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Notes Scanning Force Microscopy Characterization of an Elaidic Acid Monolayer Prepared on a Terbium-Containing Subphase Ve´ronique De´rue, Ste´phane Alexandre,* and Jean-Marc Valleton URA 500 CNRS, UFR des Sciences, Universite´ de ROUEN, 76821 Mont-Saint-Aignan, France Received January 10, 1996. In Final Form: May 7, 1996
Introduction Langmuir-Blodgett (LB) films have been studied for two decades because of their potential applications in the domains of molecular electronics, nonlinear optics, and sensors.1,2 However LB films are characterized by low thermal and mechanical stabilities; this is a major drawback for many applications. The polymerization of LB films3 and the elaboration of the Langmuir films on a counterions-containing subphase are possible ways to improve the stability. When fatty acids are spread on an aqueous solution, protons can dissociate from the acid groups. This dissociation depends on the individual acid pKa value. If counterions are present in the subphase, the pKa of the acid depends on the counterion used. These counterions lead to competitive adsorption or binding. Studies have been reported on the effect of pH and ion concentrations on the monolayer structure.4-7 It has been found that the metal ion-carboxylate interaction depends on the physical and chemical properties of the ions. A great interest has been shown for the interactions of Langmuir-Blodgett films with ions dissolved in the subphase, since these films containing metal ions may have extraordinary electric and magnetic properties.8-10 In order to show the organization of fatty acid molecules in the presence of ions, the transferred films may be studied by scanning force microscopy (SFM), which is shown to be a privileged tool.11 In this paper, we are interested in the study of the interaction between an elaidic acid monolayer and Tb3+ ions. From the literature, it is well-known that these ions enhance the stability of the elaidic acid monolayer and condense it.12 The surface topography of 13 elaidic acid layers deposited from a TbCl3 subphase was studied by SFM, and dimensions corresponding approximately to a (1) Barraud, A. Thin Solid Films 1983, 99, 317 (2) Barraud, A. J. Chim. Phys. 1985, 82, 683. (3) Miyashita, T. Prog. Polym. Sci. 1993, 18, 263-294. (4) Linden, D. J. M.; Peltonen, J. P. K.; Rosenholm, J. B. Langmuir 1994, 10, 1592-1595. (5) Linden, D. M. J.; Peltonen, J. P. K.; Fagerholm, H.; Gyo¨rvary, E.; Eriksson, F. Thin Solid Films 1994, 242, 88-91. (6) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. J. Chem. Phys. 1992, 96, 1556-1559. (7) Bettarini, S.; Bouasi, F.; Gabrielli, G.; Martini, G.; Puggelli, M. Thin Solid Films 1992, 210/211, 42-45. (8) Roberts, G. Langmuir-Blodgett Films; Plenum: New York, 1990. (9) Dong, J. A.; Franses, E. L. J. Chem. Phys. 1991, 95, 8486-8493. (10) Schwartz, D. K.; Viswanathan, R.; Garnaes, J.; Zasadzinski, J. A. J. Am. Chem. Soc. 1993, 115, 7374-7380. (11) Frommer, J. Scanning Microscopy and Atomic Force Microscopy in Organic Chemistry. Angew. Chem., Int. Ed. Engl. 1992, 31,12981328. (12) Peltonen, J. P. K.; He, P.; Rosenholm, J. B. Langmuir 1993, 9, 2363-2369.
