pubs.acs.org/Langmuir © 2010 American Chemical Society
Magnetic Self-Orientation of Lyotropic Hexagonal Phases Based on Long Chain Alkanoic (Fatty) Acids Jean-Paul Douliez* UR1268 Biopolym eres Interactions Assemblages, INRA, equipe Interfaces et Syst emes Dispers es, rue de la G eraudi ere, F-44316 Nantes, France Received March 3, 2010 It is presently shown that long chain (C14, C16, and C18) alkanoic (saturated fatty) acids can form magnetically oriented hexagonal phases in aqueous concentrated solutions in mixtures with tetrabutylammonium (TBAOH) as the counterion. The hexagonal phase occurred for a molar ratio, alkanoic acid/TBAOH, higher than 1, i.e., for an excess of fatty acid. The hexagonal phase melted to an isotropic phase (micelles) upon heating at a given temperature depending on the alkyl chain length. The self-orientation of the hexagonal phase occurred upon cooling from the “hightemperature” isotropic phase within the magnetic field. The long axis of the hexagonal phase was shown to self-orient parallel to the magnetic field as evidenced by deuterium solid-state NMR. This finding is expected to be of interest in the field of structural biology and materials chemistry for the synthesis of oriented materials.
Introduction Surfactants are known to self-assemble in water into a large variety of supramolecular assemblies among which are liquid crystalline phases such as nematic, lamellar, and hexagonal phases.1,2 Such systems are of both fundamental and industrial interest and may find applications in a large variety of domains such as materials chemistry. Direct hexagonal phases are made of cylinders (elongated rigid micelles) stacked in an hexagonal array. Such phases can be used as sacrificial templates for the synthesis of materials.3 In addition, recent findings have revealed that hexagonal phases can be oil-swollen and then that the core of their cylinder could be used as nanoreactors.4 This appears very attractive for organic chemistry or for other synthesis processes which require an hydrophobic environment.5 Moreover, the shape of the resulting product may be driven in the confined cylinder. This should be powerful for the synthesis of for instance metal or silica nanotubes or nanowires.6-8 Self-orientation of materials is also a matter of intense research, mainly because it provides additional properties, at least in the field of optics or for the synthesis of ordered (aligned) mesoporous materials.9 Then, the availability of a simple system consisting of cylinders that would self-orient would be of practical value for numerical applications. This has been achieved in the past in *Corresponding author: Ph þ33 240 67 50 83, Fax þ33 240 67 50 84, e-mail
[email protected]. (1) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992; p 450. (2) Binnemans, K. Chem. Rev. 2005, 105 (11), 4148-4204. (3) Attard, G. S.; Leclerc, S. A. A.; Maniguet, S.; Russell, A. E.; Nandhakumar, I.; Bartlett, P. N. Chem. Mater. 2001, 13 (5), 1444-1446. (4) Pena dos Santos, E.; Tokumoto, M. S.; Surendran, G.; Remita, H.; Bourgaux, C.; Dieudonne, P.; Prouzet, E.; Ramos, L. Langmuir 2005, 21 (10), 4362-4369. (5) Pindzola, B. A.; Hoag, B. P.; Gin, D. L. J. Am. Chem. Soc. 2001, 123 (19), 4617-4618. (6) Huang, L.; Wang, H.; Wang, Z.; Mitra, A.; Zhao, D.; Yan, Y. Chem. Mater. 2002, 14 (2), 876-880. (7) Bouchama, F.; Thathagar, M. B.; Rothenberg, G.; Turkenburg, D. H.; Eiser, E. Langmuir 2003, 20 (2), 477-483. (8) Ganesh, V.; Lakshminarayanan, V. Langmuir 2006, 22 (4), 1561-1570. (9) Wan, Y.; Zhao, D. Chem. Rev. 2007, 107 (7), 2821-2860. (10) Quilliet, C.; Ponsinet, V.; Cabuil, V. J. Phys. Chem. 2002, 98 (14), 3566-3569.
