In Situ Studies of Metal Coordinations and ... - ACS Publications

Huijin Liu, Haifu Zheng, Wangen Miao and Xuezhong Du*. MOE Key Laboratory of Mesoscopic Chemistry, State Key Laboratory of Coordination Chemistry, and...
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Langmuir 2009, 25, 2941-2948

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In Situ Studies of Metal Coordinations and Molecular Orientations in Monolayers of Amino-Acid-Derived Schiff Bases at the Air-Water Interface Huijin Liu, Haifu Zheng, Wangen Miao, and Xuezhong Du* MOE Key Laboratory of Mesoscopic Chemistry, State Key Laboratory of Coordination Chemistry, and School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed December 2, 2008. ReVised Manuscript ReceiVed January 7, 2009 The surface behaviors of monolayers of amino-acid-derived Schiff bases, namely, 4-(4-(hexadecyloxy)benzylideneamino)benzoic acid (HBA), at the air-water interface on pure water and ion-containing subphases (Cu2+, Ca2+, and Ba2+) have been clarified by a combination of surface pressure-area isotherms and surface plasmon resonance (SPR) technique, and the metal coordinations and molecular orientations in the monolayers have been investigated using in situ infrared reflection absorption spectroscopy (IRRAS). The presence of metal ions gives rise to condensation of the monolayers (Cu2+, pH 6.1; Ca2+, pH 11; Ba2+, pH 10), even leading to the formation of three-dimensional structures of the compressed monolayer in the case of Ba2+ (pH 12). The metal coordinations with the carboxyl groups at the interface depend on the type of metal ions and pH of the aqueous subphase. The orientations of the aromatic Schiff base segments with surface pressure are elaborately described. The spectral behaviors of the Schiff base segments with incidence angle in the case of Ba2+ (pH 12) have so far presented an excellent example for the selection rule of IRRAS at the air-water interface for p-polarization with vibrational transition moments perpendicular to the water surface. The chain orientations in the monolayers are quantitatively determined on the assumption that the thicknesses of the HBA monolayers at the air-water interface are composed of the sublayers of alkyl chains and Schiff base segments.

Introduction Schiff bases are one of the most important classes of organic compounds and have extensive applications in biological functional materials,1-3 polymer ultraviolet stabilizers,4,5 laser dyes,6 and molecular switches in logic and memory circuits,7 however, their applications are restricted due to their poor stability. Langmuir monolayer and Langmuir-Blodgett (LB) film techniques are powerful means to control molecular orientation and packing at the molecular level necessary for molecular devices and sensors.8,9 It is shown that their stability and properties are greatly increased after the introduction of Schiff base units into the Langmuir monolayers at the air-water interface because of the suppression of hydrolysis.10 To date, LB films of Schiff base units incorporated in different kinds of amphiphiles have been investigated.10-17 Amino-acid-derived Schiff bases are a kind * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 86-25-83317761. (1) Rousso, L.; Friedman, N.; Sheves, M.; Ottolenghi, M. Biochemistry 1995, 34, 12059. (2) Bassov, T.; Sheves, M. Biochemistry 1986, 25, 5249. (3) Lanyi, J. K. Biochim. Biophys. Acta 1993, 1183, 241. (4) Das, K.; Sarkar, N.; Ghosh, A. K.; Majumdar, D.; Nath, D. N.; Bhattacharya, K. J. Phys. Chem. 1994, 98, 9126. (5) Fore´s, M.; Duran, M.; Sola`, M.; Orozco, M.; Luque, F. J. J. Phys. Chem. A 1999, 103, 4525. (6) Acun˜a, A. U.; Amat-Guerri, F.; Costela, A.; Douhal, A.; Figuera, J. M.; Florido, F.; Sastre, R. Chem. Phys. Lett. 1991, 187, 98. (7) Nishiya, T.; Yamauchi, S.; Hirota, N.; Baba, M.; Hanazaki, I. J. Phys. Chem. 1986, 90, 5730. (8) Ulman,A.AnIntroductiontoUltrathinOrganicFilmsfromLangmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (9) Leblanc, R. M. Curr. Opin. Chem. Biol. 2006, 10, 529. (10) Nagel, J.; Oertel, U.; Friedel, P.; Komber, H.; Mo¨bius, D. Langmuir 1997, 13, 4693. (11) Kawamura, S.; Tsutsui, T.; Saito, S.; Murao, Y.; Kina, K. J. Am. Chem. Soc. 1988, 110, 509. (12) Chen, X.; Xue, Q.-B.; Yang, K.-Z.; Zhang, Q.-Z. Langmuir 1995, 11, 4082.

