Superlattice Formation in Fatty Acid Monolayers on a Divalent Ion

The Qxy resolution provided by a Soller collimator in front of the PSD was 0.007 Å-1. From the Qxy position of ...... Juan Torrent-Burgués. Colloids...
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Superlattice Formation in Fatty Acid Monolayers on a Divalent Ion Subphase: Role of Chain Length, Temperature, and Subphase Concentration Vincent Dupres,† Sophie Cantin,† Fewzi Benhabib,† Franc¸ oise Perrot,*,† Philippe Fontaine,‡ Michel Goldmann,‡,§ Jean Daillant,‡ and Oleg Konovalov| Laboratoire de Physico-Chimie des Polyme` res et des Interfaces (LPPI, EA 2528), Universite´ de Cergy-Pontoise, Neuville sur Oise, 95 031 Cergy-Pontoise Cedex, France, Laboratoire pour l’Utilisation du Rayonnement Electromagnetique (LURE, UMR CNRS 130), Centre Universitaire Paris Sud, BP 34, 91 898 Orsay Cedex, France, Laboratoire des Objets Complexes et Interfaces d’Inte´ reˆ t Biologique (OCIIB, FRE CNRS 2303), Universite´ Rene´ Descartes, 45 rue des Saints-Pe` res, 75270 Paris Cedex 06, and European Radiation Synchrotron Facilities (ESRF), BP 220, 38043 Grenoble Cedex, France Received February 18, 2003. In Final Form: July 29, 2003 We report the study of behenic acid Langmuir monolayers spread over chloride salts solutions of cadmium, lead, magnesium, or manganese. These monolayers were investigated by means of surface pressure-area isotherms and grazing incidence X-ray diffraction (GIXD). The effect of the concentration of cations in the subphase on the structure of the monolayers was probed at room temperature and for three different subphase pHs (5.5, 7.5, 10.5). A threshold in subphase concentration is detected for the formation of a superlattice structure corresponding to an inorganic organized layer in addition to the ordered behenic acid monolayer. This threshold is shown to strongly depend on the cation and the subphase pH. Above the threshold, the superstructure is independent of both the cation concentration and the pH. Below the threshold, the ions are disordered but induce a condensing effect on the fatty acid molecules, which is more or less pronounced depending on the ions. Moreover, the combination of isotherms and GIXD allows us to show that the existence of superstructures can be predicted from the shape of the isotherms. Indeed, a good agreement is obtained between the thresholds determined by the two experimental techniques.

I. Introduction Divalent metal ions are extensively used to make Langmuir-Blodgett (LB) multilayers of fatty acid salts.1,2 It is known that their interaction with the polar heads of fatty acid molecules leads to more stable and well-ordered monolayers, improving the LB transfer.3,4 The effect of various divalent metal cations on the structure of fatty acid Langmuir monolayers (i.e., at the air-water interface) has been the subject of several recent studies.5-13 In a * Corresponding lpms.u-cergy.fr. † LPPI. ‡ LURE. § OCIIB. | ESRF.

author:

E-mail

Francoise.Perrot@

(1) Peng, J. B.; Barnes, G. T.; Gentle, I. R. Adv. Colloid Interface Sci. 2001, 91, 163. (2) Schwartz, D. K. Surf. Sci. Rep. 1997, 27, 241. (3) Claesson, P. M.; Berg, J. M. Thin Solid Films 1989, 176, 157. (4) Schwartz, D. K.; Garnaes, J.; Viswanathan, R.; Zasadzinski, J. A. N. Science 1992, 257, 508. (5) Leveiller, F.; Jacquemain, D.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. Science 1991, 252, 1532. (6) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. J. Chem. Phys. 1992, 96, 1556. (7) Leveiller, F.; Bo¨hm, C.; Jacquemain, D.; Mo¨hwald, H.; Leiserowitz, L.; Kjaer, K.; Als-Nielsen, J. Langmuir 1994, 10, 819. (8) Datta, A.; Kmetko, J.; Yu, C.-J.; Richter, A. G.; Chung, K.-S.; Bai, J.-M.; Dutta, P. J. Phys. Chem. B 2000, 104, 5797. (9) Kmetko, J.; Datta, A.; Evmenenko, G.; Durbin, M. K.; Richter, A. G.; Dutta, P. Langmuir 2001, 17, 4697. (10) Ren, Y.; Mufazzal Hossain, Md.; Iimura, K.-I.; Kato, T. Chem. Phys. Lett. 2000, 325, 503. (11) Kmetko, J.; Datta, A.; Evmenenko, G.; Durbin, M. K.; Dutta, P. J. Phys. Chem. B 2001, 105, 10818. (12) Weissbuch, I.; Buller, R.; Kjaer, K.; Als-Nielsen, J.; Leiserowitz, L.; Lahav, M. Colloids Surf., A: Physicochem. Eng. Aspects 2002, 208, 3.

