Effective and Efficient Surfactant for CO2 Having Only Short

Jun 27, 2012 - ... Craig James , Atsushi Yoshizawa , Azmi Mohamed , Sarah E. Rogers , Richard K. Heenan , Ci Yan , Jocelyn Alice Peach , and Julian Ea...
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Effective and Efficient Surfactant for CO2 Having Only Short Fluorocarbon Chains Masanobu Sagisaka,*,† Shuho Iwama,† Atsushi Yoshizawa,† Azmi Mohamed,‡,§ Stephen Cummings,‡ and Julian Eastoe‡ †

Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan ‡ School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom § Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, Tanjong Malim, Perak 35900, Malaysia ABSTRACT: A previous study (Langmuir 2011, 27, 5772) found the fluorinated double-tail sulfogulutarate 8FG(EO)2 to act as a superefficient solubilizer for water in supercritical CO2 (W/CO2) microemulsions. To explore more economic CO2-philic surfactants with high solubilizing power as well as rapid solubilization rates, the effects of fluorocarbon chain length and linking group were examined with sodium 1,5-bis(1H,1H,2H,2Hperfluoroalkyloxy)-1,5-dioxopentane-2-sulfonates (nFG(EO)2, fluorocarbon chain length n = 4, 6, 8) and sodium 1,4-bis(1H,1H,2H,2Hperfluoroalkyloxy)-1,4-dioxobutane-2-sulfonate (nFS(EO)2, n = 4, 8). Visual observation and UV−vis spectral measurements with methyl orange as a reporter dye indicated a maximum water-to-surfactant molar ratio (W0) in the microemulsions, which was 60−80 for nFG(EO)2 and 40−50 for nFG(EO)2. Although it is normally expected that high solubilizing power requires long fluorocarbon surfactant chains, the shortest fluorocarbon 4FG(EO)2 interestingly achieved the highest W0 (80) transparent single-phase W/CO2 microemulsion. In addition, a very rapid solubilization of loaded water into CO2 was observed for 4FG(EO)2 even at a high W0 of ∼80.

1. INTRODUCTION With increasing interest in environmental concerns, the need to switch from volatile organic compounds (VOCs) to environmentally benign solvents in the chemical industries accelerates year on year. One practical substitute solvent is supercritical CO2 (scCO2), which has been well studied and applied since 1978 for polymerization for PTFE, extraction of valuable components from plants, and so on.1 However, there are limitations of scCO2 for industrial applications because of its weak solvent power for high molecular weight and/or polar compounds. Formation of reversed micelles with aqueous cores in scCO2, that is, water in CO2 microemulsions (W/CO2μEs), is able to improve the limited solubility of scCO2 for polar solutes and will broaden applications of CO2 across various fields of the chemical industry.2 For example, for reactions and extractions carried out in W/CO2μEs, the products and extracts can be easily separated from the solvent CO2 simply by decreasing pressure. This could lead to energy-saving and environmentally friendly chemical processes that do not require energy-intensive solvent removal. Therefore, W/CO2μEs will hopefully be VOC-free and energy-saving solvents for nanomaterial synthesis, enzymatic reactions, dry cleaning, dyeing, and preparation of inorganic/organic hybrid materials.2 For such W/CO2μEs to be considered as viable green solvents, the amount of surfactant used should be as small as possible, and © 2012 American Chemical Society

this can be achieved by developing optimized and highly efficient surfactants. In addition to a high solubilizing power, an important property of the surfactant is a fast solubilization rate to generate W/CO2μEs, which can be attained by rapid adsorption of surfactant at the water/CO2 interface. A faster solubilization rate contributes to a higher efficiency of mass transport in any potential manufacturing process using W/ CO2μEs. Much effort has been directed toward development of surfactants that stabilize W/CO2 μEs, and importantly, development of CO2-philic hydrocarbon surfactants is desired for economic and environmental reasons.3−8 However, most commercial and known hydrocarbon surfactants are insoluble and inactive in W/scCO2 systems.3−8 In this regard, it became apparent that conventional surfactant-design theory cannot be applied to W/CO2 systems directly and that CO2-philicity is not directly comparable to oleophilicity. Therefore, advancing molecular-design theory for CO2-philic surfactants has required new directions and paradigms in the field of surfactants. Only a few hydrocarbon surfactants have been reported to stabilize W/CO2 μE in earlier papers. One of these successful Received: March 29, 2012 Revised: June 8, 2012 Published: June 27, 2012 10988