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hexagonal phases but by using magnetic nanoparticles10 or upon shearing.11,12 There is an additional domain for which oriented assemblies are of interest. This is in the field of structural biology and solid-state NMR. Indeed, the shape of a spectrum and the position of a peak depend on the orientation with respect to the magnetic field,13 allowing for instance to determine whether a protein helix is oriented parallel or perpendicular to a bilayer.14-17 Most of the systems forming lyotropic hexagonal phases are built using “classical” surfactants such as SDS or CTAB, and surprisingly, long chain alkanoic acids have been poorly employed to date.2 This is mainly because metal salts of long chain alkanoic acids (LC2A) tend to crystallize in water at room temperature. Then, several studies have reported on the self-assembly of liquid crystalline phases using only short chain (200 mg/mL) of various alkanoic acids: myristic (tetradecanoic), palmitic (hexadecanoic), and stearic (octadecanoic) acids neutralized by TBAOH (molar ratio 1). (11) Schmidt, G.; Muller, S.; Lindner, P.; Schmidt, C.; Richtering, W. J. Phys. Chem. B 1998, 102 (3), 507-513. (12) Ramos, L.; Molino, F.; Porte, G. Langmuir 2000, 16 (14), 5846-5848. (13) Davis, J. Biochim. Biophys. Acta 1983, 737 (1), 117-171. (14) Glaubitz, C.; Watts, A. J. Magn. Reson. 1998, 130 (2), 305-316. (15) Cross, T. A.; Opella, S. J. Curr. Opin. Struct. Biol. 1994, 4 (4), 574-581. (16) De Angelis, A. A.; Opella, S. J. Bicelle Samples Solid-State NMR Membr. Proteins 2007, 2(10), 2332–2338. (17) Ramamoorthy, A.; Marassi, F. M.; Zasloff, M.; Opella, S. J. J. Biomol. NMR 1995, 6 (3), 329-334. (18) Berejnov, V.; Cabuil, V.; Perzynski, R.; Raikher, Y. J. Phys. Chem. B 1998, 102 (37), 7132-7138. (19) Zana, R. Langmuir 2004, 20, 5666-5668. (20) Zana, R.; Schmidt, J.; Talmon, Y. Langmuir 2005, 21 (25), 11628-11636.
Published on Web 03/23/2010
DOI: 10.1021/la100885e
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Figure 1. Pseudo-phase diagram of the palmitic acid-TBAOH system. The hexagonal phase domain (H) was characterized by both polarized microscopy (A) and X-ray scattering experiments √ (B). The position of the three diffraction peaks in a ratio 1: 3:2 (arrows) demonstrates the existence of the hexagonal phase. I: iso; C: crystal.
At those concentrations and room temperature, the alkanoic acid salts of TBAOH yielded isotropic solutions, the viscosity of which increased with the alkyl chain length to form a gel for stearic acid (the sample tube could be turned upside down without any flow). This finding shows, as in dilute systems,19,20 that counterion prevents the crystallization of alkanoic acids forming stable micelles, probably elongated since the systems are concentrated. Hexagonal phases were produced for those LC2A by varying the alkanoic acid/TBAOH molar ratio. In the pseudo-phase diagram, the hexagonal phase was unambiguously identified by its birefringence (both by cross-polarized visual inspection and microscopy), viscosity (transparent gel), and X-ray data (see Figure 1). That phase occurs for a relatively broad range of composition (molar ratio and dilution) and temperature. Interestingly, it is formed for an excess of alkanoic acid (vs TBAOH), implying that it requires a mixture of carboxylic and carboxylate groups (alkanoic acids and alkanoate). Then, the (carboxylic) alkanoic acids probably act as a cosurfactant. That system is then equivalent to a ternary system (TBAOH, alkanoic acid, and alkanoate) in which a cosurfactant as a long chain alkanol is included.18 Besides, preliminary experiments using long chain alkanol and the alkanoate/TBAOH (then at the neutralization point without excess of alkanoic acid) mixtures also show the formation of hexagonal phases. Interestingly, mixtures of alkanoate/ alkanoic acids have been already shown to be determinant in the self-assembly of such surfactants in dilute systems.21-23 For instance, the formation of vesicles only occurs in such a case because a network of hydrogen bonds stabilizes the self-assembly. Above a given temperature which depends on the alkyl chain length, the hexagonal phase melted to an isotropic phase. For instance, for a molar ratio alkanoic acid/TBAOH of 1.2, the systems made with the myristic, palmitic, and stearic acids formed an hexagonal phase below 15, 32, and 45 °C, respectively, and an isotropic phase at higher temperature. Tuning that phase (21) Apel, C. L.; Deamer, D. W.; Mautner, M. N. Biochim. Biophys. Acta 2002, 1559 (1), 1-9. (22) Hargreaves, W.; Deamer, D. W. Biochemistry 1978, 17 (18), 3759-3768. (23) Morigaki, K.; Walde, P. Curr. Opin. Colloid Interface Sci. 2007, 12 (2), 75-80.
