Aromatic Side Chain-Functionalized Long Chain Acid Salts: Structural

The structural factors influencing the self-assembly of aromatic side chain functionalized long chain acid salts into lyotropic liquid crystal (LLC) p...
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Langmuir 2002, 18, 7415-7427

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Aromatic Side Chain-Functionalized Long Chain Acid Salts: Structural Factors Influencing Their Lyotropic Liquid-Crystalline Behavior† Weiqiang Gu‡ and Douglas L. Gin* Department of Chemistry & Biochemistry, and Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0215 Received February 26, 2002. In Final Form: July 12, 2002 The structural factors influencing the self-assembly of aromatic side chain functionalized long chain acid salts into lyotropic liquid crystal (LLC) phases were examined. Two sets of amphiphiles were synthesized containing two different ionic headgroups (i.e., sodium carboxylate and sodium sulfonate) and four different aromatic side chains (i.e., 4-styryl ether, 4-ethylphenyl ether, 1-naphthyl ether, and 4-phenylazophenyl ether) positioned at various locations along the amphiphile backbone. Systematic variation of the type of the aromatic side chain and its relative position on the amphiphile as a function of the two headgroups revealed that the position of the side chain on the amphiphile is the most important structural factor influencing the type and stability of the LLC phases formed. The nature of the aromatic side chain and the nature of the anionic headgroup play much more subtle roles in the LLC behavior of this particular amphiphile structural motif. Noncovalent aromatic-aromatic interactions are known to play important roles in the aggregation behavior of amphiphiles containing aromatic units along the alkyl chains; however, such interactions could not be detected spectroscopically in the LLC phases of our aromatic side chain amphiphiles. We speculate that the differences in alkyl chain interdigitation and packing efficiency arising from having the aromatic unit as a side chain rather than along the alkyl chains are responsible for this discrepancy. Control experiments with aliphatic ether side chain amphiphiles also corroborate the fact that aromatic interactions do not play a significant role in the aggregation behavior of these side chain amphiphiles. Side chain amphiphiles lacking an aromatic unit are still able to form the inverted hexagonal LLC phase, so long as the extent of branching and volume of the aliphatic side chain are sufficient.

Introduction Amphiphilic molecules have the ability to spontaneously organize into a variety of phase-separated aggregate structures in the presence of water. These aggregate structures can range from relatively simple individual entities such as micelles and vesicles to extended assemblies such as interfacial Langmuir-Blodgett (LB) films and highly ordered lyotropic liquid crystal (LLC) phases (Figure 1).1,2 It is well-known that many of these organized structures that are based on lipids play very important roles in the machinery of biological systems (e.g., cell membranes).3 Organized amphiphile assemblies have also come to play increasingly important roles in the development of advanced man-made materials. For example, micelles and microemulsions are used for the controlled formation of colloidal particles,4 vesicles have been investigated for drug delivery,5 LB films have been explored for electrochemical applications,6 and LLC phases † Dedicated to our former Ph.D. advisors, Professors Robert H. Grubbs and Richard G. Weiss, on the occasion of their 60th birthdays. * To whom correspondence should be addressed. ‡ Current address: Waters Corporation, 177 Robert Treat Paine Drive, Myles Standish Industrial Park, Taunton, MA 02780.

(1) Winsor, P. A. Chem. Rev. 1968, 68, 1. (2) For reviews of LLC phases, see: (a) Tiddy, G. J. T. Phys. Rep. 1980, 57, 1. (b) Seddon, J. M. Biochim. Biophys. Acta 1990, 1031, 1. (3) Brown, G. H.; Wolken, J. J. Liquid Crystals and Biological Structures; Academic Press: New York, 1979. (4) For recent reviews, see: (a) Bonnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 10, 2455. (b) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7 (7), 607. (c) Murray, M. J.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 54, 73. (5) Ringsdorf, H.; Schlarb, B.; Venzmer. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (6) For a review, see: Tieke, B. Adv. Mater. 1990, 2, 222.

have been used as templates for forming functional inorganic7 and organic nanomaterials.8 Undoubtedly, a better understanding of how amphiphiles are able to form these diverse and important aggregate structures is of great fundamental and applied interest. The self-association of amphiphiles is commonly thought to be directed by a combination of three general phenomena: (1) hydrophobic effects,9 (2) amphiphile molecular shape/packing preferences,10 and (3) interfacial energy and intrinsic curvature.11 In addition to these three parameters, Whitten and co-workers have recently shown that noncovalent aromatic-aromatic interactions act as an additional driving force for the self-organization of amphiphiles containing aromatic units. Through detailed spectroscopic and modeling studies of fatty acid and phospholipid amphiphiles containing aromatic chromophores integrated into the linear hydrocarbon chains (Figure 2a), Whitten and co-workers found that intermolecular aromatic aggregation plays a major role in how these amphiphiles self-organize to form vesicles and LB films.12-15 For example, there is spectroscopic evidence (7) For a recent review, see: Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. Engl. 1999, 38, 56. (8) For a recent review, see: Gin, D. L.; Gu, W.; Pindzola, B. A.; Zhou, W.-J. Acc. Chem. Res. 2001, 34, 973. (9) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980. (10) Israelachvili, J. N. Intermolecular and Surface Forces with Applications to Colloidal and Biological Systems; Academic: London, 1985; pp 249-257. (11) Gruner, S. M. J. Chem. Phys. 1989, 93, 7562. (12) Song, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 9144. (13) Song, X.; Geiger, C.; Farashat, M.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 12481. (14) Song, X.; Perlstein, J.; Whitten, D. G. J. Phys. Chem. A 1998, 102, 5440.

10.1021/la0202069 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/24/2002

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Gu and Gin

Figure 1. Schematic representations of common aggregate structures (top row) and LLC phases (bottom row) formed by amphiphiles in the presence of water.

Figure 2. Schematic representations of (a) aromatic “main chain”- and (b) aromatic side chain-substituted amphiphiles.

that edge-to-face attractive interactions between adjacent aromatic units in these amphiphiles lead to “extended glide” and “herringbone” arrangements of the aromatic units in the assemblies.15 Although the importance of noncovalent aromatic interactions has been established for “main chain” aromatic amphiphiles during vesicle and LB film formation, the importance of these interactions has not yet been investigated for other types of aromatic amphiphiles and in the formation of ordered LLC phases. Recently, our research group developed a series of polymerizable LLCs based on styrene (i.e., 4-vinylphenyl) and styryl ether (i.e., 4-vinylphenoxy) side chain-functionalized long chain carboxylic and sulfonic acid salts (1a, 2a, and 3) (Figure 3). These aromatic side chain amphiphiles form the inverted hexagonal (HII) phase in the presence of 5-20% water and can be cross-linked with retention of phase microstructure in the presence of divinylbenzene.16-20 The resulting nanostructured polymer networks have been used as templates for nanocomposite synthesis16,17 and as heterogeneous organic catalysts.18,19 We have also demonstrated that control over the size of several nanoscale features in these systems is (15) Whitten, D. G.; Chen, L.; Geiger, C.; Perlstein, J.; Song, X. J. Phys. Chem. B 1998, 102, 10098.

Figure 3. Structures of styrene side chain LLC monomers that form HII phases.

possible by varying the position of the aromatic side chain along the backbone of the amphiphile.20 The ability of aromatic side chain-derivatized long chain amphiphiles (Figure 2b) to form the HII phase has been known for some time (e.g., regioisomers of calcium p-styrylundecanoate derived from 10-undecenoic acid,21 and regioisomers of lithium phenylstearate22). In fact, (16) Gray, D. H.; Hu, S.; Juang, E.; Gin. D. L. Adv. Mater. 1997, 9, 731. (17) Gray, D. H.; Gin, D. L.Chem. Mater. 1998, 10, 1827. (18) Miller, S. A.; Kim, E.; Gray, D. H.; Gin, D. L. Angew. Chem., Int. Ed. 1999, 38, 3021. (19) Gu, W.; Zhou, W.-J.; Gin, D. L. Chem. Mater. 2001, 13, 1949. (20) Reppy, M. A.; Gray, D. H.; Pindzola, B. A.; Smithers, J. L.; Gin, D. L. J. Am. Chem. Soc. 2001, 123, 363. (21) Herz, J.; Reiss-Husson, F.; Rempp, P.; Luzzati, V. J. Polym. Sci. C 1963, 4, 1275. (22) Harrison, W. J.; MacDonald, M. P. J. Phys. Chem. 1991, 95, 4136.

