Formation of Carbon Dioxide in Water Miniemulsions Using the Phase

and Department of Chemical Engineering, University of South Florida, Tampa, Florida 33620. Received November 12, 2001. In Final Form: January 25, 2002...
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Langmuir 2002, 18, 3039-3046

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Formation of Carbon Dioxide in Water Miniemulsions Using the Phase Inversion Temperature Method Petros A. Psathas,† Michelle L. Janowiak,‡ Luis H. Garcia-Rubio,*,‡ and Keith P. Johnston*,† Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, and Department of Chemical Engineering, University of South Florida, Tampa, Florida 33620 Received November 12, 2001. In Final Form: January 25, 2002 The curvature of an emulsion of CO2 and brine stabilized with a triblock copolymer of poly(dimethylsiloxane) (PDMS) and poly(ethylene oxide) (PEO), PEO-b-PDMS-b-PEO, was inverted from H2Oin-CO2 to CO2-in-H2O (C/W) by reducing the temperature. A “v-shaped” trough was observed in the interfacial tension between the aqueous and CO2 phases versus temperature at constant CO2 density. The minimum in the interfacial tension coincided with the phase inversion temperature (PIT) where the curvature of the emulsion inverts. Novel C/W miniemulsions consisting of 200 nm droplets, measured by multiwavelength turbidimetry, were formed by low-shear stirring in the low interfacial tension PIT region and then cooling to 25 °C. These droplets were 3 times smaller than those produced isothermally by high shear at a temperature below the PIT region.

Introduction The formation of an emulsion, for example, of water and oil, usually requires a surfactant, which facilitates droplet disruption by lowering the interfacial tension.1,2 The emulsion (as well as microemulsion) preferred morphology is governed by the affinity of the surfactant for each phase.3-8 Temperature can have a large effect on the preferred curvature,6,9-15 for example, due to changes in hydrogen bonding between the ethylene oxide groups and H2O for nonionic surfactants.1,6,16-19 Recently, com* To whom correspondence should be addressed. † University of Texas at Austin. ‡ University of South Florida. (1) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems: Formulation, Solvency and Physical Properties; Marcel Dekker: New York, 1988; Vol. 30. (2) Walstra, P. Formation of Emulsions. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; Vol. 1, pp 57-127. (3) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1988, 24, 654-668. (4) Kellay, H.; Binks, B. P.; Hendrikx, Y.; Lee, L. T.; Meunier, J. Adv. Colloid Interface Sci. 1994, 49, 85-112. (5) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Surfactant Molecular Geometry within Planar and Curved Monolayers in Relation to Microemulsion Phase Behavior. In The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E., Eds.; NATO ASI Series Vol. 324; Kluwer: Amsterdam, 1990; pp 557-581. (6) Kahlweit, M.; Strey, R.; Schomacker, R.; Haase, D. Langmuir 1989, 5, 305-315. (7) Ruckenstein, E. Langmuir 1996, 12, 6351-6353. (8) Anton, R. E.; Graciaa, A.; Lachaise, J.; Salager, J.-L. J. Dispersion Sci. Technol. 1992, 13, 565-579. (9) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D. I. J. Chem. Technol. Biotechnol. 1990, 48, 161-171. (10) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1 1990, 86, 3111-3115. (11) Aveyard, R.; Binks, B. P.; Lawless, T. A.; Mead, J. Can. J. Chem. 1988, 66, 3031-3037. (12) Aveyard, R.; Lawless, T. A. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2951-2963. (13) von Rybinski, W.; a. F., Th. Prog. Colloid Polym. Sci. 1998, 109, 126-135. (14) Zarur, A. J.; Mehenti, N. Z.; Heibel, A. T.; Ying, J. Y. Langmuir 2000, 16, 9168-9176. (15) Shinoda, K.; Friberg, S. Emulsions and Solubilization; John Wiley and Sons: New York, 1986. (16) Alexandridis, P. Macromolecules 1994, 27, 2414-2425.

pressed liquid or supercritical CO2 has been utilized as the nonaqueous phase “oil” at pressures from 50 to 350 bar. Both water-in-CO2 (W/C)20,21 and CO2-in-water (C/ W)22 emulsions have been reported. In addition, polymer latex dispersions have been produced by dispersion polymerization from homogeneous solutions of monomers such as styrene and methyl methacrylate in CO2.23 A critical flocculation density has been characterized for the stabilities of emulsions of liquid acrylate oligomers in CO2 based on light scattering measurements.24,25 A CO2-based emulsion of a desired curvature may be formed by shear for a fixed set of formulation variables (e.g., temperature, pH, salinity, etc.).20-22,26-28 The phase preferred by the surfactant is usually the continuous phase, according to Bancroft’s rule, although exceptions have been observed and explained for oil-water emulsions.29 Alternatively, an emulsion may be subjected to a change in a formulation variable resulting in a gradual transition to the opposite morphology. The mechanism of this “dynamic” or transitional type of inversion is complex and influenced by the emulsion composition and the (17) Kunieda, H.; Taoka, H.; Iwanaga, T.; Harashima, A. Langmuir 1998, 14, 5113-5120. (18) Theodorou, D. N. Macromolecules 1988, 21, 1422-1436. (19) Tadros, T. F. J. Phys. Chem. 1980, 84, 1575-1580. (20) Lee, C. T.; Psathas, P. A.; Johnston, K. P. Langmuir 1999, 15, 6781-6791. (21) Psathas, P. A.; da Rocha, S. R. P.; Lee, C. T., Jr.; Johnston, K. P.; Lim, K. T.; Webber, S. E. Ind. Eng. Chem. Res. 2000, 39, 2655-2664. (22) da Rocha, S. R. P.; Psathas, P. A.; Klein, E.; Johnston, K. P. J. Colloid Interface Sci. 2001, 239, 241-253. (23) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Science 1994, 265, 356. (24) O’Neill, M. L.; Yates, M. Z.; Johnston, K. P.; Smith, C. D.; Wilkinson, S. P. Macromolecules 1998, 31, 2848-2856. (25) Johnston, K. P.; Holmes, J. D.; Jacobson, G. B.; Lee, C. T.; Li, G.; Psathas, P. A.; Yates, M. Z. Reactions and Synthesis in Microemulsions and Emulsions in Carbon Dioxide. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, 2000. (26) Lee, C. T. J.; 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, 11094-11102. (27) Psathas, P. A.; Sander, E. A.; Lim, K. T.; Johnston, K. P. J. Dispersion Sci. Technol., submitted. (28) da Rocha, S. R. P.; Harrison, K. L.; Johnston, K. P. Langmuir 1999, 15, 419-428. (29) Ruckenstein, E. Adv. Colloid Interface Sci. 1999, 79, 59-76.

