Lyotropic Liquid-Crystalline Phase Behavior of Some

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Lyotropic Liquid-Crystalline Phase Behavior of Some Alkyltrimethylphosphonium Bromides Brad A. Pindzola and Douglas L. Gin* Department of Chemistry, University of California, Berkeley, California 94720 Received January 31, 2000. In Final Form: May 8, 2000 The lyotropic liquid crystal (LLC) behavior of a homologous series of alkyltrimethylphosphonium bromide salts is described. While alkyltrimethylammonium salts are common LLC mesogens that have served in mesoporous materials, phase-transfer catalysis, and polymer-surfactant assemblies, the LLC behavior of the analogous phosphonium salts has been relatively unexplored. Dodecyl- (C12TMPB), tetradecyl(C14TMPB), and hexadecyltrimethylphosphonium bromide (C16TMPB) were found to exhibit hexagonal, lamellar, and cubic mesophases similar to their ammonium counterparts; however, there is a key difference: Changing to a phosphorus-based headgroup appears to stabilize the cubic phase of C12TMPB and C14TMPB such that the phases extend down to room temperature. Some trends in the lattice parameters of the resulting LLC phases of the alkyltrimethylphosphonium bromides with respect to phase composition and temperature are also discussed. Finally, preliminary use of 31P NMR spectroscopy, as a method of structural characterization of the phases, showed variation in the peak widths for the different phases but not the expected chemical shift anisotropy seen in the more commonly studied phosphate-based systems; an explanation for this difference is proposed.

Introduction Alkyltrimethylammonium salts are common lyotropic liquid-crystalline (LLC) mesogens that have found many applications in areas such as templates for mesoporous materials,1 phase-transfer catalysis,2 and polymer-surfactant assemblies.3 While phosphonium salts in general are common in organic chemistry and play vital roles as synthetic organic reagents, the alkyltrimethylphosphonium salts are relatively unexplored in the literature. The salts have been prepared previously;4-8 however, little has been done with them beyond their synthesis. One example is the use of the octadecyltrimethylphosphonium ion to make TCNQ-containing Langmuir-Blodgett films.9 More recently, the thermotropic liquid-crystalline character of several dialkyldimethylphosphonium compounds has been explored.10 To our knowledge there are no previous examples of studies of the LLC character of alkyltrimethylphosphonium salts, even though the LLC behavior of the corresponding ammonium salts has been the subject of intense interest.11-16 Phosphorus-based analogues of the alkyltrimethylammonium salts may (1) Ying, Y. J.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. Engl. 1999, 38, 56-77. (2) Makosza, M.; Fedorynski, M. Pol. J. Chem. 1996, 70, 1093-1110. (3) Ober, K. C.; Wegner, G. Adv. Mater. 1997, 9, 17-31. (4) Hays, H. R. J. Org. Chem. 1966, 31, 3817-3820. (5) Hays, H. R.; Laughlin, R. G. J. Org. Chem. 1967, 32, 1060-1063. (6) Laughlin, R. G. J. Org. Chem. 1965, 30, 1322-1324. (7) Angeletti, E.; Tundo, P.; Venturallo, P. J. Chem. Soc., Perkin Trans. 1 1982, 4, 993-997. (8) Smith, R. N.; Hansch, C. Biochemistry 1973, 12, 4924-4937. (9) Vandevyer, M.; Richard, J.; Barraud, A.; Ruaudel-Teixier, A.; Lequan, M.; Lequan, R. M. J. Chem. Phys. 1987, 87, 6754-6763. (10) Kanazawa, A.; Tsutsumi, O.; Ikeda, T.; Nagase, Y. J. Am. Chem. Soc. 1997, 119, 7670-7675. (11) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147-1160. (12) Raimondi, M. E.; Seddon, J. M. Liq. Cryst. 1999, 26, 305-339. (13) McGrath, K. M. Langmuir 1995, 11, 1835-1839. (14) Boden, N.; Radley, K.; Homes, M. C. Mol. Phys. 1981, 42, 493496. (15) Auvray, X.; Petipas, C.; Anthore, R.; Rico, I.; Lattes, A. J. Phys. Chem. 1989, 93, 7458-7464. (16) Husson, P. F.; Mustacchi, H.; Luzzati, V. Acta Crystallogr. 1960, 13, 668-676.