Lyotropic Liquid-Crystalline Behavior of Acid Salts

these amphiphiles were among the first systems known to exhibit this type of LLC phase behavior. However, the importance of the aromatic side chain in the self-assembly behavior of these and related HII phase-forming amphiphiles has not been thoroughly investigated. We wanted to see if aromatic-aromatic interactions also play a significant role in the self-organization behavior of these side chain-functionalized amphiphiles, especially in the formation of ordered LLC phases. Whitten and co-workers observed that the position of the aromatic chromophore in the alkyl chain of some of their “main chain” amphiphiles (Figure 2a) plays important roles in controlling the strength of the aggregation.12 Incorporation of the aromatic moiety as a side chain rather than along the alkyl chain (Figure 2b) may result in differences in aromatic-aromatic aggregation behavior between these two amphiphile platforms. Herein, we present the synthesis and LLC phase characterization of two series of single-chain amphiphiles containing different aromatic ether side chains at various positions along the amphiphile backbone. The relative importance of the type of aromatic group, the position of the side chain, and the nature of the ionic headgroup are compared. UV-visible spectroscopy was also performed on the LLC phases formed by the aromatic side chain amphiphiles, to detect the presence of aromatic-aromatic aggregrates in the assemblies. From these investigations, it was found that the relative position of the aromatic side chain on the amphiphile backbone is the most important structural factor in determining the type and stability of the LLC phase formed by these amphiphiles. It was also found that the type of aromatic-aromatic interactions and aggregate species previously identified in aromatic “main chain”-functionalized amphiphiles (Figure 2a) does not play a significant role in the LLC behavior of aromatic side chain-functionalized amphiphiles. In fact, control experiments with amphiphiles containing aliphatic ether side chains revealed that an aromatic unit is not even necessary for these amphiphiles to form the HII and other LLC phases. Experimental Section General Procedures and Materials. All common solvents and anhydrous reagents were purchased from Aldrich or Fisher Scientific and used as received unless otherwise indicated. C18reverse phase silica gel (230-400 mesh) and C18-reverse phase nano-silica gel TLC plates were purchased from Fluka. Normal phase column chromatography was performed using 230-400 mesh silica gel purchased from EM Science. Unless otherwise specified, organic extracts were dried over anhydrous Na2SO4. Solvents were removed using a rotary evaporator at aspirator pressure, followed by drying on a vacuum line (e10-4 Torr). Instrumentation. 1H and 13C NMR spectra were acquired using Bruker AMX-300 (300 MHz) and AMX-400 (400 MHz) spectrometers, as well as a Varian 500 (500 MHz) spectrometer. 1H NMR chemical shifts are reported in parts per million using the residual perprotio solvent peaks in the following deuterated solvents as references: (CDCl3, δ 7.26; CD3OD, δ 3.31). 13C NMR shifts are reported in parts per million using the following perdeuterio solvent peaks as references: (CDCl3, δ 77.23; CD3OD, δ 49.15). Low-angle X-ray diffraction (XRD) profiles were obtained using an Inel CPS 120 powder X-ray diffractometer system employing monochromated Cu KR radiation. XRD measurements were all performed at ambient temperature (23 ( 1 °C). Polarized optical microscopy (POM) was performed using a Leica DMRXP POL microscope equipped with a Linkam THMSE 600 hot/cold stage for variable temperature experiments. FT-IR spectra were obtained using a Perkin-Elmer 1616 series spectrometer. The FT-IR samples were analyzed as thin films on KBr crystals or as KBr mulls. Elemental analyses were performed at the Microanalytical Facility at the University of

Langmuir, Vol. 18, No. 20, 2002 7417 California, Berkeley, CA, and at Galbraith Laboratories, Knoxville, TN. High-resolution mass spectral analysis was performed by the Central Analytical Facility in the Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO. Photopolymerizations were performed using a Cole-Parmer 9815 series 6 W UV (365 nm) lamp with a nominal flux of 1600 µW/cm2 at the sample surface. Light fluxes were measured using a Spectroline DRC-100X digital radiometer equipped with a DIX-365 UV-A sensor. UV-vis spectra were acquired using a HewlettPackard HP 8452A spectrophotometer. Preparation of LLC Phases and Photopolymerization. LLC mixtures were prepared as follows: Appropriate amounts of the amphiphiles, water, and other additives were combined in a small test tube and mixed with a spatula. The samples were then centrifuged three times at 3800 rpm for 20 min, with spatula mixing after each centrifugation run. The resulting pasty samples were then allowed to equilibrate at room temperature for approximately 1 h before POM or XRD analysis. Styryl etherbased LLC mixtures containing 2-hydroxy-2-methylpropiophenone photoinitiator (e1 wt %) were photopolymerized with 365 nm light under a nitrogen atmosphere at ambient temperature for ∼12 h. The degree of polymerization (i.e., double bond conversion) in these samples was determined to be ∼80% by monitoring the diminishment of the 988 and 896 cm-1 FT-IR bands (olefin C-H out-of-plane bending) relative to the 832 cm-1 band (para-substituted aromatic C-H out-of-plane bending).16,20 Amphiphile Synthesis. The detailed synthesis and characterization of compounds 1a-4, 1a-10, 1a-16, and 2a-10 and their precursors have been reported previously.19,20 The other aromatic derivatives of 1 and 2 were synthesized using procedures similar to those employed for the series 1a compounds and compound 2a-10. Representative syntheses for the new aromatic derivatives are included below. Spectral characterization data for all new compounds synthesized in this study are also included. The detailed syntheses and characterization data for the aliphatic ether model compounds 4 and 5 are also presented below. All compounds with one chiral center (i.e., the series 1 and 2 amphiphiles) were synthesized as racemic mixtures. Compound 5, which has two chiral centers, was synthesized as a mixture of four stereoisomers (stereoisomeric ratio not experimentally determined).23 Full XRD peak listings for all the LLC mixtures of these compounds can be found in the Supporting Information. Methyl 4-(4-Ethylphenoxy)octadecanoate. Under nitrogen, 1,1′-(azodicarbonyl)dipiperidine (0.74 g, 2.96 mmol) was dissolved in dry toluene (∼60 mL) in an oven-dried flask. Upon cooling to 0 °C, tri(n-butyl)phosphine (0.75 mL, 2.96 mmol) was added. After the mixture was stirred for 1 h at 0 °C, methyl 4-hydroxyoctadecanoate20 (0.62 g, 1.97 mmol) and 4-ethylphenol (0.29 g, 2.36 mmol) were added. The reaction mixture was stirred overnight, during which the temperature was allowed to rise gradually to room temperature. The reaction mixture was then poured into pentane (∼150 mL) and subsequently cooled to 0 °C. The cooled mixture was then filtered, and the solid was washed with cold pentane. Removal of the solvent yielded a pale yellow oil, which was purified by silica gel chromatography with CHCl3 as the eluent to yield a colorless oil (0.617 g, 75%). 1H NMR (CDCl3, 300 MHz): δ 7.10 (d, J ) 8.7 Hz, 2H), 6.80 (d, J ) 8.7 Hz, 2H), 4.23 (m, 1H), 3.64 (s, 3H), 2.59 (m, 2H), 2.42 (m, 2H), 1.97 (m, 2H), 1.60 (m, 2H), 1.26 (m, 27H), 0.88 (t, J ) 7.0 Hz, 3H). 4-(4-Ethylphenoxy)octadecanoic Acid. Methyl 4-(4-ethylphenoxy)octadecanoate (0.399 g, 0.957 mmol) and NaOH (0.181 g, 4.53 mmol) were refluxed in 1/1 MeOH/H2O (v/v) (30 mL) for 2 h. After it was cooled to room temperature, the reaction mixture was acidified with 2 M aqueous HCl to pH 1-2. The crude product (23) The effect of chirality on thermotropic LC phases is well-known, but the effect of chirality on amphiphile assemblies and LLC phases has only been studied in a select number of systems. For example, enantiomeric excess in chiral amphiphiles that form bilayer tubules, lamellar assemblies, and chiral nematic micellar systems is known to induce supramolecular helical structures with different handedness. See: (a) Nakashima, N.; Asakuma, S.; Kim, J.-M.; Kunitake, T. Chem. Lett. (Chem. Soc. Jpn) 1984, 1709. (b) Fuhrhop, J.-H.; Schnieder, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387. (c) Hiltrop, K. In Chirality in Liquid Crystals; Kitzerow, H.-S., Bahr, C., Eds.; Springer-Verlag: New York, 2001; pp 447-480. However, the effect of chirality on high curvature condensed LLC phases such as the HII phase has not been studied extensively.

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Langmuir, Vol. 18, No. 20, 2002

