Fibers and Other Aggregates of ω-Substituted Surfactants - American

Langmuir 1997, 13, 5563-5569

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Fibers and Other Aggregates of ω-Substituted Surfactants David A. Jaeger,* Guowen Li,1a Witold Subotkowski,1b and Keith T. Carron Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071

Michael W. Bench Center for Interfacial Engineering, University of Minnesota, Minneapolis, Minnesota 55455 Received February 28, 1997. In Final Form: July 23, 1997X The aggregation in dilute aqueous solutions of several ω-substituted single-chain quaternary ammonium surfactants has been studied by Krafft temperature and critical aggregation/micelle concentration measurements and by cryo-transmission electron microscopy: (16-carboxyhexadecyl)trimethylammonium bromide (1a); (16-methoxycarbonylhexadecyl)trimethylammonium bromide (1b); (17,18-dihydroxyoctadecyl)trimethylammonium bromide (1c). Octadecyltrimethylammonium bromide (2) was included for comparison. The aggregation of 1a was also studied by Raman spectroscopy. Surfactant 1a at pH 6.8 forms ribbonlike fibers with lengths of up to 1.2 µm and cross sections of 3-4 nm × 12-16 nm; at pH 2.2, ribbon to rodlike fibers; and at pH 11.5, wormlike aggregates. Surfactants 1b and 1c at pH 6.8 form rod-shaped aggregates. The Raman spectroscopy results indicated that 1a is not in a fully extended conformation within its fibers at pH 6.8. Tentative structural models have been proposed for the aggregates of surfactants 1.

Introduction The formation of fibers from amphiphiles in aqueous solution is the subject of considerable current interest,2,3a in the broad context of the relationship between amphiphile structure and aggregate morphology.3b,4 Several structural features2,3a,5,6 have been used to organize amphiphiles into fibers, especially hydrogen bonding among secondary amide groups.2,3a,5 Herein we report a study of the use of electrostatic and hydrogen bonding interactions among quaternary ammonium and carboxyl and carboxylate groups for the formation of fibers. In particular, this study involves a characterization of the aggregates formed by ω-substituted single-chain quaternary ammonium surfactants 1 in dilute aqueous solutions. Surfactant 1a contains a carboxyl group, 1b a methoxy-

carbonyl group, and 1c a vic-diol group. Unfunctionalized surfactant 2 was included for comparison. Scheme 1a

a

(a) Me3N, MeCN; (b) KOH, H2O, EtOH; (c) HBr, H2O. Scheme 2a

* To whom correspondence should be addressed. Telephone: 307766-4335. FAX: 307-766-2807. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, October 1, 1997. (1) (a) Visiting scientist on leave from Jilin University, China. (b) Present address: Chemsyn Science Laboratories, Lenexa, KS. (2) For comprehensive reviews, see: (a) Fuhrhop, J.-H.; Helfrich, W. Chem. Rev. 1993, 93, 1565. (b) Fuhrhop, J.-H.; Svenson, S.; Luger, P.; Andre´, C. Supramol. Chem. 1993, 2, 157, and references therein. (3) Fuhrhop, J.-H.; Ko¨ning, J. Membranes and Molecular Assemblies: The Synkinetic Approach; Royal Society of Chemistry: Cambridge, England, 1994: (a) Chapter 5; (b) Chapter 3. (4) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York 1992; Chapter 17. (5) For examples, see: (a) Fuhrhop, J.-H.; Spiroski, D.; Boettcher, C. J. Am. Chem. Soc. 1993, 115, 1600. (b) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812. (c) Imae, T.; Takahashi, Y.; Muramatsu, H. J. Am. Chem. Soc. 1992, 114, 3414. (d) Newkome, G. R.; Baker, G. R.; Arai, S.; Saunders, M. J.; Russo, P. S.; Theriot, K. J.; Moorefield, C. N.; Rogers, L. E.; Miller, J. E.; Lieux, T. R.; Murray, M. E.; Phillips, B.; Pascal, L. J. Am. Chem. Soc. 1990, 112, 8458. (e) Frankel, D. A.; O’Brien, D. F. J. Am. Chem. Soc. 1991, 113, 7436. (f) Fuhrhop, J.-H.; Blumtritt, P.; Lehmann, C.; Luger, P. J. Am. Chem. Soc. 1991, 113, 7437. (6) (a) Okahata, Y.; Kunitake, T. J. Am. Chem. Soc. 1979, 101, 5231. (b) Fuhrhop, J.-H.; Bindig, U.; Siggel, U. J. Am. Chem. Soc. 1993, 115, 11036. (c) Komatsu, T.; Yamada, K.; Tsuchida, E.; Siggel, U.; Bo¨ttcher, C.; Fuhrhop, J.-H. Langmuir 1996, 12, 6242. (d) Menger, F. M.; Lee, S. J. J. Am. Chem. Soc. 1994, 116, 5987. (e) Sakaiguchi, Y.; Shikata, T.; Urakami, H.; Tamura, A.; Hirata, H. J. Electron Microsc. 1987, 36, 168. (f) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474. (g) Yanagawa, H.; Ogawa, Y.; Furuta, H.; Tsuno, K. J. Am. Chem. Soc. 1989, 111, 4567. (h) Arimori, S.; Takeuchi, M.; Shinkai, S. J. Am. Chem. Soc. 1996, 118, 245.

S0743-7463(97)00225-4 CCC: $14.00

a (a) H SO , Me CO; (b) LiAlH , Et O; (c) MeSO Cl, Et N, 2 4 2 4 2 2 3 CH2Cl2; (d) LiBr, THF; (e) HBr, H2O, EtOH; (f) Me3N, MeOH.