exhibit differences in LLC phase behavior that could be valuable in templating mesoporous materials or other applications. In this paper, we present the LLC phase behavior of a homologous series of three alkyltrimethylphosphonium bromides and compare their behavior to that of their better known nitrogen analogues. Additionally, the 31P NMR behavior of one member of the series was investigated to explore the potential utility of 31P NMR spectroscopy in characterizing the phases in these systems. Experimental Section General Procedures and Materials. Reagents and solvents were obtained from commercial suppliers and were used without further purification. 1H NMR and 13C NMR spectra were obtained using a Bruker AMX-300 spectrometer (300 MHz) or Bruker DRX-500 spectrometer (500 MHz). 31P NMR spectra were obtained with a Bruker AMX-400 spectrometer (400 MHz). All NMR spectra were obtained in CDCl3 unless otherwise noted. Chemical shifts for 1H and 13C NMR spectra are reported in parts per million (ppm) relative to internal TMS, and 31P NMR spectra are reported in ppm relative to external 85% phosphoric acid in water. Melting points were obtained using a capillary melting-point apparatus and are uncorrected. Low-angle X-ray diffraction patterns were obtained using an Inel CPS-120 powder X-ray diffractometer system using monochromated Cu KR radiation. Polarized light microscopy (PLM) was performed with a Leica DMRXP POL light microscope equipped with a Linkam THMSE 600 hot stage for variable-temperature experiments. Dodecyltrimethylphosphonium Bromide (C12TMPB).4 To a 150 mL, heavy-wall, round-bottom flask equipped with a stir bar and threaded Teflon cap was added 2.00 g (8.04 mmol) of 1-bromododecane in 2-propanol (75 mL). The solution was cooled to 0 °C in an ice bath before adding the trimethylphosphine (12 mL, 12 mmol) as a 1 M solution in toluene. The flask was tightly sealed and placed behind a blast shield. The temperature was then raised to 70 °C for 16 h. After cooling to 0 °C, the reaction mixture was transferred to a Schlenk flask. The solvent and remaining trimethylphosphine were then removed in vacuo, leaving a white crystalline solid. The powder was recrystallized from dry acetone to give 1.83 g (70%) of the desired product: mp 211-213 °C (lit. 210 °C). 1H NMR (300 MHz): δ 0.88 (t, J ) 6.6 Hz, 3H), 1.25 (m, 16H), 1.55 (m, 4H), 2.19 (d, J ) 14.1 Hz, 9H), 2.43 (m, 2H). 13C NMR (125 MHz): δ 8.91 (d, J ) 54.3 Hz), 14.04, 21.68 (d, J ) 4.8 Hz), 22.60, 23.75 (d, J ) 51.6 Hz), 29.00, 29.23,

10.1021/la000121c CCC: $19.00 © 2000 American Chemical Society Published on Web 07/06/2000

