Adsorption of Inorganic Polyanions along a Langmuir Film - American

Interactions of large inorganic polyanions along a positively charged Langmuir film of dimethyldio- ctadecylammonium bromide were studied by Brewster ...
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Langmuir 1998, 14, 5573-5580

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Adsorption of Inorganic Polyanions along a Langmuir Film: A Brewster Angle Microscopic Study N. Cuvillier, R. Bernon, J.-C. Doux, P. Merzeau, C. Mingotaud,* and P. Delhae`s Centre de Recherche Paul PascalsCNRS, Avenue A. Schweitzer, F-33600 Pessac, France Received May 8, 1998. In Final Form: July 9, 1998 Interactions of large inorganic polyanions along a positively charged Langmuir film of dimethyldioctadecylammonium bromide were studied by Brewster angle microscopy (BAM). Because of their large size (>10 Å) and high refractive index, their adsorption along the organic monolayer induces an increase of the interface reflectivity and can be directly monitored by BAM. Furthermore, they strongly modify the morphology of the film and a coexistence of three phases (gaseous, liquid expanded, and liquid condensed) is easily obtained at a temperature close to 20 °C. Around this “triple point”, nucleation of liquid-condensed domains is observed along the gaseous/liquid-expanded interface, inducing unusual structures such as strings of liquid-expanded domains. Those experiments demonstrate that inorganic/organic Langmuir films based on the semiamphiphilic ionic association could present a variety of morphologies as rich as those of classical amphiphilic molecules at the gas/water interface.

Introduction Since the early work of Langmuir on stearic acid,1 the influence of inorganic ions on a monolayer spread at the gas-water interface has been mainly conducted on fatty acids and phospholipids. On one hand, the effect of cations on various Langmuir films and in particular the comparison between alkaline ions such as Li+, Na+, K+ and divalent cations such as Cd2+ or Mn2+ are readily available in the literature.2-5 Because interactions between natural membranes and ions play a considerable role in biological phenomena, the effect of calcium ions6,7 or other cations such as UO22+ or Th4+ 8-10 on phosphatidylcholine or phosphate derivative has been intensively described. In the other hand, interactions of inorganic anions with a Langmuir film have been not so largely studied. Simple monovalent anions, like halide ions, have some condensing effects on positively charged films11,12 or seem to expand the Langmuir films of some phospholipids.13,14 A few other anions (such as HAsO42- or MoO42-) present some interesting interactions with a zwitterionic lipid.15-17 * Address correspondence to this author. (1) Langmuir, I.; Schaefer, V. J. J. Am. Chem. Soc. 1937, 59, 2400. (2) Virden, J. W.; Berg, J. C. Langmuir 1992, 8, 1532. (3) Sanderson, P. W.; Lis, L. J.; Quinn, P. J.; Williams, W. P. Biochim. Biophys. Acta 1991, 1067, 43. (4) MacDonald, R. C.; Simon, S. A.; Baer, E. Biochemistry 1976, 15, 885. (5) Zasadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. K. Science 1994, 263, 1726. (6) Seimiya, T.; Ishiwata, S.; Watanabe, N.; Miyasaka, H.; Iwahashi, M.; Tajima, K.; Hayashi, M.; Muramatsu, T. J. Colloid Interface Sci. 1984, 101, 267. (7) Colacicco, G.; Buckelew, A. R. J.; Scarpelli, E. M. J. Colloid Interface Sci. 1974, 46, 147. (8) Hayashi, M.; Muramatsu, T.; Hara, I. Biochim. Biophys. Acta 1972, 255, 98. (9) Peng, J. B.; Prakash, M.; Macdonald, R.; Dutta, P.; Ketterson, J. B. Langmuir 1987, 3, 1096. (10) Gorwyn, D.; Barnes, G. T. Langmuir 1990, 6, 222. (11) Marra, J. J. Phys. Chem. 1986, 90, 2145. (12) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D. Thin Solid Films 1994, 2424, 195. (13) Lo¨sche, M.; Helm, C.; Matted, H. D.; Mo¨hwald, H. Thin Solid Films 1985, 133, 51. (14) Mingotaud, C. Thesis, University of Paris XI, 1992. (15) Davion Van Mau, N.; Issaurat, B.; Amblard, G. J. Colloid Interface Sci. 1984, 101, 1. (16) Tocanne, J. F.; Ververgaert, P. H.; Verkleij, A. J.; van Deenen, L. L. Chem. Phys. Lipids 1974, 12, 201.

