Lyotropic Phases Reinforced by Hydrogen Bonding - Langmuir (ACS

May 3, 2005 - This behavior was not observed for the homologous sodium alkylbenzenesulfonates, indicating that hydrogen bonding, mediated by the guani...
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Lyotropic Phases Reinforced by Hydrogen Bonding Stephen M. Martin and Michael D. Ward* Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455 Received February 11, 2005. In Final Form: March 31, 2005 Amphiphilic guanidinium alkylbenzenesulfonates (GCnBS; n ) number of carbons in the alkyl chain) exhibited lyotropic behavior in aqueous and organic solvents. The GCnBS compounds formed gel-like phases in certain cyclic organic solvents (e.g. p-xylene, cyclohexane) through the formation of swollen interdigitated lamellar phases reinforced by hydrogen bonding between the guanidinium ions and sulfonate moieties. This behavior was not observed for the homologous sodium alkylbenzenesulfonates, indicating that hydrogen bonding, mediated by the guanidinium (G) ion, was required for gel formation. Infrared spectroscopy unambiguously demonstrated the existence of the quasihexagonal hydrogen-bonded sheet typically adopted by G ions and the sulfonate groups in layered, solvent-free crystalline phases of the compounds, supporting lamellar structures in the gels. Small-angle X-ray scattering analysis of these gels revealed GCnBS lamellar phases with interlayer spacings (d) that increased with increasing temperature, consistent with increased absorption of solvent by the nonpolar regions of the gelator. At the lower gelator concentrations, the increase in d-spacing achieved at the higher temperatures exceeded the sum of the alkylbenzene chain lengths, suggesting either long-range interactions between the GS sheets or undulation stabilized lamellae, which have been reported in aqueous lamellar gels. The GCnBS compounds also formed lyotropic phases in water, but the phase behavior was more complex than that of the organogels. The rheology suggested gel-like behavior associated with entangled wormlike micelles at these higher concentrations. These lyotropic phases were reminiscent of crystalline layered and tubular architectures exhibited by various guanidinium organomonosulfonate compounds. These lyotropic phases expand the liquid crystal behavior observed for GS compounds beyond recently observed thermotropic smectic phases, adding to the portfolio of phase behavior exhibited by these materials.

Introduction The physical properties of molecular materials are strongly influenced by the spatial arrangement of the molecular components. Intermolecular interactions such as hydrogen bonding have been employed to influence molecular organization in crystalline materials,1 oligomers and polymers,2 and thermotropic liquid crystals.3 The delicate interplay between the hydrophilic and hydro* Author to whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Ermer O, Lindenberg L. Helv. Chim. Acta 1991, 74, 825. (b) Brunet, P.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc. 1997, 119, 2737. (c) Endo, K.; Ezuhara, T.; Koyanagi, M.; Masuda, H.; Aoyama, Y. J. Am. Chem. Soc. 1997, 119, 499. (d) Caira, M. R.; Nassimbeni, L. R.; Toda, F.; Vujovic, D. J. Am. Chem. Soc. 2000, 122, 9367. (e) Biradha, K.; Dennis, D.; MacKinnon, V. A.; Sharma, C. V. K.; Zaworotko, M. J. J. Am. Chem. Soc. 1998, 120, 11895. (f) Harris, K. D. M.; Thomas, J. M.; J. Chem. Soc., Faraday Trans. 1990, 86, 2985. (g) Langley, P. J.; Hulliger, J. Chem. Soc. Rev. 1999, 28, 279. (h) Krische, M. J.; Lehn, J.-M.; Kyritsakas, N.; Fischer, J. Helv. Chim. Acta 1998, 81, 1909. (2) (a) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278, 1601. (b) Wurthner, F.; Thalacker, C.; Sautter, A. Adv. Mater. 1999, 11, 754. (c) Schoonbeek, F. S.; van Esch, J. H.; Wegewijs, B.; Rep, D. B. A.; de Haas, M. P.; Klapwijk, T. M.; Kellog, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 1999, 38, 1393. (d) Rep, D. B. A.; Roelfsema, R.; van Esch, J. H.; Schoonbeek, F. S.; Kellog, R. M.; Feringa, B. L.; Palstra, T. T. M.; Klapwijk, T. M. Adv. Mater. 2000, 12, 563. (e) Schenning, I. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 409. (3) (a) Paleos, C. M.; Tsiourvas, D. Liq. Cryst. 2001, 28, 1127. (b) Kato, T.; Frechet, J. M. J.; Wilson, P. G.; Saito, T.; Uryu, T.; Fujishima, A.; Jin, C.; Kaneuchi, F. Chem. Mater. 1993, 5, 1094. (c) Brienne, M. J.; Gallard, J.; Lehn, J. M.; Stibor, J. J. Chem. Soc., Chem. Commun. 1989, 1868. (d) van Dorn, H. A.; van der Heijden, A. M.; de Goede, A. T. J. W.; van Rantwijk, F. van Bekkum, H. Liq. Cryst. 2000, 27, 63. (e) Hein, M.; Miethchen, R.; Schwaebisch, D.; Schick, C. Liq. Cryst. 2000, 27, 163. (f) Praefcke, K.; Levelut, A. M.; Kohne, B.; Eckert, A. Liq. Cryst. 1989, 6, 263. (g) Kanie, K.; Nishii, M.; Yasuda, T.; Taki, T.; Ujiie, S.; Kato, T. J. Mater. Chem. 2001, 11, 2875. (h) Heentrich, F.; Diele, S.; Tschierske, C. Liq. Cryst. 1994, 17, 827.