of “push-pull” molecules with electron-donating and attracting groups and exhibit a high quadratic hyperpolarizability, which have been the focus of nonlinear optical (NLO) studies for potential applications.18 Langmuir monolayers and LB films of salicylideneamino benzoic acid amphiphiles, namely, 4-(4dodecyloxy-2-hydroxybenzylideneamino)benzoic acid (DSA), have been studied in detail recently.19,20 The amphiphiles contain two moieties available for metal ions to coordinate with. In the presence of metal ions (Ca2+, Co2+, Zn2+, and Cu2+) in the subphases, the monolayers were expanded, and the long axes of the aromatic Schiff base segments were inclined with respect to the monolayer normal depending on metal ion at pH 5.3-8.2.20 Cu2+ ions coordinated not only to the carboxylate groups but also to the salicylideneamino moieties, while Ca2+, Co2+, and Zn2+ ions coordinated only to the carboxylate groups.19 The method for quantitative determination of chain orientation in the monolayers at the air-water interface using infrared reflection absorption spectroscopy (IRRAS) has been developed on the assumption that the monolayer thickness was composed of the sublayers of alkyl chains and Schiff base segments.20 IRRAS has been a leading structural method for the in situ characterization of the monolayers at the air-water interface.21-33 The IRRAS (13) Liu, M.; Xu, G.; Liu, Y.; Chen, Q. Langmuir 2001, 17, 427. (14) Dhathathreyan, A.; Mary, N. L.; Radhakrishnan, G.; Collins, S. J. Macromolecules 1996, 29, 1827. (15) Jiao, T.; Liu, M. Langmuir 2006, 22, 5005. (16) Jiao, T.; Zhang, G.; Liu, M. J. Phys. Chem. B 2007, 111, 3090. (17) Hindo, S. S.; Shakya, R.; Rannulu, N. S.; Allard, M. M.; Heeg, M. J.; Radgers, M. T.; da Rocha, S. R. P.; Verani, C. N. Inorg. Chem. 2008, 47, 3119. (18) Sliwa, M.; Le´tard, S.; Malfant, I.; Nierlich, M.; Lacroix, P. G.; Asahi, T.; Masuhara, H.; Yu, P.; Nakatani, K. Chem. Mater. 2005, 17, 4727. (19) Liu, H.; Du, X.; Li, Y. J. Phys. Chem. C 2007, 111, 17025. (20) Liu, H.; Miao, W.; Du, X. Langmuir 2007, 23, 11034. (21) Mendelsohn, R.; Braunder, J. W.; Gericke, A. Annu. ReV. Phys. Chem. 1995, 46, 305. (22) Gericke, A.; Michailov, V.; Hu¨hnerfuss, H. Vib. Spectrosc. 1993, 4, 335.

10.1021/la803976c CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

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technique can provide abundant information on chain conformation, headgroup structure, and molecular orientation.

In this paper, the monolayers from the counterpart of DSA, 4-(4-(hexadecyloxy)benzylideneamino)benzoic acid (HBA), at the air-water interface have been investigated using the in situ IRRAS technique together with surface pressure-area (π-A) isotherms and surface plasmon resonance (SPR) technique. The SPR technique can directly detect change in surface refractive index only to a depth of about 200 nm from the sensor surface with submonolayer sensitivity in real time, and the obtained surface refractive index can be converted into film thickness.34 The presence and absence of a hydroxyl group (DSA and HBA) results in an obvious difference in the deprotonation of carboxyl groups. On one hand, the deprotonation ability of the carboxyl groups in HBA is weak in comparison with DSA; on the other hand, the coordination ability of metal ions is varied. Most stable complexes are formed for Cu2+ ions, irrespective of the nature of coordination ligand or the number of ligand molecules involved.35,36 The carboxyl groups in the HBA monolayers may interact with metal ions in the subphases at pH higher than 10 (Ca2+ and Ba2+) except for Cu2+ (pH 6.1), while the metal coordinations occur at intrinsic and neutral pH in the case of DSA.19 The HBA monolayers are condensed in the presence of metal ions in the subphases, in contrast to the expansion of the DSA monolayers.19 In the case of Ba2+ (pH 12), the observed spectral behaviors present an excellent example for the selection rule of IRRAS at the air-water interface for p-polarization with vibrational transition moments perpendicular to the water surface, which have not been clearly observed before. Furthermore, the chain orientations at a surface pressure of 20 mN/m are quantitatively determined on the basis of the method developed recently.20