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Table 1. Surfactant Properties of nFS(EO)2 (m = 1) and nFG(EO)2 (m = 2) in Water at 35 and 75 °C, Respectively

surfactants is the nonionic TMN-6,9−11 which has highly branched alkyl tails and ∼8 oxyethylene units: TMN-6 was reported to solubilize water up to a water-to-surfactant molar ratio, W0, of 30. Custom-made anionic surfactants with highly branched double or triple tails (sodium bis(3,5,5-trimethyl-1hexyl) sulfosuccinate12−14 or (sodium 1,4-bis(neopentyloxy)-3(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate 15−17 ) were also found to be soluble in scCO2 and yield W/CO2 μEs with W0 less than 15. Note that the commercial analogue of these surfactants, Aerosol-OT (sodium bis(2-ethyl-1-hexyl) sulfosuccinate), is inactive and insoluble scCO2 and hence is ineffective at stabilizing W/CO2 μEs. Hence, surfactants with highly branched tails, especially methyl branches, have been accepted as CO2-philic. On the other hand, ester and ether groups have been reported to increase solubility in scCO2 as well as methyl branches, and then high CO2-philic and W/CO2interfacially active copolymers with those groups have been developed and used for emulsification and polymerization.18−20 Unfortunately, an efficient and cost-effective hydrocarbon stabilizer for W/CO2 μEs, like the Aerosol-OT used commonly for W/O μEs,21,22 has not yet been found. In earlier studies, several fluorocarbon (FC) surfactants were found to dissolve in CO2 and have a high interfacial activity at the water/CO2 interface, suggesting the feasibility of forming W/CO2 μEs.23−26 Among others, surfactants described below are noteworthy for generating microemulsions. Earlier a perfluoropolyether (PFPE) surfactant23 was found to stabilize transparent single-phase W/CO2 μEs (Winsor-IV W/CO2 μE or IVμE) but with only a low water-to-surfactant molar ratio, W0 = 21 (also expressed as a corrected water-to-surfactant molar ratio by subtracting the background water solubility in CO2, W0c = 14). Afterward, numerous reports dealing with W/ CO2 μEs focused on PFPEs.23−26 Another successful class of CO2-philic surfactants is the hydrocarbon (HC)−FC hybrids, for example, sodium 1pentadecafluoroheptyl-1-octanesulfate (F7H7, (C7H15)(C7F15)CHOSO3Na): these have both a HC and a FC chain in the same molecule. As such F7H7 is able to microemulsify up to W0 = 35 (or W0c = 32) to form stable Winsor-IV W/CO2 μEs.27 Further studies28−40 with hybrid surfactants related to F7H7 but with different FC and HC chain lengths found formation of Winsor-IV W/CO2 μEs for most of the analogues, but unfortunately the attainable W0 values in the μEs were smaller than those found for F7H7. Other investigations41−47 studied a class of fluorinated Aerosol-OT (AOT) analogues, for example, sodium bis(1H,1H,5H-octafluoropentyl)-2-sulfosuccinate (di-HCF4), which yield a Winsor-IV W/CO2 μE phase with W0 = 30 (W0c ≈ 20). In addition, double-FC-tail phosphate surfactants were also found to be efficient microemulsion stabilizers,45,48−50 the most favorable case stabilizing W/CO2 μEs with W0 up to 45.58 Another fluorinated AOT analogue sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2)was found to stabilize Winsor-IV W/CO2 μEs effectively and reach a maximum W0 of 46 (W0c ≈ 32) before phase separation.51 Recently, with the aim of optimizing surfactant structure for W/CO2 μEs an 8FS(EO)2 analogue surfactant was synthesized and evaluatedsodium 1,5-bis[(1H,1H,2H,2Hperfluorodecyl)oxy]-1,5-dioxopentane-2-sulfonate (8FG(EO)2)which has an extra methylene spacer in the sulfosuccinate group of 8FS(EO) 2 (see Table 1 for structures).52,53 Phase behavior and UV−vis absorption spectra

