Langmuir 2000, 16, 10189-10192
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Two-Dimensional Mixtures of Stearic Acid and Partially Fluorinated Amphiphilic Molecule: A Grazing Incidence X-ray Diffraction Study V. Dupres,† S. Cantin,† F. Benhabib,† F. Perrot,*,† P. Fontaine,‡ and M. Goldmann‡,§ Laboratoire de Physique des Mate´ riaux et des Surfaces, Universite´ de Cergy-Pontoise, Neuville sur Oise, 95 031 Cergy-Pontoise Cedex, France, LURE, Centre Universitaire Paris Sud, BP 34, 91 808 Orsay Cedex, France, and GRPB, Universite´ Paris V, 45 rue des Saints-Pe` res, 75270 Paris Cedex 06, France Received February 17, 2000. In Final Form: September 18, 2000 We report a grazing incidence X-ray diffraction (GIXD) study of mixed Langmuir monolayers of stearic acid (ST) and a fluorinated amphiphilic molecule (FEP). The mixing behavior of the two components was investigated at two different surface pressures, 5 and 30 mN/m. The ST molecules are found to be in the L2 mesophase at 5 mN/m and in the LS mesophase at 30 mN/m, whereas the FEP molecules appear disordered. In the mixture, the GIXD measurements, associated with previous thermodynamical and optical microscopy results, show that in the whole range of molar ratio, the two molecules are completely phase-separated. Also, we observed at 5 mN/m, relaxation in the pure ST monolayer and in the mixtures rich in ST; the tilt angle of the molecules and the rectangular distortion of the 2D-lattice decrease. By comparing mixed films and pure ST films, this relaxation could be correlated to the size of the domains.
I. Introduction Insoluble monomolecular films of amphiphilic molecules spread at the air-water interface allow for the study of the physical properties of two-dimensional (2D) systems. In the case of binary mixtures on the surface of water, the mixing behavior of various systems was studied using different techniques (isotherms, optical microscopy),1 showing that the miscibility of the two components strongly depends on the structure of the surfactant molecules and also on the thermodynamical parameters such as temperature, surface pressure, and pH. X-ray diffraction has been used only in the case of completely miscible 2D-mixtures2 in order to compare structural transitions in the mixture to that of the pure components. Whereas a complete phase separation of the components on the whole range of molar compositions is very difficult to achieve, such a system could be used as a model for 2D emulsions. The stability of such a 2D emulsion arises, on one hand, from the slower diffusion of the molecules in 2D than in 3D, and on the other hand, from the molecular arrangement which induces long-range repulsive electrostatic forces between the molecular dipoles.3 These emulsions allow us to study of the interactions between molecules in 2D and if possible, to vary them. Also, these emulsions may be used to modify solid surfaces by a controlled way. The aim of this paper is to examine the mixing behavior, at the molecular level, of two insoluble amphiphilic †
Universite´ de Cergy-Pontoise. Centre Universitaire Paris Sud. § Universite ´ Paris V. *To whom correspondence should be addressed. ‡
(1) (a) Subramaniam, S.; McConnel, H. M. J. Phys. Chem. 1987, 91, 1715. (b) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591. (c) Seul, M.; Morgan, N. Y.; Sire, C. Phys. Rev. Lett. 1994, 73, 2284. (d) Angelova, A.; Van der Auweraer, M.; Ionov, R.; Vollhardt, D.; De Schryver, F. C. Langmuir 1995, 11, 3167. (e) Sadrzadeh, N.; Yu, H.; Zografi, G. Langmuir 1998, 14, 151. (2) (a) Shih, M. C.; Durbin, M. K.; Malik, A.; Zsonack, P.; Dutta, P. J. Chem. Phys. 1994, 101, 9132. (b) Kaganer, V. M.; Mo¨hwald, M.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779 and references therein. (3) Andelman, D.; Brochard, F.; Joanny, J.-F J. Chem. Phys. 1987, 86, 3673.