phobic moieties of amphiphilic molecules has long been known to produce a wide variety of self-assembled supramolecular structures in both aqueous and organic solvents.4 The introduction of directional noncovalent intermolecular interactions, such as hydrogen-bonding, into amphiphilic systems could lead to a preference for certain micellar or liquid crystalline structures, such as lamellar phases or wormlike micelles. Our laboratory has reported numerous crystalline materials with layered architectures enforced by the presence of an infinite and well-defined two-dimensional (2D) hydrogen-bonded network of guanidinium (G) ions (C(NH2)3+) and the sulfonate (S) moieties of organomonosulfonates or organodisulfonates.5-11 The 2D network usually adopts a quasi-hexagonal hydrogen-bonding motif due to the three-fold symmetry and chemical complementarity of the G and S ions (Figure 1). Guanidinium organomonosulfonates commonly form one of two layered crystal architectures: (i) the bilayer (B) architecture, in which the organic residues of the organomonosulfonates all project from the same side of the 2D sheet and interdigitate with the organic residues of an adjacent sheet to form a discrete bilayer, and (ii) the continuously (4) Evans, D. F.; Wennerstrom, H. The Colloidal Domain; WileyVCH: New York, 1999. (5) Russell, V. A.; Etter, M. C.; Ward, M. D. Chem. Mater. 1994, 6, 1206. (6) Russell, V. A.; Etter, M. C.; Ward, M. D. J. Am. Chem. Soc. 1994, 116, 1941. (7) Russell, V. A.; Ward, M. D. J. Mater. Chem. 1997, 7, 1123. (8) Russell, V. A.; Evans, C. C.; Li, W.; Ward, M. D. Science (Washington, D. C.) 1997, 276, 575. (9) Holman, K. T.; Ward, M. D. Angew. Chem., Int. Ed. 2000, 39, 1653. (10) Holman, K. T.; Martin, S. M.; Parker, D. P.; Ward, M. D. J. Am. Chem. Soc. 2001, 123, 4421. (11) Holman, K. T.; Pivovar, A. M.; Swift, J. A.; Ward, M. D. Acc. Chem. Res. 2001, 34, 107.

10.1021/la050393l CCC: $30.25 © 2005 American Chemical Society Published on Web 05/03/2005

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lamellae with interlayer spacings (d) that increased with temperature, signifying increased solvent absorption. The lamellar structure of these phases was enforced by intermolecular GS hydrogen bonding. Upon heating in water, the GCnBS compounds formed spherical micelles, which upon cooling formed transparent gels with properties that suggest the formation of extended wormlike micelles in solution. These lyotropic phases were reminiscent of crystalline layered and tubular architectures exhibited by various guanidinium organomonosulfonate compounds. These lyotropic phases expand the liquid crystal behavior observed for GS compounds beyond recently observed thermotropic smectic phases, adding to the portfolio of phase behavior exhibited by these materials. Experimental Section

Figure 1. (a) Schematic of the “quasi-hexagonal” 2D sheet formed by complementary hydrogen bonding between guanidinium (G) and organosulfonate (S) ions in the solid state. (b) Schematic of the bilayer (B) architecture. (c) Schematic of the continuously initerdigitated layer (CIL) architecture. (d) Schematic of the tubular inclusion compound (TIC) architecture, which forms only in the presence of certain guest molecules (denoted as g).

interdigitated layer (CIL) architecture, in which the organic residues project from both sides of the sheet, interdigitating to form a continuous structure. The layered structure of these materials resembles smectic-like ordering in thermotropic liquid crystal phases. Indeed, upon heating to temperatures above T ) 100 °C, certain guanidinium alkylbenzenesulfonate (GCnBS) and guanidinium alkylbiphenylsulfonate (GCnBPS) compounds produced smectic liquid crystalline phases, in which the smectic architecture was enforced and stabilized by GS hydrogen bonding.12,13 The guanidinium organomonosulfonates also form inclusion compounds with organic guests that can be viewed as crystalline analogues of lyotropic liquid crystals (i.e., surfactant-solvent mixtures). These crystalline phases exhibit lamellar structures with flat GS sheets, as well as hexagonal phases, in which the GS sheets curl into closed cylinders that resemble cylindrical micelles (depicted schematically in Figure 1).14 Herein, we report the phase behavior of a series of guanidinium alkylbenzenesulfonates (GCnBS) in organic

