Effect of Surfactants on the Interfacial Tension between Supercritical Carbon Dioxide and Polyethylene Glycol Kristi L. Harrison, Keith P. Johnston,* and Isaac C. Sanchez Department of Chemical Engineering, University of Texas, Austin, Texas 78712 Received November 7, 1995 X The effects of various surfactants on the interfacial tension between supercritical CO2 and 600 MW polyethylene glycol (PEG) are reported at 45 °C on the basis of measurements with a novel tandem variable- volume pendant drop tensiometer. The interfacial tension of the CO 2-polyethylene glycol binary system decreases from 9.0 dyn/cm at 85 bar to 3.1 dyn/cm at 300 bar, primarily because of the increase in the density and likewise the free energy density of the CO 2 phase. This result is predicted quantitatively with a gradient model using the lattice fluid equation of state. The experimental results for the effects of three surfactants on the interfacial tension are described by a modified Winsor R theory in terms of the molecular interactions on each side of the interface and the surfactant solubility in each phase. At 276 bar, the addition of 1 wt % ammonium carboxylate perfluoropolyether (PFPE) surfactant reduces the interfacial tension from 3.2 to 2.1 dyn/cm and the interfacial area of the surfactant is 437 Å 2 /molecule. In contrast, surface activity is not observed for a hydrocarbon polyether (Brij30), since it favors the organic phase, or for a high molecular weight fluoropolymer (PolyFOA), since it favors the CO 2 phase. Because PFPE is interfacially active, it stabilizes PEG-in-CO2 microemulsions. Introduction Surfactants are beginning to play an important role in supercritical fluid (SCF) science and technology. The early work in this field addressed reverse micelles in SCFs and water-in-SCF microemulsions, for fluids such as ethane and propane, 1-5 as reviewed recently. 6,7 Various experi- mental techniques were used to determine the micro- emulsion properties including droplet size, water-to- surfactant ratio, phase behavior, curvature of the interface, polarity and degree of water motion in the droplet interior, and the solubilization and partitioning of various probes. For SCF microemulsions, models have been developed to understand and predict the phase behavior, interfacial tension, bending moment, drop size, and droplet-droplet interactions. 8,9 Supercritical carbon dioxide (T c ) 31 °C, P c ) 73.8 bar) is an attractive alternative to liquid and SCF organic solvents because it is nontoxic, nonflammable, and inex- pensive. However, because of its very low dielectric con- stant, ǫ, and polarizability per volume, R/v, CO 2 is a poor solvent for water and most nonvolatile lipophilic or hydro- philic solutes. Consequently, it may be considered a third type of condensed phase, which is very different from either lipophilic or hydrophilic phases. Thus, reverse micelles, microemulsions, and emulsions with either lipophilic or hydrophilic cores may be formed in CO 2 . These organized molecular assemblies should be able to solubilize a wide variety of new compounds into a CO 2 continuous phase. Because of the low values of ǫ and R/v for CO 2 , the most “CO 2 -philic” types of functional groups for a surfactant tail are expected to have low cohesive energy densities, e.g. fluorocarbons, fluoroethers, and siloxanes. 10-14 Although water-in-SCF microemulsions have been formed readily in SCF alkanes, it is very difficult to form them in CO 2 . More than 150 surfactants have been tested in systematic studies, 15-18 but the first indications of microemulsions have been reported only recently. 12,14,19-21 For example, FTIR, UV-visible absorbance, fluorescence, and EPR experiments have demonstrated the existence of an aqueous domain in CO 2 with a polarity approaching that of bulk water with a perfluoropolyether ammonium carboxylate surfactant. 21 For almost all of the previously mentioned surfactants, water did not dissolve in the CO 2 phase. Progress was slow, and the negative results offered few guidelines for designing better surfactants. What is really needed to advance the effective utilization of surfactants in SCF science and technology is an experimental and a theoretical description of the inter- facial activity of surfactants in these systems. The interfacial tension, γ, is a key property of interest not only for microemulsions but for all surfactant-based SCF technology. Recently, inverse emulsion 20 and dispersion 22 X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1988, 92, 2903-2907. (2) Fulton, J. L.; Blitz, J. P.; Tingey, J. M.; Smith, R. D. J. Phys. Chem. 1989, 93, 4198-4204. (3) Johnston, K. P.; McFann, G. J.; Lemert, R. M. In Pressure Tuning of Reverse Micelles for Adjustable Solvation of Hydrophiles in Super- critical Fluids; Johnston, K. P., Penninger, J. M. L., Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989; pp 140-164. (4) McFann, G. J.; Johnston, K. P. J. Phys. Chem. 1991, 95, 4889- 4896. (5) McFann, G. J. Ph.D. Thesis, The University of Texas at Austin, 1993. (6) Bartscherer, K. A.; Minier, M.; Renon, H. Fluid Phase Equilib. 1995, 107, 93-150. (7) McFann, G. J.; Johnston, K. P. In Supercritical Microemulsions; Kumar, P., Mittal, K. L., Eds.; Marcel Dekker: New York, in press. (8) Peck, D. G.; Johnston, K. P. J. Phys. Chem. 1991, 95, 9549-9556. (9) Peck, D. G.; Johnston, K. P. J. Phys. Chem. 1993, 97, 5661-5667. (10) Hoefling, T. A.; Newman, D. A.; Enick, R. M.; Beckman, E. J. J. Supercrit. Fluids 1993, 6, 165-171. (11) Newman, D. A.; Hoefling, T. A.; Beitle, R. R.; Beckman, E. J.; Enick, R. M. J. Supercrit. Fluids 1993, 6, 205-210. (12) (a) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945-947. (b) Fulton, J. L.; Pfund, D. M.; McClain, J. B.; Romack, T. J.; Maury, E. E.; Combes, J. R.; et al. Langmuir 1995, 11, 4241-4249. (13) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworths: Stonham, MA, 1994. (14) Harrison, K.; Goveas, J.; Johnston, K. P.l O’Rear, E. A. Langmuir 1994, 10, 3536-3541. (15) Oates, J. Ph.D. Thesis, The University of Texas at Austin, 1989. (16) Iezzi, A.; Enick, R.; Brady, J. In Direct Viscosity Enhancement of Carbon Dioxide; Johnston, K. P., Penninger, J. M. L., Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989; pp 122-139. (17) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51-65. (18) McFann, G. J.; Johnston, K. P.; Howdle, S. M. AIChE J. 1994, 40, 543. (19) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127-7129. (20) Adamsky, F.; Liu, W.; Lepilleur, C.; Enick, R. M.; Beckman, E. J. Microemulsions in CO2. 3rd International Symposium on Supercritical Fluids, Strasbourg, France, 1994. (21) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science, in press. 2637 Langmuir 1996, 12, 2637-2644 S0743-7463(95)01013-4 CCC: $12.00 © 1996 American Chemical Society