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Bolaamphiphilic Single-Chain Bis-Schiff Base Derivatives: Aggregation and Thermal Behavior in Aqueous Solution Xuefen Wang, Yingzhong Shen, Yi Pan, and Yingqiu Liang* Laboratory of Mesocopic Materials and Chemistry and State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China Received July 31, 2000. In Final Form: February 15, 2001 Four single-chain bolaamphiphiles, in which the hydrophobic chain was made of the flexible dodecamethylene unit and the bissalicylideneimine unit, were designed and synthesized. These bolaamphiphiles can form two types of stable ordered monolayer membranes in dilute aqueous solution, linear and curved monolayers (vesicles), which have been characterized by transmission electron microscopy and X-ray diffraction. The aggregate structure and thermal behavior of monolayer membranes from these bolaamphiphiles have been investigated by variable temperature 1H NMR, Fourier transform infrared and UV-vis spectra, and differential scanning calorimetry. The results indicated that the large rigid segments (bissalicylideneimine unit) in the middle stacked densely to establish ordered packing backbones for these bolaamphiphiles to form monolayer membranes, and the alkyl chains packed loosely in dilute aqueous solutions. Also, the problem of these bolaamphiphile monolayers possessing no marked gel-to-liquid crystalline phase transition has been explained from the structural nature of them.
1. Introduction The term “bolaamphiphile” describes a molecule that consists of two hydrophilic headgroups connected by one or two long hydrophobic chains. Bolaamphiphiles can form monolayer membranes instead of bilayer membranes. Kunitake et al. provided the first report on the formation of aqueous monomolecular membranes from such bolaform amphiphiles.1 Since then, a number of laboratories have reported the synthesis and self-assembly of bolaamphiphiles with different headgroups that include acyclic2-10 as well as macrocyclic structures.11,12 Compared to the case for the single-headed amphiphiles, the introduction of a second headgroup generally induces a higher solubility in water, an increase in the critical micelle concentration (cmc), and a decrease in the aggregation number.13 The aggregate morphologies of bolamphiphiles include spheres, tubes, lamellae, large cylinders, small or large disks, and vesicles. Monolayer lipid membranes formed by these bolaamphiphiles provide more intriguing properties than the usual bilayer membranes. For example, strangely, the gel-to-liquid crystal phase transition has not been observed so far in most of the monolayer membranes formed by symmetrical bolaamphiphiles4,7,12,14 * Corresponding author. (1) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1979, 101, 5231. (2) Fuhrhop, J.-H.; Mathieu, J. J. Chem. Soc., Chem. Commun. 1983, 144. (3) Wang, K.; Munoz, S.; Zhang, L.; Castro, R.; Kaifer, A. E.; Gokel, G. W. J. Am. Chem. Soc. 1996, 118, 6707. (4) Liang, K.; Hui, Y. J. Am. Chem. Soc. 1992, 114, 6588. (5) Visscher, I.; Engberts, J. B. F. N. Langmuir 2000, 16, 52. (6) Meglio, C. D.; Rananavare, S. B.; Svenson, S.; Thompson, D. H. Langmuir 2000, 16, 128. (7) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812. (8) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (9) Matsui, H.; Gologan, B. J. Phys. Chem. B 2000, 104, 3383. (10) Duivenvoorde, F. L.; Feiters, M. C.; van der Gaast, S. J.; Engberts, J. B. F. N. Langmuir 1997, 13, 3737. (11) Munoz, S.; Mallen, J.; Nakano, A.; Chen, Z.; Gay, A.; Echegoyen, L.; Gokel, G. W. J. Am. Chem. Soc. 1993, 115, 1705. (12) Fuhrhop, J.-H.; David, H. H.; Mathieu, J.; Liman, U.; Winter, H.-J.; Boekema, E. J. Am. Chem. Soc. 1986, 108, 1785. (13) Nagarajan, R. Chem. Eng. Commun. 1987, 55, 251. (14) Masuda, M.; Vill, V.; Shimizu, T. J. Am. Chem. Soc. 2000, 122, 12327.