was extracted with diethyl ether (3 × 10 mL), and the combined ether layers were dried over anhydrous Na2SO4. Upon removal of the solvent, 0.368 g of 4-(4-ethylphenoxy)octadecanoic acid as a colorless oil was obtained (95%). 1H NMR (CDCl3, 300 MHz): δ 7.05 (d, J ) 8.7 Hz, 2H), 6.80 (d, J ) 8.7 Hz, 2H), 4.24 (m, 1H), 2.59 (m, 2H), 2.42 (m, 2H), 1.97 (m, 2H), 1.60 (m, 2H), 1.26 (m, 27H), 0.88 (t, J ) 7.0 Hz, 3H). 1b-4. 4-(4-Ethylphenoxy)octadecanoic acid (0.133 g, 0.329 mmol) in MeOH (5 mL) was neutralized with a 0.279 M methanolic NaOH solution (1.18 mL, 0.329 mmol). After it was stirred for 30 min, the solution was concentrated to yield a white, sticky, gummy solid. After this solid was dried on the vacuum line for >2 days to a constant weight, 0.140 g (100%) of 1b-4 was obtained. FT-IR (KBr, cm-1): 2925, 2854, 1560, 1508, 1466, 1414, 1296, 1237, 1175, 947, 827, 720. 1H NMR (CD3OD, 300 MHz): δ 7.17 (d, J ) 8.6 Hz, 2H), 6.81 (d, J ) 8.6 Hz, 2H), 4.26 (m, 1H), 2.54 (p, J ) 7.5 Hz, 2H), 2.25 (m, 2H), 1.95 (m, 2H), 1.60 (m, 2H), 1.26 (m, 27H), 1.16 (t, J ) 7.5 Hz, 3H), 0.88 (t, J ) 7.0 Hz, 3H). 13C NMR (CD OD, 100 MHz): δ 182.6, 158.2, 137.4, 129.4, 117.4, 3 78.9, 35.1, 33.2, 32.2, 30.9, 30.9, 30.9, 30.9, 30.9, 30.8, 30.8, 30.8, 30.8, 30.7, 29.2, 26.4, 23.9, 16.7, 14.7. Anal. Calcd for C26H43NaO3: C, 73.20; H, 10.16. Found: C, 73.17; H, 10.40. Methyl 10-(4-Ethylphenoxy)octadecanoate. This compound was synthesized using the same procedure described for methyl 4-(4-ethylphenoxy)octadecanoate but using methyl 10hydroxyoctadecanoic acid20 instead of methyl 4-hydroxyoctadecanoic acid (79% yield). 1H NMR (CDCl3, 300 MHz): δ 7.07 (d, J ) 8.7 Hz, 2H), 6.80 (d, J ) 8.7 Hz, 2H), 4.15 (m, 1H), 3.66 (s, 3H), 2.59 (q, J ) 7.6 Hz, 2H), 2.29 (t, J ) 7.4 Hz, 2H), 2.42 (m, 2H), 1.59 (m, 6H), 1.26 (m, 23H), 0.87 (t, J ) 7.0 Hz, 3H). 10-(4-Ethylphenoxy)octadecanoic Acid. This compound was synthesized from methyl 10-(4-ethylphenoxy)octadecanoate using the same procedure described for 4-(4-ethylphenoxy)octadecanoic acid (85% yield). 1H NMR (CDCl3, 300 MHz): δ 7.05 (d, J ) 8.7 Hz, 2H), 6.80 (d, J ) 8.7 Hz, 2H), 4.15 (m, 1H), 2.59 (q, J ) 7.6 Hz, 2H), 2.34 (t, J ) 7.4 Hz, 2H), 1.58 (m, 6H), 1.26 (m, 25H), 0.87 (t, J ) 7.0 Hz, 3H). 1b-10. This compound was synthesized from 10-(4-ethylphenoxy)octadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 2929, 2854, 1565, 1510, 1444, 1422, 1296, 1238, 1175, 1115, 983, 926, 828, 722. 1H NMR (CD3OD, 300 MHz): δ 7.06 (d, J ) 8.6 Hz, 2H), 6.76 (d, J ) 8.6 Hz, 2H), 4.20 (m, 1H), 2.55 (q, J ) 7.5 Hz, 2H), 2.12 (t, J ) 7.5 Hz, 2H), 1.57 (m, 6H), 1.27 (m, 22H), 1.16 (t, J ) 7.5 Hz, 3H), 0.88 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 183.3, 158.2, 137.5, 129.8, 117.1, 79.1, 39.4, 35.2, 35.2, 33.2, 31.0, 31.0, 30.9, 30.8, 30.8, 30.8, 30.5, 29.1, 27.9, 26.6, 26.5, 23.9, 16.6, 14.6. Anal. Calcd for C26H43NaO3: C, 73.20; H, 10.16. Found: C, 73.13; H, 10.24. Methyl 16-(4-Ethylphenoxy)octadecanoate. This compound was synthesized from methyl 16-hydroxyoctadecanoic acid20 using the same procedure described for methyl 4-(4ethylphenoxy)octadecanoate (76% yield). 1H NMR (CDCl3, 300 MHz): δ 7.09 (d, J ) 8.7 Hz, 2H), 6.80 (d, J ) 8.7 Hz, 2H), 4.15 (m, 1H), 3.66 (s, 3H), 2.59 (q, J ) 7.6 Hz, 2H), 2.29 (t, J ) 7.4 Hz, 2H), 1.65 (m, 6H), 1.26 (m, 25H), 0.94 (t, J ) 7.0 Hz, 3H). 16-(4-Ethylphenoxy)octadecanoic Acid. This compound was synthesized from methyl 16-(4-ethylphenoxy)octadecanoate using the same procedure described for 4-(4-ethylphenoxy)octadecanoic acid (97% yield). 1H NMR (CDCl3, 300 MHz): δ 7.09 (d, J ) 8.7 Hz, 2H), 6.80 (d, J ) 8.7 Hz, 2H), 4.15 (m, 1H), 2.59 (q, J ) 7.6 Hz, 2H), 2.29 (t, J ) 7.4 Hz, 2H), 1.65 (m, 6H), 1.26 (m, 25H), 0.94 (t, J ) 7.0 Hz, 3H). 1b-16. This compound was synthesized from 16-(4-ethylphenoxy)octadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 2920, 2850, 1560, 1509, 1466, 1421, 1296, 1238, 1176, 1118, 965, 923, 828, 720. 1H NMR (CD3OD, 300 MHz): δ 7.05 (d, J ) 8.6 Hz, 2H), 6.76 (d, J ) 8.6 Hz, 2H), 4.20 (m, 1H), 2.55 (q, J ) 7.5 Hz, 2H), 2.13 (t, J ) 7.5 Hz, 2H), 1.56 (m, 6H), 1.25 (m, 22H), 1.17 (t, J ) 7.5 Hz, 3H), 0.90 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 183.3, 158.2, 137.6, 129.8, 117.1, 80.3, 39.4, 34.7, 31.0, 31.0, 31.0, 31.0, 30.9, 30.9, 30.9, 30.9, 30.8, 30.8, 29.1, 28.0, 27.8, 26.5, 16.6, 10.0. Anal. Calcd for C26H43NaO3: C, 73.20; H, 10.16. Found: C, 72.94; H, 9.99.

Gu and Gin Methyl 4-(1-Naphthoxy)octadecanoate. This compound was synthesized using the same procedure described for methyl 4-(4-ethylphenoxy)octadecanoate but using 1-naphthol instead of 4-ethylphenol (80% yield). 1H NMR (CDCl3, 300 MHz): δ 8.25 (d, J ) 7.5 Hz, 1H), 7.77 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.85 (d, J ) 7.0 Hz, 1H), 4.58 (m, 1H), 3.62 (s, 3H), 2.53 (m, 2H), 2.15 (m, 2H), 1.77 (m, 2H), 1.48 (m, 2H), 1.20 (m, 22H), 0.89 (t, J ) 7.0 Hz, 3H). 4-(1-Naphthoxy)octadecanoic acid. This compound was synthesized from methyl 4-(1-naphthoxy)octadecanoate using the same procedure described for 4-(4-ethylphenoxy)octadecanoic acid (97% yield). 1H NMR (CDCl3, 300 MHz): δ 8.25 (d, J ) 7.5 Hz, 1H), 7.77 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.82 (d, J ) 7.0 Hz, 1H), 4.55 (m, 1H), 2.56 (m, 2H), 2.15 (m, 2H), 1.77 (m, 2H), 1.48 (m, 2H), 1.20 (m, 22H), 0.88 (t, J ) 7.0 Hz, 3H). 1c-4. This compound was synthesized from 4-(1-naphthoxy)octadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 3054, 2925, 2853, 1578, 1508, 1461, 1400, 1267, 1237, 1155, 1095, 1066, 1018, 938, 791, 771, 721. 1H NMR (CD3OD, 300 MHz): δ 8.20 (d, J ) 7.5 Hz, 1H), 7.77 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.89 (d, J ) 7.0 Hz, 1H), 4.58 (m, 1H), 2.35 (m, 2H), 2.12 (m, 2H), 1.77 (m, 2H), 1.48 (m, 2H), 1.20 (m, 22H), 0.89 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 182.4, 155.6, 136.4, 128.5, 127.8, 127.2, 127.2, 125.9, 123.3, 120.7, 107.2, 78.9, 35.1, 25.0, 33.2, 32.1, 30.9, 30.9, 30.9, 30.9, 30.8, 30.8, 30.8, 30.7, 30.6, 26.5, 23.9, 14.6. Anal. Calcd for C28H41NaO3: C, 74.96; H, 9.21. Found: C, 75.07; H, 9.46. Methyl 10-(1-Naphthoxy)octadecanoate. This compound was synthesized using the same procedure described for methyl 10-(4-ethylphenoxy)octadecanoate but using 1-naphthol instead of 4-ethylphenol (71% yield). 1H NMR (CDCl3, 300 MHz): δ 8.30 (d, J ) 7.5 Hz, 1H), 7.79 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.82 (d, J ) 7.0 Hz, 1H), 4.58 (m, 1H), 3.62 (s, 3H), 2.30 (t, J ) 7.5 Hz, 2H), 1.50 (m, 28H), 0.89 (t, J ) 7.0 Hz, 3H). 10-(1-Naphthoxy)octadecanoic Acid. This compound was synthesized from methyl 10-(1-naphthoxy)octadecanoate using the same procedure described for 10-(4-ethylphenoxy)octanoic acid (96% yield). 1H NMR (CDCl3, 300 MHz): δ 8.30 (d, J ) 7.5 Hz, 1H), 7.79 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.82 (d, J ) 7.0 Hz, 1H), 4.58 (m, 1H), 2.29 (t, J ) 7.5 Hz, 2H), 1.50 (m, 28H), 0.88 ppm (t, J ) 7.0 Hz, 3H). 1c-10. This compound was synthesized from 10-(1-naphthoxy)octadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 3052, 2927, 2853, 1560, 1508, 1460, 1438, 1401, 1266, 1237, 1155, 1095, 1062, 1018, 789, 770. 1H NMR (CD3OD, 300 MHz): δ 8.21 (d, J ) 7.5 Hz, 1H), 7.75 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.85 (d, J ) 7.0 Hz, 1H), 4.50 (m, 1H), 2.13 (t, J ) 7.0, 2H), 1.75 (m, 4H), 1.45 (m, 6H), 1.24 (m, 18H), 0.84 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 183.3, 155.6, 136.4, 128.6, 127.9, 127.3, 127.2, 126.0, 123.2, 120.8, 107.1, 79.1, 39.4, 35.2, 35.1, 33.2, 31.0, 31.0, 30.9, 30.8, 30.8, 30.7, 30.5, 28.0, 26.7, 26.6, 23.9, 14.6. Anal. Calcd for C28H41NaO3: C, 74.96; H, 9.21. Found: C, 75.08; H, 9.33. Methyl 16-(1-Naphthoxy)octadecanoate. This compound was synthesized using the same procedure described for methyl 16-(4-ethylphenoxy)octadecanoate but using 1-naphthol instead of 4-ethylphenol (71% yield). 1H NMR (CDCl3, 300 MHz): δ 8.30 (d, J ) 7.5 Hz, 1H), 7.77 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.80 (d, J ) 7.0 Hz, 1H), 4.40 (m, 1H), 3.66 (s, 3H), 2.30 (t, J ) 7.5 Hz, 2H), 1.60 (m, 8H), 1.27 (m, 20H), 1.00 (t, J ) 7.0 Hz, 3H). 16-(1-Naphthoxy)octadecanoic Acid. This compound was synthesized from methyl 16-(1-naphthoxy)octadecanoate using the same procedure described for 4-(4-ethylphenoxy)octanoic acid (96% yield). 1H NMR (CDCl3, 300 MHz): δ 8.30 (d, J ) 7.5 Hz, 1H), 7.77 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.80 (d, J ) 7.0 Hz, 1H), 4.40 (m, 1H), 2.31 (t, J ) 7.5 Hz, 2H), 1.60 (m, 8H), 1.26 (m, 20H), 1.00 (t, J ) 7.0 Hz, 3H). 1c-16. This compound was synthesized from 16-(1-naphthoxy)octadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 3051, 2920, 2848, 1560, 1459, 1420, 1400, 1266, 1236, 1095, 1061, 1017, 957, 789, 770. 1H NMR (CD OD, 300 MHz): δ 8.21 (d, J ) 7.5 Hz, 1H), 7.75 (d, 3 J ) 7.5 Hz, 1H), 7.37 (m, 4H), 6.84 (d, J ) 7.0 Hz, 1H), 4.45 (m, 1H), 2.16 (t, J ) 7.5 Hz, 2H), 1.75 (m, 4H), 1.56 (m, 2H), 1.41 (m, 2H), 1.22 (m, 20H), 1.00 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 183.3, 155.6, 136.4, 128.6, 127.8, 127.3, 127.1, 125.9,