recent paper,11 Kmetko et al. performed a heneicosanoic acid monolayers structure study at low temperature (10 °C) and pH 8.5 with chloride salts of various divalent cations in the subphase. This study indicates that divalent ions can be divided into two categories: those that induce the classical S phase for the fatty acid molecules and those that lead to superstructures, i.e., an ordered inorganic lattice in addition to the organic lattice. The first kind of cations includes nickel, barium, cobalt, and copper, whereas manganese and magnesium belong to the second category. In previous experiments performed with chloride salts of cadmium and lead in the subphase of arachidic and heneicosanoic acid Langmuir monolayers at low temperature and pH between 7 and 9, superlattice structures were also evidenced.5,7-9 In the case of zinc and calcium dissolved in the aqueous subphase of heneicosanoic acid Langmuir monolayers, no superlattice structure has been observed.6,8 The evidence of such superstructures should provide information about more general subjects such as, for example, biomineralization processes.13,14 Indeed, in many biological assemblies (bone, teeth, shell...), the nucleation and growth of an inorganic mineral phase is directed by an organic layer. A better understanding of the mechanisms of mineralization at an organic template is of great interest for the development of new biomimetic composite materials. In the case of superstructures, the parameters that govern their formation are still unknown. Many questions remain unanswered. Why do only some divalent cations (13) Kmetko, J.; Yu, C.; Evmenenko, G.; Kewalramani, S.; Dutta, P. Phys. Rev. Lett. 2002, 89, 186102. (14) Aksay, I. A.; Weiner, S. Curr. Opin. Solid State Mater. Sci. 1998, 3, 219.

10.1021/la030057+ CCC: $25.00 © 2003 American Chemical Society Published on Web 11/15/2003

Superlattice Formation in Fatty Acid Monolayers

induce superstructures? What is the chemical composition of the two-dimensional inorganic lattice? What is the role of subphase pH and temperature? Indeed, in the case of nonsupersaturated ion solutions, the superstructures have only been detected at low temperature (around 10 °C). Also, in most of the reported studies, the concentration of divalent ions in the subphase is generally high, within the range of 10-3 to 10-4 mol/L. It should be interesting to probe systematically the influence of the subphase concentration on the structure of the fatty acid monolayer. One can wonder if a threshold of concentration exists for the superlattice formation and what is then the structure of the monolayer below the threshold. In view of LB transfer, the formation of superstructures in fatty acid monolayers is a drawback. Very rigid monolayers are thus obtained even at a surface pressure close to zero. Consequently, these films cannot be compressed to high surface pressures. Indeed, compressed arachidic acid monolayers with 5 × 10-4 mol/L of cadmium ions in the subphase have been shown to buckle in the third dimension.15 This effect has also been evidenced recently in heneicosanoic acid monolayers with manganese and magnesium ions in the subphase.11 As a result, such monolayers cannot be transferred to solid substrates by the LB technique. Nevertheless, for the purpose of obtaining well-anchored and ordered LB films, it should be useful to use low concentrations of divalent ions such as cadmium to elaborate more condensed Langmuir monolayers but with no surperstructures that cannot be transferred. In this paper, we focus on behenic acid (BA) monolayers with chloride salts of divalent cations in the subphase. Four cations have been used: cadmium (Cd), lead (Pb), magnesium (Mg), and manganese (Mn). Behenic acid, which presents a longer chain than the fatty acids previously studied in the presence of divalent ions, is welladapted to obtain good-quality LB films since, at room temperature, it displays more condensed phases over pure water. We measured the surface pressure-area isotherms of BA Langmuir monolayers as a function of the concentration of divalent ions in the subphase. The experiments have been carried out at room temperature and at different subphase pHs (5.5, 7.5, and 10.5). For each of the four studied ions (except Cd and Mn at pH 5.5), a concentration threshold below which the film is very rigid is evidenced. This threshold strongly depends on the cation and the subphase pH. Then, we investigated the structure of these monolayers by means of grazing incidence X-ray diffraction (GIXD).16 The thresholds measured by isotherms are confirmed by GIXD. Above the thresholds, superstructures are observed; GIXD allows determination of both the arrangement of fatty acid chains and the two-dimensional lattice formed by the ions. Below the thresholds, the ions are disordered but induce a condensing effect on the BA molecules with respect to over pure water. We should specify that we performed the experiments (isotherms, GIXD) in three Langmuir troughs of different geometries (different area/volume ratios). The results (in particular the thresholds) were observed to depend on the trough geometry. The reproducibility from one trough to another was obtained provided that the number of ions per BA molecule was the same. As a result, the concentration of ions is indicated in the paper as a number of cations per fatty acid molecule. (15) Fradin, C.; Braslau, A.; Luzet, D.; Alba, M.; Gourier, C.; Daillant, J.; Gru¨bel, G.; Vignaud, G.; Legrand, J. F.; Lal, J.; Petit, J. M.; Rieutord, F. Physica B 1998, 248, 310. (16) Daillant, J.; Alba, M. Rep. Prog. Phys. 2000, 63, 1725.

