Langmuir 2001, 17, 7191-7193
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Water in Carbon Dioxide Macroemulsions and Miniemulsions with a Hydrocarbon Surfactant Keith P. Johnston,* Dongman Cho, Sandro R. P. DaRocha, Petros A. Psathas, and Won Ryoo Department of Chemical Engineering, University of Texas, Austin, Texas 78712
Stephen E. Webber Department of Chemistry and Biochemistry, University of Texas, Austin, Texas 78712
Julian Eastoe and Audrey Dupont School of Chemistry, University of Bristol, Bristol BS8 ITS, United Kingdom
David C. Steytler School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom Received July 9, 2001. In Final Form: August 31, 2001 Dilute water in carbon dioxide miniemulsions and macroemulsions are formed with a hydrocarbon surfactant, sodium bis(3,5,5-trimethyl-l-hexyl)sulfosuccinate. The emulsion droplets do not appear to flocculate at 30-60 °C and 29 MPa indicating that solvation of the tails by CO2 is sufficient to screen the interdroplet attractive interactions. The solvation of the tails is enhanced by the low cohesive energy density methyl groups that interact more favorably with CO2 than methylene groups.
Introduction Microemulsions1,2 and emulsions3,4 of water in compressed carbon dioxide may be utilized as environmentally benign solvents in extraction and cleaning processes,5 phase transfer reactions and catalysis,6,7 enzymatic catalysis,8,9 and as a template in materials formation.10,11 It is challenging to form stable micro- and macroemulsions in CO2 due to strong interactions between the water droplets resulting from weak solvation of the surfactant tails.12-14 This weak solvation arises in part from the low polarizability density of CO2.15 To date, only surfactants with fluorocarbon tails, which also have low polarizability (1) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624-626. (2) Eastoe, J.; Cazelles, B. M. H.; Steytler, D. C.; Holmes, J. D.; Pitt, A. R.; Wear, T. J.; Heenan, R. K. Langmuir 1997, 13, 6980-6984. (3) da Rocha, S. R. P.; Harrison, K. L.; Johnston, K. P. Langmuir 1999, 15, 419-428. (4) Lee, C. T.; Psathas, P. A.; Johnston, K. P.; deGrazia, J.; Randolph, T. W. Langmuir 1999, 15, 6781-6791. (5) Campbell, M. L.; Apodaca, D. L.; Yates, M. Z.; McCleskey, T. M.; Birnbaum, E. R. Langmuir 2001, 17, 5458-5463. (6) Jacobson, G. B.; Lee, C. T.; da Rocha, S. R. P.; Johnston, K. P. J. Org. Chem. 1999, 64, 1207-1210. (7) Jacobson, G. B.; Lee, C. T., Jr.; Johnston, K. P.; Tumas, W. J. Am. Chem. Soc. 1999, 121, 11902-11903. (8) Kane, M. A.; Baker, G. A.; Pandey, S.; Bright, F. V. Langmuir 2000, 16, 4901-4905. (9) Holmes, J. D.; Steytler, D. C.; Rees, G. D.; Robinson, B. H. Langmuir 1998, 14, 6371-6376. (10) Ji, M.; Chen, X.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631-2632. (11) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langmuir 1999, 15, 6613-6615. (12) Peck, D. G.; Johnston, K. P. Macromolecules 1993, 26, 1537. (13) Meredith, J. C.; Johnston, K. P. Langmuir 1999, 15, 80378044. (14) Lee, C. T., Jr.; Johnston, K. P.; Dai, H. J.; Cochran, H. D.; Melnichenko, Y. B.; Wignall, G. D. J. Phys. Chem. B 2001, 105, 35403548.
densities, have been shown to stabilize w/c microemulsions.14 Either perfluoropolyether4 or poly(dimethylsiloxane)3,16 surfactant tails are able to stabilize concentrated w/c macroemulsions for many hours, although to some extent flocculation has always been present. Such concentrated emulsions have not been reported for surfactants with hydrocarbon tails, which have higher cohesive energy densities. Recently, there has been great interest in forming microemulsions and emulsions in CO2 with hydrocarbon surfactants. Dilute w/c emulsions have been formed with a triblock copolymer hydrocarbon surfactant, poly(propylene oxide-b-ethylene oxide-b-propylene oxide), Pluronic 17R4.3 The pendant methyl groups in the outer blocks have a much lower surface energy (cohesive energy density) than CH2 groups favoring solvation by CO2.15 Random copolymers composed on PO and carbonates have been shown to be unusually soluble in CO2 due to a combination of low cohesive energy density and favorable specific polar interactions.17 The addition of methyl groups to sulfosuccinate surfactants has enhanced solubility markedly and led to the formation of reverse micelles in pure CO2, as demonstrated with small-angle neutron scattering for sodium bis(3,5,5-trimethyl-l-hexyl)sulfosuccinate (AOT-TMH).18 Here, we demonstrate with static and dynamic light scattering and microscopy that this low molecular weight (15) 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, 3067-3079. (16) Psathas, P. A.; da Rocha, S. R. P.; Lee, C. T.; Johnston, K. P.; Lim, K. T.; Webber, S. Ind. Eng. Chem. Res. 2000, 39, 2655-2664. (17) Sarbu, T.; Styranec, T.; Beckman, E. J. Nature 2000, 405, 165168. (18) 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, 988-989.
