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Room-Temperature and Low-Ordered, Amphotropic-Lyotropic Ionic Liquid Crystal Phases Induced by Alcohols in Phosphonium Halides Kefeng Ma,† Astghik A. Shahkhatuni,†,‡ B. S. Somashekhar,§ G. A. Nagana Gowda,§,| YuYe Tong,† C. L. Khetrapal,§ and Richard G. Weiss*,† Department of Chemistry, Georgetown UniVersity, Washington, D.C. 20057-1227, USA and Center of Biomedical Magnetic Resonance, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow-226 014, India ReceiVed May 23, 2008. ReVised Manuscript ReceiVed July 1, 2008 Tri-n-decylmethylphosphonium chloride and bromide (1P10X) salts are not liquid crystalline. However, mesophases are induced by adding very small amounts of an alcohol or water. The temperature ranges of the induced smectic A2 (SmA2) liquid-crystalline phases can be very broad and the onset temperatures can be below room temperature depending upon the concentration of the alcohol or water and the structure of the alcohol. At least one molar equivalent of hydroxyl groups is necessary to convert the 1P10X completely into a liquid crystal. Strong association between the hydroxyl groups of an alcohol or water and the head groups of the 1P10X is indicated by spectroscopic, diffraction, and thermochemical data. Unlike many other smectic phases, those of the 1P10X/alcohol complexes are easily aligned in strong magnetic fields and the order parameters of selectively deuterated alcohols as measured by 2H NMR spectroscopy, ∼10-2, are much lower than the values found when the host is a commonly employed thermotropic liquid crystal. The dependence of the specific values of the order parameters on temperature, the nature of the halide anion, and the structure and concentration of the alcohol are reported. In sum, a detailed picture is presented to explain how and why an alcohol or water induces liquid crystallinity in the 1P10X salts. The data also provide a blueprint for designing media with even lower order parameters that can be hosts to determine the conformations and shapes of guest molecules.
Introduction Many applications of ionic liquids (ILs), commonly defined as salts that melt at less than 100 °C,1 as ‘task-specific’ ‘green’ solvents2 for diverse catalytic3 and synthetic reactions,4 for aids in separations and extractions,5 for nanotechnological,6 bio* To whom correspondence should be addressed. E-mail: weissr@ georgetown.edu. † Georgetown University. ‡ Permanent address: Molecule Structure Research Center, National Academy of Sciences, Yerevan 0014, Armenia. § Sanjay Gandhi Post Graduate Institute of Medical Sciences. | Current address: Department of Chemistry, Purdue University, West Lafayette, IN 47907. (1) (a) Rogers, R. D.; Voth, G. A. Acc. Chem. Res. 2007, 40, 1077–1078. (b) Rogers, R. D.; Seddon, K. R., Eds.; Ionic Liquids III: fundamentals, progress, challenges, and opportunities (ACS Symposium Series 901-902); American Chemical Society: Washington, DC, 2005. (2) (a) Plechkova, N. V.; Seddon, K. R. In Methods and Reagents for Green Chemistry, Tundo, P.; Perosa, A.; Zecchini, F. Eds.; Wiley: Hoboken, NJ, 2007, pp 103-130. (b) Rogers, R. D.; Seddon, K. R., Eds., Ionic Liquids as Green SolVents, Progress and Prospects (ACS Symposium Series 856); American Chemical Society: Washington, DC, 2003. (c) Rogers, R. D.; Seddon, K. R.; Volkov, S. Green Industrial Applications of Ionic Liquids; Kluwer Academic Publishers: Norwell, MA, 2003. (d) Rogers, R. D.; Seddon, K. R., Eds.; Ionic Liquids. Industrial Applications for Green Chemstry (ACS Symposium Series 818); American Chemical Society: Washington, DC, 2002. (e) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391–1398. (3) (a) Parvulescu, V. I.; Hardacre, C. Chem. ReV. 2007, 107, 2615–1665. (b) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667–3692. (4) (a) Miao, W.; Chan, T. H. Acc. Chem. Res. 2006, 39, 897–908. (b) Malhotra. S. V., Ed. Ionic Liquids in Organic Synthesis (ACS Symposium Series 950); American Chemical Society: Washington, DC, 2007. (c) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, 2003. (5) (a) Han, X.; Armstrong, D. W. Acc. Chem. Res. 2007, 40, 1079–1086. (b) Anjan, S. T. Chem. Eng. Prog. 2006, 102, 30–39. (c) Zhao, H.; Xiao, S.; Ma, P. J. Chem. Technol. Biotechnol. 2005, 80, 1089–1096. (d) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H.; Rogers, R. D. EnViron. Sci. Technol. 2002, 36, 2523–2529. (6) (a) Ichikawa, T.; Yoshio, M.; Hamasaki, A.; Mukai, T.; Ohno, H.; Kato, T. J. Am. Chem. Soc. 2007, 129, 10662–10663. (b) Biswas, K.; Rao, C. N. R Chem.-Eur. J. 2007, 13, 6123–6129. (c) Wang, Y.; Yang, H. J. Am. Chem. Soc. 2005, 127, 5316–5317. (d) Itoh, H.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 2004, 126, 3026–3027. (e) Zhu, Y.; Wang, W.; Qi, R.; Hu, X. Angew. Chem., Int. Ed. 2004, 43, 1410–1414.
