pubs.acs.org/Langmuir © 2009 American Chemical Society
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Evolution toward the X Phase of Fatty Acid Langmuir Monolayers on a Divalent Cation Solution Sophie Cantin,*,† Sebastien Peralta,† Philippe Fontaine,‡ Michel Goldmann,§, and Franc- oise Perrot†
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† Laboratoire de Physico-Chimie des Polym eres et des Interfaces (LPPI, EA 2528), Universit e de Cergy-Pontoise, 5 mail Gay-Lussac Neuville/Oise, 95031 Cergy-Pontoise Cedex, France, ‡Synchrotron Soleil, L’Orme des Merisiers, Saint Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France, §Institut des Nanosciences de Paris (INSP,), Campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France, and UFR Biom edicale, Universit e Paris Descartes, 45 rue des St p eres 75006 Paris, France
Received June 25, 2009. Revised Manuscript Received August 10, 2009 The structure of docosanoic acid monolayers spread over chloride salt solutions of copper was investigated by means of isotherm measurements, grazing incidence X-ray diffraction, and Brewster angle microscopy, as a function of the ion concentration and at two subphase pHs (5.5 and 7.5). The X phase is evidenced immediately above a concentration threshold which depends on the pH. The sequence of phases leading to this rigid phase involves two different processes depending on the pH. The initial L2h phase evolves toward an X-like phase through a phase transition which is first order at pH 7.5 while it is second order at pH 5.5. The transition is then followed by a continuous evolution toward the X phase.
I. Introduction Langmuir monolayers of amphiphilic molecules spread over metal salt solutions provide interesting models for studying organic-inorganic interactions.1,2 Organic-inorganic interfaces are involved in many natural materials such as nacre or bone,3 justifying extensive investigations of such systems. Among amphiphilic molecules, saturated fatty acids appear as the simplest molecules. However, their interaction with divalent cations seems quite subtle. Indeed, different structural organizations have been evidenced by means of grazing incidence X-ray diffraction (GIXD) depending on the ion, subphase pH, temperature, or fatty acid chain length.2,4-10 Even for dilute metal solutions, drastic modifications of Langmuir monolayer phases may be induced by ion adsorption. Indeed, when lead, cadmium, magnesium, or manganese ions are dissolved in the aqueous subphase, a superlattice structure of a thin organized inorganic layer under the fatty acid monolayer is evidenced by means of GIXD.7 This superstructure can be observed at room temperature provided that the fatty acid chain is long enough.8 Experiments performed at low temperature (10 °C) with *Corresponding author. E-mail
[email protected]. (1) Kmetko, J.; Yu, C.; Evmenenko, G.; Kewalramani, S.; Dutta, P. Phys. Rev. B 2003, 68, 085415. (2) Leveiller, F.; Bohm, C.; Jacquemain, D.; M€ohwald, H.; Leiserowitz, L.; Kjaer, K.; Als-Nielsen, J. Langmuir 1994, 10, 819. (3) Song, F.; Soh, A. K.; Bai, Y. L. Biomaterials 2003, 24, 3623. (4) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. J. Chem. Phys. 1992, 96, 1556. (5) 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. (6) Kmetko, J.; Datta, A.; Evmenenko, G.; Durbin, M. K.; Richter, A. G.; Dutta, P. Langmuir 2001, 17, 4697. (7) Kmetko, J.; Datta, A.; Evmenenko, G.; Dutta, P. J. Phys. Chem. B 2001, 105, 10818. (8) Dupres, V.; Cantin, S.; Benhabib, F.; Perrot, F.; Fontaine, P.; Goldmann, M.; Daillant, J.; Konovalov, O. Langmuir 2003, 19, 10808. (9) Cantin, S.; Pignat, J.; Perrot, F.; Fontaine, P.; Goldmann, M. Phys. Rev. E 2004, 70, 050601(R). (10) Pignat, J.; Cantin, S.; Liu, R. C. W.; Goldmann, M.; Fontaine, P.; Daillant, J.; Perrot, F. Eur. Phys. J. E 2006, 20, 387.