10.1021/la015677u CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002

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Figure 1. Schematic representation of the effect of formulation variables on the transitional inversion in curvature and interfacial tension of a CO2/brine/nonionic surfactant system.

pathways for the changes in formulation variable(s), as has been shown for oil-water systems.30-33 The interfacial tension (γ), which is known to exhibit a minimum at the inversion point, as depicted in Figure 1, is a key property for characterizing emulsion curvature and dynamic inversions.11,15,34-38 At low temperatures, a certain ethoxylated surfactant partitions more strongly to the aqueous phase due to hydrogen bonding, resulting in an O/W (or C/W) emulsion.1,6,16-19 At high temperatures, where the water-ethylene oxide hydrogen bonds are weaker, the surfactant may be expected to partition toward the oil resulting in a W/O (or W/C) emulsion. During a continuous increase in temperature, the emulsion morphology passes through a very low stability region in which γ is a minimum. A three-phase emulsion may be expected21 that consists of top and bottom excess CO2 and H2O phases, respectively, and a middle surfactant-rich phase that can solubilize maximum amounts of CO2 and H2O. This region is the balanced state of the system (also denoted as WIII39 for water-oil systems). The temperature at which the emulsion morphology inverts is called the phase inversion temperature (PIT).15 Shinoda et al.15,34 first pointed out that one may take advantage of the ultralow interfacial tension of the PIT region to form miniemulsions of very fine droplets in the nanometer size range. In water-oil systems containing mixtures of ethoxylated surfactants with different hydrophilic-lipophilic balance (HLB) values, miniemulsions were formed with droplet sizes in the range of 100-500 nm.15,40-46 This “emulsification by the PIT method” (30) Salager, J.-L.; Marquez, L.; Pena, A. A.; Rondon, M.; Silva, F.; Tyrode, E. Ind. Eng. Chem. Res. 2000, 39, 2665-2676. (31) Minana-Perez, M.; Jarry, P.; Perez-Sanchez, M.; RamirezGouveia, M.; Salager, J.-L. J. Dispersion Sci. Technol. 1986, 7, 331343. (32) Smith, D. J. Phys. Chem. 1990, 94, 3746-3752. (33) Zerfa, M.; Sajadi, S.; Brooks, B. W. Colloids Surf., A 2001, 178, 41-48. (34) Shinoda, K.; Shibata, Y. Colloids Surf. 1986, 19, 185-196. (35) Wade, W. H.; Morgan, J.; Jacobson, J.; Salager, J. L.; Schechter, R. S. J. Soc. Pet. Eng. 1978, 18, 242. (36) Sottmann, T.; Strey, R. J. Chem. Phys. 1997, 106, 8606-8615. (37) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Langmuir 1989, 5, 1210-1217. (38) Pouchelon, A.; Meunier, J.; Langevin, D.; Chatenay, D.; Cazabat, A. M. Chem. Phys. Lett. 1980, 76, 277-281. (39) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworth: London, 1954. (40) Minana-Perez, M.; Gutron, C.; Zundel, C.; Anderez, J. M. a.; Salager, J.-L. J. Dispersion Sci. Technol. 1999, 20, 893-905. (41) Forster, T.; Schambil, F.; von Rybinski, W. J. Dispersion Sci. Technol. 1992, 13, 183-193. (42) Forster, T.; von Rybinski, W.; Wadle, A. Adv. Colloid Interface Sci. 1995, 58, 119-149.

