Complexed Bilayer Membranes with Novel Structural Features

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Langmuir 1997, 13, 533-538

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Complexed Bilayer Membranes with Novel Structural Features Formed by Single-Chain Amphiphiles Xianchun Lu, Zhiqiang Zhang, and Yingqiu Liang* Department of Chemistry, Coordination Chemistry Institute and State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China Received June 27, 1996. In Final Form: October 28, 1996X Stable synthetic bilayer membranes with Cu2+-coordinated headgroups can be formed by the series of single-chain amphiphiles, CnH2n+1NHC2H4NH2 (n ) 8, 12, 14, 16, 18), in dilute aqueous Cu(NO3)2. The novel structural features of the membrane system have been investigated thoroughly via X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) and infrared (FT-IR), and electronic reflectance spectroscopies. Two bilayer packing fashions were conclusively proposed: the tail-to-tail model for the n ) 8 amphiphiles and the interdigitation one for the n > 8 amphiphiles. It was revealed at the molecular level that the balance and matching between the headgroup coordination and the hydrophobic assembly of the alkyl chains endow the complexed bilayer membranes with structural varieties.

1. Introduction The formation of bilayer membranes from single-chain amphiphiles in dilute aqueous solutions is mainly determined by the balance between the interaction among the hydrophobic tails and that among the hydrophilic headgroups, and the molecular geometry as well. Through any of the following ways to enhance the assembly between amphiphiles, such as the introduction of a rigid segment,1,2 a hyperextended alkyl chain,3 a perfluorinated chain,4 electrostatic attraction between the oppositely charged headgroups,5 hydrogen bonding,6,7 protonation of the headgroups,6 coordination8 and polymerization,9 etc., stable bilayer membranes are obtainable. Therefore special properties different from those of isolated amphiphilic molecules are produced, which offers important ways to structurally design fabricated two-dimensional supermolecular systems with predictable functions.10,11 In a previous communication we reported that complexed bilayer membranes are formed from a series of single-chain amphiphiles CnH2n+1NHC2H4NH2 (n ) 8, 12, 14, 16, 18) dispersed in dilute aqueous Cu(NO3)2, and the membrane properties are strongly dependent on their structures.12 In this paper, detailed studies through X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) and infrared (FT-IR), and electronic spectroscopies are performed to reveal the structure-function relationship of the novel membrane system. It is brought to light at the molecular level that competition and balance between the hydrophobic assembly of the alkyl tails and the coordination structures of the Cu2+-complexed headgroups lead to various bilayer structures, which explains the * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, January 1, 1997. (1) Kunitake, T.; Okahata, Y.; Shimomura, M.; Yasunami, S.; Takerabe, K. J. Am. Chem. Soc. 1981, 103, 5401. (2) Liang, Y.; Wu, L.; Tian, Y.; Zhang, Z.; Chen, H. J. Colloid Interface Sci. 1996, 178, 703. Liang, Y.; Zhang Z.; Wu, L.; Tian, Y.; Chen, H. J. Colloid Interface Sci. 1996, 178, 714. (3) Menger, F. M.; Yamasaki, Y. J. Am. Chem. Soc. 1993, 115, 3840. (4) Kraft, M.-P.; Giulieri, F.; Riess, J. G. Angew. Chem., Int. Ed. Engl. 1993, 32, 741. (5) Fukuda, H.; Kawata, K.; Okuda, H.; Regen, S. L. J. Am. Chem. Soc. 1990. 112, 1365. (6) Lu, X.; Zhang Z.; Liang, Y. Langmuir 1996, 12, 5501. (7) Kunitake, T.; Yamada, N.; Fukunaga, N. Chem. Lett. 1984, 1089. (8) Suh, J.; Lee, K. J.; Kwon, O.-B.; Oh, S. Langmuir 1995, 11, 2626. (9) Dodrer, N.; Regen, S. L. J. Am. Chem. Soc. 1990, 112, 2829. (10) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (11) Ringsdorf, H. Supramol. Sci. 1994, 1, 5. (12) Lu, X.; Zhang, Z.; Liang, Y. J. Chem. Soc., Chem. Commun. 1994, 2731.