Lyotropic Liquid-Crystalline Behavior of Acid Salts 123.2, 120.8, 107.1, 80.2, 39.5, 34.5, 31.0, 30.9, 30.9, 30.9, 30.9, 30.8, 30.8, 30.8, 30.7, 30.7, 28.0, 27.8, 26.5, 10.1. Anal. Calcd for C28H41NaO3: C, 74.96; H, 9.21. Found: C, 75.06; H, 9.46. Methyl 4-(4-Phenylazophenoxy)octadecanoate. This compound was synthesized using the same procedure described for methyl 4-(4-ethylphenoxy)octadecanoate but using 4-phenylazophenol instead of 4-ethylphenol (68% yield). 1H NMR (CDCl3, 300 MHz): δ 7.88 (m, 4H), 7.52 (m, 3H), 7.00 (m, 2H), 4.50 (m, 1H), 3.66 (s, 3H), 2.50 (m, 2H), 2.00 (m, 2H), 1.70 (m, 2H), 1.26 (m, 24H), 0.87 (t, J ) 6.5 Hz, 3H). 4-(4-Phenylazophenoxy)octadecanoic Acid. This compound was synthesized from methyl 4-(4-phenylazophenoxy)octadecanoate using the same procedure described for 4-(4ethylphenoxy)octadecanoic acid (99% yield). 1H NMR (CDCl3, 300 MHz): δ 7.89 (m, 4H), 7.52 (m, 3H), 7.00 (m, 2H), 4.44 (m, 1H), 2.54 (m, 2H), 2.06 (m, 2H), 1.70 (m, 2H), 1.26 (m, 24H), 0.87 (t, J ) 6.5 Hz, 3H). 1d-4. This compound was synthesized from 4-(4-phenylazophenoxy)octadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 3068, 2923, 2851, 1597, 1570, 1498, 1439, 1414, 1299, 1246, 1140, 1103, 931, 836, 766, 720, 687. 1H NMR (CD3OD, 300 MHz): δ 7.85 (m, 4H), 7.50 (m, 3H), 7.08 (m, 2H), 4.50 (m, 1H), 2.28 (m, 2H), 1.96 (m, 2H), 1.70 (m, 2H), 1.25 (m, 24H), 0.87 (t, J ) 7.0 Hz, 3H). 13C NMR (CD OD, 100 MHz): δ 182.2, 163.2, 154.2, 148.0, 131.6, 3 130.3, 125.9, 123.6, 117.0, 79.1, 35.0, 34.9, 33.2, 32.1, 30.9, 30.9, 30.9, 30.9, 30.9, 30.8, 30.7, 30.7, 30.6, 26.3, 23.9, 14.6. Anal. Calcd for C30H43N2NaO3: C, 71.68; H, 8.62; N, 5.57. Anal. Calcd for C30H43N2NaO3‚1/2H2O: C, 70.42; H, 8.67; N, 5.48. Found: C, 70.06; H, 8.85; N, 5.39. Methyl 10-(4-Phenylazophenoxy)octadecanoate. This compound was synthesized using the same procedure described for methyl 10-(4-ethylphenoxy)octadecanoate but using 4-phenylazophenol instead of 4-ethylphenol (40% yield). 1H NMR (CDCl3, 300 MHz): δ 7.87 (m, 4H), 7.51 (m, 3H), 7.00 (m, 2H), 4.42 (m, 1H), 3.66 (s, 3H), 2.29 (t, J ) 7.5, 2H), 1.60 (m, 6H), 1.30 (m, 22H), 0.87 ppm (t, J ) 7.0 Hz, 3H). 10-(4-Phenylazophenoxy)octadecanoic Acid. This compound was synthesized from methyl 10-(4-phenylazophenoxy)octadecanoate using the same procedure described for 4-(4ethylphenoxy)octadecanoic acid (97% yield). 1H NMR (CDCl3, 300 MHz): δ 7.88 (m, 4H), 7.51 (m, 3H), 7.00 (m, 2H), 4.33 (m, 1H), 2.33 (t, J ) 7.5, 2H), 1.60 (m, 6H), 1.30 (m, 22H), 0.87 (t, J ) 7.0 Hz, 3H). 1d-10. This compound was synthesized from 10-(4-phenylazophenoxy)octadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 3068, 2925, 2852, 1599, 1563, 1498, 1442, 1418, 1297, 1248, 1139, 979, 924, 837, 766, 720, 687. 1H NMR (CD3OD, 300 MHz): δ 7.85 (m, 4H), 7.50 (m, 3H), 7.04 (m, 2H), 4.42 (m, 1H), 2.12 (t, J ) 7.5 Hz, 2H), 1.60 (m, 6H), 1.30 (m, 22H), 0.87 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 183.3, 163.2, 154.3, 148.1, 131.7, 130.3, 126.0, 123.7, 117.0, 79.4, 39.5, 35.2, 35.2, 33.2, 31.0, 31.0, 30.9, 30.9, 30.8, 30.8, 30.5, 28.0, 26.6, 26.5, 23.9, 14.7. Anal. Calcd for C30H43N2NaO3: C, 71.68; H, 8.62; N, 5.57. Found: C, 71.33; H, 8.64; N, 5.32. Methyl 16-(4-Phenylazophenoxy)octadecanoate. This compound was synthesized using the same procedure described for methyl 16-(4-ethylphenoxy)octadecanoate but using 4-phenylazophenol instead of 4-ethylphenol (63% yield). 1H NMR (CDCl3, 300 MHz): δ 7.90 (m, 4H), 7.50 (m, 3H), 7.00 (m, 2H), 4.28 (m, 1H), 3.66 (s, 3H), 2.30 (t, J ) 7.5 Hz, 2H), 1.66 (m, 4H), 1.30 (m, 24H), 0.98 (t, J ) 7.0 Hz, 3H). 16-(4-Phenylazophenoxy)octadecanoic Acid. This compound was synthesized from methyl 16-(4-phenylazophenoxy)octadecanoate using the same procedure described for 4-(4ethylphenoxy)octadecanoic acid (97% yield). 1H NMR (CDCl3, 300 MHz): δ 7.90 (m, 4H), 7.50 (m, 3H), 7.00 (m, 2H), 4.28 (m, 1H), 2.34 (t, J ) 7.5 Hz, 2H), 1.69 (m, 8H), 1.30 (m, 20H), 0.98 (t, J ) 7.0 Hz, 3H). 1d-16. This compound was synthesized from 16-(4-phenylazophenoxy)octadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 3066, 2919, 2848, 1599, 1561, 1498, 1466, 1420, 1297, 1248, 1140, 965, 923, 836, 765, 720, 687. 1H NMR (CD3OD, 300 MHz): δ 7.85 (m, 4H), 7.50 (m, 3H), 7.04 (m, 2H), 4.37 (m, 1H), 2.12 (d, J ) 7.5 Hz,