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involves the formation of an emulsion at or close to the PIT to produce small domains of water and oil, followed by rapid cooling (or heating) 20 °C away from the PIT to reduce coalescence and raise the stability. It is reasonable to assume that this concept is also applicable to the waterCO2 interface, as we wish to demonstrate in this work. During the cooling (heating) process, the water (CO2) drops, initially present in the middle phase of the threephase emulsion, gradually disappear and nanometer-sized droplets of CO2 (H2O) “nucleate”. This nucleation is a result of the sharper curvature of the monolayer around CO2 (H2O). In the low-γ PIT region, these fine CO2 droplets can easily coalesce, potentially because of nucleation of holes in the flexible monolayer47 and inefficient Marangoni-Gibbs gradients at the interface. By drawing the system away from the PIT region, the fine CO2 droplets are stabilized against coalescence resulting in a C/W miniemulsion. In emulsions containing CO2, the use of isothermal high shear and various polymeric surfactants has produced stable W/C20,21,27,28 and C/W22 emulsions of 1-5 µm in size. For these emulsions, the dynamic phase inversion technique has not been applied and miniemulsions have not been observed. Interfacial tension measurements have been used to locate the minimum in γ by changing the surfactant structure (i.e., mole percent of ethylene oxide in block and graft copolymers) and consequently the emulsion curvature.28 Moreover, either W/C or C/W emulsions have been formed for ionizable surfactants by varying the pH.21 One side of the “v-shaped” trough in γ in Figure 1 has been measured for this system and others.27 The mapping of a complete transitional inversion in γ with a formulation variable for a given surfactant, that is, both sides of the trough in γ, has not been reported for a CO2-based system. Recently, it has been shown that high internal phase C/W emulsions may be used as templates to prepare well-defined porous polymers via polymerization.48 The first objective of this study is to demonstrate a transitional inversion with temperature for a CO2/brine/ PEO-b-PDMS-b-PEO (PDMS, poly(dimethylsiloxane); PEO, poly(ethylene oxide)) surfactant system. The surfactant structure was R1-(Si(Me)2O)14-Si(Me)2-R1 where R1 ) (CH2)3(OCH2CH2)12OH. The inversion is described in terms of both emulsion curvature (W/C vs C/W emulsion) and a v-shaped interfacial tension trough. The second objective is to utilize this understanding of the inversion region to develop a PIT method to produce a C/W miniemulsion with nanometer-sized droplets. These emulsions contained equal amounts of brine and CO2. The interfacial tension was measured with a high-pressure pendant drop tensiometer at a constant CO2 density, both with deionized water and brine, in an attempt to locate the PIT region characterized by a minimum in γ. The droplet size distribution was measured from the analysis of UV-vis turbidimetry from 200 to 800 nm in terms of base functions with a desired set of properties. The droplet size, size distribution, and emulsion stability are reported as a function of relevant variables in the PIT method including the surfactant concentration, cooling time, initial (43) Forgiarini, A.; Esquena, J.; Gonzalez, C.; Solans, C. Prog. Colloid Polym. Sci. 2000, 115, 36-39. (44) Wadle, A.; Forster, T.; von Rybinski, W. Colloids Surf., A 1993, 76, 51-57. (45) Taisne, L.; Cabane, B. Langmuir 1998, 14, 4744-4752. (46) Landfester, K.; Willert, M.; Antonietti, M. Macromolecules 2000, 33, 2370-2376. (47) Kabalnov, A.; Wennerstrom, H. Langmuir 1996, 12, 276-292. (48) Butler, R.; Davies, C. M.; Cooper, A. I. Adv. Mater. 2001, 12, 1459-1463.

Carbon Dioxide in Water Miniemulsions

temperature, and degree of shear. The mechanisms of emulsion stability, inversion, and miniemulsion formation are discussed given insight provided by the interfacial tension data. Experimental Section The triblock PEO-b-PDMS-b-PEO (Dow Corning Q4-3667) surfactant, R1-(Si(Me)2O)14-Si(Me)2-R1 where R1 ) (CH2)3(OCH2CH2)12OH, was purchased from Dow Corning and used as received. The overall Mn was 2268 g/mol. This architecture with 48.2 wt. % PDMS may be expected to be well-balanced with respect to partitioning between water and carbon dioxide on the basis of related surfactants.28 Deionized water (Nanopure II, 16 µS/cm, Barnstead) was used as indicated. Instrument-grade CO2 (Praxair) was passed through an oxytrap (Oxyclear, model no. RGP-31-300) and an activated carbon filter prior to use. The interfacial tension was measured at a constant CO2 density of 0.842 g/mL using a high-pressure pendant drop apparatus.27 Pendant drops were formed at the end of a surface-modified silica capillary27 (Western Analytical Products) with 183 µm o.d. and 99 µm i.d. The cell was thermostated within 0.1 °C by an Omega temperature controller, heating tape, and fiberglass insulation. The water was introduced in the measurement cell with a constaMetric 3200 HPLC pump (ThermoQuest, Inc.) by setting a desired pressure typically 7-14 bar above the pressure of the measurement cell. Once a suitable drop was formed, the six-port switching valve connecting the pump to the cell was closed and timing of the drop age was started. Several images were recorded until the value of interfacial tension reached equilibrium and stayed constant within 1% of previous recordings. The apparatus and procedure used to form the emulsions by isothermal high shear through a 127 µm capillary are described elsewhere.21 An automatic syringe pump (Isco, model 260D) was substituted for the manual pressure generator in some experiments. Moreover, in one particular experiment, a homogenizer (Avestin, model C-5) was used to form the emulsion by pumping the bottom phase through an adjustable homogenizing valve at a typical pressure of 15 000 psia for 20 min. The temperature was controlled within 0.1 °C by submerging both the view cell and the homogenizer in water baths equipped with temperature controllers (Julabo, Inc.). Electrical conductivity was measured during the experiments in order to determine the emulsion morphology (W/C or C/W), by using a pair of high-pressure platinized stainless steel electrodes with a cell constant of 3.7 cm-1, as described previously.21 The contents in the variablevolume view cell were also stirred with a magnetic stir bar. The criterion used to determine the stability was the formation of a 20% volume of excess CO2 on the top (or water on the bottom) of the cell, after stopping both recirculation and stirring. PIT Emulsification Protocol. The emulsion was formed by a magnetic stir bar for 3 min without recirculation through the capillary, at the specified starting conditions (50 °C/4000 psia or 65 °C/5000 psia). The temperature was measured by a thermocouple inserted into the metal wall of the pressure vessel. It was stirred while cooling to 25 °C and a specified pressure in a period of 2 or 8 min. The pressure was gradually decreased during cooling to final values of 1850 or 1693 psia, respectively, so that a constant CO2 density of 0.856 or 0.842 g/mL was maintained between initial and final conditions. In some experiments, during cooling the emulsion was also recirculated and sheared through a 127 µm capillary. Rapid cooling was achieved by decreasing the water bath temperature with ice. A stable pressure reading after cooling was used as an indication that cell contents had reached an equilibrium temperature. In a few experiments, hexane was injected to achieve a concentration of 3.6 wt % in CO2 utilizing a six-port valve (Valco, Inc.) equipped with a 150 µL external loop. Turbidity Measurements. The high-pressure optical cell with a path length of 137 µm21 was mounted in a UV-vis spectrophotometer (Cary 3E UV-vis) and thermostated at 25 °C with a temperature controller within 0.1 °C. The emulsion sample was obtained by recirculation through a 100 µL external loop in a Valco six-port valve. The emulsion sample was recirculated through the optical cell utilizing a second flow loop which was already filled with a 0.5 wt % surfactant brine solution (0.1 M