LLC Behavior of Alkyltrimethylphosphonium Bromides 29.24, 29.43, 29.51, 30.48, 31.82. 31P NMR (162 MHz): δ 25.40. Anal. Calcd. for C15H34PBr: C, 55.38; H, 10.53; P, 9.52. Found: C, 55.31 H, 10.77 P, 9.35. Tetradecyltrimethylphosphonium Bromide (C14TMPB).8 C14TMPB was synthesized following the same procedure as 1 using 1-bromotetradecane in place of 1-bromododecane. mp 217219 °C (lit. 218-220 °C). 1H NMR (300 MHz): δ 0.87 (t, J ) 6.6 Hz, 3H), 1.25 (m, 18H), 1.51 (m, 4H), 2.21 (d, J ) 14.2 Hz, 9H), 2.44 (m, 2H). 13C NMR (125 MHz): δ 8.90 (d, J ) 54.3), 14.04, 21.67 (d, J ) 4.6), 22.60, 23.73 (d, J ) 51.4), 29.01, 29.24, 29.27, 29.44, 29.52, 29.56, 29.59, 30.48, 31.84. 31P NMR (162 MHz): δ 25.37. Anal. Calcd. for C17H38PBr: C, 57.78; H, 10.84; P, 8.77. Found: C, 57.44 H, 11.11 P, 8.52. Hexadecyltrimethylphosphonium Bromide (C16TMPB).7 C16TMPB was synthesized following the same procedure as 1 using 1-bromohexadecane in place of 1-bromododecane. mp 229231 °C (lit. 197-200 °C). 1H NMR (300 MHz): δ 0.87 (t, J ) 6.8 Hz, 3H), 1.25 (m, 24H), 2.20 (d, J ) 14.1 Hz, 9H), 2.44 (m, 2H). 13C NMR (125 MHz): δ 8.88 (d, J ) 54.3 Hz), 14.04, 21.67 (d, J ) 4.5 Hz), 22.60, 23.67 (d, J ) 51.4 Hz), 29.02, 29.25, 29.27, 29.45, 29.52, 29.57, 29.59, 29.61, 30.49, 31.84. 31P NMR (162 MHz): δ 25.40. Anal. Calcd. for C19H42PBr: C, 59.83; H, 11.10; P, 8.12. Found: C, 59.89 H, 11.21 P, 8.00. 31P NMR of LLC Phases. Phases were prepared in glass capillaries by addition of appropriate amounts of the C14TMPB and distilled water. The capillaries were then sealed and homogenized by repeated cycles of centrifugation (3800 rpm, 1030 min) and heating (110 °C). After standing at room temperature for at least 16 h, the capillaries were placed in 5 mm NMR tubes, and proton-decoupled 31P NMR spectra were acquired. For spectra acquired at elevated temperatures, the samples were allowed to equilibrate at the acquisition temperature ((0.2 °C) for 5 min before acquisition. Phase Diagrams. LLC samples of each phosphonium salt were prepared for a number of different compositions. Appropriate amounts of the salt and distilled water were weighed into a vial, sealed with Parafilm, centrifuged (3800 rpm, 10 min), handmixed, resealed, and then centrifuged a second time. Each sample was then allowed to equilibrate at room temperature for at least 16 h before being used in any measurements. The extent of each LLC phase was determined using variabletemperature PLM. Specimens for PLM were prepared by pressing samples of each composition between microscope coverslips and sealing them with RTV102, a silicone rubber adhesive. Transitions from one texture to another were recorded on the heating cycle and were repeated at least three times. The identity of each observed phase was then confirmed by powder X-ray diffraction. Specimens for X-ray diffraction were prepared in 1.0 mm internal diameter glass capillary tubes using centrifugation or positive pressure to load the LLC phases into the capillaries. Some phases were too viscous to fill the capillary in this manner. In these cases, the phosphonium salt and distilled water were weighed directly into the capillary tube. The capillary tube was sealed, and the components were homogenized by repeated heating and cooling cycles.

Results and Discussion The extents of the LLC phases formed by these compounds were determined by locating transition temperatures for a series of amphiphile/water concentrations under a polarizing light microscope. The temperature regime examined was constrained to 25-80 °C in order to limit the effects of water loss on the phase behavior. The identities of the phases were then confirmed by powder X-ray diffraction. Some characteristic diffraction patterns are shown in Figure 1. Dodecyltrimethylphosphonium bromide (C12TMPB) exhibited three LLC phases (Figure 2). At amphiphile ratios between 53 and 80 wt % amphiphile, a regular hexagonal phase (HI) was observed at all temperatures in the area of interest. Increasing the amphiphile/water ratio leads to the formation of a viscous isotropic cubic phase (Qa) that persists to about 85 wt % amphiphile. Finally, above 85% a lamellar phase (La) was observed at tem-

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Figure 1. Representative X-ray diffraction patterns obtained for the LR (a), QR (b), and HI (c) LLC phases of the alkyltrimethylphosphonium bromides. Patterns presented here were acquired with C12TMPB at concentrations and temperatures of (a) 90:10/65 °C, (b) 80:20/25 °C, and (c) 55:65/25 °C.

Figure 2. Phase diagram for C12TMPB/water. Diagonal hashing indicates areas of coexistence for two or more liquidcrystalline phases. Vertical hashing indicates areas of coexistence of crystal and liquid crystal phases. I indicates a fluid isotropic phase (either micellar or solution), and X indicates a crystalline phase.

peratures greater than 63 °C. Below that temperature only a crystalline phase or mixture of crystals and liquid crystal was observed. This phase behavior is similar to that seen for dodecyltrimethylammonium bromide (DTAB).13 The most notable difference is the larger area covered by the Qa phase of the C12TMPB, which in this case exists down to room temperature. Tetradecyltrimethylphosphonium bromide (C14TMPB) behaves similarly to C12TMPB (Figure 3). The HI domain is slightly larger, covering 36-72 wt % surfactant at room temperature and decreasing to 48-72% at 80 °C. The Qa regime is 75-88 wt % amphiphile over most of the temperature range, and again the La phase only exists at elevated temperatures. To our knowledge, no complete phase diagram has been mapped for the LLC behavior of myristyltrimethylammonium bromide (MTAB) in water. LLC mesophases have been reported, and the presence of hexagonal and lamellar phases was indicated.14 Hexadecyltrimethylphosphonium bromide (C16TMPB) also exhibits three LLC phases (Figure 4). Its HI phase is by far the most extensive of the three compounds, existing between 22 and 66 wt % amphiphile at room temperature and 30-73% at 80 °C. In this case, the Qa phase only exists at temperatures above 43 °C between 75 and 83 wt % amphiphile. Below that exists a mixture of crystal and liquid crystal. The La phase extends from the edge of the cubic phase to 88 wt % amphiphile and above 38 °C. The observed phase behavior for C16TMPB/