Langmuir, Vol. 18, No. 20, 2002 7419 2H), 1.66 (m, 4H), 1.45 (m, 2H), 1.30 (m, 22H), 0.97 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 183.3, 163.4, 154.2, 148.0, 131.6, 130.3, 126.0, 123.6, 118.0, 80.3, 39.4, 34.6, 31.0, 30.9, 30.9, 30.9, 30.9, 30.9, 30.8, 30.8, 30.8, 30.8, 28.0, 27.9, 26.4, 10.0. Anal. Calcd for C30H43N2NaO3: C, 71.68; H, 8.62; N, 5.57. Found: C, 71.59; H, 8.72; N, 5.42. 2a-4. In an oven-dried flask, 4-(4-vinylphenoxy)octadecanoic acid20 (0.33 g, 0.82 mmol), dicyclohexylcarbodiimide (0.169 g, 0.82 mmol), N-hydroxysuccinimide (0.094 g, 0.82 mmol), and a catalytic amount of N,N′-dimethylaniline (10 mg) were dissolved in methylene chloride (30 mL). After 4 h of stirring, the solution was filtered and the solid was washed with an additional 20 mL of methylene chloride. The colorless oil, obtained after removal of solvent, was dissolved in 50 mL of 2/1 (v/v) THF/H2O. Taurine (0.154 g, 1.23 mmol) and Na2CO3 (0.131 g, 1.23 mmol) were added to the above solution. After it was stirred overnight, the reaction mixture was diluted with water (50 mL) and then loaded onto a reverse phase C18-coated silica gel column equilibrated with 1/1 (v/v) MeOH/H2O. The compound was eluted with 9/1 MeOH/ H2O. Removal of the solvent and drying on the vacuum line for >2 days afforded 0.351 g of 2a-4 as a white solid (81%). FT-IR (KBr, cm-1): 3322, 2924, 2853, 1655, 1606, 1546, 1508, 1466, 1245, 1214, 1173, 1061, 988, 897, 834, 744, 722, 615. 1H NMR (CD3OD, 300 MHz): δ 7.31 (d, J ) 6.7 Hz, 2H), 6.84 (d, J ) 6.7 Hz, 2H), 6.63 (dd, J ) 11.0, 17.6 Hz, 1H), 5.58 (d, J ) 17.6 Hz, 1H), 5.05 (d, J ) 11.0 Hz, 1H), 4.32 (m, 1H), 3.55 (t, J ) 7.2 Hz, 2H), 2.92 (t, J ) 7.2 Hz, 2H), 2.28 (m, 2H), 1.93 (m, 2H), 1.60 (m, 2H), 1.28 (m, 24H), 0.88 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 175.7, 159.9, 137.8, 132.6, 128.6, 117.0, 111.7, 78.2, 51.6, 36.8, 35.0, 33.3, 33.2, 31.2, 31.0, 31.0, 31.0, 30.9, 30.9, 30.8, 30.8, 30.7, 30.7, 26.4, 23.9, 14.6. Anal. Calcd for C28H46NNaO5S: C, 63.25; H, 8.72; N, 2.63; S, 6.03. Found: C, 63.59; H, 8.91; N, 2.75; S, 6.22. 2a-16. This compound was synthesized from 16-(4-vinylphenoxy)octadecanoic acid20 using the same procedure described for 2a-4 (75% yield). FT-IR (KBr, cm-1): 3324, 2921, 2851, 1647, 1607, 1559, 1507, 1465, 1245, 1203, 1175, 1064, 988, 967, 897, 833, 806, 721, 623. 1H NMR (CD3OD, 300 MHz): δ 7.29 (d, J ) 6.7 Hz, 2H), 6.81 (d, J ) 6.7 Hz, 2H), 6.63 (dd, J ) 11.0, 17.6 Hz, 1H), 5.58 (d, J ) 17.6 Hz, 1H), 5.05 (d, J ) 11.0 Hz, 1H), 4.21 (m, 1H), 3.56 (t, J ) 7.0 Hz, 2H), 2.92 (t, J ) 7.0 Hz, 2H), 2.15 (t, J ) 7.8, 2H), 1.63 (m, 6H), 1.25 (m, 22H), 0.92 (t, J ) 7.5 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 176.3, 160.1, 137.9, 131.8, 128.6, 117.0, 111.6, 80.2, 51.7, 37.4, 36.8, 34.7, 30.9, 30.9, 30.9, 30.9, 30.8, 30.8, 30.8, 30.7, 30.7, 30.5, 27.9, 27.1, 26.5, 10.1. Anal. Calcd for C28H46NNaO5S: C, 63.25; H, 8.72; N, 2.63; S, 6.03. Found: C, 63.37; H, 8.98; N, 2.78; S, 6.14. 2b-4. This compound was synthesized from 4-(4-ethylphenoxy)octadecanoic acid using the same procedure described for 2a-4 (68% yield). FT-IR (KBr, cm-1): 3320, 2925, 2854, 1651, 1548, 1509, 1465, 1376, 1233, 1199, 1175, 1060, 828, 743, 721, 620. 1H NMR (CD3OD, 500 MHz): δ 7.07 (d, J ) 8.5 Hz, 2H), 6.81 (d, J ) 8.5 Hz, 2H), 4.26 (m, 1H), 3.56 (t, J ) 6.5 Hz, 2H), 2.93 (t, J ) 6.5 Hz, 2H), 2.56 (q, J ) 7.5 Hz, 2H), 2.31 (m, 2H), 1.95 (m, 2H), 1.60 (m, 2H), 1.20 (m, 24H), 1.16 (t, J ) 7.5 Hz, 3H), 0.88 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 125 MHz): δ 175.7, 158.0, 137.8, 129.9, 117.1, 78.3, 51.5, 36.7, 35.0, 33.2, 33.2, 31.2, 30.9, 30.9, 30.9, 30.9,30.9, 30.8, 30.8, 30.8, 30.6, 29.2, 26.4, 23.9, 16.7, 14.6. Anal. Calcd for C28H48NNaO5S: C, 63.01; H, 9.06; N, 2.62; S, 6.01. Found: C, 62.81; H, 9.22; N, 2.54; S, 5.75. 2b-10. This compound was synthesized from 10-(4-ethylphenoxy)octadecanoic acid using the same procedure described for 2a-4 (57% yield). FT-IR (KBr, cm-1): 3317, 2928, 2854, 1648, 1560, 1509, 1458, 1375, 1293, 1237, 1211, 1175, 1064, 987, 827, 722. 1H NMR (CD3OD, 300 MHz): δ 7.06 (d, J ) 8.6 Hz, 2H), 6.76 (d, J ) 8.6 Hz, 2H), 4.20 (m, 1H), 3.57 (t, J ) 6.9 Hz, 2H), 2.93 (t, J ) 6.9 Hz, 2H), 2.55 (q, J ) 7.5 Hz, 2H), 2.15 (t, J ) 7.5 Hz, 2H), 1.57 (m, 6H), 1.27 (m, 22H), 1.16 (t, J ) 7.5 Hz, 3H), 0.88 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 176.3, 158.3, 137.7, 129.9, 117.2, 79.2, 51.7, 37.4, 36.8, 35.3, 33.2, 30.9, 30.8, 30.7, 30.5, 30.5, 30.5, 30.5, 29.2, 29.2, 29.1, 27.1, 26.6, 23.9, 16.7, 14.6. Anal. Calcd for C28H48NNaO5S: C, 63.01; H, 9.06; N, 2.62; S, 6.01. Found: C, 63.11; H, 9.12; N, 2.69; S, 6.19. 2b-16. This compound was synthesized from 16-(4-ethylphenoxy)octadecanoic acid using the same procedure described for 2a-4 (66% yield). FT-IR (KBr, cm-1): 3330, 2922, 2851, 1644,