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Figure 2. Deuterium solid-state NMR spectra of the TBAOH/ palmitic acid system at different temperatures upon heating A (293 K) and B (305 K) and upon cooling C (293 K).
transition is then possible by varying the alkyl chain length and appears as an advantage compared to other surfactants. The hexagonal phase was further characterized by deuterium solid-state NMR using deuterated fatty acids. This represents an additional advantage of using alkanoic acids as liquid crystals for fundamental studies since other surfactants are rarely available in their deuterated form. This may be of interest for neutron scattering experiments for which the contrast of the signal may be varied by mixing deuterated and protonated components. Doping such hexagonal phases with proteins or other host molecules should then allow a deep characterization in such a confined and “membrane”-like environment. The NMR data are displayed Figure 2 as a function of temperature in the case of the palmitic acid system at a molar ratio alkanoic acid/TBAOH of 1.2. The spectrum at room temperature (293 K, Figure 2A) is characteristic of an hexagonal phase13 with a relatively sharp powder pattern which is averaged (compared to that of a bilayer assembly) by the rotation of the cylinders along their long axis. Several qudrupolar splittings, Δν90, can be measured on that spectrum, the largest having a value of 11 kHz. Upon heating, the system transits to an isotropic phase as illustrated by the isotropic line (305 K, Figure 2B). Upon cooling from that temperature to the hexagonal phase, the spectrum appears rather different than that acquired prior heating (293 K, Figure 2C). Indeed, it is now composed of well-resolved peaks, and the quadrupolar splittings are now twice that of Figure 2A. Indeed, the largest splitting is now of 22 kHz. This unambiguously demonstrate that the cylinders are now oriented parallel to the magnetic field. Over the 15 labeled positions of palmitic acid, 14 peaks can be assigned. The orientation feature is also evident by using the selectively labeled palmitic acid at position 2 (Figure 3). In the oriented spectrum (bottom), one can measure a splitting of 22 kHz. That value corresponds on the perdeutrated system (see Figure 2) to the largest quadrupolar splitting, allowing the assignment of that position to carbon C2, C1 being the carboxylic group. This is a common feature that the first labeled position exhibits the largest quadrupolar splitting. In the spectrum shown in Figure 3, the broad isotropic peak in the middle of the spectrum (top) stands for the signal of water Langmuir 2010, 26(13), 11397–11400
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Figure 4. Deuterium solid-state NMR spectra for the systems using myristic (top) and stearic acids (bottom) at 280 and 310 K, respectively (see text for details).
Figure 3. Spectra obtained in the palmitic acid system using selectively labeled palmitic acid on position 2: (top) at 293 K and (bottom) at the same temperature but after heating (305 K) and cooling the sample within the magnetic field.
since one used nondepleted deuterated water. That peak is splitted in the oriented spectrum (bottom), showing that water is also in average self-oriented in the field in the confined environment of the interstices of the hexagonal phase. Similar spectra were obtained with tetradecanoic and octadecanoic acids (Figure 4). In the first case, the transition from the hexagonal phase to the isotropic phase is below the room temperature at which the NMR sample is placed within the coil probe so that one can only acquire an oriented spectrum upon cooling. In the case of the longer chain (C18), one obtained a powder spectrum similar to that of Figure 2 for the palmitic acid (not shown), and upon heating (330 K) and cooling back, this afforded the oriented spectrum. We have already observed such a feature in a different system made of fatty acid and monoglyceride mixtures;24 however, only lumps of aggregated materials were obtained at that time. In the present study, the hexagonal phase is homogeneous in the sample gel. As previously mentioned, self-orientation of such edifices might be of interest, and we are further checking that the present hexagonal phases can be oil-swollen with the aim of synthesizing oriented materials. Next, such systems may be an alternative to the use of for instance bicelles16,25,26 or oriented bilayers between glass plates14 which are commonly used in the solid-state NMR community for determining biological structural parameters. For (24) Douliez, J.-P. Langmuir 2004, 20, 1543-1550. (25) Marcotte, I.; Auger, M. Concepts Magn. Reson., Part A 2005, 24A (1), 17-37. (26) Carlotti, C.; Aussenac, F.; Dufourc, E. J. Biochim. Biophys. Acta 2002, 1564 (1), 156-164.
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instance, we could take the advantage that the hexagonal phase is a gel; that is, the orientation is “fixed” within the sample. Rotating the sample within the NMR coil probe should then allow to investigate various angles (as for samples oriented between glass plates), and we are exploring this avenue in the future. We also have to search in the future similar systems which can be fully deuterated (actually, only the alkanoic acid is deuterated), varying the nature of the counterion (and using a counterion which is commercially available under its deuterated form). This should allow to gain information on protonated host molecules (such as peptides or proteins) by high-resolution proton NMR. In summary, it is presently shown that saturated long chain alkanoic acids may form liquid crystalline phases such as hexagonal phases that self-orient in a magnetic field. It is believed to be of particular interest since compared to “classical” surfactants, there exist a large variety of alkanoic (fatty) acids (including chiral), and moreover, there are natural generic biomolecules. Varying the alkyl chain length allows to easily tune the transition temperatures. Since alkanoic acids are available under their carboxylic form, the nature of the counterion may be widely varied compared to the case of “classical” surfactants such as SDS or CTAB, and we have shown in the past that it strongly influences their polymorphism in diluted systems.27-29 The additional promising feature in such systems is the self-orientation of the hexagonal phase in a magnetic field. This potentially represents oriented nanoreactors which could be oil-swollen for materials chemistry.4 Moreover, oriented materials may find applications in optics, in materials chemistry, and for structural solid-state NMR measurements of host molecules.15,16,30 Altogether, this shows that such “simple” liquid crystals made of saturated long chain alkanoic acids are of both practical and fundamental interest.