Langmuir, Vol. 18, No. 8, 2002 3041 NaCl) and pressurized with CO2 at the desired level. This process diluted the emulsion by 15 times. A rapid increase in the optical density indicated the arrival of the emulsion sample in the optical cell, and the emulsion was recirculated further for 2 min and multiple scans were recorded for each condition to ensure representative sampling. Although the size of the emulsion droplets may change during transport from the sample loop to the optical cell, the change may be expected to be modest for the following reasons. The shear in this secondary recirculation loop was small compared to that in the primary recirculation loop. The average droplet sizes were reproducible (within approximately 10%) and did not change significantly with time during recirculation through the optical cell. Droplet Size Assessment. Multiwavelength turbidity measurements can be used to obtain the average droplet size, size distribution, and concentration of droplet dispersions.49,50 For a polydisperse system, the turbidity is given by

τ(λ) ) lNp

π 4

∫

∞

0

Q(m,R)D2 f(D) dD

(1)

where l is the path length, Np is the number of droplets, Q(m,R) is the extinction efficiency, D is the diameter, and f(D) is the droplet size distribution.51 For particles that both absorb and scatter light, the extinction efficiency is expressed in the following way:

Qext ) Qsca + Qabs

(2)

where Qsca and Qabs are the scattering and absorption efficiencies, respectively. These efficiencies can be calculated using Mie theory.52 The real component of the complex refractive index obtained at one wavelength can be calculated for all the wavelengths of interest through the Kramers-Kronig transforms.53 The droplet size distribution may be approximated by base functions having a desirable set of properties. Log-normal and gamma probability density functions have been extensively used for this purpose. The parameters of the base functions are calculated by minimizing the error between the calculated and experimental spectra. The fitting is done using a standard nonlinear Marquardt-Levenberg algorithm. The base function selected for the droplet size distribution in this study is the lognormal probability density function (PDF),54

f(D) )

1

[

exp -

x2πσD

]

(ln Dg - D) σ2

(3)

The nth moment of the distribution is defined as Jn ) E(Dn) ) exp[n(ln Dg) + (nσ)2/2]. The averages are defined below from the moments of the distribution:

DS ) D32 ) DN )

J3 J2

J1 J0

Dτ ) Dw )

() J6 J3

1/2

J4 J3

The number average is usually more representative of the smaller sizes, while the weight average is more representative of the larger droplets. (49) Elicabe, E. G.; Garcia-Rubio, L. H. Approximations to the Refractive Index for Light Scattering Measurements; Center for Materials Development, University of South Florida: Tampa, FL, 1990. (50) Crawley, G.; Cournil, M.; Di Beneditto, D. Powder Technol. 1997, 91, 197-208. (51) Kerker, M. The Scattering of Light, and Other Electromagnetic Radiation; Academic Press: New York, 1969. (52) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley-Interscience: New York, 1998. (53) Elicabe, G. E.; Garcia-Rubio, L. H. J. Colloid Interface Sci. 1989, 129, 192-200. (54) Bruder, L. M.; Garcia-Lopez, A.; Garcia-Rubio, L. H. Systematic Study of the Sensitivity of Multiwavelength Spectroscopy for Particle Characterization. XXIII Congress in Chemical Engineering, Chisa, 1992.

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Figure 2. Stability of a 50:50 (mass) CO2/0.1 M NaCl emulsion versus temperature, formed isothermally with 3 wt % PEOb-PDMS-b-PEO surfactant at various CO2 densities using a homogenizer. The symbols merely indicate conditions where stability experiments were performed. Solid lines represent isostability contours for 20% volume creaming (C/W emulsions) or settling (W/C emulsions). At the balanced state, the emulsions were highly unstable.