Chart 1
except those with hydrophobic chain substituted headgroups (gemini surfactants).5,6 The photochromism of bis-Schiff bases has been studied extensively. It has been confirmed that the appearance of the photochromism is accompanied by intramolecular proton transfer from the hydroxyl group to the imino nitrogen atom, followed by cis-trans isomerization around the carbon-nitrogen double bond.15 Long-chain bis-Schiff base derivatives may have some interesting functions. In the present work, four single-chain bolaamphiphiles I (Chart 1), containing a symmetrical bissalicylideneimine unit with two attached dodecmethylene chains terminated by quaternary ammonium headgroups, were designed and synthesized. Two salicylideneimine units were connected by a rigid chromophore group (oxydiphenylene or phenylene) or a flexible short methylene chain (ethylene or hexamethylene). These bolaamphiphiles present two rigid Schiff base segments and possess strong hydrogen-bond capacity,16,17 which will facilitate formation of ordered monolayer membranes. The aggregate structure and specific properties will be affected by the structural elements of the bolaamphiphiles. Herein, we present a careful investigation on the aggregation and thermal behavior of I by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), temperature-dependent 1H NMR, Fourier transform infrared (FT-IR) and UVvis spectra, and differential scanning calorimetry (DSC). (15) Kawato, T.; Kanatomi, H.; Koyama, H.; Igaraski, T. J. Photochem. 1986, 33, 199. (16) Hoshino, N.; Inabe, T.; Mitani, T.; Maruyama, Y. Bull. Chem. Soc. Jpn. 1988, 61, 4207. (17) Guo, Y.; Wang, F.; Fan, M. Acta Phys.-Chim. Sin. 1991, 7, 593.
10.1021/la001084s CCC: $20.00 © 2001 American Chemical Society Published on Web 04/24/2001
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Also, we elucidate the effect of the structural elements of the bolaamphiphiles on the aggregate structure and morphology, especially to reveal the specific properties of these bolaamphiphile monolayer membranes. 2. Experimental Section Bolaamphiphiles Ia-d were synthesized by condensation of 4-bromododecyloxy-salicylaldehyde and appropriate diamines (a, 4,4′-oxydiphenylenedimine; b, p-phenylenediamine; c, ethylenedimine; d, hexamethylenedimine) (the molecular ratio of aldehyde and diamine is 2:1), followed by quaternization with trimethylamine. The final products were identified by FT-IR (Bruker IFS-66V) and 1H NMR (Bruker Am-300). Ib, as an example, was synthesized by the following procedure. 4-Bromododecyloxysalicylaldehyde. According to a general method,18 2,4-dihydroxy-benzaldehyde (2.76 g, 20 mmol) was dissolved in 30 mL of methanol containing 1.3 g (23 mmol) of KOH and refluxed under an N2 atmosphere for 1 h. Then, this mixture was added dropwise to a stirred methanol solution containing 9.84 g (30 mmol) of 1,12-dibromododecane and refluxed for 20 h under an N2 atmosphere. The solution was concentrated to dryness to give a mixture as a red-brown powder. The product was purified by column chromatography (silica gel GF254; eluent, light petroleum/ethyl acetate/chloroform from 30:1:5 to 30:8:2 (v/v/v)) to give 1.62 g (21%) of 4-bromododecyloxy-salicylaldehyde as a white granular solid. FT-IR (KBr pellet): 3437, 3010, 2920, 2852, 1630, 1577, 1470, 1221, 1189, 1119, 720, and 645 cm-1. 