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Articles Effect of Counterions on the Organized Structure of Cu2+-Coordinated Bilayer Membranes Formed by Monoalkyl Derivatives of Ethylenediamine Chun Li, Xianchun Lu, and Yingqiu Liang* Lab of Mesoscopic Materials Science and State Key Lab of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China Received June 22, 2001. In Final Form: October 18, 2001 The complexation of monoalkyl derivatives of ethylenediamine, CnH2n+1NHCH2CH2NH2 (n ) 8, 12, 14, 16, and 18), to CuX2 (X- ) Cl- or ClO4-) in dilute aqueous dispersions can form stable synthetic bilayer membranes. The aggregate structure and properties of the bilayer membranes have been investigated thoroughly by small-angle X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, electronic reflectance spectroscopy, and differential scanning calorimetry (DSC). The results confirm a strong influence of the counterions on the organized structure of Cu2+-coordinated bilayer membranes. When X- ) Cl-, the Cu2+-coordinated headgroups adopt a trans configuration, which gives a compressed tetrahedral CuN4 environment, and the two tails in the complex take an approximately diagonal position with tilted tail-to-tail packing fashion in bilayer assemblages. When X- ) ClO4-, a cis configuration is adopted in the square plane of the Cu2+-coordinated headgroups, and the two tails are located at the same side of the plane and parallel to each other, and the interdigitated chain packing model was adopted in the bilayer membranes. The bilayer membranes formed in these two systems exhibit different appearances and gel-to-liquid crystal phase-transition temperatures due to their different organized structures.
Introduction The design and construction of supramolecular organized assemblages from amphiphiles by noncovalent intermolecular interactions, such as hydrogen bonding, metal-ligand interactions, van der Waals interactions, π∠π stackings, and so on, have become a topic of increasing interest in recent years, due to their fascinating structures and novel functionalities.1-6 Usually, the organized structure and macroscopic morphology of these aggregates are determined not only by the chemical structures of small molecules but also by some environmental factors. From the chemical point of view, the highly ordered structure of molecular assemblages is decisively governed by the chemical structures of the component amphiphiles. For elucidating the relationship between the structure of amphiphiles and the properties of the supramolecular and mesoscopic aggregates, some systematic work has been conducted by several individual research groups during the past two decades.4-7 Kunitake and co-workers4,8,9 studied the self-organization of various kinds of synthetic amphiphiles to discuss To whom correspondence should be addressed: e-mail yqliang@ nju.edu.cn. (1) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1980. (2) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (3) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89. (4) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (5) Fuhrhop, J. H.; Helfrich, W. Chem. Rev. 1993, 93, 1565. (6) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (7) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (8) Okahata, Y.; Kunitake, T. J. Am. Chem. Soc. 1979, 101, 5231. (9) Kunitake, T.; Okahata, Y.; Simomura, M.; Yasunami, S.; Takarabe, K. J. Am. Chem. Soc. 1981, 103, 5401.
the relationship between aggregate morphology and chemical structure of the amphiphiles in aqueous media.They found that several kinds of structural units, including hydrophilic headgroups, spacers, mesogenic rigid segments, and hydrocarbon tails, determine the aggregate morphology of amphiphiles and their stability.4,9 Later, some works about the formation of stable fibrous assemblies with various well-defined nano- and microstructures from aldonamide amphiphiles,10-12 amino acid derivatives,13,14 some phospholipid derivatives,15,16 and bolaamphiphiles17-20 in aqueous media were reported, which demonstrate that the intermolecular hydrogen bonding markedly stabilizes the molecular assemblages. These works were expanded more recently into organic media by reporting on the formation of organogels based on the formation of hydrogen-bonded fibrous nanoscale networks.6,7,21-24 Moreover, self-assembly amphiphilic (10) Fuhrhop, J. H.; Schnieder, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387. (11) Fuhrhop, J. H.; Schnieder, P.; Boekema, E.; Helfrich, W. J. Am. Chem. Soc. 1988, 110, 2861. (12) Fuhrhop, J. H.; Svenson, S.; Boettcher, C.; Ro¨ssler, E.; Vieth, H. M. J. Am. Chem. Soc. 1990, 112, 4307. (13) Nakashima, N.; Asakuma, S.; Kunitake, T. J. Am. Chem. Soc. 1985, 107, 509. (14) Imae, T.; Takahashi, Y.; Muramatsu, H. J. Am. Chem. Soc. 1992, 114, 3414. (15) Schnur, J. M. Science 1993, 262, 1669. (16) Spector, M. S.; Selinger, J. V.; Singh, A.; Rodriguez, J. M.; Price, R. R.; Schnur, J. M. Langmuir 1998, 14, 3493. (17) Fuhrhop, J. H.; Spiroski, D.; Boettcher, C. J. Am. Chem. Soc. 1993, 115, 1600. (18) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812. (19) Nakazawa, I.; Masuda, M.; Okada, Y.; Hanada, T.; Yase, K.; Asai, M.; Shimizu, T. Langmuir 1999, 15, 4757. (20) Shimizu, T.; Iwaura, R.; Masuda, M.; Hanada, T.; Yase, K. J. Am. Chem. Soc. 2001, 123, 5947.