molecules at the air-water interface: stearic acid CH3(CH2)16COOH (ST) and a fluorinated molecule CF3(CF 2 ) 5 -(CH 2 ) 2 -S-CH 2 -CHOH-CH 2 -O-CH 2 -CH(C2H5)(C4H9) (FEP). In an earlier study,4 we investigated in detail Langmuir monolayers made of ST and FEP on macroscopic and mesoscopic scales. Two amphiphilic molecules spread at the air-water interface may exhibit either an ideal behavior or attractive or repulsive interactions. Attractive interactions between the different molecules lead to their miscibility and repulsive interactions result in a partial or a complete phase separation. To determine the mixing behavior of ST/FEP Langmuir monolayers as a function of the ST molar fraction x and the surface pressure, we performed a thermodynamical analysis based on the study of the surface pressure versus molecular area isotherms. These measurements were associated to optical observations by means of fluorescence microscopy and Brewster angle microscopy (BAM). It was shown that this system displays, on a mesoscopic scale, a phase separation in the whole range of molar fractions and even at very low surface pressure. Small circular domains were visualized. For x e 0.7, a rather homogeneous size about 10 µm in diameter was observed, whereas for x g 0.8, the polydispersity was higher with larger domains. With BAM equipped with an analyzer on the path of the reflected light, the domains were observed to present an optical anisotropy due to the tilt of the molecules. The texture appeared the same as that of the domains of the L2 mesophase in pure ST monolayers. Nevertheless, we cannot be certain that the domains were formed exclusively of ST molecules by BAM observations, i.e., that the system is partially or completely phase separated. However, it is important to check the mixture on a smaller length scale. For this purpose, grazing incidence X-ray diffraction experiments (GIXD)5 are well adapted because the molecular arrangement of the molecules in (4) Dupre´s, V.; Cantin, S.; Benhabib, F.; Perrot, F. Europhys. Lett. 1999, 48, 86.
10.1021/la000239t CCC: $19.00 © 2000 American Chemical Society Published on Web 12/02/2000
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the mixed films can be obtained and compared to that of the pure ST and FEP monolayers. Hydrogenated and fluorinated amphiphilic molecules have significantly different chain sections,4,6 leading to well-separated diffraction patterns. So, if the mixed film presents a complete phase separation, we will find the diffraction peaks corresponding to the pure components. In the case of partial phase separation, shift or disappearance of the diffraction peaks should be observed.2 Consequently, the mixing behavior of the two molecules can be determined unambiguously, even at a small fraction of one of the two components. We report in this paper GIXD experiments on mixed ST/FEP Langmuir monolayers. We have chosen to study the films at two different surface pressures, 5 and 30 mN/ m; 30 mN/m is the surface pressure at which we would like to transfer the films on solid surfaces, and 5 mN/m was chosen because domains were observed by optical means even though this is a low surface pressure. By comparing the molecular arrangements of the pure films and of the mixed films, domains of pure ST have been highlighted at the two surface pressures. Also, at 5 mN/ m, a relaxation effect was observed in the pure ST monolayer and in the mixed films rich in ST. During the slow relaxation in the pure ST monolayer the tilt angle decreases from about 25° to a value nonmeasurable by GIXD. This phenomenon is not frequently mentioned.7 II. Experimental Setup and Materials The grazing X-ray diffraction experiments were carried out on the D41B beamline at the LURE synchrotron source (Orsay, France). The X-ray wavelength λ ) 1.646 Å was selected using a focusing Ge(111) crystal. The angle of incidence was fixed at R ) 0.85 Rc using a mirror; Rc is the critical angle for total external reflection of X-rays at the air-water interface; the value is 2.8 mrad at λ ) 1.646 Å. The intensity of the X-ray beam diffracted by the monolayer was monitored vertically using a CO2-filled position sensitive detector (PSD), as a function of the in-plane component of the scattering vector, Qxy, selected by means of a Soller collimator. The Qxy resolution was 0.007 Å-1. The study of the Qxy-pattern integrated over the vertical wave vector component Qz allows us to determine the 2D lattice, while the position of the Bragg rods integrated over the corresponding Qxy gives information about the tilt angle.8 In the present study, two distinct phases have been observed, the so-called LS and L2 phases. When molecules are parallel to the surface normal and free to rotate around their long axis, the lattice is hexagonal and the {1,0}, {0,1}, and {1,1 h } reflections are degenerate (LS phase). Thus, the Qxy pattern presents a single diffraction peak and, out of the plane, the Bragg rod has its maximum at Qz ) 0 Å-1. When molecules are tilted toward a nearest neighbor (L2 phase), the hexagonal lattice is distorted to a rectangular structure. Such a cell yields two diffraction peaks, one corresponding to the {1, 1 h } reflection and the other to {1,0} and {0,1}. The Bragg rod of the nondegenerate peak is located at Qz ) 0 Å-1, while the Bragg rod of the two degenerate reflections {1,0} and {0,1} is centered at Qz g 0 Å-1. The position of the center of this peak allows us to estimate the tilt angle. (5) (a) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Laxbauer, L. A.; Mo¨hwald H. Phys. Rev. Lett. 1987, 58, 2224. (b) Barton, S. W.; Thomas, B. N.; Flom, E. B.; Rice, S. A.; Lin, B.; Peng, J. B.; Ketterson J. B.; Dutta, P. J. Chem. Phys. 1988, 89, 2257. (c) Kenn, R. M.; Bo¨hm, C.; Bibo, A. M.; Peterson, I. R.; Mo¨hwald, H.; Als-Nielsen, J.; Kjaer K. J. Phys. Chem. 1991, 95, 2092. (6) Goldmann, M.; Nassoy, P.; Rondelez, F.; Renault, A.; Shin, S.; Rice, S. A. J. Phys. II 1994, 4, 773. (7) (a) Schlossman, M. L.; Schwartz, D. K.; Pershan, P. S.; Kawamoto, E. H.; Kellogg, G. J.; Lee, S. Phys. Rev. Lett. 1991, 66, 1599. (b) Buontempo, J. T.; Rice, S. A.; Karaborni, S.; Slepman, J. I. Langmuir 1993, 9, 1604. (c) Bommarito, G. M.; Foster, W. J.; Pershan, P. S.; Schlossman, M. L. J. Chem. Phys. 1996, 105, 5265. (d) Foster, W. J. Ph.D. Thesis; Harvard University, Cambridge, MA, 1995. (8) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep 1994, 246, 251.
Dupres et al. Surface pressure measurements were carried out using a filter paper Wilhelmy plate, in a thermostated Teflon trough thoroughly cleaned with dichloromethane and ethanol (Merck) and rinsed with pure water. The incident beam was conducted through vacuum tubes and the trough was placed in a box filled with a flow of water-saturated helium. The temperature of the subphase was set to 20 ( 0.5 °C. The surface pressure was kept constant during a scan. Unless otherwise specified, the desired surface pressure was reached by compression of the monolayer. Stearic acid (ST), CH3(CH2)16COOH, was purchased from Fluka (>99.5% grade) and 1-(2′-F-hexylethylthio)-3-(2′′-ethylhexyloxy)-2-ol-propane (FEP), CF3-(CF2)5-(CH2)2-S-CH2CHOH-CH2-O-CH2-CH-(C2H5)(C4H9), was a gift from L’Ore´al. Both molecules were used without further purification and spread onto a subphase of ultrapure water (MilliRO-MilliQ system). Chloroform (Merck, Pro Analysis grade) was used as spreading solvent, both components of the films being dissolved in the same chloroform solution with different molar fractions.