1H NMR (CDCl3, 30 °C, ppm): 1.27 (m, CH2, 18H), 1.79 (m, CH2, 2H), 3.40 (t, CH2Br, 2H), 4.0 (t, OCH2, 2H), 6.41-7.43 (m, C6H3, 3H), 9.70 (s, CHN, 1H), and 11.48 (s, OH, 1H). N,N′-Bis[4-bromododecyloxysalicylaldehyde]-p-phenylenediamine. p-Phenylenediamine (0.27 g, 2.5 mmol) dissolved in ethanol was added to a ethanol solution of 4-bromododecyloxysalicylaldehyde (1.93 g, 5 mmol), and the solution was gently stirred for 6 h in the presence of a small amount of acetic acid at room temperature. The pale yellow solid was filtered off and recrystallized with ethyl acetate. Yield: 82%. FT-IR (KBr pellet): 3453, 2954, 2919, 2850, 1625, 1573, 1472, 1289, and 1198 cm-1. Elem. Anal. Calcd for C44H62N2O2Br2: C, 62.71; H, 7.42; N, 3.32. Found: C, 62.71; H, 7.45; N, 3.28. Then, N,N′bis[4-bromododecyloxysalicylaldehyde]-p-phenylenediamine was quaternized by trimethylamine in ethanol in a sealed ampule at 110 °C for 60 h. Solvent was removed in vacuo, and the precipitates were washed with dichloromethane several times, recrystallized with ethanol/dichloromethane (3:1), and then dried in a vacuum at 50 °C for 24 h to obtain Ib. Yield: 78%. FT-IR (KBr pellet): 3411, 3010, 2922, 2852, 1614, 1472, 1295, 1248, and 1194 cm-1. Elem. Anal. Calcd for C50H80O4N4Br2: C, 62.49; H, 8.39; N, 5.83. Found: C, 62.02; H, 7.98; N, 5.74. Bolaamphiphiles Ia-d were readily dissolved in water to give 10 mM Ia or Ib (translucent) and 20 mM Ic or Id (transparent) dispersions by sonication. TEM was conducted on a JEOL model JEM-200CX instrument by the negative post staining method. Specimens were prepared by applying one drop of the dispersions onto a Cu grid coated with a conductive polymer film and by blotting off excess liquid with filter paper. Subsequently, a drop of 2% aqueous uranyl acetate was placed on each of the grids and immediately drained off. Then, these specimens were kept in a desiccator over silica gel until transfer to the electron microscope. The cast films were prepared by spreading a few drops of the dispersions on CaF2 plates (for FT-IR) or glass ones (for XRD) and slowly drying in atmosphere at room temperature for measurements. FT-IR spectra were recorded on a Bruker IFS 66V spectrophotometer equipped with a temperature control attachment. The film spectrum was obtained by subtraction of the CaF2 spectrum from the sample spectrum. The temperaturedependent FT-IR spectra were measured in the range from 0 to 100 °C. The small-angle XRD patterns of the cast films were measured on a Rigaku model D/max-RA diffractometer. An aqueous sonicated sample, 0.5 mL of 2 wt %, was sealed in a stainless steel pan for DSC measurement, which was performed on a SETARAM microcalorimeter at a heating rate of 1 °C/min. (18) Menczel, J. D.; Leslie, T. M. Thermochim. Acta 1990, 166, 309.
Langmuir, Vol. 17, No. 11, 2001 3163 Samples for 1H NMR were prepared by dissolving four bolaamphiphiles with sonication in deuterium oxide (D2O) to obtain 10 mM Ia and Ib and 20 mM Ic and Id dispersions. All 1H NMR spectra were obtained with a Bruker Avances-300 NMR spectrometer equipped with a temperature control attachment. The temperature-dependent 1H NMR spectra were measured in the range from 20 to 85 °C. The aqueous solution of each bolaamphiphile (1 × 10-4 M) was maintained at 5 °C for 30 min by a water bath after sonication and then subjected to measurements of temperature-dependent absorption spectra using a Shimadzu UV-240 spectrometer equipped with a temperature control attachment.