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systems with metal binding sites have been under investigation for many years due to their wide applications in catalysis and material science.25-29 Recently, considerableeffort has been directed toward control of the organized structure and macroscopic morphology of the molecular assemblages by modulating the component molecules through changing physical or chemical environments, including heat treatment,30 light irradiation,30-32 pH variation,28,33 and change of counterion type.34-38 In our previous papers, we have designed and synthesized a kind of simple amphiphile, monoalkylethylenediamine, to construct bilayer membranes,39-42 which have shed some new insights on the relationship between monomolecular structure and function. Especially for the [Cu(CnH2n+1NHCH2CH2NH2)2](NO3)2 complexed bilayer membranes formed by the serial single-chain amphiphiles CnH2n+1NHCH2CH2NH2 (n ) 8, 12, 14, 16, and 18) dispersed in dilute aqueous Cu(NO3)2, the membrane structure and properties are strongly dependent on the alkyl chain length.41 To deepen the understanding of the aggregate properties of this series of homologues, in the current study, we present results on organized structure of monoalkyl derivatives of ethylenediamine, CnH2n+1NHCH2CH2NH2 (n ) 8, 12, 14, 16, and 18), in dilute aqueous CuX2 with different X- ions (X- ) Cl- and ClO4-). The results confirm a strong influence of counterions on the organized structure and properties of Cu2+-coordinated bilayer membranes. The configuration of the Cu2+coordinated headgroups and packing model of aliphatic chains in bilayer membranes can be easily modulated by varying the counterions. Although the aggregate morphology is preserved upon switching between the two coordinated configurations of the headgroups, their molecular organized structure and physicochemical properties change correspondingly with varying the counterions. Experimental Section Sample Preparations. The serial amphiphiles, CnH2n+1NHCH2CH2NH2 (n ) 8, 12, 14, 16, 18), were synthesized by the method described in our previous paper.41 Unless otherwise (21) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1949. (22) Luo, X.; Li, C.; Liang, Y. Chem. Commun. 2000, 2091. (23) Luo, X.; Lui, B.; Liang, Y. Chem. Commun. 2001, 1556. (24) Esch, J. V.; Feringa, B. L. Angew. Chem., Int. Ed. Engl. 2000, 39, 2263. (25) Sasaki, D. Y.; Shnek, D. R.; Pack, D. W.; Arnold, F. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 905. (26) Scrimin, P.; Ghirlanda, G.; Tecilla, P.; Moss, R. A. Langmuir 1996, 12, 6235. (27) Bhattacharya, S.; Snehalatha, K.; George, S. K. J. Org. Chem. 1998, 63, 27. (28) Sommerdijk, N. A. J. M.; Booy, K. J.; Pistorius, A. M. A.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. Langmuir 1999, 15, 7008. (29) Mancin, F.; Tecilla, P.; Tonellato, U. Langmuir 2000, 16, 227. (30) Aoki, K.; Nakagawa, M.; Ichimura, K. J. Am. Chem. Soc. 2000, 122, 10997. (31) Spector, M. S.; Price, R. R.; Schnur, J. M. Adv. Mater. 