Langmuir 1996, 12, 5501-5503
5501
Bilayer Formation in Dilute Aqueous Solution from Monoalkylethylenediamine Xianchun Lu, Zhiqiang Zhang, and Yingqiu Liang* Department of Chemistry, Coordination Chemistry Institute and State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, Peoples’ Republic of China Received March 18, 1996. In Final Form: July 29, 1996
Introduction It is usually considered that a rigid segment (e.g., biphenyl unit, azobenzene unit, diphenylazomethine unit, etc.) is essential for a pure single-chain amphiphile to form stable bilayer membranes in dilute aqueous dispersion,1,2 except for very few cases such as amphiphiles with a hyperextended alkyl chain3 or a perfluorinated chain,4 or monoglycerides.5 To our knowledge, reports concerning bilayer formation in dilute aqueous solution from a simple synthetic amphiphile with a common hydrocarbon chain and possessing no rigid segment are very few. We present herein that stable bilayer membranes can be selfassembled in pure water and in acidic solution from a series of amphiphiles with monoalkyl chain in common length and without a rigid segment, CnH2n+1NHC2H4NH2 (n ) 8, 12, 14, 16, 18). It may provide some new insight into the mechanisms of bilayer formation. Experimental Section The amphiphiles were prepared by alkyl halides reacting directly with excess ethylenediamine as follows 110 °C NaOH
CnH2n+1Br + H2NC2H4NH2 9 898 8h CH3(CH2)n-3CH2CH2NHCH2CH2NH2 c d a b The column chromatography for purifying the crude products was performed using 100-200 mesh alkaline Al2O3 and methanol-ether solution (1:1 vol). The final products were verified by thin layer chromatography and by proton NMR (Bruker AM500). 1H NMR (CDCl3): 2.81 (t, 2H, H-d), 2.67 (t, 2H, H-c), 2.60 (t, 2H, H-b), 1.78 (broad, 3H, N-H), 1.47 (m, 2H, H-a), 1.29-1.25 (m, (2n-6)H, -(CH2)n-3-), 0.88 (t, 3H, -CH3). The samples were prepared by dispersing each of them in doubly distilled water and 20 mM hydrochloric solution, respectively, and after sonication to make pure aqueous dispersion A, and acidic dispersion B (CnH2n+1NHC2H4NH2‚2HCl). All of the stock dispersions (10 mM) were stable, uniform, transparent, or translucent. B was negatively stained by 2 wt % uranyl acetate; their morphology observation was carried out with a JEOL-200CX transmission electron microscope (TEM). While as to A, the TEM morphologies have not been investigated, for the aggregate structure of A is sensitive to pH values and therefore nondestructive staining is nearly impossible. Differential scanning calorimetry (DSC) measurements of the above aqueous samples were performed with a SETARAM microcalorimeter at a heating rate of 1 °C min-1. The cast films of both samples were prepared by spreading a few drops of the aqueous samples on glass plate and keeping the * To whom correspondence should be addressed. (1) Kunitake, T.; Okahata, Y.; Shimomura, M.; Yasunami, S.; Takarabe, K. J. Am. Chem. Soc. 1981, 103, 5401. (2) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (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) Adam, C. D.; Durrant, J. A.; Lowry, M. R.; Tiddy, G. J. J. Chem. Soc., Faraday Trans. 1 1984, 80, 789.
S0743-7463(96)00257-0 CCC: $12.00
Figure 1. Transmission electron micrographs of acidic aqueous samples of the CnH2n+1NHC2H4NH2 amphiphiles: (a, top) n ) 8; (b, middle) n )12; (c, bottom) n ) 18. plate in vacuo (ca. 20 mmHg) at room temperature.6,7 Then their X-ray diffraction (XRD) patterns were measured on a Rigaku Model D/max-RA diffractionmeter.
Results and Discussion Typical electron micrographs of sample B are illustrated in Figure 1. It is seen that they all display vesicular morphologies, indicating directly that this series of amphiphiles self-assemble into bilayer membranes in acidic aqueous solutions. Figures 2 and 3 display the DSC curves of all aqueous samples and the XRD profiles of (6) Nakashima, N.; Ando, R.; Kunitake, T. Chem. Lett. 1983, 1577. (7) Kunitake, T.; Shimomura, M.; Kajiyama, T.; Harada, A.; Okuyama, K.; Takayanagi, M. Thin Solid Films 1984, 121, L89.
© 1996 American Chemical Society
5502 Langmuir, Vol. 12, No. 22, 1996
Notes
Figure 4. Plots of Tc as a function of the carbon number n in the tail. Figure 2. DSC curves of aqueous bilayers: (a) sample A; (b) sample B.
Figure 3. Small-angle XRD profiles of cast films of C12H25NHC2H4NH2 from (a) pure aqueous solution and (b) acidic solution. Table 1. DSC Results on the Phase Transition Temperatures of Aqueous Solutions of CnH2n+1NHC2H4NH2 in Samples A and B, along with Small-Angle XRD Results of Their Cast Films
amphiphile
evaluated molecular length, Å
n)8 n ) 12 n ) 14 n ) 16 n ) 18
16 21 23.5 26 28.5
phase transition Tc, °C (∆H, kJ/mol) A B 36 (3.97) 47 (16.28) 54 (20.19) 58 (25.91) 63 (31.27)
25 (2.83) 39 (20.42) 46 (27.63) 52 (34.76) 58 (41.68)
long spacing Dn, Å A B 32.11 42.27 47.25 52.32 57.40
28.64 35.96 39.49 43.23 46.61
cast films for samples A and B of n ) 12 amphiphile, respectively. The phase transition temperatures (Tc) and enthalpy (∆H) values from DSC experiments are listed in Table 1, together with the values of long spacing in XRD patterns (Dn), and the plots of Tc as a function of the carbon number n in the tail are given in Figure 4. From Table 1 and Figures 2 and 3, it can be seen that both samples exhibit phase transition temperatures in their DSC curves and also periodic peaks in diffraction profiles of their cast films similar to those reported in literature.8,9 On the other hand, under the same condition the series of amphiphiles fail to disperse uniformly in dilute aqueous NaOH solution even after being sonicated for a long time, and the gel-to-liquid crystal phase transitions (8) Ishikawa, Y.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 621. (9) Shimomura, M.; Aiba, S.; Tajima, N.; Inoue, N.; Okuyama, K. Langmuir 1995, 11, 969.