3. Results and Discussion 3.1. TEM Images of Aggregates. The resulting solutions of four bolaamphiphiles, clear or translucent, are easy to foam by shaking, indicating their surface activity properties. The appearance of these dispersions is also very similar to that of aqueous solutions of lecithin lipsomes such as Kunitake’s bolaamphiphiles.1 Two types of aggregate morphologies can be observed by transmission electron microscopy. Bolaamphiphiles Ia and Ib with bent and linear rigid segments in the middle of the molecule, respectively, produce well-developed linear aggregates as shown in Figure 1, parts A (tubular) and B (binded fibers). The rigid aggregate structure appears to become flexible when the relatively short methylene chain occupies the center of the molecule as in Ic and Id. Ic and Id with a “flexible rigid segment” produce regular spherical vesicles as shown in Figure 1, parts C and D, respectively. These phenomena indicated that the linear aggregate structure can be bent into a vesicle structure by increasing the flexible moiety of the middle rigid segment. This means that the relative short methylene chain as connector at the center of the molecule may promote the flexibility of the molecular chain and create a curvature suitable for vesicle formation. 3.2. X-ray Diffraction of Cast Films. It has been established in many experiments that the self-aggregate structure of amphiphiles in dilute aqueous dispersions has been kept in the water-cast films in various aspects.19-21 Therefore, to obtain the structural features of the aqueous monolayer membranes, the small-angle XRD method can be applied to study the long spacing of the cast monolayer membranes of bolaamphiphiles. Small-angle regions of the X-ray diagrams for Ia-d revealed a series of sharp reflection peaks indicative of ordered structures similar to those of glucosamide bolaamphiphiles7 (Figure 2). According to the Bragg equation, the layer spacings were calculated to be 3.61, 5.30, 3.36, and 3.72 nm for Ia-d, respectively. The molecular lengths of the corresponding bolaamphiphiles were estimated to be about 5.6, 5.1, 5.0, and 5.5 nm for Ia-d, respectively, by the Corey-Pauling-Koltun model. This strongly implied that monomolecular ordered membranes were formed from them in water. In the case of Ia, the molecules are bent at the rigid segment because of the single oxygen atom connection of the two rigid segments; the molecular length of Ia is larger than the layer spacing of the corresponding monolayers. In contrast, Ib with a linear rigid segment allows the molecular chain to be parallel to the monolayer normal, and thus the layer thickness is very close to the corresponding molecular length. As to Ic and Id with the flexible rigid segment, their layer spacings are lower than their corresponding molecular lengths, (19) Lu, X.; Zhang, Z.; Liang, Y. Langmuir 1997, 13, 533. (20) Ishikawa, Y.; Kunitake, T. J. Am. Chem. Soc. 1986, 108, 8300. (21) Shimomura, M.; Aiba, S.; Tajima, N.; Inoue, N.; Okuyama, K. Langmuir 1995, 11, 969.
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Figure 1. The TEM images of I dispersed in water (stained by 2% aqueous uranyl acetate): (A) Ia, (B) Ib, (C) Ic, and (D) Id.
Figure 2. XRD of the cast films of bolaamphiphiles: (A) Ia, (B) Ib, (C) Ic, and (D) Id.
which suggested a tilted monolayer arrangement of the molecular chains. 3.3. 1H NMR Spectroscopy. The restricted molecular motion in synthetic ordered membranes is readily reflected by the line broadening phenomenon in the 1H NMR spectra.8,22-25 In the membranes, the highly ordered molecular lateral stacking makes the movement of amphiphilic molecules anisotropic, thus leading to the reduction in spin-spin relaxation time and the increase (22) Ulmius, J.; Wennerstrom, H. J. Magn. Reson. 1977, 28, 309. (23) Zhang, Z.; Wu, L.; Liang, Y.; Yin, Q. J. Colloid Interface Sci. 1997, 188, 501. (24) Yamada, N.; Koyama, E.; Kaneko, M.; Seki, H.; Furuse, T. Chem. Lett. 1995, 387. (25) Okahata, Y.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 550.
Figure 3. 1H NMR spectra of bolaamphiphiles I in D2O solutions: (A) Ia, (B) Ib, (C) Ic, and (D) Id. (An asterisk designates solvent).
in proton NMR line width. The tighter the packing of membrane-forming molecules, the broader the line width, and vice versa. Therefore, NMR spectroscopy is a sensitive nondestructive technique for studying membrane systems about molecular lateral stacking. Figure 3 shows 1H NMR spectra of bolaamphiphiles I in D2O dispersions at ambient temperature (30 °C). As can be seen, they all gave strong signs of N-methyl (ca. 3.1 ppm) and relatively narrow signals of methylene (1.31.4 ppm), but the signs of salicylideneimine units (6-9 ppm) were almost indiscernible for four bolaamphiphiles. The 1H NMR spectral experiments on bolaamphiphiles I can be clarified satisfactorily by the above explanation. Strong signals of N-methyl indicated that the trimethylammonium headgroups in water had high mobility, relatively narrow signals of methylene illustrated that the alkyl chains were less restricted and packed loosely
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Figure 4. Variable-temperature 1H NMR spectra of Ib in D2O solution. Figure 5. FT-IR spectra in the 3100-2700 and 1700-1100 cm-1 regions of bolaamphiphiles I cast from H2O on CaF2 plates at room temperature: (A) Ia, (B) Ib, (C) Ic, and (D) Id.
