Langmuir 2003, 19, 8161-8167
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Effects of CO2-philic Tail Structure on Phase Behavior of Fluorinated Aerosol-OT Analogue Surfactant/Water/ Supercritical CO2 Systems Masanobu Sagisaka,† Satoshi Yoda,‡ Yoshihiro Takebayashi,‡ Katsuto Otake,*,‡ Yukishige Kondo,§,| Norio Yoshino,§,| 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, Ibaraki 305-8565, Japan, 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 December 20, 2002. In Final Form: July 3, 2003 The effects of the structural factors of surfactants such as CO2-philic tail length and molecular structure on the formation of water-in-supercritical CO2 (W/scCO2) type microemulsions were examined at various temperatures and pressures for fluorinated Aerosol-OT (AOT) analogue surfactants, sodium bis(1H,1H,5Hoctafluoropentyl)-2-sulfosuccinate (di-HCF4), sodium bis(1H,1H,7H-dodecafluoroheptyl)-2-sulfosuccinate (di-HCF6), sodium bis(1H,1H,9H-hexadecafluorononyl)-2-sulfosuccinate (di-HCF8), sodium bis(1H,1H,2H,2Hheptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2), and sodium bis((1H,1H,2H,2H-heptadecafluorodecyl)oxyethylene)-2-sulfosuccinate (8FS(EO)4). With the all surfactants, a transparent single phase of water, a surfactant, and scCO2 mixture, or a W/scCO2 microemulsion phase, was formed under certain conditions. With an increase in the water composition, the microemulsion phase became a turbid W/scCO2 macroemulsion phase or completely separated into two phases. On the other hand, when pressure (CO2 density) or temperature was increased, the macroemulsion phase turned into a microemulsion phase. At CO2 densities 1.96 mol %. With the other surfactants having fluorocarbon chains longer than that of di-HCF4, the cloud pressure was significantly lower than that for di-HCF4. Unfortunately, a large increase in the water content caused precipitation of the surfactants di-HCF6, di-HCF8, and 8FS(EO)m. The amount of precipitates increased with an increase in the water content, a fact suggesting a decrease in the number of surfactant molecules that participate in reverse micelle formation. The critical packing parameter and HLB theories32-34 predict that the mean curvature of surfactant aggregates in oil approaches zero (i.e., like a lamellar LC) with increase in the water content. Actually, for the W/O systems with a large amount of ionic surfactant, an LC phase was rarely found to appear subsequent to a single microemulsion phase when the water content was increased.36,37 In addition, our earlier reports demonstrated that the fluorinated AOT analogue surfactants used in this study prefer to form aggregates of a small curvature.23-31 To examine whether the precipitate is an LC or not, the residue obtained after slow depressurization was observed with the polarized optical microscope. The observation revealed the existence of an LC in the residue, thus indicating the formation of LC in the W/scCO2 system. In general, the solvent power of scCO2 depends on its density. Figure 2 shows the cloud densities of CO2 with 1.22 mol % water in the presence of the surfactants. Because of low concentrations of water and surfactant, the CO2 density is calculated to be pure.38 At densities higher than those on each curve, a transparent microemulsion was formed. In the series of di-HCFn, the surfactant having longer fluorocarbon chains formed microemulsion at a lower CO2 density. Since fluorocarbon groups show weak van der Waals forces and have no dipole moment like CO2, they are CO2-philic10-24 and exhibit little mutual or other interactions between them. The specific solute-solvent interactions between CO2 and fluorinated compounds were also reported.39 Thus, the existence of longer fluorinated chains in their molecules means a stronger CO2-philicity of the surfactants. In addition, a (36) Shinoda, K.; Friberg, S. Emulsion and Solubilization; WileyInterscience: New York, 1986. (37) Lindman, B.; Shinoda, K.; Jonstromer, M.; Shinohara, A. J. Phys. Chem. 1988, 92, 4702. (38) Span, R.; Wagner, W. J. Phys. Chem. Ref. Data 1996, 25, 1509. (39) Dardin, A.; DeSimone, J. M.; Samulski, E. T. J. Phys. Chem. B 1998, 102, 1775.
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fluorocarbon group is bulkier than a hydrocarbon group, and is suitable for the formation of microemulsions. These characteristics of the surfactants are the source of their water/CO2 interfacial activity and prevention of aggregation and fusion of microemulsion droplets, and the activity becomes stronger with an increase in the fluorocarbon chain length. In Figure 2, γcmc is the interfacial tension of the air/ aqueous solution interface at the critical micelle concentration (cmc), and the values are given in Table 1. Recently, Eastoe et al. have found a clear relationship between γcmc and cloud pressure (or CO2 density).40 Our results confirmed the correlation as seen in Figure 2. Figure 3 shows W0c-temperature phase diagrams for W/scCO2/di-HCFn mixtures at various CO2 densities. In the figure, E, µE, and P represent the phases of macroemulsion, microemulsion, and microemulsion with precipitated LC-like surfactant, respectively. No microemulsion phase was formed for di-HCF4 at CO2 densities
