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Notes Phosphate Surfactants for Water-in-CO2 Microemulsions David C. Steytler,* Emily Rumsey, and Matthew Thorpe School of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, United Kingdom Julian Eastoe* and Alison Paul School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom Richard K. Heenan ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, OXON OX11 0QX, United Kingdom Received May 23, 2001. In Final Form: August 16, 2001
Introduction In recent years, there has been a significant advance in the design and synthesis of fluorocarbon-based CO2compatible amphiphiles including both surfactants for water-in-CO2 (w/CO2) microemulsions1-6 and larger block copolymers, which form micelles and stabilize emulsions in CO2.7-10 However, in comparison with microemulsion formation in oil media, relatively few surfactants have to date proven effective at stabilizing w/CO2 microemulsions without addition of cosurfactant. Johnston initially reported microemulsion formation using a “hybrid” dichained sulfate2 (C7H15 C7F15‚CHSO4Na) and later a single-chain, commercial grade anionic PFPE carboxylate (CF3(CF2CF(CF3)-O-)3CF2CO2- NH4+).3 More recently a range of dichained sodium sulfosuccinate compounds (ROOCCH2CH(SO3-Na+)COOR have been studied to determine the * Corresponding authors. David C. Steytler: tel, UK + 1603 592033; fax, UK + 1603 25985; e-mail,
[email protected]. Julian Eastoe: tel, UK + 117 9289180; fax, UK + 117 9250612; e-mail,
[email protected]. (1) Beckman, E. J.; Hoefling, T. A.; Enick, R. M. J. Phys. Chem. 1991, 95, 7127-7129. (2) Harrison, K.; Goveas, J.; Johnston, K. P.; O’Rear, E. A., III Langmuir 1994, 10, 3536-3541. (3) 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. (4) 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. (5) (a) Eastoe, J.; Downer, A.; Paul, A.; Steytler, D. C.; Rumsey, E.; Penfold, J.; Heenan, R. K. Phys. Chem. Chem. Phys. 2000, 2, 52355342. (b) Liu, Z.-T.; Erkey, C. Langmuir 2001, 17, 274-277. (6) 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, 11094-11102. (7) McClain, J. B.; Betts, D. E.; Canelas, D. A.; Samulski, E. T.; DeSimone, J. M.; Londono, J. D.; Cochran, H. D.; Wignall, G. D.; ChilluraMartino, D.; Triolo, R. Science 1996, 274, 2049-2053. (8) Londono, J. D.; Dharmapurikar, R.; Cochran, H. D.; Wignall, G. D.; McClain, J. B.; Betts, D. E.; Canelas D. A.; DeSimone, J. M.; Samulski, E. T.; Chillura-Martino, D.; Triolo, R. J. Appl. Crystallogr. 1997, 30, 690-695. (9) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Science 1994, 265, 356-359. (10) O’Neill, M. L.; Yates, M. Z.; Harrison, K. L.; Johnston, K. P.; Canelas, D. A.; Betts, D. E.; DeSimone, J. M.; Wilkinson, S. P. Macromolecules 1997, 30, 5050-5059.