to each other, and no signals of salicylideneimine units implied that the large rigid segments were in dense stacking. These results suggested that the large rigid segments stacked densely to establish an ordered packing backbone for bolaamphiphiles to form monolayer membranes in dilute aqueous solution. The variations of 1H NMR spectra of bolaamphiphilic monolayer membranes with temperature were also studied, and drastic changes in proton absorption line width with increasing temperature were observed. As an example, the 1H NMR spectra of Ib in D2O at different temperatures are shown in Figure 4. As can be seen , the NMR peaks became narrower and stronger at elevated temperatures. At low field, no signals were observed at 30 °C. With increasing temperature, weak and broad signals emerged (at 50 °C) and these signals became stronger and narrower (at 80 °C). The gradual change on the NMR signals about salicylideneimine units implied that the densely stacking rigid segments dissolved gradually with temperature. This led to the less restricted molecular motion of the bolaamphiphiles as shown by the NMR signals of the methylene protons, which were enhanced at elevated temperatures. Attention must be paid to the changes in the chemical shift of the NMR peaks with temperature. The NMR peaks shifted downfield with increasing temperature. This phenomenon has also been observed in structural transitions of nonionic peptide aggregates in an aqueous medium,26 probably as a result of the enhancement of the solvent effect. At high temperature, the bolaamphiphiles’ motion is less restricted, and the interaction between the bolaamphiphiles and D2O was enhanced. Thus, the bolaamphiphilic proton must be strongly shifted to a lower field by the paramagnetic deshielding of oxygen atoms of D2O molecules. 3.4. FT-IR Spectroscopy. IR spectroscopy has proved to be a powerful tool to study thermotropic phase transition of surfactant-H2O binary systems at the molecular level.27-29 Because the self-aggregate structure of amphiphiles in dilute aqueous dispersions has been kept in the water-cast films in various aspects,19-21 one can investigate the details of the order-disorder phase transition by monitoring the temperature-dependent alterations in the alkyl chain of the cast films with IR spectroscopy. Figure 5 displays IR spectra of bolaamphiphiles I monolayer membranes-H2O on CaF2 plates at room
temperature. It is known that the frequencies of the bands due to CH2 antisymmetric and symmetric modes (νa(CH2) and νs(CH2)) of the alkyl chain usually appear at ∼2918 and ∼2848 cm-1, corresponding to highly ordered hydrocarbon chains in the gel state with an all-trans conformation; the increases in the frequencies of the two bands indicate the introduction of gauche conformers into the alkyl chains, thus leading them to some disorder of the liquid crystalline state.30,31 According to the result of 1H NMR spectra of bolaamphiphiles I, the alkyl chains were less restricted to display a less ordered alignment, which may produce some gauche conformers in the chain. As shown in Figure 5, even at room temperature the CH2 antisymmetric and symmetric stretching bands appear at 2921-2923 and 2851-2853 cm-1 for Ia-d, which indicates that the methylene chains are certainly in some short-range disorder similar to a liquid crystalline state. In fact, no marked gel-to-liquid crystal phase transition temperature characterized by abrupt changes of νa(CH2) and νs(CH2) frequencies of the methylene chain was observed in variable temperature (0-100 °C) FT-IR spectra of cast films of the four bolaamphiphiles. In addition, the broad absorption peaks at ∼1608 and ∼1616 cm-1 for Ia and Ib, respectively, are assigned to the carbon-nitrogen double bond overlapped with the aryl ring absorption. However, for substituted phenylene or oxydiphenylene groups with the methylene chain group in the center of the rigid segment, only sharp single peaks are observed at 1629 and 1627 cm-1, characteristic of the carbon-nitrogen double bond for Ic and Id, respectively. Compared with the free carbon-nitrogen double bond (∼1640 cm-1), the lower frequencies of the carbonnitrogen double bond absorption peaks in the four bolaamphiphiles indicated that these groups are all hydrogen bonded16 with the adjacent hydroxy group, as illustrated in Chart 1. 3.5. UV-Vis Spectroscopy. UV-vis spectroscopy has been extensively used for the determination of the apparent thermal behavior of ordered aggregates.32,33 So, the temperature dependence of the dispersions probably gives more information about the intriguing thermotropic behavior of monolayer membranes from bolaamphiphiles.
