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Incorporation of Fluorocarbon in Fluorinated Surfactant Based Liquid Crystals M. H. Ropers* and M. J. Ste´be´ Equipe Physico-Chimie des Colloı¨des, UMR 7565, Faculte´ des Sciences, BP 239, F-54506 Vandoeuvre-les-Nancy, France Received July 15, 2002. In Final Form: January 22, 2003 The solubilization of a fluorocarbon oil in a nonionic fluorinated surfactant/water system is investigated by small-angle X-ray scattering to understand its role in the phase transitions between liquid crystals and to compare hydrogenated and fluorinated systems. The analysis of the phase behavior investigated between 5 and 40 °C shows the disappearance of the sponge phase even at low oil fractions whereas the lamellar and cubic phases are preserved and the hexagonal phase appears at high oil fractions due to the vanishing of the physical frustration of the curvature. The rate of oil incorporation is a function of both water-tosurfactant ratio and temperature, especially in the lamellar and cubic phases. Perfluorodecalin is mainly incorporated as a free film and induces a strong surfactant chain ordering in the hexagonal phase. In addition to to the lamellar/cubic transition previously pointed out in the binary system, the incorporation of oil induces new epitaxial relationships between the hexagonal phase and the two cubic structures of space groups Ia3d and Pn3m. Finally, the whole phase transitions in the liquid crystal domain of this ternary system offer a better understanding of the similarities in the local structure of the liquid crystal structures.
Introduction Surfactant/water systems form, at high surfactant concentrations, liquid crystals of various structures tuned by temperature and composition.1-3 The addition of truly lipophilic compounds in the organized structures is often possible, but the capacity of incorporation depends on the molecular geometry of the solubilized compound and may induce phase transitions.4,5 As concerns the nonionic surfactants, numerous studies were developed by Friberg: 3,6-9 the solubilization of hydrocarbons is drastically influenced by temperature10 and has been generalized under the PIT (phase inversion temperature) concept.3 More recently, the incorporation of hydrocarbons in liquid crystals of polyoxyethylenealkyl ether C12En/water systems has received attention to analyze the phase transitions through the changes in the interfacial film.11 The substitution of decane for xylene induces opposite changes in the curvature of the surfactant film due to the higher capacity of alkyl chains to incorporate xylene than decane. The peculiar class of fluorinated surfactants has also been considered. They form organized molecular systems in a * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Ekwall, P. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: 1975; Vol. 1. (2) Tiddy, G. J. T. Phys. Rep. 1980, 57, 1. (3) Lyotropic Liquid Crystals; Friberg, S., Ed.; Advances in Chemistry Series No. 1521; American Chemical Society: Washington, DC, 1976. (4) Tardieu, A.; Luzzati, V. Biochim. Biophys. Acta 1970, 219, 11. (5) Maddaford, P. J.; Toprakcioglu, C. Langmuir 1993, 9, 2868. (6) Friberg, S.; Mandell, L.; Fontell, K. Acta Chem. Scand. 1969, 23, 1055. (7) Emulsions and Solubilization; Shinoda, K., Friberg, S., Eds.; J. Wiley: New York, 1986. (8) Friberg, S.; Flaim, T. In Inorganic Reactions in Organized Media; Holt, L., Ed.; ACS Symposium Series 177; American Chemical Society: Washington, DC, 1982. (9) Ward, A. J.; Friberg, S.; Larsen, D. W. In Macro- and Microemulsions: Theory and Applications; Shah, D. O., Ed.; ACS Symposium Series 272; American Chemical Society: Washington, DC, 1985. (10) (a) Saito, H.; Shinoda, K. J. Colloid Interface Sci. 1967, 24, 10. (b) Saito, H.; Shinoda, K. J. Colloid Interface Sci. 1968, 26, 70. (11) Kunieda, H.; Ozawa, K.; Huang, K.-L. J. Phys. Chem. B 1998, 102, 831.
Figure 1. Temperature-composition phase diagram of the surfactant C6F13C2H4SC2H4(OC2H4)2OH in water (adapted from ref 16).