1999, 11, 337. (32) Ichimura, K. Chem. Rev. 2000, 100, 1847. (33) Jonas, U.; Shah, K.; Norvez, S.; Charych, D. H. J. Am. Chem. Soc. 1999, 121, 4580. (34) Nascimento, D. B.; Rapuano, R.; Lessa, M. M.; Carmona-Ribeiro, A. M. Langmuir 1998, 14, 7387. (35) Paleos, C. M.; Kardassi, D.; Tsiourvas, D. Langmuir 1999, 15, 282. (36) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447. (37) Buwalda, R. T.; Stuart, M. C. A.; Engberts, J. B. F. N. Langmuir 2000, 16, 6780. (38) Ikeda, Y.; Imae, T.; Iida, M.; Koine, N.; Kaizaki, S. Langmuir 2001, 17, 361. (39) Lu, X. C.; Zhang, Z. Q.; Liang, Y. Q. J. Chem. Soc., Chem. Commun. 1994, 2731. (40) Lu, X. C.; Zhang, Z. Q.; Liang, Y. Q. Langmuir 1996, 12, 5501. (41) Lu, X. C.; Zhang, Z. Q.; Liang, Y. Q. Langmuir 1997, 13, 533. (42) Liang, Y. Q.; Lu, X. C.; Li, C. Acta Chim. Sin. 2000, 58, 742.
Li et al. specified, all chemicals were analytical grade and were used as received. Double-distilled water was used in the preparation of the aqueous solutions and dispersions. A mixture of 1.00 mmol of CnH2n+1NHCH2CH2NH2 (n ) 8, 12, 14, 16, 18) and 50 mL of water was sonicated (CSF-1B, 250 W, Shanghai) at 40 °C for 2 h to give 2 × 10-2 M stable and uniform aqueous dispersions. By mixing each of them with 1 × 10-2 M CuX2 (X- ) Cl-, ClO4-) solution (1:1 by volume) and sonicating the resultant dispersions at 50 °C for ca. 2 h and then at 5 °C for 30 min, the 5 × 10-3 M dispersions of the Cu2+-coordinated amphiphiles, [Cu(CnH2n+1NHCH2CH2NH2)2]X2 [designated as dispersions A (X- ) Cl-) and B (X- ) ClO4-), respectively, hereafter], were obtained. The cast films were prepared by spreading a few drops of the dispersions on CaF2 plates [for Fourier transform infrared (FT-IR) spectral measurements] or glass ones [for X-ray diffraction (XRD) characterization] and slowly drying in a vacuum; then the obtained cast films were kept in a saturated water atmosphere for 48 h at room temperature. Transmission Electron Microscopic Measurements. TEM was conducted on a JEOL model JEM-200CX instrument by the negative staining method. Each aqueous dispersion was mixed with 2 wt % aqueous uranyl acetate of equal volume and sonicated for 30 min at 50 °C and then 30 min at 5 °C. One drop of the dispersion was placed on a Cu grid coated with a conductive polymer film, which was dried in air, and then the Cu meshes were subjected to TEM observation with magnifications of 10000100000. Small-Angle XRD Measurements. The XRD patterns of cast films were measured on a Rigaku model D/max-RA diffractionmeter. X-ray was generated with a Cu anode, and the Cu KR beam (λ ) 1.5418 Å) was taken out via a graphite monochromator. The long spacing (Dn) in each pattern was calculated from the mean of all the peaks, excluding those whose relative errors are above 0.5%. DSC Measurements. DSC was performed on a Setaram microcalorimeter. The dispersions (0.9 mL) were sealed in sample tubes, with an equal volume of pure water as reference, and DSC thermograms were scanned at a heating rate of 1 °C min-1. Each DSC curve was reproducible. IR Spectral Measurements. FT-IR spectra were recorded with a Bruker IFS66V spectrometer equipped with a DTGS detector. All spectra were collected for 200 interferograms with a resolution of 2 cm-1. The band intensities were taken as peak heights measured from a baseline determined individually for each spectrum. Electronic Spectral Measurements. The solid powders of the complexed bilayers were obtained by scraping the cast films off the glass plates. The electronic reflectance spectra of the powders were measured with a Shimadzu UV-240 spectrometer, with BaSO4 as reference.
