Langmuir 1999, 15, 6781-6791
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Water-in-Carbon Dioxide Emulsions: Formation and Stability C. Ted Lee, Jr., Petros A. Psathas, and Keith P. Johnston* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712
Janet deGrazia and Theodore W. Randolph Department of Chemical Engineering, The University of Colorado at Boulder, Boulder, Colorado 80309 Received March 24, 1999. In Final Form: June 8, 1999 Stable water-in-carbon dioxide (W/C) emulsions, for either liquid or supercritical CO2 containing up to 70 vol % water, are formed with various molecular weight perfluoropolyether ammonium carboxylate surfactants. Water droplet sizes ranging from 3 to 10 µm were determined by optical microscopy. From conductivity measurements, an inversion to C/W emulsions results from a decrease in CO2 density or salinity at constant pressure, a decrease in surfactant molecular weight, or an increase in temperature. Emulsions become more stable with a change in any of these formulation variables away from the balanced state, which increases interfacial tensions and interfacial tension gradients, enhancing Marangoni-Gibbs stabilization. This type of stability is enhanced with an increase in the molecular weight of the surfactant tails, which increases the thickness of the stabilizing films between droplets. W/C emulsions formed with the 7500 molecular weight surfactant were stable for several days.
Introduction Together, water and carbon dioxide (CO2) constitute the two most abundant and environmentally benign solvents on earth. Liquid or supercritical CO2 (Tc ) 31 °C, Pc ) 73.8 bar) exhibits solvent properties that are tunable with pressure, and it is essentially nontoxic and nonflammable. Dense CO2 is nonpolar (unlike water) and has weak van der Waals forces1 (unlike oils) and as such may be considered a third type of fluid phase in nature, somewhat similar to fluorocarbons. Dispersions of water-in-CO2, whether on the nanometer (microemulsions) or micrometer (emulsions) scale, offer new possibilities for separations on the basis of polarity, and as media for reactions between polar and nonpolar molecules.2-5 Conventional hydrocarbon surfactants used in oil/water (O/W) systems often exhibit low solubilities in CO2 and are incapable of solubilizing a significant amount of water.6-8 Surfactants with fluoroalkyl and fluoroether tails are quite soluble in CO2,8-10 because the weak dispersion forces for these tails are well matched to those of CO2.11 (1) O’Shea, K.; Kirmse, K.; Fox, M. A.; Johnston, K. P. J. Phys. Chem. 1991, 95, 7863. (2) Jacobson, G. B.; Lee, C. T.; daRocha, S. R. P.; Johnston, K. P. J. Org. Chem. 1999, 64, 1207-1210. (3) Jacobson, G. B.; Lee, C. T.; Johnston, K. P. J. Org. Chem. 1999, 64, 1201-1206. (4) Holmes, J. D.; Steytler, D. C.; Rees, G. D.; Robinson, B. H. Langmuir 1998, 14, 6371-6376. (5) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 119, 6399-6406. (6) Iezzi, A.; Enick, R.; Brady, J. In Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J. M. L., Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989. (7) Oates, J. PhD Dissertation, University of Texas: Austin, TX, 1989. (8) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51-65. (9) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127-7129. (10) Hoefling, T. A.; Beitle, R. R.; Enick, R. M.; Beckman, E. J. Fluid Phase Equilib. 1993, 83, 203-212. (11) 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.