830 DOI: 10.1021/la9022823
henicosanoic acid monolayers as organic matrix show that the other divalent metal ions only lead to a condensation of the fatty acid monolayer, without any ion ordering.7 Barium, nickel, and cobalt induce the classical high-pressure S phase of fatty acid monolayer spread over pure water while calcium, zinc, and copper lead to the formation of a new fatty acid structure called X.4,5,7 This X phase exhibits untilted molecules arranged on a highly distorted hexagonal lattice with a GIXD peaks indexation opposite to the S phase one. Its cell parameters are rather close to those corresponding to a pseudo-herringbone (PHB) ordering of the backbone planes of the molecules. This PHB structure is specific to two-dimensional films. Indeed, most of the non-rotator fatty acid monolayer phases present a herringbone structure (HB) which is very common in three-dimensional packing of aliphatic chain derivatives.11 For fatty acid monolayers on pure water, such a PHB structure was only previously observed in a tilted phase, the L2h phase. This highlights the strong influence of calcium, zinc, and copper on the fatty acid headgroups. It should be emphasized that the X phase, in the same way as the superstructures, is detected only if the fatty acid chain is long enough or the temperature sufficiently low.12,13 Indeed, IRRAS measurements performed on octadecanoic acid monolayers spread over aqueous CuCl2 subphases have evidenced highly crystalline fatty acid domains even at large molecular areas.13 Nevertheless, the chain packing was determined as hexagonal. Thus, increasing the chain length allows the observation of more ordered phases at ambient temperature. Until now, attempts to correlate the observed organic-inorganic structures to specific ion properties have been unsuccessful. For the cations leading to superstructure formation, the chemical composition of the inorganic species ordered at the interface as well as the nature of the cation-headgroup interaction remains still unknown. Grazing incidence X-ray absorption fine structure (11) Kaganer, V. M.; M€ohwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779. (12) Gericke, A.; H€uhnerfuss, H. Thin Solid Films 1994, 245, 74. (13) Simon-Kutscher, J.; Gericke, A.; H€uhnerfuss, H. Langmuir 1996, 12, 1027.
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data obtained when lead is dissolved in the subphase have nevertheless allowed proposing a model for the hydrolysis product, including a covalent headgroup-cation bonding.14 Studies of the adsorption kinetics leading to the different structures observed in the presence of divalent cations may provide new insights into understanding headgroup-cation interactions. For the cations that lead to superstructure formation, an ion concentration threshold necessary to immediate superlattice observation was demonstrated.8 Its value strongly depends on the ion and subphase pH. Below this threshold, the superstructure is formed through a kinetic process which has been analyzed in the presence of cadmium and magnesium by coupling GIXD and Brewster angle microscopy (BAM).9,10 The initial fatty acid L2 phase detected over divalent cation-free subphase has been observed to evolve toward the superstructure through a new weakly ordered fatty acid structure called the intermediate I phase. Moreover, with chloride salts of magnesium, the subphase pH has been shown to influence the sequence of phases. The role of subphase pH, not only on the ability for the ions to form superstructure but also on the kinetic process, could be due to the gap between the interfacial pH and the subphase pH; this gap strongly depends on the cation as shown by polarization-modulated infrared reflection-absorption spectroscopy.15 No kinetic studies of the sequence of phases that leads to the final structure have been conducted in the presence of the other metal ions, in particular those that induce the fatty acid X phase. In this paper, we focus on docosanoic acid (or behenic acid) monolayers spread over chloride salt solutions of copper, at room temperature, and on two different subphase pH (5.5 and 7.5). Surface pressure-molecular area isotherms were measured as the function of ion concentration in the subphase. The X phase was observed for the first time by means of Brewster angle microscopy (BAM). Below a concentration threshold deduced from isotherms and GIXD experiments, the adsorption kinetics of Cu2þ ions was followed by GIXD. The data were then correlated to BAM observations which can be very useful in the analysis of the phase transitions occurring as a function of time. In particular, domains resulting from either molecular tilt or lattice anisotropy can be evidenced.