Materials. Alkylbenzenes were purchased from Sigma-Aldrich (Milwaukee, WI) and used without further purification. Guanidinium tetrafluoroborate (GBF4) was prepared by the neutralization of tetrafluoroboric acid with stoichiometric quantities of guanidinium carbonate. Guanidinium 4′-Alkyl-4-benzenesulfonate (GCnBS). GCnBS compounds were prepared by the direct sulfonation of alkylbenzenes with chlorosulfonic acid followed by precipitation from acetone with GBF4 and recrystallization in methanol, as reported previously.10 Sample Preparation. Samples containing GCnBS and organic solvents were prepared by mixing the GCnBS and solvent in a vial and heating to 90 °C for 5 min, whereupon a gel-like phase formed. Following this, the samples were allowed to cool to room temperature. Samples containing GCnBS and water were prepared by mixing in a vial and heating to 90 °C until the GCnBS dissolved. The aqueous gels and viscous fluids were formed upon cooling of these solutions. Materials Characterization. All rheological measurements were performed using an ARES II rheometer (Rheometric Science) equipped with a fluid bath temperature controller and a controlled environment stage. Oscillatory shear measurements were performed using a 25 mm diameter parallel plate geometry and strains between 0.3% and 0.5%. Steady shear measurements were performed using either a 25 or 50 mm diameter parallelplate geometry and shear rates between 0.01 and 20 s-1. Fourier transform infrared spectroscopy was performed using a Nicolet Magna 550 FT-IR equipped with a Harrick heating stage. Smallangle X-ray diffraction (SAXS) was performed on an Anton-Paar SAXSesse instrument and on a 2D-SAXS line equipped with a copper rotating anode X-ray source, a Rigaku 2D area detector, a heating stage, and Bruker SAXS software. Both SAXS instruments are located at the University of Minnesota Characterization Facility. Cryogenic transmission electron microscopy (Cryo-TEM) was performed using a JEOL 1210 transmission electron microscopy equipped with a Gatan cryotransfer stage and a Gatan Multiscan CCD camera located at the University of Minnesota Characterization Facility. Freeze-dried specimens of the organogels were prepared by applying a thin layer of the gel to an SEM sample stub, placing this assembly in a test tube, plunging the test tube into liquid nitrogen, and then applying a vacuum.

Results and Discussion

and aqueous solvents. GCnBS compounds with n g 10 carbons formed gel-like phases in certain cyclic organic solvents upon heating. In contrast to the many lowmolecular-mass organogelators that adopt fibrillar structures,15 these phases consisted of solvent-swollen GCnBS (12) Martin, S. M.; Yonezawa, J.; Horner, M. J.; Macosko, C. W.; Ward, M. D. Chem. Mater. 2004, 16, 3045. (13) Mathevet, F.; Masson, P.; Nicoud, J.-F.; Skoulios, A. Chem. Eur. J. 2002, 8, 2248.

Organogelation with GCnBS. GCnBS compounds (4 e n e 16) were each mixed with toluene, p-, o-, or m-xylene, mesitylene, or cyclohexane and heated to 90 °C. Upon heating in these organic solvents, GCnBS compounds with n g 10 carbons and [GCnBS] g 5 wt% formed translucent, gel-like phases characterized by resistance to flow and substantial yield stresses. The gel phases persisted upon cooling to room temperature. Although complete gelation was not observed for mixtures (14) Horner, M. J.; Holman, K. T.; Ward, M. D. Angew. Chem., Int. Ed. 2001, 40, 4045.

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Figure 2. (A) Polarized optical micrograph of a 10% mixture of GC11BS in p-xylene at 25 °C. (B) Image of the 10% GC11BS/ p-xylene sample exhibiting gel-like behavior. (C) SEM of a freeze-dried sample of a gel prepared from 4% GC11BS in o-xylene. Table 1. Locations of the νN-H Peaks for Pure GCnBS Compounds and [GCnBS] ) 10 wt% Mixtures in p-Xylene compound

νN-H crystalline GCnBS

νN-H 10 wt% GCnBS in p-xylene

GC10BS GC11BS GC12BS GC14BS GC16BS

3369 cm-1, 3342 cm-1 3370 cm-1, 3344 cm-1 3367 cm-1, 3336 cm-1 3370 cm-1, 3342 cm-1 3367 cm-1, 3337 cm-1

3369 cm-1, 3335 cm-1 3368 cm-1, 3335 cm-1 3369 cm-1, 3336 cm-1 3369 cm-1, 3336 cm-1 3368 cm-1, 3335 cm-1

with [GCnBS] < 5 wt%, a significant thickening of the mixtures was observed. The ability to form a gel was sensitive to the solvent used, as GCnBS compounds could not be dispersed in solvents such as alcohols, certain cyclic organics,16 and surprisingly, alkanes, despite prolonged heating and sonication. GCnBS compounds caused thickening of benzene, ethylbenzene, propylbenzene, cumene, and t-butylbenzene but did not produce mixtures with perceptible yield stresses. The homologous sodium salts of the GCnBS compounds did not form gel-like phases or exhibit thickening when mixed with toluene, p-, o-, and m-xylene, mesitylene, or cyclohexane, which indicated that hydrogen-bonding between the guanidinium ions and sulfonate moieties was necessary for gel formation. The gels were translucent in appearance and exhibited optical birefringence when placed between crossed polarizers, consistent with the existence of lamellar phases and the presence of mesoscale ordered domains that trap pockets of solvent (Figure 2). Freeze-dried samples of the organogelators exhibited large lamellar slabs organized in stacks, consistent with the existence of a lamellar architecture in the gel state. The Fourier transform infrared (FTIR) spectra of the gels prepared from GCnBS and p-xylene revealed the presence of two νN-H peaks at νN-H ≈ 3369 and 3335 cm-1 (Table 1). The peaks maintained their positions and shapes up to temperatures as high as 90 °C (Figure 3). The locations and shapes of the νN-H peaks, which previous work in our laboratory has established as a fingerprint of the quasihexagonal GS sheet, were nearly identical to those observed for the solvent-free crystalline GCnBS materials, which exhibit lamellar ordering in the crystalline state.12,13 In contrast, we have observed that the νN-H peaks of the crystalline GCnBS compounds broaden significantly upon heating above their crystalline-smectic transitions due to disorder or disruption of the quasihexagonal GS motif.12 The νN-H peaks of the organogels (15) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133.