10.1021/la0110388 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001
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Langmuir, Vol. 17, No. 23, 2001
Figure 1. Photomicrographs of an emulsion in carbon dioxide at 30 °C and 29 MPa with 1 wt % water and 1 wt % AOT-TMH.
surfactant, AOT-TMH, stabilizes dilute w/c miniemulsions and macroemulsions with average diameters of 50 nm and 4 µm, respectively. The macroemulsion droplets do not appear to flocculate at 30-60 °C and 29 MPa, unlike the case for each of the emulsions cited above which was stabilized with a polymeric surfactant. For systems with 1 wt % water, w/c emulsions are formed whereas those containing equal amounts of water and carbon dioxide lead to c/w emulsions. The reduction in interdroplet interactions resulting from the increased solvation of the tails is sufficient for the formation of the w/c emulsions. Results and Discussion A new batch of the above surfactant AOT-TMH was synthesized as described before.19 Equilibrium surface tension-concentration behavior, measured using a Lauda TVTI drop volume tensiometer at 25 °C, was identical to that determined before, indicating a high surface chemical purity of the new surfactant. The phase behavior of AOTTMH was measured in pure CO2. It dissolved at 50 °C and 31 MPa after several hours of stirring with a magnetic stir bar for a loading of 0.1 wt %. Phase separation conditions, that is, slightly turbid cloud points, were observed upon cooling to 40 °C at a constant pressure of 34.5 MPa and upon reducing pressure to 29.0 MPa at a constant temperature of 80 °C. The lower solubility in this study relative to that of Eastoe et al.19 at 50 MPa is due to the lower pressure. In contrast, the widely used surfactant bis(2-ethyl-l-hexyl)sulfosuccinate (AOT) does not even melt in the presence of CO2 under these conditions and is extremely insoluble. Dilute w/c emulsions were formed in a 28 mL variable volume view cell by recirculation through a capillary tube (0.13 mm internal diameter by 5 cm length) with an HPLC pump (Thermoquest) as described previously.16 At first, the emulsion was formed with 1 wt % of water and 1 wt % surfactant. Since an excess amount of surfactant was loaded, relative to its solubility in pure CO2, only part of it dissolved. The emulsion phase was formed in the presence of an excess phase containing water and surfactant. The emulsion droplets were observed in an external microscopy cell with a path length of 0.13 mm. As shown in Figure 1, approximately 4 µm water droplets were observed at 30 °C and 29 MPa. The water droplets were relatively uniform in size and did not flocculate. At 29 MPa, the water droplet sizes did not change signifi(19) Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733-8740.
Letters
Figure 2. Average droplet sizes measured by dynamic light scattering for w/c emulsions as a function of elapsed time after recirculation was stopped (T ) 35 °C, P ) 31 MPa, 1 wt % 0.1 M NaCl brine, 0.1 wt % AOT-TMH).
cantly over a temperature range of 22-60 °C. Upon lowering the pressure to 9 MPa, the droplets settled but did not appear to flocculate. The droplet sizes of the emulsions were measured with dynamic light scattering (DLS), as has been done with a similar apparatus for organic emulsions in CO2.20 The blackened stainless steel cylindrical light scattering cell had two fixed parallel circular sapphire windows (2.54 cm diameter and 0.70 cm thickness) with a path length of 0.7 cm. For DLS measurements, the emulsion apparatus above included a porous metal filter (Valco; 0.5 µm pore size). The filter contributed to the shear of the emulsion droplets along with the capillary tube. The light source was a He-Ne laser with a wavelength of 632.8 nm, and the scattered light signal was detected utilizing a fiber with an avalanche photodioide detector. The data were analyzed by using a digital autocorrelator (Brookhaven; BI-9000AT) with 200 real-time channels and the CONTIN program. The measured average droplet sizes were essentially constant as the scattering angle was varied from 15° to 10° with a reproducibility of less than 20%. With DLS, three different droplet sizes were observed at 35 °C and 31 MPa as shown in Figure 2. The largest droplets were 4 µm in agreement with the above microscopy results. The diameter of the smallest droplets was 50 nm, in the miniemulsion range, and approached the microemulsion range. They were several times greater than those observed for previously reported w/c microemulsions with aqueous cores (∼10 nm).2,21 The larger droplets are consistent with the larger water-to-surfactant ratio in the new miniemulsions relative to the earlier transparent single-phase microemulsions. Due to the excess phase, the water-to-surfactant ratio in the droplets was unknown. The diameter of these droplets was 20-fold that of the dry reverse micelle diameter of AOT-TMH (2.8 nm) formed in supercritical carbon dioxide.18 The number fraction and the volume fraction of the 50 nm droplets was above 99%; the remainder of the distribution was composed of the other two fractions. In addition, midsize (20) Yates, M. Z.; O’Neill, M. L.; Johnston, K. P.; Webber, S.; Canelas, D. A.; Betts, D. E.; DeSimone, J. M. Macromolecules 1997, 30, 50605067. (21) Lee, C. T.; Psathas, P. A.; Ziegler, K. J.; Johnston, K. P.; Dai, H. J.; Cochran, H. D.; Melnichenko, Y. B.; Wignall, G. D. J. Phys. Chem. B 2000, 104, 11094.