technological,7 and electrochemical processes,8 and for other uses in engineering9 have been developed. Liquid crystals (LCs)10 have been employed as organized media for many applications as well, including to impart regio- and stereoselectivity during reactions of solutes11 and as templating agents to form mesoporous12 and nanostructured13 materials. Ionic liquid crystals (ILCs), materials that combine the interesting solvent properties of ILs and the self-organizing behavior of LCs, may be useful in fundamental and applied applications where neither ILs nor LCs are effective.14 (7) (a) van Rantwijk, F.; Sheldon, R. A. Chem. ReV. 2007, 107, 2757–2785. (b) Raab, T.; Bel-Rhlid, R.; Williamson, G.; Hansen, C.; Chaillot, D. J. Mol. Catal. B: Enzym 2007, 44, 60–65. (c) Sanfilippo, C.; D’Antona, N.; Nicolosi, G. Biotechnol. Lett. 2004, 26, 1815–1819. (d) Park, S.; Kazlauskas, R. J. Curr. Opin. Biotechnol. 2003, 14, 432–437. (8) (a) Sakaebe, H.; Matsumoto, H.; Tatsumi, K. Electrochim. Acta 2007, 53, 1048–1054. (b) MacFarlane, D. R.; Forsyth, M.; Howlett, P. C.; Pringle, J. M.; Sun, J.; Annat, G.; Neil, W.; Izgorodina, E. I. Acc. Chem. Res. 2007, 40, 1165– 1173. (c) Zhao, H. Chem. Eng. Commun. 2006, 193, 1660–1677. (d) Ohna, H., Ed. Electrochemical Aspects of Ionic Liquids; Wiley: Hoboken, NJ, 2005. (9) Zhao, H. Chem. Eng. Commun. 2006, 193, 1660–1677. (10) (a) Singh, S. Liquid Crystals: Fundamentals, World Scientific: Singapore, 2002. (b) Demus, D.; Goodby J.; Gray, G. W.; Spiess, H. W.; and Vill, V. Handbook of Liquid Crystals, Wiley-VCH: Weinheim, NY, 1998. (c) Stegemeyer, H. Liquid Crystals, Darmstadt: Steinkopff, New York: Springer, 1994. (11) (a) Lemieux, R. P. Acc. Chem. Res. 2001, 34, 845–853. (b) Reichardt, C. SolVent and SolVent effects in Organic Chemistry, 3rd ed.; Wiley-VCH: Weinheim, 2003. (c) Ichimura, K. Chem. ReV. 2000, 100, 1847–1873. (d) Kansui, H.; Hiraoka, S.; Kunieda, T. J. Am. Chem. Soc. 1996, 118, 5346–5352. (e) Weiss, R. G. Tetrahedron 1988, 44, 3413–3475. (12) (a) Brennan, T.; Hughes, A. V.; Roser, S. J.; Mann, S.; Edler, K. Langmuir 2002, 18, 9838–9844. (b) Lin, H.-P; Mou, C -Y Acc. Chem. Res. 2002, 35, 927– 935. (c) Attard, G. S.; Leclerc, S. A. A.; Maniguet, S.; Russell, A. E.; Nandhakumar, I.; Gollas, B. R.; Bartlett, P. N. Microporous Mesoporous Mater. 2001, 44, 159– 163. (13) (a) Yamauchi, Y.; Momma, T.; Yokoshima, T.; Kuroda, K.; Osaka, T J. Mater. Chem. 2005, 15, 1987–1994. (b) Wang, L.; Chen, X.; Zhan, J.; Chai, Y; Yang, C.; Xu, L.; Zhang, W.; Jing, B. J. Phys. Chem. B 2005, 109, 3189–3194. (c) Li, L.-S; Walda, J.; Manna, L.; Alivisatos, A. P. Nano Lett. 2002, 2, 557–560. (14) (a) Binnemans, K. Chem. ReV. 2005, 105, 4148–4204. (b) Faul, C. F. J.; Antonietti, M. AdV. Mater. 2003, 15, 673–683. (c) Kato, T. Science 2002, 295, 2414–2418.
10.1021/la801594q CCC: $40.75 2008 American Chemical Society Published on Web 08/08/2008
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Typically, lyotropic liquid crystals consist of amphiphilic molecules in a very large volume excess of a low-molecularmass isotropic liquid (frequently water).15 When the mixtures contain large amounts of an amphiphile, lamellar phases can be formed.15 Since the first report of ILs in 1888,16 the number of examples has grown astronomically. However, very few ILs have been found to be amphotropic with organic liquids.17 During the past decade, we have characterized the neat phases of a large number of phosphonium salts, [H(CH2)n]3P+[(CH2)mY] X- (mPnX), where n ) 10, 14, 18, m ) 0-5 when Y ) H and X- is a halide, NO3-, ClO4-, BF4-, PF6-, etc.18 Recently, we investigated the influence of added methanol or a covalently attached methanol-like group (i.e., m ) 2 and Y ) OH) with X ) Br- and Cl-.19 Almost all of these mPnX salts have lamellar packing structures in their solid phases and, in those cases where single crystal data are available (N.B., salts with 4 equivalent long n-alkyl chains),18e the molecules are organized in bilayers with cations and anions alternating within ionic planes separating the long lipophilic chains. In the liquid-crystalline phases of the mPnX, the molecular directors are found to be normal to the plane of the bilayers, resulting in enantiotropic phases which are most precisely characterized as bilayer smectic A (SmA2). These salts are easily synthesized and stable in air up to 200 °C. Because of the many modifications that can be incorporated within the general structural motif, the completely saturated nature of their cations and the simplicity of their anions, their transparency in the visible and near UV regions, and their generally broader mesomorphic temperature ranges and better stabilities than those of analogous ammonium salts,18 we envision a wide variety of applications for the mPnX salts. We report here the properties of the smectic A2 phases of mixtures of a phosphonium halide, [H(CH2)10]3P+CH3Cl(1P10Cl) or [H(CH2)10]3P+CH3Br- (1P10Br) (neither of which forms a liquid-crystalline phase when neat), and a small amount of an alcohol or water (Figure 1); the compositions investigated are rich in salt and poor in alcohol or water. The structures and concentrations of the hydroxyl-containing adduct have been correlated with the temperature ranges and transition temperatures of the phases. The structural characteristics of the phases themselves have been determined at the microscopic and macroscopic levels using a variety of spectroscopic, diffraction, and thermochemical techniques.