Figure 3. Infrared spectra of crystalline GC11BS displaying N-H stretching peaks at νN-H ) 3370 and 3344 cm-1 and [GC11BS] ) 10 wt% in p-xylene at 25 and 90 °C with νN-H ) 3368 and 3335 cm-1. The nearly identical spectra of the crystalline layered phases and organogel phase argues that the GS hydrogen-bonded network is very similar in both materials. In contrast, the thermotropic smectic phase, obtained by heating neat crystalline GC11BS to 170 °C, loses the νN-H features associated with hydrogen bonding.

were observed at lower energy compared with free G protons, consistent with complete participation of all the hydrogen-bonding donors and acceptors.12,17,18 Collectively, these results indicate that the quasihexagonal GS hydrogen-bond network was preserved in the gel-like phase in which all the hydrogen-bonding donors and acceptors (guanidinium hydrogens and sulfonate oxygens, respectively) are involved in hydrogen bonding. The persistence of the GS sheet is not unexpected in a nonpolar organic solvent, which is unlikely to perturb the hydrogen bonding. Small-angle X-ray scattering (SAXS) patterns of GCnBS/xylene gels exhibited a single low-angle peak at amphiphile concentrations as low as [GCnBS] ) 2 wt%. For example, the low-angle peak observed for GC11BS/ p-xylene gels at room temperature corresponded to a d spacing of 40 Å. This value excceds those for the crystalline and smectic phases of GC11BS recently reported by our laboratory (23 and 31 Å, respectively),12 which indicates some swelling of the lamellae by solvent. This swelling is reminiscent of behavior observed for inorganic clay platelets that, when decorated with surfactants, can be swollen by organic solvents with retention of lamellar ordering.19 Related swelling behavior also has been observed for clays swollen by water.20 Although the SAXS data, which exhibit only a single diffraction peak, do not (16) The following solvents were tested and did not exhibit gel formation: hexane, octane, dodecane, tetradecane, hexadecane, hexanol, ethylene glycol, 2-chlorotoluene, benzyl alcohol, benzaldehyde, aniline, bromobenzene, chlorobenzene, pyridine, and dimethylnaphthalene. (17) νN-H ) 3419 cm-1 for guanidine hydrochloride, in which the guanidinium ions do not participate significantly in intermolecular hydrogen bonding. (18) Russell, V. A. Hydrogen Bonding and Control of Molecular Packing in the Organic Solid State with Implications for Materials Design. Ph.D. Thesis, University of Minnesota, Minneapolis, 1995; p 271. (19) Ho, D. L.; Briber, R. M.; Glinka, C. J. Chem. Mater. 2001, 13, 1923.

Lyotropic Phases Reinforced by Hydrogen Bonding

allow for an unambiguous assignment of microstructure, the absence of higher-order peaks in lamellar gels is not unusual.21 For example, large undulation fluctuations can destroy the long-range order to the extent that higher order reflections become difficult to observe.22 The observation of optical birefringence rules out a spherical (body-centered cubic) microstructure, but a hexagonal phase, which would be expected to exhibit birefringence, cannot be excluded on the basis of the SAXS data alone. Attempts to assign the microstructure using simple scaling laws (d spacing vs 1/volume fraction of the gelator at a fixed temperature) were inconclusive, which suggested that the d-spacing changes were associated with a complex temperature-dependent uptake of solvent (see below) rather than an ideal, uniform swelling. Nonetheless, when combined with the marked tendency of the GS sheet to enforce lamellar structures in the solid and smectic liquid crystal state, the optical birefringence, the large lamellar slabs in freeze-dried specimens, and the FT-IR data, which are a fingerprint of the quasihexagonal GS sheet, the assignment of a lamellar LR phase seems most reasonable. Wide-angle X-ray scattering measurements did not reveal any diffraction peaks at higher 2θ values (large q) that would signify crystalline order like that observed in the aforementioned solvent-free crystalline analogues of these compounds. This is not surprising, as disorder of the loosely packed organosulfonate chains and solvent is to be expected in the gel state, as observed for other hydrogenbonded gels.23 The interlayer d spacing of the GCnBS lamellar phases, measured by SAXS, exhibited a marked dependence on temperature. At T < 30 °C, d ≈ 40 Å for GC11BS in p-xylene with 6 wt% e [GC11BS] e 40 wt% (Figure 4). As the temperature was increased above 30 °C, the d spacings for [GC11BS] < 40 wt% increased, with a noticeable increase in the peak widths. The magnitude of the temperature dependence of d increased with decreasing [GC11BS]. At [GC11BS] ) 6 wt%, the lowest concentration examined, d increased from 40 to 70 Å when heated from 30 to 90 °C. This was accompanied by a substantial broadening of the diffraction peak, suggesting either a broader distribution of the interlamellar spacings or smaller coherence length for the lamellar ordering. In contrast, the change in d with temperature was negligible for [GC11BS] ) 40 wt% over the same temperature range (higher concentrations were not examined because large viscosities prevented preparation of uniform mixtures). The increase in the volume of the GC11BS lamellae with temperature was larger than that expected solely from the increase in the molar volume of the solvent; whereas the lamellar volume increased by 75% as the temperature was increased from 30 to 90 °C (estimated from the change in d spacing) the anticipated increase in the molar volume of p-xylene is only 6.4%.24 Therefore, the primary cause for the increase in d spacing can be assigned to increased absorption of solvent by the GC11BS lamellar phases (Figure 4, inset). At low temperatures, the gels consisted of lamellar domains in equilibrium with solvent trapped between the domains. As the temperature was increased, the lamellar domains absorbed more solvent with a (20) Ramsay, J. D. F.; Swanton, S. W.; Bunce, J. J. Chem. Soc., Faraday Trans. 1990, 86, 3919. (21) Warriner, H. E.; Keller, S. E.; Idziak, S. H.; Slack, N. L.; Davidson, P.; Zasadzinski, J. A.; Safinya, C. R. Biophys. J. 1998, 75, 272. (22) Mahjoub, H. F.; Bourgaux, C.; Sergot, P.; Kleman, M. Phys. Rev. Lett. 1998, 81, 2076. (23) Wang, G.; Hamilton, A. D. Chem. Eur. J. 2002, 8, 1954-1961. (24) Daubert, T. E.; Danner, R. P. Data Compilation Tables of Properties of Pure Compounds; Design Institute for Physical Property Data, American Institute of Chemical Engineers: New York, 1985.