Results Syntheses. The syntheses of 1a and 1b are given in Scheme 1. Bromo ester 3, prepared from 17-bromoheptadecanoic acid, was converted into 1b with Me3N in MeCN. Saponification of 1b, followed by acidification, gave 1a. The synthesis of 1c is outlined in Scheme 2. Racemic diol acid 4, obtained from methyl 17-octadecenoate, was converted with H2SO4 and Me2CO into the corresponding ketal acid, which was reduced to yield ketal alcohol 5. This alcohol was then transformed into the corresponding bromo ketal, which was hydrolyzed to give bromo diol 6, followed by its conversion into (()-1c with Me3N in MeOH. Aggregate Characterization. The aggregates of surfactants 1 and 2 in H2O and various buffers were characterized by Krafft temperature (Tk) and critical aggregation/micelle concentration (cac/cmc) measurements and by cryo-transmission electron microscopy (cryo© 1997 American Chemical Society

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Table 1. Critical Aggregation/Micelle Concentrations of 1a-c and 2 compd

mediuma,b

temp, °C

probec

103 cac, Md

1a

pH 1.0 pH 2.2 pH 6.8 pH 11.0 pH 11.5 pH 6.8 pH 6.8 pH 6.8 H2O H2O

87 82 65 82 82 65 65 65 65 65

PER PER DPH DPH DPH DPH DPH DPH DPH PER

1.0 ( 0.1 1.7 ( 0.1 1.2 ( 0.1 1.9 ( 0.1 2.2 ( 0.2 0.72 ( 0.02 1.8 ( 0.1 0.028 ( 0.001 0.27 ( 0.02 0.28 ( 0.01

1b 1c 2

a pH 1, 0.10 M HCl; pH 2.2, HCl/KCl buffer (I ) 0.030); pH 6.8, Tris buffer (I ) 0.030); pH 11.0 and 11.5, carbonate buffers (I ) 0.040 and 0.046, respectively); see Experimental Section for detailed compositions. b Contained 0.01% (v/v) THF. c [PER] ) 5.0 × 10-7 M; [DPH] ) 5.2 × 10-7 M. d Limits of error are average deviations for two runs with different samples.

Figure 2. Cryo-TEM micrograph of 1a (1 wt %) in the pH 6.8 buffer vitrified from 65 °C. The structures in the upper and lower left corners correspond to the lacy support film.

Figure 1. Plot of relative fluorescence vs the log of the concentration of 1a in the pH 6.8 buffer.

TEM). The aggregates of 1a in one of the buffers were also characterized by Raman spectroscopy. Tk and Cac/Cmc Measurements. The Tk values in H2O of 1a-c and 2 are 55, 26, 40, and 37 °C, respectively. The cacs/cmcs of 1a-c and 2 under various conditions, determined by the fluorescence probe method7 with 1,6diphenyl-1,3,5-hexatriene (DPH) and perylene (PER) as probes, are given in Table 1. The break in the plot of relative fluorescence vs the log of the concentration of surfactant corresponds to the cac/cmc. A representative plot for 1a in the pH 6.8 buffer is given in Figure 1. Cryo-TEM. The aggregates of 1a-c and 2, formed under the conditions of the cac/cmc measurements in several of the buffers, were imaged by cryo-TEM.8 The concentration of each surfactant was 1 wt %, which is above its cac/cmc. Solutions of 1a were vitrified from the following temperatures: pH 6.8 buffer, 65 °C; pH 2.2 buffer, 82 °C; pH 11.5 buffer, 82 °C. Solutions of 1b, 1c, and 2 in the pH 6.8 buffer were uniformly vitrified from 65 °C. The cryo-TEM micrographs of 1a-c and 2 are presented in Figures 2-7. The cryo-TEM micrograph of 1a in the pH 6.8 buffer, Figure 2, contains ribbonlike fibers of irregular curvature (7) (a) Chattopadhyay, A.; London, E. Anal. Biochem. 1984, 139, 408. (b) Goodling, K.; Johnson, K.; Lefkowitz, L.; Williams, B. R. J. Chem. Educ. 1994, 71, A8. (c) Mast, R. C.; Haynes, L. V. J. Colloid Interface Sci. 1975, 53, 35. (8) Vinson, P. K.; Bellare, J. R.; Davis, H. T.; Miller, W. G.; Scriven, L. E. J. Colloid Interface Sci. 1991, 142, 74.

Figure 3. Cryo-TEM micrograph of 1a (1 wt %) in the pH 2.2 buffer vitrified from 82 °C. The three globular features are frost.

with lengths of up to 1.2 µm and nonuniform cross sections of 3-4 nm (thickness) × 12-16 nm (width). Note that the ribbons in Figure 2 are variously oriented with respect to the smaller and larger cross sectional dimensions. The cryo-TEM micrograph of 1a in the pH 2.2 buffer, Figure 3, contains ribbon to rodlike fibers, with lengths of up to ca. 0.25 µm, which in some cases are splayed at one end. Note that a few of the aggregates of 1a in the pH 6.8 buffer exhibit similar splaying (Figure 2). The micrograph of 1a in the pH 11.5 buffer, Figure 4, contains wormlike aggregates, with lengths of up to 30-40 nm and diameters of ca. 4 nm. These aggregates appear to be much less rigid than those of 1a in the pH 6.8 (Figure 2) and pH 2.2 buffers (Figure 3). The cryo-TEM micrograph of 1b in the pH 6.8 buffer, Figure 5, contains rod-shaped aggregates with diameters of ca. 4 nm and lengths of 20-50 nm. The micrograph of 1c in the pH 6.8 buffer, Figure 6, contains rod-shaped aggregates throughout the frozen film with lengths of 20-40 nm and diameters of 4-5.5 nm and larger rod-shaped aggregates adjacent to the lacy support film with lengths of 60-120 nm and diameters of 4-6 nm. The micrograph of 2 in the pH 6.8 buffer, Figure 7, contains spherical micelles with diameters of ca. 4 nm.