Finally, one should note that besides those studies, a large variety of organic ions or metal complexes have been organized along a Langmuir film.18-23 Recently, polyanions selected in the polyoxometalate series have been used in Langmuir and LangmuirBlodgett films.24,25 Known for well over a century, those inorganic compounds can be depicted as molecular fragments of close-packed metal oxides of formula XaMbOcn(M ) Mo, W, V, ...; X ) P, Si; B, Co, ...).26 The best known structural type is the so-called Keggin ions (see Figure 1) but more complex structures can easily be obtained. As an example, Figure 1 shows the tetranuclear magnetic cluster used in this work and based on a Co4O16 group between two trivacant ligands PW9O349-. Electrostatic interactions between those bulky (volume larger than 1000 Å3) and highly charged (usually between -3 and -10) inorganic clusters and a positively charged monolayer have been used in order to organize the polyanions along the interface. Indeed, as shown by X-ray reflectivity,27 a dense inorganic monolayer (thickness about 10 Å, volume fraction of polyanions larger than 50%) is formed at the gas-water interface even for a low surface charge density. This adsorbed inorganic sheet enables the formation of multilayers, leading to new hybrid inorganic/organic materials after transfer onto a solid substrate.28 (17) Angelova, A.; Petrov, J.; Kuleff, I. Langmuir 1992, 8, 213. (18) Aiai, M.; Ramos, J.; Mingotaud, C.; Amiell, J.; Delhaes, P. Chem. Mater. 1998, 10, 728. (19) Lehmann, U. Thin Solid Films 1988, 160, 257. (20) Brynda, E.; Kminek, I.; Nespurek, S. J. Mater. Sci. 1989, 24, 4164. (21) Berndt, P.; Kurihara, K.; Kunitake, T. Langmuir 1992, 8, 2486. (22) Asano, K.; Miyano, K.; Ui, H.; Shimomura, M.; Ohta, Y. Langmuir 1993, 9, 3587-93. (23) Xia, W.-S.; Huang, C.-H.; Zhou, D.-J. Langmuir 1997, 13, 80. (24) Clemente-Leon, M.; Mingotaud, C.; Agricole, B.; Gomez-Garcia, C. J.; Coronado, E.; Delhaes, P. Angew. Chem., Int. Ed. Engl. 1997, 36, 1114. (25) Clemente-Leon, M.; Mingotaud, C.; Gomez-Garcia, C. J.; Coronado, E.; Delhaes, P. Thin Solid Films, in press. (26) Polyoxometalates: from platonic solids to anti-retroviral activity; Pope, M. T., Mu¨ller, A., Eds.; Kluwer Academic Publishers: Dordrecht, 1994; Vol. 10. (27) Cuvillier, N.; Bonnier, M.; Rondelez, F.; Paranjape, D.; Sastry, M.; Ganguly, P. Prog. Colloid Polym. Sci. 1997, 105, 1118. (28) Clemente-Leon, M.; Agricole, B.; Mingotaud, C.; Gomez-Garcia, C. J.; Coronado, E.; Delhaes, P. Langmuir 1997, 13, 2340.

S0743-7463(98)00544-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/27/1998

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Figure 2. Schematic principle of the Brewster angle microscope used in this work.

Figure 1. Structure of the polyoxometalates used in this work.

