Langmuir 2008, 24, 3759-3766
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Dynamics of Spontaneous Vesicle Formation in Fluorocarbon and Hydrocarbon Surfactant Mixtures Thomas M. Weiss,† Theyencheri Narayanan,*,† and Michael Gradzielski‡ European Synchrotron Radiation Facility, BP 220, F-38042 Grenoble, France, and Stranski Laboratorium fu¨r Physikalische und Theoretische Chemie, Institut fu¨r Chemie, Technische UniVersita¨t Berlin, Sekr. TC7, Strasse des 17. Juni 124, D-10623 Berlin, Germany ReceiVed NoVember 12, 2007. In Final Form: December 30, 2007 The spontaneous self-assembly of unilamellar vesicles was investigated by means of time-resolved synchrotron small-angle X-ray scattering. The self-assembly process was initiated by rapid mixing of anionic surfactant micelles with either zwitterionic or cationic surfactant micelles in equimolar ratio using a stopped-flow device. For the zwitteranionic systems, transient disklike mixed micelles are observed as structural intermediates prior to the onset of vesiculation. These disklike micelles display an exponential growth law, and above a critical size they close to form unilamellar vesicles. In the catanionic system, the earliest observable structures within the mixing time of 4 ms are unilamellar vesicles. Nevertheless, in both systems a narrow distribution of the vesicle size was observed at the initial stages of their formation. The subsequent evolution of the vesicle size distribution depends on the subtle differences in the bilayer composition and properties.
Introduction The self-assembling properties of amphiphilic molecules are increasingly exploited in nanotechnology, for instance in the bottom-up approach for forming nanostructured systems.1-4 While the equilibrium microstructures such as micelles, vesicles, lamellar, and other lyotropic phases including cubic and hexagonal phases have been extensively investigated over the past decades,5-7 the transient intermediate states have become the focus of research only more recently.8-18 Understanding the kinetic pathways of self-assembly is important in exploiting these systems in practical applications such as detergency, encapsulation, controlled release, stabilization mechanisms of colloidal dispersions, and nanoreactors.19 Micellar self-assembly can be induced in many * Corresponding author. E-mail:
[email protected]. † European Synchrotron Radiation Facility. ‡ Technische Universita ¨ t Berlin. (1) Tredgold, R. H. Order in Thin Organic Films; Cambridge University Press: Cambridge, 1994. (2) Ulman, A. Thin Films: Self-Assembled Monolayers of Thiols; Academic Press: San Diego, CA, 1998. (3) Cheng, J. Y.; Mayes, A. M.; Ross, C. A. Nat. Mater. 2004, 3, 823. (4) Gradzielski, M.; Mu¨ller, M.; Bergmeier, M.; Hoffmann, H.; Hoinkis, E. J. Phys. Chem. B 1999, 103, 1416. (5) Chevalier, Y.; Zemb, Th. Rep. Prog. Phys. 1990, 53, 279. (6) Laughlin, R.G. The Aqueous Phase BehaVior of Surfactants; Academic Press: San Diego, CA, 1994. (7) Micelles, Membranes, Microemulsions, and Monolayers; Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds.; Springer: New York, 1994. (8) O’Connor, A. J.; Hatton, T. A.; Bose, A. Langmuir 1997, 13, 6931. (9) Egelhaaf, S. U.; Schurtenberger, P. Phys. ReV. Lett. 1999, 82, 2804. (10) Jung, H.-T.; Coldren, B.; Zasadzinski, J. A.; Iampietro, D. J.; Kaler, E. W. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 1353. (11) Schmo¨lzer, S.; Gra¨bner, D.; Gradzielski, M.; Narayanan, T. Phys. ReV. Lett. 2002, 88, 258301. (12) Jung, H.-T.; Lee, S. Y.; Kaler, E. W.; Coldren, B.; Zasadzinski, J. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 15318. (13) Shioi, A.; Hatton, T. A. Langmuir 2002, 18, 7341. (14) Leng, J.; Egelhaaf, S. U.; Cates, M. E. Biophys. J. 2003, 85, 1624. (15) Wang, H.; Nieh, M. P.; Hobbie, E. K.; Glinka, C. J.; Katsaras, J. Phys. ReV. E 2003, 67, 060902. (16) Weiss, T. M.; Narayanan, T.; Wolf, C.; Gradzielski, M.; Panine, P.; Finet, S.; Helsby, W. I. Phys. ReV. Lett. 2005, 94, 038303. (17) Bryskhe, K.; Bulut, S.; Olsson, U. J. Phys. Chem. B 2005, 109, 9265. (18) Micali, N.; Villari, V.; Consoli, G. M. L.; Cunsolo, F.; Geraci, C. Phys. ReV. E 2006, 73, 051904. (19) Patarin, J.; Lebeau, B.; Zana, R. Curr. Opin. Colloid Interface Sci. 2002, 7, 107.