reason for different membrane properties. The structural diversity of complexed bilayer membranes can be of great significance in the molecular design of membrane-forming amphiphiles. Also metal complexes possess many special optical, electric, magnetic, and thermal properties, so the introduction of them into a membrane system may result in membrane materials with particular functions. 2. Experimental Section The amphiphiles were synthesized by alkyl bromides reacting directly with ethylenediamine in a molar ratio of 1:8 as follows 110 °C, 8 h NaOH

CnH2n+1Br + H2NC2H4NH2 (excess) 9898 CnH2n+1NHC2H4NH2 (n ) 8, 12, 14, 16, 18) The column chromatography for purifying the crude products was performed using 100-200 mesh alkaline Al2O3 and methanol-ether solution (1:1 by volume). The final products were verified by thin-layer chromatography and by 1H NMR (Bruker AM-500). Sonication of these compounds in distilled water gives 2 × 10-2 M stable and uniform aqueous dispersions. By mixing each of them with 1 × 10-2 M Cu(NO3) solution (1:1 by volume) and sonicating the resultant solutions for ca. 2 h, the 5 × 10-3 M dispersions of the Cu2+-coordinated amphiphiles, [Cu(CnH2n+1NHC2H4NH2)2](NO3)2, were obtained. These dispersions are clear to translucent, depending on the tail length of the amphiphile, and are stable, uniform for months without apparent changes. Each sample was stained with 2 wt % aqueous uranyl acetate of equal volume and then subjected to transmission electron microscopy (TEM) measurements (JEOL Model JEM-200CX). Differential scanning calorimetry (DSC) measurements were performed on a SETARAM microcalorimeter at the heating rate 1 °C min-1. 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 vacuum; then they were kept in a moist atmosphere for about 48 h at room temperature. FT-IR spectra of the films were recorded on a Bruker IFS-66v spectrophotometer. The small-angle XRD patterns of the cast films were measured on a Rigaku Model D/max-RA diffractionmeter with Cu-KR radiation (λ ) 1.5418 Å). All of the diffraction profiles showed more than five periodic reflection peaks characteristic of a lamellar structure, and 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%. 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 using BaSO4 as reference. FT-

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Figure 1. TEM morphologies of the complexed bilayer membranes of [Cu(CnH2n+1NHC2H4NH2)2](NO3)2: (a) n ) 8; (b) n ) 12; (c) n ) 14; (d) n ) 16; (e) n ) 18. Raman spectra of the powders were investigated on a Bruker RFS-100 spectrophotometer, and band intensities were taken as peak heights measured from a baseline determined individually for each spectrum. All of the above spectra were obtained at room temperature.

3. Results and Discussion 3.1. Novel Complexed Bilayer Membranes. Sonication of this series of amphiphiles in aqueous solutions of Cu(NO3)2 results in stable and uniform dispersions. However, drastic differences appear between samples with n ) 8 and n ) 12-18 in the appearance of the dispersions: the former gives a clear blue solution, while the latter gives a translucent light-purple emulsion. TEM observations all showed vesicular morphologies with bilayer walls

(Figure 1). As can be seen from Figure 1, the density of the vesicles is increased and the bilayer structure improved with extending the alkyl tail length, showing that the coordination of the headgroups with Cu2+ (designated as CuN4) and hydrophobic assembly of the alkyl chains result in a novel kind of complexed bilayer membranes. Shown in Figure 2 is the DSC curves of the complexed bilayer dispersions. The endothermic peaks in the curves indicate the gel-to-liquid crystal phase transition, which is one of the basic physicochemical properties of bilayer membranes.10 The values of the phase transition temperatures (Tc) of the n ) 12-18 bilayers exhibit fairly good linearity with n as depicted in Figure 2, indicating the similar lateral packing in the membrane structure of