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II. Experimental Setup and Materials Behenic acid (BA, CH3-(CH2)20-COOH) (Sigma, purity g 99%) was dissolved in chloroform. CdCl2, PbCl2, MgCl2, and MnCl2 were purchased from Sigma (purity 99.99%). The experiments have been carried out at the air/solutions interface and room temperature (20 °C). The pH was either unadjusted (5.5) or adjusted to 7.5 with NaHCO3 (Sigma) or to 10.5 with KOH (Sigma). The use of different species to adjust the pH allows one to obtain stable values in time. The surface pressure-area isotherms were performed on a Nima Langmuir trough (Nima, 601BAM). The volume of the trough is 0.32 L. The compression speed was 0.6 Å2‚(molecule‚min)-1. The GIXD measurements were performed at the “Troı¨ka II” (ID10-B) beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) and on the D41B beamline at the LURE synchrotron source (Orsay, France). The used Langmuir trough, mounted on a vibration isolation system, was equipped with a single barrier for the monolayer compression and with a Wilhelmy balance for the surface pressure control. The volume of the trough was 0.32 L at ESRF and 0.96 L at LURE. The wavelength (λ ) 1.552 Å at ESRF and λ ) 1.605 Å at LURE) was selected using a two-crystal diamond (111) monochromator at ESRF and a focusing Ge (111) crystal at LURE. To fix the angle of incidence of the X-ray beam to 0.85 Rc, where Rc is the critical angle for total internal reflection, we used a double-mirror setup which strongly suppresses the higher harmonics. To reduce the scattering by the air above the monolayer, a flux of He gas was maintained inside the Langmuir trough. The diffracted intensity was recorded by a vertical Ar/ CO2-filled position-sensitive detector (PSD). The GIXD signal was averaged over a monolayer area close to 50 mm2. The measurement of the intensity as a function of the in-plane component Qxy of the scattering vector, integrated over the vertical component, Qz, yields Bragg peaks. The Qxy resolution provided by a Soller collimator in front of the PSD was 0.007 Å-1. From the Qxy position of the peaks, the parameters of the unit cell can be deduced. By extracting the Qz dependence of the intensity across each Bragg peak (Bragg rod profiles), the tilt direction and the tilt angle can be determined.