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water is nearly identical to the behavior observed in the analogous cetyltrimethylammonium bromide (CTAB)/ water system.15,16 It seems reasonable that the modest effect of increasing headgroup size, by replacement of nitrogen with phosphorus, on the phase behavior observed in the shorter chain analogues is being overwhelmed in this case by the longer alkyl chain. Unfortunately, the viscous isotropic cubic phases (QR) observed in these systems cannot be conclusively indexed to a particular space group from the two observed diffraction peaks. The observed data, however, are consistent with the Ia3 h d space group, which is commonly observed in the corresponding alkyltrimethylammonium bromide/water systems at the same relative location in the phase diagram.13,15 The trend in the lattice parameter, for all studied systems, is toward smaller dimensions with increasing temperature and amphiphile content. Figure 5 illustrates typical changes in the lattice parameter with respect to temperature for a series of three C16TMPB/water concentrations (two in the HI domain and one in the LR domain). The decrease in lattice parameter (d100/cos 30° for HI, d100 for LR, and d211x6 for QR) with increasing temperature can be rationalized as an effective shortening of the alkyl chains due to an increase in available thermally accessible gauche conformations. Figure 6 shows the change in lattice parameter with respect to phase composition for C12TMPB and C16TMPB at room temperature. For C12TMPB at room temperature, the lattice parameter of the HI phase decreases by 8 Å

Pindzola and Gin

Figure 5. Plot of temperature versus d100 for three compositions of C16TMPB and water: [, 45% C16TMPB (HI), 9 65.1% C16TMPB (HI), and 2 84.7% C16TMPB (LR).

Figure 6. Plot of composition versus d100 for 9 C12TMPB/ water (HI) and [ C16TMPB/water (HI) at room temperature.

Figure 7. Cross-section of rodlike micelles in the HI phase. If l is the length of the all-trans amphiphile, then 2l is equal to the lattice parameter a when the aggregates are in contact. When a is greater than 2l, however, the aggregates cannot be in contact and the extra distance must be occupied by associated water molecules.

with increasing amphiphile concentration. For C16TMPB, in the same phase, the lattice parameter drops by 25 Å, from 79 to 54 Å. The second observation at first seems surprising for two reasons. First, and most obvious, is the two and a half nanometer change in the unit cell. The second is the rather large size of the unit cell for this phase across the entire range of concentration. The length of the all-trans conformation of the amphiphile from MM2 calculations is approximately 25 Å, including the bromide ion. If the rodlike micellar aggregate walls are touching, then the lattice parameter should be roughly twice the length of the molecules (Figure 7). This is only the case for the smallest observed unit cells. The larger observed dimensions are significantly greater than twice the all-

LLC Behavior of Alkyltrimethylphosphonium Bromides

Figure 8. 31P NMR spectra of three different LLC phases: (a) 80:20 C14TMPB:water, 19 °C (QR); (b) 60:40 C14TMPB:water, 19 °C (HI); (c) 90:10 C14TMPB:water, 92 °C (LR).