Experimental Section Materials. The compound HBA was synthesized according to a procedure similar to that reported recently.191H NMR (500 MHz, DMSO-d6): 8.5 (s, 1H, -CHdN); 7.05-8.0 (m, 8H, phenyl); 4.05 (t, 2H, -OCH2-); 1.2-2.1 (m, 28H, -(CH2)14-); 0.85 (t, 3H, -CH3). Octadecanethiol (ODT, >95% GC) was purchased from Fluka. The other chemical reagents used were of analytical grade, and water used was double-distilled (pH 5.6; resistivity, 18.2 MΩ cm) after a deionized exchange. The pH values of metal ion-containing solutions (1 mmol/L) were adjusted with or without NaOH. (23) Gericke, A.; Hu¨hnerfuss, H. J. Phys. Chem. 1993, 97, 12899. (24) Gericke, A.; Hu¨hnerfuss, H. Langmuir 1995, 11, 225. (25) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027. (26) Flach, C. R.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. B 1997, 101, 58. (27) Gericke, A.; Flach, C. R.; Mendelsohn, R. Biophys. J. 1997, 73, 492. (28) Bi, X.; Taneva, S.; Keough, K. M. W.; Mendelsohn, R.; Flach, C. R. Biochemistry 2001, 40, 13659. (29) Ren, Y.; Kato, T. Langmuir 2002, 18, 6699. (30) Du, X.; Miao, W.; Liang, Y. J. Phys. Chem. B 2005, 109, 7428. (31) Wang, Y.; Du, X.; Guo, L.; Liu, H. J. Chem. Phys. 2006, 124, 134706. (32) Dyck, M.; Kerth, A.; Blume, A.; Lo¨sche, M. J. Phys. Chem. B 2006, 110, 22152. (33) Zheng, J.; Leblanc, R. M. Infrared reflection absorption spectroscopy of monolayers at the air-water interface. In AdVanced Chemistry of Monolayers at Interfaces; Imae, T., Ed.; Elesevier B. V.: Amsterdam, 2007; Chapter 10. (34) Jung, L. S.; Campbell, C. T.; Chinowsky, T. M.; Mar, M. N.; Yee, S. S. Langmuir 1998, 14, 5636. (35) Irving, H.; Williams, R. J. P. J. Chem. Soc. 1953, 3192. (36) Hemakanthi, G.; Dhathathreyan, A.; Mo¨bius, D. Colloids Surf. A 2002, 198-200, 443.