III. Results and Discussion A. Surface Pressure-Area Isotherms. We measured the surface pressure-area isotherms of BA Langmuir monolayers spread over chloride salts solutions of Cd, Pb, Mg, and Mn. The number of deposited fatty acid molecules was 1.37 × 1017, leading to an initial molecular area A ) 37 Å2. The concentration of divalent cations was systematically varied from 10-7 to 10-2 mol/L, corresponding to a variation of the number of cations per BA molecule between about 14 × 10-2 and 14 × 103. Three different subphase pHs were probed: 5.5, 7.5, and 10.5. In the case of MnCl2 solutions, no experiment has been performed at pH 10.5 since a precipitate was observed. Figure 1 shows a part of the isotherms obtained at different pHs, as a function of the ion concentration, compared to that measured over pure water at the same pH. Except in the case of Cd and Mn at pH 5.5 (not shown), a common characteristic of the isotherms obtained with the four cations at the three studied pHs is the existence of a threshold of ion concentration above which the monolayer behaves in a drastically different way. This threshold is strongly dependent on both the cation and the subphase pH. Above the ion concentration threshold, the film is very rigid, even at a surface pressure π ) 0 mN/m.15 The monolayers cannot be compressed to surface pressures higher than a few mN/m. They display a solid behavior, carrying away the paper Wilhelmy plate. Then the slope of the isotherm is observed to change and the area per molecule does not correspond any longer to a monolayer: multilayers are probably formed. Moreover, the shape of

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Figure 1. Surface pressure vs mean molecular area isotherms for BA monolayers spread over chloride salts of Cd, Pb, Mg, and Mn, as a function of the ion concentration (different broken lines) compared to that measured over pure water (straight line) and for different subphase pH (as indicated).

the isotherms was strongly dependent on the barrier speed, with a higher surface pressure reached for faster compression speeds. Below the concentration threshold, the film is clearly less rigid and can be compressed until high surface pressure. One can notice on the isotherms that, even at low surface pressure, the packing density of molecules is higher than that obtained over pure water; this indicates a contraction of the monolayer due to the interaction of cations with BA molecules. This effect is observed for each of the four ions but is even more pronounced in the case of Pb. For Cd and Mn at pH 5.5, even at high concentration, only a condensing effect of the ions is observed but the films do not display the solid behavior evidenced at higher pH. These results seem to indicate the presence of superstructures in BA monolayers spread at room temperature over chloride salt solutions of Cd, Pb, Mg, and Mn above a well-defined concentration threshold. These observations have to be confirmed by structural characterizations using GIXD. The thresholds measured within a precision of 10% by means of isotherms are reported in Table 1. One can observe that the values vary strongly both for one cation to the other and also as a function of pH. For the four cations, the threshold decreases with increasing pH. In the case of Cd and Mn at pH 5.5, it may be possible that

Table 1. ion Cd Pb Mg Mn

subphase pH threshold (number of ions per BA molecule) 5.5 7.5 10.5 5.5 7.5 10.5 5.5 7.5 10.5 5.5 7.5

no superstructure 3.5 1.7 59 2.4 2.1 11230 3510 1.4 no superstructure 7

superstructures exist at very high concentration (>10-2 mol/L corresponding to more than 14 × 103 ions per BA molecule). However, at these concentrations, the complete dissolution of the cations is impossible. B. Grazing Incidence X-ray Diffraction. We performed GIXD experiments on BA monolayers spread on chloride salt solutions of Cd, Pb, Mg, and Mn, adjusted to pH 5.5, 7.5, or 10.5. Above the threshold, GIXD experiments were performed on uncompressed monolayers at an area per molecule between 36 and 51 Å2. For each of the cations, the diffraction patterns measured above the thresholds are independent of both the ion concentration and the sub-

Superlattice Formation in Fatty Acid Monolayers

Figure 2. X-ray diffraction data in the horizontal plane, integrated over Qz (a), and contours of equal intensity vs the in-plane and out-plane scattering vector components Qxy and Qz (b) for an uncompressed BA monolayer with Cd ions in the subphase (150 ions per BA molecule), pH ) 7.5.