a

Critical micelle concentration. bSurface tension at CMC. cArea per surfactant headgroup obtained using Gibbs adsorption equation and surface tension data. Properties for 8FS(EO)2 and 8FG(EO)2 have already been reported previously.32

of the polar dye methyl orange (MO) in 8FG(EO)2/W/CO2 mixtures were observed/measured at various W0 values. Interestingly, upon addition of aqueous MO solutions to the 8FG(EO)2/scCO2 mixtures at 350 bar, transparent single phases were obtained and the absorbance attributed to MO increased until a maximum W0 value of ∼60 was attained (W0c ≈ 52). Since such a high W0 is unusual in this field, being the largest ever reported, it is as much as an attainable W0 value for typical W/O μE systems formed with an efficient solubilizer like Aerosol-OT. However, this microemulsion still has cost and environmental problems from the fluorocarbon tails. More recently, to obtain more economic and environmentally friendly surfactants for W/CO2 μEs, the minimum necessary fluorine content of a surfactant has been investigated54 it was found that at least two fluorinated carbons (CF3CF2−) are required for sufficient CO2-philicity to stabilize μEs. An increase in fluorine level leads a lower aqueous surface tension at CMC and a lower stabilization cloud pressure for the W/CO2 μEs. Over recent years, many scientists in this field have attempted to understand and explain why FCs act as such efficient CO2-philic groups whereas common HC surfactants are much less effective (so as to be essentially inactive in this high-pressure solvent). Recent molecular simulation studies55,56 have elucidated the reasons: as compared with HC chains, FC groups have (1) a stronger interaction with CO2 via quadrupolar and dispersion interactions and (2) weaker FC− FC chain−chain interactions which are down to a weak repulsion, electrostatic in origin. These properties conspire together to give FC surfactant reversed micelles better solvation by CO2, and this causes lower surfactant interfacial packing densities (higher critical packing parameters) and weaker attractive intermicellar interactions as compared with hydrocarbon surfactant analogues. Thus, a fluorocarbon chain number of two (CF3−CF2−) is the minimum level needed to generate a viable CO2-philic surfactant. To explore more efficient surfactants having 8FG(EO)2-like high solubilizing for W/CO2 μEs power requires advances in surfactant design theory, and it must be supported by more effort to examine aggregation behavior of nFG(EO)2 and nFS(EO)2 with different FC lengths. Here, in this study new CO2-philes have been synthesized (nFG(EO)2) with only short FC chains (FC length n = 4 and 6), and their aggregation behavior and solubilizing power in W/CO2 μEs have been compared with control analogues nFS(EO)2. The significance of this study to the field of surfactant science is that optimal surfactant chemical structures are now available for efficient and effective stabilization of W/CO2 μEs. Through advancing and 10989