Liu et al. Isotherm Measurements. The π-A isotherms were recorded on a Nima 611 Langmuir trough (Nima, England). A Wilhelmy plate (filter paper) was used as the surface pressure sensor and situated in the middle of the trough. Two mobile barriers compressed or expanded symmetrically at a rate of 0.022 nm2/molecule/min. Monolayers were obtained by spreading the chloroform solution of HBA on pure water and ion-containing subphases at 22 °C. SPR Measurements. The commercial Spreeta sensors (Texas Instruments) were used for the SPR measurements.37,38 These devices combine the sensor surface with all the optic and electronic components required for the SPR experiments in a compact and lightweight assembly39 with a p-polarized near-infrared light of 840 nm as the light source. The SPR sensor was first cleaned using an aqueous solution of 1% Triton X-100 and 0.1 M NaOH followed by double-distilled water. Its sensing gold surface was made hydrophobic by immersion in a 2 mM ODT solution in absolute ethanol for 20 min followed by rinsing with absolute ethanol and double-distilled water, respectively. The SPR sensor was then dried and positioned above the monolayer-free air-water interface. The SPR sensor was initialized in air and calibrated in double-distilled water, and a SPR baseline was obtained in water. A monolayer of HBA was spread until the desired surface pressure was reached, and then it was allowed for relaxation for 30 min. The ODT-modified SPR sensor mounted in a holder was slowly lowered into contact with the monolayer and brought to a depth of ca. 2 mm. This procedure was equivalent to a horizontal monolayer transfer to a solid substrate but without ever removing the substrate (i.e., the sensor) carrying the immobilized monolayer from the trough.37,38 Upon contact of the SPR sensor with the monolayer, a step increase of the SPR signal was recorded, and a new SPR baseline was established for a period of time to ensure the integrity of the immobilized monolayer. IRRAS Measurements. IRRAS spectra of the monolayers at the air-water interface were recorded on an Equinox 55 FTIR spectrometer (Bruker, Germany) connected to an XA-511 external reflection attachment with a shuttle trough system suitable for the air-water interface experiments. Sample (film-covered surface) and reference (film-free surface) troughs were fixed on a shuttle device driven by a computer-controlled stepper motor for allowing collection from the two troughs in an alternating fashion. A KRS-5 polarizer was used to generate a perpendicularly polarized IR beam. The incident IR beam was conducted out of the spectrometer through the KRS-5 polarizer and focused onto the air-water interface. The reflectedIRbeamwenttoaliquid-nitrogen-cooledmercury-cadmiumtelluride (MCT) detector. These experiments were carried out at 22 °C. The film-forming molecules were spread from the chloroform solutions of desired volumes, and 20 min was allowed for solvent evaporation. The measurement system was then enclosed for humidity equilibrium and monolayer relaxation for 4 h prior to compression. The monolayers were then compressed discontinuously from the initial surface pressure of ∼0 mN/m. After 30 min of relaxation, the two barriers were stopped, and the monolayer areas were kept constant. The spectra were recorded with a resolution of 8 cm-1 by coaddition of 1024 scans. A time delay of 30 s was allowed for film equilibrium between trough movement and data collection. Spectra were acquired using a p-polarized radiation followed by data collection using an s-polarized one. The angles of incidence measured by the instrument are corrected to increase by 1.0° in comparison with the preset angles of incidence.

Results and Discussion Surface Behaviors of the Monolayers. Figure 1 shows π-A isotherms of the monolayers of HBA on pure water and ioncontaining subphases, respectively. On pure water, the monolayer exhibits some expansion below 25 mN/m followed by a plateau region around 25-28 mN/m, and then an obvious rise in surface pressure occurs. The limiting areas below and above the plateau (37) Du, X.; Hlady, V.; Britt, D. Biosens. Bioelectron. 2005, 20, 2053. (38) Wang, Y.; Du, X. Langmuir 2006, 22, 6195. (39) Weimar, T. Angew. Chem., Int. Ed. 2000, 39, 1219.

BehaVior of HBA Monolayers at the Air-Water Interface

Figure 1. π-A isotherms of the HBA monolayers on pure water and ion-containing subphases, respectively: compression rate, 0.022 nm2/ molecule/min; 22 °C.