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1556, 1510, 1467, 1380, 1237, 1215, 1177, 1064, 965, 827, 721. 1H NMR (CD OD, 500 MHz): δ 7.06 (d, J ) 8.5 Hz, 2H), 6.78 (d, 3 J ) 8.5 Hz, 2H), 4.16 (m, 1H), 3.57 (t, J ) 7 Hz, 2H), 2.95 (t, J ) 7 Hz, 2H), 2.55 (q, J ) 7.5 Hz, 2H), 2.15 (t, J ) 7.5 Hz, 2H), 1.57 (m, 6H), 1.27 (m, 22H), 1.17 (t, J ) 7.5 Hz, 3H), 0.94 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 125 MHz): δ 176.2, 158.2, 137.6, 129.8, 117.2, 80.3, 51.6, 37.4, 36.7, 34.7, 30.9, 30.9, 30.9, 30.9, 30.9, 30.8, 30.8, 30.8, 30.6, 30.5, 29.2, 27.9, 27.0, 26.5, 16.7, 10.1. Anal. Calcd for C28H48NNaO5S: C, 63.01; H, 9.06; N, 2.62; S, 6.01. Found: C, 63.18; H, 9.35; N, 2.59; S, 5.78. 2c-4. This compound was synthesized from 4-(1-naphthoxy)octadecanoic acid using the same procedure described for 2a-4 (75% yield). FT-IR (KBr, cm-1): 3300, 3052, 2924, 2852, 1652, 1577, 1540, 1507, 1463, 1401, 1267, 1237, 1214, 1176, 1095, 1060, 790, 771. 1H NMR (CD3OD, 500 MHz): δ 8.22 (d, J ) 7.5 Hz, 1H), 7.76 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.89 (d, J ) 7.0 Hz, 1H), 4.56 (m, 1H), 3.57 (t, J ) 7 Hz, 2H), 2.90 (t, J ) 7 Hz, 2H), 2.35 (m, 2H), 2.10 (m, 2H), 1.75 (m, 2H), 1.42 (m, 2H), 1.20 (m, 22H), 0.89 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 125 MHz): δ 175.6, 155.3, 136.4, 128.6, 127.8, 127.4, 127.2, 126.1, 123.3, 121.0, 107.1, 78.3, 51.5, 36.8, 34.9, 33.3, 33.2, 31.1, 31.0, 30.9, 30.9, 30.9, 30.8, 30.8, 30.7, 30.7, 30.6, 26.4, 23.9, 14.6. Anal. Calcd for C30H46NNaO5S: C, 64.84; H, 8.34; N, 2.52; S, 5.77. Found: C, 64.65; H, 8.44; N, 2.44; S, 5.59. 2c-10. This compound was synthesized from 10-(1-naphthoxy)octadecanoic acid using the same procedure described for 2a-4 (76% yield). FT-IR (KBr, cm-1): 3304, 3052, 2927, 2853, 1648, 1578, 1542, 1508, 1460, 1401, 1267, 1237, 1209, 1176, 1095, 1063, 790, 770. 1H NMR (CD3OD, 300 MHz): δ 8.21 (d, J ) 7.5 Hz, 1H), 7.75 (d, J ) 7.5 Hz, 1H), 7.36 (m, 4H), 6.85 (d, J ) 7.0 Hz, 1H), 4.56 (m, 1H), 3.55 (t, J ) 7 Hz, 2H), 2.90 (t, J ) 7.0 Hz, 2H), 2.13 (t, J ) 7.0, 2H), 1.75 (m, 4H), 1.45 (m, 6H), 1.24 (m, 18H), 0.84 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 176.1, 155.6, 136.4, 128.6, 127.8, 127.3, 127.1, 125.9, 123.2, 120.8, 107.1, 79.1, 51.5, 37.3, 36.7, 35.0, 35.0, 33.1, 30.9, 30.8, 30.7, 30.6, 30.4, 30.4, 30.4, 30.3, 26.9, 26.5, 23.8, 14.6. Anal. Calcd for C30H46NNaO5S: C, 64.84; H, 8.34; N, 2.52; S, 5.77. Found: C, 64.75; H, 8.45; N, 2.54; S, 5.69. 2c-16. This compound was synthesized from 16-(1-naphthoxy)octadecanoic acid using the same procedure described for 2a-4 (88% yield). FT-IR (KBr, cm-1): 3318, 3052, 2922, 2851, 1645, 1578, 1558, 1506, 1462, 1401, 1266, 1236, 1213, 1177, 1095, 1064, 1017, 952, 789, 770. 1H NMR (CD3OD, 500 MHz): δ 8.20 (d, J ) 7.5 Hz, 1H), 7.75 (d, J ) 7.5 Hz, 1H), 7.37 (m, 4H), 6.84 (d, J ) 7.0 Hz, 1H), 4.44 (m, 1H), 3.57 (t, J ) 7 Hz, 2H), 2.96 (t, J ) 7 Hz, 2H), 2.15 (t, J ) 7.5 Hz, 2H), 1.75 (m, 4H), 1.56 (m, 2H), 1.41 (m, 2H), 1.22 (m, 20H), 1.00 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 125 MHz): δ 176.2, 155.6, 136.4, 128.6, 127.9, 127.3, 127.1, 126.0, 123.3, 120.8, 107.2, 80.2, 51.6, 39.5, 37.4, 36.7, 34.6, 31.1, 30.9, 30.9, 30.9, 30.8, 30.7, 30.6, 30.5, 28.0, 27.8, 27.0, 26.5, 10.2. Anal. Calcd for C30H46NNaO5S: C, 64.74; H, 8.34; N, 2.52; S, 5.77. Found: C, 64.19; H, 8.59; N, 2.45; S, 5.81. 2d-4. This compound was synthesized from 4-(4-phenylazophenoxy)octadecanoic acid using the same procedure described for 2a-4 (72% yield). FT-IR (KBr, cm-1): 3306, 3068, 2925, 2853, 1644, 1600, 1495, 1297, 1249, 1216, 1140, 1061, 924, 839, 766, 721, 687. 1H NMR (CD3OD, 300 MHz): δ 7.86 (m, 4H), 7.50 (m, 3H), 7.08 (m, 2H), 4.49 (m, 1H), 3.56 (t, J ) 7.0 Hz, 3H), 2.95 (t, J ) 7.0 Hz, 3H), 2.36 (m, 2H), 2.00 (m, 2H), 1.68 (m, 2H), 1.24 (m, 24H), 0.87 (t, J ) 7.0 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 175.5, 162.9, 154.2, 148.2, 131.7, 130.3, 126.0, 123.7, 117.0, 78.5, 51.5, 36.8, 34.9, 33.2, 33.1, 31.1, 30.9, 30.9, 30.9, 30.9, 30.9, 30.8, 30.7, 30.7, 30.6, 26.3, 23.9, 14.6. Anal. Calcd for C32H48N3NaO5S: C, 63.03; H, 7.93; N, 6.89; S, 5.26. Found: C, 62.92; H, 8.11; N, 6.74; S, 4.92. 2d-10. This compound was synthesized from 10-(4-phenylazophenoxy)octadecanoic acid using the same procedure described for 2a-4 (67% yield). FT-IR (KBr, cm-1): 3308, 3068, 2927, 2853, 1644, 1599, 1538, 1497, 1442, 1416, 1297, 1249, 1207, 1139, 1062, 980, 922, 838, 765, 721, 688. 1H NMR (CD3OD, 300 MHz): δ 7.86 (m, 4H), 7.50 (m, 3H), 7.04 (m, 2H), 4.44 (m, 1H), 3.56 (t, J ) 7.0 Hz, 3H), 2.93 (t, J ) 7.0 Hz, 3H), 2.17 (d, J ) 7.5 Hz, 2H), 1.60 (m, 6H), 1.30 (m, 20H), 0.87 (t, J ) 7.0 Hz, 3H). 13C NMR (CD OD, 100 MHz): δ 176.1, 163.1, 148.0, 131.6, 130.3, 3 125.9, 123.6, 116.9, 79.3, 51.5, 37.3, 36.7, 35.1, 35.1, 33.1, 30.8, 30.8, 30.7, 30.7, 30.6, 30.5, 30.5, 30.4, 27.0, 26.4, 26.4, 23.8, 14.6.

Gu and Gin Anal. Calcd for C32H48N3NaO5S: C, 63.03; H, 7.93; N, 6.89; S, 5.26. Found: C, 62.97; H, 7.73; N, 6.81; S, 5.02. 2d-16. This compound was synthesized from 16-(4-phenylazophenoxy)octadecanoic acid using the same procedure described for 2a-4 (50% yield). FT-IR (KBr, cm-1): 3318, 3069, 2928, 2852, 1644, 1601, 1558, 1498, 1465, 1416, 1302, 1254, 1204, 1140, 1064, 966, 836, 765, 722, 687. 1H NMR (CD3OD, 300 MHz): δ 7.85 (m, 4H), 7.50 (m, 3H), 7.04 (m, 2H), 4.37 (m, 1H), 3.56 (t, J ) 7.0 Hz, 3H), 2.94 (t, J ) 7.0 Hz, 3H), 2.12 (d, J ) 7.5 Hz, 2H), 1.66 (m, 4H), 1.45 (m, 2H), 1.30 (m, 22H), 0.97 (t, J ) 7.0 Hz, 3H). 13C NMR (CD OD, 100 MHz): δ 176.3, 163.2, 154.2, 148.0, 131.7, 3 130.3, 126.0, 123.6, 117.0, 80.4, 51.6, 37.4, 36.7, 34.6, 31.0, 30.9, 30.9, 30.9, 30.9, 30.8, 30.8, 30.7, 30.6, 30.5, 27.9, 27.1, 26.4, 10.0. High-resolution MS (LSIMS+): calcd for [C32H48N3NaO5S], 609.3212; calcd for [(C32H48N3NaO5S) + Na], 632.3110; found, 632.3126. Methyl 10-Hexyloxyoctadecanoate. An oven-dried flask was charged with methyl 10-hydroxyoctadecanoate (0.35 g, 1.11 mmol), 1-iodohexane (0.82 mL, 5.55 mmol), silver oxide (1.29 g, 5.55 mmol), CaSO4 (6.0 g), and glass beads (6.0 g). The reaction mixture was stirred and heated at 120 °C overnight in the dark. After the reaction mixture was cooled back to room temperature, CHCl3 (30 mL) was added. The mixture was then filtered, and the residue was washed with additional CHCl3 (50 mL). After removal of the solvent, a yellow oil was obtained. Purification of the oil via column chromatography on silica gel using CH2Cl2 as the eluent lead to 0.29 g of methyl 10-hexyloxyoctadecanoate as a colorless oil (66%). 1H NMR (CDCl3, 500 MHz): δ 3.66 (s, 3H), 3.38 (t, J ) 7.0 Hz, 2H), 3.16 (m, 1H), 2.30 (t, J ) 7.5 Hz, 2H), 1.61 (m, 2H), 1.56 (m, 2H), 1.27 (m, 34H), 0.88 (m, 6H). 10-Hexyloxyoctadecanoic Acid. Methyl 10-hexyloxyoctadecanoate (0.27 g, 0.68 mmol) was refluxed in 1/1 MeOH/H2O (15 mL) containing 0.5 g of NaOH for 2 h. After the reaction mixture was cooled to room temperature, it was acidified to pH 1-2. The mixture was then extracted with diethyl ether (3 × 10 mL), and the combined ether layers were dried over anhydrous Na2SO4. After the removal of the solvent, the residue was purified by column chromatography on silica gel using 1/1 ethyl acetate/ hexanes as the eluent to obtain 0.24 g of 10-hexyloxyoctadecanoic acid as a colorless oil (93%). 1H NMR (CDCl3, 500 MHz): δ 3.40 (t, J ) 7.0 Hz, 2H), 3.20 (m, 1H), 2.37 (t, J ) 7.5 Hz, 2H), 1.65 (m, 2H), 1.56 (m, 2H), 1.27 (m, 34H), 0.88 (m, 6H). Sodium 10-Hexyloctadecanoate (4). This compound was synthesized from 10-hexyloxyoctadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 2927, 2854, 1570, 1466, 1444, 1421, 1377, 1347, 1232, 1098, 926, 723, 699. 1H NMR (CD3OD, 500 MHz): δ 3.43 (t, J ) 7.0 Hz, 2H), 3.25 (m, 1H), 2.15 (t, J ) 7.5 Hz, 2H), 1.60 (m, 2H), 1.54 (m, 2H), 1.27 (m, 34H), 0.92 (m, 6H). 13C NMR (CD3OD, 125 MHz): δ 183.3, 81.0, 70.1, 39.4, 35.2, 35.2, 33.2, 33.0, 31.4, 31.1, 31.1, 31.1, 31.0, 31.0, 30.9, 30.8, 30.6, 28.0, 27.3, 26.7, 26.6, 23.9, 14.6, 14.6. High-resolution MS (LSIMS+): calcd for [C24H47NaO3], 406.3423; calcd for [(C24H47NaO3) + Na], 29.3321; found, 429.3311. Methyl 10-(2-Ethylhexyloxy)octadecanoate. This compound was synthesized using the same procedure described for methyl 10-hexyloxyoctadecanoate but using 2-ethyl-1-iodohexane instead of 1-iodohexane (31% yield). 1H NMR (CDCl3, 500 MHz): δ 3.66 (s, 3H), 3.25 (m, 2H), 3.15 (m, 1H), 2.28 (t, J ) 7.5 Hz, 2H), 1.60 (m, 2H), 1.27 (m, 38H), 0.90 (m, 6H). 10-(2-Ethylhexyloxy)octadecanoic Acid. This compound was synthesized from methyl 10-(2-ethylhexyloxy)octadecanoate using the same procedure described for 10-hexyloxyoctadecanoic acid (70% yield). 1H NMR (CDCl3, 500 MHz): δ 3.26 (m, 2H), 3.15 (m, 1H), 2.34 (t, J ) 7.5 Hz, 2H), 1.60 (m, 2H), 1.27 (m, 38H), 0.90 (m, 6H). Sodium 10-(2-Ethylhexyl)octadecanoate (5). This compound was synthesized from 10-(2-ethylhexyloxy)octadecanoic acid using the same procedure described for 1b-4 (quantitative yield). FT-IR (KBr, cm-1): 2929, 2854, 1572, 1466, 1442, 1420, 1378, 1347, 1092, 927, 723, 699. 1H NMR (CD3OD, 500 MHz): δ 3.27 (m, 2H), 3.20 (m, 1H), 2.15 (t, J ) 7.0 Hz, 2H), 1.60 (m, 2H), 1.27 (m, 38H), 0.91 (m, 6H). 13C NMR (CD3OD, 125 MHz): δ 183.3, 79.6, 71.0, 40.4, 38.2, 33.9, 33.9, 31.9, 30.6, 29.8, 29.7, 29.7, 29.7, 29.6, 29.6, 29.5, 29.3, 29.2, 26.7, 25.4, 25.3, 23.9, 23.1,