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bilayer membranes, the small-angle XRD method was applied to study the long spacing of the cast bilayer membranes. Periodic peaks indicative of ordered structures, which are similar to those reported in the literature,17,18 are observed in the XRD profiles of the cast films of the series of complexed bilayer membranes. The XRD patterns of the n ) 8 and 16 amphiphiles are exemplified in Figure 3, and shown in Figure 3 is the tail length (n) dependence of the bilayer thickness, i.e., the long spacing (Dn) of the cast bilayer membranes. It is seen that the bilayer thickness shows good linearity with tail length when n ) 12, 14, 16, and 18, while D8 is far above the straight line. The results reflect the structural discrepancy between the cast bilayer membranes formed by n ) 8 and n > 8 amphiphiles. Comparing the Dn values of the cast bilayers obtained by the XRD methods with the evaluated molecular length of the corresponding amphiphiles (Ln) by the CPK model results in the following two different equations:

Figure 2. DSC curves of the complexed bilayer membranes (upper); phase transition temperature (Tc) values as functions of the tail length (lower).

Figure 3. XRD patterns of the cast complexed bilayers from the n ) 8 (a) and n ) 16 (b) amphiphiles (upper); bilayer thickness (Dn) values as functions of the tail length (lower).

these amphiphiles. On the other hand, the Tc value of the n ) 8 amphiphile is far below that extrapolated from the straight line, suggesting that its molecular packing mode is deviant from that of the n > 8 amphiphiles. 3.2. XRD Patterns of Cast Films from the Complexed Bilayer Membranes. It has been established by many experiments that the self-aggregate structure of amphiphiles in dilute aqueous dispersions has been kept in the water-cast films in various aspects.13-17 Therefore in order to obtain the structural features of the aqueous

D8 ) 1.63L8

(1)

Dn ) 1.06n + 6.24 < Ln (n ) 12-18)

(2)

The n ) 8 cast bilayer thickness falls between the monomolecular and bimolecular length of the amphiphile, suggesting that the complexed bilayer membranes bear the tail-to-tail packing model, while in the case of n > 8 the bilayer thickness is smaller than the corresponding evaluated molecular length, implying that the molecules probably assume the extensively tilted tail-to-tail chain packing model or the interdigitated one. The chain tilt angles (with respect to the bilayer normal) for the two models, calculated from eq 2,19 are 65° and 33.4°, respectively. It has been shown that in many cases6,10,14,20 the extensively tilted tail-to-tail type bilayer assemblages (the J-like aggregates) 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. Thus, in consideration of the DSC results that all of the Tc values for the n > 8 molecules are larger than 84 °C, it is preferable that the n > 8 molecules adopt the interdigitated chain packing model in the bilayer membranes. That is to say there exist two quite different structures in the cast films from the complexed bilayer membranes. Good agreements can be found between the tail length dependence of the Tc value of the complexed bilayer membranes in aqueous solutions through the DSC curves and that of the Dn value of the cast films from the aqueous bilayers through the XRD measurements. Combining the results obtained via these two methods and our previous research,6,12 it can be concluded that (1) the aggregate characteristics of this series of single-chain amphiphiles formed in aqueous dispersions are basically maintained (13) Nakashima, N.; Ando, R.; Kunitake, T. Chem. Lett. 1983, 1577. (14) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1134. (15) Kunitake, T.; Shimomura, M.; Kajiyama, T.; Harada, A.; Okuyama, K.; Takayanagi, M. Thin Solid Films 1984, 121, L89. (16) Ishikawa, Y.; Kunitake, T. J. Am. Chem. Soc. 1986, 108, 8300. (17) Shimomura, M.; Aiba, S.; Tajima, N.; Inoue, N.; Okuyama, K. Langmuir 1995, 11, 969. (18) Ishikawa, Y.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 621. (19) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (20) Ishikawa, Y.; Nishimi, T.; Kunitake, T. Chem. Lett. 1990, 165. Okuyama, K.; Ikeda, M.; Yokoyama, S.; Ochiai, Y.; Hamada, Y.; Shimomura, M. Chem. Lett. 1988, 1013.