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Figure 4. Cryo-TEM micrograph of 1a (1 wt %) in the pH 11.5 buffer vitrified from 82 °C. The structure in the upper right corner corresponds to the lacy support film.

Figure 6. Cryo-TEM micrograph of 1c (1 wt %) in the pH 6.8 buffer vitrified from 65 °C. The vertical structure on the left side corresponds to the lacy support film.

Figure 5. Cryo-TEM micrograph of 1b (1 wt %) in the pH 6.8 buffer vitrified from 65 °C. The three large features are frost.

Figure 7. Cryo-TEM micrograph of 2 (1 wt %) in the pH 6.8 buffer vitrified from 65 °C. The dark structure at the top is the lacy support film, and the two large spherical features are frost.

Raman Spectroscopy. The conformation of the methylene chain of 1a in the pH 6.8 buffer was investigated by Raman spectroscopy, which is known to be sensitive to differences between trans and gauche conformations of carbon-carbon bonds of hydrocarbon chains.9 Raman spectra of 1 wt % 1a in the pH 6.8 buffer, taken over the range of 850 to 1200 cm-1 at temperatures from 40 to 70 °C, are shown in Figure 8. This spectral region is dominated by carbon-carbon stretches. For comparison, spectra of liquid and crystalline hexadecane (HD) taken at 25 and -50 °C, respectively, are shown in Figure 9. An unsuccessful attempt was made to determine the orientation of 1a within its ribbonlike aggregates in the pH 6.8 buffer by electron diffraction. The sample was sensitive to electron beam damage; the ribbons were destroyed at electron doses much lower than needed to obtain diffraction information. Discussion For 1a, in buffers of comparable ionic strength (I ) 0.030 to 0.046) covering the pH range of 2.2 to 11.5, the (9) (a) Okabayashi, H. Z. Naturforsch. 1977, 32A, 1569. (b) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1976, 80, 1462.

lowest cac value (1.2 × 10-3 M, pH 6.8) differs from the highest (2.2 × 10-3 M, pH 11.5) by less than a factor of 2. The cac of 1a decreases on going from pH 2.2 (1.7 × 10-3 M) to pH 1.0 (1.0 × 10-3 M) by less than a factor of 1 /2. This decrease may simply reflect the higher ionic strength (0.10) of the pH 1.0 solution.10 Overall, the cac of 1a varies little with pH. The carboxyl group of 1a should be fully deprotonated at pH 11.5 and undissociated at pH 1.0, and the extent of its deprotonation should increase on going from pH 1.0 to 2.2 to 6.8. The insensitivity of the cac values to the ionization state of the carboxyl group suggests that, within the aggregates of 1a formed at the various pHs, the carboxyl/carboxylate groups are within hydrophilic microenvironments.11 For n-alkyl univalent ionic surfactants, the cmc of a given homologue is typically halved upon the addition of (10) For examples, see: Mukerjee, P.; Mysels, K. J. Natl. Stand. Ref. Data Ser. (U. S. Natl. Bur. Stand.) 1977, NSRDS-NBS 36. (11) Menger, F. M.; Mounier, C. E. J. Am. Chem. Soc. 1993, 115, 12222.

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Figure 8. Raman spectra of 1a in the pH 6.8 buffer as a function of temperature (°C). Peaks for gauche and all-trans conformations are indicated by G and T, respectively.

Figure 9. Raman spectra of liquid (top) and solid (bottom) hexadecane taken at 25 and -50 °C, respectively. See the text for assignments of the indicated peaks.

a methylene group to the hydrocarbon chain.12 Thus the cmc of 7, the unfunctionalized parent surfactant of 1a-c, is estimated to be 5.6 × 10-5 M in the pH 6.8 buffer at 65 °C, based upon the value for 2 under the same conditions (2.8 × 10-5 M, Table 1). In the pH 6.8 buffer at 65 °C, the cac values of the functionalized surfactants 1a, 1b, and 1c are greater than the cmc of 7 by factors of 21, 13, and 32, respectively. These larger values reflect the presence of polar/ionic functional groups within 1a-c, in addition to the quaternary ammonium group found in parent surfactant 7.

The cmc of 2 in H2O at 65 °C, measured with the DHP and PER probes, is 2.7(2.8) × 10-4 M (Table 1), and that of 2 in H2O at 25 °C, measured by surface tensiometry, is 1.5 × 10-4 M.13 The cmc values of alkyltrimethylammonium bromides increase with temperature.14 For example, the cmc of 8 in H2O increases from 0.955 to 1.55 m on going from 25 to 65 °C.14 These comparisons suggest (12) Clint, J. H. Surfactant Aggregation; Blackie: Glasgow, Scotland, 1992; Chapter 5. (13) Padday, J. F. J. Phys. Chem. 1967, 71, 3488. (14) Evans, D. F.; Allen, M.; Ninham, B. W.; Fouda, A. J. Solution Chem. 1984, 13, 87.