Until now, no information is available concerning the in-plane morphology of this new system in which a monolayer of large inorganic polyanions is adsorbed along an organic layer. Fortunately, direct observations of the monolayer texture can easily be performed by Brewster angle microscopy (BAM). This technique, introduced a few years ago,29,30 allows one to probe the layer without the usual drawbacks of the fluorescence microscopy. In this paper, we describe the observed morphology of a dimethyldioctadecylammonium bromide (DODA) monolayer in interaction with two types of polyanions (see Figure 1): a cobalt Keggin ion (noted Keg formulated as K5HCoW12O40) and a highly charged ion (noted Co4Keg formulated as K10[Co4(H2O)2(PW9O34)2]). These 2D systems, in which the adsorbed polyanion layer is directly visible by BAM, appear to have a rich surface morphology with, in particular, a triple point near room temperature. Experimental Section Brewster Angle Microscope. Reflection of a p-polarized beam onto water is minimized when the angle of incidence is equal to a specific angle (θB ) 53.2°). Using such a property, Meunier et al.29 developed a microscope to image a monolayer at the gas-water interface. Although a resolution better than a few micrometers is achieved, the acquisition time of their apparatus is close to 2.5 s. Because a Langmuir film (especially in the gaseous phase) is highly fluid and mobile, fast imaging of the surface is essential. Thus, the instrument has been slightly modified in order to form the final image in a single step: our imaging can be performed in less than 1/3 s with a resolution better than 2 µm over the whole field of view (0.7 × 1 mm2). General Setup. The laboratory-made BAM is fully controlled by a Power Macintosh. Its modular architecture enables simultaneous operations of the various subsystems. The black thermostated Langmuir trough (70 × 15 × 1 cm3) is equipped with two Teflon-coated barriers for a symmetric compression (29) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (30) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590.

with a compression ratio larger than 30. dc motors control the barrier positions. The surface pressure is measured by the Wilhelmy method. All these functions are supervised by a standalone analogical module working either in a “constant area” or “constant pressure” mode. This module receives the compression parameters from the central computer, which in return collects the isotherm data. The temperature of the subphase is regulated by a Huber cryostat equipped with an external temperature probe emerged in the trough. The working temperature is set also by the computer. An optical sensor measures continuously the water level in the trough (long-term stability better than 0.5 µm) and commands a Teflon valve to refill the trough in order to compensate the loss of water due to evaporation. A more precise system (based on the same technology) is used to evaluate the distance between the surface and the objective for a fine control of the image focus: this device compensates the residual variations of the water level and also the slight angle between the plane of motion of the optical system and the liquid interface. The optical setup is linked to a two axis translation stage which enables us to scan over the whole surface of the Langmuir trough. Another translation axis controls the altitude of the microscope above the water. Step-by-step motors are used for a high reproducibility of the displacements. Optical Setup. The light source is a polarized helium-neon laser (10 mW) that produces a fairly parallel beam used without correction. The beam passes through a GLAN polarizer and illuminates the water interface. To select the incident angle, both the laser and the polarizer are mounted on a rotating arm controlled by a step-by-step motor (accuracy 0.01°). The detection system is placed on a second optical rail, the orientation of which is modified by a second motor. The reflected beam passed first through a microscope objective (magnification ×20, NA 0.3). The working distance of this objective (17 mm) is long enough to avoid contact with the water level even at the highest angle of incidence. The objective is mounted on a translation stage powered by two linear step-by-step motors. Its position with respect to the water surface is then adjustable. After the objective, the beam passes through an analyzer (identical to the polarizer) and a plano convex lens (f ) 10 mm) and is then detected by a high-sensitivity CCD camera (Hamamatsu C5985, sensitivity 0.2 lux at 1/25 s). As the objective is corrected for infinity, the image is independent of its distance from the second lens. Therefore, additional accessories (such as filters) could easily be placed into the optical pathway. Finally, a mobile linear shutter can be placed few millimeters ahead of the CCD chip in order to select only a thin strip (adjustable width) of the image. This shutter plays a key role in the formation of an image focused on its whole surface, as described in Figure 2. Due to the oblique optical axis (about 53° for a water surface), the focus plane of the optical system intersects the interface only as a single line (in A in Figure 2). The image (A′) of this line coincides with the plane of the CCD. Outside this limited area (for example, in B), the image (B′) of the water surface is out of the CCD plane and therefore fuzzy. Taking account of the finite depth of field (3 µm), the in-focus part of the image corresponds to a thin stripe.

