Langmuir 2004, 20, 9953-9959
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Hybrid Fluorocarbon-Hydrocarbon CO2-philic Surfactants. 1. Synthesis and Properties of Aqueous Solutions Audrey Dupont, Julian Eastoe,* Martin Murray, and Laura Martin School of Chemistry, University of Bristol, Bristol, BS8 1TS United Kingdom
Frederic Guittard and Elisabeth Taffin de Givenchy Laboratoire de Chimie des mate´ riaux Organiques et Me´ talliques (C.M.O.M.), Faculte´ des Sciences, Parc Valrose, 06108 Nice, France
Richard K. Heenan ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, OXON OX11 0QX United Kingdom Received June 23, 2004. In Final Form: August 25, 2004 Six different hybrid fluorocarbon-hydrocarbon (F-H) sulfate and sulfonate surfactants, with variations in the relative F/H carbon chain length, have been synthesized and characterized in aqueous solution. These compounds have been targeted for potential activity in densified CO2. Tensiometric data and chemical analyses were consistent with surfactants of high chemical purity. Fluorination in terms of the F/H ratio exerts a strong control over all the surfactant physicochemical properties, including critical micelle concentrations (cmc’s) and adsorption isotherms. One of these partially fluorinated surfactants (the sulfonate φ-F6H4) achieves very low surface tensions in water (γcmc ∼ 19 mN m-1) more reminiscent of fully fluorinated double-chain compounds. Detailed 19F NMR studies revealed that ω′-CF3 groups can exhibit separate signals for monomeric and micellized forms, hence facilitating cmc determinations. Small-angle neutron scattering investigations confirmed the presence of ellipsoidal or extended disklike micelles, depending on the F-H chain asymmetry. For example, a symmetric hybrid F8H8 generates disklike micelles, whereas chain asymmetry in F8H4 or φ-F6H4 tends to drive cylindrical aggregation structures. These changes are consistent with variations in the surfactant packing parameter caused by the different chain F/H ratios. Hence, adsorption and aggregation are shown to respond in a predictable way to the molecular structure of these unusual surfactants.
Introduction Hybrid fluorocarbon-hydrocarbon (F-H) compounds possess separate hydrocarbon and fluorocarbon chains in the same molecule, and owing to this unusual chemical structure they exhibit interesting physicochemical properties.1-16 In particular, previous studies have shown that * To whom correspondence should be addressed. Tel: UK + 117 928 9180. Fax: UK + 117 925 0612. E-mail: julian.eastoe@ bristol.ac.uk. (1) Guo, W.; Li, Z.; Fung, B. M. J. Phys. Chem. 1992, 96, 6738. (2) Yoshino, N.; Hamano, K.; Omiya, Y. Langmuir 1995, 11, 466. (3) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1996, 12, 5768. (4) Ito, A.; Kamogawa, K.; Sakai, H.; Hamano, K.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1997, 13, 2935. (5) Harrison, K.; Goveas, J.; Johnston, K. P.; O’Rear, E. A. Langmuir 1994, 10, 3536. (6) Eastoe, J.; Bayazit, Z.; Martel, S.; Steytler, D. C.; Heenan, R. K. Langmuir 1996, 12, 1423. (7) Keiper, J. S.; Simhan, R.; DeSimone, J. M.; Wignall, G. D.; Melnichenko, Y. B.; Frielinghaus, H. J. Am. Chem. Soc. 2002, 124, 1834. (8) Senapati, S.; Keiper, J. S.; DeSimone, J. M.; Wignall, G. D.; Melnichenko, Y. B.; Frielinghaus, H.; Berkowitz, M. L. Langmuir 2002, 18, 7371. (9) Keiper, J. S.; Behles, J. A.; Bucholz, T. L.; Simhan, R.; DeSimone, J. M.; Wignall, G. D.; Lynn, G. W., Melnichenko, Y. B.; Frielinghaus, H. Langmuir 2004, 20, 1065. (10) Sagisaka, M.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kiatiyanan, B.; Kondo, Y.; Yoshino, N.; Takebayashi, K.; Sakai, H.; Abe, M. Langmuir 2003, 19, 220. (11) Salaniwal, S.; Cui, S. T.; Cummings, P. T.; Cochran, H. D. Langmuir 1999, 15, 5188.