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Figure 4. FT-Raman spectra of the solid powders of the complexed bilayers from the n ) 8 (a) and 16 (b) amphiphiles.

Figure 5. FT-IR spectra of the cast complexed bilayers from the n ) 8 (a) and 16 (b) amphiphiles.

in the immobilized films, i.e., the water-cast films,13-17 and (2) the complexed bilayer membranes formed by the amphiphiles hold two entirely different types of structures. 3.3. FT-Raman and FT-IR Spectra of Cast Films from the Complexed Bilayer Membranes. XRD measurements can investigate the orientation and arrangement of the tail chains in films, while the information about the intermolecular chain packing and the intramolecular chain conformation of the membrane-forming amphiphiles can be obtained by vibrational spectra.21-25 Therefore, the Fourier transform Raman and infrared spectroscopies are used to investigate the cast bilayer films. In order to enhance the Raman signal, solid powders were gathered as the Raman scattering samples. According to the conclusion of section 3.2, the structural information obtained from the cast films holds true in principle for the aqueous complexed bilayer membranes. The FT-Raman spectra of the solid powders of the cast complexed bilayers of the n ) 12, 14, 16, and 18 amphiphiles are nearly identical to each other but different from that of the n ) 8 amphiphile. The spectra of the n ) 8 and 16 amphiphiles are illustrated in Figure 4. The strong multiple peaks between 3000 and 2800 cm-1 are assigned to the CH stretching vibrations, and those around 1450 and 1296 cm-1 to the CH2 deformation and twisting vibrations.23,24 The C-C stretching modes, mixed with the C-N ones, appear in the region 1130-1062 cm-1,26 and the band features of both spectra in this region are characteristic of all-trans hydrocarbon chains.24 It has

been established that the band intensity ratio of the methylene symmetric (ca. 2850 cm-1) over antisymmetric (ca. 2880 cm-1) vibrations, I2850/I2880, is sensitive to the intermolecular chain packing order, and the ratio decreases with increasing order.23,24 The ratios are 0.97 for spectrum a (n ) 8) and 0.7 for spectrum b (n ) 16). For comparision the FT-Raman spectra of the CCl4 solutions of the same series of amphiphiles, representative of the molecularly dispersed state, are recorded (not shown here), and the I2850/I2880 ratios are estimated to be about 1.5 for all samples. So, the n ) 8 complexed bilayer membranes are in a somewhat orderly state but possess looser lateral chain packing than the n ) 16 ones. It is reasonable, as we known from XRD, that the tail chains in the membranes are packed in the tail-to-tail and interdigitation models, respectively, in the case of the two amphiphiles. The splitting of the CH2 deformation mode at ca. 1450 cm-1 in spectrum b may also suggest stronger chain interaction.27 Moreover, in Figure 4 the strong peak at ca. 1050 cm-1 and the weak one at ca. 720 cm-1 are characteristic of the free NO3- bands.28,29 It gives spectral evidence for our assumption in the previous communication12 that the NO3ions do not coordinate with Cu2+, while two water molecules act as the axial ligands of the CuN4 headgroup. Very similar to the FT-Raman spectra, the FT-IR spectra of the cast complexed bilayers of the serial amphiphiles can be categorized into two types, and shown in Figure 5 are the IR spectra of the n ) 8 (a) and 16 (b) cast bilayers. The two spectra have common features: the bands in the region 3300-3100 cm-1 are attributed to the N-H stretching vibrations overlapped by the absorption of water in the cast films, and the latter also gives a weak band