II. Experimental Setup and Materials Docosanoic acid (or behenic acid noted BA, CH3-(CH2)20COOH) (Sigma, purity >99%) was first dissolved in chloroform (Merck, analytical grade) to a concentration of 0.389 g L-1. The monolayer was then spread over CuCl2 (Sigma, purity 99,99%) solutions prepared with ultrapure deionized water from a Milli-Q Millipore system. The subphase pH was either unadjusted (5.5) or adjusted to 7.5 with NaHCO3 (Sigma). The temperature was set to 20 °C using a water refrigerated/heating circulator. The deposited volume of behenic acid solution was chosen in order to obtain an initial mean molecular area of 0.37 nm2. For the GIXD or BAM studies, the monolayer was then compressed to a mean molecular area of 0.22 nm2, corresponding to a surface pressure of 0 mN/m and to the end of the gas-L2h phase transition plateau over a divalent cation-free subphase. The surface pressure-area isotherms were performed on a Nima Langmuir trough (Nima, 601BAM). The grazing incidence X-ray diffraction (GIXD) experiments reported here were carried out at the LURE synchrotron on the D41 beamline and at Hasylab facility on the BW 1 beamline. The (14) Boyanov, M. I.; Kmetko, J.; Shibata, T.; Datta, A.; Dutta, P.; Bunker, B. A. J. Phys. Chem. B 2003, 107, 9780. (15) Le Calvez, E.; Blaudez, D.; Buffeteau, T.; Desbat, B. Langmuir 2001, 17, 670.
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Article experimental setups were previously described.16,10 The trough was moved by 2 mm horizontally after each scan in order to avoid the influence of the monolayer degradation due to X-ray exposure. However, this limits the number of scans on the same film. Brewster angle microscopy takes advantage of the reflectivity properties of an interface illuminated at the Brewster angle with light polarized in the plane of incidence. Optical anisotropies due to molecular tilt, or lattice anisotropy in the untilted phases, can be observed by adding an analyzer on the path of the reflected light.17,18 The Langmuir trough geometries (volume V, surface S) were different for one experimental setup to another: V=367 mL, S= 324 cm2 for BAM experiments, V = 250 mL, S = 450 cm2 for GIXD at Hasylab, V=1000 mL, S=690 cm2 for GIXD at LURE, and V = 305 mL, S = 518 cm2 for isotherm measurements. Consequently, in order to ensure time coherence between all these experiments, we used an experimental unit expressing the salt concentration as a number of ions per behenic acid molecule.9
III. Results and Discussion The structure of BA monolayers spread over CuCl2 solutions was first studied as a function of subphase concentration at pH 5.5 and 7.5. The parameters that lead to the BA X phase were thus characterized. The kinetic process of the X phase formation was then studied coupling GIXD and BAM. A. X-Phase Characterization: Effects of pH and Subphase Concentration. a. Surface Pressure-Area Isotherms. Surface pressure-area isotherms of BA Langmuir monolayers spread over chloride salts solutions of copper adjusted to pH 7.5 or pH 5.5 were measured at different subphase concentrations. The concentration was systematically varied from 10-7 to 5 10-3 mol/L, leading to a variation of the number of cations per BA molecule between about 1310-2 and 6700. Figure 1 presents the corresponding isotherms, compared to those measured over divalent cation-free subphase at the same pH. With increasing ion concentration, the shape of the isotherms evolves in a rather similar way at the two studied pH; only a shift of the number of Cu2þ ions is observed. A strong condensation of the BA monolayer is first evidenced at low ion concentration. The isotherms become steeper, and a phase transition remains visible only at concentrations lower than 0.5 