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Figure 3. Compression isotherm of DODA: on pure water (open circle) or on a 10-6 mol‚L-1 Co4Keg solution (full circle). The linear shutter is then positioned in order to select this stripe and mask the fuzzy part of the image. When the objective is moved (see Figure 2), the new in-focus object (B) has an image in B′′ that can be selected by the translated shutter. Then, to obtain a full image (instead of only a thin stripe), the shutter displacement is synchronized with the continuous translation of the objective, selecting always the in-focus part of the image. By choosing an exposition time for the CCD camera larger than the scan time, one single frame corresponds to a full image that is totally in focus. The width of the shutter should correspond to a compromise between the resolution of the microscope and the luminosity of the collected image, but the minimum time for the capture of an image is mainly dependent on the mechanical constraints on the shutter and objective motions. In our simple setup, an image of 0.7 × 1.5 mm2 size is captured in less than 0.3 s with a resolution better than 2 µm on its whole surface. The performances of this instrument can be easily improved by changing the mechanical translation of the shutter and objective. The final image is then digitized (739 × 509 pixels, 8 bits) and transferred to the computer through the camera controller. A geometrical correction (using the NIH Image 1.61 software) is applied on the digitized image to eliminate the distortion due to the tilted axis of view. No further manipulation of the images was made. Finally, whiter stripes appear in some of the pictures presented below. Those artifacts were due to a defective motor that was replaced afterward. The changes in morphology described in this paper and induced by a variation of the surface pressure, the molecular area, or the temperature are perfectly reproducible and reversible (with some hysteresis in the case of thermal changes). Injections of polyanions into the subphase were performed with a syringe containing 10 mL of a concentrated solution of the polyanions, behind the barrier (i.e., through the gas/pure water interface and not through the monolayer). The exact concentration of this solution was calculated in order to get a final and average concentration of 10-6 mol‚L-1 of polyoxometalate in the subphase. Materials. The synthesis of the polyoxometalates is described elsewhere.31,32 Their purity was checked by chemical analysis and magnetic measurements. Sodium tungstate (dihydrate) was purchased from Sigma. Dimethyldioctadecylammonium bromide was obtained from Kodak (purity higher than 99%) and used without further purification. Chloroform (HPLC grade from Prolabo) was used as the spreading solvent. The Millipore Q-grade water for the subphase had a resistivity higher than 18 MΩ cm.

Results and Discussion Effect of the Polyanions on the Monolayer Morphology. When spread on pure water at room temperature, the positively charged lipid DODA presents a compression isotherm without a plateau or a kink in the curve (Figure 3). However, a plateau appears in the (31) Galan-Mascaros, J. R.; Gimenez-Saiz, C.; Triki, S.; Gomez-Garcia, C. J.; Coronado, E.; Ouahab, L. Angew. Chem., Int. Ed. Engl. 1995, 34, 1460. (32) Finke, R. G.; Droege, M. W.; Domaille, P. J. Inorg. Chem. 1987, 26, 3886.