phase pH. The structure of both the BA molecules and the ion complexes just vary according to the used cation. The pH seems to influence exclusively the value of the threshold, probably in terms of number of adsorbed ions. Below the threshold, inorganic diffraction peaks are no longer detected. Upon compression of the monolayers, the observed phases and the surface pressures of transitions depend on the cation, its concentration, and the subphase pH. A condensing effect of the ions is systematically evidenced. As shown below, this phenomenon is very pronounced in the case of Pb, even with very low ion concentrations. An important result is that superstructures are evidenced above the concentration thresholds determined by isotherms, provided that the thresholds are expressed as a number of cations per BA molecule. This means that the isotherms are a useful guide to know whether superstructures are formed. Within a precision of 10%, the thresholds measured by means of GIXD correspond to that obtained with isotherms. To our knowledge, this is the first evidence of superstructures at ambient temperature (in the case of non supersatured cations solutions). Moreover, the existence of a concentration threshold for their formation, which can be very low depending on the cation and the subphase pH, is demonstrated. In the following, we present the results obtained with two of the cations, Cd at a subphase pH of 7.5, and Pb at a subphase pH of 5.5. 1. Effect of Cd Ions at pH 7.5. Above the threshold, the diffraction pattern is invariant over the studied concentration range. Figure 2a shows the diffraction peaks obtained within the range 1.37 e Qxy e 1.63 Å-1. The three most intense peaks correspond to the packing of the chains of the BA molecules, whereas the weak peaks are attributed to an organized layer of cadmium ions or their hydratation products (superlattice structure).7,8 The pres-

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ence of three peaks indicates an oblique unit cell with dimensions a′ ) 4.59 Å, b′ ) 4.90 Å, and γ′ ) 121.6°, corresponding to an equivalent pseudo-rectangular unit cell with lattice parameters a ) 4.59 Å, b ) 8.38 Å, and γ ) 93.7°. Figure 2b shows the contours of equal diffracted intensity as a function of the in-plane and out-of-plane scattering vector components Qxy and Qz. The three peaks are out of the plane, indicating tilted molecules. From the Bragg rod profiles, we found that the chains are tilted almost in the direction of the longer b axis of the rectangular cell, with a tilt magnitude of about 9.5°. The three peaks have a resolution-limited width, indicating a correlation length larger than about 1000 Å. We obtain with behenic acid exactly the same diffraction pattern as that reported by Leveiller et al.7 with arachidic acid spread at 9 °C over a 10-3 M CdCl2 subphase adjusted to pH 8.8 and also by Datta et al.8 in a heneicosanoic acid monolayer at 9.2 °C with 10-4 M CdCl2 in the subphase and pH within the range of 7-9. One can observe that the chain length of the molecule influences the experimental conditions under which a superlattice is formed. Indeed, the diffraction peaks indicative of a superlattice structure appear while cooling the subphase at about 12 °C in arachidic acid monolayers;7 in contrast, these peaks are present at ambient temperature in BA monolayers. We also performed GIXD experiments at ambient temperature with a fatty acid with a shorter chain, stearic acid; no superstructure has been detected even at high Cd concentration. This is in agreement with the results obtained with arachidic acid monolayers and seems to indicate a correspondence between the chain length and the temperature for the superstructure formation, in the same way as for the phase diagram on pure water.17 These results indicate that within the ion concentration range generally used in the literature (10-3 to 10-4 mol/ L), BA monolayers display superstructures at surface pressures close to zero, even at room temperature. As a result, very few studies have been devoted to the study of LB films of BA with Cd ions.10,18-21 We underline that a degradation of the films has been noticed as a function of time, which was all the more fast as the Cd concentration increases. Both at LURE and ESRF, we could not perform GIXD measurements over a large Qxy range on the same monolayer. In particular, for the higher studied Cd concentrations, this degradation did not allow us to monitor more than one or two peaks. Above the threshold, it manifests itself as the disappearance of all the peaks corresponding to the superlattice structure and for all the studied concentrations the diffraction peaks corresponding to the packing of the BA chains were replaced by a single broad and asymmetric in-plane peak at Qxy ) 1.52 Å-1 (Figure 3). Below the threshold, the superlattice structure is no longer observed. We measured the diffraction patterns at different Cd concentrations, as a function of the surface pressure. In the following, we present the results obtained for a subphase concentration corresponding to 2.9 Cd ions per BA molecule. For a subphase containing 2.9 Cd ions per BA molecule and whatever is the surface pressure, two lower-order (17) Bibo, A. M.; Peterson, I. R. Adv. Mater. 1990, 2, 309. (18) Fromherz, P.; Oelschla¨gel, U.; Wilke, W. Thin Solid Films 1987, 146, L15. (19) Buhaenko, M. R.; Grundy, M. J.; Richardson, R. M.; Roser, S. J. Thin Solid Films 1988, 159, 253. (20) Grundy, M. J.; Musgrove, R. J.; Richardson, R. M.; Roser, S. J.; Penfold, J. Langmuir 1990, 6, 519. (21) Riegler, J. E. J. Phys. Chem. 1989, 93, 6475.