variety comparable to that of the hydrogenated surfactants. For example, polyoxyethylenefluoroalkyl ethers exhibit a similar phase sequence as their hydrogenated analogues.12-17 Apart from their well-known properties such as their high efficiency at interfaces and their wetting properties, they enable solubilization of fluorocarbon and gas in water used in biomedical applications. The cosolubilization of fluorocarbon and water by means of fluorosurfactants can be described in terms of the PIT.15 The incorporation of fluorocarbon into liquid crystals has rarely been considered, whereas numerous techniques such as X-ray scattering, NMR, or vibrational spectroscopies could be very helpful. (12) Achilefu, S.; Selve, C.; Ste´be´, M. J.; Ravey, J. C.; Delpuech, J. J. Langmuir 1994, 10, 2131. (13) Kratzat, K.; Guittard, F.; De Givenchy, E. T.; Cambon, A. Langmuir 1996, 12, 6346. (14) Ravey, J. C.; Ste´be´, M. J. Prog. Colloid Polym. Sci. 1987, 73, 127. (15) Ravey, J. C.; Ste´be´, M. J. Colloids Surf., A 1994, 84, 11. (16) Ropers, M. H.; Ste´be´, M. J. Phys. Chem. Chem. Phys. 2001, 3, 4029. (17) Ravey, J. C. In Microemulsions: Structures and Dynamics; Friberg, S., Bothorel, P., Eds.; CRC Press: Boca Raton, FL, 1987; pp 93-117.
10.1021/la026243h CCC: $25.00 © 2003 American Chemical Society Published on Web 03/05/2003
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Figure 2. Partial ternary phase diagram (surfactant-rich corner) of the system CF6 ΣE2/H2O/PFD (w/w %) at several temperatures: (a) 5 °C, (b) 10 °C, (c) 20 °C, (d) 30 °C, and (e) 40 °C. The symbols H2, L2, V2, and LR refer to the reverse hexagonal phase, the reverse micellar phase, the reverse bicontinuous cubic phase, and the lamellar phase, respectively. The dotted line in the cubic domain separates the regions described by the space groups Pn3m (noted P) and Ia3d (noted I).
The present study aims to understand the incorporation of perfluorocarbon (perfluorodecalin) in the liquid crystal domain of a system composed of a fluorinated nonionic surfactant, C6F13C2H4SC2H4(OC2H4)2OH, and water. The solubilization is analyzed in terms of phase transitions and changes in structural parameters. This surfactant exhibits in water a hydrophobic character by forming at high fractions a sponge phase L3, a bicontinuous cubic phase V2, a lamellar phase LR, and a reverse micellar phase L2 (Figure 1).16 The bicontinuous cubic domain is divided into two domains characterized by two space groups Ia3d and Pn3m. The thin tongue at the highest water fractions corresponds to the cubic symmetry Pn3m, and the bigger domain is associated to the cubic symmetry Ia3d. Smallangle X-ray scattering (SAXS) and Raman spectroscopy previously showed that the hydrophobic and ethylene oxide layers are unmodified with water dilution in the whole liquid crystal domain. The fluorinated chains adopt an all-trans conformation, while the hydrophilic chains prefer the meander conformation independently of the composition. The PIT of this system in the presence of perfluorodecalin was empirically evaluated at -88 °C,15 so the
investigated temperature range here characterizes the system over the PIT. In the first part, partial phase diagrams of the ternary system are reported and analyzed; then in the second part the structural parameters and relationships are determined and discussed. Materials and Methods Materials. The nonionic fluorinated surfactant C6F13C2H4SC2H4(OC2H4)2OH, noted CF6 ΣE2 (MS ) 512 g/mol, VS ) 343.8 cm3/mol at 25 °C), was synthesized in our laboratory according to a method described by Cambon and co-workers.18 The identity and purity of the product were checked by thin-layer chromatography and NMR experiments. Water with a low resistivity comes from a Milli-Q system. Perfluorodecalin C10F18, noted PFD, is a mixture of the cis and trans isomers in the ratio 45/55. It is supplied from Fluorochem Ltd. with a purity attested to 99% (MO ) 462 g/mol, VO ) 236.9 cm3/mol at 25 °C). Phase Behavior. The phase behavior was obtained by the classical method. A large number of samples of various compositions covering the whole liquid crystal domain were prepared. First the surfactant CF6 ΣE2 and water are mixed, and then PFD (18) Cambon, H.; Delpuech, J. J.; Matos, L.; Serratrice, G.; Szyonyi, S. Bull. Soc. Chim. Fr. 1986, 6, 965.
Incorporation of Fluorocarbon in Liquid Crystals is added. Samples were stored for several days before the temperature scan. The phase diagram was established between 5 and 40 °C by steps of 5 °C. Phases were first identified by visual inspection with a polarizing light microscope and then characterized by SAXS. SAXS. X-ray measurements on the lamellar phase were carried out using a home-built apparatus, equipped with a classical tube (λ ) 1.54 Å). The X-ray beam is focused by means of a curved gold/silica mirror on the detector placed at 500 mm from the sample holder, which is thermally controlled with an accuracy of 0.5 °C. The unambiguous characterization of the cubic phases requires synchrotron radiation. Experiments were performed at LURE (Orsay, France) on beam line D22 of the DCI synchrotron source, whose characteristics are the same as employed previously.16 Before measurements, samples were stored at the desired temperature and then introduced either in 0.5 or 1 mm thick capillaries or placed between two 20 µm thick Mylar films, spaced with a 1 mm thick Teflon ring. Thermal equilibrium was ensured by introducing systematically a delay time of more than 1 h before recordings.