Figure 1. Representation of a 2-dimensional slice of the proposed packing arrangement for the layered solid and LC (smectic A2) phases of 1P10X•ROH (R is H or an alkyl group and X- ) Cl- or Br-). The long alkyl chains between two bilayers may be somewhat interdigitated. The acronym 1P10X•ROH indicates that one molar equivalent of ROH has been added to the 1P10X salt. Addition of other molar equivalents, m, is noted by 1P10X•mROH.
and orientation can then be studied by NMR spectroscopy.20 In that regard, we find that the overall order parameters of the SmA2 phases and the alcohol molecules within them are very low. Especially when the number of nuclei in a guest molecule is large, the large number of lines from residual dipolar couplings requires that media with very low order parameters be used to obtain simple NMR spectra and, thereby, to be able to perform the necessary spectral analyses.21 Aqueous-based lyotropic liquid crystals (N.B. ‘bicellar’ media) and compressed hydrogels22 have been exploited for this purpose to a much greater extent than ordered media with organic liquids because the order parameters of the organic-based systems are usually too high to yield simple spectra of solutes.23–25 This report includes our efforts to develop nonaqueous, nonpolymeric media with low order parameters for the purpose of 3-dimensional structural analyses of organic solute molecules.
Experimental Section
One potential application for the ionic liquid crystals reported here is as hosts of small molecules whose molecular geometry
Instrumentation. 1H NMR spectra of molecules in CDCl3 solutions were measured with a Varian Unity Inova 300 MHz spectrometer and referenced to an internal tetramethylsilane (TMS) standard. DMSO-d6 in a sealed capillary was inserted in the NMR tube containing samples of neat or hydrated phosphonium halides as an external lock signal. Thirty-two free induction decays (FIDs) were accumulated. 2H NMR spectra were recorded at various temperatures on a Bruker AVANCE 400 spectrometer operating at 61.4 MHz or a Varian Unity Inova 500 spectrometer at 76.7 MHz.
(15) (a) Forrest, B. J.; Reeves, L. W. Chem. ReV. 1981, 81, 1–14. (b) Kuzma, M. R.; Saupe, A. Handbook of Liquid Crystal Research, Collings, P. J.; Patel, J. S., Eds.; Oxford University Press: New York, 1998, Chapter 7. (c) Collings, P. J. Liquid Crystals, 2nd ed.; Prince University Press: NJ, 2002; Chapter 8. (16) (a) Gabriel, S.; Weiner, J. Ber. 1888, 21, 2669–2679. (17) (a) See, for example Zhang, G.; Chen, X.; Xie, Y.; Zhao, Y.; QIu, H. J. Colloid Interface Sci. 2007, 315, 601–606. (b) Asaro, F.; Pellizer, G.; Pergolese, B. Mol. Cryst. Liq. Cryst. 2003, 394, 127–139. (c) Gu, W.; Gin, D. L. Langmuir 2002, 18, 7415–7427. (d) Attard, G. S.; Fuller, S.; Howell, O.; Tiddy, G. J. T. Langmuir 2000, 16, 8712–8718. (e) Stella, I.; Mu¨ller, A. Colloids Surf A 1999, 147, 371–374. (f) Auvray, X.; Petipas, C.; Lattes, A.; Rico-Lattes, I. Colloids Surf A 1997, 123-124, 247–251. (g) Ylihautala, M.; Ingman, P.; Jokisaari, J.; Diehl, P. Appl. Spectrosc. 1996, 50, 1435–1438. (18) (a) Abdallah, D. J.; Robertson, A.; Hsu, H.; Weiss, R. G. J. Am. Chem. ¨ ; onen, Z. S.; Weslowski, Soc. 2000, 122, 3053–3062. (b) Chen, H.; Kwait, D. C.; G B. T.; Abdallah, D. J.; Weiss, R. G. Chem. Mater. 2002, 14, 4063–4072. (c) Nagana Gowda, G. A.; Chen, H.; Khetrapal, C. L.; Weiss, R. G. Chem. Mater. 2004, 16, 2101–2106. (d) Nagana Gowda, G. A.; Khetrapal, C. L.; Lu, L.; Wauters, H. C.; Abdallah, D. J.; Weiss, R. G. Proc. Ind. Nat. Sci. Acad. 2004, 70A, 627– 634. (e) Abdallah, D. J.; Lu, L.; Cocker, M. T.; Bachman, R. E.; Weiss, R. G. Liq. Cryst. 2000, 27, 831–837. (19) Ma, K.; Somashekhar, B. S.; Nagana Gowda, G. A.; Khetrapal, C. L; Weiss, R. G. Langmuir 2008, 24, 2746–2758.