III. Results and Discussion: Molecular Structure of the Mixed Monolayers a. Study at a Surface Pressure Π ) 30 mN/m. Figure 1a shows the diffraction data for the two pure monolayers, ST and FEP, at 30 mN/m. For the FEP monolayer, the Qxy pattern shows no diffraction peak over the whole range studied (1-1.6 Å-1). Thus, the FEP monolayer appears disordered probably because of its particular structure (a large polar head with a long branched aliphatic chain). On the contrary, GIXD data for the ST monolayer at 30 mN/m reveal a single in-plane diffraction peak at Qxy ) 1.52 Å-1. The out-plane scattering vector component Qz has its maximum around 0 Å-1, which is typically the case of a hexagonal lattice with molecular axis perpendicular to the water surface. It is consistent with the fact that at 30 mN/m the ST monolayer is known to be in the LS phase, which is untilted and hexagonal.9 One can deduce the cell parameters for the hexagonal lattice (a ) b, γ ) 120°); the intermolecular spacing is calculated as 4.77 Å from the peak position and the mean molecular area is ∼19.7 Å2, which coincides with isotherms results. For mixed Langmuir monolayers at 30 mN/m, a single in-plane diffraction peak can be noticed (Figure 1) over the whole x range. From x ) 0.1 to 0.9, the position and the shape of the single peak are always the same as the pure ST peak. Thus, the mixed ST/FEP Langmuir monolayers at 30 mN/m present domains of pure ST over the entire range of molar fraction. This completes previous results obtained on this system:4 (i) thermodynamical measurements have shown the additivity of the mean molecular areas; and (ii) BAM observations have indicated the presence of domains with the same texture due to the tilt of the molecules, whatever the fraction of ST molecules. Moreover, we have observed that the continuous phase is occupied, in all the different mixtures, by the disordered FEP rich phase. These previous results and X-ray data mean that (i) the introduction of FEP molecules in ST domains seems to be unlikely. Indeed, the section of an FEP molecule is larger than that of a ST molecule. Thus, their presence would induce disorder in the domains and consequently displacement and/or broadening of the diffraction peaks. Moreover, long-range order of the tilt as observed by BAM would decrease and/or disappear. (ii) The dipolar interactions between ST molecules are strong so that it is not likely that ST molecules dissolve in the FEP phase. (9) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P. Phys. Rev. 1992, A45, 5734.
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Figure 2. X-ray diffraction data in the horizontal plane (left) and contours of equal intensity vs the in-plane and out-plane scattering vector component Qxy and Qz (right) over several hours for ST monolayer at room temperature and Π ) 5 mN/m. (a) Just after deposition; (b) second scan after deposition; (c) third scan after deposition; (d) fourth scan after deposition; (e) fifth scan after deposition.
Figure 1. Grazing incidence X-ray diffraction data for pure and different ST/FEP mixed Langmuir monolayers at room temperature and 30 mN/m (x: molar fraction of ST). Note the different scales of intensity depending on x. (a) ST (solid inverted triangle) and FEP (open circle); (b) x ) 0.1; (c) x ) 0.5; (d) x ) 0.9.
At 30 mN/m, all of these results allow us to conclude that a complete phase separation of the ST/FEP mixture occurs at all molar fractions. b. Study at a Surface Pressure Π ) 5 mN/m. After the study at 30 mN/m, it is interesting to study the mixing behavior of ST and FEP at lower surface pressure, i.e., 5 mN/m, where domains have been observed by optical means. In particular, we noted a striking observation relative to the relaxation of the films, which has been studied very little previously.7 Concerning the FEP monolayer, the Qxy pattern keeps the same aspect as at 30 mN/m, and no diffraction peak appears.