are estimated from the π-A curve by extrapolating the corresponding linear regions to 0 mN/m to be 0.306 and 0.205 nm2, respectively. The crystal structure of 4-(benzylideneamino)benzoic acid was early determined by X-ray diffraction.40 The N-phenyl plane shows a twist angle of 41.1° relative to the azomethine (-CHdN-) plane, and the C-phenyl plane displays a twist angle of 13.7°.40 That means that the aromatic Schiff base segments take a nonplanar conformation in the crystalline state. The crystal structure of methyl 4-(2,4-dihydroxybenzylideneamino)benzoate was recently determined by X-ray diffraction with the typical distortion of anils: the mean planes of the sixmembered rings make an angle of 47.69°.18 The density of the compound is 1.446 g/cm3, which is among the three highest values known for anils without heavy atoms,18 and the crosssectional area of the close-packed Schiff bases is estimated to be about 0.214 nm2 along the short axis from the crystal structure. Obviously, the cross-sectional area of 4-(benzylideneamino)benzoic acid along the short axis should be very close to the value for methyl 4-(2,4-dihydroxybenzylideneamino)benzoate. The limiting area of 0.205 nm2 above the plateau region is close to the cross-sectional area of a hydrocarbon chain and smaller than that of the close-packed Schiff base. It is most likely that the plateau region is due to the monolayer collapse. The similar cases were reported in the literature for the occurrence of the film collapses ranging from 5 to 42 mN/m.41-44 The presence of metal ions in the subphases gives rise to condensation of the monolayers to different degrees. In the case of Cu2+ (pH 6.1), the monolayer exhibits a slight condensation with a limiting area of 0.301 nm2 in comparison with that on pure water. This is different from the DSA monolayer, in which a significant expansion occurs in the presence of Cu2+.19 In the case of Ca2+ (pH 11), a liquid-condensed monolayer is formed with a limiting area of 0.242 nm2; however, the monolayer gradually collapses above 35 mN/m. The presence of Ba2+ (pH 10) is similar to the case of Ca2+ (pH 11), with a limiting area of 0.249 nm2 except for a collapse pressure of about 50 mN/m. It is obvious that the long axes of the Schiff base segments are oriented almost perpendicular to the water surface in the two cases. Upon further (40) Bu¨rgi, H. B.; Dunitz, J. D. HelV. Chim. Acta 1970, 53, 1747. (41) Jiao, T.; Liu, M. J. Phys. Chem. B 2005, 109, 2532. (42) Huang, X.; Jiang, S.; Liu, M. J. Phys. Chem. B 2005, 109, 114. (43) Hühnerfuss, H.; Neumann, V.; Stine, K. J. Langmuir 1996, 12, 2561. (44) Nuckolls, C.; Katz, T. J.; Verbiest, T.; Elshocht, S. V.; Kuball, H.-G.; Kiesewalter, S.; Lovinger, A. J.; Persoons, A. J. Am. Chem. Soc. 1998, 120, 8656.

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Figure 2. Changes in SPR angle of the HBA monolayers on pure water and the aqueous Ba2+ subphase (pH 12).

increase in pH of aqueous Ba2+ solution, the isotherm displays extremely compressed characteristics with a limiting area of 0.160 nm2, much smaller than the cross-sectional areas of the closepacked benzylideneaniline nucleus (0.214 nm2) and saturated hydrocarbon chains (about 0.20 nm2), which suggests the formation of three-dimensional structures of the compressed monolayer. To clarify the surface behaviors of the monolayers on pure water and the Ba2+-containing subphase (pH 12), the corresponding film thicknesses are estimated. Figure 2 shows the changes of SPR angles for the monolayers on pure water and the aqueous Ba2+ subphase (pH 12). According to the method developed by Jung et al.,34 the film thickness can be estimated from the following equation:

∆R ) m(ηa - ηs)[1 - exp(-2da ⁄ Id)] exp(-2db ⁄ Id) where ∆R is defined as the SPR response only to the adsorbate a (HBA monolayers herein) after adsorbate b (premodified ODT monolayer) is already present, m is the slope of the calibration plot (SPR angle versus refractive index of ethanol/water solutions, 138° per RIU), ηa is bulk refractive index of adsorbed materials (ηa ) 1.5), ηs is refractive index of bulk liquid solution, da and db are the thicknesses of adsorbates a and b, respectively, and Id is a characteristic decay length indicating that the evanescent electromagnetic field decays away exponentially into the medium. Id is a key parameter in the calculation and can be accurately estimated from the Maxwell’s equations using the complex dielectric constant of the metal (εmetal) at the wavelength45 (λ ) 840 nm) and the effective refractive index of the sample (ηeff) in question.34,46 The latter is measured experimentally using the SPR technique. In fact, Id, a value of ca. 380 nm, varies only weakly with ηeff. The film thicknesses are estimated to be 2.32 and 4.70 nm on pure water at 20 and 30 mN/m, respectively. Note that the estimated thickness is the average effective thickness of pure substance.34 The film thickness of 4.70 nm at 30 mN/m is larger than the extended length of the HBA molecule of 3.19 nm. It is clear that the monolayers can be formed on pure water below 25 mN/m and start to collapse above 25 mN/m. This demonstrates that the plateau region in the isotherm in the vicinity of 25 mN/m is due to the collapse of the monolayer but not due to the phase transition of the monolayer. The film thickness on the Ba2+-containing subphase (pH 12) is estimated to be 9.17 (45) Innes, R. A.; Sambles, J. R. J. Phys. F: Met. Phys. 1987, 17, 277. (46) Johnston, K. S.; Karlsen, S. R.; Jung, C. C.; Yee, S. S. Mater. Chem. Phys. 1995, 42, 242.