Figure 4. Typical morphologies of a DODA monolayer observed on pure water at room temperature: (A) 86 Å2/molecule and ca. 10 mN/m; (B) 70 Å2/molecule and ca. 20 mN/m; (C) 58 Å2/molecule and ca. 30 mN/m.

isotherm when the subphase temperature is decreased,33 suggesting a phase transition from a liquid-expanded (LE)

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Figure 5. Typical morphologies of a DODA monolayer on a 10-6 mol‚L-1 Co4Keg solution almost at null surface pressure and room temperature: (A) ca. 138 Å2/molecule; (B) ca. 99 Å2/ molecule.

phase to a liquid-condensed (LC) phase occurring during the compression. BAM demonstrates that such a transition still occurs at room temperature, even if it is not revealed in the isotherm measurement. Indeed, the BAM images of a DODA monolayer at low surface pressure are homogeneous and with an overall slight reflectivity corresponding to a LE phase. When the surface pressure is increased, small brighter spots are observed on the background of the LE phase (see Figure 4). The size and eventually the number of such nuclei are increasing during compression. At the collapse pressure, the monolayer is still biphasic. This peculiar phenomenon has been already reported.12 The lack of a plateau in the isotherm and of a homogeneous phase near the collapse pressure may be due to important kinetic effects related to the strong repulsion forces between polar heads. When spread on a 10-6 mol‚L-1 Co4Keg solution, the DODA isotherm is shifted toward small areas per molecule (see Figure 3). This “condensing” effect has been already observed with the Keg polyanion.28 Surely, it is related

Figure 6. Typical morphologies of a compressed DODA monolayer on a 10-6 mol‚L-1 Co4Keg solution at room temperature: (A) ca. 83 Å2/molecule and 0 mN/m; (B) ca. 73 Å2/ molecule and 1 mN/m; (C) ca. 45 Å2/molecule and 30 mN/m.

(33) Taylor, D. M.; Dong, Y.; Jones, C. C. Thin Solid Films 1996, 284-285, 130.

to the adsorption of the ions along the interface, which then counterbalance (at least partially) the repulsion

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Figure 7. Reflectivity of a DODA Langmuir film (initial surface pressure: 2 mN/m, null final surface pressure) after injection of a concentrated polyoxometalate solution into the subphase (see text): Na2WO4 (full circle); Co4Keg (open circle).

between charged ammoniums. At null surface pressure and for similar molecular areas, a transition is monitored on a polyanion subphase when on pure water just a LE phase was observed. Indeed, Figure 5A shows that large and smooth domains presenting a small reflectivity are observed during the compression and cover totally the interface when the density continues to be increased (see Figure 5B). In Figure 5A, the black area corresponds to a phase having a reflectivity (quasi null) clearly lower than that of the DODA LE phase on pure water. Then, it should correspond to a gaseous (G) phase. The whiter phase, which has no correspondence on pure water is surely associated to the polyanion. It can be related to a LE phase where the DODA molecules are interacting with the Co4Keg. When the compression continues, white dots are appearing and are ever more dense at the interface for an increasing surface pressure (see Figure 6). Therefore, a phase transition from a LE to a LC is observed and may be linked to the slight bump in the compression isotherm around 15-20 mN/m (Figure 3). Thus, for the relatively high surface pressure, the morphology of the DODA monolayer on pure water or on a Co4Keg subphase is very close. The only noticeable difference is that the BAM images present on a Co4Keg subphase a background whiter than on pure water (see Figures 4A and 6A). The lipid density in those two pictures is similar: the observed changes could not be due to some modifications in the optical properties of the homogeneous organic layer. However, it should be directly related to the inorganic layer adsorbed along the interface. Indeed, the reflectivity of an interface depends on the respective refractive indexes of water (n1), air (n2), and the layer (n(z)):29

r(θB) R

(n(z)2 - n12)(n(z)2 - n22)

∫-∞+∞

n(z)2

dz

For the organic layer, the refractive index should be close to 1.5 and the thickness to ca. 20 Å. Those two values have the same order of magnitude as that expected for a close-packed layer of polyanions: the diameter of Keggin ions is ca. 10 Å, and a saturated solution of a commercially available Keggin compound (formulated as H4SiW12O40) has a refractive index close to 1.6. Then, the reflectivity of the interface should be increased because of the adsorbed inorganic layer, which thickens the high-index region. This inorganic layer cannot be seen easily by BAM with “regular” anions (such as chloride, bromide, ...) because of their small size and the lower refractive index of the corresponding aqueous solution. To confirm the formulated hypothesis, a concentrated solution of polyanions was injected into the subphase after spreading and compression of the monolayer (see Experimental Section).