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pair of elaidic acid molecules were observed.12 Here, we elaborated a monolayer of elaidic acid on a Tb3+-containing subphase. Then the monolayer was transferred on solid substrates for SFM and IR spectroscopy studies. For the first time, the complex between Tb3+ ions and elaidic acid molecules was observed. Materials and Methods Materials. The fatty acid, trans-9-octadecenoic (elaidic) acid (99%) was obtained from Sigma and used without further purification. Terbium chloride (TbCl3) (99.9%) was obtained from Aldrich and used as the subphase salt. Monolayer Formation and LB Film Deposition. The experiments were carried out at 22 °C using a LangmuirBlodgett trough (Atemeta, Paris, France). The dimensions of the through are 50 cm × 6.5 cm and the volume of the subphase is 250 cm3. This system uses a mobile barrier for compressing the fatty acid molecules and a Wilhelmy balance for measuring the interfacial pressure. The subphase was a TbCl3 solution with a concentration of 0.5 × 10-3 M prepared with water obtained from a MilliQ system from Millipore involving deionization, reverse osmosis, and filtration. The pH of the subphase was 5.2. An amount of 0.1 mL of an elaidic acid solution (10-4 M on chloroform) was spread with a micropipet (Nichyiro, 100 mL) and after 15 min the elaidic acid molecules were compressed to 10 mN/m. After the compression, the film was transferred at a pressure of 10 mN/m using the vertical method. The transfers were performed on muscovite for a study by SFM and on ZnS slides for FTIR spectroscopic analysis. Muscovite samples (12 mm × 12 mm) were obtained from Metafix (Montdidier, France). Muscovite was freshly cleaved before any transfer and used without any particular processing. Samples were prepared by both downward and upward strokes of muscovite slides through the interface. Muscovite slides were initially in the air and the downward stroke was faster than the upward stroke (1 cm/min). Since muscovite is hydrophilic, a single layer was deposited during the upward stroke. The feedback loop (control of the mobile barrier by the signal produced by the Whilhelmy balance) was kept closed in order to operate at a constant interfacial pressure. Samples obtained were dried under vacuum. ZnS slides were obtained from Sorem (Pau, France). These slides (35 mm × 9 mm × 2 mm) were cleaned with chloroform in an ultrasonic bath and then dried for several hours under vacuum. On these hydrophilic slides, 15 layers were transferred as a Y-type deposition. Scanning Force Microscopy. Scanning force microscopy (SFM) experiments were performed with a Nanoscope II from Digital Instruments (Santa Barbara, CA), in the contact mode, with a 1 µm scanner. Measurements were achieved in the air. The cantilevers used were characterized by a low spring constant of about 0.06 N/m. All the measurements were performed with the feedback loop on (constant force ) 10-9 to 10-8 N). FTIR Spectroscopy. The transmission infrared spectra of transferred films were measured with a Nicolet 510M FTIR spectrophotometer. The IR spectra were obtained by collecting and averaging out 200 scans at a resolution of 4 cm-1. Each ZnS slide has its reference spectrum. The final film spectra were obtained by subtraction of the reference spectra from the sample spectra. On a pure water subphase, a single layer of elaidic acid could be deposited. The signal of one layer being not sufficient for reliable IR spectroscopy measurements, the pure elaidic acid spectrum was obtained after drying a droplet of the elaidic acid solution in chloroform deposited on a ZnS slide.
Results and Discussion The pressure/area isotherms of elaidic acid for a cationfree subphase and for a 0.5 mM Tb3+-containing subphase have been studied (Figure 1). The isotherm for the cation© 1996 American Chemical Society
Notes
Figure 1. Pressure/area isotherms of elaidic acid on a 0.5 mM TbCl3-containing subphase (s) and on a pure water subphase (- - -).
Figure 2. Infrared sprectra: (a) elaidic acid monolayer prepared on a TbCl3-containing subphase and transferred at a surface pressure of 10 mN/m on a ZnS slide; (b) elaidic acid droplet dried on a ZnS slide.