trans conformation. The observed unit cells, however, compare well with those observed for CTAB/water mixtures at similar concentrations.15 Still, since the all-trans conformation of the amphiphile represents the maximum possible length of the molecule, the minimum distance between rodlike micellar aggregates for the 30% amphiphile phase is 29 Å. This suggests that the amphiphiles are able to associate water molecules out to a distance of at least 1.5 nm from the “edge” of the micellar aggregates. 31P NMR spectroscopy has been used to identify the structure of LLC phases formed from natural17,18 and unnatural phospholipids,19,20 di-n-alkyl phosphates,21 and classical ammonium surfactants with phosphate counterions.22 The 31P NMR behavior of one of the phosphonium salts, C14TMPB, was investigated in order to explore the potential utility of this method for characterizing our LLC phases. The proton-decoupled 31P NMR spectra obtained for three different surfactant/water ratios, one in each phase domain, are presented in Figure 8. Figure 8a is of a 80% C14TMPB/20% water phase and shows the expected isotropic shape and line width (0.06 ppm, fwhm) for a cubic phase. Figure 8b is of a 60% C14TMPB/40% water mixture that forms a HI phase, and Figure 8c is for 90% C14TMPB/10% water mixture that forms a LR phase. It is important to note that the 31P NMR spectrum for the 90:10 composition was acquired at 92 °C in order to access the LR phase. The line widths (fwhm) for the HI and LR phases are 1.6 and 2.5 ppm, respectively. (17) Tilcock, C. P. S.; Cullis, P. R.; Gruner, S. M. Chem. Phys. Lipids 1986, 40, 47-56. (18) Ora¨dd, G.; Lindblom, G.; Fontell, K.; Ljusberg-Wahren, H. Biophys. J. 1995, 68, 1856-1863. (19) Srisiri, W.; Sisson, T. M.; O’Brien, D. F.; McGrath, K. M.; Han, Y.; Gruner, S. M. J. Am. Chem. Soc. 1997, 119, 4866-4873. (20) Lee, Y.-S.; Yang, J.-Z.; Sisson, T. N.; Frankel, D. A.; Gleeson, J. T.; Aksay, E.; Keller, S. L.; Gruner, S. M.; O’Brien, D. F. J. Am. Chem. Soc. 1995, 117, 5573-5578. (21) Hirata, H.; Maegawa, K.; Kawamatsu, T.; Kaneko, S.; Okabayashi, H. Colloid Polym. Sci. 1996, 274, 654-661. (22) Tansho, M.; Imae, T.; Tanaka, S.; Ohki, H.; Ikeda, R.; Suzuki, S. Colloids Surf. B 1996, 7, 281-286.

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Typically, LLC phase structure assignments are made based on the sign and magnitude of the chemical shift anisotropy (CSA).17 In our system, the characteristic anisotropic line shapes observed in the phosphate-based systems are not apparent. The lack of expected asymmetry is likely due to the relative magnitude of the chemical shielding tensor in the phosphate and phosphonium headgroup.23 A phosphorus atom in a phosphate experiences a much more polar environment owing to the phosphorus-oxygen double bond than it would as a tetraalkylphosphonium group. Thus, the phosphonium group has a much smaller chemical shielding tensor and therefore displays a much smaller CSA. The CSA does still exist, however, and while the peak shape anisotropy cannot be resolved, the line width differences that it creates are still observable. The differences observed in the line widths therefore correspond to the differences in the environment around the phosphorus nucleus due to the differing phase geometry. The difference observed between the HI and LR spectra does not correspond to the expected factor of 2, but the elevated temperature for the LR sample can easily account for the discrepancy. 31P NMR analysis can therefore be a useful method of characterizing LLC systems of this type. However, the small CSA inherent in the tetraalkylphosphonium headgroup makes it difficult to determine phase structure independent of other means. Conclusions The alkyltrimethylphosphonium bromides are readily synthesized LLC mesogens that nicely complement the LLC behavior of their ammonium-based analogues. In addition, the presence of the 100% abundant 31P nucleus provides a useful NMR handle to aid in studies of the LLC environment in these systems. While the phosphonium salts have similar overall phase behavior to the nitrogen salts, there is a key difference. Changing the headgroup from a quaternary ammonium group to a quaternary phosphonium group appears to stabilize the cubic phase of C12TMPB and C14TMPB such that they extend down to room temperature. This effect, however, does not apply to C16TMPB. The chain length, in this case, dominates the properties of the molecule, and so it has a nearly identical phase diagram to its nitrogen analogue, CTAB. We are currently looking at polymerizable analogues of these phosphonium bromides for LLC phase behavior and potential use in forming organic/organic and organic/ inorganic nanostructured materials. Acknowledgment. Primary financial support from the U.S. Dept. of Energy (DE-AC03-76SF00098) and the National Science Foundation (DMR-9625433) is gratefully acknowledged. We also thank the Raychem Corporation, 3M, and Rohm and Haas for research gifts. B.A.P. thanks Eli Lilly for a graduate fellowship, and D.L.G. thanks the Alfred P. Sloan Foundation for a Research Fellowship. LA000121C (23) Seelig, J.; Seelig, A. Q. Rev. Biophys. 1980, 13, 19-61.

Lyotropic Liquid-Crystalline Phase Behavior of Some

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