Recently, the formation of water-in-CO2 (W/C) microemulsions has been demonstrated for fluoroether or hybrid fluorocarbon-hydrocarbon surfactants.12-15 These microemulsions are thermodynamically stable, isotropic mixtures of water, CO2, and surfactant, and typically consist of dispersed phase droplets from 2 to 50 nm in diameter. In contrast to microemulsions, (macro)emulsions contain relatively large droplets (>0.1 µm) which, although no longer thermodynamically stable, may be kinetically stable for long periods. Furthermore, macroemulsions may be formed with higher interfacial tensions between water and oil (or CO2) than in the case of microemulsions, and thus lower values of surfactant adsorption at the interface. Therefore, emulsions may be formed for a wider variety of surfactants than microemulsions, and with lower surfactant concentrations. This becomes especially important when dealing with expensive fluorinated surfactants often used in CO2. Emulsions are inherently unstable because of the large interfacial free energy. This thermodynamic instability is manifested in the various mechanisms of emulsion destabilization: aggregation, coalescence, sedimentation, and Ostwald ripening. Aggregation is the result of attractive forces between droplets and can be prevented by supplying a sufficiently strong repulsive force between the droplets. For W/C emulsions, electrostatic repulsion is negligible because of the very short Debye length resulting from low dielectric constants. Therefore, other repulsive forces must be present to stabilize W/C emulsions, including steric forces and stabilizing forces due to (12) 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. (13) Harrison, K.; Goveas, J.; Johnston, K. P.; O’Rear, E. A. Langmuir 1994, 10, 3536-3541. (14) Eastoe, J.; Cazalles, B. M. H.; Steytler, D. C.; Holmes, J. D.; Pitt, A. R.; Wear, T. J.; Heenan, R. K. Langmuir 1997, 13, 6980-6984. (15) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. Langmuir 1997, 13, 3934-3937.
10.1021/la9903548 CCC: $15.00 © 1999 American Chemical Society Published on Web 08/26/1999
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interfacial tension gradients in the thin film separating approaching droplets. Two approaching emulsion droplets may be stabilized against aggregation by the use of macromolecular surfactants that provide steric stabilization.16 Under good solvent conditions, the surfactant tails are extended, which results in a repulsive force acting over relatively large distances. Decreasing the solvent power results in rapid aggregation (flocculation) of droplets at the critical flocculation point as the solvent expands away from the surfactant tails, resulting in a strong attractive force between the surfactant tails. For high molecular weight stabilizers, the critical flocculation temperature (CFT) often occurs within a few degrees Celsius of the θ-temperature (the temperature at which an infinite molecular weight polymer becomes insoluble in the solvent when dissolved at infinite dilution) of the stabilizing moiety of the surfactant dissolved in the continuous phase.16 For low molecular weight stabilizers, the CFT often occurs at temperatures significantly before the θ-temperature.17 Furthermore, droplet aggregation (coagulation) may result for low molecular weight steric stabilizers even under good solvent conditions, because the relatively short tails cannot provide a repulsive force over a sufficient distance where the van der Waals attractive forces become important (510 nm). Here, following the La Mer convention,18 flocculation refers to droplet aggregation due to an attractive force between the tails, whereas coagulation refers to aggregation as a result of the van der Waals attractive force between the droplets, excluding the tails. Recently, a lattice-fluid self-consistent field theory (LFSCF) was developed to describe the mechanism of steric stabilization of emulsions and latexes in supercritical fluids as a function of the extension of the surfactant tails.19-22 According to LF-SCF theory and computer simulations,23 these colloids flocculate at the same density (critical flocculation density, or CFD) where the stabilizer moiety of the surfactant phase separates from the SCF in bulk solution. This principle has been verified experimentally24,25 and is useful for designing surfactants for emulsions and polymer latexes26-28 in supercritical fluids. As two droplets approach each other, tangential flow of the liquid film between the droplets may generate surfactant concentration gradients along the interface. The resulting unfavorable interfacial tension gradients resist drainage of the film and may prevent aggregation and/or coalescence (Marangoni-Gibbs stabilization). Bulk diffusion of surfactant to the regions of low concentration reduces the interfacial tension gradient, eliminating this type of stabilization. If the surfactant is more soluble in the dispersed phase, the diffusion length is small and the (16) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (17) Cowell, C.; Li-In-On, R.; Vincent, B. J. Chem. Soc., Faraday Trans. 1 1978, 74, 337-347. (18) La Mer, V. K. J. Colloid Sci. 1964, 19, 291. (19) Peck, D. G.; Johnston, K. P. Macromolecules 1993, 26, 1537. (20) Peck, D. G.; Johnston, K. P. J. Phys. Chem. 1993, 97, 5661. (21) Meredith, J. C.; Johnston, K. P. Macromolecules 1998, 31, 55185528. (22) Meredith, J. C.; Johnston, K. P. Macromolecules 1998, 31, 55075517. (23) Meredith, J. C.; Sanchez, I. C.; Johnston, K. P.; de Pablo, J. J. J. Chem. Phys. 1998, 109, 6424-6434. (24) Heitner-Wirguin, C. Polymer 1979, 20, 371-374. (25) Robert, A.; Tondre, C. J. Colloid Interface Sci. 1984, 98, 515522. (26) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Science 1994, 265, 356-359. (27) Lepilleur, C.; Beckman, E. J. Macromolecules 1997, 30, 745756. (28) O’Neill, M. L.; Yates, M. Z.; Johnston, K. P.; Smith, C. D.; Wilkinson, S. P. Macromolecules 1998, 31, 2848-2856.
