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Modification of Black Film Hydration by Infrared Irradiation J.-J. Benattar,*,† Q. Shen,†,| S. Bratskaya,† V. Petkova,† M. P. Krafft,‡ and B. Pucci§ CEA-Saclay, DSM/DRECAM, Service de Physique de l’Etat Condense´ , F-91191 Gif sur Yvette Cedex, France, Chimie des Syste` mes Associatifs, Institut Charles Sadron (CNRS), 6, rue Boussingault, F-67083 Strasbourg Cedex, France, and Universite´ d’Avignon, Laboratoire de Chimie Bioorganique et des Syste` mes Mole´ culaires Vectoriels, Faculte´ des Sciences, 33 rue Louis Pasteur, F-84000 Avignon, France Received November 19, 2003. In Final Form: December 9, 2003 The hydration of black films is an essential parameter which governs their structure. We present a new method to modify their water core using IR irradiation. The absorption of IR by water increases the film temperature and induces reversible processes changing the film thickness. These film modifications are controlled using X-ray reflectivity during irradiation. In Newton black films where the aqueous core is reduced to a hydration layer, IR provokes evaporation of the bonded water through the hydrophobic chain walls. In common black films containing liquid water, IR induces more complex hydrodynamic processes.
Introduction A black film is a very thin structure which consists of two well-organized monolayers of surfactant molecules separated by a water core. Black films have been studied extensively,1 and the primary emphasis was on the nature and range of molecular forces, the thinning, and the factors affecting their stability. A good understanding of these films is important since they play a major role in foams and in emulsion stability. Two different equilibrium black films can be obtained: common black films (CBFs) and Newton black films (NBFs). CBFs, whose interactions can be described by the colloid stability theory,2 have a rather large equilibrium thickness due to a central core containing liquid water, whereas NBFs are much thinner with a central core reduced to a hydration layer of bonded water molecules.3,4 The water content of a black film is an essential parameter, which governs its structure as well as its molecular interactions. For example, the phase transition CBF-NBF can be realized by reducing the aqueous core of a CBF; the reduction of hydration in a NBF could also significantly modify its ordering. To date, it appears that no studies have been made concerning the effect of infrared irradiation on the hydration of black films. In the present paper, we investigate the water transport properties in black films induced by IR irradiation. These water core modifications provoked by the infrared source are continuously followed by means of X-ray reflectivity. This effect could be used as the basis of a new and simple method to control the water core of black films by a near low-power infrared * Corresponding author. E-mail:
[email protected]. † CEA-Saclay, DSM/DRECAM, Service de Physique de l’Etat Condense´. ‡ Chimie des Syste ` mes Associatifs, Institut Charles Sadron (CNRS). § Universite ´ d’Avignon, Laboratoire de Chimie Bioorganique et des Syste`mes Mole´culaires Vectoriels, Faculte´ des Sciences. | Permanent address: School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P.R. China. (1) Mysels, K. J.; Shinoda, K.; Frankel, S. Soap Films; Pergamon Press: New York, 1959. (2) Verwey, E. J. W.; Overbeeck, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1949. (3) Be´lorgey, O.; Benattar, J. J. Phys. Rev. Lett. 1991, 66, 313. (4) Sentenac, D.; Benattar, J. J. Phys. Rev. Lett. 1998, 81, 160.