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Figure 3. X-ray diffraction data in the horizontal plane, integrated over Qz (a), and contours of equal intensity vs the in-plane and out-plane scattering vector components Qxy and Qz (b) for a degraded BA monolayer with Cd ions in the subphase (2.9 ions per BA molecule), pH ) 7.5.

Figure 4. In-plane Bragg peak positions versus surface pressure for a BA monolayer with Cd ions in the subphase (2.9 ions per BA molecule), pH ) 7.5. The empty triangles indicate the position of the nondegenerate (02) peak, while the filled circles indicate the position of the degenerate (11) peak.

diffraction peaks are observed, indicating a centered rectangular (distorted hexagonal) unit cell. In terms of this rectangular cell, the peaks can be assigned to the degenerate (11) and (11 h ) and to the nondegenerate (02) Bragg reflections.22 Figures 4 and 5 show the in-plane Qxy positions of the peaks, the rectangular cell parameters a and b, the mean molecular area A, and the tilt angle as a function of π. At surface pressures lower than 14 mN/m (Figure 6a), the peak obtained at lower Qxy is in the plane of the water surface whereas the second peak is out of the plane, showing that the molecules are tilted toward a nearest neighbor (NN tilt). The more intense, in-plane, peak is the nondegenerate (02) peak, and the out-of-plane peak corresponds to the degenerate (11) and (11 h ) reflections. One can notice that the (02) peak is resolution-limited, implying a crystallization direction along the longer b axis of the unit cell, perpendicular to the tilt direction. The (22) Kaganer, V. M.; Mo¨hwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779.

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packing of the molecular chains can be determined by calculating the parameters aT and bT of the rectangular cell projected onto a plane normal to the long axis of the molecules (transverse cell).23The transverse parameters aT ) 4.5 Å and bT ) 8.6 Å deduced from the diffraction data correspond to a pseudo-herringbone (PHB) backbone arrangement of the carbon chains. This is characteristic of the L2h phase. For surface pressures in the range 14-26 mN/m, both peaks are out of the plane (Figure 6b). The Qz position of the more intense peak corresponds to one-half the Qz value of the other peak, indicating a tilt of the chains toward a next-nearest neighbor (NNN tilt). The peaks can be thus assigned respectively to the degenerate (11) and (11 h ) and to the nondegenerate (02) Bragg reflections. The parameters of the transverse rectangular unit cell are aT ) 5 Å and bT ) 7.8 Å, corresponding to a herringbone (HB) arrangement of the hydrocarbon chains. None of the peak widths is resolution-limited. The HB packing of the chains indicates the presence of a crystallization direction that should be perpendicular to the molecular tilt. This cannot be evidenced by the diffraction data since we did not succeed in detecting the higher-order (20) peak that should be resolution-limited. These data are characteristic of the L′2 phase. At surface pressures higher than 26 mN/m, the two diffraction peaks are found in the plane of the water surface, indicating a centered rectangular packing of untilted molecules (Figure 6c). The parameters of the unit cell are characteristic of a HB packing of the chains. Both peaks are broad, meaning that the crystallization is probably along the a direction, as in the L′2 phase. These diffraction data are in agreement with a packing of the molecules in the S phase.24 The results obtained for BA monolayers with 2.9 Cd ions per BA molecule in the subphase can be compared to the data obtained on pure water and ambient temperature.25,26 Over pure water, the sequence of phases and the characteristics of the transitions are exactly the same as over chloride salt solutions of Cd. There is a discontinuity of the rectangular cell parameters a and b at the L2-L′2 phase transition, whereas the L′2 - S transition appears continuous. The tilt angle continuously decreases with increasing surface pressure, until it reaches zero in the S phase. However, some differences can be noticed. The two phase transitions occur at lower π on the CdCl2 subphase, indicating the condensing effect of the ions. This is confirmed by the values of the mean molecular area A in the L2 and L′2 phases which are slightly lower than that obtained on pure water. The tilt angles are lower when cadmium is present in the subphase. As a result, there is no crossing of the Qxy positions of the two peaks in the L2 phase as observed on pure water. Over the two subphases, the transverse PHB unit cell of the L2 phase extends in the direction of the parameter bT. However, the NN tilt reduces the stretching of the in-plane cell. With Cd ions in the subphase, the tilt angle is lower than about 20° and the in-plane cell remains stretched in the direction of the parameter b. In contrast, on pure water, the higher tilt angle leads to a jump of the stretching direction from b to a when decreasing π. As a conclusion, (23) Kuzmenko, I.; Kaganer, V. M.; Leiserowitz, L. Langmuir 1998, 14, 3882. (24) Durbin, M. K.; Richter, A. G.; Yu, C.-J.; Kmetko, J.; Bai, J. M.; Dutta, P. Phys. Rev. E 1998, 58, 7686. (25) 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. (26) Fradin, C.; Daillant, J.; Braslau, A.; Luzet, D.; Alba, M.; Goldmann, M. Eur. Phys. J. B 1998, 1, 57.