Results and Discussions TEM Images of the Organized Assemblages. Sonication of this series of amphiphiles, CnH2n+1NHCH2CH2NH2 (n ) 8, 12, 14, 16, 18), in dilute aqueous CuX2 (X) Cl-, ClO4-) gives the dispersions of the Cu2+-coordinated amphiphiles, [Cu(CnH2n+1NHCH2CH2NH2)2]X2. The dispersions are clear to translucent, depending on the tail length of the amphiphiles, and are stable and uniform for months without apparent changes. Figure 1 shows the typical transmission electron micrographs for these dispersions, which all revealed the presence of vesicular morphologies with diameters of 150-500 nm. However there are distinct differences in the appearance between the dispersions with Cl- and ClO4- as counterions. In the former case the bilayer aggregates are blue, while the latter are purple. Obviously for both systems, the only difference lies in the X- ion type; therefore, it can be inferred that the type of counterion is of vital importance to the structure of complexed headgroups, and the bilayer membranes can be categorized into two structurally differing types.
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Figure 2. XRD profiles of the cast Cu2+-coordinated bilayers of the n ) 14 amphiphiles: (A) X- ) Cl-; (B) X- ) ClO4-.
Figure 1. TEM images of the aggregates of C14H29NHCH2CH2NH2 in aqueous CuX2 (stained with 2 wt % uranyl acetate): (A) X- ) Cl-; (B) X- ) ClO4-.
XRD of Cast Films from the Cu2+-Coordinated Bilayer Membranes. It has been demonstrated by many experiments that in the cast films the self-aggregate structure of amphiphiles in dilute aqueous dispersions is well preserved.39-44 To obtain the organized structure of the assemblages in aqueous solution, the small-angle XRD method was applied to study the long-range structure of the cast bilayer membranes, and the representative XRD patterns of cast films (n ) 14) for dispersions A (Cl-) and B (ClO4-) are illustrated in Figure 2. It can be seen from this figure that both samples exhibit periodic peaks corresponding to bilayers with ordered structures, which are similar to those reported in our previous papers.39-42 Figure 3 shows the tail length (n) dependence on the long spacing (Dn) of the cast bilayer membranes. For each system the long spacing shows good linearity with tail length, indicating similar packing modes in the bilayer membranes irrespective of alkyl chain length, and the linear relationships between Dn and n for samples A and B are written as
(A) Dn (Cl-) ) 1.34n + 7.39 (B) Dn (ClO4-) ) 0.97n + 6.24 The slope and intercept of the equations reflect the (43) Asakuma, S.; Okada, H.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 1749. (44) Shimomura, M.; Aiba, S.; Tajima, N.; Inoue, N.; Okuyama, K. Langmuir 1995, 11, 969.
Figure 3. Bilayer thickness (Dn) values as functions of the tail length (n).
contributions of alkyl chain and headgroup to bilayer thickness (Dn), respectively. For X- ) Cl-, the long spacing (Dn), i.e., the cast bilayer thickness, falls between the monomolecular and bimolecular length of the amphiphile; while in the case of X- ) ClO4-, the bilayer thickness is smaller than the corresponding molecular length (Ln, shown in Table 1), as estimated from a CPK molecular model. These results indicate that the amphiphilic molecules probably assume the extensively titled tail-to-tail chain packing model or the interdigitated one in bilayer membranes. It is inferred from the phase-transition temperatures (Tc) of bilayer membranes discussed below that in dispersions A (X- ) Cl-) the amphiphiles adopt the tilted tail-to-tail chain packing model, whereas in dispersions B (X- ) ClO4-) the interdigitated chain packing one was adopted. On the basis of the packing model of aliphatic chains in the bilayer membranes, the chain orientation angle (with respect to the bilayer normal) for each system was calculated from the slope of the linear equations45,46 to be 58.2° (Cl-) and 40.2° (ClO4-). Although the radius of Cl- ion (1.81 Å) is less than that of ClO4- ion (2.76 Å), the intercept of eq A is larger than that of eq B, indicating that the contribution of Cl- ion to the headgroup layer thickness is larger than that of ClO4- ion. These results may be due to the different position of both ions and the different complexed configuration of headgroups as discussed below. (45) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (46) Lu, X. C. Ph.D. Thesis, Nanjing University, China, 2000.