Figure 8. Typical morphologies of a DODA monolayer (at ca. 130 Å2/molecule and 0 mN/m) on a 10-6 mol‚L-1 Co4Keg solution: (A) T ) 23 °C; (B) T ) 8 °C.

The initial concentration of this solution was chosen to have a final 10-6 mol‚L-1 polyoxometalate subphase. The DODA Langmuir film is in a homogeneous state before injection. The reflectivity of the monolayer evaluated through the BAM images is then monitored versus time (see Figure 7). A large increase of this reflectivity is observed presumably when the diffusion front passes through the observation area. A similar behavior is recorded with the Keg polyanion (in that case, the maximum reflectivity is ca. 5-10% less than that observed with Co4Keg). When Na2WO4 is used instead of the large polyanions, only a small jump in the reflectivity is recorded. The size of this last anion is too small to induce a large change in the monolayer reflectivity. This experiment confirms that the inorganic adsorbed layer is responsible for an increase of the Langmuir film reflectivity. In other words, the BAM allows us to “see” the adsorbed inorganic layer itself. Effect of the Temperature on the Monolayer Morphology. Pressure-area isotherms for the polyanion/DODA system show no significant change with temperature in the range 9-28 °C, although a clearly temperature-dependent morphology is observed. All the experiments described below are performed at low surface density in the zero surface pressure region: for high surface pressures, the monolayer presents a homogeneous reflectivity associated with the LC phase. Just after spreading at a temperature higher than 23 °C and with an area per molecule ca. 200-100 Å2, the

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Figure 9. Typical morphologies of a DODA monolayer (at ca. 130 Å2/molecule and ca. 0 mN/m) on a 10-6 mol‚L-1 Co4Keg solution: (A) T ) 14 °C (with increasing temperature); (B) T ) 12 °C (with decreasing temperature).

monolayer is clearly biphasic (see Figure 8A), as expected from the results described above. For such a low surface pressure (lower than 1 mN/m), a continuous gas phase corresponding to a relatively low density of the molecules appears in black while the LE domains are in gray because of the higher reflectivity due to the adsorbed polyanions. The rounded shapes of these LE “bubbles” indicate that the line tension of the LE/G interface (noted λLE/G) is quite large, whereas the repulsion between domains due to dipole-dipole interactions prevents coalescence.34,35 At temperatures lower than 10 °C and for similar molecular densities, the system is also clearly biphasic with large and optically isotropic domains surrounded by the gaseous phase (see Figure 8B). These domains are brighter than the LE phase. Their higher reflectivity is linked to the higher surface density of the LC phase. They also act like a solid with an intrinsic rigidity and no detectable interactions between different domains. The LC/G interface is rougher than the LE/G one, suggesting that the line tension λLC/G should be lower than λLE/G. Such a change from a LE/G to a LC/G coexistence depending on the temperature is expected.36 At intermediate temperature (ca. 11-14 °C), large changes in the morphology are observed. When the (34) McConnell, H. M. Annu. Rev. Phys. Chem. 1991, 42, 171. (35) Knobler, C. M. Science 1990, 249, 870. (36) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces, 1st ed.; Interscience Publishers: New York, 1966; p 386.