(20) (a) Diehl, P.; Khetrapal, C. L. In NMR Basic Principles and Progress, Vol. 1; Diehl, P.; Fluck, E.; Kosfeld, R., Eds.; Springer-Verlag: New York, 1969. (b) Jacobsen, J. P.; Schaumburg, K. J. Magn. Reson. 1977, 28, 191–201. (c) Meiboom, S.; Snyder, L. C. Acc. Chem. Res. 1971, 4, 81–87. (21) (a) Bax, A.; Grishaev, A. Curr. Opin. Struct. Biol. 2005, 15, 563–570. (b) Vivekanandan, S.; Joy, A.; Suryaprakash, N. J. Mol. Struct. 2004, 694, 241– 247. (c) Tjandra, N.; Bax, A. Science 1997, 278, 1111–1114. (22) (a) Prestegard, J. H.; Bougault, C. M.; Kishore, A. I. Chem. ReV. 2004, 104, 3519–3540. (b) Bax, A. Protein Sci. 2003, 12, 1–16. (c) Gronenborn, A. C. R. Biol. 2003, 325, 957–966. (d) Gronenborn, A.M. C. R. Biol. 2002, 325, 957–966. (e) Shahkhatuni, A. A.; Shahkhatuni, A. G. Russ. Chem. ReV. 2002, 71, 1005– 1040. (23) (a) Sobajima, S. J. Phys. Soc. Jpn. 1967, 23, 1070–1078. (b) Panar, M.; Phillips, W. D. J. Am. Chem. Soc. 1968, 90, 3880–3882. (c) Abe, A.; Yamazaki, T. Macromolecules 1989, 22, 2145–2149. (d) Ginzburg, B. M.; Shepelevskii, A. A. J. Macromol. Sci. B: Phys. 2003, B42, 1–56. (24) (a) Czarniecka, K.; Samulski, E. T. Mol. Cryst. Liq. Cryst. 1981, 63, 205–214. (b) Sarfati, M.; Lesot, P.; Merlet, D.; Courtieu, J. Chem. Commun. 2000, 2069–2081. (c) Gronenborn, A. M. C. R. Biol. 2002, 325, 957–966. (i) Bax, A. Protein Sci. 2003, 12, 1–16. (l) Verdier, L.; Sakhaii, P.; Zweckstetter, M.; Griesinger, C. J. Magn. Reson. 2003, 163, 353–359. (25) (a) Yan, J.; Zartler, E. R. Magn. Reson. Chem. 2005, 43, 53–64. (b) Frudenberger, J. C.; Kno¨r, S. Angew. Chem., Int. Ed. 2005, 44, 423–426. (c) Haberz, P.; Farjon, J.; Griesinger, C. Angew. Chem., Int. Ed. 2005, 44, 427–429.
Amphotropic-Lyotropic Ionic Liquid Crystal Phases A relaxation delay of 1 s and 32 K data points were used. Depending on the concentration of deuterated alcohols in a sample, 256-1024 FIDs were accumulated and a spectral width of 30-60 KHz was used. Samples of the 1P10X salts with varying concentrations of a deuterated alcohol for the 2H NMR studies were prepared in 5-mm NMR tubes under a nitrogen atmosphere, and the tubes were flamesealed immediately. Phase transition temperatures (corrected) were measured and polarized optical micrographs (POMs) were recorded on a Leitz 585 SM-LUX-POL microscope with crossed polarizers, a Leitz 350 heating stage, a Photometrics CCD camera interfaced to a computer, and an Omega HH503 microprocessor thermometer connected to a J-K-T thermocouple. In a drybox, a small amount of sample was placed in the center of a cover glass and another was placed gently on top; the edges around the coverslip sandwich were then wrapped with a narrow strip of Teflon film and ‘super glue’ was brushed over the edges of the Teflon film to seal the samples and avoid alcohol loss via evaporation or hydration on the heating stage of the microscope. In this way, transition temperatures could be reproduced in at least three consecutive heating and cooling cycles. Selected subambient temperatures were maintained by altering the flow of dry nitrogen gas (which had passed through a 2 m long copper coil that was immersed in liquid nitrogen) through the hollow block of the stage. To avoid moisture condensation on the cooling stage and optics, the microscope was enclosed in a plastic container through which a positive pressure of dry nitrogen gas was slowly passed. X-ray diffractometry (XRD) was conducted on a Rigaku RAPID/ XRD image plate system using Cu KR radiation (λ ) 1.54056 Å) from a Rigaku generator. Temperature control, based on flowing nitrogen gas that was heated over a filament or cooled by liquid nitrogen and passes through a thermocouple with a feedback loop, was estimated to be ( 0.5 °C. Samples were loaded into 0.5 mm glass capillaries (W. Mu¨ller, Scho¨nwalde, FRG) in a drybox and then the capillaries were flame-sealed immediately. Diffractions in degrees (2θ) were converted to distances (d) using Bragg’s equation (λ ) 2d sin θ).26 Thermal gravimetric analyses (TGA) were performed under a nitrogen atmosphere at a 5 °C/min heating rate on a TA Q50 thermogravimetric analyzer interfaced to a computer. Differential scanning calorimetry (DSC) was conducted on a TA Q200 DSC cell base interfaced to a TA Thermal Analyst 3100 controller; samples were sealed in aluminum pans under a nitrogen atmosphere. A refrigerated cooling system (RCS90) was used to control the cooling rates. Sample Preparations. Details of syntheses and characterizations of the molecules employed here are collected in the Supporting Information file. Aliquots of the hygroscopic 1P10X salts with different weight percents (wt %) of water or alcohols were prepared in a drybox under a nitrogen atmosphere and then immediately flamesealed in glass tubes. The tubes were immersed in a hot-water bath at temperatures corresponding to the isotropic phases for at least 5 h (to ensure homogeneity), allowed to cool slowly to room temperature with the water in the bath, and kept for at least 2 days at room temperature. The tubes were opened in a drybox and aliquots were removed for different experiments, as needed.
Results and Discussions Water Uptake by 1PnX Salts. Neither 1P10Cl nor 1P10Br is a liquid crystal,18b,19 but both are very hygroscopic and gradually convert from solids to liquid crystals when left in air. The uptake of water by the 1P10X salts was examined by 1H NMR spectroscopy on neat samples that had been exposed to the air for different periods. To increase resolution, each sample was heated to a temperature at which it was isotropic (Figure 2). Thermal gravimetric analyses (TGA) showed a 3.4% continuous weight loss between 94 and 115 °C from the sample labeled as monohydrated 1P10Cl in Figure 2; 3.5 wt% loss corresponds to (26) Giacovazzo C. Fundamentals of Crystallography, 2nd ed.; Oxford University Press: New York, 2002; Chap 3.