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Table 1. XRD and DSC Results on the Bilayer Thickness Values and the Phase-Transition Temperatures of Amphiphilic Aggregates of CnH2n+1NHCH2CH2NH2 in Aqueous CuX2 appearance
Tc, °C (DSC)
chain packing mode and orientation angle
Dn, Å (XRD)
n
Cl-
ClO4-
Cl-
ClO4-
Cl-
ClO4-
Lna, Å
Cl-
ClO4-
8 12 14 16 18
clear, blue translucent, blue emulsion, blue emulsion, blue emulsion, blue
clear, purple translucent, purple emulsion, purple emulsion, purple emulsion, purple
39 51 56 61 65
47 61 67 72 78
18.15 23.51 26.21 28.92 31.58
14.08 18.01 19.92 21.85 23.77
16 21 23.5 26 28.5
tail-to-tail, 58.2°
interdigitated, 40.2°
a
Evaluated molecular length.
Figure 4. DSC curves of the Cu2+-coordinated bilayer membranes: (A) X- ) Cl-; (B) X- ) ClO4-.
Figure 5. Phase-transition temperature (Tc) values dependence on the tail length (n).
DSC of Aqueous Dispersion. Differential scanning calorimetry experiments of the complexed bilayer dispersions showed endothermic peaks in the curves (Figure 4), indicating the gel-to-liquid crystal phase transition, which is one of the basic physicochemical properties of bilayer membranes.4 The values of the phase-transition temperature (Tc) for each system exhibit fairly good linearity with n as depicted in Figure 5, which is in agreement with XRD results, indicating similar lateral packing in the membrane structure of each system. It is well-known that the phase-transition temperature is strongly dependent on the aliphatic chain packing mode. It is clear from Figure
5, for X- ) Cl- samples (A), that the Tc values increase with extending alkyl chain length and increase to 65 °C as n ) 18, which are similar to the values of a model bilayer membrane with tail-to-tail packing reported in the literature.4,47-49 In the case of X- ) ClO4- (B) the Tc values are higher than those of dispersions A with the same chain length, and for n ) 18 amphiphile, the Tc value increases to 78 °C. It has been shown that the extensively tilted tail-to-tail-type bilayer assemblages possess relatively fluid chain packing and exhibit relatively low Tc, whereas the interdigitated-type bilayer membranes have tighter chain packing and display higher Tc values than the tail-to-tail ones.1,2,4,48-51 Thus, by combining XRD and DSC results, it can be reasonably inferred that in dispersions A (X- ) Cl-) the amphiphiles adopt the tilted tail-to-tail chain packing model, whereas in dispersions B (X- ) ClO4-) the interdigitated chain packing model was adopted in the bilayer membranes. That is to say, there exist two quite different structures both in aqueous dispersions and in the cast films from the complexed bilayer membranes with different counterions. The dependence of structure and properties of bilayer membranes on counterions is summarized in Table 1. IR Spectra of Cast Films from the Cu2+-Coordinated Bilayer Membranes. XRD is a powerful investigative tool in elucidating long-range structures of molecular ordered assemblages. However, further insights into the structural properties of the intramolecular chain conformation and intermolecular chain packing of the membrane-forming amphiphiles can be obtained by vibrational spectroscopy.41,52,53 In the present work, FTIR spectra were measured to obtain this information. For each system, the IR spectra of the cast bilayers are nearly identical to each other irrespective of the tail length. Figure 6 shows the IR spectra of n ) 16 cast bilayer membranes with Cl- (A) and ClO4- (B) as