Figure 10. Formation of LC phase in a LE/G biphasic system: (A) excess of the LE phase (observation made at 75 Å2/molecule, 2.8 mN/m, and 18 °C); (B) excess of the G phase (observation made at 300 Å2/molecule, 0 mN/m, and 19 °C).

temperature of the monolayer is increasing, “melting” of the white solid domains forms digits then stripes of LC phases (see Figure 9A). These thin stripes lead finally to “bubbles” of LC then LE domains when the temperature of the monolayer continues to rise. This behavior appears to be typical of the kind of monolayer used in this work. Indeed, changing the polyoxometalate from Co4Keg to Keg, which is less charged than the previous one, leads to similar morphologies but at a temperature slightly smaller than that for Co4Keg. Finally, the specificity of the polyoxometalate/DODA system is that the three phases (G, LE, and LC) are simultaneously observed (see Figure 9B) when the system is cooled. This coexistence is stable for at least 24 h. The relative proportions of LE and LC domains depend on the temperature and on the kind of polyanions: the proportion of LC domains increases with decreasing temperature and for a given temperature when Co4Keg is used instead of Keg. These experimental conditions seem to correspond to the triple point of the monolayer. However, even if the monolayer is spread at a constant temperature (i.e., without thermal hysteresis) well above the triple point (e.g., 19-22 °C), a small quantity of LC phase still

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Figure 11. Model of nucleation for a LC domain in a biphasic LE/G system (see text).

coexists with the expected G/LC phases. These domains are stable for at least a few hours and may be formed during spreading because of a local decrease of the temperature induced by the solvent evaporation and because of the gain in line tension energy (see below). Nucleation in the Triphasic Monolayer. Threephase coexistences in monolayers are rarely described in the literature. The extensive study of pentadecanoic acid on water had pointed out such a G-LE-LC triple point in the Langmuir film.37 More recently, a three-phase intersection point was analyzed for a mixed monolayer.38 The observations are generally made after a large expansion or a temperature quench of the film, both conditions leading to a nucleation of gas domains at the edge of the most condensed phases. The behavior of our system is slightly different. Nucleation of LC domains is mainly observed along the LE/G interface, leading to bright areas of a few micrometers thick around the gray domains (see Figure 10). Such preferential sites for nucleation may be understood in terms of a balance between the line energy (Eλ) and the electrostatic energy (Eel). Indeed, three cases can be considered. The nucleation of a small circular LC domain within a LE domain (see Figure 11A) will result in a large increase in the Eel value because of the dipoledipole repulsions and an increase of the line energy equal to πdλLC/LE, where d is the diameter of the LC nucleus. If the LC domain is formed (symmetrically) at the boundary line between the LE and G phases (see Figure 11B), the overall electrostatic repulsion is then decreased when compared to the previous case and the change in energy due to the line tension is d[(π/2)(λLC/LE + λLC/G) - λLE/G]. Finally, the formation of a LC domain in the gaseous phase (see Figure 11C) corresponds again to a decrease of the electrostatic repulsion between the LE and LC domains (we neglect the interactions between the gaseous phase and the more condensed phases) and to an increase of the line energy of πdλLE/G. Then, if the electrostatic interactions are dominant, the LC phase should nucleate in the gaseous phase. Experimentally, this is clearly not the case. If the line tension energy is the main factor, a minimum of energy can be found when the nucleation occurs at the LE/G interface. It will indeed happen if the line tension λLE/G is larger than λLC/LE + λLC/G. This is compatible with the observed roughness of the various interfaces. When the LE phase is in excess in the LE/G biphasic system (see Figure 10A), the nucleation at the interface forms continuous and relatively thick (>15 µm) lines. Furthermore, nuclei appear inside the large LE domains. When the G phase is in excess (see Figure 10B), the LC domains are covering the boundaries of the LE “bubbles” with the gaseous phase: they forms dots and somewhat continuous thin stripes of less than 5 µm thick. They also (37) Moore, B. G.; Knobler, C. M.; Akamatsu, S.; Rondelez, F. J. Phys. Chem. 1990, 94, 4588. (38) Hagen, J. P.; McConnel, H. M. Colloids Surf. A 1995, 102, 16772.