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Figure 2. 1H NMR spectra of neat and hydrated 1P10Cl in their isotropic phases. The signal at ∼3.2 ppm is attributed to water, and the others listed as R are from groups attached directly to P+. The quantities of water are approximated from integrations of peak areas. Samples were kept at isotropic temperatures in these experiments for no more than 5 min to minimize loss of water.
1.0 equivalent of water. Samples of the 1P10X with different degrees of hydration were also prepared by mixing weighed amounts of water and neat 1P10X (vide infra). These hygroscopic properties of the phosphonium halides are attributed to strong H-bonding interactions between halide anions and hydroxyl protons of water, as well as between phosphorus cations and hydroxyl oxygen atoms. Consistent with expectations based upon the sizes and ‘softness’ of the two halides,27 1P10Br is qualitatively less hygroscopic than 1P10Cl. In both salts, the strong interactions retain the complexed water molecules to temperatures above their bulk boiling temperature. Regardless, because the samples were prepared and heated to their isotropic phases in sealed tubes, any water loss from the salt is reabsorbed when the temperature is lowered. Optical Textures and Phase Transition Temperatures from Polarized Optical Microscopy (POM). Upon being cooled from their isotropic phases, liquid-crystalline samples sandwiched between cover glasses spontaneously adopt a homeotropic alignment. The micrograph in Figure 3A indicates that one-half equivalent of water is insufficient to make 1P10Cl a liquid crystal at room temperature: no flow was detected when a strong lateral force was placed on the coverslips of this sample and a soft material with a compact, fan-shaped texture was observed upon cooling from its isotropic phase. However, The POM of 1P10Cl•H2O in Figure 3B, made upon shearing and then cooling the isotropic phase, is a focal conical fan texture characteristic of a Smectic A phase. Liquid crystallinity is attributed to the formation of complexes between 1P10X and water molecules at the head groups of the salts. An analogous induction of liquid crystallinity has been reported to occur with the nonionic molecule, 2-(3,4-dihexadecyloxy)-imidazo[4,5-f]-1,10-phenanthroline.28 Oily streak patterns were observed when samples were sheared at temperatures within their liquid-crystalline phases (Figure 3C and D). The liquid-crystalline phases of the 1P10X with water or an alcohol are assigned as SmA2 based upon POM and X-ray diffraction (vide infra) data. The solid 1P10X were previously identified as bilayered lamellar structures.18,19 Clearing temperatures (TC) from either SmA2fisotropic or solidfisotropic transitions of heated samples were determined by optical microscopy. Virtually no supercooling was observed (27) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447–4454. (28) Cardinaels, T.; Ramaekers, J.; Nockemann, P.; Driesen, K.; Van Hecke, K.; Van Meervelt, L.; Lei, S.; De Feyter, S.; Guillon, D.; Donnio, B.; Binnemans, K. Chem. Mater. 2008, 20, 1278–1291.
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Figure 3. Polarized optical micrographs (POMs) of hydrated samples after being cooled from their isotropic phases: (A) fan-shaped texture of 1P10Cl•0.5H2O in its solid phase at 23.7 °C; (B) focal conical fan texture of 1P10Cl•H2O in its liquid-crystalline phase at 23.7 °C; (C) oily streak texture of 1P10Cl•C2H5OH in its liquid-crystalline phase at -13.3 °C; (D) oily streak texture of 1P10Cl•CH3(CH2)2OH in its liquid-crystalline phase at -17.4 °C.
by optical microscopy (as well as by DSC and 2H NMR analyses (vide infra)) during isotropicfSmA2 or isotropicfsolid transitions. Upon cooling, many of the 1P10X•ROH complexes underwent solid-liquid phase separations at a characteristic temperature, TS, and phase homogeneity could be regained only by heating to above the melting temperature of the 1P10X component. In only some cases could the liquid-crystal-tobiphasic transitions be observed by optical microscopy; they were always detectable by X-ray and 2H NMR analyses. However, because the increments between NMR spectra or X-ray diffractograms are g5 °C, the transition temperatures measured by POM (where observable) are more precise. Recently, we reported that both the 1P10Cl and 1P10Br become liquid-crystalline at room temperature when they are associated with one equivalent of methanol molecules.19 Evidence for 1P10X•CH3OH complex formation (as well as for other 1P10X•H(CH2)nOH) is found from XRD and 2H NMR data that will follow. The possible formation of liquid-crystalline phases by 1P10X•H(CH2)nOH (where n is as many as 6 carbon atoms) and 1P10X•m(CH2OH)2 (where m ) 0.5 or 1.0; i.e., containing one-half or one molar equivalent of ethylene glycol) has been examined here (Figure 4). The mesophases span a wide temperature range, from ca. -30 to 50 °C, depending on the specific X and H(CH2)nOH. Whereas liquid crystallinity of 1P10Cl was not induced by added alcohols with n > 4 or branched alcohols, such as isopropyl alcohol or t-butyl alcohol, 1P10Br was able to form liquid-crystalline phases with all of the alcohols with n e 6, including those that are branched. The mesomorphic temperature ranges of both salts could be ‘tuned’ by altering the structure of the added alcohol. Both the clearing and solidifying temperatures (where a solid was formed within the temperature range examined) of the 1P10Br•H(CH2)nOH decreased as n increased from 0 to 3 (i.e., water > methanol > ethanol >1-propanol), and then increased to n ) 6 while the mesophase temperature range decreased. A
