Langmuir 2003, 19, 8615-8617
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Bilayer in a Liquid Self-Supported Film Charles E. H. Berger,*,† Vance Bergeron,‡ Bernard Desbat,§ Daniel Blaudez,† Hamid Kellay,† and Jean-Marie Turlet† Centre de Physique Mole´ culaire Optique et Hertzienne, CNRS UMR 5798, Universite´ Bordeaux I, 33405 Talence Cedex, France, Laboratoire de Physique, CNRS UMR 5672, Ecole Normale Supe´ rieure de Lyon, 69364 Lyon Cedex 07, France, and Laboratoire de Physico-Chimie Mole´ culaire, CNRS UMR 5803, Universite´ Bordeaux I, 33405 Talence Cedex, France Received April 23, 2003. In Final Form: August 15, 2003 The drainage of vertical soap films formed from nonionic surfactant solutions of pentaethylene glycol monododecyl ether (C12E5) was studied using Fourier transform infrared spectroscopy. At relatively low surfactant concentrations in the bulk solution (but above the critical micelle concentration), a transient surfactant bilayer was observed within these soap films. The expulsion of water and of a surfactant bilayer from the film was followed as a function of time. The bilayer found within the film displayed a much higher degree of molecular order than the two outer monolayers of the soap film. This study can serve as a model for similar studies on biomembranes.
Introduction Many thin film experiments are aimed at studying a system as close as possible to the naturally occurring bilayer of a biomembrane. Because such a molecular system can only exist in an aqueous environment, different model systems are used, such as deposited bilayers1 or inverted bilayers.2 Possibilities of using Fourier transform infrared (FTIR) spectroscopy in an aqueous environment are often limited because of the high IR absorption of water. In this paper, we present a study of a transient molecular structure confined in a thin layer of water between two outer surfactant monolayers. Due to the relatively low amount of water, this system can be quantitatively studied by FTIR spectroscopy. The spectral contribution of the outer monolayers can be separated from that of the confined structure by subtraction. This approach allows revealing any differences in their molecular organization. Specific information concerning the molecular orientation of the surfactant monolayers and the type of macromolecular surfactant structures (i.e., micellar, bilayer, etc.) confined within the film can be inferred. The transient structure turns out to be a bilayer. This information provides new in situ data important for our understanding of surfactant phase behavior in confined geometries and the stability of thin liquid films (e.g., foams, emulsions). Experimental Section Pentaethylene glycol monododecyl ether (C12E5) was obtained from Sigma (France) and Nikko Chemical Co. (Japan) and used without further purification. All solutions were made with * To whom correspondence should be addressed. Present address: Netherlands Forensic Institute, Ministry of Justice, P.O. Box 3110, 2280 GC Rijswijk, The Netherlands. † Centre de Physique Mole ´ culaire Optique et Hertzienne, CNRS UMR 5798, Universite´ Bordeaux I. ‡ Laboratoire de Physique, CNRS UMR 5672, Ecole Normale Supe´rieure de Lyon. § Laboratoire de Physico-Chimie Mole ´ culaire, CNRS UMR 5803, Universite´ Bordeaux I. (1) Sackmann, E. Science 1996, 271, 43. (2) Lhert, F.; Capelle, F.; Blaudez, D.; Heywang, C.; Turlet, J.-M. J. Phys. Chem. B 2000, 104, 11704. Chattopadhyay, A. J. Chem. Educ. 2000, 77, 1339. Chattopadhyay, A. Langmuir 1999, 15, 7881. Yamanaka, T.; Tano, T.; Kamegaya, O.; Exerowa, D.; Cohen, R. Langmuir 1994, 10, 1871. Umemura, J.; Matsumoto, M.; Kawai, T.; Tanaka, T. Can. J. Chem. 1985, 63, 1713.
Figure 1. A typical set of FTIR spectra obtained at 40 s intervals during the draining of a C12E5 soap film. In addition to the gradual decrease in time of the intensity of the O-H stretching bands, an abrupt transition is clearly indicated by a large shift of the baseline. ultrapure water (Milli-Q from Millipore). Vertical soap films were prepared by partly withdrawing a rectangular glass frame (3 cm wide) from a surfactant solution. The film and solution were kept in a cylindrical cell of constant humidity. Absorbance spectra were recorded on a Nicolet 670 FTIR spectrometer in transmission mode at a resolution of 4 cm-1. The infrared radiation with a beam diameter of 1 mm passed through the soap film and through two CaF2 windows (20 mm diameter). With the enclosure partly filled with solution, a saturated atmosphere at room temperature (around 23 °C) was approached. On rare occasions, there was some condensation on the windows, but this was very easily noticeable and no measurements were carried out under such circumstances.