Cu2þ ion/BA molecule at pH 7.5 or 25 Cu2þ ion/BA molecule at pH 5.5. Then above a concentration threshold close to 5 Cu2þ ions/BA molecule at pH 7.5 or 100 Cu2þ ions/BA molecule at pH 5.5, the shape of the isotherms changes drastically. Even at a surface pressure Π=0 mN/m, the Wilhelmy paper of the pressure sensor is kept prisoner in a very rigid film, leading to erroneous surface pressure measurements. The surface pressure does not exceed a few mN/m at molecular areas which do not correspond any longer to a monolayer. The shape of the isotherms is yet reproducible but depends on the compression speed. Such particular isotherm shapes were previously evidenced for BA Langmuir monolayers spread over chloride salt solutions of Cd2þ, Mg2þ, Mn2þ, and Pb2þ, above a concentration threshold corresponding to immediate superstructure formation.8 Nevertheless, with shorter fatty acids and at low temperature, Cu2þ ions do not form any ordered lattice under the organic monolayer. They only induce a condensation of the fatty acid molecules according to a new X phase.7 The shape of the isotherms measured above the concentration thresholds evidenced at the two studied pH indicates the presence of a very rigid film. These (16) Fontaine, P.; Goldmann, M.; Bordessoule, M.; Jucha, A. Rev. Sci. Instrum. 2004, 75, 3097. (17) Riviere, S.; Henon, S.; Meunier, J.; Schwartz, D. K.; Tsao, M.-W.; Knobler, C. M. J. Chem. Phys. 1994, 101, 10045. (18) Riviere-Cantin, S.; Henon, S.; Meunier, J. Phys. Rev. E 1996, 54, 1683.
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Figure 1. Surface pressure vs mean molecular area isotherms for BA monolayers spread over CuCl2 solutions adjusted to pH 7.5 (left) and 5.5 (right) as a function of the ion concentration (different broken lines) compared to that measured on divalent cation-free subphases (straight line) at the same pH.
Figure 2. 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 a BA monolayer spread over a CuCl2 solution 10 Cu2þ ions/BA molecule at pH 7.5 and Π = 0 mN/m.
thresholds probably correspond to the Cu2þ subphase concentration necessary to immediate X phase formation. At the two investigated pH, the isotherm networks obtained as a function of concentration are similar, except that the concentration threshold corresponding to immediate X phase formation is a factor of 20 higher at pH 5.5 than at pH 7.5. The main physicochemical difference between the two studied pH is the carboxylic headgroup dissociation state.19-21 Indeed, the headgroup is partially dissociated at pH 7.5 while it is probably not ionized at pH 5.5, leading to a stronger headgroup-cation interaction at pH 7.5. b. Characterization of the Final Structure. GIXD experiments were then performed at a molecular area A=0.22 nm2 and different subphase concentrations. The subphase pH was set to either 5.5 or 7.5. The diffraction patterns obtained above the concentration thresholds evidenced by means of isotherms at 5 Cu2þ ions/BA and 100 Cu2þ ions/BA at respectively pH 7.5 and 5.5 are independent of the ion concentration (Figure 2) and of the subphase pH. Two in-plane first-order diffraction peaks are detected, indicating a distorted hexagonal lattice (or equivalently rectangular lattice). The peak positions are similar to those reported for henicosanoic acid monolayers spread over 10-4 M CuCl2 solutions at low temperature (10 °C).7 No inorganic peak is detected. The diffraction profiles are characteristic of the fatty (19) Johann, R.; Vollhardt, D.; M€ohwald, H. Colloids Surf., A 2001, 12, 311. (20) Datta, A.; Kmetko, J.; Richter, A. G.; Yu, C.; Dutta, P.; Chung, K.-S.; Bai, J.-M. Langmuir 2000, 16, 1239. (21) Johann, R.; Brezesinski, G.; Vollhardt, D.; M€ohwald, H. J. Phys. Chem. B 2001, 105, 2957.