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Figure 4. (A) The dependence of the interlayer spacing, d, as measured by SAXS, on temperature and GC11BS weight fraction in mixtures of GC11BS and p-xylene. The increase in d with temperature suggests a swelling of the lamellae by excess solvent (green circles), an effect that decreases with increased amounts of surfactant. (B) Representative data illustrating the shift in the single Bragg peak for two different concentration of the GC11BS organogelator. In this particular data set for 6 wt% GC11BS, a small peak remains at the higher 2θ value due to a portion of the gelator that is not swollen. This is not always observed, and it can be attributed to slow kinetics of solvent absorption by the lamellae. (C) Schematic of the solvent-induced swelling of the GCnBS lamellar phase as the temperature is increased.

concomitant increase in the interlayer spacing (d). As the concentration of amphiphile was increased, the amount of solvent available relative to the lamellar phase decreased, decreasing the effect of temperature on d. For example, at [GC11BS] ) 40 wt%, d did not increase as the temperature was raised, indicating that all the available solvent had been absorbed by the lamellar phase at 25 °C. Whereas the increase in d spacing with decreasing gelator concentration and increasing temperature is consistent with swelling of a lamellar LR phase, this combination eliminates the possibility of a bicontinuous lamellar L3 “sponge” phase, which would be insensitive to temperature.25 Inspection of the SAXS data for [GC11BS] ) 40 wt% actually reveals two diffraction peaks (see Supporting Information), one corresponding to the swollen lamellar phase with d ) 40 Å and the other to the initial lessswollen lamellar phase with d ) 31 Å. While this behavior is common for for [GC11BS] ) 40 wt%, it occurs only occasionally for lower gelator concentrations, its appearance varying sample-to-sample. For example, for the particular [GC11BS] ) 6 wt% sample in Figure 4, a small scattering peak at the initial 2θ value persists as the temperature is increased (similar behavior was exhibited by the [GC11BS] ) 15 wt% sample including in the Supporting Information). The persistent peak at higher 2θ can be attributed to kinetic effects associated with swelling, which can be difficult to reproduce because the samples were simply hand-mixed. Nonetheless, this is always observed for [GC11BS] ) 40 wt%, which is to be expected as the amount of solvent, relative to the amount of gelator, is reduced. (25) Gomati, R.; Bouguerra, N.; Gharbi, A. Physica B 2001, 299, 101.

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The length of the undecylbenzene residue is approximately 24 Å (assuming an all-trans configuration); therefore, interdigitation of the alkyl chains is not possible for d > 48 Å, a condition that is achieved for gelator concentrations e20% above some temperature. Although the swelling of surfactant-modified clays by organic solvents has not been examined extensively, the lamellar spacing in xylene-swollen montmorillonite does not exceed the value required for interdigitation of tallow surfactant chains on opposing clay platelets. In this respect, the retention of the lamellar architecture in the GC11BS organogels at d spacings exceeding the interdigitation limit appears to be unusual for organogels. Large d spacings for LR hydrogels, however, are well established and have been attributed to lamellae with low bending moduli that become stabilized by undulation.26-28 It is reasonable to suggest that similar effects may be operative for the GCnBS organogels, given the established compliance of the GS sheet and its ionic character. Electrostatic stabilization also has been invoked for LR systems.29 Regardless of the underlying contributions, the retention of lamellar ordering at large d spacings is consistent with GS sheets that are sufficiently large to sustain lamellar ordering. Using the d spacing measured at 90 °C for 6% GC11BS, a rudimentary calculation of the Hamaker constant30 for opposing GC11BS sheets separated by p-xylene suggested that the interaction energy between the sheets, assuming sheet areas of 104 nm2, was comparable to kT. Large sheet areas are not unexpected, as the edge energy of the GS sheet is probably significant. Indeed, crystalline salts of these compounds tend to adopt habits in which the faces exposing the edge of the sheet are small. Furthermore, previous studies in our laboratory revealed that smectic GCnBS compounds, including GC11BS, formed large-area smectic domains, either alone or when blended with polymers.31 The rheology of the organic gels was characterized using oscillatory and steady, unidirectional shear. Oscillatory shear experiments on a sample consisting of 10 wt% GC11BS in p-xylene resulted in curves of elastic (G′) and loss (G′′) moduli vs oscillatory frequency (ω) in which G′ was greater than G′′ over the range of frequencies between ω ) 10-2 s-1 and ω ) 4.64 s-1 (Figure 5), indicating a solidlike response to small strains at low frequencies. This behavior is characteristic of gel-like systems. The response of samples containing GC11BS in p-xylene to the startup of steady, unidirectional shear was examined (26) Helfrich, W. Z. Naturforsch A 1978, 33, 305. (27) Safinya, C. R.; Sirota, E. B.; Roux, D.; Smith, G. S. Phys. Rev. Lett. 1989, 62, 1134. (28) Safinya, C. R.; Roux, D.; Smith, G. S.; Sinha, S. K.; Dimon, P.; Clark, N. A.; Bellocq, A. M. Phys. Rev. Lett. 1986, 57, 2718. (29) Roux, D.; Safinya, C. R. J. Phys. (France) 1998, 49, 307. (30) The interaction energy per surface area between two parallel planes separated by a distance D is defined as W/area ) (-A)/(12πD2), where A is the Hamaker constant. Hamaker constants were calculated using previously reported values of the dielectric constant (e), index of refraction (n) and UV absorption frequency (ne) for p-xylene (see Lide, D. R. Handbook of Chemistry and Physics, 74th ed.; CRC Press: Boca Raton, FL 1994) and estimated values of (e),(n), and (ne) for the opposing GS sheets. The Hamaker constant for the interaction of two similar objects (designated as subscript 1) separated by an different intervening material (subscript 2) is A ) (3/4)kT[(e1 - e2)/(e1 + e2)]2 + (3hνe)/(16x2) [(n12 - n22)2/(n12 + n22)3/2]; see Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: San Diego, CA, 1991. The Hamaker constant for GS sheets separated by p-xylene is A ) 1.02 × 10-21 J at 90 °C. The interlayer spacing at 90 °C for 6 wt% GC11BS in p-xylene was 70 Å, leading to an interaction energy per area W/area ) -5.5 × 10-27 J/Å2. If a lamellar sheet is assumed to have dimensions of 100 nm × 100 nm (area ) 106 Å2), the interaction energy between opposing lamellae would be W ) -5.5 × 10-21 J. This value is comparable to kT(90 °C) ) 5.01 × 10-21 J. (31) Yonezawa, Y.; Martin, S. M.; Macosko, C. W.; Ward, M. D. Macromolecules 2004, 7, 6424.