Results and Discussion Emulsification with Shearing and Phase Behavior. The stability of a 50:50 (mass) CO2/0.1 M NaCl brine emulsion, formed with 3 wt % PEO-b-PDMS-b-PEO surfactant, is depicted in the CO2 density versus temperature diagram of Figure 2. The Avestin homogenizer was used to form the emulsion, typically with a differential pressure of 15 000 psia. The contours inside the graph enclose density regions of similar stability, indicated above each curve. This system showed a novel transition with temperature for a CO2-based emulsion. At low temperatures (i.e., 25 and 35 °C) throughout the entire density range studied, a C/W emulsion is observed with stabilities exceeding 24 h at densities above 0.92 g/mL. The curvature was assigned on the basis of observed creaming (rising CO2 bubbles) and conductivities presented below. In visual observation, the emulsion appeared to be very homogeneous and milky white, with 100% of the CO2 phase emulsified in the aqueous phase. The stability was gradually reduced to approximately 1 h by reducing the CO2 density below 0.8 g/mL, presumably as a result of larger CO2 droplets that rose to the surface faster. Lower densities also decreased the homogeneity of the C/W emulsion. Here they appeared to be foamy, consisting of CO2 droplets that coalesce faster to form an upper excess phase. Upon increasing the temperature to 40 °C, the system morphology changed distinctly. In less than 5 s, the emulsion coalesced and separated from both top and bottom forming a thin middle phase estimated to constitute 1-3% of the total volume. While vigorous stirring was maintained, the morphology resembled a semitransparent, very low viscosity one-phase system. On the other hand, upon isothermal high-shear recirculation, a stronger white tint was observed, indicating greater emulsification. Regardless of stirring or high-shear recirculation, the system coalesced within 5 s. This morphology was consistently maintained up to 59 °C with no change in stability and thickness of the thin middle phase. Upon increasing the temperature above 60 °C, the emulsion inverted to CO2-continuous, verified by conductivity (shown below) and the settling of the water droplets from the top down. It had a strong white appearance and stability greater than 4 h at CO2 densities higher than 0.77 g/mL. Even though 100% of the H2O was emulsified

Psathas et al.

Figure 3. Electrical conductivity of a 50:50 (mass) CO2/0.1 M NaCl emulsion versus temperature, formed with 3 wt % PEOb-PDMS-b-PEO surfactant at a constant CO2 density of 0.842 g/mL, using a homogenizer.

in CO2, the emulsion appeared visually to be less homogeneous than the C/W emulsions above. The existence of these inhomogeneities was confirmed by optical microscopy (photomicrographs not shown) in the form of droplet aggregates. These aggregates reflect the inability of the 1000 g/mol PDMS tail to provide complete steric stabilization against the attractive van der Waals forces.55,56 On the basis of previous characterizations of the degree of flocculation with optical microscopy,21 the W/C emulsion may be categorized as moderately flocculated at the highest CO2 density (i.e., 0.85 g/mL) and highly flocculated at the lower densities. The high stability of the emulsion is due in part to gelation from flocculation and hindered sedimentation as described previously for similar PDMS-based surfactants.21 The inversion of this system from C/W to W/C at higher temperatures was also monitored with electrical conductivity (k) measurements, as depicted in Figure 3. The transition through the balanced state into the W/C region resulted in a reduction in conductivity by 3 orders of magnitude, reflecting a low dielectric constant CO2continuous phase. Similar results were formed by shearing the emulsion with an HPLC pump (Thermoquest, Inc.) through a capillary instead of the Avestin homogenizer.21 The stabilities of the emulsions formed with the two techniques were nearly identical in the C/W region at a density of 0.95 g/mL. However, the W/C emulsion formed with the capillary was slightly less stable, approximately 2 h at the highest density, indicating that the weaker shear produced less efficient breakup of the aggregates. The fact that these changes were small is consistent with facile drop breakup due to the low values of interfacial tension reported below. The observed effect of temperature on the emulsion stabilized by the PEO-b-PDMS-b-PEO surfactant is consistent with the above studies of emulsions composed of water and oil. The transfer of surfactant from water (C/W emulsion) to a middle phase with an increase in temperature is due to weakening of hydrogen bonds between water and the ether oxygens. The increase in temperature from 35 °C, where the surfactant prefers water, to a temperature within the range 40-60 °C results in a more balanced monolayer with similar affinities for (55) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (56) Yates, M. Z.; Shah, P. S.; Johnston, K. P.; Lim, K. T.; Webber, S. E. J. Colloid Interface Sci., in press.

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Figure 4. Interfacial tension versus temperature at 0.05 wt % PEO-b-PDMS-b-PEO surfactant concentration and a constant CO2 density of 0.842 g/mL with deionized water.

Figure 5. Interfacial tension versus temperature at 0.05 wt % PEO-b-PDMS-b-PEO surfactant concentration and a constant CO2 density of 0.842 g/mL with 0.1 M NaCl.