(21) Hill, I. R.; Levin, I. W. J. Chem. Phys. 1979, 70, 842. (22) Snyder, R. G. J. Chem. Phys. 1979, 71, 3229. (23) Gaber, B. P.; Peticolas, W. L. Biochim. Biophys. Acta 1977, 465, 260. (24) Yellin, N.; Levin, I. W. Biochim. Biophys. Acta 1977, 489, 177. (25) Harrand, M.; Masson, M. J. Chem. Phys. 1987, 87, 5176. (26) Fleming, G. D.; Shepherd, R. E. Spectrochim. Acta 1987, 43A, 1141.

(27) Boerio, F. J.; Koening, J. L. J. Chem. Phys. 1970, 52, 3425. (28) Irish, D. E.; Walrafen, G. E. J. Chem. Phys. 1967, 46, 378. (29) Degen, I. A.; Newman, J. A. Spectrochim. Acta 1993, 49A, 859.

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at ca. 1640 cm-1; the strong absorptions at around 2920 and 2850 cm-1 are assigned to the antisymmetric and symmetric CH2 stretching modes of all-trans chains,30,31 which are in agreement with the Raman spectral results; the single sharp band at ca. 1590 cm-1 is assigned to the N-H deformation mode; the bands at 1440-1430 cm-1 may be assigned to the scissoring mode of CH2 connected with N atoms; the 1451 cm-1 band in spectrum a is the absorption of the asymmetric deformation mode of the terminal CH3 groups; and the absorption of the NO3- ions appears as strong bands in the region 1380-1350 cm-1.32 However, two drastic differences are observed. One is that the CH2 scissoring mode of the alkyl chain appears as a singlet at 1467 cm-1 in spectrum a, while it is a doublet near 1461 and 1471 cm-1 in spectrum b. The CH2 scissoring band has been known to be sensitive to the intermolecular interaction;33,34 its singlet appearance is characteristic of hydrocarbon chains with a hexagonal subcell packing, while the splitting of the mode is indicative of an orthorhombic subcell packing. It agrees well with the above Raman results, for the former subcell packing is looser compared with the latter. The other difference is that the absorption of N-H stretching vibrations in spectrum a is broader and stronger and shifts to lower wavenumber than that of spectrum b, while the N-H deformation band in spectrum a is sharper and shifts to higher wavenumber than that of spectrum b. This kind of spectral change is attributed to the stronger hydrogen bonding32 in the n ) 8 bilayers, which may result from the structural variations in the Cu2+-coordinated headgroups. 3.4. Structure of Complexed Bilayer Membranes. It has been found that stable bilayer membranes can be formed from this series of single-chain amphiphiles, CnH2n+1NHC2H4NH2 (n ) 8, 12, 14, 16, 18), both in pure water and in acidic aqueous solutions.6 In those cases, both the Tc from DSC and Dn from XRD display fairly good linearity with the tail length (n) of the amphiphiles, and the deviation of Tc (n ) 8) and D8 from the corresponding straight lines does not take place, indicating that the serial amphiphiles form bilayer structures bearing a similar packing mode either in pure water or in acidic dispersion.6 The results are drastically different from that of the Cu2+-complexed bilayer membranes, in which, as stated above, two types of membrane structures are discovered. Therefore, it is clearly seen that the coordination of the headgroups with Cu2+ ions is of vital importance to the determination of the membrane structures. As asymmetric derivatives of ethylenediamine, ML2 type complexes of the Cu2+-coordinated amphiphiles usually have planar CuN4 structures with cis- and trans-isomers.35 It has been established that complexes with cis-configuration show higher σ-σ* transition energies than those with trans-arrangements,36,37 and square-coplanar CuN4 complexes show d-d transition energies in the range (1820) × 103 cm-1 while the range for tetrahedral CuN4 is (12-16) × 103 cm-1.38 Figure 6 displays the electronic (30) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, N.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56. (31) Zhang, Z.; Liang, Y. J. Colloid Interface Sci. 1995, 169, 220. (32) Nakanishi, K.; Solomon, P. H. Infrared Absorption Spectroscopy, 2nd ed.; Holden-Day Inc.: San Francisco, CA, 1977. (33) Tasumi, M.; Shimanouchi, T.; Miyazawa, T. J. Mol. Spectrosc. 1962, 9, 261. (34) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. (35) Kennedy, B. P.; Lever, A. B. P. J. Am. Chem. Soc. 1973, 95, 6907. (36) Leigh, G. J.; Mingos, D. M. P. J. Chem. Soc. A 1970, 587. (37) Textor, M.; Ludwig, W. Helv. Chim. Acta 1972, 55, 184. (38) Hathaway, B. J. J. Chem. Soc., Dalton Trans. 1972, 1196.