free subphase is in agreement with the data from the literature.13 For the Tb3+-containing subphase, a strong condensing effect of the elaidic acid monolayer is observed. This condensing effect has already been reported for elaidic acid monolayers with Tb3+- (ref 12) and also with Pb2+(ref 14) containing subphases while only a weak condensing effect was observed with Cd2+.12 This condensation of elaidic acid corresponds to an increase in the order of the alkyl chains of elaidic acid. On a pure water subphase, the film of elaidic acid is in a liquid expanded state (the molecular area is 37 ( 1 Å2 at 10 mN/m) while, on the Tb3+-containing subphase, the film of elaidic acid is in a solid state (the molecular area is 21 ( 1 Å2 at 10 mN/m). The IR spectrum of pure elaidic acid (absence of TbCl3) (Figure 2a) shows a peak due to the CdO stretching vibration (1712 cm-1) corresponding to a monoprotonated carboxyl group (COOH). In the presence of Tb3+, the peak is no longer located at 1712 cm-1 but at 1538 cm-1 (Figure 2b) and corresponds to the CdO stretching vibration of a carboxylate group (COO-). Therefore this indicates that (13) Mingotaud, A. F.; Mingotaud, C.; Patterson, L. K. Handbook of Monolayers; CRC Press and Academic Press: Boca Raton, FL, and San Diego, CA, 1993. (14) Gericke, A.; Hu¨hnerfuss, H. Langmuir 1995, 11, 225-230.
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Figure 3. Scanning force microscopy image (15 nm × 15 nm) of one layer of elaidic acid deposited on muscovite from a TbCl3containing subphase at a surface pressure of 10 mN/m. A filtered image (selection of the main frequencies on the Fourier transform in Figure 4) emphasizing the structure of the complex is in the left inset.
Figure 4. 2D Fourier transform of the image in Figure 3 and schematic diagram of the unit cell. The Miller’s indices (h,k) are shown on the Fourier transform.
the presence of Tb3+ ions leads to a complete dissociation of the carboxyl groups. The absence of a broad band at 3100-3400 cm-1 characteristic for hydroxyl groups shows that the Tb3+ ions are not hydroxylized. Therefore these ions interact with the dissociated elaidic acid molecules. A sample prepared by the transfer of an elaidic acid layer on muscovite has been imaged by SFM. At the nanometer scale (Figure 3), the sample exhibits structures in the form of clover leaves ordered along a definite axis. Each structure can be assumed to correspond to a complex formed between one Tb3+ ion and three elaidic acid molecules. A filtered image obtained after selecting the main characteristic frequencies on the Fourier transform (left in Figure 4) emphasizes the structure of the complex (left inset in Figure 3). Since the odd values of h + k have zero intensity on the Fourier transform, the unit cell (right in figure 4) is a perfect centered rectangular lattice. The unit cell dimensions are 0.90 ( 0.02 nm and 1.41 ( 0.01 nm. From this unit cell, we calculated the area of the complex, and we found an area of 63 ( 2 Å2. The complex being constituted of three elaidic acid molecules, the area for one elaidic acid molecule is 21.0 ( 0.7 Å2. This area is in agreement with the area deduced from the pressure/area isotherm of elaidic acid (Figure 1). Therefore these results confirm that the clover leaf structure corresponds to a complex between three elaidic acid molecules and one Tb3+ ion.
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At a different time, less resolved images with the same lattice were obtained (not shown). However, on these images the three elaidic acid molecules which constitute the complex are not resolved. This difference of resolution between the images may be due to a change in the interactions between the tip and the sample. Similar less resolved images have already been published: an elaidic acid LB film prepared in the presence of TbCl312 and a stearic acid LB film prepared in the presence of TbCl35 have been imaged by SFM. In both cases, the structure of the complex was not resolved. The image of Figure 3 exhibiting the structure of the complex is, to our knowledge, the first image in which the detailed structure of a multivalent ion/fatty acid complex
Notes
has been observed. It allows a better understanding of the images previously obtained and establishes clearly the stoichiometry of the complex formed between Tb3+ ion and three elaidic acid molecules, which was also confirmed by IR results. Acknowledgment. V.D. acknowledges the Ministe`re de la Recherche et de l’Enseignement Supe´rieur for its financial support. We thank Prof. J. F. Verche`re for helpful discussions concerning the formation of terbium complexes. LA9600311