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15.3 mmol) in water (30 cm3) was added to this mixture. The reaction mixture was stirred under reflux for 20 h. After solvents were evaporated the white solid residue was washed with 1,4-dioxane to remove the unreacted diester. The product was extracted in a Soxhlet unit with dry acetone to remove excess NaHSO3 and recrystallized from acetone. It afforded a white powder, 4FG(EO)2 (with no regioisomers as C4F9CH2CH2OCOCH2CH(−SO3Na)CH2COOCH2CH2C4F9), after vacuum drying (yield 1.47 g, 50.4%). 1 H NMR (500 MHz, CF3COOD, TMS), (δH/ppm): 2.74 (a, m, 4H), 3.10 (d, dd, 2H, J = 6.1, 16.9 Hz), 3.38 (c, dd, 2H, J = 7.3, 16.9 Hz), 4.41 (e, m, 1H), 4.74 (b, t, 4H, J = 6.30 Hz) for C4F9CH2aCH2bOCOCH2cCH2dCHe(SO3Na)COOCH2bCH2aC4F9. IR (KBr) νmax/cm−1: 1739, 1358, 1226, 1135, 1061,879, 720. Anal. Calcd for C17H13O7F18Na: C, 28.1; H, 1.8.; S, 4.4. Found: C, 27.3; H, 2.1; S, 4.9. 2.2.2.2. Synthesis of Sodium Bis(1H,1H,2H,2H-tridecafluorooctyl)2-sulfoglutarate, 6FG(EO)2. Bis(1H,1H,2H,2H-tridecafluorooctyl) glutaconate (11.00 g, 13.38 mmol) was dissolved in 1,4-dioxane (225 cm3); then the mixture was heated to 50 °C. A solution of sodium hydrogensulfite (5.4 g, 51.7 mmol) in water (90 cm3) was added to this mixture. Purification of the reaction mixture was carried out as described in the synthesis of 4FG(EO)2. It afforded a white powder, 6FG(EO)2 (with no regioisomers as C 6 F 13 CH 2 CH 2 OCOCH 2 CH(−SO 3 Na)CH 2 COOCH 2 CH 2 C 6 F 13 ), after vacuum drying (yield 5.90 g, 47.6%). 1H NMR (500 MHz, CF3COOD, TMS), (δH/ppm): 2.80 (a, m, 4H), 3.16 (d, dd, 2H, J = 6.0, 16.9 Hz), 4.43 (c, dd, 2H, J = 7.2, 16.9 Hz), 4.46 (e, m, 1H), 4.80 (b, t, 4H, J = 6.30 Hz) for C 6 F 13 CH 2 a CH 2 b OCOCH 2 c CH2dCHe(SO3Na)COOCH2bCH2aC6F13. IR (KBr) νmax/cm−1: 3573, 3540, 2980, 1728, 1464, 1430, 1368, 1320, 1237, 1144, 1082, 1052, 1006, 955, 901, 841, 781, 735, 698, 653. Anal. Calcd for C21H13O7F26Na: C, 27.2; H, 1.4.; S, 3.5. Found: C, 27.0; H, 1.5; S, 3.2. 2.3. Surface Tension Measurements. Surface tensions of aqueous surfactant solutions were measured using a Wilhelmy tensiometer (CBVP-Z, Kyowa Interface Science) equipped with a platinum plate. Measurements were performed at 35 ± 0.1 °C until constant values of the surface tension of the aqueous surfactant solutions were obtained; the experimental error was less than 0.1 mN/ m. The critical micelle concentration (CMC) was obtained from the point of intersection of the curves in the graph of surface tension versus logarithm of the surfactant concentration. 2.4. Phase Behavior and UV−Vis Absorption Spectral Measurements. A high-pressure vessel with an optical window and a moveable piston inside the vessel was used to observe the phase behavior of surfactant/water/scCO2 mixtures with varying pressure and temperature but constant composition. A detailed description of the experimental apparatus and procedures for the measurements can be found elsewhere.52,53,58 In order to examine formation of aqueous cores in W/CO2 μEs, UV−vis absorption spectroscopy measurements were performed using methyl orange (MO) as a trace marker dye on a double-beam spectrometer (Hitachi High-Technologies, Co., U-2810) with a quartz window pressure cell (volume 1.5 cm3), which was connected to the experimental apparatus. The cell was made of stainless steel (SUS316) and had three quartz windows with a thickness of 8 mm. Each window had an inner diameter of 10 mm; the windows were positioned so as to provide a perpendicular 10 mm optical path. Each window was attached to the stainless steel body of the cell using PTFE kel-F packing. The windows were fastened tightly to the steel body, thereby compressing the packing between the stainless steel parts and the quartz window and providing excellent sealing (tested up to 400 bar). The temperature of the cell was controlled by circulating water with a thermostat bath. Spectroscopic measurements were performed, and the resulting absorption spectra of the cell windows were compared with those of a standard quartz cell for an aqueous MO solution at ambient pressure; it was observed that both spectra were in good agreement with each other. The measurements of the water/surfactant/scCO2 systems were performed at temperatures of 35−75 °C and pressures lower than 400 bar. In addition to a raw water-to-surfactant molar ratio (W0), the

improving molecular design of CO2-philic surfactants, W/CO2 μEs could become industry-acceptable universal solvents.