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nm. It is obvious that the three-dimensional structures of the compressed monolayer are formed in this case. Metal Coordinations in the Monolayers. IRRAS data are defined as plots of reflectance-absorbance (RA) versus wavenumber. RA is defined as -log(R/R0), where R and R0 are the reflectivities of the film-covered and film-free surfaces, respectively. The different selection rules of the IRRAS for the monolayers at the air-water interface are first presented.21,27,29,47 For the s-polarized radiation, the electric field vector is perpendicular to the plane of incidence, i.e., parallel to the water surface. The bands are always negative and their intensities decrease with increasing angle of incidence. The electric field vector in the p-polarized radiation is parallel to the plane of incidence. For the vibrations with their transition moments parallel to the water surface, the bands are initially negative and their intensities increase with increasing angle of incidence and reach a maximum, then a minimum in the reflectivity is found at the Brewster angle.7 Above the Brewster angle the bands become positive and their intensities decrease upon further increase of incidence angle.7 For the vibrations with their transition moments perpendicular to the water surface, the bands are positive first and then become negative above the Brewster angle. If the vibrational transition moments are tilted to the air-water interface, the intensities of the bands are weak and even zero.47 Figures 3a,b show p-polarized IRRAS spectra of the HBA monolayers on pure water below the Brewster angle at various surface pressures in different regions, respectively. The two bands at 2922 and 2853 cm-1 in the vicinity of about 0 mN/m are due to the antisymmetric and symmetric CH2 stretching vibrations [νa(CH2) and νs(CH2)] of hydrocarbon chains, respectively. It is known that the νa(CH2) and νs(CH2) frequencies are sensitive to the conformation order of alkyl chains. Lower wavenumbers are characteristic of preferential all-trans conformations in highly ordered chains, while the number of gauche conformers increases with frequency and width of the bands. In this case, the alkyl chains are clearly populated with the gauche conformations. A band at 1680 cm-1 is assigned to the CdO stretching vibration [ν(CdO)] of the carboxyl groups.48,49 The bands at 1602, 1576, and 1512 cm-1 can be attributed to the skeleton stretching vibration of the phenyl rings [ν(CdC)].48-53 The spectral baseline is distorted in the regions between 3800-3000 cm-1 and between 1800-1500 cm-1 due to the altered structure of the water adjacent to the headgroups of the film constituents, so that no ν(CdN) band can be clearly identified. The band at 1469 cm-1 is due to the CH2 scissoring mode [δ(CH2)], indicative of a hexagonal subcell structure of hydrocarbon chains,54 mixed with the ν(CdC) vibration. A strong band at 1259 cm-1 is assigned to the dC-O stretching vibration.49-51 It is difficult to observe the coupled band due to the C-O stretching and OH deformation modes of the carboxyl groups in the monolayer at the air-water interface (around 1300 cm-1 for the LB films of octadecanoic acid55). The peak at 1163 cm-1 is due to the dC-H in-plane bending mode [β(C-H)] of the phenyl rings.49,53 Below 25 mN/m, with increasing surface pressure, the νa(CH2) and νs(CH2) bands (47) Wang, C.; Zheng, J.; Oliveira, O. N., Jr.; Leblanc, R. M. J. Phys. Chem. C 2007, 111, 7826. (48) Kutsumizu, S.; Kato, R.; Yamada, M.; Yano, S. J. Phys. Chem. B 1997, 101, 10666. (49) Tao, Y.-T.; Lin, W.-L.; Hietpas, G. D.; Allara, D. L. J. Phys. Chem. B 1997, 101, 9732. (50) Meic´, Z.; Baranovic´, G. Pure Appl. Chem. 1989, 61, 2129. (51) Kozhevina, L. I.; Prokopenko, E. B.; Rybachenko, V. I.; Titov, E. V. J. Mol. Struct. 1993, 295, 53. (52) Teyssie, P.; Charette, J. J. Spectrochim. Acta 1963, 19, 1407. (53) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (54) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116. (55) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96.