both CO2 and H2O. Consequently, a middle phase (WIII) emulsion is formed in the “PIT region” or “balanced state”. In systems without added salt, temperature alone was not sufficient to invert the emulsion to W/C up to 65 °C. A salt concentration of 0.1 M NaCl was required to lower the PIT region within the range 40-60 °C (mean value of 50 °C), as shown in Figure 2. When the aqueous phase was deionized water without added salt, the onset of the PIT region was estimated to be well above 65 °C. Indeed, a stable C/W emulsion was formed up to 65 °C without any large changes from 35 to 65 °C. However, above 55 °C (at a CO2 density of 0.85 g/mL) the stability decreased slightly from approximately 24 to 2-3 h. This indicated a decrease in the affinity of the monolayer toward H2O. Increasing salinity to 0.05 M NaCl lowered the onset of the PIT region to approximately 52 °C, with a mean value in this region of 60 °C. The phase behavior of this system and the stability of the emulsion followed Figure 2 closely except for the shift of the PIT region to higher temperatures. Interfacial Activity as a Function of Temperature. The interfacial tension was measured at a constant CO2 density of 0.842 g/mL (except at 80 and 85 °C where CO2 densities were 0.81 and 0.79 g/mL, respectively, due to pressure limitations) and 0.05 wt % surfactant concentration, as shown in Figure 4. The aqueous phase was deionized water. Throughout the range 25-65 °C, γ remains constant between 0.38 and 0.45 mN/m. The surfactant prefers the H2O phase, and the curvature is not affected by the decrease in solvation of PEO by water at higher temperatures. The monolayer prefers to curve around CO2, forming a C/W emulsion, and both the γ and emulsion data suggest that the system is still not close to the balanced state, even at 65° C. Thus, instead of raising the temperature further we chose to add salt, which is known to drive ethylene oxide groups away from water.8,12,57,58 When 0.1 M NaCl was used in the aqueous phase, the PIT region shifted to lower temperatures (i.e., 40-60 °C), as shown in Figure 5 for the same surfactant concentration. Here, γ passes through a deep minimum at the balanced state where the surfactant partitions equally between brine and CO2. This is the first example to report a full v-shaped trough for γ versus a formulation variable for a single surfactant in a CO2-based system. The shape of the curve clearly indicates the presence of a PIT.

The mapping of the PIT region with interfacial tension has been the focus of many studies involving water-oil systems.9,12,36 Typically, γ changes by 2 or 3 orders of magnitude near the minimum in the PIT region. On the other hand, in Figure 5, γ decreases only by a factor of 3 between 25 and 41 °C. It was not possible to accurately measure values below 0.2 mN/m with our pendant drop technique even for a capillary with an inside diameter of 50 µm as the drops simply streamed out of the tip. The smallest value of approximately 0.1 mN/m was measured at 42 °C by using a 50 µm i.d. tip. The accuracy of this particular measurement was insufficient to include it in Figure 5 due to streaming of the droplet. The lifetime of the droplet on the tip was only 20 s. Upon equilibration, which is likely to take 5 min, it may be presumed that γ would be much lower than this reported value. Therefore, another technique would be required to further narrow the temperature range at the minimum in γ. However, the reported measurements are clearly sufficient to indicate that the PIT minimum is present. Furthermore, the temperature range of the minimum has been located to within 1 °C. Emulsification with the PIT Method. The implementation of the PIT method to form emulsions of H2O and CO2 has not been reported previously. Concentrated (50:50 by mass) C/W emulsions22 formed with ethoxylated hydrocarbon surfactants and W/C20,21 emulsions formed with anionic perfluoropolyether and PDMS-based ionomer surfactants consist of 1-5 µm droplets as determined by optical microscopy. This optical technique does not allow accurate assessment of the polydispersity, nor does it detect sizes below approximately 600 nm at the maximum magnification. Thus, in this study, turbidity measurements of droplets in a range of 200-800 nm are used to determine the number-average and weight-average droplet diameter and the droplet size distribution. All of the experiments were performed at two surfactant concentrations, that is, 1 and 3 wt % (based on both water and CO2). At each concentration, the effect of cooling time (i.e., 2 or 8 min), initial temperature (i.e., 50 or 65 °C), and the presence of shear during cooling were tested. Figure 6 depicts the stability of a representative 50:50 C/W emulsion formed with the PIT method in a CO2 density versus temperature diagram. The emulsion consists of 0.1 M NaCl as the aqueous phase and a 1 wt % overall PEO-b-PDMS-b-PEO surfactant concentration. The system was first heated to 50 °C and 4000 psia (CO2 density is 0.856 g/mL) and stirred vigorously with the stir bar for 3 min without recirculation through the capillary. Then, while stirring it was cooled within 2 min to 25 °C and 1850 psia in order to maintain constant density. In

(57) Aarra, M. G.; Hoiland, H.; Skauge, A. J. Colloid Interface Sci. 1999, 215, 201-215. (58) Binks, B. P.; Fletcher, P. D. I.; Taylor, D. F. J. Langmuir 1997, 13, 7030-7038.

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Psathas et al. Table 1. Comparison between the Isothermal High-Shear Recirculation Emulsification Method and the PIT Method Cs (wt %)

method

avg Dn (nm)

emulsion stability

1 1 3 3

high shear PIT high shear PIT

701 215 711 240

∼2 h >30 min ∼2 h >40 min

Figure 6. Stability of a 50:50 (mass) CO2/0.1 M NaCl emulsion versus temperature, formed with 1 wt % PEO-b-PDMS-b-PEO surfactant concentration, using the PIT method at various CO2 densities. Starting emulsification conditions are 50 °C and 4000 psia, where the CO2 density is 0.856 g/mL. The final conditions are indicated by the symbols.

visual observations, the emulsion appeared very fine having a very strong milky white appearance. It was stable for approximately 30 min at which a 20 vol % of excess H2O phase formed on the bottom. This emulsion stability at a density of 0.856 g/mL is 30 min compared with a value of 2 h in Figure 2 for an emulsion formed by isothermal high shear at a constant temperature of 25 °C. This modest loss in stability is impressive given that so little shear is produced by the magnetic stirrer. Magnetic stirring may not be sufficient to produce an emulsion as homogeneous as in the case of the homogenizer. The viscosity of the emulsion, and not just that of the continuous phase (in this case H2O), determines the magnitude of the shear stress applied to the droplets.59,60 The ultralow viscosity of CO2 greatly weakens shear forces that facilitate droplet deformation and disruption relative to an emulsion containing hydrocarbon oil in the balanced state region. The deformation depends on the ratio of the external stress over the Laplace pressure, expressed in a dimensionless Weber number, given by

