Langmuir 1996, 12, 5751-5755
5751
Microtubes Created in Thin Liquid Films during Bilayer Adhesion and Fusion Vance Bergeron Equipe Mixte RP-CEA, Service de Chimie Mole´ culaire, CEA Saclay, 91191 Gif sur Yvette, France Received August 12, 1996. In Final Form: October 7, 1996X Subsurface bilayers are found to exist at the air-water interface of spontaneously forming vesicular solutions of sodium bis(2-ethylhexyl)sulfosuccinate, (AOT). The bilayers produce long-range repulsive forces that increase the stability of foam and emulsion films formed from these solutions. In addition, during film formation and adhesion of two interfaces possessing subsurface bilayers, tubular networks of wrinkled bilayers become trapped within the film. This phenomenon is analogous to pocket formation in lipid membrane multilayers seen during a binding transition, and the resulting networks slowly evolve through processes that mimic two-dimensional foam coarsening.
It is now recognized that diverse phenomena such as cell adhesion and fusion,1,2 foam and emulsion stability,3,4 and wetting behavior5,6 are all governed by surface-force interactions. The majority of existing experimental information concerning these forces has been obtained by two techniques: the surface force apparatus (SFA), where mica-supported interfaces are brought into contact,7 and the osmotic stress technique, which squeezes together membranes freely suspended in the bulk solution.8 Here, a complementary technique is used, the thin-film balance (TFB), which brings two individual fluid interfaces into contact. Previously this technique has been used to study the forces and interactions across foam films formed from simple surfactant solutions9-11 and was instrumental in elucidating and quantifying the structural forces created from micellar solutions.12,13 In this Letter the method is applied to spontaneously forming vesicular solutions of a common double-chained surfactant. The results obtained from these solutions provide us with evidence of bilayer adsorption at a fluid interface, confirm previously proposed membrane-binding mechanisms, and reveal new phenomena (micro-tube formation) that can occur during bilayer adhesion and/or fusion. These findings are important to a broad class of problems concerning biological function, structure and dynamics in complex fluids, and foam and emulsion stability. The thin-film balance we use to study the surface forces and film-thinning dynamics between two fluid interfaces is based on the original device of Mysels and Jones.9 Isolated liquid films (e.g., foam, emulsion films) are formed and held in a hole (e.g., 0.1-1 mm diameter) drilled X Abstract published in Advance ACS Abstracts, November 1, 1996.
(1) Cowely, A. C.; Fuller, N. L.; Rand, R. P.; Parsegian, V. A. Biochemistry 1978, 17, 3163. (2) Helm, C. A.; Israelachvili, J. N.; McGuiggan, P. M. Science 1989, 246, 919. (3) Israelachvili, J. In Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992. (4) Kruglyakov, P. M. In Thin Liquid Films; Fundamentals and applications; Ivanov, I. B., Ed.; Marcel Dekker, Inc.: New York, 1988; Chapter 11. (5) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (6) Sackman, E. Science 1996, 271, 43. (7) Israelachvili, J. N.; Adams, G. E. Nature 1976, 262, 774. (8) Parsegian, V. A.; Fuller, N.; Rand, R. P. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 2750. (9) Mysels, K. J.; Jones, M. N. Discuss. Faraday Soc. 1966, 42, 42. (10) Exerowa, D.; Kolarov, T.; Khristov, K. H. R. Colloids Surf. 1987, 22, 171. (11) Bergeron, V.; Waltermo, A.; Claesson, P. Langmuir 1996, 12, 1336. (12) Bergeron, V.; Radke, C. J. Langmuir 1993, 8, 3020. (13) Pollard, M. L.; Radke, C. J. J. Chem. Phys. 1994, 101, 6979.
S0743-7463(96)00796-2 CCC: $12.00
through a solution-permeable porous disk that is connected to a reservoir containing the solution under study.12 By control of the pressure difference between a drop of fluid in the hole and the reservoir (i.e., capillary pressure), solution is drawn through the porous disk into the reservoir, eventually producing a free-standing thin film suspended in the hole. The device then relies on a balance between the imposed capillary pressure and the so-called disjoining pressure14 within the film. The later is equivalent to an osmotic pressure and arises from surface-force interactions created by having the interfaces in close proximity. Thus, for flat films in equilibrium, the disjoining pressure is directly determined by measurements of the capillary pressure (10-100000 ( 1 Pa). Film thicknesses to within (0.5 nm are simultaneously measured by microinterferometric methods15 in conjunction with video microscopy.12 Combining measurements of the pressure and thickness leads to equilibrium disjoiningpressure isotherms which are equivalent to force isotherms obtained by other techniques (e.g., SFA). In addition, the TFB allows us to quantitatively study three-dimensional film-thinning dynamics as two fluid interfaces approach. This is accomplished by videoenhanced microscopy (VEM). An incident-light microscope, coupled with a CCD detector, is focused on the film, and the intensity of the light reflected from the two film interfaces is used to determine the film-thickness profile and to reconstruct an image of the film. All of the dynamic events observed are stored on standard video recording devices for convenient digital analysis. This type of information cannot be determined by the SFA or osmotic stress techniques which makes the TFB a powerful complementary tool. Further experimental details can be found elsewhere.12 In this work we exploit the advantages of the TFB by measuring both the disjoining pressure isotherms and film-thinning dynamics for vesicular solutions of a simple anionic double-tailed surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT). This surfactant was obtained from Sigma and purified using the methods outlined by Ficheux.16 Figure 1a contains two foam-film (e.g., airsolution-air) disjoining-pressure isotherms obtained from 5 mM solutions of purified AOT. The open circles identify data obtained from pure AOT solutions while the filled squares correspond to solutions with 0.1 M added NaCl. The circles represent an average of 12 independent (14) Derjaguin, B.; Churaev, N. V. J. Colloid Interface Sci. 1978, 66, 389. (15) Sheludko, A. Adv. Colloid Interface Sci. 1967,1, 391. (16) Ficheux, M. F. Ph.D. Thesis, Universite´ Bordeaux I, 1995.