Letters
Figure 3. Turbidity at λ ) 500 nm as a function of elapsed time after shearing and recirculation were stopped. (T ) 50 °C, P ) 31 MPa, 1 wt % brine (0.1 M NaCl), 0.1 wt % AOT-TMH).
droplets were observed with a diameter of approximately 300 nm. The droplet sizes were not measured in the first 30 min after recirculation due to potential convection and kinetic instabilities in the emulsions, for example, Ostwald ripening. At the shortest times in Figure 2, all three droplet diameters increased. After 50 min, the sizes became relatively constant for the smallest droplets and decreased slowly for the intermediate-sized droplets. At 170 min, the largest droplets had settled. The turbidity of the emulsions was measured as a function of time to further characterize the stability by recirculating the emulsions through a cell with a path length of 2 cm and measuring the absorbance with a Cary 3E spectrophotometer. The aqueous phase included 0.1 M NaCl to make the aqueous phase less favorable for the surfactant. The emulsion turbidity increased for an hour after the shearing and stirring were stopped as shown in Figure 3. This initial increase in turbidity is consistent with the growth in particle diameters observed in the DLS measurements. The decrease in turbidity at later times is consistent with the settling of the larger particles, which was also observed with microscopy and DLS. To understand the phase behavior of these emulsions further, a 50 wt % mixture of water and carbon dioxide
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was formed with 0.1 wt % surfactant. This 50/50 water to carbon dioxide ratio is useful for determining the natural curvature of the interface, since the entropy of mixing either phase into the other is similar.22 For a temperature range of 10-65 °C and a pressure range of 100-275 bar, a very homogeneous milky white opaque c/w emulsion was formed. The emulsions were c/w and not w/c since the meniscus moved upward leaving an excess bottom phase, indicating creaming of CO2 droplets.23 After 7 min, the clear region in the bottom phase reached 20%. After 10 min, the emulsion separated into a clear bottom phase and an upper slightly foamy phase of equal volume, which underwent gradual coalescence. Lowering the temperature to 10 °C increased the stability only slightly, as the density of the bubbles became closer to that of water. This formation of a c/w emulsion with a low molecular weight surfactant complements an early study which formed these emulsions with block copolymer hydrocarbon surfactants.23 The addition of 0.1 or 0.36 M salt drove the surfactant out of the water phase into a thin middle phase, with clear excess water and CO2, but not into the CO2 phase. These results indicate that the curvature in the dilute w/c emulsions formed in the light scattering and microscopy studies was opposite the natural curvature. This type of curvature inversion is well-known for water/oil emulsions22 since the entropy of mixing a small amount of water in a large amount of continuous phase is highly favorable. The DLS, turbidity, and microscopy results indicate consistently that AOT-TMH, which is substantially more active at the water-CO2 interface than AOT, can stabilize w/c miniemulsions and w/c and c/w macroemulsions. Acknowledgment. This material is based upon work supported in part by the STC Program of the National Science Foundation under Agreement No. CHE-98766674, the Separations Research program at the University of Texas at Austin, the Welch Foundation, and EPSRC Grants GR/L05532 and GR/L25653. A.D. thanks EPSRC and the University of Bristol for a studentship. LA0110388 (22) Salager, J.-L.; Marquez, L.; Pena, A. A.; Rondon, M.; Silva, F.; Tyrode, E. Ind. Eng. Chem. Res. 2000, 39, 2665-2676. (23) da Rocha, S. R. P.; Psathas, P. A.; Klein, E.; Johnston, K. P. J. Colloid Interface Sci. 2001, 239, 241-253.