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Figure 5. Rectangular cell parameters, mean molecular area, and tilt angle vs surface pressure for a BA monolayer with Cd ions in the subphase (2.9 ions per BA molecule), pH ) 7.5.

Figure 6. X-ray diffraction data in the horizontal plane, integrated over Qz (left), and contours of equal intensity vs the in-plane and out-plane scattering vector components Qxy and Qz (right) for BA monolayers with Cd ions in the subphase (2.9 ions per BA molecule), pH ) 7.5, at different surface pressures: 7.5 (a), 20 (b) and 35 mN/m (c).

below the threshold, the monolayer displays the same sequence of phases than over pure water but with a condensing effect of the Cd ions. To obtain more information about the transition between superstructures and L2 phase, we approached the concentration threshold value (3.5 ions per BA molecule) by concentration steps of 0.4 ions per BA molecule, at zero surface pressure and A ) 51 Å2. Below the threshold, the tilt angle decreases slowly with increasing number of ions, from about 30° over pure water to about 23° for a subphase containing 3.2 Cd ions per BA molecule. Above the threshold, the value of the tilt angle is about 9.5° independent of the number of ions. We should specify that no coexistence between superstructures and L2 phase has been observed. 2. Effect of Pb Ions at pH 5.5. Above the concentration threshold, the measured diffraction pattern is independent of the ion concentration. We investigated the diffraction

peaks for Qxy in the range 0.3-2.1 Å-1, and Figure 7 shows the results obtained within the range 1.4 e Qxy e 1.72 Å-1 (Figure 7a,b) and 0.33 e Qxy e 0.55 Å-1 (Figure 7c). The three most intense peaks (Figure 7a) correspond to the structure of the BA molecules. They indicate an oblique unit cell with dimensions a′ ) 4.52 Å, b′ ) 4.97 Å, and γ′ ) 121.9°, corresponding to an equivalent pseudo-rectangular unit cell with lattice parameters a ) 4.52 Å, b ) 8.47 Å, and γ ) 95.0°. The three peaks are in the plane of the water surface, meaning that the BA chains are untilted (Figure 7b). The weak peaks correspond to the organization of a thin layer of Pb ions or their hydrolysis products. Figure 7c shows the three first-order inorganic peaks leading to a unit cell area 14 times larger than that of the BA molecules. The diffraction pattern is exactly the same as that reported by Kmetko et al.9 in a heneicosanoic acid Langmuir monolayer spread at 10 °C over a 10-5 M PbCl2 subphase at pH 5.5. Nevertheless, we detected two more peaks corresponding to this superstructure at Qxy ) 1.756 Å-1 and Qxy ) 1.775 Å-1, indexed respectively (1,4) and (4,1 h ) in terms of the superlattice cell. Below the threshold, the behavior is strongly different from that obtained for Cd ions. Indeed, the structure is independent of the ion concentration in the studied range and also of the surface pressure. One obtains systematically two in-plane diffraction peaks at Qxy ) 1.505 Å-1 and Qxy ) 1.622 Å-1. This pattern is indicative of the packing of untilted BA molecules in a centered rectangular (distorted hexagonal) unit cell (Figure 8). The positions of the two peaks are very close to that measured in the S phase obtained over pure water at high surface pressure. Moreover, the two peaks are broad as in the S phase. Consequently, the peaks probably correspond, respectively, to the degenerate (11) and (11 h ) and to the nondegenerate (02) Bragg reflections. The parameters of the rectangular cell are then a ) 4.96 Å and b ) 7.75 Å, indicating a HB packing of the chains. These results show that Pb ions induce a strong condensing effect on the BA molecules. Indeed, even at zero surface pressure, the BA monolayer displays the structure of the high-pressure S phase of BA monolayer spread over pure water. Moreover, the contraction of the BA lattice is independent of the ion concentration. Among the four studied cations, Pb is the only one that induces such an important effect. In the same way as for the Cd ions, we approached the threshold value (59 ions per BA molecule) by concentration steps of 1.5 ions per BA molecule at zero surface pressure