We )

GηcR γ

(4)

where ηc is the shear viscosity of the continuous phase, G is the velocity gradient (duz/dy), and R is the droplet radius. According to Walstra et al.,2 the main mechanism of disruption of the emulsion droplets formed by stirring is elongation in laminar flow (viscous forces) or turbulence. In the homogenizer, on the other hand, cavitation together with elongation and turbulence have the potential to produce much smaller droplets. Table 1 presents a direct comparison between the number-average droplet diameters of the emulsions formed by the low-shear PIT and the isothermal highshear recirculation methods. On average, at least four measurements were performed for each condition, and the reproducibility was within 10% of the Dn. Notice that the PIT method produced nanometer-sized droplets in the range of 215-240 nm. These droplets were approximately 3 times smaller than those produced by isothermal high (59) Binks, B. B. Emulsions - Recent Advances in Understanding. In Modern Aspects of Emulsion Science; Binks, B. P., Ed.; The Royal Society of Chemistry: Cambridge, U.K., 1998. (60) Edwards, D. A.; Brenner, H.; Wasan, D. T. Interfacial Transport Properties and Phenomena; Butterworth-Heinemann: Boston, 1991.

Figure 7. Droplet size distributions obtained by the turbidity technique of an emulsion formed with 3 wt % PEO-b-PDMSb-PEO: (a) isothermal high shear at 0.964 g/mL (25 °C and 4000 psia) and (b) the PIT method at a constant CO2 density of 0.856 g/mL for initial and final temperatures of 50 and 25 °C, respectively.

shear. The effect of surfactant concentration on the droplet diameter and the emulsion stability was minimal. Thus, a 1 wt % concentration produced sufficient coverage of the emulsion droplets. As shown in Figure 7, the polydispersity was lower for the PIT emulsions. From this result, it may be inferred that the ultralow γ in the balanced state for the PIT method favors more uniform formation of the emulsion droplets. The reduction in shear and droplet disruption of the emulsion due to the low viscosity of CO2 (ca. 10-3 cP) may contribute to the high polydispersity in the isothermal formation method. In the PIT method where the Laplace pressure is much lower due to the lower γ and where droplets are formed by nucleation, the low viscosity of CO2 does not limit the ability to form a good emulsion. In studies of O/W emulsions, nanometer-sized emulsion droplets have been produced with similar PIT method protocols. Minana-Perez et al.40 found a decrease in droplet size in some cases by 1 order of magnitude using carefully selected ethoxylated sorbitan surfactants that exhibited inversion with temperature. Moreover, Forster et al.41,42,44 used the PIT method in a ternary system containing a

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Table 2. Effect of Cooling Time (Initial and Final Temperatures of 50 and 25 °C, Respectively) on Emulsions Formed by the PIT Method

Table 4. Emulsions Formed by the PIT Method with and without High-Shear Recirculation during Cooling (initial T of 50 °C and 2 min Cooling Time to 25 °C)

Cs (wt %)

cooling time (min)

avg Dn (nm)

emulsion stability

Cs (wt %)

high shear

avg Dn (nm)

emulsion stability

1 1 3 3

2 8 2 8

287 299 253 175

∼30 min ∼20 min ∼24 min ∼20 min

1 1 3 3

with without with without

407 287 438 253

∼35 min ∼30 min ∼30 min ∼24 min

Table 3. Effect of Initial Temperature on Emulsions Formed by the PIT Method for 2 min Cooling Time to the Final Temperature of 25 °C

Table 5. Effect of Hexane as a Hydrophobe in the CO2 Phase on Emulsions Formed by the PIT Method (Initial Temperature of 50 °C and Cooling Time of 2 min)

Cs (wt %)

initial T (°C)

avg Dn (nm)

emulsion stability

Cs (wt %)

hexane

avg Dn (nm)

emulsion stability

1 1 3 3

50 65 50 65

287 521 253 240a

∼30 min ∼25 min 24 min 15 min

1 1 3 3

with without with without

302 287 470 253

∼20 min ∼25 min ∼20 min ∼24 min

a

Very polydisperse sample.