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Figure 5. Elastic (G′, 0) and loss (G′′, O) moduli for a 10% mixture of GC11BS in p-xylene at 25 °C. G′ is greater than G′′ over a large range of frequencies (the crossover occurs at 4.64 s-1), indicating gel-like behavior for small strains.

Figure 6. The response of samples, viscosity (η) vs time (t) containing GC11BS in water to the startup of steady, unidirectional shear. (A) [GC11BS] ) 10 wt% sample in water sheared at a shear rate of γ ) 10-1 s-1 and T ) 25 °C twice in succession exhibited severe hysteresis. (B) [GC11BS] ) 10 wt% and [GC11BS] ) 5 wt% samples in water sheared at γ ) 10-1 s-1 and T ) 25 °C exhibited a strong dependence of η on concentration. (C) [GC11BS] ) 10 wt% sample in water sheared at different strain rates (γ ) 10-1 s-1, γ ) 1 s-1, and γ ) 10 s-1) and T ) 25 °C exhibited shear thinning behavior. (D) [GC11BS] ) 10 wt% sample in water sheared at γ ) 10-1 s-1 and T ) 25 °C exhibited a slight increase in plateau viscosity (ηplateau) with temperature and a decrease in the magnitude of the viscosity overshoot.

for [GC11BS] ) 5 wt% and [GC11BS] ) 10 wt% in the range 25 °C e T e 55 °C. In general, samples subjected to steady, unidirectional shear exhibited a sharp increase in the viscosity (η) immediately after the startup of shear, eventually reaching a plateau value at long times (Figure 6). The η overshoot, defined as maximum value of η divided by the plateau value (ηplateau), and the magnitude of ηplateau, depended on test conditions. Mixtures with [GC11BS] ) 10 wt% in p-xylene sheared at a strain rate of γ ) 10-1 s-1 at T ) 25 °C produced a curve of η with a small overshoot followed by a steady decline to ηplateau ≈ 2 × 103 Pa‚s (Figure 6a). Shearing was halted, and the sample was allowed to rest for 1 min, after which, the sample was again subjected to startup of steady shear at a γ ) 10-1 s-1. The next trace of η produced a larger overshoot but a lower ηplateau ≈ 6 × 102 Pa‚s. Further shear cycles

Lyotropic Phases Reinforced by Hydrogen Bonding

Figure 7. Phase diagram depicting the solution behavior of GC11BS in water (temperature vs concentration). For c ) 2 and 5 wt%, the GC11BS precipitated from a nonviscous solution at a temperature between 30 and 40 °C. The solutions with c ) 10, 15, and 20 wt% became viscous and formed a transparent gel-like phase upon cooling from 70 to 45 °C. The GC11BS precipitated slowly from the gel-like phase at a temperature below 30 °C. The phase boundaries depicted are estimations only.