Results In Figure 1, a typical series of FTIR spectra of a draining vertical C12E5 film at a concentration of 5 × 10-4 M (above the critical micelle concentration (cmc), 6.5 × 10-5 M)3 is (3) Deguchi, K.; Meguro, K. J. Colloid Interface Sci. 1972, 38, 596.
10.1021/la0346853 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/18/2003
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Figure 2. The C-H stretching region of the last eight spectra before and first eight spectra after the transition, subtracting the contributions of water and induced background. The decrease in intensity is clear and abrupt and amounts to a factor of 2. Sets of spectra like this one were obtained routinely and reproducibly.
presented. At this concentration (0.02 wt %), the solution is in the micellar region of the phase diagram.4,5 All absorbance spectra are measured relative to a background spectrum without a film. Sensitivity was sufficient for films thinner than 1 nm, and saturation of the absorbance did not occur, even for the thickest films. Spectra were obtained every 40 s with the absorbance decreasing as the film drained. The very wide and intense peak in the 3700-2800 cm-1 region is due to O-H stretching bands and arises from the water in the film. The water core thickness of a film was derived from the intensity of this band using Fresnel’s calculations and literature values for the optical constants of water.6 The much smaller peaks around 2900 cm-1 are the C-H stretching bands associated with the surfactant molecules. Finally, CO2 bands are seen around 2350 cm-1 arising from CO2 in the atmosphere surrounding the film in the first eight spectra. When the film reaches a thickness of the order of 100 nm, a large, sudden shift in the baseline is observed; in Figure 1 this happens after the first eight spectra. The fact that the baseline is raised in the beginning is explained by less incident radiation reaching the detector due to scattering losses, resulting in an increased apparent absorbance relative to the background spectrum without a film. The appearance of the CO2 bands (around 2350 cm-1) in the first eight spectra has the same cause: without scattering, they are invisible due to compensation with the background measurement, but when scattering results in miscompensation, a portion of these strong bands becomes apparent. Figure 2 shows the C-H stretching bands (3000-2800 cm-1 region) taken from the same set of spectra, in which the O-H stretching and baseline contributions have been subtracted. The total C-H stretching integrated intensity always showed an abrupt stepwise decrease by a factor of 2 within experimental accuracy that coincided with the (4) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (5) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (6) Bertie, J. E.; Lan, Z. Appl. Spectrosc. 1996, 50, 1047.
Letters
Figure 3. The averages of the C-H stretching region for the system with and without the inner bilayer (accumulated for several series of spectra). The difference between these two gives us the spectrum of the inner bilayer separately. Even though peak shapes and positions are different, the integrated intensity of the contributions of inner bilayer and outer monolayers are equal. Fits are shown as a guide to the eye.
sudden disappearance of the scattering during the course of film drainage. The transition always happens abruptly at a water core thickness near 100 nm. The intensity and shape of the C-H bands do not change after the transition, while the water core continues to drain until it is thinner than 2 nm, showing that only the two surface monolayers remain after the transition. This is consistent with the system changing from a film with surface monolayers and an inner surfactant structure to one without this inner structure. The factor of 2 indicates that the surface monolayers and the inner surfactant structure must have a very similar amount of surfactant and suggests that the inner structure is a bilayer. When considering the presence of a bilayer in the film, the above-mentioned light scattering could have two possible origins: defects in the bilayer on the micrometer scale or Helfrich type undulations7 of the bilayer which could scatter part of the incident infrared radiation. These undulations might also explain the long-range interactions implied by the removal of the inner bilayer at a film thickness of around 100 nm. Another possibility is that long-range double layer forces could arise from a very low salt level in the nonionic systems which provides long Debye lengths. In Figure 3, the C-H stretching region before the transition is shown (averaged from many spectra) as well as the average of the spectra after the transition. With the former corresponding to a system with a bilayer and the latter to a system without, subtraction of the two gives the spectrum for the bilayer only. This subtraction method relies on the reasonable assumption that the outer monolayers are not changed by the transition. The spectral contribution of the C12E5 bilayer inside the film is very different from that of the surface monolayers. The symmetric and antisymmetric methylene bands are positioned at 2849 and 2919 cm-1, respectively, both shifted 5 cm-1 downward relative to the case for the outer monolayers. These band positions indicate that there are very few gauche defects in the hydrocarbon chains.8,9 (7) Helfrich, W. Z. Naturforsch. 1978, 33a, 305. (8) Ricard, L.; Abbate, S.; Zerbi, G. J. Phys. Chem. 1985, 89, 4793.