hydrocarbon surfactant/cosurfactant mixture to produce approximately 200 nm monodisperse emulsions of longterm stability. In both studies, increasing surfactant concentration to 5 wt % or higher produced smaller and more monodisperse emulsions. On the other hand, similar droplet sizes were obtained for surfactant concentrations of 1 and 3 wt % in our study. It is likely that the low surfactant requirement for water/CO2 systems is a result of the low γ for the water-CO2 binary system as has been demonstrated previously for W/C microemulsions.61 As a consequence of this low γ, relatively little surfactant is needed to lower the γ to a level sufficiently low for emulsion formation. Another critical parameter in the production of miniemulsions with the PIT method is the rate of cooling as shown in Table 2 at a constant CO2 density of 0.842 g/mL. The initial temperature was kept constant for all experiments reported at 50 °C. As was expected from similar studies in water-oil systems,40,42,44 the effect of cooling time is statistically insignificant. Intuitively, one may expect the formation of smaller droplets with the fastest cooling times due to more intense nucleation and more efficient preservation of the nucleated minidroplets during the inversion. Apparently, the cooling rate was sufficiently rapid in both cases to form miniemulsions. From a practical point of view, the insensitivity of the PIT method to the cooling rate over this range makes the process more robust for scale-up purposes. Table 3 summarizes the results of the effect of initial temperature, that is, 50 or 65 °C for a fixed cooling time of 2 min and constant CO2 density of 0.842 g/mL. The droplet size distribution is almost identical to Figure 7b for a starting temperature of 50 °C. In water-oil systems, when starting at a temperature within the PIT region, the droplet size is smaller than when starting from the W/O region and crossing the entire PIT zone.40 Similarly, in our study for the 1 wt % surfactant concentration, a transition from 65 to 25 °C produced droplets almost twice as large as those formed when starting at 50 °C. When the entire transition from W/C (at 65 °C) to C/W (at 25 °C) is allowed to take place, the time for cooling in the low-γ region is much larger compared to the case where the initial condition is 50 °C. As a result, the nucleated emulsion droplets coalesce into larger ones that skew the size distribution. In both cases, the emulsion was not recirculated through the capillary. The shear produced by the stir bar was relatively low. Thus, most of the small (61) da Rocha, S. R. P.; Johnston, K. P. Langmuir 1999, 16, 36903695.

emulsion droplets were formed by phase inversion during cooling and not shear. Unfortunately, the emulsion formed with 3 wt % surfactant was highly polydisperse. The PIT method was also used to form W/C miniemulsions by heating the stirred system from 50 to 65 °C at constant density with a salinity of 0.1 M NaCl. The emulsion unfortunately was very flocculated and inhomogeneous, indicative of poor steric stabilization by the short (i.e., 1000 g/mol) PDMS tail. During heating from 50 to 65 °C, extensive association of the small droplets into aggregates was observed, which could not be broken by stirring. The use of high shear through the homogenizer produced only slightly less flocculated W/C emulsions than those formed with the HPLC pump and capillary tube. The effect of high-shear recirculation during cooling through a 127 µm capillary on the droplet size is shown in Table 4. The cooling rate was kept at 2 min, the initial temperature was 50 °C, and the CO2 density was constant at 0.842 g/mL. At both surfactant concentrations, highshear recirculation during the cooling from 50 to 25 °C resulted in larger droplets by a factor of 2, with the same polydispersity. It is likely that the imposed shear stress on the emulsion droplets in the low-γ region during cooling induces coalescence.62 Given the ultralow γ, the structure approaches that of a bicontinuous microemulsion with very elastic surfactant monolayers separating the newly formed droplets. When the droplets are forced to pass through the narrow capillary, the film of the continuous phase (i.e., water at 25 °C) between the droplets is greatly deformed and can rupture more easily through nucleation of a hole, resulting in coalescence.47,63 Moreover, the Marangoni-Gibbs gradients become less efficient in this low-γ regime in preventing coalescence. Finally, the 3.6 wt % hexane was added to CO2 in an attempt to suppress Ostwald ripening. This emulsion destabilization mechanism is based on the molecular diffusion of the dispersed phase through the continuous medium from smaller to larger droplets. A substance that is soluble in the dispersed phase (in our case CO2) can generate an osmotic force preventing shrinking of the smaller droplets.45,46,64,65 The results are depicted in Table 5. The initial temperature was 50 °C, the cooling time was 2 min, and the CO2 density was kept constant at 0.842 g/mL. In the case of 3 wt % surfactant, the presence of hexane produced droplets twice as large, which is counterintuitive based on the arguments above. Perhaps (62) Borwankar, R. P.; Lobo, L. A.; Wasan, D. T. Colloids Surf. 1992, 69, 135-146. (63) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 1931-1935. (64) Webster, A. J.; Cates, M. E. Langmuir 2001, 17, 595-608. (65) Langevin, D. Adv. Colloid Interface Sci. 2000, 88, 209-222.

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the hexane lowered the adsorption of the surfactant at the interface causing it to partition toward the continuous phase. Conclusions The multiwavelength turbidity technique is a highly useful technique for the assessment of the droplet size distribution of emulsions under pressure. C/W miniemulsions composed of 200 nm droplets are formed with the PIT emulsification method with low shear. A characteristic v-shaped trough in γ in the PIT region with a minimum of 0.2 mN/m at 41 °C serves as a road map to design pathways to produce stable miniemulsions. The temperature-induced inversion in the curvature of the CO2/brine/ PEO-b-PDMS-b-PEO system coincides with a minimum in interfacial tension in the range 40-60 °C. The C/W miniemulsions were formed by low-shear stirring in the PIT region (three-phase system), followed by rapid cooling (e.g., 25 °C/min) to 25 °C. The formation of nanometer-

Psathas et al.

sized droplets is facilitated by the ultralow γ in the PIT region which favors droplet nucleation at the onset of inversion to the C/W morphology. In the cases where highshear recirculation was added during the PIT cooling process, shear-induced coalescence skewed the distribution to larger sizes. At 25 °C, the average droplet size was over 3 times smaller for emulsions formed by the PIT method versus those formed isothermally under highshear recirculation. In the PIT method, smaller droplets were produced for an initial temperature in the PIT region relative to the W/C emulsion region. Acknowledgment. This material is based upon work supported by the STC Program of the National Science Foundation under Agreement No. CHE-9876674 and the Separations Research Program at the University of Texas. We thank KSV Ltd. for their invaluable help in upgrading the pendant drop interfacial tension apparatus. LA015677U