2. EXPERIMENTAL SECTION 2.1. Materials. The surfactants used in this study were sodium 1,5bis[(1H,1H,2H,2H-perfluorohexyl)oxy]-1,5-dioxopentane-2-sulfonate (4FG(EO)2), sodium 1,5-bis[(1H,1H,2H,2H-perfluorooctyl)oxy]-1,5dioxopentane-2-sulfonate (6FG(EO) 2 ), sodium 1,5-bis[(1H,1H,2H,2H-perfluorodecyl)oxy]-1,5-dioxopentane-2-sulfonate (8FG(EO)2), sodium 1,4-bis[(1H,1H,2H,2H-perfluorohexyl)oxy]-1,4dioxobutane-2-sulfonate (4FS(EO) 2 ), and sodium 1,4-bis[(1H,1H,2H,2H-perfluorodecyl)oxy]-1,4-dioxobutane-2-sulfonate (8FS(EO)2). Three nFS(EO)2 (n = 4 and 8) and 8FG(EO)2 were synthesized and evaluated on their interfacial properties previously.52,53,57 Synthesis and purification procedures of the new glutarate-type surfactants, nFG(EO)2 (n = 4 and 6), were described in section 2.2. Fluorinated alcohols, 1H,1H,2H,2H-tridecafluoro-1octanol (Wako Pure Chemical Industries) and 1H,1H,2H,2H-nonafluoro-1-hexanol (Tokyo Chemical Industry), and glutaconic acid (Aldrich) were used without further purification. Reagent-grade acetone, ethyl acetate, 1,4-dioxane, toluene, p-toluene sulfonic acid monohydrate, and sodium hydrogensulfite were commercially obtained from Wako Pure Chemical Industries and employed as received. Surfactant structures are shown in Table 1 with interfacial properties of aqueous solutions obtained by standard measurements. As compared to nFS(EO)2, nFG(EO)2 has an extra −CH2− spacer between the FC and the sulfonate group. Ultrapure water with a resistivity of 18.2 MΩ cm, obtained from a Millipore Milli-Q Plus system, was used in the experiments. CO2 with 99.99% purity (Ekika Carbon Dioxide Co., Ltd.) and methyl orange (MO; Acros Organics) were used without further treatment. 2.2. Synthesis. 2.2.1. Synthesis of Bis(fluoroalkyl) Glutaconate. 2.2.1.1. Bis(1H,1H,2H,2H-nonafluorohexyl) Glutaconate. A mixture of 2.46 g of 1H,1H,2H,2H-nonafluoro-1-hexanol (9.32 mmol), 0.60 g of glutaconic acid (4.64 mmol), and 0.20 g of p-toluene sulfonic acid monohydrate (1.05 mmol) in 100 cm3 toluene was refluxed under stirring at 130 °C for 48 h. During reaction the water liberated was removed azeotropically from the reaction system to shift the equilibrium of the esterification reaction. After the reaction was complete, the mixture was washed sufficiently with brine to remove ptoluene sulfonic acid, unreacted glutaconic acid, and the glutaconic acid monoester; then, it was dried over Na2SO4. After removal of the drying agent, toluene and 1H,1H,2H,2H-nonafluoro-1-hexanol in the mixture were evaporated under vacuum, and then a transparent sticky liquid, bis(1H,1H,2H,2H-nonafluorohexyl) glutaconate, was obtained (yield 2.50 g, 86.3%). 1H NMR (500 MHz, CDCl3, TMS), (δH/ppm): 2.50 (a, multiplet(m), 4H), 3.29 (c, double doublet (dd), 2H, J = 1.5, 7.2 Hz), 4.44 (b, triplet (t), 2H, J = 5.4 Hz), 4.46 (f, t, 2H, J = 5.3 Hz), 5.96 (e, double triplet (dt), 1H, J = 1.6, 15.8 Hz), 7.03 (d, dt, 1H, J = 7.2, 15.8 Hz) for C4F9CH2aCH2bOCOCH2cCHd CHeCOOCH2fCH2aC4F9. IR (KBr) νmax/cm−1: 1747, 1224, 1135, 1081, 878, 749. 2.2.1.2. Bis(1H,1H,2H,2H-tridecafluoroocttyl) Glutaconate. A mixture of 4.0 g of 1H,1H,2H,2H-tridecafluoro-1-octanol (11.0 mmol), 0.65 g of glutaconic acid (5.00 mmol), and 0.40 g of ptoluene sulfonic acid monohydrate (2.10 mmol) in 20 cm3 toluene was refluxed with a Dean−Stark apparatus under stirring at 130 °C for 48 h. Purification of the reaction mixture was carried out as described in the synthesis of bis(1H,1H,2H,2H-nonafluorohexyl) glutaconate, and then a transparent sticky liquid, bis(1H,1H,2H,2H-tridecafluorooctyl) glutaconate, was obtained (yield 2.52 g, 61.3%). 1H NMR (500 MHz, CDCl3, TMS), (δH/ppm): 2.50 (a, m, 4H), 3.28 (c, dd, 2H, J = 1.5, 7.1 Hz), 4.44 (b, t, 2H, J = 5.1 Hz), 4.46 (f, t, 2H, J = 5.2 Hz), 5.96 (e, dt, 1H, J = 1.6, 15.8 Hz), 7.02 (d, dt, 1H, J = 7.2, 15.7 Hz) for C6F13CH2aCH2bOCOCH2cCHdCHeCOOCH2fCH2aC6F13. 2.2.2. Synthesis of Bis(fluoroalkyl)-2-sulfoglutarate. 2.2.2.1. Synthesis of Sodium Bis(1H,1H,2H,2H-nonafluorohexyl)-2-sulfoglutarate, 4FG(EO)2. Bis(1H,1H,2H,2H-nonafluorohexyl) glutaconate (2.50 g, 4.02 mmol) was dissolved in 1,4-dioxane (70 cm3); then the mixture was heated to 50 °C. A solution of sodium hydrogensulfite (1.60 g, 10990