Liu et al.

Figure 3. p-Polarized IRRAS spectra of the HBA monolayers on pure water at various surface pressures at an incidence angle of 30° at 22 °C: (a) in the region 3000-1100 cm-1; (b) in the region 4000-2600 cm-1.

increase in intensity and shift to lower frequencies, respectively. It is obvious that the chain order increases gradually with surface pressure. The growth in band intensity with surface pressure results from the increase of surface molecular density and decrease of molecular tilt. The bands related to the Schiff base units, such as the ν(CdO), ν(CdC), ν(C-O), and β(C-H) vibrations, remain almost unchanged in intensity with surface pressure. Obviously, these spectral features indicate that the long axes of the aromatic Schiff base segments take a reduction in tilt angle relative to the surface normal with surface pressure. The positive increments of these bands due to their directional transition moments preferentially along the surface normal might almost compensate their negative increments due to the increase of surface density in this case. At 30 mN/m above the plateau region, all of the bands are significantly intensified; moreover, the relative intensity of the νa(CH2) band to the νs(CH2) one [νa(CH2)νs(CH2)] is reduced, which is clearly different from those below 25 mN/m. This indicates that the C-C-C planes of the alkyl chains are oriented preferentially parallel to the water surface.30,56 The δ(CH2) band appears at 1471 cm-1, indicative of a triclinic subcell structure of alkyl chains in a parallel arrangement.57 It is obvious that the collapsed film at 30 mN/m has different chain packing and orientation from the monolayers at low surface pressures. In addition, the ν(CdO) band is observed to shift to higher (56) Wang, Y.; Du, X.; Miao, W.; Liang, Y. J. Phys. Chem. B 2006, 110, 4914. (57) Holland, R. F.; Nielsen, J. R. J. Mol. Spectrosc. 1962, 9, 436.

BehaVior of HBA Monolayers at the Air-Water Interface

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Figure 4. p-Polarized IRRAS spectra of the HBA monolayers on the ion-containing subphases at various surface pressures at an incidence angle of 30° at 22 °C: (a) Cu2+, pH 6.1; (b) Ca2+, pH 11; (c) Ba2+, pH 10; (d) Ba2+, pH 12.

frequency and become sharp, indicating the dehydration of the hydrated carboxyl groups. This reflects that a strong interaction occurs between the adjacent carboxyl groups at 30 mN/m, forcing the hydrated water away from them. Blume and co-workers investigated the relation between the intensity of the OH stretching band at approximately 3600 cm-1 and the film thickness at the air-water interface and correlated the increase in the intensity of the band directly with the increase of film thickness.58 The IRRAS spectra of the OH stretching bands on pure water with surface pressure are presented in Figure 3b. The magnitude of the change in the OH stretching band intensity with surface pressure is nearly identical to those of the two CH2 stretching band intensities. The film collapse is further verified with the considerably enhanced intensity of the OH stretching band above the plateau region on pure water. For the aqueous Ca2+ solution, the pH of which is not adjusted with NaOH, the spectrum of the monolayer is the same as that on the pure water. Upon increase to pH 11 (Figure 4a), the ν(CdO) band of the carboxyl groups observed on the pure water disappears completely, and some new bands appear in the two regions 1600-1500 and 1450-1400 cm-1 primarily due to the antisymmetric and symmetric stretching vibrations of carboxylate groups [νa(COO) and νs(COO)], respectively. The 1546 cm-1 band is unambiguously assigned to the νa(COO) vibration. The strong band at 1589 cm-1 is primarily due to the νa(COO) vibration mixed with the ν(CdC) mode. Below the Brewster angle, two positive peaks around 1449 and 1434 cm-1 appear, and they (58) Hussain, H.; Kerth, A.; Blume, A.; Kressler, J. J. Phys. Chem. B 2004, 108, 9962.

