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Langmuir 1996, 12, 1423-1424
1423
Droplet Structure in a Water-in-CO2 Microemulsion Julian Eastoe,* Zeynep Bayazit, and Sarah Martel School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
David C. Steytler School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
Richard K. Heenan ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K. Received October 31, 1995. In Final Form: January 25, 1996X High-pressure small-angle neutron-scattering measurements show that a dichain hydrocarbonfluorocarbon surfactant ((C7H15)(C7F15)CHSO4-Na+) stabilizes spherical droplets, with mean water core radius around 25 Å, in a water-in-CO2 microemulsion (temperature ) 25 °C, pressure ) 500 bar). At lower pressures droplet clustering is observed. Our structural work on this system builds on initial phase stability studies by Johnston et al. (Langmuir 1994, 10, 3536).
Introduction Liquid or supercritical CO2 is a nontoxic, chemically inert, nonflammable fluid and as such is preferable to conventional petrochemical solvents as a reaction/extraction medium. The main disadvantage of CO2 is that it is a ‘poor’ solvent for polar solutes. The formation of surfactant aggregates in CO2 offers a promising approach for enhancing solubility levels, since such organized fluids can behave as “universal solvents”. In this respect a waterin-CO2 (w/c) microemulsion system, with similar properties to the more conventional water-in-oil microemulsions1 (w/o or L2 phases), would be ideal. Recently the formation of w/o phases in low density butane, propane, and ethane at elevated pressures has been demonstrated using hydrocarbon surfactants such as AOT, DDAB, and C12E52-10 (see ref 10 for a recent review). The small-angle neutron-scattering (SANS) measurements show that spherical droplets are present in the single-phase region. As the fluid density approaches the lower limit of F ∼ 0.5 g cm-3, a cloud point is reached at a critical pressure Pc and temperature Tc. Close to this critical point droplet clustering occurs. Recent theoretical work is able to explain the formation and interfacial properties of these systems.11 In order to overcome the problem of low solubility of hydrocarbon surfactants in CO2,12 Beckman et al. used * Author to whom correspondence should be addressed. Telephone: UK + 117 9289000 ext. 4726. Fax: UK + 117 9250612. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, March 15, 1996. (1) Robinson, B. H. Chem. Brit. 1990, 26, 342. (2) Gale, R. W.; Fulton, J. L.; Smith, R. D. J. Am. Chem. Soc. 1987, 109, 920. (3) Eastoe, J.; Robinson, B. H.; Steytler, D. C. J. Chem. Soc. Faraday Trans. 1990, 86, 511. (4) Smith, R. D.; Tingey, J. M.; Fulton, J. L.; Watson, D. M. J. Phys. Chem. 1991, 95, 1445. (5) McFann, G. J.; Johnston, K. P. Langmuir 1993, 9, 2942. (6) Blitz, J. P.; Smith, R. D.; Fulton, J. L. J. Phys. Chem. 1988, 92, 2707. (7) Eastoe, J.; Robinson, B. H.; Young, W. K.; Steytler, D. C. J. Chem. Soc., Faraday Trans. 1990, 86, 2883. (8) Kaler, E. W.; Bilman, J. F.; Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1991, 95, 458. (9) Eastoe, J.; Robinson, B. H.; Steytler, D. C.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1994, 90, 3121. (10) Bartscherer, K. A.; Renon, H.; Minier, M. Fluid Phase Equilib. 1995, 107, 95. (11) Peck, D. G.; Johnson, K. P. J. Phys. Chem. 1993, 97, 5661.
fluorocarbon analogues.13 After this initial work Johnston et al. demonstrated that a dichain hybrid surfactant ((C7H15)(C7F15)CHSO4-Na+ or H-F) can microemulsify slightly more than its own weight (∼2%) of water in CO2.14 The surfactant H-F is only soluble in CO2 on addition of water, and as is found for w/o microemulsions (e.g., ref 3), the w/c phase stability is sensitive to both P and T. The unusual molecular structure meets two important criteria for promoting reversed micelle formation in CO2: (i) a voluminous hydrophobe that induces the surfactant film to curve toward water and (ii) an enhanced compatibility in CO2 which is assisted by the fluorocarbon moiety.13,14 Here we report SANS measurements as a function of pressure, on a w/c phase stabilized by the H-F surfactant. The data and analysis clearly show that spherical droplets are formed, making this the first direct observation of droplet structure in a CO2 microemulsion. Experimental Section The H-F surfactant was obtained as described by O’Rear et al.15 The chemical purity was 98-99%, as told by 1H, 19F, and 13C NMR (Jeol GX400) as well as elemental microanalysis. Until used, the surfactant was stored in a dessicator, over refreshed P2O5, at -20 °C. Both duNouy ring tensiometry (Kruss K12) and electrical conductivity (Jencon electrochemical analyzer) gave the aqueous phase critical micelle concentration (cmc) as (4.7 ( 0.10) × 10-4 mol dm-3 at 25 °C, in good agreement with previous work.15 D2O (Fluorochem 99% D-atom) and CO2 (B.O.C.) were used as received. SANS measurements were performed on the LOQ instrument at the Rutherford Appleton Laboratory at ISIS, using a high-pressure cell described before (maximum operating pressure 500 bar).9 The measurements determine the absolute scattering cross section I(Q) (cm-1) as a function of momentum transfer Q (Å-1) ) (4π/λ) sin(θ/2) with λ the incident neutron wavelength (2.2 f 10 Å) and θ the scattering angle (