certain hybrid F-H compounds lower significantly surface and interfacial tensions [e.g., ref 3]. Such enhanced properties are rarely obtained when mixing individual fluorocarbon and hydrocarbon surfactants together, owing to a natural incompatibility of H and F chains. Usually mixtures of separate H and F surfactants tend to form mixed or partially segregated micelles.17 Hence, “anchoring” H and F groups together is the driver for the unique properties of these F-H hybrid surfactants. This paper describes aqueous solution, adsorption, and aggregation properties of two series of F-H hybrid anionic surfactants, sulfates denoted FmHn’s and sulfonates termed φ-FmHn’s, both shown in Figure 1. The objectives of this study were twofold: it was of interest to understand the effect of chemical structure on surfactant physicochemical properties and to highlight the individual contribution of fluorocarbon and hydrocarbon fragments on dilute surfactant phase behavior. In pursuit of this, drop volume tensiometry, 19F NMR spectroscopy, and (12) Salaniwal, S.; Cui, S.; Cochran, H. D.; Cummings, P. T. Ind. Eng. Chem. Res. 2000, 39, 4543. (13) Salaniwal, S.; Cui, S. T.; Cochran, H. D.; Cummings, P. T. Langmuir 2001, 17, 1773. (14) Salaniwal, S.; Cui, S. T.; Cochran, H. D.; Cummings, P. T. Langmuir 2001, 17, 1784. (15) Guo, W.; Li, Z.; Fung, B. M.; O’Rear, E. A.; Harwell, J. H. J. Phys. Chem. 1992, 96, 6738. (16) Guo, W.; Li, Z.; Fung, B. M.; O’Rear, E. A. J. Phys. Chem. 1992, 96, 10068. (17) Guo, W.; Guzman, E. K.; Heavin, S. D.; Li, Z.; Fung, B. M.; Christian, S. D. Langmuir 1992, 8, 2368.
10.1021/la048447z CCC: $27.50 © 2004 American Chemical Society Published on Web 10/06/2004
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Langmuir, Vol. 20, No. 23, 2004
Figure 1. Molecular structures of the surfactants.
small-angle neutron scattering (SANS) were employed. These F-H surfactants were then assessed as stabilizers for water-in-CO2 (w/c) microemulsions, and this work will be reported in part 2. Related hybrid surfactants have been investigated for activity in CO2 elsewhere.4-14 Hence, taken together with previous research, the studies of both aqueous and CO2 phase behavior give valuable, new insight to assist in rational design of efficient surfactants for diverse applications in dense fluid CO2. Experimental Section Chemicals. Hybrid sulfate surfactants (F7H4, F8H4, F7H7, and F8H8) were synthesized according to the procedures given by Guo et al.1 Hybrid sulfonates (φ-F4H4 and φ-F6H4) were prepared as detailed by Yoshino et al.2 Purities of all the surfactants and their various synthetic intermediates were assessed by standard spectroscopic and analytical methods (1H and 19F NMR, mass spectrometry, and elemental analysis). Results were in agreement with previous work and indicative of organically pure compounds (e.g., refs 1 and 2). Detail on the syntheses, characterizations, and chemical identifications can be found in the Supporting Information. Sample Preparation. All surfactant solutions, in H2O or D2O, were prepared by mass by dilution of the appropriate stock solutions. When necessary, concentrations were corrected for density (Paar digital density meter DMA 35). All water used for solution preparation and glassware cleaning/rinsing was obtained from a R0100HP Purite system of resistivity 18.2 MΩ cm. Instruments and Methods. Drop Volume Tensiometry. Surface tension measurements were performed at 25.0 °C, using a Lauda TVT1 drop volume tensiometer as described elsewhere.18 Tensiometer components were kept in Micro 90 solutions, and prior to use they were rinsed copiously with large amounts of pure water. Measurements were carried out in dynamic mode; hence time-dependent effects were accounted for by following γ as a function of drop formation time, until stable equilibrium tensions were achieved. The instrument was calibrated with aqueous solutions of absolute ethanol: agreement with literature values was typically (0.2 mN m-1.19 Electrical Conductivities. Conductivity measurements were made at 25.0 °C, using a Jenway electrochemistry analyzer. Surfactant solutions were allowed to reach thermal equilibrium in a water bath at 25.0 ( 0.5 °C, before introducing the conductivity probe. 19F NMR Spectra. NMR measurements of dilute surfactant solutions were performed at 25 °C, using a JEOL Lambda 300 spectrometer operating at 282.65 MHz. The 19F chemical shift data, expressed in parts per million (ppm), were calibrated relative to the external reference trifluoroacetic acid (TFA, δ ) -79.45 ppm). Small-Angle Neutron Scattering Experiments. Deuterium oxide, 99.9% in D-atom, was purchased from Fluorochem Ltd. SANS was used to characterize aqueous micellar aggregates of selected hybrid surfactants (F8H8, F8H4, and φ-F6H4). Experiments were conducted on the time-of-flight small-angle diffractometer, LOQ, at ISIS, Rutherford Appleton Laboratory, Didcot, U.K.20 D2O provided contrast against the surfactant aggregates. Samples were held in Hellma quartz cells and thermostated at (18) Eastoe, J.; Paul, A.; Rankin, A.; Wat, R.; Penfold, J.; Webster, J. R. P. W. Langmuir 2001 17, 7873. (19) Bircumshaw, L. L. J. Chem. Soc. Trans. 1922, 1, 887. (20) Information on SANS instrumentation can be found at http:// www.isis.rl.ac.uk.
Dupont et al. 25 ( 1 °C. The instrument was calibrated using a blend of protonated and deuterated polystyrene homopolymers.21 Neutron counts accumulated over periods of 40 min provided sufficient statistical quality; a 10-min run was allowed for transmissions. Following standard procedures,22 raw data were treated to yield normalized scattering intensities I(Q) in cm-1, where the momentum transfer Q ) (4π/λ) sin(θ/2) and θ is the scattering angle (