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Figure 3. BAM images obtained for a BA monolayer spread over a CuCl2 solution at pH 7.5 and Π = 0 mN/m: (a) 10 Cu2þ ions/BA molecule, X phase observed with analyzer; (b) 35 Cu2þ ions/BA molecule, inhomogeneities in the ion layer thickness; (c, d) 10 Cu2þ ions/BA molecule, jagged shapes of the X phase condensed domains; (e) X phase rectangular cell showing the angles between reticular planes. The white bar represents 100 μm.
acid X phase. The peaks are respectively assigned to the nondegenerate (02) and the degenerate (11) and (11) Bragg reflections in terms of the rectangular unit cell. The deduced rectangular lattice parameters are a = 0.427 nm and b = 0.897 nm. These values are quite close to those corresponding to a PHB ordering of the fatty acid backbone planes (a=0.44 nm and b= 0.87 nm).11 By fitting the peaks with Lorentzian shapes, the positional correlation length L can be deduced from the full width at halfmaximum (FMWH) according to L = 2/FMWH. One obtains Langmuir 2010, 26(2), 830–837
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Figure 4. X-ray diffraction data obtained for a BA monolayer over a CuCl2 solution 1 Cu2þ ions/BA molecule at Π = 0 mN/m, pH 7.5. (a, b) Scans measured at t = 45 min: (a) scan of intensity as a function of Qxy integrated over all Qz values; (b) (11)/(11) rod scan of intensity as a function of Qz. (c) Evolution with time of the L2h phase rectangular cell parameters a and b (open symbols) and of the tilt angle (solid symbols).
L02 =42.4 nm and L11 =33.0 nm in the (02) and (11)/(11) directions, respectively. This highly distorted X phase, displaying an opposite peak indexation compared to the S phase, is thus also evidenced at room temperature provided that the fatty acid chain is enough long.12 It is not present in the generic fatty acid phase diagram measured for a pure water subphase, indicating a strong influence of the headgroup-cation interactions. BAM observations were performed at a molecular area A = 0.22 nm2 and above the concentration thresholds evidenced by means of isotherms at 5 Cu2þ ions/BA and 100 Cu2þ ions/BA at pH 7.5 and 5.5, respectively. The images obtained above the concentration threshold are independent of the subphase concentration and pH. Figure 3a shows an image obtained at a 10 Cu2þ ions per BA subphase concentration, with an analyzer on the path of the reflected light. Small dark gaseous holes are still visible. In addition, domains displaying slightly different reflectivities can be noticed in the condensed phase. This weak optical anisotropy is similar to the one detected in the high-pressure S phase and results from different lattice orientations.17 This is in agreement with GIXD data indicating untilted BA molecules arranged on a distorted hexagonal lattice. Inhomogeneities in the thickness of the ion layer can nevertheless be detected at concentrations well above the threshold, as shown in Figure 3b obtained without analyzer at pH 7.5 for 35 Cu2þ ions/BA molecule. The different shades of gray correspond to various thicknesses of the ion layer. The crystallinity and rigidity of the X phase induced by Cu2þ ions is confirmed by the observation of fractures in the condensed film leading to jagged domain shapes (Figure 3c,d). The angles measured on the images are close to those between reticular planes, as shown on the lattice depicted on Figure 3e. The fractures have thus occurred along reticular planes of the BA crystal. Langmuir 2010, 26(2), 830–837