reproduced the second η curve. The difference between the η curves produced by the initial shearing and subsequent shearing steps was attributed to a shearinduced alignment of the lamellar domains, similar to the behavior of the thermotropic smectic liquid crystalline phases formed by GCnBS.12 The initial trace of η could be reproduced by separating and resetting the rheometer plates, as this process regenerates disorder of the domains. The response of samples containing GC11BS in p-xylene to the startup of steady shear exhibited a significant concentration dependence. The overall shape of the η curve remained the same, but the magnitude of η varied with ηplateau ≈ 6 × 102 Pa‚s for [GC11BS] ) 10 wt% and ηplateau ≈ 1 × 102 Pa‚s for [GC11BS] ) 5 wt% at T ) 25 °C (Figure 6b). The gels also exhibited strong shear thinning with ηplateau ≈ 6 × 102 Pa‚s at γ ) 10-1 s-1 decreasing to ηplateau ≈ 2 × 101 Pa‚s at γ ) 10 s-1 for [GC11BS] ) 10 wt% in p-xylene at T ) 25 °C (Figure 6c); similar behavior was observed for [GC11BS] ) 5 wt% in p-xylene. The steady shear response of the GC11BS samples exhibited only a slight temperature dependence. The overshoot, apparent at 25 °C in the [GC11BS] ) 10 wt% in p-xylene sample, was less distinct at T ) 35 and 55 °C (Figure 6d). The viscosity of the samples increased slightly with T from ηplateau ≈ 6 × 102 Pa‚s at T ) 25 °C to ηplateau ≈ 8 × 102 Pa‚s at T ) 35 °C, consistent with an increase in the volume fraction of the lamellar phase due to increased absorption of solvent. As the volume fraction of the lamellar domains increased with temperature, the interactions between the domains increased, leading to an increase in viscosity. GCnBS Gels in Water. GCnBS compounds with n g 10 exhibited the ability to form gel-like phases when mixed with water. In contrast to the organic gels described above, the aqueous gels were transparent and did not exhibit optical birefringence. At high temperatures (T ) 90 °C), the GCnBS compounds with n < 14 exhibited clear solutions with low viscosities for [GCnBS] ) 2-20 wt%. The samples containing GCnBS with n g 10 exhibited complex phase behavior that depended on temperature, concentration, and n. A representative example is GC11BS (Figure 7), for which a solution with low viscosity predominated at high temperatures over the entire range of concentrations (2 wt% e [GC11BS] e 20 wt%). When [GC11BS] ) 2-5 wt%, GC11BS precipitated from the lowviscosity solution upon cooling below 40 °C. When [GC11BS] ) 10-20 wt%, however, the viscosity increased and the solutions eventually gelled as they were cooled from 70 to 45 °C. GC11BS precipitated slowly from the

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Figure 8. Phase diagram depicting the solution behavior of [GCnBS] ) 10 wt% in water (temperature vs alkyl chain length n). The phase boundaries depicted estimations only.

gel-like phases when the temperature was decreased below 30 °C. No agitation was required to induce gel formation or precipitation. It is important to note that the sodium salt of dodecylbenzenesulfonate (NaC12BS) exhibited neither viscosification nor gel formation, only forming clear solutions with low viscosities at high temperatures (40 °C e T e 90 °C) for all concentrations examined ([NaC12BS] e 20 wt%). NaC12BS precipitated at temperatures below 40 °C. This indicates that the viscosification and gelation exhibited by GCnBS compounds in water is mediated by hydrogen-bonding interactions between the G ion and the sulfonate amphiphile. The phase behavior of the GCnBS compounds in aqueous solution depended not only on the concentration in solution but also on the length of the alkyl substituent. For [GCnBS] ) 10 wt%, the viscosity increased with chain length, n (Figure 8). Gel phases were observed for n ) 11 and 12 between 30 °C e T e 45 °C and for n ) 16 at T g 45 °C. The temperature at which precipitation occurs from the gel increases with n from T ≈ 25 °C for n ) 11 and 12 to T ≈ 40 °C for n ) 14 and 16. The rheological behavior of GC11BS in water was examined in order to elucidate the lyotropic structures formed in solution. Oscillatory shear with [GC11BS] ) 10 wt % and T ) 50 °C revealed a nearly constant value of G′ for ω > 1 s-1, which decreased with decreasing frequency for ω < 1 s-1 (Figure 9a). In contrast, G′′ exhibited a maximum at ω ≈ 1 s-1, and G′ was greater than G′′ for ω > 1 s-1. This behavior is in reasonable agreement with a single-relaxation-time Maxwell model with a time constant τ ) 1 s. Similar response to oscillatory shear has been reported for amphiphilic solutions containing entangled networks of long wormlike micelles.32 As the sample temperature was decreased the crossover point at which G′ became greater than G′′ occurred at lower frequencies until crossover was no longer observed over the range of frequencies tested (Figure 9c and 6d), indicating that the samples reacted like solids to small strains at low temperatures (T e 30 °C). The response of [GC11BS] ) 10 wt% in water to the startup of steady unidirectional shear at 50 °C was measured over the range of strain rates 0.05 s-1 e γ˘ e 5 s-1 (Figure 10). At low strain rates (γ˘ ) 0.05 s-1), η increased rapidly upon startup of shearing and reached a plateau value of ηplateau ) 200 Pa‚s. As the strain rate was increased, the plateau value decreased (ηplateau ) 20 Pa‚s at a strain rate of 5 s-1), indicating that the fluid was (32) Larson, R. G. The Structure and Rheology of Complex Fluids; Oxford University Press: New York, 1999; p 565.

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Figure 9. Elastic (G′, 0) and loss (G′′, O) moduli vs oscillatory frequency (ω) for a 10% mixture of GC11BS in water at 50, 40, 30, and 25 °C. The fluid exhibits a Maxwellian response (single relaxation time) at 50 °C. As the temperature is decreased, the response to shear becomes more gel-like until the G′ is greater than G′′ over the entire range of frequencies measured. A schematic of the putative wormlike micelle is depicted in the 40 °C panel. The blue and gray spheres are intended to represent guanidinium ions and sulfonate headgroups, respectively.