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4FG(EO)2 and 4FS(EO)2 were almost the same at 45 °C. This implies that nFG(EO)2 exhibits a greater ability to decrease the water/scCO2 interfacial tension at higher temperature. In other words, the glutarates nFG(EO)2 should exhibit a greater ability to stabilize W/CO2 μEs than those with the smaller headgroup sulfosuccinate nFS(EO)2. 3.2. Phase Behavior of the nFS(EO)2 and nFG(EO)2/W/ CO2 Mixtures. Figure 2 shows the phase transition pressures

parameter W0c was used to express the true water-to-surfactant molar ratios. The solubility of water in scCO2 was calculated as52,53

W 0c =

[water]0 − [water]S [water]S = W0 − [surfactant]0 [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 background CO2, and [surfactant]0 is the number of moles of surfactant. The value of [water]S was obtained from literature data.59,60 The densities of CO2 were calculated using the Span−Wagner equation of state (EOS).61 Predetermined amounts of surfactant and CO2 (20.0 g), where the molar ratio of surfactant to CO2 was fixed at 8 × 10−4, were loaded into a variable-volume high-pressure optical cell. Then water or an aqueous MO solution (3 mmol L−1) was added into the optical cell through a six-port valve until the clear Winsor-IV W/scCO2 μE (IVμE) solution became a turbid macroemulsion or a precipitated hydrated surfactant. Surfactant molar concentration was in range between 10 and 20 mmol L−1, for example, 16.7 mmol L−1 at 45 °C and 350 bar, as the inner volume of the cell was varied by changing experimental pressure and temperature. During spectroscopic measurements the scCO2 mixture was stirred and circulated between the optical vessel and the quartz window cell until a constant absorbance was attained. Circulation was then discontinued, the valves between the vessel and the quartz window cell were closed, and measurement was performed. The physical properties of the continuous phase of scCO2 were assumed to be equivalent to those of pure CO2.

