LETTER pubs.acs.org/JPCL
A Nonaqueous Liquid Crystal Emulsion: Fluorocarbon Oil in a Hexagonal Phase in an Ionic Liquid Suraj Chandra Sharma and Gregory G. Warr* School of Chemistry, The University of Sydney, Sydney, NSW, 2006, Australia ABSTRACT: The formation of highly stable emulsions of perfluoromethyldecalin (PFMD) dispersed in a hexagonal liquid crystal continuous phase comprising a 1:1 mass ratio of hexaethylene glycol monohexadecyl ether (C16E6) in the room-temperature ionic liquid ethylammonium nitrate (EAN) is reported. The hexagonal phase structure is shown to persist up to at least 80% w/w dispersed PFMD by polarized optical microscopy and small-angle X-ray scattering. SECTION: Macromolecules, Soft Matter
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nvironmental concerns are today a major driving force for the incorporation of green solvents into formulations for pharmaceutical, personal care, and household products. Many ionic liquids—liquids composed entirely of ions and molten below 100 °C—have good environmental pedigrees with low energy intensity for their production, negligible vapor pressures, and ease of reuse. Among room-temperature ionic liquids, ethylammonium nitrate (EAN) has been studied most extensively, particularly since the work of Evans et al. in 1980s1 3 on amphiphile self-assembly into micelles and liquid crystals in EAN.4 Previous work from our group has demonstrated the self-assembly of numerous polyoxyethylene nonionic surfactants (CnEm) into micelles and lyotropic liquid crystals in EAN and related this to their well-known behavior in aqueous systems.5,6 The use of lyotropic liquid crystal continuous phases to stabilize emulsions was first reported by Friberg et al.7 These form in the regions of a ternary phase diagram where an isotropic phase to be dispersed coexists with the liquid crystals. The liquid crystals mesostructure surrounds the emulsion droplets, stabilizing them against coalescence and creaming. Although there have been numerous studies on both direct and inverted lyotropic liquid crystals as continuous phases in aqueous emulsions,8 12 there are as yet no reports using a lyotropic liquid crystal in a room-temperature ionic liquid. The mixing between hydrocarbons and fluorocarbons is known to be highly nonideal so a fluorocarbon oil such as perfluoromethyldecalin (PFMD) is an ideal candidate for emulsification without the complication of swelling the surfactant alkyl chains, and possibly modifying the structure of lyotropic liquid crystal. Here we report the formation of long-term stable emulsions using a hexagonal liquid crystal of C16E6 in EAN to emulsify PFMD up to high dispersed-phase volume fractions. The phase diagram of the ternary C16E6/EAN/PFMD system at 25 °C is shown in Figure 1, with the emulsification path r 2011 American Chemical Society
indicated by the arrow. EAN and PFMD, as well as C16E6 and PFMD are mutually immiscible. With increasing concentration of C16E6 in EAN, first an isotropic micellar (L1) solution, then discrete cubic (I1), direct hexagonal (H1), and bicontinuous cubic (V1) phases are found in order before solid C16E6 is encountered.5 We have chosen a hexagonal phase of a fixed composition (C16E6/EAN = 50/50) as a continuous phase in which to emulsify PFMD. The pattern of phase behavior of polyoxyethylene nonionic surfactants in EAN is remarkably similar to that in water.13 These surfactants are less amphiphilic in EAN than in water: hydrocarbons are more soluble in EAN, and hence longer hydrocarbon chains are necessary to drive the formation of liquid crystals in EAN. Conversely, EAN is a poorer solvent for polyoxyethylene chains,14 and consequently they adopt a less extended conformation at an interface,15 which leads to higher interfacial areas.16,17 This has consequences for molecular packing constraints: e.g. the binary phase diagram of C16E6/EAN closely resembles that of C12E8/H2O. Figure 1a shows a typical texture seen for hexagonal liquid crystals of C16E6 in EAN under polarizing optical microscopy. This hexagonal phase cannot be swollen by solubilization of even small amounts (1 wt %) of PFMD due to the immiscibility of the hydrocarbons and fluorocarbons. However, more than 80 wt % of PFMD can readily be dispersed into the hexagonal liquid crystal matrix, and forms liquid crystals emulsions that remain stable for at least two months. The emulsions formed are highly viscous, stiff, and do not flow when the tube is inverted. Polarizing optical micrographs show that the emulsified PFMD droplets are polydisperse. At low PFMD weight fractions, Received: June 15, 2011 Accepted: July 18, 2011 Published: July 18, 2011 1937
dx.doi.org/10.1021/jz200806p | J. Phys. Chem. Lett. 2011, 2, 1937–1939
The Journal of Physical Chemistry Letters
LETTER
Figure 1. The ternary phase diagram of C16E6/EAN/PFMD system at 25 °C, showing emulsification path. Optical micrographs show (a) base hexagonal liquid crystal at weight fraction of PFMD, Wo = 0; emulsions for (b) Wo = 0.303, (c) Wo = 0.505, and (d) Wo = 0.807.