similar, but less clear, trend was found for the 1P10Cl•H(CH2)nOH. Somewhat surprisingly, 1-decanol, an alcohol whose chain length is the same as that of the long alkyl chains of 1P10X, did not induce a liquid crystal phase in either salt. Secondary and tertiary alcohols decrease both TC and TS of the 1P10X•ROH much more than their primary alcohol isomers. Ethylene glycol, with two hydroxyl groups, lowers TC of the liquid-crystalline 1P10X•ROH more than any of the alcohols with one hydroxyl group, and it narrows the mesophase range. However, one-half equivalent of ethylene glycol, corresponding to one equivalent of hydroxyl groups, provides 1P10X•0.5(CH2OH)2 whose mesophase ranges are among the broadest observed. This observation and other evidence that will be presented subsequently point to specific interactions between hydroxyl groups and the charged head groups of the 1P10X salts. As shown in the phase diagrams for 1P10X in the presence of different concentrations of methanol, 1-butanol, and ethylene glycol (Figure 5 and Figure S1 of Supporting Information), liquidcrystalline phases persist in some cases when much more than one molar equivalent of hydroxyl groups is added (Tables S1 and S2 of Supporting Information). At temperatures below TS of 1P10Cl•mCH3OH, phases with even less than one equivalent of methanol separate into solid 1P10Cl and methanol; 1P10Br•mCH3OH (m e 1.0) remain one (solid) phase19 and separate discernibly into a solid and a liquid only at m > 1.0. From the phase diagrams in Figures 5A and 5B, it is clear that at least one molar equivalent of methanol is necessary to induce formation of a homogeneous smectic phase; the 1P10X are converted only partially to liquid crystals at lower methanol concentrations. The solidifying temperature decreases less rapidly with increasing methanol concentration than the clearing temperature. The congruence between the TC and TS lines occurs for both salts at ca. 4 equivalents of methanol; the temperatures are -38.6 to -47.9 °C in 1P10Cl, and -42.7 to -48.8 °C in 1P10Br.
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Figure 4. Phase transition temperatures of 1P10Cl (A) and 1P10Br (B) in the presence of one equivalent of water or an alcohol, or one-half equivalent of ethylene glycol. The transition temperatures of neat 1P10X and with one equivalent of methanol are from ref 19, TC ) clearing temperature on heating; TS ) solidification temperature of 1P10Br and solidification with liquid phase separation of 1P10Cl on cooling.
However, liquid crystallinity could not be retained even at 1.5 equiv of 1-butanol (Figure S1 of Supporting Information) or ∼2 equiv of ethylene glycol (Figure 5C) in 1P10Br. The phase diagram in Figure 5C indicates that only one-half molar equivalent of ethylene glycol, corresponding to one molar equivalent of hydroxyl groups, is needed to convert 1P10Br into a completely liquid-crystalline phase, and the same result is found with 1P10Cl (Figure 4A). In the homogeneous liquid-crystalline phases at the higher ethylene glycol concentrations, both, one or neither of the hydroxyl groups from one molecule may be associated with salt head groups; our current data do not allow a distinction among these possibilities. The fact that the temperature at which liquid crystallinity of 1P10Br is lost (at ca. 2.0 mol equiv or 4.0 equiv of hydroxyl groups), -28.1 to -37.4 °C, is somewhat higher than when methanol is the added alcohol is attributed to the covalent linkage between the hydroxyl groups in ethylene glycol; the decreased motional freedom of the linked hydroxyl groups should allow the head groups of the salt to remain organized to a higher temperature. Differential Scanning Calorimetry (DSC). Cooling thermograms were examined from the highest temperatures within the isotropic phases to the lowest temperatures within the solid phases (Table 1). At the lowest temperature explored, -80 °C, all of the samples with phosphonium salt and an alcohol became
Figure 5. Phase diagrams of 1P10Cl (A) and 1P10Br (B) with different concentrations of methanol and 1P10Br (C) with different concentrations of ethylene glycol. I ) isotropic, K ) solid of 1P10X, K′ ) solid of 1P10Br•CH3OH, LC ) liquid crystalline, L ) liquid of the solute. Lines between points are drawn as a visual aid and are not based on a physical model. Points for solid lines were determined by optical microscopy; points for dashed lines were determined (less precisely) using X-ray diffractometry and 2H NMR data (vide infra).
phase-separated; only upon reheating these samples to their isotropic phase and recooling could homogeneous liquidcrystalline phases be restored (Figure S2 of Supporting Information and vide infra). In addition, a solid-to-solid phase transition was observed on cooling most of the samples with an alcohol to low temperatures (-25 °C, were broader (>300 Hz for the line width of the -CD3 doublets) than in the SmA2 phase, as expected if methanol molecules are still entrapped in the ionic headgroup areas of solid 1P10Br and suffer increasingly restricted motion (and very rapid spin-lattice relaxation35). Specific association between 1P10Br salts and methanol molecules, like that in the liquid-crystalline phase, is retained in the K2 phase. However, phase separation between 1P10Br and methanol molecules does occur at the K2 f K1 transition (vide ante). The ordered content of methanol molecules is defined here as the ratio between the peak area of the stronger quadrupolar doublet (from the ordered -CD3 groups) and the sum of that area and 75% of the areas of the two, nearly coincident central singlets (from isotropic -CD3 and -OD components). The TC values of 1P10Br•1.5CD3OD (and other samples with more than one equivalent of methanol) derived from the 2H NMR spectra are lower than the temperatures from optical microscopy and X-ray diffractometry data. This discrepancy indicates that methanol molecules exchange positions rapidly within the regions of the ionic layer planes at temperatures above TC measured by 2H NMR spectroscopy but below the TC measured by X-ray and OM methods. Thus, the magnitude of the discrepancy between the TC for the 1P10X•mROH (m g 1) measured by NMR and the other two methods increases with increasing m as a result of increasing rates of site exchanges by alcohol molecules. Because the NMR method senses changes within a phase of the 1P10Br matrix that do not involve first-order transitions, we prefer to use the TC from OM and X-ray data when more than 1 equiv of alcohol is present. The spectra in the isotropic phases of the 1P10X media with a rapidly reorienting deuterated solute consist of singlets. As temperature is lowered and approaches TC, the central singlet becomes smaller and one or more quadrupolar doublets, depending on the number of chemically inequivalent deuterons and the orientations of the individual chemically equivalent deuterons (e.g., the -CD2 deuterons of 1P10Cl•C2D5OD), appear. The appearance of the doublets coincides with the TC measured (35) (a) Liu, K.; Ryan, D.; Nakanishi, K.; McDermott, A. J. Am. Chem. Soc. 1995, 117, 6897–6906. (b) Olsson, U.; Wong, T. C.; So¨derman, O. J. Phys. Chem. 1990, 94, 5356–5361.