counterions, respectively. It is clear from this figure that the spectra are quite different; however, there is one important feature in common: two strong bands appear near 2918 and 2850 cm-1, being assigned to the asymmetric and symmetric CH2 stretching modes of long hydrocarbon chains, respectively.54-56 It has been well documented for longchain hydrocarbon molecules that the frequencies of the (47) Kodama, M.; Kunitake, T.; Seki, S. J. Phys. Chem. 1990, 94, 1550. (48) Streefland, L.; Yun, F.; Rand, P.; Hoekstra, D.; Engberts, J. B. F. N. Langmuir 1992, 8, 1715. (49) Ghosh, P.; Sengupta, S.; Bharadwaj, P. K.; Hoekstra, D. Langmuir 1998, 14, 5712. (50) Huang, C.; Mason, J. T.; Levin, I. W. Biochemistry 1983, 22, 2775. (51) Shimomura, M.; Aiba, S.; Tajima, N.; Inoue, N.; Okuyama, K. Langmuir 1995, 11, 969. (52) Yellin, N.; Levin, I. W. Biochim. Biophys. Acta 1977, 489, 177. (53) Gaber, B. P.; Peticolas, W. L. Biochim. Biophys. Acta 1979, 465, 260. (54) Snyder, R. G.; Hou, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (55) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145.
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Figure 7. Electronic reflectance spectra of the powders of the n ) 16 cast Cu2+-coordinated bilayers: (A) X- ) Cl-; (B) X- ) ClO4-. Inset: Coordinated configuration of the CuN4 headgroups.
Figure 6. FT-IR spectra of the n ) 16 cast bilayers: (A) X- ) Cl-; (B) X- ) ClO4-.
Cu2+-coordinated
asymmetric and symmetric CH2 stretching vibrations are conformation-sensitive due to the perturbation by Fermi resonance interaction with the methylene bending vibration.54-56 Lower wavenumbers (2918 and 2850 cm-1) are characteristic of highly ordered conformers with preferential all-trans chains, which indicates that the hydrocarbon chains of amphiphiles in both dispersions take on a close-packed all-trans conformation at room temperature. It is also known that for hydrocarbon chains the intensity ratio of the methylene asymmetric mode (2918 cm-1) over the symmetric mode (2850 cm-1) is sensitive to the intermolecular chain packing order and the ratio increases with increasing order.53 Analysis of dispersion A gave an intensity ratio (asymmetric to symmetric) of 1.15, whereas the value obtained for dispersion B was 1.60. These results demonstrated a substantially higher ordering of the alkyl chains in dispersions B compared to A. The bands in the region 3300-3100 cm-1 are attributed to the N-H stretching vibrations. It is clear that the absorption bands of N-H stretching vibrations in spectrum A (X- ) Cl-) (3222 and 3133 cm-1) are broader and shifted to lower wavenumbers than those of spectrum B (X- ) ClO4-) (3324 and 3280 cm-1). The single sharp bands at ca. 1595 (A) and 1576 cm-1 (B) are assigned to the N-H deformation mode, in which the former takes an upshift by 19 cm-1 relative to the latter. Both νNH and δNH changes [compared with free νNH (3500 and 3400 cm-1) and δNH (1560 cm-1)] are attributed to the formation of hydrogen bonding,57 and the changes in system A are stronger than those in system B, which results from the different configuration of the Cu2+-coordinated headgroups as explained below. Organized Structure of the Cu2+-Coordinated Bilayer Membranes. As discussed in the above sections, both the Tc from DSC and Dn from XRD display fairly (56) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334. (57) Nakanishi, K.; Solomon, P. H. Infrared Absorption Spectroscopy, 2nd ed.; Holden Day Inc.: San Francisco, CA, 1977.