B. Kinetics of the X Phase Formation. The kinetic process of the X phase formation was then studied at pH 7.5 and 5.5, for subphase ion concentrations below the thresholds previously described. The evolution with time of the BA monolayer was investigated by means of BAM and GIXD. The ion concentration was only observed to modify the time scale of the kinetic process without any change in the sequence of phases. a. pH 7.5. We describe the results obtained for a CuCl2 salt concentration corresponding to 1 Cu2þ ion per BA molecule. The evolution of the diffraction pattern is reported for Qxy in the range 1.25-1.75 A˚-1. Three stages are evidenced during ion adsorption. Stage 1: In a first stage, the diffraction profile is characteristic of the L2h phase, in the same way as over a divalent cation-free subphase. Figure 4a,b shows the pattern measured at t=45 min. Two lower-order diffraction peaks are detected on the scan of intensity as a function of Qxy integrated over all Qz values (Figure 4a). These peaks correspond to the in-plane (02) peak (at lower Qxy) and the degenerate out-of-plane (11)/(11) peak, according to a centered rectangular cell of parameters a = 0.482 nm and b = 0.864 nm. From the Qz component of the (11)/(11) peak (Figure 4b), one can deduce that the BA molecules are tilted toward a nearest-neighbor with an angle of about 20° with respect to the surface normal. Also, 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 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).11 The deduced transverse parameters aT = 0.453 nm and bT=0.864 nm correspond to a PHB backbone packing of the carbon chains. This is characteristic of the L2h phase. Observed with BAM, the film is characteristic of the L2h phase. Indeed, with an analyzer on the path of the reflected light, the DOI: 10.1021/la9022823
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Figure 5. X-ray diffraction data obtained for a BA monolayer over a CuCl2 solution 1 Cu2þ ions/BA molecule at Π=0 mN/m, pH 7.5, and t=132 min. (a) Scan of intensity as a function of Qxy integrated over all Qz values. (b) Contours of equal intensity vs the in-plane and out-plane scattering vector components Qxy and Qz. (c) L2h phase (11)/(11) rod scan of intensity as a function of Qz.
condensed domains appear divided into regions showing different shades of gray. These regions correspond to different tilt-azimuthal orientations of the BA molecules. The evolution with time of the rectangular cell parameters and of the tilt angle during this first stage is shown in Figure 4c. One can observe a continuous decrease of the tilt angle, down to 15° at t=95 min. The observed decrease of the a cell parameter is related to the tilt angle decrease. In contrast, the b cell parameter remains nearly constant in agreement with the crystallization direction along the b axis. Stage 2: In a second step reached at t=95 min, two diffraction peaks are detected in addition to the two Bragg reflections corresponding to the L2h phase (Figure 5). These peaks are inplane and measured at Qxy =14.32 nm-1 and Qxy =15.93 nm-1, indicating untilted BA molecules organized on a distorted hexagonal lattice. Even though this phase presents a strongly distorted hexagonal lattice, it does not exactly correspond to the final X phase and will be called X1. The peak indexation can be deduced from the evolution toward the X phase. It is the same as in the X phase and leads to rectangular cell parameters a = 0.441 nm and b=0.877 nm. The Bragg reflections corresponding to the L2h phase indicate BA molecules arranged on a rectangular cell of parameters a = 0.467 nm and b=0.861 nm and tilted toward a nearest-neighbor; the tilt angle is close to 15° with respect to the surface normal. During all this second stage (from t=95 min to t=363 min), the four diffraction peaks are detected and their positions remain fixed. As shown on the pattern obtained at t=309 min, one can observe that with time the intensity of the L2h phase peaks decreases to the advantage of that of the X1 phase peaks (Figure 6). At t = 363 min, the diffraction profile only displays the X1 phase peaks. As a result, a first-order phase transition takes place between the L2h and X1 phases. 834 DOI: 10.1021/la9022823
Figure 6. X-ray diffraction data in the horizontal plane, integrated over Qz for a BA monolayer over a CuCl2 solution 1 Cu2þ ions/BA molecule at Π = 0 mN/m, pH 7.5, and t = 309 min.