Figure 13. Changes of ordered contents of -CD3 deuterons (O; see text) and 2H quadrupolar splittings of deuterons of 1P10Cl with 1 equiv of ROH as a function of temperature: (A) 1P10Cl•D2O [3.8 wt% D2O in two environments, 9 and •], (B) 1P10Cl•C2D5OD [9.3 wt% CD3CD2OD; -CD3, 1; two -CD2 environments, 2 and •; -OD, 9], (C) 1P10Cl•CH3CH2CD2OH [10.9 wt% CH3CH2CD2OH; -CD2, 9], (D) 1P10Cl•CH3(CH2)2CD2OH [13.1 wt% CH3(CH2)2CD2OH, -CD2, 9]. The transition temperatures of the corresponding undeuterated 1P10Cl•ROH noted along the X-axis are from POM. Lines between points are drawn as a visual aid and are not based on a physical model.
by OM and XRD (Figure 4, and Figures S6-S8 of Supporting Information). For the primary alcohols explored here at one temperature, the magnitudes of the quadrupolar splittings increase but the intensities of their peaks decrease in the order, -CD3 to -CD2 to -OD (Figure 12). The signals from the -OD deuterons are either weak (broadened) or undetectable generally at temperatures slightly above TS due to decreased motional averaging.35 In most 1P10X•ROH samples, a new central single peak appears at still lower temperatures in the spectra while the quadrupolar doublets decrease in intensity, broaden, and increase their splitting. The singlet is a result of separation of the
Amphotropic-Lyotropic Ionic Liquid Crystal Phases
Langmuir, Vol. 24, No. 17, 2008 9853
Figure 14. Examples of possible exchanging orientations for C2D5OD molecules at the headgroup region in the SmA2 phase of 1P10Cl•C2D5OD.
1P10X•ROH into a solid phase (the 1P10X salt) and an isotropic liquid phase (the alcohol). By contrast, 1P10Br•C2D5OD (as well as 1P10Br•CD3OD19) remains monophasic throughout the cooling from its SmA2 phase to below TS; the ethanol molecules remain associated with the cationic and anionic groups in the ionic planes of 1P10Br. The presence of two quadrupolar doublets of very different intensities for the chemically equivalent deuterons in the spectra of the liquid-crystalline phases of 1P10Cl•CD3OD19 and 1P10X•C2D5OD (Figures 12C and 13B, and Figure S16B of Supporting Information) indicate that the alcohol molecules interconvert slowly between two distinctly different chemical environments (Figure 14). An interesting question arising from these observations, “Are the smectic phases completely homogeneous?”, cannot be answered definitively with the available data because the minor doublet may be a consequence of either kinetic or thermodynamic factors. The kinetic attribution is less likely because the samples were annealed for long periods before commencing measurements and the two doublets are present at common temperatures approached by both cooling and heating. The intermolecular interactions with hydroxyl groups are stronger and the ionic headgroup areas are smaller for chloride anions than for bromide ions. Both factors should allow the small alkyl groups of methanol and ethanol to orient within the ionic parts of 1P10Cl layers better than within 1P10Br layers. Consistent with the X-ray data in Figure 9 and the 2H NMR data mentioned above, the alkyl groups of longer homologues, such as 1-propanol and 1-butanol, appear to arrange themselves more parallel to the n-decyl chains of both 1P10X.
∆)
( )
3 e2qQ SXD 2 h
1 SXD ) (3cos2 β - 1)SC3 2
(1) (2)
The quadrupolar splitting (∆) of any given deuteron in the medium of a liquid crystalline phase can be related to the degree of orientation of its X-D bond (where X ) C or O) with respect to the applied magnetic field direction, SXD, by eq 1 (in which (e2qQ/h) is the deuteron quadrupolar coupling constant along the X-D bond, eQ is the electric quadrupole moment of a deuteron, and eq is the electric field gradient experienced by the deuteron).20 The quadrupolar coupling constants of deuterons are 223 KHz for water,36 and 155 KHz for the methyl group of methanol (31 °C),20b 183 KHz for the methyl group and 177 KHz for the methylene group of ethanol (20-41 °C),37 and 170 KHz for the R-methylene groups of 1-propanol and 1-butanol.38 These constants vary slightly with temperature and are assumed to be constant throughout the temperature ranges explored here. The (36) Wong, T. C.; Hakala, M. R. Chem. Phys. Let. 1985, 119, 85–88, Temperatures not stated are presumed to be room temperature. (37) Lickfield, G. C.; Beyerlein, A. L.; Savitsky, G. B. J. Phys. Chem. 1984, 88, 3566–3570.