structure-function relationship between the perfluoro hydrophobe (R) identity and microemulsion stability in CO2.5 Chemical modification of the PFPE carboxylates has now been also proved successful in producing a cationic surfactant capable of stabilizing w/CO2 microemulsions.6 The design of hydrocarbon surfactants for use in CO2 represents a considerable challenge, but significant inroads have recently been made in establishing the chemical groups required for CO2-compatible hydrophobes.11,12 On the basis of the success of straight-chain perfluoro analogues of the surfactant Aerosol OT (sodium diethylhexyl sulfosuccinate) at stabilizing w/CO2 microemulsions, we here examine related perfluoro analogues of the surfactant NH4DEHP (ammonium diethylhexyl phosphate) that has, like AOT, proved highly effective for solubilization of water in oil media.13 For water-in-oil microemulsions stabilized by NH4DEHP, it has been shown that a low level of free diethylhexyl phosphoric acid dramatically reduces stability such that phase separation can be induced even by contact with gaseous CO2. Fortuitously, the inductive effect of fluorine in the fluorocarbon analogues lowers the pKa and facilitates their application in dense CO2 where, dependent upon temperature and pressure, the pH of water droplets is in the range 3-3.5. Although supercritical conditions are useful in fractionation processes, the cost and engineering requirement for scale-up of practical applications of CO2 are much simplified when liquid CO2 is used at its vapor pressure. The ability to stabilize microemulsions under this condition is therefore an important feature of surfactant design. In this regard, we have recently reported a significant lowering of both the air-water limiting surface tensions and w/CO2 microemulsion phase boundary pressures for CF3-terminated sulfosuccinates compared with analogous CHF2-terminated surfactants.5a One such compound (diCF4) was found to stabilize w/CO2 microemulsions at CO2 vapor pressure, but this result was tempered by the high cost of the precursor alcohol. In this communication, we report formation of w/CO2 microemulsions by two CHF2terminated dialkyl phosphate surfactants (Figure 1). Significantly, one of the compounds examined (di-HCF6P) forms stable systems in liquid CO2 at vapor pressure and can be obtained at a fraction of the cost of di-CF4. Experimental Section Surfactant Synthesis. Dialkyl phosphoric acids based on the alcohols 1H,1H,5H-octafluoro-n-pentanol and 1H,1H,7Hdodecafluoro-n-heptanol were synthesized following a standard procedure14 with benzotrifluoride replacing benzene as the organic solvent. The dialkyl phosphate synthesis allows the free acid form of the surfactant to be isolated and washed with water (11) Sarbu, T.; Styranec, T.; Beckman, E. J. Nature 2000, 405, 165168. (12) 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. (13) Steytler, D. C.; Sargeant, D. L.; Welsh, G. E.; Robinson, B. H.; Heenan, R. K. Langmuir 1996, 12, 5312-5318. (14) Peppard, D. F.; Mason, G. W.; Giffin, G. J. Inorg. Nucl. Chem. 1965, 27, 1683-1691.
10.1021/la010758b CCC: $20.00 © 2001 American Chemical Society Published on Web 11/06/2001
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Figure 2. Air-water surface tension vs surfactant concentration for the perfluoro dialkyl phosphate surfactants di-HCF4-P and di-HCF6-P. T ) 25 °C. Figure 1. Chemical structures and abbreviations used for the perfluoro dialkyl phosphate surfactants and related sulfosuccinate surfactants. to remove any inorganic material. This is an important feature as it is known that small amounts of salt impurity have a significant influence on the phase behavior of microemulsions formed by ionic surfactants in CO2.15 The ammonium salts were prepared by neutralizing a methanol solution of the acids with concentrated ammonium hydroxide to pH ∼ 9. The solvent and excess ammonia were then removed under vacuum at 50 °C on a rotary evaporator, and the resulting product was dried in a desiccator over P2O5. Purity was established by elemental analysis, and the absence of monoester was confirmed by pH titration. In accord with our previous nomenclature adopted for sulfosuccinate surfactants, we designate these phosphate surfactants as di-HCF4-P and di-HCF6-P, respectively. Surface Tension. Measurements were made by the duNouy ring method on thermostated solutions using a Kruss K10ST tensiometer with standard correction procedures applied for the ring configuration. Phase Behavior. The P-T microemulsion phase stability was determined by visual inspection using a stirred high-pressure optical cell that has been described elsewhere.16 The sample composition is defined in terms of surfactant concentration and a water-to-surfactant molar ratio, w, which represents the solubilization capacity of the microemulsion. Since the cell has variable volume, surfactant concentrations depend on pressure and are quoted based on a cell volume of 12 cm3. Small-Angle Neutron Scattering (SANS). The experiments were performed using a high-pressure optical cell fitted with sapphire windows giving a path length of 1 cm. Measurements were made on the LOQ time-of-flight instrument at ISIS17 to give the scattering cross section I(Q) (cm-1) as a function of momentum transfer Q (Å-1) ) (4π/λ) sin(θ/2); λ is the incident neutron wavelength (2.2 f 10 Å), and θ is the scattering angle (