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that the DHP and PER probes have accurately reported the cmc value of 2 at 65 °C, and by implication, the cac values of 1a-c. In the Raman spectra of hexadecane (Figure 9), the salient features for the solid are two prominent peaks at 1062 and 1136 cm-1 and for the liquid a prominent peak at 1080 cm-1. The peaks for the former can be assigned to the all-trans (solid) conformation and that for the latter to gauche conformations. Taken together, the spectra indicate that liquid hexadecane contains a mixture of chains in the all-trans conformation and those with gauche conformations. In particular, note the presence of the all-trans peak at 1136 cm-1 in the liquid spectrum. In the Raman spectrum of 1a at 40 °C (Figure 8), the peaks at 1110 and 1136 cm-1, designated T, are assigned to the all-trans conformation of its methylene chain, consistent with assignments for the spectrum of solid hexadecanamide (not shown). Note that the intensities of these two peaks decrease as the temperature increases, becoming negligible at 55 and 70 °C. Also note that a peak at ca. 1080 cm-1, assigned to gauche conformations and designated G in Figure 8, is present at 55 and 70 °C, but is absent or only a shoulder at 40-50 °C. These observations indicate that within its aggregates at 55 and 70 °C, 1a is not in an all-trans conformation. The changes in the conformational character of 1a on going from 40 to 70 °C are indeed associated primarily with the formation of its aggregates, as its Tk value (55 °C in H2O; see above) is exceeded, and not with rotational energy barrier and entropy considerations. Calculations demonstrate that for a carbon-carbon bond within a hydrocarbon chain, the trans/gauche conformation ratio would change by only about 3% on going from 25 to 70 °C due to these considerations alone.15 In the solid state the methylene chain of the related compound 11-aminoundecanoic acid hydrobromide hemihydrate is in an all-trans conformation.16 As noted above, the ribbonlike fibers of 1a in the pH 6.8 buffer in Figure 2 have nonuniform cross sections of 3-4 nm × 12-16 nm and lengths of up to 1.2 µm. Several tentative models are possible for the organization of 1a within these aggregates, within which the carboxyl groups are most likely partially dissociated. The Raman spectroscopy results preclude models (not shown) in which 1a is in a fully extended (all-trans) conformation, including those containing rectangular monolayers/bilayers of 1a stacked one upon another with the long molecular axis of 1a perpendicular to the long axis of the ribbon and those in which the long molecular axis of 1a is parallel to the long axis of the ribbon. In model 1, illustrated in Figure 10a, a rectangular bilayer membrane is formed by back-to-back disposition of 1a in a U-shaped conformation, which places the quaternary ammonium and carboxyl/carboxylate head groups at the two faces of the bilayer, where they are in contact with H2O. The bilayer membranes are stacked one upon another, with the plane of a membrane perpendicular to the long axis of the ribbon. By an inspection of a CPK molecular model, the length of 1a (without Br-) in a U-shaped conformation is ca. 1.4 nm. Thus the 3-4 nm width noted above is consistent with a bilayer of U-shaped 1a. Electrostatic interactions between the quaternary ammonium and carboxylate groups and hydrogen bonding among the carboxyl and carboxylate groups occur both within and between bilayers on the two larger cross sectional surfaces. The stacking of rectan(15) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; Wiley-Interscience: New York, 1994; p 601. (16) Sim, G. A. Acta Crystallogr. 1955, 8, 833.

Fibers and Other Aggregates

Figure 10. Models for surfactant aggregation; see the text for descriptions: (a) model 1; (b) model 2; (c) model 3.

gular bilayers results in hydrophobic interactions among the folded methylene chains both within and between adjacent bilayers but is accompanied by unfavorable exposure of these chains to H2O along the two smaller cross sectional surfaces. The aggregates of 1a in the pH 2.2 buffer (Figure 3) are more complex than those in the pH 6.8 buffer (Figure 2). As noted above, the aggregates in the former are ribbon to rodlike and are in some cases splayed at one end into membrane patches of unknown nature. At pH 2.2, the carboxyl groups of 1a are probably largely undissociated. Thus on going from the aggregates at pH 6.8 to those at pH 2.2, some of the carboxyl-carboxylate hydrogen bonding interactions and quaternary ammonium-carboxylate electrostatic interactions are replaced by weaker carboxyl-carboxyl hydrogen bonding interactions and quaternary ammonium-carboxyl ion-dipole interactions, respectively. Nevertheless, it is likely that the gross ordering of 1a within the ribbons/rods at pH 2.2 is closely related to that in the ribbons at pH 6.8. Thus the above ribbon model 1 (Figure 10a) can be considered here also. To accommodate rods instead of ribbons, the rectangular cross section of model 1 is replaced by the circular cross section of related model 2, illustrated in Figure 10b. In going from model 1 to 2, unfavorable hydrocarbon-H2O contact along the ribbon edges is eliminated. Model 3, illustrated in Figure 10c, is an alternative to rod model 2. Circular bilayers of 1a in a largely, but not completely, extended conformation are stacked one upon another, with the long molecular axis of 1a perpendicular to the long axis of the rod. The rod core would likely contain H2O, consistent with the cac results, which suggest that within all of the aggregates formed at the various pHs the carboxyl/carboxylate groups are in contact with H2O. At least near the outer aggregate-H2O interface, the methylene chains are folded, consistent with the Raman spectroscopy results, to facilitate maximum hydrophobic interactions among them. The disposition of the quaternary ammonium and carboxyl/carboxylate head groups could be either statistical or asymmetric. The former would allow electrostatic interactions between the quaternary ammonium and carboxylate groups and hydrogen bonding among the carboxyl and carboxylate groups both on the surface of a rod and within its core. An asymmetric disposition would place the larger quaternary ammonium head groups on the surface. Models comparable to models 2 and 3 been proposed by Okahata and Kunitake6a for a

