Langmuir 2002, 18, 9611-9612
9611
Spontaneous Formation of Vesicles from Octadecylamine in Dilute Aqueous Solution Induced by Ag(I) Ion Xuzhong Luo, Wangen Miao, Sanxie Wu, and Yingqiu Liang* Lab of Mesoscopic Materials Science and State Key Lab of Coordination Chemistry, Nanjing University, Nanjing 210093, P. R. China Received July 1, 2002. In Final Form: September 27, 2002
The self-organized structure of vesicles is a fascinating phenomenon that attracted enormous interest not only in colloid chemistry but also in biochemistry, in a wide variety of chemical disciplines, and in material science.1-3 In recent years, synthetic amphiphiles that can self-organize into vesicles in dilute aqueous solution have been reported with different chemical structures and composition of headgroups and hydrophobic tail(s).4-11 It has been known that common single-tail hydrocarbon amphiphiles usually form micelles in dilute aqueous solution other than vesicles. The formation of vesicles from single-chain amphiphiles in dilute aqueous solution 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 large rigid segments (e.g. biphenyl, azobezene, etc.),12,13 a hyperextended hydrocarbon chain,14 and electrostatic attraction between oppositely charged headgroups,5,15 stable vesicles are obtainable. Sasaki et al.16 and another research group17 report that vesicle-forming synthetic double-tail amphiphiles can assemble into a higher ordered self-assembled structure by metal ion recognition at the membrane surface. Several recent reports reveal that simple micelle-forming amphiphiles with a common hydrocarbon chain form vesicles induced by metal ions in dilute aqueous * E-mail:
[email protected]. (1) Lasic, D. D. Liposomes: From Physics to Applications; Elsevier: Amsterdam, 1993. (2) Sackmann, E. In Structure and Dynamics of Membranes; Sackmann, E., Lipowsky, R., Eds.; Elsevier/North-Holland: Amsterdam, 1995; Chapter 1. (3) Knight, C. G. Liposomes: From Physical Structure to Therapeutic Applications; Elsevier/North-Holland: Amsterdam, 1981. (4) Martinez, J. S.; Zhang, G.; Holt, P.; Jung, H.-T.; Carrano, C. J.; Haygood, M. G.; Butler, A. Science 2000, 287, 1245. (5) Putlitz, B. Z.; Hentz, H. P.; Landfester, K.; Antonietti, M. Langmuir 2000, 16, 3003. (6) Bandyopadhyay, P.; Bharadwaj, P. K. Langmuir 1998, 14, 7537. (7) Bhattacharya, S.; De, S.; George, S. K. J. Chem. Soc., Chem. Commun. 1997, 2287. (8) Kimizuka, N.; Wakiyama, T.; Miyauchi, H.; Yoshimi, T.; Tokuhiro, M.; Kunitake, T. J. Am. Chem. Soc. 1996, 118, 5808. (9) Ghosh, P.; Khan, T. K.; Bharadwaj, P. K. J. Chem. Soc., Chem. Commun. 1996, 189. (10) Shimizu, T.; Masuda, M.; Shibakami, M. Chem. Lett. 1997, 267. (11) Schenning, A. P. H.; Bruin, B. De; Feiters, M. C.; Nolte, R. J. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1662. (12) Kunitake, T.; Okahata, Y.; Shimomura, M.; Yasunami, S.; Takarabe, K. J. Am. Chem. Soc. 1993, 115, 3840. (13) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (14) Menger, F. M.; Yamasaki, Y. J. Am. Chem. Soc. 1993, 115, 3840. (15) Fukuda, H.; Kawata, K.; Okuda, H. J. Am. Chem. Soc. 1990, 112, 1635. (16) Waggoner, T. A.; Last, J. A.; Kotula, P. G.; Sasaki, D. Y. J. Am. Chem. Soc. 2001, 123, 496. (17) Constable, E. C.; Meier, W.; Nardin, C.; Mundwiler, S. J. Chem. Soc., Chem. Commun. 1999, 1483.