Materials and Methods Sample Preparation. Protonated (Sigma-Aldrich, both 99% purity), perdeuterated (Eurisotop) alkanoic acids, and selectively (27) Douliez, J.-P.; Navailles, L.; Nallet, F. Langmuir 2006, 22 (2), 622-627. (28) Gaillard, C.; Novales, B.; Jer^ome, F.; Douliez, J.-P. Chem. Mater. 2008, 20, (4), 1206-1208. (29) Novales, B.; Navailles, L.; Axelos, M.; Nallet, F.; Douliez, J.-P. Langmuir 2008, 24 (1), 62-68. (30) Tjandra, N.; Bax, A. Science 1997, 278 (5340), 1111-1114.
DOI: 10.1021/la100885e
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Article labeled palmitic acid on position 2 (Sigma-Aldrich) were used. Concentrated stock solutions of alkanoic acid/TBAOH were prepared by mixing the desired volume of a 1 M stock solution TBAOH (Sigma) so that the molar ratio was 1. Samples were heated and vortexed at 70 °C until all the alkanoic acid material was fully dispersed. Then, the pseudo-phase diagrams were built by adding various amount of each alkanoic acids to the corresponding stock solution and further diluted. The highest concentration is limited by the use of a 1 M stock solution of TBAOH. Solid-State NMR. Deuterium solid-state NMR experiments were performed at several temperatures from 20 to 60 °C on a 400 MHz Bruker spectrometer operating at 61 MHz for deuterium using a static double-channel probe. The sample coil of the probe was adapted to load a 7 mm rotor such as those used for magic angle spinning probes equipped with a stretched stator. Typically, lipid dispersions were previously heated to 60 °C, and a volume of ca. 500 μL transferred into the rotor which was sealed and then end-capped. A Hahn quadrupolar echo sequence19 was used with an inter pulse delay of 40 μs. 4K points in 2K (16K in the case of the selectively labeled palmitic acid) accumulations (every 2 s) were done with a 90° pulse and spectral width of 8 μs and 250 kHz, respectively. Free induction decay signal were zero-filled to 16K points prior to Fourier transform after a broad line exponential multiplication of 20 Hz. For deuterium spectroscopy, the general theory for lipid systems can be found in the literature.13,31 Briefly, the deuterium NMR signal is composed of doublets with a splitting, Δν, which depends on the orientation of the C-D bond with respect to the (31) Seelig, J. Q. Rev. Biophys. 1977, 10 (3), 353-418.
11400 DOI: 10.1021/la100885e
Douliez magnetic field. In an anisotropic but disoriented medium, all the orientations are allowed, and these doublets are superimposed to form a powder spectrum having two main peaks with an increased intensity corresponding to the 90° orientation, separated by Δν90. The edge of the spectrum corresponds to the 0° orientation, with a splitting Δν0 equal to twice Δν90. In the case of perdeuterated systems, the spectrum is composed by the superimposition of signals from each labeled position. Cross-Polarized Microscopy. Observations were made at room temperature at 20 magnification using an optical microscope in the cross polarized mode (Nikon Eclipse E-400, Tokyo, Japan) equipped with a 3-CCD JVC camera allowing digital images (768 512 pixels) to be collected. A drop of the lipid dispersion (about 20 μL) was deposited on the glass slide surface (76 26 1.1 mm, RS France) and covered with a cover slide (22 22 mm, Menzel-Glaser, Germany). The glass slides were previously cleaned with ethanol and acetone. X-ray. Diffraction diagrams were monitored by recording X-ray diffraction diagrams every 30 min on a Bruker D8 Discover diffractometer. Cu KR1 radiation (Cu KR1, 1.5405 A˚) produced in a sealed tube at 40 kV and 40 mA was selected and parallelized using a G€ obel mirror parallel optics system and collimated to produce a 500 mm beam diameter. Samples were transferred into a 1.5 diameter capillary which was further sealed.
Acknowledgment. I thank Bruno Pontoire for his assistance during the X-ray experiments and Laurence Navailles, Frederic Nallet, and Olivier Mondain-Monval from the CRPP, Pessac, for powerful suggestions and discussions on liquid crystals. Access to the NMR facilities to the BIBS platform of INRA Angers-Nantes was greatly appreciated by the author.
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