Lyotropic Liquid-Crystalline Behavior of Acid Salts

Langmuir, Vol. 18, No. 20, 2002 7421 Scheme 1. Synthesis Scheme for Series 1 and 2 Amphiphiles Containing Aromatic Ether Side Chains at the C10 and C16 Positionsa

Figure 4. Structures of series 1 and series 2 aromatic side chain amphiphiles synthesized in this study. 22.6, 13.3, 10.4. High-resolution MS (LSIMS+): calcd for [C26H51NaO3], 434.3736; calcd for [(C26H51NaO3) + Na], 457.3634; found, 457.3655.

Results and Discussion Amphiphile Synthesis. To determine the importance of the aromatic side chain on the LLC behavior of these substituted long chain acid salts, two sets of 12 compounds containing different aromatic groups (4-styryl ether, 4-ethylphenyl ether, 1-naphthyl ether, and 4-phenylazophenyl ether) were synthesized with the side chain at three positions along the amphiphile backbone (Figure 4). One set of amphiphiles is based on an 18-carbon, long chain carboxylic acid sodium salt (compounds starting with the name prefix 1). The second set of amphiphiles (compounds starting with the name prefix 2) is similar to the first set except it contains an extended sodium sulfonate headgroup instead of a sodium carboxylate headgroup. Compounds within these two series are designated with a letter identifying the type of aromatic ether side group (a ) 4-styryl; b ) 4-ethylphenyl; c ) 1-naphthyl; d ) 4-phenylazophenyl (i.e., azobenzene)), followed by a number specifying the position of the side group on the alkyl chain with respect to the carbonyl carbon designated as C1. For example, the compound 1c10 refers to the octadecanoic acid sodium salt containing a 1-naphthyl ether side chain at the C10 position relative to the carbonyl group (see Figure 4). Depending on the attachment point of the side chain, these aromatic side chain amphiphiles were synthesized using two different procedures as shown in Schemes 1 and 2, using protocols similar to those described in our previous publications.19,20 For the series 1 sodium carboxylate amphiphiles with the aromatic ether side chain at the C10 and C16 positions, commercially available ω-hydroxy fatty acids were first esterified, and then the resulting ω-hydroxy esters were oxidized to form the corresponding aldehydes. Addition of n-alkyl Grignard

a Reaction conditions: (a) MeOH, H2SO4. (b) PCC. (c) CH3(CH2)nMgX (n ) 1 or 7). (d) ADDP, Bu3P, ArOH. (e) (i) NaOH, MeOH, H2O; (ii) H3O+. (f) 1 equiv of NaOH. (g) N-Hydroxysuccinimide, DCC, DMAP. (h) NH2(CH2)2SO3H, Na2CO3.

reagents to the crude aldehydes afforded the secondary alcohols as racemic mixtures, with the individual positions of the hydroxy groups determined by the choice of starting fatty acids and alkyl Grignard reagents (Scheme 1). For the series 1 amphiphiles with the aromatic ether side chain at the C4 position, the secondary alcohol intermediate was synthesized by coupling n-tetradecanyl Grignard with methyl 4-chloro-4-oxobutyrate to form the corresponding ketone compound, followed by reduction with NaBH4 to generate racemic methyl 4-hydroxyoctadecanoate (Scheme 2). For all the compounds, the desired side chains were then attached to the secondary alcohol intermediates via a modified Mitsonobu reaction using the appropriate phenols. Hydrolysis of the resulting aromatic ethersubstituted octadecanoic esters, followed by neutralization of the resulting carboxylic acids with NaOH, subsequently yielded the series 1 amphiphiles (Schemes 1 and 2). The corresponding series 2 sodium sulfonate amphiphiles were generated by grafting 2-aminoethanesulfonic acid (i.e., Taurine) onto the series 1 amphiphiles (see Schemes 1 and 2). The substituted octadecanoic acids were first reacted with N-hydroxysuccinimide to form the corresponding activated esters, and then they were reacted with 1.5 equiv of 2-aminoethanesulfonic acid in the presence of 1.5 equiv of Na2CO3.

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Gu and Gin

Scheme 2. Synthesis Scheme for Series 1 and 2 Amphiphiles Containing Aromatic Ether Side Chains at the C4 Positiona

Figure 5. XRD profiles of LLC phases formed by some of the series 2 amphiphiles in the presence of 6% H2O: (a) 2c-16, L phase; (b) 2c-10, HII phase; (c) 2c-4, HII phase. The numbers above the peaks are d spacings in Å.

a Reaction conditions: (a) CH (CH ) MgX, Bu P. (b) NaBH . 3 2 13 3 4 (c) ADDP, Bu3P, ArOH. (d) (i) NaOH, MeOH, H2O; (ii) H3O+. (e) 1 equiv of NaOH. (f) N-Hydroxysuccinimide, DCC, DMAP. (g) NH2(CH2)2SO3H, Na2CO3.

Scheme 3. General Synthetic Approaches to Aliphatic Ether Side Chain Amphiphilesa

a Reaction conditions: (a) 5 equiv of Ag O; 5 equiv of R-I; 2 120 °C. (b) (i) NaOH, CH3OH, H2O; (ii) H3O+. (c) 1 equiv of NaOH.

To gauge the importance of the aromatic side chains in the LLC behavior of these amphiphiles, two sodium octadecanoate amphiphiles containing aliphatic ether side chains were synthesized and studied as control experiments. Compound 4 contains an n-hexyl ether side chain located at the C10 position of the octadecanoate amphiphile backbone, and compound 5 contains a 2-ethylhexyl ether side chain also at the C10 position. The synthesis of these two control compounds is depicted in Scheme 3. LLC Phase Behavior. To compare the LLC behavior of the amphiphiles listed above, the compounds were examined by polarized optical microscopy (POM) and lowangle powder X-ray diffraction (XRD) in their neat forms and as mixtures containing 6, 12, and 24 wt % water. POM was used to rapidly detect the presence of LLC phases in the amphiphile mixtures and to qualitatively determine any phase changes with changes in system composition. The observed LLC phases were then identified quantitatively using XRD by correlating the pattern of observed d spacings with those characteristic of common LLC phases. For example, LLC samples with low water

content that exhibit d spacings that proceed as 1, 1/x3, 1/2, 1/x7, ... correspond to an HII phase. Samples that exhibit XRD d spacings that proceed as 1, 1/2, 1/3, 1/4, ... are indicative of an L phase.2 Some illustrative XRD profiles of LLC phases formed by the aromatic side chain amphiphiles prepared in this study are presented in Figure 5. There are essentially three orthogonal structural parameters being compared in our study of the LLC behavior of the 24 compounds in series 1 and 2: (1) the relative position of the aromatic side chain on the amphiphile, (2) the nature of the aromatic side chain; and (3) the nature of the ionic headgroup. Consequently, the LLC results are presented and organized according to these three parameters. Table 1 summarizes the LLC behavior of the carboxylate- and sulfonate-based amphiphiles with the aromatic side chains all located near the end of the amphiphile backbone (C16). As can be seen from Table 1, all of these amphiphiles form the L phase regardless of the nature of the aromatic side chain, the nature of the headgroup, or the amount of water used to form the phases in this study. In addition, there does not appear to be any correlation between the layer spacing (i.e., d100 in the XRD profiles) of these L phases and either the size of the aromatic units or the water content in the phases. In contrast, amphiphiles with the aromatic side chain near the middle of the amphiphile display a strong tendency to adopt the HII phase. As can be seen in Table 2, the HII phase is the predominant assembly formed by these amphiphiles when there is less than 24 wt % water in the mixtures, regardless of the type of aromatic side chain. With 24 wt % water in the mixtures, most of the amphiphiles tend to form the L phase and mixed phases. Another subtle trend that can be discerned from the LLC data is that the midchain-substituted molecules containing 4-ethylphenoxy and 1-naphthoxy units show an earlier onset of HII phase destabilization with increasing water content compared to their styrene and azobenzene analogues. Amphiphiles 1b-10, 1c-10, 2b-10, and 2c-10 begin to show signs of HII phase instability (i.e., a tendency to form other LLC phases and mixed phases) at 12 wt % water, whereas the other analogues do not exhibit this behavior until there is 24 wt % water in the mixtures.