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fiber-forming bolaamphiphile with two quaternary ammonium head groups. The wormlike aggregates of 1a at pH 11.5 (Figure 4) are less rigid than the ribbons/rods of 1a at pH 2.2 and 6.8, appearing to be curved/bent along their lengths. As noted above, at pH 11.5, 1a should be fully deprotonated. Thus among the head groups, only quaternary ammonium-carboxylate electrostatic interactions are possible within the aggregates. The less rigid nature of the aggregates at pH 11.5 may result from the lack of carboxyl-carboxylate and carboxyl-carboxyl hydrogen bonds. Models 2 and 3 can be considered here also. Models 2 and 3 can be considered for the ordering of monomers within the rods of 1b. In model 2, both the quaternary ammonium and methoxycarbonyl groups are found at the aggregate-H2O interface, resulting from the U-shaped conformation of 1b. In model 3, the quaternary ammonium and methoxycarbonyl groups are likely localized at the surface of a rod and within its core, respectively, reflecting the substantially greater hydrophilicity of the former. This model corresponds to that of a rod-shaped micelle formed by an unfunctionalized quaternary ammonium surfactant,4 with perhaps the inclusion of some H2O within the center of a circular bilayer cross section. Models 2 and 3 can also be considered for the ordering of monomers within the rods of 1c. In model 2, both the quaternary ammonium and vic-diol groups are found at the aggregate-H2O interface, resulting from the U-shaped conformation of 1b. This model is consistent with the likely folding of (threo) 9 within micelles to place both its quaternary ammonium and vic-diol groups at the micelleH2O interface.17 In model 3, the distribution of the quaternary ammonium and vic-diol groups of 1c is probably more statistical than that of the terminal groups of 1b, given the greater hydrophilicity of the vic-diol group of the former compared to the methoxycarbonyl group of the latter. Of course, the placement of a vic-diol group at the surface of a rod necessitates that of a quaternary ammonium group within the core of the circular bilayer cross section. In this case, it is probable that water is included within the core.

The observation of spherical micelles (ca. 4 nm diameter) for 2 in the pH 6.8 buffer vitrified from 65 °C is consistent with that by cryo-TEM of spherical micelles (5-6 nm diameter) for 8 (1 wt %) in H2O vitrified from ca. 25 °C.8 The smaller diameter for 2 likely reflects a reduced aggregation number at the higher temperature.14 It is interesting to compare the lengths of the aggregates of surfactants 1a-c. The aggregates of 1a are considerably longer in the pH 6.8 buffer than in the pH 2.2 and 11.5 buffers, perhaps due to a number of factors: a more favorable mix of head group interactions at pH 6.8, the lower temperature from which the pH 6.8 buffer was vitrified (65 vs 82 °C), and the different buffer components. The binding forces between the head groups are primarily responsible for the structural integrity of the ribbons/ rods of 1a, which appear to be on the borderline between fluid and solid aggregates, as discussed by Fuhrhop and Helfrich.2 In the pH 6.8 buffer, the ribbons of 1a are decidedly longer than the rods of 1b and 1c. This difference reflects the stronger interactions involving the carboxyl and carboxylate groups of 1a, compared to those (17) Jaeger, D. A.; Sayed, Y. M. J. Org. Chem. 1993, 58, 2619.

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involving the methoxycarbonyl and vic-diol groups of 1b and 1c, respectively. Fuhrhop and co-workers5a have reported the formation of monolayer rods or tubules from single-chain amphiphiles containing one amino acid head group and one primary amine head group, with one or two secondary amide groups, respectively, along the chain connecting the two head groups. In these rods/tubules, the bolaamphiphiles are in all-trans conformations, in contrast to the nonlinear conformation of 1a in its ribbons. Although the dominant structural feature of the rods/tubules is hydrogen bonding among the amide groups,5a some of the interactions among the amino acid and amine head groups should be comparable to those among the carboxyl/ carboxylate and quaternary ammonium head groups of 1a. As noted above, fibers have been formed from a bolaamphiphile with two quaternary ammonium head groups.6a Other examples of fiber formation in aqueous media not involving hydrogen bonding among amide groups include those of Fuhrhop and co-workers6b,c (hydrogen bonding among ammonium and amine groups; association of hydrophobic porphyrin centers), Menger and Lee6d (hydrogen bonding among carboxyl/carboxylate groups), Sakaiguchi and co-workers6e and Miller and coworkers6f (ion pairing between a quaternary ammonium head group and salicylate ion), Yanagawa co-workers6g (hydrogen bonding between nucleic acid bases), and Shinkai and co-workers6h (interactions among boronate ester complexes with saccharides). Summary The Tk values in H2O of surfactants 1 and 2, except for 1b, are decidedly above room temperature. The cac results suggest that, within the aggregates formed by 1a in various buffers over a wide pH range, the carboxyl/carboxylate groups are in contact with H2O. Surfactant 1a at pH 6.8 forms ribbonlike fibers with lengths of up to 1.2 µm and cross sections of 3-4 nm × 12-16 nm; at pH 2.2, ribbon to rodlike fibers; and at pH 11.5, wormlike aggregates. Surfactants 1b and 1c at pH 6.8 form rod-shaped aggregates. Within its fibers at pH 6.8, 1a is not in a fully extended conformation. Tentative structural models have been proposed for the aggregates of surfactants 1, in which their folded chains are arranged in layers perpendicular to the long axes of their fibers/rod-shaped aggregates. Experimental Section General Procedures and Materials. 1H and 13C NMR spectra were recorded in CDCl3 unless noted otherwise with Me4Si and CDCl3 (center line at δ 77.00 ppm relative to Me4Si) as internal standards, respectively, in CD3OD with Me4Si and CD3OD (center line at 49.00 ppm relative to Me4Si) as internal standards, respectively, in 4:1 CDCl3-DMSO-d6 with Me4Si and CDCl3 (center line at δ 77.00 ppm relative to Me4Si) as internal standards, respectively, and in D2O (1a, with (CH2)12 at δ 1.25 as internal standard). All NMR spectra were recorded at 27 °C unless noted otherwise, and J values are in hertz. Krafft temperatures were determined according to a literature procedure.18 1,6-Diphenyl-1,3,5-hexatriene and perylene (Aldrich) were used as received. The following buffers, in addition to 0.10 M HCl (pH 1.0, I ) 0.10) and H2O (HPLC grade), were used in the cac/cmc measurements: pH 2.2, 0.0078 M HCl, 0.022 M KCl (I ) 0.030); pH 6.8, 0.030 M tris(hydroxymethyl)aminomethane (Tris), 0.028 M HCl, 0.0020 M KCl (I ) 0.030); pH 11.0, 0.013 M KHCO3, 0.014 M KOH (I ) 0.040); pH 11.5, 0.013 M KHCO3, 0.020 M KOH (I ) 0.046). Tetrahydrofuran (THF) was distilled from LiAlH4. Extracts were dried over Na2SO4 or MgSO4. All melting points were taken with open capillary tubes and are (18) De´marcq, M.; Dervichian, D. Bull. Soc. Chim. Fr. 1945, 939.