Letters
The width of the antisymmetric and symmetric methylene bands is smaller for the inner bilayer (24 and 14 cm-1, respectively) than for the surface monolayers (31 and 25 cm-1, respectively). Their narrow width shows a high degree of positional order of the chains (i.e., the local environments are very similar for all molecules).10,11 The total intensity of the bands reveals that the orientation and the density of the molecules in the confined bilayer are not very different from those of the molecules in the surface monolayers (most likely perpendicular to the plane of the film). A confined micellar layer would have given a much smaller contribution to the C-H band intensity than the one observed, since many of the surfactant molecules in it would not be oriented perpendicularly to the film. Taken together, these results indicate that the surfactant structure inside the film has a higher degree of molecular order than the surface monolayers. This strongly supports that the inner structure is a bilayer. This is significant, as it represents the first direct evidence of bilayers within thin liquid films formed from micellar solutions. Although evidence of micellar layers in thin films formed from micellar surfactant solutions is wellestablished,12,13 it has only been conjectured that surfactant bilayers can be seen within films from these solutions.13 Indeed, bilayer or micellar structure formation in confined films is not mutually exclusive; certain systems could possibly contain both structures simultaneously. Here we show the existence of a transient bilayer in the film without indication of micellar structuring. If micelles were present, surfactant molecules not oriented perpendicularly to the surface of the film would be revealed by a lower contribution to the C-H stretching bands in our spectra. In addition to the study of the surfactant structures, information can also be obtained on the water core of the film and, more specifically, on how water drains from the film. Figure 4 shows the evolution of the water core thickness obtained from the intensity of the O-H stretching band centered around 3400 cm-1 during film drainage. Note that no clear discontinuity occurs, unlike the abrupt transition seen in the C-H stretching region in Figure 2. Conclusion Our experimental data indicate the presence of a highly organized transient bilayer inside the water core of a draining vertical soap film. This bilayer can be character(9) Flach, C. R.; Brauner, J. W.; Mendelsohn, R. Biophys. J. 1993, 65, 1994. (10) Tasumi, M.; Shimanouchi, T. J. Chem. Phys. 1965, 43, 1245. (11) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta 1978, 34A, 395. (12) Nikolov, A. D.; Kralchevsky, P. A.; Ivanov, I. B.; Wasan, D. T. J. Colloid Interface Sci. 1989, 133, 3. (13) Bergeron, V.; Radke, C. J. Langmuir 1992, 8, 3020.
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Figure 4. Water core thickness of the soap film as determined from Fresnel’s calculations. The results are corrected for the scattering contribution to the absorbance. The arrow indicates between which datapoints the bilayer disappeared.
ized as having a high degree of positional order, a low number of gauche defects, and an orientation that is predominantly perpendicular to the plane of the film (like in the surrounding surface monolayers). The sudden disappearance of this bilayer does not seem to influence the draining of the water from the film. An exciting aspect of this method is that it allows one to spectroscopically investigate a system close to a biomembrane: a free bilayer in an aqueous environment. Since the surface monolayers that hold the system together can easily be studied separately, we can readily separate their spectral contribution from that of the bilayer. Furthermore, this study provides the first direct evidence that surfactant bilayers can occur in thin liquid films formed from micellar solutions. The presence of such bilayers suggests that systems displaying micellar structuring may also simultaneously contain bilayer structure. This generalized point of view offers a larger range of possibilities to explain foam-film stability in concentrated micellar solutions. Acknowledgment. This work was supported by the EC under the “TMR” network: foam stability and wetting transitions. LA0346853