source. We separately study the infrared action on liquid water and bonded water on CBFs and NBFs, respectively. To generalize our method, we study films stabilized with various surfactants (ionic, nonionic, and zwitterionic). Experimental Section A reflectivity experiment involves the measurement of the ratio R(θ) ) I(θ)/I0 at various incidence angles θ between the intensity I0 of the incident X-ray beam and that, I(θ), reflected by the film. The experiments were performed using a highresolution reflectometer (OptiX from Nonius) with a copper tube as an X-ray source (λ ) 1.5405 Å). A reflectivity curve provides the electron density profile along the film normal. The film being considered as a succession of homogeneous slabs, for each slab thickness, electron density and interfacial roughness can be derived from the experimental profile.5 The experimental reflectivity curves were fitted using a symmetrical five-layer film model:6 two slabs for the alkyl chains, two for the polar heads, and one for the central core. The vertical films (4 × 0.5 cm2) were drawn from the different solutions; the films were enclosed in an airtight cell to maintain a saturated vapor atmosphere.6 To provide a low-power IR light irradiation on the black films, we used a lamp made of a low-pressure tube (Black Light Blue 4W, from Vilber Lourmat, whose UV (λ ) 356 nm) line was filtered). The light intensity through the cell is less than 100 mW. The measurement of the spectrum shows a very small quantity of near-IR sufficiently weak to prevent the film bursting. The films were irradiated during the whole experiment at a distance of 20 cm (Figure 1). Different surfactants were used at concentrations (c) above the critical micelle concentration: hexaethylene glycol monododecyl ether (C12E6, c ) 0.5 g/L), sodium dodecyl sulfate (SDS, c ) 1 g/L), and the phospholipids dimyristoyl-phosphatidylcholine (DMPC (zwitterionic), c ) 0.5 g/L) and dimyristoyl-phosphatidylethanolamine (DMPE (negatively charged at pH 12), c ) 0.5 g/L). They were purchased from Sigma and used without further purification. Perfluoralkylated-phosphatidylcholine (F-PC, c ) 0.25 g/L) was synthesized according to ref 7.7 Liposomal dispersions of small unilamellar vesicles (SUVs) of DMPC and DMPE (5) Born, M.; Wolf, E. Principles of Optics, 6th ed.; Pergamon Press: London, 1984. (6) Cuvillier, N.; Millet, F.; Petkova, V.; Nedyalkov, M.; Benattar, J.-J. Langmuir 2000, 16, 5029. (7) Santaella, C.; Vierling, P.; Riess, J. G. New J. Chem. 1991, 15, 685.
10.1021/la036174m CCC: $27.50 © 2004 American Chemical Society Published on Web 01/13/2004
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Figure 1. Schematics of the frame section installed at the center of the reflectometer. (a) Side view of the cell; the light only illuminates the black film surface such that there is a temperature gradient between the film (Tf) and the bulk (Tb). (b) Top view of the cell, which shows the X-ray geometry. Figure 3. Time dependence of the overall thickness of a SDS common black film under irradiation. Time zero is chosen before the source is switched off. During irradiation, one can observe a large decrease (≈55 Å) reaching equilibrium thickness (70 Å). When the irradiation is stopped (arrow), the thickness increases until the equilibrium state of the film is reached. Table 1. Equilibrium NBF Thickness (d) before and under Light Irradiation for Different Surfactants
Figure 2. Experimental reflectivity profiles of DMPC Newton black film recorded before (open circles) and under irradiation (solid circles). The shift of the interference “Kiessig fringes” toward higher angles, observed under irradiation, evidences the thinning of the initial film from 59.3 to 54.1 Å due to a loss of water of the central core. The variation of the overall thickness obtained by heating the whole cell is represented in the inset. in ultrapure water (Millipore Alpha-Q) were obtained by the Bangham method.8 The DMPE dispersions9 were prepared at pH ) 12.
Results and Discussion (1) Irradiation of NBFs. The DMPC reflectivity curves recorded at equilibrium before and under IR are reported in Figure 2. Under IR, we observe a shift of the whole curve toward the higher angles indicative of a film thinning. This process does not result from any chain transformation (the initial state of the DMPC film was chosen at T ) 28 °C, above the gel-LC transition (Tm ) 23 °C) induced by the chain melting10,11). We must emphasize that when the irradiation is stopped, the film rapidly recovers its initial state. This process is fully reversible and perfectly reproducible. A similar thinning of DMPC films can be obtained when the temperature of the whole cell is increased (i.e., the film, the reservoir, and the vapor are at the same temperature); we found a thickness decrease of 8 Å for a temperature increase of 17 °C (see the inset of Figure 2). Although we cannot compare the film irradiation process and the heating of the whole cell, this gives a rough idea of the temperature increase. (8) Szola, F., Jr.; Papahadjopoulos, D. Annu. Rev. Biophys. Bioeng. 1980, 9, 467. (9) Lasic, D. D. Liposomes: From Physics to Applications; Elsevier Science B.V.: Amsterdam, 1993. (10) Cevc, G.; Marsh, D. Phospholipid Bilayers: Physical Principles and Models; John Wiley & Sons: New York, 1987. (11) Nikolova, A.; Exerowa, D. Langmuir 1992, 8, 3102.