For the pure ST monolayer, a striking behavior can be noticed. Immediately after the film deposition, the GIXD data present two well-defined peaks (Figure 2a) in good agreement with the fact that at 20 °C and 5 mN/m, the ST monolayer is in the L2 mesophase (distorted hexagonal lattice with molecules tilted toward a nearest neighbor).9 A series of five scans over several hours for the same monolayer shows a time evolution of the monolayer structure (Figure 2). The two peaks come close to each other; at the fourth scan (each scan lasting approximately an hour and a half), only a single asymmetrical peak is detected in the diffraction pattern (Figure 2d). The next scans present only one symmetrical peak centered at Qz ) 0 Å-1 (Figure 2e). Our results show, for the first scan, a tilt angle of approximately 25°. Then, the tilt angle decreases little by little to 24° in the second scan and 19° in the third scan. On the fourth scan (Figure 2d), the two peaks are so close to each other that the tilt angle is not measurable; it becomes lower than the resolution of GIXD for tilt angle measurements, i.e., about 10°.10 In particular, this limit is due to the low statistics of the out-plane data. (10) Fontaine, P. Ph.D. Thesis, Lille I, 1995.
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This is consistent with previous observations using Brewster angle microscopy with analyzer (PBAM) that revealed that the ST monolayer at 5 mN/m always presents an optical anisotropy related to a tilt of the molecules after several hours. Note that the intensity of the outplane peak decreases compared to the intensity of the in-plane peak, the integrated intensity being approximately constant. We also performed an experiment on a pure ST monolayer previously compressed at a surface pressure higher than 5 mN/m and then expanded to 5 mN/m. Only one peak centered at Qz ) 0 Å-1 and at the same Qxy as in Figure 2e is observed, with no time evolution. For mixed monolayers (Figure 3), the time evolution is only observed for a part of molar fractions. For x e 0.7, the Qxy pattern (Figure 3c) reveals only a single peak at the same position as for the ST monolayer (Figure 2e); moreover, no time evolution is observed. For x g 0.8, GIXD data show two peaks. The film then relaxes and we finally observe a single peak after several scans corresponding to the single peak of Figure 3c. It should be noted that the evolution time of the monolayer is as slower as the mixture is richer in ST. By optical observations,4 we previously observed that the size of the ST domains is rather homogeneous for x e 0.7, of order of 10 µm, and heterogeneous and larger in average for x g 0.8. The observed phenomenon could be interpreted as a relaxation phenomenon, because the relaxation time varies with the size of the domains. In the mixtures, we observed than the relaxation time increases with the domains size. In pure ST films, the relaxation time is much larger than in the mixtures; it is consistent with the fact that the L2 phase is continuous. These results, correlated to previous thermodynamical and optical results, indicate that the mixed monolayers are completely phase-separated at the pressure of 5 mN/ m. IV. Conclusion GIXD experiments performed on mixed monolayers of ST and FEP spread at the air-water interface complete our previous observations with optical and thermodynamical techniques. The GIXD experiments allow us to observe pure ST domains. From all of these studies, we can conclude that a complete phase separation takes place on the whole range of molar compositions and at least for surface pressures higher than 5 mN/m. To our knowledge, such 2D completely separated emulsions have not been reported in the literature. Moreover, at 5 mN/m, we noticed a strong relaxation of the tilt angle of the ST molecules in the pure ST monolayers as well as in the mixtures rich in ST. After several hours, the tilt angle is not measurable with GIXD and is consequently lower than the experimental resolution, i.e., 10°. It is consistent with BAM observations which indicate the presence of an optical anisotropy due to the tilt of the molecules, but does not allow for quantitative
Figure 3. GIXD data for ST/FEP mixed monolayers at Π ) 5 mN/m for different x molar fractions. (a) x ) 0.9 just after deposition; (b) x ) 0.9 third scan after deposition; (c) x ) 0.4.
tilt angle measurements. The mixed monolayers rich in FEP were observed to be immediately relaxed, probably because the size of the ST domains is much smaller on average. Because of the duration of the scans and the sensitivity of the GIXD experiment, more refined experiments are needed to study these relaxation effects. Acknowledgment. We would like to thank G. Vanlerberghe from L’Ore´al for the gift of the FEP molecules. LA000239T