3. RESULTS AND DISCUSSION 3.1. Interfacial Properties of nFS(EO)2 and nFG(EO)2 in Water. Surface tension γ of an aqueous surfactant solution was studied as a function of concentration to determine the CMC and surface tension γCMC at the CMC. The parameter γCMC is especially important in predicting the microemulsifying ability of the surfactant in scCO2;36,43,44,54,62 this is because (1) the water/air and water/CO2 interfacial properties of the surfactant are correlated and (2) a microemulsion generally forms at an interfacial tension below 1 mN/m.63,64 Figure 1 shows the surface tension of the surfactants nFS(EO)2 and nFG(EO)2 at various concentrations. The

Figure 2. Phase boundary pressures where a transparent single phase becomes a turbid macroemulsion on decreasing pressure at different temperatures for nFG(EO)2 and nFS(EO)2. (a) Difference in pressures for each surfactant mixture at the same W0 (9.2), and (b− f) pressures for each surfactant at different W0 values. Molar ratio of surfactant-to-CO2 was fixed at 8 × 10−4.

(Ptrans) where a transparent single phase (P > Ptrans) becomes a visibly turbid phase (P ≤ Ptrans) for surfactant/W/CO2 mixtures. The difference in Ptrans for each surfactant was evaluated at the same W0 in Figure 2. The single phases were attributed to Winsor-IV W/CO2 μEs (IVμEs) if there was a presence of surfactant solubilized higher water content than the background solubility in pure scCO2, suggesting generation of water pools in surfactant reversed micelles. Under the same W0 condition, Ptrans curves are almost the same for 4FG(EO)2 and 4FS(EO)2, suggesting a small effect of the extra −CH2− group on Ptrans. Earlier studies found that the lower the γCMC the lower the Ptrans,36,43,44,54,62 then the similar Ptrans values for 4FG(EO)2 and 4FS(EO)2 were in good agreement with that trend. Comparing the results for the nFG(EO)2 series the longer fluorocarbon nFG(EO)2 required higher Ptrans to stabilize IVμEs. In general, a longer oleophilic (or CO2-philic) chain equips a surfactant molecule with not only the ability to lower W/O (or W/CO2) interfacial tension but also a more cylindrical molecular structure due to the increased attractive interactions between tails. According to critical packing

Figure 1. Change in surface tension of aqueous solutions of nFG(EO)2 and nFS(EO)2 as a function of log[surfactant concentration]. Data for n = 4 were measured at 35 °C in this study, but data for n = 8 were at 75 °C and already reported in an earlier paper.32

surface tension was a decreasing function of concentration; however, it was invariant above the concentration indicated by the arrow in Figure 1. The threshold concentration was defined as the CMC. The values of CMC and γCMC are listed in Table 1. A comparison between nFG(EO)2 and nFS(EO)2 revealed that the CMC and area per headgroup, Ah,st, were almost the same at 35 and 75 °C. However, γCMC of 8FG(EO)2 was lower by 6 mN/m than that of 8FS(EO)2 at 75 °C, though those for 10991