compound
d (Å) before irradiation
d (Å) under irradiation
∆d (Å)
hydration number
DMPC F-PC C12E6 SDS
59.3 56 55 33
54.1 53 52 33
5 3 3 0
12 (see ref 6) 12 13 (see ref 14) 3 (see ref 3)
It has been shown12 that the soap films absorb IR and the absorption spectrum depends on the film thickness. Here the IR effect is to increase the temperature of the film. The temperature of the vapor remains almost unchanged (e2 °C) due to thermal exchanges with the cell. The bulk solution, not directly irradiated, remains also at the initial temperature. Thus the film, locally heated by the light absorption by water, is at a higher temperature than the bulk (Figure 1). Note that the thickness of a NBF is so small that high temperature of the film is reached quite instantaneously. The energy supplied by IR, which is enough to excite the water molecules, allows evaporation. This local heating is responsible for the thickness reduction due to water evaporation through the alkyl chain walls (the energy necessary for a water molecule to pass through a lipid membrane is 26 kJ/mol (10 kT)13). To generalize the process, we made experiments on NBFs with different surfactants: F-PC (zwitterionic phospholipid with highly hydrophobic chains), C12E6 (nonionic), and SDS (anionic). The results are summarized in Table 1. In all the cases except for SDS, we observe a thinning of the film under irradiation. For SDS, it seems extremely difficult to remove bonded water molecules due to a low hydration number. (2) Irradiation of CBFs. For better understanding of the process of water extraction under IR, we used CBFs which contain a rather thick water core. The irradiation effect on the film thickness of the SDS CBF is reported in Figure 3. The experimental curve confirms the thickness decrease as well as the reversibility and the reproducibility of the water extraction process observed in the NBF. Here the main difference is that for the CBF the thickness decrease is steeper than for the NBF (DMPC, C12E6, F-PC). The thinning mechanism is obviously different from that (12) Huibers, P. D. T.; Shah, D. O. Langmuir 1997, 13, 5995. (13) Marrink, S.-J.; Berendesen, H. J. J. Phys. Chem. 1994, 98, 4155.
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Figure 4. (a) Time dependence of the overall thickness of a DMPE common black film. Time zero is chosen when the film is at equilibrium before the irradiation. When the irradiation is stopped (arrow), the thickness increases until the equilibrium state of the film is reached. In the inset is reported the evolution of the thickness of another DMPE common black film when the whole cell is heated (i.e., the film, the vapor, and the reservoir are at the same temperature). This curve displays a discontinuity at ≈29.5 °C corresponding to a melting transition of the chains of DMPE at pH 12. (b) Schematic drawing of the film connected to the bulk solution. In a transient regime, the chain melting transition starts first in the film (T1) where the temperature is higher than at the surface of the solution (T2) and induces a surface tension gradient (∆σ b) and a surface flow. The related Marangoni effect is accompanied by a liquid flow (in the opposite direction of the surface flow) responsible for the thickening.
observed for the NBF. It consists of two stages with different rates: faster, when the water is in its liquid state, and slower, when the water is bonded or more organized. There are two ways in which the thinning occurs: liquid flow toward the meniscus and evaporation. The absence of free liquid core in the NBF means that the water extraction must occur only by evaporation through the alkyl chain walls. The rate of liquid extraction from the CBF shows that the main process is the water flow from the film to the meniscus. When all the liquid water is removed, evaporation starts playing the major role. Derjaguin15 introduced the concept of thermo-osmotic flow in capillaries induced by the presence of a temperature gradient. Here the gradient is due to a temperature difference between the film and the solution (which remains at room temperature). This gradient is the driving force of a thermo-osmotic flow toward the meniscus, responsible for the fast film thinning. It should be emphasized that the maximum temperature reached by a thick film is probably higher than that reached in the same conditions by the NBF due to higher water content with respect to a constant film area responsible for the cooling. To compare the behavior of a CBF with that of the NBF of phospholipid, we used DMPE which is similar to DMPC but with a more hydrophilic polar head. The result of the thinning under irradiation is presented in Figure 4a. During irradiation, three stages are observed (whereas only two are observed for SDS): (i) a striking increase of the thickness of 33 Å during the first 30 min; (ii) a steeper decrease of 65 Å during the next 30 min; (iii) a slower decrease of 18 Å reaching the equilibrium thickness of the NBF (60 Å) after 2 h. The thinning process is also reversible. To analyze the unexpected increase of the thickness at the beginning of irradiation, it is necessary to compare the film behavior under irradiation with that obtained by heating the whole cell. The latter, represented in the inset of Figure 4a, does not display any thickening as observed (14) Schalchli, A. The`se de l’Universite´ de Paris VI, Paris, 1994. (15) Derjaguin, B. V.; Churaev, N. V.; Muller, V. M. Surface Forces; Consultant Bureau: New York, 1987.