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Figure 6. Electronic reflectance spectra of the powders of the complexed bilayers from the n ) 8 (a) and 16 (b) amphiphiles. Upper part: coordination configuration of the CuN4 headgroups.

reflectance spectra of the powders prepared from the blue (n ) 8) and light-purple dispersions (n ) 16 being used as an example), respectively. From the absorption maxima in the figure the d-d transition energies are obtained as 15.5 × 103 cm-1 for the former and 18.1 × 103 cm-1 for the latter; and the σ-σ* transition energies are obtained as 42.9 and 30.3 × 103 cm-1 for the former and 44.4 and 35.7 × 103 cm-1 for the latter. Both the d-d and σ-σ* transition energies of the n ) 8 bilayer membranes are lower than those of the n ) 16 ones. It can be inferred that when n ) 8, the complex adopts a trans-configuration with the headgroups coordinated so as to give a compressed tetrahedral CuN4 environment, and the two tails in the [Cu(C8H17NHC2H4NH2)2]2+ complex take the approximately diagonal position; while, for n ) 12-18, a cisconfiguration is adopted with the copper square-coplanar, and the two tails are located at the same side of the plane and parallel to each other. The coordination structures of the headgroups are also schematically depicted in Figure 6. Therefore it is seen that the headgroup structures will greatly influence the packing fashion of the hydrocarbon tails in the bilayer membranes. Combining the XRD analyses on the lamellar structure of the cast films, it can be deduced that when n ) 8, the two spatially separated aliphatic chains extend obliquely into the tail gap of the neighboring amphiphiles in the same layer to form the tail-to-tail bilayer structure and the compressed tetrahedral CuN4 lies along the membrane surface with its central plane vertical to the membrane normal; in the case of n ) 12-18, the chains strongly interdigitate in the bilayer and tilt, so leading to a membrane thickness smaller than the monomolecular length of the corresponding amphiphile, and the CuN4 coordination planes align along the membrane normal and parallel to each other.12 Therefore it is seen that the competition between the steric isomerization of the CuN4 headgroups and the spontaneous assembly of the aliphatic tails result in the structural diversity in the novel complexed bilayer membranes. Also, the speculated structures can give a reasonable answer for the IR spectral differences between these two types of bilayer membranes in N-H stretching and deformation modes in Figure 5: the accessibility of the nitrogen atoms in structure a will inevitably lead to stronger hydrogen bonding than in structure b.

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4. Concluding Remarks The novel kind of synthetic bilayer membranes, bearing the Cu2+-coordinated headgroups, is formed by the series of single-chain amphiphiles, CnH2n+1NHC2H4NH2 (n ) 8, 12, 14, 16, 18), in dilute aqueous Cu(NO3)2. The balance and matching between the headgroup coordination and the hydrophobic assembly of the alkyl chains endow the complexed bilayer membranes with structural varieties,

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which may make possible the molecular design of superamolecular membrane materials with predictable functions. Acknowledgment. We thank the State Science and Technology Commission of China for a grant in support of this work. LA960643L