Figure 1. Transmission electron micrograph of the aqueous sample from a dispersion of octadecylamine in dilute AgNO3 solution (poststained with 2 mass % uranyl acetate).
solution.4,18 Herein, we present that, in dilute aqueous solution, octadecylamine, which forms micelles in dilute acidic aqueous solution, assembles into vesicles induced by Ag(I) ion. All chemicals (octadecylamine, copper sulfate, and silver nitrate, etc.) used are of analytical reagent grade. It is known that octadecylamine forms micelles in dilute acidic aqueous solution.19 To study the influence of adding metal ions on the aggregate behavior of octadecylamine in water, the 5.0 mmol‚L-1 (10 mL) dispersion (2C18H37NH2‚AgNO3) was prepared by mixing octadecylamine (0.10 mmol), AgNO3 (0.05 mmol), and doubly distilled water under sonication. The obtained dispersion was investigated by transmission electron microscopy (JEOL Model JEM200CX) and differential scanning calorimetry (SETA-RAM MODEL Micro-DSC), and the cast films13,20 were studied by small-angle X-ray diffraction (Rigaku Model D/MaxRA). Sonication of octadecylamine in dilute aqueous solutions of AgNO3 results in a stable and uniform white emulsion. Transmission electron micrography of the dispersions (shown in Figure 1) reveals that the dispersion displays vesicular morphologies. The XRD pattern (shown in Figure 2) of the corresponding cast film from the dispersion exhibits periodic peaks, demonstrating the well-ordered structure. The long spacing (D ) 4.55 nm) of the aggregates obtained by small-angle X-ray diffraction experiments is slightly smaller than twice the evaluated monomolecular length obtained with the CPK model (2.42 nm) but much larger than the monomolecular length of the surfactant, indicating that the amphiphile assembles into bilayer membranes bearing a slightly tilted tail-to-tail packing model in the dilute AgNO3 solution. Shown in Figure 3 is the DSC curve of the dispersions. The endothermic peaks in the curve indicate the gel-toliquid crystal phase transition, which is one of the basic physicochemical properties of bilayer membranes.13,21 On heating, the dispersion exhibits a gel-to-liquid crystal phase transition (Tc transition) at 69 °C (∆H ) 20.2 kJ‚mol-1). The relatively low phase transition temperatures imply that in the bilayer the hydrocarbon chains of (18) Luo, X.; Wu, X.; Liang, Y. J. Chem. Soc., Chem. Commun. 2002, 492. (19) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982; p 8. (20) Nakashima, N.; Ando, R.; Kunitake, T. Chem. Lett. 1983, 1577. (21) Kunitake, T.; Kimizuka, N.; Higashi, N.; Nakashima, N. J. Am. Chem. Soc. 1984, 106, 1978.
10.1021/la0261646 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/29/2002
9612
Langmuir, Vol. 18, No. 24, 2002
Figure 2. XRD pattern of the cast film from a dispersion of octadecylamine in dilute AgNO3 solution.
Figure 3. DSC curve of the dispersion of octadecylamine in a dilute AgNO3 solution.
amphiphiles adopt z loose packing mode (tail-to-tail packing model).13,22 This finding is consistent with the result obtained by XRD. In this experiment, we found that no stable dispersions and no vesicles could be obtained by dispersing octadecylamine (0.10 mmol) in 10 mL of an aqueous solution of KNO3 (5 mmol‚L-1) or Na2SO4 (2.5 mmol‚L-1) under sonication. This fact may help us to understand the (22) Spiker, C. G.; Parry, R. W. J. Am. Chem. Soc. 1953, 75, 2726.
Notes
Figure 4. Schematic representation of the micelle formation from octadecylamine in acidic aqueous solution and the bilayer formation upon coordination of Ag(I) to octadecylamine in aqueous solution. The relative headgroup size between micelle and vesicle is not draw to scale.
mechanism of the vesicle formation of octadecylamine in dilute AgNO3 solution. It was known that Ag(I) ions can coordinate with octadecylamine to form stable complexes,23 while K(I) and Na(I) ions cannot. Therefore, it is reasonable to deduce that the coordination between octadecylamine and Ag(I) ion is responsible for the vesicle formation. Israelachvili and co-workers22 proposed the shapestructure or packing parameter concept, which would predict micelles for single-chain surfactants, and bilayers for double-chain surfactants. In the dispersion of octadecylamine in AgNO3 solution, a Ag(I) ion coordinates with two octadecylamine molecules and thus they form pseudo-double-tailed surfactants with a coordinated headgroup, which may self-assemble into sphereical vesicles in aqueous solution. The micelle formation in dilute acidic aqueous solution and bilayer formation of octadecylamine in dilute AgNO3 solution are illustrated in Figure 4. In conclusion, we have shown that, in dilute aqueous solution, octadecylamine, an amphiphile which only forms micelles in dilute acidic aqueous solution, assembles into vesicles induced by Ag(I) ion. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (NSFC, NO. 20273029). LA0261646 (23) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