(26) Murugesan, M.; Aulice Scibioh, M.; Jayakumar, R. Langmuir 1996, 12, 5467. (27) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56. (28) Cameron, D. G.; Umemura, J.; Wong, P. T. T.; Mantsch, H. H. Colloids Surf. 1982, 4, 131. (29) Liang, Y.; Li, G.; Zhang, D. Acta Chim. Sin. 1992, 50, 461.
(30) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (31) Du, X.; Shi, B.; Liang, Y. Spectrosc. Lett. 1999, 32, 1. (32) Liang, Y.; Zhang, Z.; Wu, L.; Tian, Y.; Chen, H. J. Colloid Interface Sci. 1996, 178, 714. (33) Liang, Y.; Wu, L.; Tian, Y.; Zhang, Z.; Chen, H. J. Colloid Interface Sci. 1996, 178, 703.
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Figure 7. DSC curves of 2 wt % aqueous solutions of bolaamphiphiles: (A) Ia, (B) Ib, (C) Ic, and (D) Id.
Figure 6. Temperature-dependent absorption spectra of 1 × 10-4 M bolaamphiphiles: (A) Ia, (B) Ib, (C) Ic, and (D) Id.
Figure 6 describes the temperature dependence of the absorption spectra of aqueous bolaamphiphiles Ia-d. Obviously, transition points are observed in dispersions of Ia (∼70 °C) and Ib (∼55 °C) (containing oxydiphenylene or p-phenylene groups between two Schiff base groups) but not in those of Ic and Id (containing flexible methylene chains between two Schiff base groups). The spectra of bolaamphiphiles Ia (Figure 6A) and Ib (Figure 6B) display two bands at 292 and 318 nm and 287 and 365 nm, respectively, which are assigned to the nonplanar and planar conformations of the diphenylazomethine, respectively.32 The spectral difference of Ia and Ib is due to the effect of the different connector between the two Schiff base segments. These two bands are almost independent of the temperature in the range below the transition points. When the temperature is raised above the transition point, the intensities in longer wavelength absorption bands characteristic of the planar conformation reduce markedly with increasing temperature. This indicated that the increased temperature leads to the conversion of the planar to the nonplanar conformation of the chromophore parts. Although the absorption spectra of bolaamphiphiles Ic (Figure 6C) and Id (Figure 6D) display similar spectral shapes, they all give the maximum at 282 nm for Ic and 283 nm for Id, which are characteristic of the monomeric chromophore band due to the substitution of the flexible methylene chain with the rigid oxydiphenylene or phenylene group, and are independent of the temperature. These indicated that no conversion of the chromophore part conformations is present in monolayer membranes of Ic or Id. 3.6. DSC Measurements. The gel-to-liquid crystal phase transition is one of the basic physicochemical properties of bilayer membranes. However, compared to the bilayer membranes, no marked transition point has been observed so far in most of the monolayer membranes formed by symmetrical bolaamphiphiles4,7,12,14 except those with hydrophobic chain substituted headgroups (gemini surfactants).5,6 In general, this is explained with the assumption that the average distance between bolaamphiphiles in curved monolayers is larger than that in bilayers and therefore causes weaker interactions.4,12,34 As a result, a favorable rotation along a single bond in the
methylene chain occurs, and so the transition point is indiscernible. To further illustrate the thermal behavior of bolaamphiphile monolayer membrane systems, the aggregates of Ia-d were examined by DSC. Ia and Ib dispersions possess single endothermic peaks at ∼76 °C (Figure 7A) and ∼61 °C (Figure 7B), respectively. The corresponding enthalpy changes are 9.2 kJ/mol for Ia and 3.4 kJ/mol for Ib; these values are considerably lower than those found for bilayer vesicles of analogous monopolar amphiphiles (∆H, 20-80 kJ/mol)35 and are close to those obtained from nonspherical aggregates of aqueous bolaamphiphilic phosphocholines (∆H,