Jaeger et al. uncorrected. Unless noted otherwise, the ratios describing the compositions of mixtures represent relative volumes. Elemental analyses were performed by Atlantic Microlab, Atlanta, GA. Octadecyltrimethylammonium Bromide (2).13 With the procedure used for the synthesis of octadecyltrimethylammonium methanesulfonate from octadecyl methanesulfonate,17 1.1 g (3.3 mmol) of 1-bromooctadecane was converted into crude product, which was recrystallized (5 °C) twice from 1:1 CHCl3-hexane to give 1.0 g (78%) of 2: mp 240-244 °C dec. Methyl 17-Bromoheptadecanoate (3).19 A mixture of 0.50 g (1.4 mmol) of 17-bromoheptadecanoic acid19 (EMKA Chemie), 200 mL of MeOH, and 0.2 mL of concentrated sulfuric acid was refluxed under N2 for 24 h and then rotary evaporated. A solution of the residue in CHCl3 was washed twice with 5% NaHCO3 and then with H2O, followed by rotary evaporation to leave 0.56 g of crude 3. This material was chromatographed on a 20-cm × 1.5cm column of silica gel (J. T. Baker 3405) packed in hexane and eluted with 1:20 EtOAc-hexane to give 0.52 g (97%) of 3: mp 46-47 °C (lit.19 mp 48.5 °C). Anal. Calcd for C18H35BrO2: C, 59.50; H, 9.71. Found: C, 59.51; H, 9.74. (16-(Methoxycarbonyl)hexadecyl)trimethylammonium Bromide (1b). A mixture of 0.51 g (1.4 mmol) of 3 and 30 mL of 2:1 Me3N-MeCN was stirred at 25 °C for 5 d under N2 and rotary evaporated to leave 0.69 g of a solid. This material was chromatographed on a 20-cm × 1.5-cm column of aluminum oxide (J. T. Baker 0537) packed in CHCl3 and eluted with 1:5:5 EtOH-MeCN-CHCl3 to give 0.55 g of product that was recrystallized (10 °C) from 1:1 CHCl3-hexane to yield 0.54 g (93%) of 1b: mp 198-204 °C; 1H NMR (270 MHz) δ 3.67 (s, 3 H, CH3O), 3.58 (m, 2 H, CH2N), 3.47 (s, 9 H, (CH3)3N), 2.30 (t, J ) 7.6, 2 H, CH2CO), 1.76 (m, 2 H, CH2CH2N), 1.61 (m, 2 H, CH2CH2CO), 1.18-1.42 (m + s at 1.25, 24 H, (CH2)12); 13C NMR (67.9 MHz) δ 174.31, 67.03, 53.32, 51.39, 34.07, 29.56, 29.38, 29.30, 29.19, 29.09, 26.13, 24.91, 23.16. Anal. Calcd for C21H44BrNO2: C, 59.70; H, 10.50. Found: C, 59.44; H, 10.56. (16-Carboxyhexadecyl)trimethylammonium Bromide (1a). A mixture of 0.25 g (0.59 mmol) of 1b, 0.28 g (5.0 mmol) of KOH, 20 mL of H2O, and 20 mL of EtOH was refluxed for 2 d and rotary evaporated. To the residue was added 20 mL of H2O, and the pH of the resultant solution was adjusted to 1 with concentrated hydrobromic acid to precipitate crude 1a. This material was collected and washed with Me2CO and then CHCl3 to give 0.22 g (91%) of solid product that was recrystallized (10 °C) from 1:1.5 MeOH-Et2O to give 1a: mp 216.5-217.5 °C; 1H NMR (270 MHz, CD3OD) δ 3.35 (m, 2 H, CH2N), 3.12 (s, 9 H, (CH3)3N), 2.26 (t, J ) 7.6, 2 H, CH2CO), 1.80 (m, 2 H, CH2CH2N), 1.60 (m, 2 H, CH2CH2CO), 1.24-1.43 (m + s at 1.29, 24 H, (CH2)12); 1H NMR (270 MHz, D O, 70 °C) δ 3.23 (m, 2 H, CH N), 3.05 (s, 2 2 9 H, (CH3)3N), 2.28 (br t, J ) 7.6, 2 H, CH2CO), 1.71 (m, 2 H, CH2CH2N), 1.51 (m, 2 H, CH2CH2CO), 1.25 (s, 24 H, (CH2)12); 13C NMR (67.9 MHz, CD3OD) δ 177.80, 67.91, 34.99, 30.72, 30.61, 30.53, 30.42, 30.24, 27.37, 26.10, 23.93; IR (KBr) 1722 cm-1 (CdO). Anal. Calcd for C20H42BrNO2: C, 58.81; H, 10.36. Found: C, 58.79; H, 10.31. (()-(17,18-Dihydroxyoctadecyl)trimethylammonium Bromide (1c). A mixture of 269 mg (0.736 mmol) of 6 and 25 mL of 25% (w/v) Me3N-MeOH was stirred at 25 °C for 5 d and rotary evaporated to leave 0.32 g of a solid. This material was washed with CHCl3 and recrystallized (10 °C) from 1:2 Et2O-MeOH to give 260 mg (83%) of 1c: mp 209-210 °C; 1H NMR (270 MHz, CDCl3-DMSO-d6) δ 3.29-3.59 (m, 5 H, OCH2CHO, CH2N), 3.14 (s, 9 H, (CH3)3N), 1.72 (m, 2 H, CH2CH2N), 1.26 (s, 28 H, (CH2)14); 13C NMR (67.9 MHz, CDCl -DMSO-d ) δ 70.37, 64.98, 51.56, 3 6 31.96, 28.32, 28.09, 27.94, 27.67, 24.78, 24.23, 21.41. Anal. Calcd for C21H46BrNO2: C, 59.42; H, 10.92; N, 3.30. Found: C, 59.35; H, 10.84; N, 3.27. (()-17,18-Dihydroxyoctadecanoic Acid (4).20 By a literature procedure,21 1.45 g (4.89 mmol) of methyl 17-octadecenoate, prepared by the procedure for the corresponding ethyl ester,22 gave a crude product that was recrystallized from EtOAc to yield 1.11 g (72%) of 4: mp 100-101 °C (lit.20 mp 105-106 °C); (19) Hunsdiecker, H.; Hunsdiecker, C. Chem. Ber. 1942, 76, 291. (20) Jennen, A.; Everaerts, F. C. R. Seances Acad. Sci. 1960, 252, 91. (21) Swern, D.; Billen, G. N.; Scanlan, J. T. J. Am. Chem. Soc. 1946, 68, 1504. (22) Richard, M. A.; Deutch, J.; Whitesides, G. M. J. Am. Chem. Soc. 1978, 100, 6613.