Lee et al.
Figure 1. Schematic representation of the (a) phase behavior and (b) interfacial tension for water/CO2/ionic surfactant mixtures as a function of formulation variables.
effect of Marangoni-Gibbs stabilization is minimal. However, if the surfactant is more soluble in the continuous phase, it must diffuse through the thin film to reach the interface, resulting in sustained interfacial tension gradients and emulsion stability.29,30 Furthermore, local fluctuations in film thickness result in interfacial tension gradients that dampen these fluctuations (Gibbs elasticity effect), provided that the surfactant cannot diffuse quickly into this concentration gradient region.31 These arguments are consistent with Bancroft’s rule, which states that the phase in which the surfactant is more soluble will be the continuous phase of the emulsion.32 The stability of O/W and W/O microemulsions and macroemulsions is highly dependent on the phase behavior, interfacial tension (γ), and natural curvature (curvature in the absence of interdroplet interactions), which are interrelated, as shown in Figure 1.29,31,33-35 Here we address emulsions composed of similar amounts of water and oil in which the droplet interactions caused by the entropy of mixing play a small role. As is well-known for water-oil emulsions and microemulsions, the phaseinversion point, e.g., the phase-inversion temperature (PIT), corresponds to a minimum in interfacial tension and zero net curvature. Here the surfactant has equal affinity for both the oil and water phases (a so-called balanced system).29,31,34,36 At the balanced state, γ and gradients in γ are small and thus emulsions tend to be very unstable.30,31,34,37,38 Changing any formulation variable away from the phase-inversion point causes the surfactant to become preferentially soluble in one phase, resulting in increased emulsion stability. At low values of the hydrophilic-lipophilic balance (HLB), the surfactant favors the oil phase, and low conductivity W/O emulsions (29) Binks, B. P. Langmuir 1993, 9, 25-28. (30) Edwards, D. A.; Brenner, H.; Wasan, D. T. Interfacial Transport Processes and Rheology; Butterworth-Heinemann: Boston, 1991. (31) Kabalnov, A.; Wennerstro¨m, H. Langmuir 1996, 12, 276-292. (32) Bancroft, W. D. J. Phys. Chem. 1913, 17, 501. (33) Binks, B. P. Modern Aspects of Emulsion Science; The Royal Society of Chemistry: Cambridge, U.K., 1998. (34) Shinoda, K.; Frieberg, S. Emulsions and Solubilization; WileyInterscience: New York, 1986. (35) Walstra, P. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1996; Vol. 4. (36) Bourrel, M.; Graciaa, A.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1979, 72, 161-163.
Formation and Stability of W/C Emulsions
are formed. For high values of HLB, the surfactant favors water, and O/W emulsions are formed with high conductivities. A limited understanding of the balanced state for surfactants in CO2, as well as the mechanism of steric stabilization in supercritical fluids, has made the formation of W/C emulsions elusive. These factors are compounded by faster thin film drainage rates and droplet settling rates that are expected in W/C relative to W/O emulsions because of the relatively low viscosity of CO2. Furthermore, faster surfactant diffusion rates, which tend to oppose the formation of interfacial tension gradients, add to these challenges. The objective of this study is to achieve a mechanistic understanding of W/C and C/W emulsion formation and stability as a function of five formulation variables: CO2 density, temperature, salinity, added alcohol, and surfactant tail length, as well as the volume fraction of water. We have chosen to examine ammonium carboxylate perfluoropolyether (PFPE-NH4) surfactants with molecular weights from 672 to 7500 to vary steric forces and to manipulate the balanced state with regard to surfactant partitioning. These surfactants have been shown to be very CO2-philic with large solubilities (.5 wt %) in CO2 at moderate pressures.5,6 Furthermore, these surfactants exhibit very low solubilities in water (e.g.,