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Figure 8. X-ray diffraction data in the horizontal plane, integrated over Qz (a), and contours of equal intensity vs the in-plane and out-plane scattering vector components Qxy and Qz (b) for BA monolayers with Pb ions in the subphase (9 ions per BA molecule), pH ) 5.5.

Figure 7. X-ray diffraction data in the horizontal plane, integrated over Qz (a, c), and contours of equal intensity vs the in-plane and out-plane scattering vector components Qxy and Qz (b) for BA monolayers with Pb ions in the subphase (150 ions per BA molecule), pH ) 5.5.

and A ) 51 Å2. The structure is strictly invariant both below and above the threshold. As in the case of Cd ions, coexistence between S phase and superstructures has never been observed. IV. Conclusions We studied the structure of BA (C22) monolayers with chloride salts of Cd, Pb, Mg, and Mn in the subphase as a function of the ion concentration and at three subphase pHs. Contrary to arachidic acid (C20) and heneicosanoic acid (C21) monolayers that display superstructures only at low temperature, a superlattice structure corresponding to an organized layer of ions or hydroxide complexes is evidenced at ambient temperature in BA monolayers. One obtains the same structures as those previously reported. These results indicate a connection between the sequence of phases of the fatty acid over pure water and the superstructure formation. The presence of a rotator phase (LS phase) at high surface pressure on pure water seems to prevent the superlattice existence. Besides, for each of

the four studied cations, the existence of a threshold of ion concentration is demonstrated for the superlattice formation. It is evidenced by surface pressure-area isotherms and confirmed by GIXD. This threshold is expressed as a number of ions per BA molecule since it was observed to depend on the volume of the Langmuir trough. Its value strongly varies with the cation and the subphase pH. Above the threshold, the superlattice parameters measured for each ion are independent of the ion concentration and the subphase pH. Below the threshold, the ions are no longer organized but induce a condensing effect on the BA molecules that could be interesting in view of LB transfer. For example, in the case of Cd, the sequence of phases is the same as that obtained over pure water but the phase transitions occur at lower surface pressure and the tilt angle of the molecular chains is lower. The effect is more pronounced for Pb since a S phase is observed even at zero surface pressure. Using GIXD and in the case of Pb at pH 5.5 and Cd at pH 7.5, we also examined the transition by approaching the threshold value until (10%. The two transitions seem to be discontinuous since the unit cell as well as the tilt angle (for Cd) change abruptly. However, to conclude about the order of the transition, the important parameter that would be interesting to measure is the ion surface concentration. Further experiments are in progress in order to study the behavior of other divalent cations, in particular calcium, which is involved in biomineralization processes. Indeed, in the case of the ions for which no superstructure has been detected, one can wonder if the reason is not the high value of the concentration threshold. As shown in this paper, surface pressure-area isotherms are simple experiments to check the existence of superstructures. Other future work would be devoted to study the influence

Superlattice Formation in Fatty Acid Monolayers

of the counterion on superstructure formation. Also, in the context of biomimetic materials, it should be interesting to replace the fatty acids of the organic monolayer by biological molecules.

Langmuir, Vol. 19, No. 26, 2003 10815

Acknowledgment. We thank M. Alba for useful advice during the synchrotron experiments. LA030057+