Figure 7 presents BAM images of a condensed domain obtained with an analyzer at t=105 min and t=210 min. One can clearly distinguish the nucleation of a new phase at the expense of the L2h phase. This phase displays a very slight optical anisotropy corresponding to a lattice anisotropy. This is in good agreement with the GIXD data obtained for the X1 phase, indicating untilted molecules organized on a distorted hexagonal lattice. During this second step, as expected for a firstorder phase transition, the surface covered by the L2h phase decreases until the domains display, as a whole, a slight optical anisotropy. Stage 3: In a third stage (from t=363 min), the two diffraction peaks of the X1 phase continuously move away from each other, until they reach the positions corresponding to the X phase. BAM Langmuir 2010, 26(2), 830–837
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Figure 7. BAM images obtained with an analyzer for a BA monolayer spread over a CuCl2 solution 1 Cu2þ ions/BA molecule at Π=0 mN/m and pH 7.5, at (a) t=105 min and (b) t=210 min. The white bar represents 100 μm.
Figure 9. BAM images obtained with an analyzer for a BA monolayer spread over a CuCl2 solution 80 Cu2þ ions/BA molecule at Π = 0 mN/m and pH 5.5, at (a) t = 2 min, (b) t = 15 min, (c) t = 120 min, and (d) t = 150 min. The white bar represents 100 μm.
Figure 8. Schematic representation of the rectangular cell at different stages of the evolution, at pH (a) 7.5 and (b) 5.5. The black solid line represents the L2h phase cell at the beginning of the evolution, the black dotted line the L2h phase cell during the L2h-X1 phase coexistence, the gray dotted line the X1 or X0 1 phase cell, and the gray solid line the X phase cell.
observations reveal a very slight increase of the optical anisotropy due to the more pronounced distortion of the X phase lattice. The BA rectangular cell deduced from the diffraction patterns is represented in Figure 8a at the different stages of the evolution. One can notice that the b cell parameter remains nearly constant during the evolution from the L2h phase to the X1 phase while the a parameter decreases. Then the evolution from the X1 phase to the X phase mainly involves an increase of the b cell parameter leading to a strong cell distortion. b. pH 5.5. Figure 9 shows BAM images obtained as a function of time for an 80 Cu2þ ions per BA subphase concentration. One can notice a gradual decrease of the optical anisotropy. Indeed, the tilt angle decreases continuously since the contrast between the domains corresponding to different tilt-azimuthal orientations decreases. At t=150 min, only a lattice optical anisotropy marking the X phase formation is observed. The absence of phase coexistence is confirmed by GIXD data showing a continuous evolution from the L2h phase toward the X phase. Figure 10a-e shows the first stage of the time evolution of the two observed Bragg reflections for a 50 Cu2þ ions per BA solution. The variations of the rectangular cell parameters and of the tilt angle are reported in Figure 10f. The evolution with time of the (11)/(11) rod scans of intensity as a function of Qz indicates a continuous Langmuir 2010, 26(2), 830–837
decrease of the tilt angle, from 18° at t=29 min down to 15° at t= 71 min, 10° at t=156 min, and 6° at t=174 min. The tilt angle then vanishes at t=186 min (Figure 10e). The BA molecules appear thus untilted and organized on a distorted hexagonal lattice of parameters a=0.457 nm and b=0.870 nm. These cell parameters have slightly different values than those measured at pH 7.5; consequently, this phase will be called X0 1. The two diffraction peaks then continuously move away from each other until the X phase is reached. The evolution with time of the rectangular cell is summarized in Figure 8b. In the same way as at pH 7.5, the evolution from the L2h phase to the X0 1 phase is mainly marked by the a cell parameter decrease. Then the final evolution toward the X phase involves both a decrease of the a cell parameter and an increase of the b cell parameter. c. Discussion. The main difference in the kinetic behavior evidenced at the two studied pH is the order of the phase transition occurring from the initial L2h phase, first-order at pH 7.5 and second order at pH 5.5. It means that the initial ionization state of the fatty acid headgroups