (18) 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. (19) Liu, Z.; Erkey, C. Langmuir 2001, 17, 274. (20) Wiebe, R. Chem. Rev. 1941, 29, 475. (21) Ito, A.; Kamogawa, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. J. Jpn. Oil Chem. Soc. 1996, 45, 479. (22) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1996, 12, 5768. (23) Yoshino, N.; Komine, N.; Suzuki, J.; Arima, Y.; Hirai, H. Bull. Chem. Soc. Jpn. 1991, 64, 3262. (24) Kondo, Y.; Yokochi, E.; Mizumura, S.; Yoshino, N. J. Fluorine Chem. 1998, 91, 147.
d
At 73 °C.
suitable steric structures for the formation of reversed micelles.28-30 Further, the fluorocarbon chain is both hydrophobic and CO2-philic, whereas the hydrocarbon chain is hydrophobic but not CO2-philic.9-19 The hydrocarbon chain length of FC6-HCn was altered in these studies with the intention of assessing its effect on W/scCO2 microemulsion formation. AOT was examined for comparison. 2. Experimental Section 2.1. Materials. The four hybrid type surfactants, FC6-HCn (hydrocarbon chain length n ) 2, 4, 6, and 8), and a fluorinated AOT analogue surfactant, 8FS(EO)2, used in this study were synthesized in our laboratory as previously reported.23-25 They were repeatedly purified until their purity became >99%. AOT (Aldrich, purity 98%) was used as a control sample to confirm the effect of substitution of F for H atoms on the formation of a W/scCO2 microemulsion. Table 1 summarizes the properties of the surfactants used in this study.21-27,31,32 Injection grade distilled water (Ohtsuka Pharmaceutical Co., Ltd., pH ) 6.5) and D2O (Wako Pure Chemical Industries Ltd.; purity 99.9%) were used. CO2 of 99.99% purity (Tomoe Shokai Co. Ltd.) was used in all experiments without further treatment. 2.2. Measurements. Formation of a microemulsion was examined by visual observation and FT-IR measurement of mixtures of water, surfactant, and CO2. Figure 1 is a schematic representation of the experimental apparatus. A high-pressure vessel having an optical window and a moving piston inside the vessel was used to observe changes of phase with varying pressure and temperature without changing composition. Surfactants (0.08 mol % in CO2) were sealed in front of the moving piston, and a known amount of CO2 was introduced into the vessel. The cell temperature was then raised to 75 °C, and the pressure was increased to 400 bar by increasing the backside pressure of the moving piston. The mixture was then kept under these conditions with vigorous stirring for 1 day to obtain a clear and transparent single phase. A known amount of water was then added to the mixture through the six-port valve. In this study, a clear mixture with water content larger than the solubility in pure scCO2 is presumed to characterize microemulsion formation. The mixture became turbid when the pressure was decreased, suggesting a transition of microemulsion to macroemulsion. The cloud pressure was determined by changing the pressure at various temperatures. Experiments were conducted at temperatures from 35 to (25) Yoshino, N.; Hamano, K.; Omiya, T.; Kondo, Y.; Ito, A.; Abe, M. Langmuir 1995, 11, 466. (26) Sagisaka, M.; Ito, A.; Kondo, Y.; Yoshino, N.; Kwon, K. O.; Sakai, H.; Abe, M. Colloids Surf., A 2001, 185, 749. (27) Abe, M.; Ito, A.; Yoshihara, K.; Ogino, K.; Yoshino, N. Mater. Technol. 1994, 12, 81. (28) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Biochim. Biophys. Acta 1977, 470, 185. (29) Cullis, P. R.; Kruijff, B. D. Biochim. Biophys. Acta 1979, 559, 399. (30) Israelachvili, J. N. Chem. Scr. 1985, 25, 7.
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Figure 1. Schematic representation of experimental apparatus.
Figure 2. Dissolution pressures for 0.08 mol % FC6-HCn at various temperatures. 75 °C and pressures up to 480 bar. The water uptake W0c, or the solubilized water molecules per one surfactant molecule, was calculated by subtracting the solubility of water in pure scCO2 from the feed composition.20 The existence of a core of bulk water in scCO2 was examined with a high-pressure FT-IR photometer (JASCO Co. Ltd., FT/IR 620) connected to the experimental apparatus with D2O. In FTIR measurements, the mixtures were circulated between the optical vessel and the FT-IR cell until the absorbance of D2O became constant. Then the valves between the vessel and the FT-IR were closed and the FT-IR spectrum was measured. After experiments, to confirm the stability of the surfactants, 1H NMR and 19F NMR spectra of residual matter in the cell were measured with a 400-MHz FT-NMR spectrometer (JEOL, JNMLA400BW). NMR spectra were taken as previously reported.23-25 Optical microscopic observation of the residues was carried out using an inverse type optical microscope (Olympus, Model IMT2) under crossed nicol.