different ways (such as changes in concentration or temperature) and the time scales of structural organization may range from millisecond up to weeks.20,21 The underlying kinetics usually involves multiple stages with several rate-limiting steps.22-24 Moreover, the complex free-energy landscape in these systems may lead to long-lived metastable states. For instance, unilamellar vesicles (ULVs) are believed to be in such a state; nevertheless, increasing evidence has emerged that at least in some circumstances ULVs may even be a thermodynamic equilibrium state.10,12,25,26 Moreover, these vesicles can be considered as models for biological cell membranes and are closely related to certain biomimetic systems.26 Therefore, ULVs are intriguing systems for exploring the dynamics of self-assembly.16 In practice, ULV can be produced by different routes, including mixing equimolar cationic and anionic micellar solutions, addition of a cosurfactant to an anionic micellar solution, the unbinding transition of a lamellar phase, sonication, or by extrusion of multilamellar vesicles or lamellar phases.10,21,26,27 Of these different methods, roughly equimolar mixing leads to a relatively narrow distribution of vesicle size that remains stable over the time scale of weeks or even months.10-12,16,21 Time-resolved small-angle X-ray scattering (SAXS) is a convenient technique to probe the transient micellar entities and their kinetics in the millisecond range and above.16 The transition can be conveniently induced by rapid mixing of equimolar amounts of two different micellar species with a stopped-flow device.8,11,16 In this study, relatively simple mixtures of cationic and anionic, and zwitterionic and anionic micellar solutions were employed to investigate the self-assembly of ULV. It should be noted that such surfactant mixtures generally exhibit strong synergism that is less in the case of zwitterionic/cationic surfactant mixtures, (20) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905. (21) Gradzielski, M. J. Phys.: Condens. Matter 2003, 15, R655. (22) Aniansson, E. A. G.; Wall, S. N. J. Phys. Chem. 1974, 78, 1024. (23) Neu, J. C.; Canizo, J. A.; Bonilla, L. L. Phys. ReV. E 2002, 66, 061406. (24) Noguchi, H.; Gompper, G. J. Chem. Phys. 2006, 125, 164908. (25) Gradzielski, M.; Bergmeier, M.; Hoffmann, H.; Mu¨ller, M.; Grillo, I. J. Phys. Chem. B 2000, 104, 11594. (26) Nieh, M.-P.; Raghunathan, V. A.; Kline, S. R.; Harroun, T. A.; Huang, C.-Y., Pencer, J.; Katsaras, J. Langmuir 2005, 21, 6656. (27) Lasic, D. D. Liposomes; Elsevier: Amsterdam, 1993.
10.1021/la703515j CCC: $40.75 © 2008 American Chemical Society Published on Web 03/08/2008
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since typical cationic head groups are only very slightly polarizable and therefore can only interact weakly with the dipolar zwitterionic head group. The choice of mixtures of hydrocarbon and fluorocarbon surfactants was primarily guided by the significant enhancement of X-ray contrast as compared to pure hydrocarbon surfactants. In addition, such mixtures with perfluorinated surfactants display a much extended vesicular phase than the equivalent hydrocarbon system.
Weiss et al. scale using the standard procedure described elsewhere.33 The normalized two-dimensional SAXS patterns were azimuthally averaged to obtain the scattered intensity as a function of q. The corresponding background intensity of the stopped-flow cell filled with water was subsequently subtracted. The resulting background subtracted scattered intensity is denoted by I(q). Data Analysis. The SAXS intensity from a suspension of identical, randomly oriented particles can be factorized as given below35 I(q) ) NV2∆F2P(q) S(q)
Experimental Materials and Method Materials. The initial micelles comprised of 50 mM solutions of tetradecyldimethylamine oxide [C14H29N(CH3)2O], lithium perfluorooctanoate (C7F15COOLi), tetradecyltrimethylammonium bromide [C14H29N(CH3)3Br], and lithium perfluorooctanesulfonate (C8F17SO3Li) are henceforth referred to as TDMAO, LPFO, TTAB, and LPFOS, respectively.28-31 TDMAO was a gift from Clariant and was purified by recrystallizing twice from acetone. TTAB was purchased from Aldrich (99%) and purified by crystallization from ether/ethanol. LPFO and LPFOS were obtained from Fluka and recrystallized from ether prior to use. All surfactant solutions were prepared using doubly distilled water. Stopped-Flow Device. The fast mixing experiments were performed using a stopped-flow device (SFM-3, Bio-Logic) that has been specifically adapted for SAXS experiments. The mixing device consisted of three motorized syringes and two turbulent mixers. More details of the apparatus can be found elsewhere.32 The scattering cell of the stopped-flow apparatus is made of a thin-walled quartz capillary with diameter of 1.6 mm and wall thickness of 10 µm. After the mixing sequences, the flow is arrested by a solenoid stopper that is activated upon the cessation of syringe movement. The minimum dead time is defined by the sum of the mixing time (