polyhedra as commonly seen in high internal-phase emulsion systems. The small-angle X-ray scattering (SAXS) patterns for the C16E6/EAN/PFMD samples along the PFMD addition path at a fixed C16E6/EAN weight ratio of 1:1 (Figure 2) show√four Bragg reflections, which index to the hexagonal phase (1, 3, 2, √ 7). With increasing PFMD content in emulsions, the number √ and intensity of higher-order reflections decreases, but the 3 peak remains visible up to at least Wo = 0.505, confirming that the hexagonal structure remains intact. The position of first peak is also unaffected by emulsification up to Wo = 0.807, consistent with retention of structure and composition of the original lyotropic phase, and of PFMD’s inability to swell the hydrocarbon cores of the C16E6 aggregates. Figure 2b shows that the interlayer spacing, d, does not change within experimental uncertainty as a function of weight fraction of PFMD. This contrasts markedly with the results of Aramaki et al.,18 who found that a hexagonal to discrete cubic phase transition occurred when dodecane was emulsified in the hexagonal liquid crystal of C16E6/ H2O. We report for the first time that a hexagonal liquid crystal formed by polyoxyethylene nonionic surfactant in the roomtemperature ionic liquid EAN can be used to emulsify a large amount of PFMD, yielding highly stable liquid crystal emulsions similar to those reported in aqueous systems. The emulsions droplets are polydisperse, becoming close-packed and deforming to polyhedra at high volume fractions of PFMD. These emulsions are potentially useful for many formulations where incorporation of green solvents is highly desirable.
Figure 2. (a) SAXS patterns and (b) d-spacing of the C16E6/EAN/ PFMD system at different weight fractions, Wo, of PFMD with C16E6/ EAN = 50/50.
Wo = 0.303, the droplets are well dispersed with diameters less than 5 μm (Figure 1b). With increasing concentration of PFMD, the droplet diameter also increases to 5 10 μm for Wo = 0.505 (Figure 1c). At higher concentration of PFMD (Wo= 0.807), the droplets are tightly packed as shown in Figure 1d. Some of the droplets are more than 10 μm in diameter and have distorted into
’ EXPERIMENTAL SECTION EAN was prepared as described previously1 by slow addition of concentrated nitric acid (Ajax Finechem Pty Ltd.) to ethylamine (Sigma) while keeping the reaction temperature below 15 °C. Excess water was removed by rotary evaporation followed by nitrogen purging and heating at 108 110 °C under a nitrogen atmosphere overnight. The water content, determined by Karl Fischer titration, was zero. Hexaethylene glycol monohexadecyl ether (C16E6) and PFMD were purchased from Nikkol and Sigma-Aldrich, respectively, and were used as received. For the preparation of emulsions, first calculated amounts of C16E6 and 1938
dx.doi.org/10.1021/jz200806p |J. Phys. Chem. Lett. 2011, 2, 1937–1939
The Journal of Physical Chemistry Letters EAN were mixed in a screw cap-test tube to make hexagonal liquid crystal. The hexagonal liquid crystal was then heated above its melting point (67 °C) followed by the addition of PFMD under continuous mixing with a vortex mixer at room temperature for 5 min. The rates of addition of PFMD and mixing were kept constant to obtain reproducible liquid crystal emulsions. The SAXS was performed on a point collimated Anton Paar SAXSess and the PW3830 laboratory X-ray generator (40 kV, 50 mA) with a long-fine focus sealed X-ray tube (CuKR wavelength of λ = 0.1542 nm) from PANalytical. Detection was performed with a high sensitivity 62 66 mm2 2D imaging- plate and radially averaged to give a q-range of 0.2 6.92 nm 1. A Leica DM2500P optical microscope with a CCD camera (Micropublisher 3.3 RTV) was used for the microscopic observation.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was funded by the Australian Research Council. S.C.S. thanks the University of Sydney for a University Postdoctoral Fellowship.
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of Reverse Micellar Cubic Liquid Crystals and Derived Emulsions. Langmuir 2007, 23, 11007–11014. (13) Mitchell, J. D.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. Phase Behaviour of Polyoxyethylene Surfactants with Water. Mesophase Structures and Partial Miscibility (Cloud Points) J. Chem. Soc., Faraday Trans. I 1983, 79, 975–1000. (14) Werzer, O.; Warr, G. G.; Atkin, R. Conformation of Poly(ethylene oxide) Dissolved in Ethylammonium Nitrate. J. Phys. Chem. B 2011, 115, 648–652. (15) Werzer, O.; Warr, G. G.; Atkin, R. Compact Poly(ethylene oxide) Structures Adsorbed at the Ethylammonium Nitrate-Silica Interface. Langmuir 2011, 27, 3541–3549. (16) Atkin, R.; Warr, G. G. Phase Behavior and Microstructure of Microemulsions with a Room-Temperature Ionic Liquid as the Polar phase. J. Phys. Chem. B 2007, 111, 9309–9316. (17) Wakeham, D.; Niga, P.; Warr, G. G.; Rutland, M. W.; Atkin, R. Nonionic Surfactant Adsorption at the Ethylammonium Nitrate Surface: A Neutron Reflectivity and Vibrational Sum Frequency Spectroscopy Study. Langmuir 2010, 26, 8313–8318. (18) Alam, M. M.; Sugiyama, Y.; Watanabe, K.; Aramaki, K. Phase Behavior and Rheology of Oil-Swollen Micellar Cubic Phase and Gel Emulsions in Nonionic Surfactant Systems. J. Colloid Interface Sci. 2010, 341, 267–272.
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