and ordered diffraction profiles of cast films cannot be obtained from them. It has been established that the gelto-liquid crystal phase transition is one of the intrinsic physicochemical properties of bilayer membranes,2,10 and the self-aggregate structure of amphiphiles in dilute aqueous dispersions has been manifested to be kept in the water-cast films in various aspects.6,7,9,11,12 Moreover, the Tc and Dn values exhibit fairly good linearity with n for both samples (see Figure 4, and infra eqs 1 and 2), suggesting that the physicochemical properties of A and B are similar to each other, and the aggregation behavior of the series of amphiphiles in pure water bears a resemblance to that in acidic dispersions. The above facts offset the lack of direct observation of electron micrographs of sample A, so it can be safely concluded that this series of simple amphiphiles self-assemble into bilayer membranes both in pure water and in aqueous solutions of inorganic acid. In addition, it can be seen from Figure 2 that when n exceeds 14, the DSC scans of both samples show weak and broad pretransitional peaks similar to those of liposomes and other synthetic bilayer membranes. The nature of pretransitions is still not clearly understood now. In comparison with the appearance of only major phase transitions for the bilayers of the amphiphiles with short chains (n ) 8, 12) in both samples A and B, it is considered that the pretransitions may be related with the periodical undulation of the relatively closely packed bilayer membranes.13 Furthermore, as can be seen from Table 1, the bilayer thickness of the cast films from A is nearly twice the evaluated molecular length by CPK model of the corresponding amphiphile, suggesting that the amphiphiles assume the H-like bilayer packing mode. On the other hand, the bilayer thickness of B is smaller than twice the evaluated length but larger than the value. Two possibilities can be proposed, one is the tilted tail-to-tail bilayer packing mode14 and the other is the part interdigitation one of the alkyl chains. Considering the close packing requirement of the alkyl chains, and also the fact that the partly interdigitated chain packing fashion is only observed in the case of asymmetric double-chain amphiphiles,15 the former mode is preferable to the latter. Therefore, the alkyl chains, adopting the tail-to-tail bilayer mode, take different orientations between cast films from pure aqueous dispersions and those from acidic ones. The (10) See for example, Okahata, Y.; Ando, R.; Kunitake, T. Ber. Bunsenges. Phys. Chem. 1981, 85, 789. (11) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsenges. Phys. Chem. 1983, 87, 1134. (12) Ishikawa, Y.; Kunitake, T. J. Am. Chem. Soc. 1986, 108, 8300. (13) Janlak, M. J.; Small, D. M.; Shipley, G. G. Biochemistry 1976, 15, 4575. (14) Lu, X.; Zhang, Z.; Liang, Y. J. Chem. Soc., Chem. Commun. 1994, 2731. (15) Streefland, L.; Yuan, F.; Rand, P.; Hoekstra, D.; Engberts, J. B. F. N. Langmuir 1992, 8, 1715.
Notes
Langmuir, Vol. 12, No. 22, 1996 5503
linear relationships between Dn and n for A and B are written as
A:
Dn ) 2.53n + 11.91
(1)
B:
Dn ) 1.80n + 14.27
(2)
The slope of the equations reflect the tilt angle of the hydrocarbon chain axes.14,16 And it is obtained that in the cast films of pure aqueous dispersions the alkyl chains of the amphilphiles are aligned approximately vertical to the membrane surface as the H-like aggregates, and in those of acidic dispersions the chains take a rather tilt orientation just like the J-aggregates and with the angle of 45° from the normal direction of the bilayer plane, the difference being due to the effect of headgroup protonation. On the other hand, the DSC results of the solutions demonstrated that the phase transition temperatures of A are higher than the corresponding values of B (Table 1). It has been pointed out that the hydrocarbon chains in the H-aggregates are packed more tightly than in the J-ones, and thereafter the former exhibit higher phase transition temperatures.2 So the agreement between the (16) Umemura, J; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62.
XRD results of cast films and the DSC values of solutions demonstrated, as the results by means of various methods in literature,6,7,9,11,12 that the cast films retain structural characteristics essentially analogous to those of the original aqueous dispersions. In conclusion, bilayer membranes can be formed, in dilute solutions of water and hydrochloric acid, from a series of single-chain, rigid-segment-free amphiphiles which are very simple in structure. Combined with our earlier work on the complexed bilayer system with Cu2+coordinated headgroup of the same series of amphiphiles,14 we deduce that one can obtain bilayer membranes in dilute aqueous solution from a single-chain amphiphile just through enhancing the interaction between its headgroups to achieve hydrophilic/hydrophobic balance: in sample A the interaction is mainly the hydrogen bonding as in the well-known case of monoglycerides,5 and in B probably a many body interaction among the protonated nitrogen atoms in the ethylenediamine headgroups and the thusintroduced counterions during protonation. And the proper cylindrical shape in molecular geometry is also demanded, as in the case of a perfluorinated chain.4 Acknowledgment. We thank the State Science and Technology Commission of China for a grant in support of this work. LA9602574