3. Results and Discussion As the initial step in this study, the solubility of FC6HCn in scCO2 in the absence of water (i.e. in dry scCO2) was examined by visual observation. Figure 2 shows the dissolution pressures of 0.08 mol % FC6-HCn at various temperatures. Except for FC6-HC2, all FC6-HCn species dissolved completely in neat scCO2 at pressures higher than those shown in the figure (dotted lines). Solid particles were observed at pressures lower than those lines. FC6HC2 was insoluble throughout the whole experimental range examined in this study. In contrast, FC6-HC6 was
Figure 3. Cloud pressures of 0.08 mol % FC6-HCn (n ) 4, 6, and 8)/scCO2 mixtures dosed with definite amounts of water and the effect of W0c on the cloud pressures as a function of temperature. The transparent single phase is above each boundary.
the most soluble and FC6-HC8 needed the highest pressures to dissolve in scCO2. These results indicate the existence of an optimum hydrocarbon chain length controlling the dissolution of the hybrid surfactant in scCO2, presumably due to differences in molecular weight, HCB (hydrophilic/CO2-philic balance),33 steric structure, and crystallinity. 8FS(EO)2 was almost insoluble and remained solid in dry scCO2 under our experimental conditions. A minute amount of 8FS(EO)2 may dissolve in scCO2, as decreasing the pressure caused a slight clouding of the mixtures. Cloud pressures of FC6-HCn (n ) 4, 6, and 8)/scCO2 mixtures with definite amounts of water were examined. Figure 3 shows the cloud pressures of FC6-HCn and the effect of W0c on the cloud pressures as a function of temperature. In the figure, a transparent single phase is observed at pressures higher than those of each cloud pressure curve. The cloud pressures increased with increasing water content, as mentioned in the earlier reports.13-19 The cloud pressures with water approached those under dry conditions with increasing temperature, and they converged at a point near W0c ) 0. This suggests that the transparent phase arose via dissolution of individual hydrated surfactant molecules. In the lowtemperature region, the explanation why an increase of (31) Antelmi, D. A.; Ke´kicheff, P. J. Phys. Chem. B 1997, 101, 8169. (32) Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733. (33) Lee, C. T., Jr.; Psathas, P. A.; Johnston, K. P.; deGrazia, J.; Randolph, T. W. Langmuir 1999, 15, 6781.
W/scCO2 Microemulsion from Fluorinated Surfactants
the hydrated surfactant dose increases the cloud pressure is that the apparent molecular weight of the hydrated surfactant is higher than that of the surfactant without water. As the water solubility in scCO2 increases on increasing the temperature, the molecular weight of surfactant/water complexes decreases, and the cloud pressure approaches that of the system without moisture. In the high-temperature range, because all water molecules are dissolved in the scCO2 phase, surfactant molecules become less associated with water, and the cloud pressure becomes lower than that without water due to the cosolvent effects of water dissolved in scCO2. This suggests that (i) water prefers to dissolve in scCO2 rather than in the reversed micelle of FC6-HCn and (ii) that reversed micelles of FC6-HCn can only take up a few water molecules at lower temperatures. Eventually, the transparent phase disappears by addition of >1.22 mol % water (i.e., 1.22 mol % water is barely soluble in pure scCO2 at the optimum condition in this study)20 and the W0c values cannot reach 7. The optimum hydrocarbon chain length of FC6-HCn to form a microemulsion was not clarified because of its low W0c. For the hybrid type surfactant, Johnston et al. reported that F7H7, whose dose not dissolve in pure scCO2, takes up more than 30 water molecules in the low-temperature range.13 A notable difference between FC6-HCn and F7H7 is the presence of a benzene ring in the former molecule. The fluorinated ionic surfactants bearing a benzene ring have been reported to be fairly soluble in scCO2 (over 10 wt %) at low temperatures ( AOT, although 8FS(EO)2 has a poorer solubility in neat scCO2 than those of FC6-HCn. The W/scCO2 microemulsion formed with 8FS(EO)2 could take up an amount of water as large as the largest previously reported amount in similar systems. 8FS(EO)2 is a useful surfactant for novel technologies employing W/scCO2 microemulsions. The aqueous core in the reversed micelle, characterized through FT-IR spectra of D2O, swells on addition of water, and shrinks under increased pressure. In this study, the main purpose was to design suitable surfactant structures capable of forming stable W/scCO2 microemulsions containing a large amount of water. It has become clear that the essential properties governing successful surfactant use in W/scCO2 microemulsions are (i) high interfacial adsorption and ability to lower the water/scCO2 interfacial tension and (ii) weak attraction, strong steric repulsion, and particularly high structural disorder between CO2-philic groups. Compared with the above factors, solubility in pure scCO2 appears to be unimportant. LA020340T