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parameter (CPP) theory,65,66 the CPP of a surfactant is defined as to be v/a0 lc, where v and lc are the volume and length of the hydrophobic tail. According to this approach reversed micelles would be formed for surfactant with CPP > 1 (reversed cones form if the double-tail orients upward) to ∼1 (cylindrical). If a longer nFG(EO)2 has a higher ability to lower W/CO2 interfacial tension, the higher Ptrans could be required to achieve CPP (>1) required for reversed micelle formation by CO2-swollen tails at higher CO2 densities.65,66 Another explanation for the higher Ptrans with the longer n is, the higher molecular weight which is expected to require higher pressure (density) to dissolve nFG(EO)2 in CO2. These masses were 0.264 g for n = 4, 0.337 g for n = 6, and 0.409 g for n = 8 to be same molar concentration; it is known that a higher P affects the solvent properties, for example, increasing the density and having a higher dielectric constant of CO2.1 The Ptrans curves were found to increase gradually with increasing W0 and become almost constant at above a certain W0. This kind of W0 dependence of the phase boundary pressure has been observed in other surfactant/W/scCO2 μE systems.52,65,66 As discussed in previous papers,52,65 the elevated Ptrans with W0 results from a promotion of reversed micelle aggregation by generating aqueous core−core interactions. With an increase in W0, the uncertainties of the phase boundary pressures became larger, for example, they were ±1 and ±20 bar at a few W0 and W0 > 60, respectively. The larger uncertainty presumably results from formation of a two-phase system instead of the IVμE at a higher W0. In this case, the twophase system would be a ‘Winsor-II type W/CO2 μE’ (i.e., a W/CO2 microemulsion having separated excess water, IIμE)52,53 or ‘liquid crystal phase,66 which have been found in other surfactant/W/scCO2 systems. For 8FG(EO)2, a precipitate was visually observed at W0 = 82.5 and temperatures < 65 °C or the higher W0, and the amount of that increased with W0. The solubilization rate of 4FG(EO)2 for IVμE was interestingly different from those of the other surfactants examined here. Figure 3 shows change in appearance of the 4FG(EO)2/W/CO2 mixture after loading water into the mixture as W0 increased from 74 to 77. Usually when water is loaded into a transparent surfactant/scCO2 mixture a turbid macroemulsion appears, and then it becomes a transparent single-phase IVμE after at least several minutes.51 Interestingly, 4FG(EO)2 was able to solubilize loaded water in several seconds and then make a clear phase as seen in Figure 3. The solubilization rate was observed to increase at elevated temperature, following increasing solubility of water and weakening hydrogen bonding between water molecules. 3.3. Solubilization of Aqueous Dye and Microenvironmental Polarity in Reversed Micelles. To confirm the solubilizing power of each surfactant 3 mmol L−1 aqueous methyl orange (MO) solution was loaded as a dispersed phase into the surfactant/CO2 mixtures, and then UV−vis adsorption spectra were measured at different W0 values. Alone, MO does not dissolve in pure CO2 but dissolves in water and is generally incorporated within the water-rich pockets of a IVμE, dyeing the solutions red.58 All nFS(EO)2/ and nFG(EO)2/CO2 mixtures were initially colorless but changed into reddish solutions by loading the MO solution. The reddish color was developed with addition of the MO solution, reflecting the reversed micelles encapsulating the loaded MO solution. Figure 4 displays changes in the spectra for 4FG(EO)2 and 4FS(EO)2 mixtures as a function of W0. A large and broad absorption peak of MO solubilized in IVμE was found at 360−

Figure 3. Change in appearance of the 4FG(EO)2/W/CO2 mixture as a function of elapsed time after loading of 20 μL of water into the Winsor-IV W/CO2 μE to increase W0 from 74 to 77 at 75 °C.

Figure 4. UV−vis absorption spectra of MO in (a) 4FG(EO)2 and (b) 4FS(EO)2/W/CO2 mixtures for different W0 values at 75 °C and 350 bar (CO2 density = 0.81 g/cm3). MO concentration in water was 3 mmol L−1. Molar ratio of surfactant-to-CO2 was fixed at 8 × 10−4. Values for each spectrum mean W0 condition.

520 nm in Figure 4. As the local environment of MO became more polar, the absorbance maximum λmax shifted to longer wavelengths;23,26,49 for example, λmax in pure methanol is 421 nm but 464 nm in pure water.23 Then, λmax can be employed as 10992

dx.doi.org/10.1021/la301305q | Langmuir 2012, 28, 10988−10996

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highly dissociated surfactant head attracting the zwitterionic MO molecules. Since the molar ratio of MO-to-sodium ion ([MO]/[Na+]) in the system is quite low (