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Ind. Eng. Chem. Res. 2002, 41, 1038-1042
Phase Behavior and Micelle Size of an Aqueous Microdispersion in Supercritical CO2 with a Novel Surfactant Xing Dong and Can Erkey University of Connecticut, Storrs, Connecticut 06269
Horng-Ji Dai and Hung-Chih Li University of Tennessee, Knoxville, Tennessee 37996-2200
Hank D. Cochran* and J. S. Lin Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6181
The sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)-2-sulfosuccinate (di-HCF4) was used as a surfactant to stabilize water-in-CO2 microemulsions under a range of conditions. The phase behavior of the surfactant/water/CO2 system was explored using cloud-point measurements for [di-HCF4] ) 0.05-0.2 M, temperatures up to 55 °C, and pressures up to 35 bar, yielding the highest water and surfactant concentration ever achieved for water-in-CO2 microemulsion systems at comparable pressures. Stable microemulsions at [di-HCF4] ) 0.1 M were studied by small-angle X-ray scattering over the range of water-to-surfactant molar ratio from 0.5 to 20. The micelles were observed to swell as water was added to the core. Introduction Recent experiments1,2 have investigated the use of carbon dioxide (CO2) as a nonpolar solvent medium for the formation of microemulsions. CO2 is an inexpensive and essentially benign alternative to potentially hazardous industrial solvents. The formation of microemulsions in CO2 might enable a wide range of industrial operations in a medium with minimal environmental impact.14 Consani and Smith3 showed that most industrially available surfactants are incapable of forming stable microemulsions in CO2 because of their negligible solubilities. Thus, a number of efforts4-9 have sought surfactants having favorable interactions with CO2. In this work, we explore the phase behavior and micelle size of water-in-carbon dioxide (W/CO2) microemulsions stabilized by a novel surfactant sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)-2-sulfosuccinate (di-HCF4) that has recently been synthesized by us.10 Cloud-point measurements were made for [diHCF4] ) 0.05-0.2 M, temperatures up to 55 °C, and pressures up to 350 bar. Small-angle X-ray scattering (SAXS) measurements were used to determine median micelle size for [di-HCF4] ) 0.1 M over the range of water-to-surfactant molar ratios, wo, from 0.5 to 20. Our experimental approach is described in the next section. Then, we present results and a discussion, and finally, we offer a few concluding remarks. Experimental Approach The surfactant di-HCF4 was synthesized at the University of Connecticut by the reaction of sodium hydrogen sulfite with bis(2,2,3,3,4,4,5,5-octafluoro-1pentyl) maleate in a mixture of 1,4-dioxane and water. The details of the synthetic procedure are given else* Corresponding author. E-mail
[email protected], telephone 865-574-6821.
where.10 Surfactant purity was confirmed by 1H NMR spectroscopy and elemental analysis. The macroscopic phase behavior of W/CO2 microemulsions was investigated, also at the University of Connecticut, using a high-pressure vessel (internal volume of 54 cm3) that was custom manufactured from 316SS and equipped with two sapphire windows (diameter ) 5.72 cm, thickness ) 1.27 cm). The windows were sealed on both sides with PEEK seals. In a typical experiment, a certain amount of surfactant, water, and a magnetic stir bar were placed in the vessel, which was then sealed. The vessel was then placed on a magnetic stir plate and heated to the desired temperature by a recirculating heater/cooler (Fischer) via a machined internal coil. The vessel was charged very slowly with CO2 from a syringe pump (2-ISCO, 100D) equipped with a cooling jacket. When an optically transparent singlephase solution was obtained, the pump was stopped. Subsequently, the vessel was slowly depressurized until the cloud point was reached. The volume lost during depressurization was around 5% of the total volume, as calculated using the density of CO2. The temperature was controlled during each experiment with a variation of (0.5 °C. The pressure was measured using a pressure transducer (6-Omega Engineering Inc., PX01K1-5KGV). The SAXS experiments were performed on the Oak Ridge National Laboratory 10m SAXS instrument11 with a sample-to-detector distance of 2.069 m using Cu KR radiation with a graphite monochromator (λ ) 1.54 Å) and a 20 × 20 cm2 area detector with a cell size of 3 mm. The SAXS data were corrected for instrument background and detector efficiency (via an 55Fe standard) on a cell-by-cell basis prior to azimuthal averaging to give a Q range of 0.007 < Q < 0.252 Å-1 (Q ) 4πλ-1 sin θ). Net intensities were converted to an absolute differential cross section with precalibrated secondary standards.12 The SAXS cell used in this study is similar to that described by Fulton.6 The monoblock stainless steel cell consists of two single-crystal diamond windows