Figure 12. Particular morphologies in a DODA monolayer spread on a 10-6 mol‚L-1 Co4Keg solution: (A) LC nucleus (designated by the arrow) sticking together two LE domains (observation made at 18 °C and 128 Å2/molecule); (B) chainlike structure of LE domains observed at 19 °C and at ca. 215 Å2/ molecule.

modify the roughness of the interfaces: the LE domains have some sharp edges instead of smooth ones. These LC lines correspond to the formation of a more organized domain at the interface between two phases. Such a behavior can then be compared to the surface melting or crystallization, which may occur between a solid or a liquid and its vapor when the bulk system is close to its triple point. As an example, surface freezing of n-alkanes or of a liquid-crystal film appears a few degrees above the bulkfreezing transition temperature.39,40 In bidimensionnal system, an equivalent process should indeed correspond to the melting or the crystallization along a line between a gaseous and a liquid (or solid) phase. In our system, the line solidification may be favored by the large decrease of the line tension when the LE domains are covered by the LC phase. Indeed, the change in energy is equal to p(λLC/LE + λLC/G - λLE/G) when the LE domain having a perimeter p is totally covered by a thin layer of the LC phase. Then, the experimental result suggests again that λLE/G is larger (39) Swanson, B. D.; Stragier, H.; Tweet, D. J.; Sorensen, L. B. Phys. Rev. Lett. 1989, 62, 909. (40) Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Ocko, B. M.; Deutsch, M. Phys. Rev. Lett. 1993, 70, 958.

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than λLC/LE + λLC/G. This corresponds to the case of total wetting, which is known in 3D system to favor surface freezing.39 Particular Morphologies Associated with the Three Phase Coexistence. These small LC nuclei along the LE/G interface have some drastic effects on the spatial distribution of the LE phase. As an example, a small LC domain (diameter: ca. 4 µm) glues two LE “bubbles” together (see Figure 12A). Large deformations of the LE domain are then induced in the vicinity of the sticking point. Whereas the electrostatic repulsion tends to move away the two LE “bubbles”, their boundaries are blocked onto the LC nuclei. The competition between the two effects leads to the observed “diabolo” shape of the LE domains near their contact point. This situation of contact is never observed in the classical case of the pure LE phase (in absence of any LC nuclei) where the LE “bubbles” prefer to lose their shapes to avoid contact (see Figure 8A). In several cases, chainlike structures of several LE “bubbles” attached together are observed (see Figure 12B). These “bubbles” have generally a small size and although the BAM resolution is not enough to observe clearly the contact point, each of the “diabolo” shapes seems to present a bright spot in the middle of them, i.e., each of these LE domains is stuck together by a small LC domain. This particular structure is similar to those observed when small latex particles interact with LC domains to form strings of circular domains.41 In these last experiments, electrostatic interactions are responsible for such behavior,

In the case of large anions such as the polyoxometalates, Brewster angle microscopy enables us to observe directly an inorganic layer adsorbed along a positively charged Langmuir film. Such adsorption, which induces large changes in the compression isotherm, modifies strongly the morphology at the gas/aqueous subphase interface. On pure water, LE and LC phases are observed for a DODA monolayer when for similar areas G, LE′, and LC′ phases are seen on a polyanion subphase. The LE′ and LC′ phases differ from the LE and LC phases mainly in the absorbed inorganic layer. When the three phases coexist in such a monolayer, the LC phase has a high tendency to be formed along the G/LE interface: this behavior could be mainly due to the line tension energy. Finally, charges and eventually shapes of the ions should be the key parameters which control the change in morphology of such a system. Further studies are currently in progress to clarify this dependency.

(41) Nassoy, P.; Birch, W. R.; Andelman, D.; Rondelez, F. Phys. Rev. Lett. 1996, 76, 455.

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whereas a decrease of the line tension energy should be the driving force in our system leading to the chainlike structure. Conclusion

Acknowledgment. We gratefully acknowledge financial support of the BAM construction by the Region Aquitaine research and technology program. We thank Dr M. Clemente-Leon, Pr. C. J. Gomez-Garcia, and Pr. E. Coronado (University of Valencia, Spain) for the polyoxometalates synthesis and for helpful discussions.