under irradiation but clearly shows the presence of a phase transition10 at 29.5 °C at pH ) 12 (the thickness vs temperature exhibits an abrupt jump around the chain melting transition temperature). In both processes, irradiation of the film and heating, the temperature is increased such as to pass through the phase transition temperature (Tm); nevertheless, only the irradiation process induces this “extra thickening”. In the case of irradiation, the film temperature is higher than that at the surface of bulk solution (Figure 4b), thus creating a transient temperature gradient. Due to its very small mass, the film heating is faster than that of the surface of the solution. This temperature gradient produces a regime in which the phase transition occurs first in the film and then at the surface monolayer. According to ref 16,16 the surface tension decreases abruptly before the transition temperature and reaches an almost constant value after. A surface tension gradient thus appears between the vertical film and the surface monolayer. In a first stage, this surface tension gradient causes a Marangoni effect (i.e., surface flow) directed from the film to the surface monolayer. This surface flow is accompanied by a liquid flow in the opposite direction,17 responsible for the extra thickening in the film (Figure 4b). Very similar processes were induced by laser at the surface of a surfactant solution.18 The second stage of the thinning is similar to that previously observed with SDS (thermo-osmosis). When the whole cell is heated (inset of Figure 4a), the thinning due to the temperature dependence of the interactions is smaller (≈25 Å) than that observed under IR (≈65 Å in Figure 4a) due to an additional thermo-osmotic flow. The third stage, when the water core thickness becomes smaller, is slow. At this level, the remaining water cannot flow since the molecules are bound to the polar heads. The energy supplied by IR is enough to remove this bonded water by evaporation through the alkyl chain walls. (16) Lee, S.; Kim, D. H.; Needham, D. Langmuir 2001, 17, 5544. (17) Levich, V. G. Physicochemical Hydrodynamics; Prentice Hall: Englewood Cliffs, NJ, 1962. (18) Gugliotti, M.; Baptista, M. S.; Politi, M. J. Langmuir 2002, 18, 9792.
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Nevertheless, it seems difficult to separate the process of osmotic flow and evaporation. Conclusions We have demonstrated that the use of a near-IR light allows the control of the hydration core in black films. This process, observed for different kinds of surfactants, is reversible and very reproducible. The film thickness decreases under IR, and the rate of this process depends on the water content within the film. Even for films in equilibrium, the light action provokes forced film drainage when the film contains liquid water or evaporation of bonded water; thus it is possible to generate the CBF to NBF transition. We have shown that the absorption of IR by water molecules almost instantaneously creates a temperature gradient between the film and the bulk. Then the resulting thermo-osmosis and evaporation processes can occur. These observations raise questions about the ability of Raman and IR spectroscopy to investigate the
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black film structure without altering the sample.19-23 Although the basis of a new method of controlling the hydration or the local temperature in black films is clearly evidenced, further improvements of this experiment are necessary such as the control of the IR flux or the temperature calibration of the film. Acknowledgment. We thank J. Prost, B. Robert, D. Joyeux, and M. Ollivon for very helpful discussions and F. Ghez (Jobin-Yvon SA) for the light spectrum. One of us (S. Bratskaya) gratefully acknowledges financial support from NATO. LA036174M (19) Lhert, F.; Blaudez, D.; Heywang, C.; Turlet, J.-M. Langmuir 2002, 18, 512. (20) Berger, C.; Turlet, J.-M.; Blaudez, D. Langmuir 2003, 19, 1. (21) Yian, Y. J. Phys. Chem. 1991, 95, 9985. (22) Corkill, J. M.; Goodman, J. F.; Ogden, C. P.; Tate, J. R. Proc. R. Soc. A 1962, 273, 84. (23) Umemura, J.; Matzumoto, M. K.; Kawai, T.; Takenaka, T. Can. J. Chem. 1985, 63, 1713.