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Figure 3. X-ray diffraction profile of the lamellar phase in the CF6 ΣE2/water/PFD system (R ) 5.5, fO ) 0.1, T ) 10 °C).
Results Phase Behavior. Partial phase diagrams of the ternary system CF6 ΣE2/H2O/PFD are reported in Figure 2 for five temperatures between 5 and 40 °C. The liquid crystal domain, formed only in the surfactant-rich corner, is composed of the lamellar and cubic phases pre-existing in the binary system and of a reverse hexagonal phase at higher oil fractions. The surfactant-to-water ratios are noted as R, and fO is the oil weight fraction. R takes the values 7, 5.5, 4, 3, and 2.3, corresponding to the surfactant weight fractions in water 0.88, 0.85, 0.8, 0.75, and 0.7, respectively. The solubilization of PFD in the CF6 ΣE2/H2O system exhibits several features: the disappearance of the sponge phase, the preservation of both lamellar and cubic phases, and the formation of the hexagonal phase at high oil fractions. The incorporation of oil in the sponge phase even at low weight fractions ( 2) and where the cubic Pn3m exists (R < 4).
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example, for R ) 4 and fO ) 0.097, d211 ) 49.5 Å and d01 ) 48.9 Å (Figure 15b), and for R ) 5.5 and fO ) 0.17, d211 (Ia3d) ) 46.2 Å and d01(H2) ) 45.9 Å. In both cases, the analogy between planes having the same repeat distances is valid only at phase borders in a reduced oil fraction range where the cubic phase melts into the hexagonal phase by increasing temperature. As the oil fraction increases, the planes are no longer identical due to the change in surfactant chain ordering. In this cubic/ hexagonal transition, some other planes of lower densities are related, such as the second diffraction plane of the H2 phase with the (220) or the third diffraction plane with the (300) plane of the Pn3m structure (Table 2). Finally, the first diffraction planes of the structure Pn3m are related to the H2 phase, whereas the second diffraction planes of the structures Ia3d and Pn3m are connected with the lamellar phase. Conclusion
Figure 16. (a) Section of the hexagonal lattice according to the plane (11). (b) Section of the bicontinuous cubic lattice of space group Pn3m according to the plane (001).
The analogy between the two planes (110) and (01) can be seen in examining the two structures. On one hand, Figure 16a exhibits the classical representation of the hexagonal phase according to the plane (11). The d10 distance corresponds to the distance between two rows of cylinders. On the other hand, Figure 16b displays the cubic structure Pn3m viewed by the ICR model and according to the plane (001). In this scheme, channels appear like ellipses, since they are not perpendicular to the section, and the plane (110) corresponds also to the distance between two rows of cylinders. Parts a and b of Figure 16 thus present similarities, suggesting that the planes are analogous. For R higher than 3.5, the structure Pn3m does not form and only the structure Ia3d is identified in the cubic domain. In this case, the structural relationship with the hexagonal phase is expressed as d01(H2) ) d211(Ia3d). For
The solubilization of perfluorodecalin in the binary system CF6 ΣE2/H2O affects the phase behavior. It cannot be solubilized in the sponge phase, whereas the liquid crystals incorporate it. The incorporated fraction depends, however, on the surfactant/water ratio R. The cubic phase forms at lower temperatures with oil, and the domain is progressively shifted toward low R ratios and low oil fractions as the temperature rises. The transition from Ia3d to Pn3m occurs with increasing the oil or aqueous fractions. In the lamellar and cubic phases, perfluorodecalin is mainly incorporated as a free film, but insertion into the fluorinated chains arranged in the cubic phase may be possible. The hydrophobic thickness is, in this case, R-dependent. The hexagonal phase incorporates oil to a greater extent than the other phases (up to 0.4) by modifying the surfactant chain ordering. Some epitaxial relationships were pointed out at the liquid crystal phase transition. The epitaxial relationship between the lamellar phase and the two cubic structures is still observed in the presence of oil. The formation of the hexagonal phases induces new ones, observed between the cubic and hexagonal phases. In contrast to the lamellar/cubic transition, the cubic/hexagonal transition involves the first diffraction planes. The (01) diffraction plane of the hexagonal phase is related to the (110) diffraction plane of the cubic structure Pn3m for R < 4 and to the (211) plane of the cubic structure Ia3d for R g 4. LA026243H