order parameter of the methyl group along its 3-fold symmetry axis, SC3, can be calculated from the coupling constants and eq 2 (where β is the angle between a C-D bond and the C3-axis, slightly fluctuating around 70°, depending on the mesomorphic properties of the host).20 The orientation factors, SCD, SOD, and SC3 (excluding SOD of alcohols due to the low intensity and very large line width of the quadrupolar splittings) for the smectic phases of the investigated samples are summarized in Table 2. Previously, we attributed the somewhat larger SCD and SC3 values for 1P10Cl•CD3OD than for 1P10Br•CD3OD to the smaller size of the chloride anion.19 The same trend in order parameters is followed by the other 1P10Cl•ROH and 1P10Br•ROH investigated here. Also, within the same temperature range and phase for a series of 1P10X•ROH, several interesting other trends are noted: (1) SCD of R-CD2 groups increase with increasing length of alkyl chains for primary alcohols; (2) the SC3 and SCD of methyl groups in ethanol are smaller than those of methanol; (3) order parameters for methanol decrease as it concentration is increased. In all cases, the order parameters of alcohols in the SmA2 phases of both 1P10X are much lower, ∼10-2, than in typical, uncharged, thermotropic liquid-crystals.39 These observations will be exploited in future studies.
Conclusions The two tri-n-decylmethylphosphonium halides investigated here do not form a thermotropic liquid-crystalline phase when neat. However, addition of one equivalent or more of one of several simple alcohols or water (corresponding to a very a low weight %) or one-half molar equivalent of ethylene glycol (i.e., corresponding to one molar equivalent of hydroxyl groups) induces liquid crystallinity in the form of smectic A2 phases. Mixtures of crystalline and liquid-crystalline materials are observed when solutes with less than one equivalent of hydroxyl groups are added to a 1P10X. At the other extreme, a mesophase can be maintained when nearly four molar equivalents of methanol (or two of the diol, ethylene glycol) are added to the salts. Data from 2H NMR spectra of deuterated alcohols added to the salts and X-ray diffractograms indicate that mesophase induction is a result of “molecular lubrication” in a cause-and-effect relationship: at the ionic planes of the crystalline 1P10X, the strong and specific P+X- electrostatic interactions are sufficiently attenuated by complexing hydroxyl groups to allow some fluidity; that, in addition to increases in the headgroup areas caused by the presence of the hydroxyl groups, allows the long chains to adopt additional gauche bends; chain bending reduces the bilayer thicknesses. The mesophase onset temperatures and temperature ranges can be ‘tuned’ by changing the structure of the added alcohol or its concentration. Both variables affect the area projected by the P+ X- headgroup onto a layer plane. Thus, the values of TC and TS are inversely proportional to the extent to which an alcohol enlarges the headgroup area (as discerned from X-ray diffraction measurements) and not only the van der Waals volume of the alcohol, itself. Shape as well as size matters: at one equivalent of H(CH2)nOH, the influence of alkyl group enlargement on the induction of liquid crystallinity (and decreases in layer thicknesses) increases as n changes from 0 to 3 and then decreases as n increases further; for the isomeric butyl alcohols, the order (38) (a) Diehl, P.; Niederberger, W. J. Magn. Reson. 1974, 15, 391–392. (b) Mantsch, H. H.; Saitoˆ, H.; Smith, I. C. P. Progress in NMR Spectroscopy, Vol. 11, Pergamon Press: Great Beritain, 1977, pp 211-271. The temperatures are not stated; they are presumed to be room temperature. (39) (a) NMR of Ordered Liquids, Burnell, E. E.; de Lange, C. A., Eds.; Kluwer Academic Publishers: Norwell, MA, 2003. (b) Khetrapal, L. C.; Kunwar, A. C. AdV. Magn. Reson. 1977, 9, 301–422. (c) Emsley, J. W.; Lindon, J. C. NMR Spectroscopy Using Liquid Crystal SolVents; Pergamon Press: Oxford, 1975.
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Table 2. Molecular Orientation Parameters of Solute Molecules in the Smectic Phases of the Salts -CD2 or -OD (only for water) sample 1P10Cl•D2O 1P10Cl•C2D5OD 1P10Cl•H(CH2)2CD2OH 1P10Cl•H(CH2)3CD2OH 1P10Br•D2O 1P10Br•0.5CD3OD 1P10Br•1.5CD3OD 1P10Br•C2D5OD 1P10Br•H(CH2)2CD2OH 1P10Br•H(CH2)3CD2OH
T range (°C)
∆ (KHz)
SCD or SOD
25 - 0 15 - -15 15 - -20 10 - -15 15 - -10 50 - -5
3.8 - 7.9 5.1 - 9.8 13.2 - 17.9 15.1 - 19.0 16.7 - 22.1 2.7 - 8.1
0.011 - 0.024 0.019 - 0.037 0.050 - 0.067 0.059 - 0.075 0.065 - 0.087 0.0081 - 0.024
30 - -25 -10 - -25 25 - -25 25 - -5
11.7 - 16.1 21.8 - 22.5 13.8 - 20.0 15.8 - 21.0
0.044 - 0.061 0.082 - 0.085 0.054 - 0.078 0.062 - 0.082
is primary > secondary > tertiary. We hypothesize that the lipophilic parts of the larger alcohols enter the regions occupied by the decyl chains of the 1P10X and, thereby, make more difficult the gauche bends that are responsible for the decreased lamellar thicknesses of the SmA2 phases with the smaller alkanols. Large differences in the magnitudes of these effects are found between the phosphonium salts with chloride and bromide as the counterion. Unlike the salts with smaller and harder chloride, those with the larger and softer bromide anion are able to sustain liquid crystallinity in the presence of one equivalent of some secondary or tertiary alcohols. Also, all of the 1P10Cl•ROH complexes investigated became phase-separated at the SmA2 f K2 phase transitions (on cooling); 1P10Br•mCH3OH (1e m