10.1021/ie010342t CCC: $22.00 © 2002 American Chemical Society Published on Web 08/24/2001
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Figure 1. Pressure-temperature phase diagram for W/CO2 microemulsions at [di-HCF4] ) 0.15 M
Figure 2. Pressure-temperature phase diagram for W/CO2 microemulsions at [di-HCF4] ) 0.20 M
for the transmission of X-rays, a sapphire view window for the determination of the number of phases, gas injection and release ports, and an internal magnetic stir bar. The sample chamber in the cell has a volume of 10 cm3 and an X-ray path length of 0.5 mm between the two diamond windows. Sample preparation involved the introduction of surfactant and water into the SAXS cell and then charging with CO2 until the experimental pressure was reached. Microemulsions of 0.1 M surfactant concentration with various water-to-surfactant molar ratios (wo ) 0, 5, 10, 14.2, 17.5, and 20) were studied at 345 bar and 27 °C. The samples in the X-ray scattering cell were stirred with the magnetic stir bar to help them equilibrate and were visually inspected to confirm that a second phase was not present. A small correction to the total scattering was made for scattering from the density fluctuations in the near-critical solvent by subtracting the scattering of pure CO2 (fitted to an Ornstein-Zernike model) in the cell at the same temperature and pressure.
Figure 3. Effect of surfactant concentration on pressuretemperature phase diagram for W/CO2 microemulsions supported by di-HCF4 at wo ) 10.
Results and Discussion Previously, we have reported the pressure-temperature phase stability diagrams for W/CO2 microemulsions formed with [di-HCF4] ) 0.05 and 0.1 M.10 New cloud-point data at [di-HCF4] ) 0.15 and 0.2 M are presented in Figures 1 and 2. Without any added water, the surfactant does not dissolve completely at these concentrations, as indicated by a separate solid phase at the bottom of the vessel. However, a cloud point can still be reached by reduction of pressure, indicating that a substantial amount of surfactant still dissolves in CO2. The addition of water stabilizes the system, and optically transparent solutions can be obtained. At these surfactant concentrations, the solution takes on a strong orange color as the cloud point is approached. The phase transformations are reversible, and as the pressure is raised, the solution color changes from orange to yellow and to clear. The cloud-point pressure increases with increasing wo at a fixed temperature, and this phenomenon has also been observed for W/CO2 microemulsions formed in the presence of F7-H75 and PFPE.13 This behavior is indicative of swelling of micelles with added water, which, in turn, strengthens attractive micellemicelle interactions. Thus, higher pressures are required to strengthen tail-solvent interactions to prevent
phase separation. For [di-HCF4] ) 0.2 M, we were not able to obtain optically clear, single-phase solutions at wo ) 15 and 20 at temperatures less than 40 °C. It seems that, beyond a certain limit, there is a slight decrease in the water solubilization capacity with an increase in surfactant concentration. In all cases, there is a slight decrease in the density (5-10%) that corresponds to the cloud-point pressure with increasing temperature. For example, at [di-HCF4] ) 0.2 M, the density at the cloud point decreases from 0.90 to 0.84 g/cm3 as the temperature increases from 15 to 55 °C. A similar behavior was also observed for W/O microemulsions created with supercritical ethane and AOT.15 The P-T phase diagram at wo ) 10 for different diHFC4 concentrations is shown in Figure 3. As the surfactant concentration increases from 0.1 to 0.4 M, there is no appreciable increase in cloud-point pressure; thus, the cloud point is nearly independent of the dispersed-phase volume fraction. This also suggests that the micelle size is independent of surfactant concentration in this region of the phase diagram. A similar behavior was also observed at low AOT concentrations in organic solvents. It is important to note that a surfactant concentration of 0.4 M and wo ) 10 translates into ∼25 wt % surfactant and 8 wt % water. It might
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Figure 4. Differential scattering cross section as a function of scattering vector.