plays a significant role. Indeed, in the absence of divalent cations, PM-IRRAS experiments performed on arachidic acid monolayers have shown that the carboxylic group is probably not ionized at pH 5.5 while it is completely dissociated above pH 10.19 The gradual dissociation of the headgroup with increasing pH also manifests itself on the apparent molecular areas as measured by means of isotherms or GIXD experiments.21,22 In the presence of divalent cations, PMIRRAS experiments indicate that the balance between the acidic and salty species at the interface is obtained for a subphase pH value which is different from the fatty acid pKa and dependent on the divalent cation.15 Thus, at low pH values, the ionization state of the headgroups changes from undissociated to dissociated in the course of ion adsorption.15,23,24 As a result, at pH 5.5, a large amount of ions is necessary to bring the fatty acid monolayer into the X phase, as shown by the ion concentration threshold higher by a factor of 20 than at pH 7.5. At pH 5.5, at the beginning of the kinetic process, all the fatty acid headgroups of the monolayer are in the same undissociated state. During ion adsorption, the changes in headgroup dissociation state occur thus simultaneously in the (22) Ariga, K.; Nakanishi, T.; Hill, J. P.; Shirai, M.; Okuno, M.; Abe, T.; Kikuchi, J.-I. J. Am. Chem. Soc. 2005, 127, 12074. (23) Bloch, J. M.; Yun, W. Phys. Rev. A 1990, 41, 844. (24) Lochhead, M. J.; Letellier, S. R.; Vogel, V. J. Phys. Chem. B 1997, 101, 10821.
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Figure 10. X-ray diffraction data obtained for a BA monolayer over a CuCl2 solution 50 Cu2þ ions/BA molecule at Π=0 mN/m and pH 5.5. (a-e) Scans measured at (a) t = 29 min, (b) t = 71 min, (c) t = 156 min, (d) t = 174 min, and (e) t = 186 min. On the left: scan of intensity as a function of Qxy integrated over all Qz values. On the right: (11)/(11) rod scan of intensity as a function of Qz. (f) Evolution with time of the L2h phase rectangular cell parameters a and b (open symbols) and of the tilt angle (solid symbols). 836 DOI: 10.1021/la9022823
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whole monolayer. Consequently, the transition toward the X0 1 phase can occur continuously. In contrast, at pH 7.5, the monolayer is initially made of patches of dissociated and undissociated headgroups. There is thus a strong electrostatic attraction of the ions toward the dissociated headgroups. During ion adsorption, the proportion of undissociated headgroups decreases at the expense of the ionized one. This could explain that the transition occurs according to a discontinuous process, i.e., first order. The evolution toward the X phase appears drastically different than the kinetics previously studied for the Mg2þ and Cd2þ cations that lead to superstructure formation as final structure. Indeed, a new intermediate weakly ordered I structure was detected during the adsorption of Mg2þ and Cd2þ while in presence of Cu2þ ions, a direct phase transition from the L2h phase toward an X-like phase is observed.
IV. Conclusion The structure of BA Langmuir monolayers was investigated during Cu2þ adsorption, at two subphase pHs (5.5 and 7.5).
Langmuir 2010, 26(2), 830–837
Article
The X phase is formed immediately at room temperature above a concentration threshold depending on the pH. Its structure is pH-independent. BAM observations indicate the high rigidity of this phase. Below the threshold, the observed kinetic process differs according to the pH. The L2h phase evolves toward the X phase through an X-like phase, called X1 at pH 7.5 and X0 1 at pH 5.5. The X1 and X0 1 phase’s structures are close to each other, with a unit cell distortion less pronounced than that of the X phase. The main difference depending on the pH is the order of the phase transition toward the X-like phase; indeed, the transition is first-order at pH 7.5 while it is second-order at pH 5.5. This could be related to the initial ionization state of the carboxylic headgroups. The second phase transition, leading to the X phase, is a continuous evolution. These results indicate that kinetic studies are as important as the characterization of the final organic-inorganic formed structure to improve the understanding of headgroup-cation interactions.
DOI: 10.1021/la9022823
837