© 1996 American Chemical Society
5752 Langmuir, Vol. 12, No. 24, 1996
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Figure 1. (a, left) Disjoining pressure versus the total film thickness (i.e., aqueous core plus adsorbed surfactant monolayers) for single foam films made from solutions containing 5 mM AOT (circles) and 5 mM AOT with 0.1 M added NaCl (squares). In both curves a strong repulsive force leads to a rather thick ∼10 nm limiting film thickness. (b, right) For the same solutions in (a), X-ray reflectivity curves off the bulk air-solution interface display Kiessig fringes indicating subsurface layer thicknesses of 8.8 nm for the pure AOT solution and 4.0 nm for the solution with 0.1 M added NaCl. Twice these layer thicknesses (17.6 and 8.0 nm) correspond well to the change in slope and limiting film thickness found in the disjoining pressure isotherms.
experiments, and the two curves sketched alongside these symbols indicate the boundaries within which all the data obtained lie. The 0.1 M added salt case was highly reproducible and in this case the error bars are smaller than the size of the square symbols representing the data. We also note that transmission electron microscopy confirmed that the salt-enriched case spontaneously forms a dilute polydisperse vesicular solution. As expected the long-range part of the disjoining-pressure isotherm for the pure AOT solutions is dominated by the electrostatic double-layer forces arising from the counterions of the negatively charged surfactant. Furthermore, the addition of salt suppresses the length scale of these forces to closer distances, consistent with Debye-Hu¨ckel theory. However, the strong repulsion producing a limiting thickness near 10 nm in both cases is rather surprising. For example, at 0.1 M added NaCl, the theoretical Debye length is of order 0.9 nm, which implies that a very thin film should be formed (∼5 nm total film thickness including the surfactant interfacial layers). Indeed, for singlechained surfactant systems (i.e., sodium dodecyl sulfate) above the critical micelle concentration (cmc) and containing similar amounts of salt, total film thicknesses of 4.4 nm are observed.10,12,17 Thus, the limiting thickness of our AOT films is nearly twice the expected value. Recent X-ray reflectivity data off the air-solution interface of our AOT solutions shed some light on these anomalous film thicknesses.18 The reflectivity data were obtained with a MICRO-CONTROLE diffractometer specially equipped to study horizontal liquid surfaces at (17) Be´lorgey, O.; Benattar, J. J. Phys. Rev. Lett. 1991, 66, 313. (18) Schalchli, A.; Sentenac, D.; Benattar, J. J.; Bergeron, V. J. Chem. Soc., Faraday Trans. 1996, 92 (4), 553; Corrigendum J. Chem. Soc., Faraday Trans. 1996, 92 (12), 2317.
grazing incidences and are reproduced in Figure 1b. The striking feature in the reflectivity curves from these airsolution surfaces is the interference fringes (i.e., Kiessig fringes) produced by large density gradients at the interface. Since the extended length of an AOT molecule is only 1.4 nm, these fringes cannot be accounted for by the surfactant monolayer at the interface. Instead, the best-fit theoretical curves using a homogeneous-slab model indicate a subsurface layer with a density 10% lower than the bulk, and 8.8 ( 0.2 nm thick for pure AOT solutions, reducing to 4.0 ( 0.2 nm thick in the solutions containing 0.1 M added NaCl (see ref 18 for details). Similar subsurface layers at an air-solution interface have recently been detected by specular reflection of neutrons off the surface of surfactant solutions above the cmc.19 Interestingly, in the present case twice the thickness of the subsurface layers agrees well with the optical thicknesses measured for the corresponding thin films in Figure 1a; 17.6 nm matches the location of the abrupt change in slope for the 5 mM AOT disjoining pressure isotherm, and 8.0 nm roughly corresponds to the limiting film thickness in both cases. The crucial information needed to interpret these findings is provided by our film thinning observations in Figure 2. This figure contains a series of snapshots that depict the film-thinning dynamics of a foam film formed from a 5 mM AOT + 0.1 M NaCl solution. The film is 0.2 mm in diameter and the photos where taken at 30-s intervals about 2 min after bringing the interfaces together. The dark regions correspond to a uniform 10 nm film thickness, while the light and colored regions range from 100 to several hundred nanometers thick. (19) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1993, 97, 13907.
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Figure 2. Images of a foam film (5 mM AOT + 0.1 M NaCl), displaying the film-thinning dynamics (binding-transition) that leads to a tubular network in the film. This film is 0.2 mm in diameter and the photos where taken at approximately 30 s intervals. The black regions correspond to a thin uniform thickness of 10 nm while the white and colored areas are thick-film regions ranging from 100 to several hundred nanometers, respectively. After nucleation of an adhesive thin-film region, excess solution is pushed in advance of its expanding perimeter. When two expanding domains meet, instead of coalescing, a stable thick-film region (tube) is created. Once formed, these tubes maintain their dimensions and only slowly (as much as an hour) get pushed into the Plateau borders of the film.
Thus, the long interconnected strands running through the film are actually thick-film regions (microtubes) suspended in the film. These structures have not been seen before. The merging of two circular thin-film regions at the bottom portion of the photos demonstrates how the tubes are formed. First, attractive van der Waals forces nucleate an adhesive thin-film transition at different positions in the film. These regions then expand, driving excess solution at their borders into contact which then forms a tubular channel embedded in the film (