Fibers and Other Aggregates 1H NMR (400 MHz, CDCl -DMSO-d ) δ 3.61 (m, 1 H, OCH CHO), 3 6 2 3.55 (m, 1 H, OCHaHbCHO), 3.38 (m, 1 H, OCHaHbCHO), 2.92 (br s, 2 H, 2 OH), 2.26 (t, J ) 7.3, 2 H, CH2CO2), 1.60 (m, 2 H, CH2CH2CO2), 1.19-1.50 (m + s at 1.25, 28 H, (CH2)14); 13C NMR (100.6 MHz, CDCl3-DMSO-d6) δ 175.19, 71.43, 66.13, 33.52, 32.61, 29.04, 28.90, 28.73, 28.59, 28.45, 24.98, 24.25. Anal. Calcd for C18H36O4: C, 68.31; H, 11.47. Found: C, 68.34; H, 11.45. (()-16-(2,2-Dimethyl-1,3-dioxolan-4-yl)hexadecanoic Acid. A mixture of 1.4 g (4.4 mmol) of 4, 1.0 mL of concentrated sulfuric acid, and 70 mL of Me2CO was stirred at 25 °C for 24 h before and 2 h after the addition of 1.0 g of K2CO3. The reaction mixture was filtered and rotary evaporated, and the residue was extracted with two 100-mL portions of cyclohexane. The combined extracts were dried and rotary evaporated to give crude product that was chromatographed on a 20-cm × 2-cm column of silica gel packed in hexane and eluted with 1:400:600 MeOH-EtOAc-hexane to give 1.3 g (82%) of the title compound: mp 68-69 °C; 1H NMR (270 MHz) δ 9.48 (br s, OH), 4.04 (m, 2 H, OCHaHbCHO), 3.50 (m, 1 H, OCHaHbCHO), 2.35 (t, J ) 7.6, 2 H, CH2CO2), 1.64 (m, 2 H, CH2CH2CO2), 1.19-1.58 (m + 3 s at 1.41, 1.36, and 1.26, 34 H, (CH3)2CO2, (CH2)14); 13C NMR (100.6 MHz) δ 179.88, 108.58, 76.16, 69.48, 34.02, 33.55, 29.60, 29.55, 29.53, 29.49, 29.40, 29.22, 29.03, 26.91, 25.72, 24.65. Anal. Calcd for C21H40O4: C, 70.74; H, 11.31. Found: C, 70.82; H, 11.37. (()-16-(2,2-Dimethyl-1,3-dioxolan-4-yl)hexadecanol (5). In standard fashion, 3.79 g (10.7 mmol) of the above ketal acid was reduced with LiAlH4 in Et2O to give crude product, which was chromatographed on a 20-cm × 2-cm column of silica gel packed in hexane and eluted with 2:3 EtOAc-hexane to yield 2.9 g (81%) of 5: mp 60.0-60.1 °C; 1H NMR (270 MHz) δ 4.06 (m, 2 H, OCHaHbCHO), 3.64 (m, 2 H, CH2OH), 3.50 (m, 1 H, OCHaHbCHO), 1.18-1.65 (m + 3 s at 1.25, 1.36, and 1.41, 37 H, (CH3)2CO2, OH, (CH2)15); 13C NMR (100.6 MHz) δ 108.57, 76.20, 69.54, 63.08, 33.61, 32.82, 29.64, 29.51, 29.43, 26.95, 25.76. Anal. Calcd for C21H42O3: C, 73.63; H, 12.36. Found: C, 73.70; H, 12.36. (()-2,2-Dimethyl-4-(16-bromohexadecyl)-1,3-dioxolane. A literature procedure23 was used to convert (100%) 5 into crude (()-16-(2,2-dimethyl-1,3-dioxolan-4-yl)hexadecyl methanesulfonate, which was used without further purification: 1H NMR (400 MHz) δ 4.22 (t, J ) 6.6, 2 H, CH2O), 4.06 (m, 2 H, OCHaHbCHO), 3.50 (m, 1 H, OCHaHbCHO), 3.00 (s, 3 H, CH3SO3), 1.76 (m, 2 H, CH2CH2O), 1.21-1.69 (m + 3 s at 1.25, 1.36, and 1.41, 34 H, (CH3)2CO2, (CH2)14). A mixture of 3.26 g (7.76 mmol) of the crude methanesulfonate, 1.0 g (12 mmol) of LiBr, and 80 mL of THF was refluxed under N2 for 24 h and rotary evaporated. A solution of the residue in CH2Cl2 was washed with H2O, dried, and rotary evaporated to give crude product that was chromatographed on a 20-cm × 2-cm column of silica gel packed in hexane and eluted with 1:10 EtOAc-hexane to yield 2.0 g (67%) of the title compound: mp 43.5-44.0 °C; 1H NMR (270 MHz) δ 4.06 (m, 2 H, OCHaHbCHO), 3.50 (m, 1 H, OCHaHbCHO), 3.41 (t, J ) 6.9, 2 H, CH2Br), 1.85 (m, 2 H, CH2CH2Br), 1.20-1.69 (m + 3 s at 1.25, 1.36, and 1.41, 36 H, (CH3)2CO2, (CH2)15); 13C NMR (67.9 MHz) δ 108.56, 76.17, 69.54, 34.02, 33.60, 32.84, 29.64, 29.53, 29.43, 28.75, 28.17, 26.95, 25.75. Anal. Calcd for C21H41BrO2: C, 62.21; H, 10.19. Found: C, 62.05; H, 10.12. (()-16-Bromo-1,2-octadecanediol (6). A mixture of 1.0 g (2.5 mmol) of the above bromo ketal, 1.0 mL of concentrated hydrobromic acid, and 50 mL of EtOH was refluxed under N2 for 12 h and rotary evaporated. The aqueous residue was extracted twice with 100-mL portions of CHCl3, and the combined extracts were washed twice with H2O, dried, and rotary evaporated to give 1.07 g of crude product, which was chromatographed on a 20-cm × 2-cm column of silica gel packed in CHCl3 and eluted