good linearity with the tail length of the amphiphiles, indicating that the serial amphiphiles form bilayer structures bearing a similar packing mode in each system. However, for these two systems, there are distinct differences in appearance, Tc, and Dn. Therefore, it can be inferred that the counterion type is of vital importance to the determination of the membrane structures. Figure 7 shows the electronic reflectance spectra of the powders prepared from the blue (X- ) Cl-) and purple (X) ClO4-) dispersions (n ) 16), respectively. The differences between the two kinds of assemblages in color and electronic spectra correspond to configurational diversities in the Cu2+-coordinated headgroups. For asymmetric derivatives of ethylenediamine, ML2 type complexes of Cu2+-coordinated amphiphiles usually have planar CuN4 structures with cis and trans isomers.58 It has been established that complexes with a cis configuration show higher σ∠σ* transition energies than those with a trans arrangement,59,60 and square-coplanar CuN4 complexes show d-d transition energies in the range of (18-20) × 103 cm-1, while the range for tetrahedral CuN4 complexes is (12-16) × 103 cm-1.61 From Figure 7, the σ∠σ* transition energies are obtained as 35.21 × 103 and 42.37 × 103 cm-1 for dispersion A and 36.50 × 103 and 43.10 × 103 cm-1 for dispersion B, and the d-d transition energy is obtained as 16.78 × 103 cm-1 for dispersion A and 19.23 × 103 cm-1 for dispersion B. Obviously, both the σ∠σ* and d-d transition energies of dispersion A are lower than those of dispersion B, indicating that when X ) Cl-, the Cu2+coordinated headgroups adopt a trans configuration, which gives a compressed tetrahedral CuN4 environment; while for X- ) ClO4-, a cis configuration is adopted in the square plane of Cu2+-coordinated headgroups. The coordination structures of the headgroups are also schematically illustrated in Figure 7. Therefore, it can be deduced that the headgroup structures will greatly influence the packing of the hydrocarbon tails in the organized aggregates: for dispersions A (X- ) Cl-) the two tails in the complexes take approximately diagonal positions; whereas in dispersions B (X- ) ClO4-) the two tails are located at (58) Kennedy, B. P.; Lever, A. B. P. J. Am. Chem. Soc. 1973, 95, 6907. (59) Leigh, G. J.; Mingos, D. M. P. J. Chem. Soc. A 1970, 587. (60) Textor, M.; Ludwig, W. Helv. Chim. Acta 1972, 55, 184. (61) Hathaway, B. J. J. Chem. Soc., Dalton Trans. 1972, 1196.
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Figure 8. Schematic illustration of packing structure of the Cu2+-coordinated bilayers: (A) X- ) Cl-; (B) X- ) ClO4-.
the same side of the plane and parallel to each other. Combining the XRD analyses on the lamellar structures of the cast films, it can be inferred that, for dispersions A, the two spatially separated aliphatic chains extend obliquely and adopt tail-to-tail packing model in bilayer assemblages; while for dispersions B, the tilted chains interdigitate in the bilayer, so leading to a membrane thickness smaller than the monomolecular length of the corresponding amphiphile. In a word, the competition between the steric isomerization of the CuN4 headgroups and the hydrophobic assembly of the aliphatic tail chains result in the structural diversity in the novel complexed bilayer membranes. Also, the speculated structures can give a reasonable explanation for the IR spectral differences between these two types of bilayer membranes in N-H stretching and deformation modes in Figure 6: the accessibility of the nitrogen atoms in structure A will inevitably lead to stronger hydrogen bonding than in structure B. By combing the results from various analytical methods, a schematically illustration of the organized structure are shown in Figure 8.
Conclusions The present work shows a very simple method, by altering the counterions, to modulate the headgroup configuration and packing model of aliphatic chain in synthetic bilayer membranes formed by the serial monoalkylethylenediamine CnH2n+1NHCH2CH2NH2 (n ) 8, 12, 14, 16, 18) in dilute aqueous CuX2 (X- ) Cl-, ClO4-). This molecular-level modification arising from the change of counterions is amplified by the self-assembly process through cooperative effects of metal-ligand in the headgroups and van der Waals interactions of alkyl chains on their basic physicochemical properties, such as appearance and gel-to-liquid transition temperature of the bilayer membranes. Acknowledgment. Financial support from the National Natural Science Foundation of China (20073019) is gratefully acknowledged. LA010949E