Martin and Ward

be obtained for a sample containing [GC11BS] ) 2 wt% in water. The micrographs revealed of spherical micelles with a diameter of D ≈ 50 Å, which is consistent with the expected value for spherical micelles formed by GC11BS amphiphiles with a length L ≈ 23 Å. Several of the micrographs also indicated the presence of large (≈5000 Å) crystallites of GC11BS in coexistence with the micellar solution, presumably due to precipitation as the sample was cooled during sample preparation. The SAXS patterns of samples containing varying concentrations of GC11BS and GC10BS were measured over the range of temperatures between 25 °C e T e 90 °C. Gels formed by GC14BS and GC16BS were not examined. Below T ) 30 °C, the GCnBS compounds precipitated and low-angle Bragg peaks appeared that were assignable to the crystalline forms of the GCnBS compounds. The SAXS patterns of samples containing [GC10BS] ) 10 wt% and [GC11BS] ) 10 wt% at T ) 50 °C exhibited a decrease in scattered intensity (I) in the low-q regime, and a broad peak between q ) 0.1 Å-1 and q ) 0.2 Å-1. The peaks corresponded to spacings of d ) 44 Å for the [GC10BS] ) 10 wt% solution and d ) 47.5 Å for the 10 wt% GC11BS solution. The shape and location of the peaks remained constant in the range 30 °C e T e 90 °C, exhibiting only a slight decrease in peak intensity at high T. The peak shape and position remained constant for concentrations as low as [GC11BS] ) 1 wt%. The broad peaks observed in the SAXS patterns of aqueous samples containing GC10BS and GC11BS were likely due to the structure factor contribution to the scattered intensity (i.e., due to the interactions between surfactant structures in solution). The scattering profile, however, did not conform to the Guinier approximation for long cylindrical micelles at low q, where I(q)q ∝ -q2. Nonetheless, the rheological response is consistent with the existence of an entangled network of extended micelles. Although speculative, the data suggest that the broad peaks in the SAXS pattern are due to diffraction from small clusters or bundles of micelles with inter-micellar distances d ≈ 43.6 Å for GC10BS and d ≈ 47.5 Å for GC11BS. These values are consistent with the size of the spherical micelles, which would presumably correspond to the diameter of wormlike micelles formed by these amphiphiles. This assignment, however, is not conclusive. Conclusion

Figure 10. Viscosity (η) vs time (t) for a 10% solution of GC11BS in water at 50 °C measured upon the startup of steady, unidirectional shear at various shear rates. The fluid exhibits significant shear-thinning, as well as a viscosity overshoot that increases as the shear rate is increased.

strongly shear-thinning. The increasing strain rate also caused a change in the response to startup. Rather than increasing sharply and immediately achieving a plateau value, η increased rapidly and overshot the plateau value before decreasing again. The magnitude of the overshoot increased with the strain rate. This rheological responses shear thinning and overshoot on startup of steady shears is reminiscent of the behavior exhibited by entangled polymer solutions and solutions containing entangled wormlike micelles.33 Due to limitations caused by the increased viscosity of the high-concentration samples ([GC11BS] g 5 wt%), cryogenic transmission electron micrographs could only (33) Larson, R. G. The Structure and Rheology of Complex Fluids, Oxford University Press: New York, 1999; p 566.

The amphiphilic nature of the GCnBS compounds, combined with their ability to form a robust network of intermolecular hydrogen bonds, produces lyotropic phases in both organic and aqueous solution. The tendency for low concentrations of GCnBS to form swollen lamellar phases in cyclic organic solvents speaks to the ability of the GS network to stabilize large sheetlike structures, while the increasing solvent uptake with temperature indicates the importance of favorable interactions between the organic solvent and the organic alkylbenzene moieties. Such inclusion behavior has already been demonstrated for GS compounds in the solid state.34 The aqueous phases formed by the GCnBS compounds exhibited a significant increase in viscosity due to the formation of micellar structures in solution. While the SAXS results are inconclusive with respect to the specific type of micelles formed, the rheological characteristics of the fluids indicate the existence of an entangled network of extended micelles (34) Horner, M. J.; Ward, M. D., manuscript in preparation.

Lyotropic Phases Reinforced by Hydrogen Bonding

formed in the absence of any excess salt in solution. The formation of these extended micellar phases is dependent on the presence of the guanidinium ion, demonstrating the strong influence that intermolecular hydrogen bonding can have on the phase behavior and micellar structure of amphiphilic molecules in both organic and aqueous media. The high gelator concentrations required for gelation of organic solvents and the precipitation of crystals upon cooling to room temperature in aqueous gels limits the applications of these materials. Nonetheless, these studies illustrate the ability to influence the formation and dimensionality of surfactant microstructures through the introduction of specific noncovalent intermolecular interactions.

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Acknowledgment. This work was supported by the National Science Foundation (DMR-0305278) and in part by the MRSEC Program of the National Science Foundation under Award No. DMR-0212302. S.M.M. gratefully acknowledges the financial support provided by a Sundahl Fellowship. We thank Jingshan Dong for his assistance with cryo-TEM imaging. Supporting Information Available: Small-angle Xray scattering data for GC11BS/p-xylene gels corresponding to the concentrations and temperatures in Figure 4. This material is available free of charge via the Internet at http://pubs. acs.org. LA050393L