Lyotropic Liquid-Crystalline Behavior of Acid Salts

Langmuir, Vol. 18, No. 20, 2002 7423

Table 1. LLC Phase Behavior of Aromatic Ether Side Chain Amphiphiles with m ) 14 and n ) 1

phasea (d100, Å) X

compound

Ar

neat

6% H2O

12% H2O

24% H2O

4-styryl 4-ethylphenyl 1-naphthyl 4-phenylazophenyl 4-styryl 4-ethylphenyl 1-naphthyl 4-phenylazophenyl

L (44.6)b L (47.2) L (44.4) L (48.2) L (57.3) L (60.6) L (59.8) L (57.7)

L (48.2)b L (45.2) L (43.6) L (47.2) L (57.6) L (60.4) L (58.0) L (57.3)

L (48.2)b L (45.2) L (43.6) L (47.3) L (56.8) L (59.0) L (58.2) L (57.7)

L (47.9)b L (46.3) L (45.3)

C(O)NH(CH2)2SO3-Na+

1a-16 1b-16 1c-16 1d-16 2a-16 2b-16 2c-16 2d-16

COO- Na+

a

L (55.9) L (59.0) L (56.1)

L ) lamellar. b See ref 20. Table 2. LLC Phase Behavior of Aromatic Ether Side Chain Amphiphiles with m ) 8 and n ) 7 phasea (d100, Å) X

compound

COO- Na+

C(O)NH(CH2)2SO3-Na+

a

1a-16 1b-16 1c-16 1d-16 2a-16 2b-16 2c-16 2d-16

Ar

neat

4-styryl 4-ethylphenyl 1-naphthyl 4-phenylazophenyl 4-styryl 4-ethylphenyl 1-naphthyl 4-phenylazophenyl

(35.3)b

HII HII (35.1) HII (33.0) HII (35.5) HII (41.5) HII (41.1) HII (39.3) HII (40.6)

6% H2O (34.7)b

HII HII (34.4) HII (31.6) HII (35.3) HII (39.5) HII (39.6) + L (43.7) HII (39.1) HII (40.4)

12% H2O

24% H2O

(32.9)b

HII HII (34.9) HII (31.6) + L (37.5) HII (35.8) HII (39.7) HII (40.1) + L (43.7) HII (38.8) + L (41.7) HII (41.2)

HII (33.0)b L (39.9) L (38.0) L (38.8) HII (40.3) + M L (45.9) L (42.7) + HII (37.1) L(45.3)

L ) lamellar; HII ) inverted hexagonal; M ) mixture of phases. b See ref 20. Table 3. LLC Phase Behavior of Aromatic Ether Side Chain Amphiphiles with m ) 2 and n ) 13 phasea (d100, Å) X

compound

Ar

neat

6% H2O

12% H2O

C(O)NH(CH2)2SO3-Na+

1a-16 1b-16 1c-16 1d-16 2a-16 2b-16 2c-16 2d-16

4-styryl 4-ethylphenyl 1-naphthyl 4-phenylazophenyl 4-styryl 4-ethylphenyl 1-naphthyl 4-phenylazophenyl

HII (30.3)b HII (27.7) M (31.8) M (27.3) M (35.8) L (34.5) M (35.7) HII (36.6)

HII (28.6)b HII (27.7) M (27.4) M (30.1) L (41.1) L (35.9) L (35.5) HII (36.5)

HII (28.6)b HII (28.2) + M (30.0) M (31.3) M (33.0) L (41.8) L (34.6) L (36.0) HII (36.3)

COO-Na+

a

L ) lamellar; HII ) inverted hexagonal; M ) mixture of phases. b See ref 20.

Table 3 summarizes the phase behavior of the various amphiphiles with the aromatic side chain attached closest to the headgroup (at C4). With this particular side chain attachment motif, there are two trends that can be discerned. First, only the sodium carboxylate amphiphiles that have the smallest aromatic moieties as side chains (i.e., 1a-4 and 1b-4) favor formation of the HII phase. The incorporation of larger aromatic units as side chains close to the carboxylate headgroup results in amphiphiles that tend to form mixed LLC phases. In contrast, the opposite behavior is observed in the sulfonate amphiphiles. Only the sulfonate derivative with the largest aromatic side chain (i.e., azobenzene ether) at the C4 position favors formation of the HII phase (i.e., 2d-4). All other sulfonate amphiphiles with an aromatic side chain at the same position favor the formation of the L phase or mixed phases. Recent work in our group has shown that the position of the aromatic side chain on the amphiphile backbone of styryl ether monomer 2a has a major effect on the type of LLC phase preferred by this particular amphiphile platform.20 Systematic analysis of the LLC behavior of a series of similar amphiphiles (Tables 1-3) suggests that the relative position of the side chain is the most important factor governing the LLC behavior of these side chain-

functionalized amphiphiles. The nature of the aromatic side chain appears to have very little effect on LLC behavior in comparison. Similarly, changing the ionic headgroup from carboxylate to an extended sulfonate group also seems to have very little effect on the LLC behavior of these amphiphiles. The phase behavior of two aliphatic ether (i.e., nonaromatic) side chain amphiphiles was also examined as a control experiment. Table 4 summarizes the LLC behavior of an n-hexyloxyoctadecanoic acid sodium salt (4) and a 2-ethylhexyloxyoctadecanoic acid sodium salt (5), both with the aliphatic ether side chain near the middle of the amphiphile (C10 position). Although 4 tends to form the L phase, compound 5 exhibits a strong tendency to form the HII phase over a range of compositions, even though the amphiphile lacks an aromatic unit. The d100 spacings for the HII phases of 5 are also very similar to those of the HII phases formed by amphiphiles with aromatic side chains at the same position (Table 2). This control experiment indicates that an aromatic moiety is not essential for HII phase formation in these side chain amphiphiles. Apparently, so long as the side chain is near the middle of the amphiphile backbone and possesses enough steric bulk to help induce interfacial curvature toward the aqueous domain, similar HII phases can be

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Langmuir, Vol. 18, No. 20, 2002

Gu and Gin

Table 4. LLC Phase Behavior of Amiphiphiles with Aliphatic Ether Side Chains (m ) 8 and n ) 7) phasea (d100, Å) X

compound

side chain

neat

6% H2O

12% H2O

24% H2O

COO-Na+

4 5

n-hexyl 2-ethylhexyl

L (42.4) HII (38.5)

L (40.7) + M HII (36.0)

L (43.3) + M HII (35.5)

L (43.4) + HII L (38.1) + M

a

L ) lamellar; HII ) inverted hexagonal; M ) mixture of phases. Table 5. Photopolymerization of the LLC Phases of the Styryl Ether Side Chain Amphiphiles Containing a Sodium Sulfonate Headgroup (2a) compound 2a-4

2a-10

Figure 6. XRD profiles of some of the LLC phases prepared from the styryl ether amphiphiles 2a before and after polymerization: (a) 2a-16 with 6% H2O and 12% divinylbenzene before polymerization; (b) 2a-16 with 6% H2O and 12% divinylbenzene after polymerization; (c) 2a-10 with 6% H2O and 12% divinylbenzene before polymerization; (d) 2a-10 with 6% H2O and 12% divinylbenzene after polymerization; (e) 2a-4 with 6% H2O; (f) 2a-4 with 6% H2O and 12% divinylbenzene before polymerization; (g) 2a-4 with 6% H2O and 12% divinylbenzene after polymerization.

formed. These results confirm that aromatic interactions are relatively unimportant in the aggregation behavior of this particular class of amphiphile. Instead, the results suggest that amphiphile shape/packing considerations10 are the dominant directing forces in the LLC phase formation of these particular systems (Table 4). With respect to LLC photopolymerization, the two subsets of styryl ether amphiphiles 1a and 2a can be crosslinked in the presence of divinylbenzene with radical photoinitiators. Prior work in our group with carboxylate amphiphiles 1a has shown that placing the styryl ether closer to the chain end and further from the headgroup leads to HII phases with an increased tendency to form the L phase, especially upon photopolymerization.20 As can be seen in the XRD profiles in Figure 6, the sulfonatebased styryl ether analogues exhibit similar behavior. The LLC behavior of these amphiphiles is summarized in Table 5. The sulfonate amphiphile with the styryl ether unit near the middle of the chain (2a-10) can be cross-linked in the HII phase with retention of phase microstructure. The amphiphile with the polymerizable group closer to the ionic headgroup (at C4) initially forms an HII assembly, but this phase is not very stable. It spontaneously rearranges into the more stable L phase during the polymerization process. The amphiphile with the styryl ether near the tail end (at C16) tends to favor the L phase instead of the HII phase, and the L phase is preserved upon cross-linking. This trend in HII phase stability during photopolymerization for both 1a and 2a parallels what has already been observed with different aromatic and aliphatic substituents in Tables 1-4: The relative position of the side chain on the amphiphile is the most important

2a-16

composition of LLC mixturea monomer (6% H2O, 12% divinylbenzene) polymer (6% H2O, 12% divinylbenzene) monomer (6% H2O, 7.5% octane, 7.5% divinylbenzene) polymer (6% H2O, 7.5% octane, 7.5% divinylbenzene) monomer (6% H2O, 7.5% xylene, 7.5% divinylbenzene) polymer (6% H2O, 7.5% xylene, 7.5% divinylbenzene) monomer (6% H2O, 12% divinylbenzene) polymer (6% H2O, 12% divinylbenzene) monomer (5% H2O, 18% divinylbenzene) polymer (5% H2O, 18% divinylbenzene) monomer (5% H2O, 25% divinylbenzene) polymer (5% H2O, 25% divinylbenzene) monomer (6% H2O, 12% divinylbenzene) polymer (6% H2O, 12% divinylbenzene)

phaseb (d100, Å) HII (34.4) L (37.1) HII (34.6) L (38.2) HII (34.5) L (38.0) HII (40.1) HII (38.5) HII (41.8) HII (38.0) HII (41.8) HII (37.7) L (58.3) L (57.4)

a Also contains