(23) Crossland, R. K.; Servis, K. L. J. Org. Chem. 1970, 35, 3195.

Langmuir, Vol. 13, No. 21, 1997 5569 with 2:3 EtOAc-CHCl3 to yield 0.67 g (74%) of 6: mp 74-75 °C; NMR (270 MHz) δ 3.69 (m, 2 H, OCHaHbCHO), 4.38-3.49 (m + t at 3.41, J ) 6.6, 3 H, OCHaHbCHO, CH2Br), 1.79-2.02 (m, 4 H, CH2CH2Br, 2 OH), 1.20-1.52 (m + s at 1.26, 28 H, (CH2)14); 13C NMR (67.9 MHz) δ 72.31, 66.86, 34.04, 33.22, 32.84, 29.63, 29.53, 29.42, 28.75, 28.17, 25.52. Anal. Calcd for C18H37BrO2: C, 59.17; H, 10.21. Found: C, 58.98; H, 10.28. Cac/Cmc Measurements. Modified literature procedures7 were followed. A given solution of the probe (DHP, 5.2 × 10-3 M; PER, 5.0 × 10-3 M) in THF (spectral grade) was stored at 5 °C and used for no longer than 3 days. A 5.0-µL portion of the probe solution was diluted to 50 mL with buffer, 0.10 M HCl, or H2O to give a stock solution used in a cac/cmc measurement. The initial surfactant solution for a cac/cmc measurement was prepared by dissolving an appropriate amount of surfactant in 2.00 mL of the stock solution. Solutions with lower [surfactant] were prepared by sequential dilution with the stock solution. Fluorescence measurements were made with a Perkin-Elmer LS-5 fluorescence spectrophotometer. The excitation wavelengths were 358 and 250 nm, and the emission wavelengths 430 and 450 nm for DPH7a,b and PER,7c,24 respectively. The excitation and emission slits were set at 3 and 5 nm, respectively. Measurements were made using a water-jacketed, 1-cm path length quartz cell (NSG Precision Cells, type T-54FL) connected to a circulating water bath. The cell, filled with a probecontaining surfactant solution and thermostated at the desired temperature, was kept in the dark in the spectrophotometer for 15 min before the excitation shutter was opened and the fluorescence intensity measured. Cryo-TEM. The sample preparation procedures were similar to those described previously in detail.8 A controlled environment vitrification system (CEVS) similar to that described by Bellare and co-workers25 was used to produce the thin vitrified samples suitable for cryo-TEM. Prior to freezing by being plunged into liquid ethane, the 1% surfactant solutions in the various buffers were allowed to equilibrate at the appropriate temperature (65 or 82 °C) in the CEVS chamber, which was maintained at 100% relative humidity. The cryo-TEM experiments were performed on a JEOL-1210 transmission electron microscope operated at 120 kV and equipped with a Gatan cryo-transfer specimen holder. The TEM images were recorded at 3-5 µm underfocus to enhance phase contrast. Raman Spectroscopy. Raman spectra were acquired with Kr+ ion laser excitation at 647 nm and liquid nitrogen-cooled CCD detection. The laser was a Spectra Physics 2025 operating at 150 mW, and the CCD detector was a Photometrics system with a TK512 CCD chip. The spectrograph was an ISA HR320 with a 1200-groove ion-etched grating. A Kaiser Supernotch Holographic filter was used to remove the laser line from the signal, and the scattering geometry was 90° with p-polarized light. The Raman spectra were calibrated with an indene standard. The 1a sample (1 wt % in the pH 6.8 buffer) was contained in the thermostated quartz used cuvette used for the cac/cmc measurements. The hexadecane and hexadecanamide samples were contained in a conventional 1-cm × 1-cm fluorescence cuvettte. Solid hexadecane was obtained by cooling with liquid N2 to ca. -50 °C. 1H

Acknowledgment. DAJ and KTC gratefully acknowledge the U. S. Army Research Office and the National Science Foundation (Grant No. OSR-950477), respectively, for the support of this research. LA970225W (24) Ogan, K.; Katz, E.; Slavin, W. J. Chromatogr. Sci. 1978, 16, 517. (25) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87.