also be possible to increase the percentage of water. To our knowledge, these are the highest water and surfactant concentrations ever achieved for water-in-CO2 microemulsion systems at comparable pressures. Figure 4a shows the SAXS results for microemulsions of di-HCF4 in CO2 at 0.1 M, 345 bar, and 27 °C with wo ) 0, 5, 10, 14.2, 17.5, and 20. These are conditions previously shown10 to produce single-phase microemulsions; this single-phase condition was verified visually prior to each SAXS experiment. The figure is a plot of the differential scattering cross section as a function of the magnitude of the scattering vector, Q, after corrections for background and scattering due to the cell and the pure solvent. The differential cross section increases and shifts to lower Q as wo increases, which is the expected result of swelling of the micelles through the addition of water to their cores. Figure 4b shows the same results in a log-log form These results were interpreted by fitting the data to models of the structure of the microdispersion. Most frequently micelles are inferred to be in the form of a spherical core and shell and can be polydisperse in the core radius. Indeed, such a model fit our data well; the parameters resulting from fitting such a model are presented in Table 1. It is seen that the resulting core radius is essentially constant and that the shell radius increases with increasing wo, a result we believe to be unphysical. A monodisperse cylindrical model also yielded reasonable but not perfect agreement with the experimental results. Figure 5a-f illustrates the degree of agreement between model and
Table 1. Micelle Size Parameters from Fitting SAXS Data to a Polydisperse Spherical Core/Shell Model and a Monodisperse Cylinder Model polydisperse spherical core/shell
monodisperse cylinder
w0
rcore/nm
rshell/nm
rcyl/nm
lcyl/nm
0.0 5.0 10.0 14.2 17.5 20.0
3.5 2.0 2.5 2.6 2.7 3.5
4.3 3.5 8.1 8.9 9.0 11.4
0.8 1.7 1.9 2.1 2.4 3.1
4.6 3.9 8.1 12.4 16.0 24.1
data. Table 1 also shows the results of this analysis. The median cylinder radius is seen to grow approximately according to r ∝ wo , and the cylinder length is seen to grow approximately according to l ∝ wo2. The fit of the cylinder model would be improved by the addition of polydispersity, especially at the higher end of the Q range, but we believe that little would be learned by adding another free parameter to the model. The micelle size and shape inferred by fitting imperfect small-angle scattering data to models cannot be unique or rigorous. Nevertheless, we tentatively conclude that the micelles have two characteristic dimensions and are most likely elongated in shape and polydisperse in size. These conclusions could be strengthened by future small-angle neutron scattering experiments with varying degrees of deuteration of the water.
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Figure 5. Fit of monodisperse cylindrical micelle model to scattering data. (a) wo ) 20.0, (b) wo ) 17.5, (c) wo ) 14.2, (d) wo ) 10.0, (e) wo ) 5.0, and (f) wo ) 0.0.
Concluding Remarks
Literature Cited
We have described microdispersions that are stabilized in CO2 by the di-HFC4 surfactant that we have synthesized. We have presented their phase behavior as a function of temperature, pressure, and water-tosurfactant ratio. We have examined these microdispersions using small-angle X-ray scattering and have characterized the micelle shape and size with variations of the water-to-surfactant ratio. In future work, we shall examine this system further using other techniques, and we shall explore some of its potential applications.
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Acknowledgment This work was sponsored in part by the Division of Chemical Sciences of the U.S. Department of Energy (DOE) at Oak Ridge National Laboratory (ORNL) and through subcontract at the University of Tennessee. ORNL is operated for the DOE by UT-Battelle, LLC, under Contract DE-AC05-00OR22725. The authors gratefully acknowledge financial support from Connecticut Innovations, Inc., Grant 98CT013.
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Johnston, K. P.; Randolph, T. W.; Bright, F. V. Water Core within Perfluoropolyether-Based Microemulsions Formed in Supercritical Carbon Dioxide. J. Phys. Chem. B 1997, 101, 6707. (14) Goetheer, E. L. V.; Vortsman, M. A. G.; Keurentjes, J. T. F. Opportunities for process intensification using reverse micelles in liquid and supercritical carbon dioxide. Chem. Eng. Sci. 1999, 54, 1589. (15) Bartscherer, K. A.; Minier, M.; Renon, H. Microemulsions in compressible fluidssA review. Fluid Phase Equilib. 1995, 107, 93.
Received for review April 17, 2001 Revised manuscript received June 15, 2001 Accepted June 22, 2001 IE010342T