Figure 5. p-Polarized IRRAS spectra of the HBA monolayers on different subphases at the surface pressure of 20 and/or 30 mN/m at an incidence angle of 60° at 22 °C.

become negative above the Brewster angle (Figure 5). The 1434 cm-1 peak is attributed to the νs(COO) vibration and the 1449 cm-1 peak to the ν(CdC) one. This indicates that the νs(COO) transition moment is nearly perpendicular to the water surface. That means that the long axes of the Schiff base segments are

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oriented almost perpendicular to the water surface. In the monolayer of DSA in the presence of Ca2+ (pH 7.5), two νa(COO) bands appear at 1551 and 1514 cm-1, and one νs(COO) band is observed at 1451 cm-1. For the monolayer of octadecanoic acid on the aqueous Ca2+ subphase, two main νa(COO) bands are observed at 1565 and 1542 cm-1.59 The 1258 cm-1 band decreases significantly in intensity in this case, which suggests that the C-O bonds are oriented nearly perpendicular to the water surface. This is further supported by the appearance of the corresponding negative peak at 1255 cm-1 above the Brewster angle (Figure 5). A similar case occurs for the peak at 1167 cm-1. These spectral features demonstrate that the long axes of the Schiff base segments are oriented almost perpendicular to the water surface. The νa(CH2) and νs(CH2) bands in the vicinity of 0 mN/m appear at 2918 and 2851 cm-1, respectively, and their frequencies are independent of surface pressure. The band intensities nearly increase in proportion with surface molecular density, which indicates that the chain orientation remains almost unchanged with surface pressure. The other bands also show a similar change trend with surface pressure. In the presence of Cu2+ (pH 6.1) (Figure 4b), the spectrum is almost similar to that in the case of Ca2+ (pH 11) except for the 1259 and 1166 cm-1 bands with relatively high intensities. This indicates that the carboxyl groups can interact completely with Cu2+ and that the long axes of the Schiff base units are tilted relative to the normal of the water surface. Moreover, the spectrum is different from that for the DSA monolayer on the Cu2+containing subphase with the coordination of Cu2+ ions not only to carboxylate groups but also to the salicylideneamino moieties.19 These spectral features reflect that Cu2+ ions coordinate only to the carboxylate oxygen atoms but not to the imino nitrogen atoms. The bands at 1586 and 1545 cm-1 are due to the νa(COO) vibrations, and the band at 1441 cm-1 is due to the νs(COO) one. These indicate that both bridging and chelating bidentate coordinations are formed in this case. On the aqueous Ba2+ subphase at pH 10 (Figure 4c), the ν(CdO) band at 1683 cm-1 is still observed, but decreases in intensity concomitant with the appearance of some new bands. These indicate that part of the carboxyl groups react with Ba2+ ions in this case. The νa(CH2) and νs(CH2) bands decrease slightly in frequency with surface pressure. The two bands at 1550 and 1515 cm-1 are assigned to the νa(COO) vibration. Simon-Kutscher et al. observed relatively weak bands between 1540 and 1560 cm-1 due to the νa(COO) vibration in the monolayer of octadecanoic acid in the presence of Ba2+ (around pH 6).25 Ren et al. used the polarized modulation IRRAS (PM-IRRAS) technique and observed a weak νa(COO) band around 1550 cm-1 from the monolayers of barium octadecanoate at pH 6.3 (ionic interaction between the carboxylate groups and Ba2+ ions),60 whereas at pH 9.5, a strong νa(COO) band at 1511 cm-1 was instead developed, indicative of a preferential covalent interaction (chelating bidendate coordination).60 In comparison with octadecanoic acid and DSA, the deprotonation of the benzoic acid groups is relatively difficult; however, similar metal complexes are formed in the case of Ba2+. It is worth noting that the 1258 cm-1 band reduces in intensity with the increase of surface pressure. This indicates that the long axes of the Schiff base segments decrease in tilt angle with increasing surface pressure. At pH 12 (Figure 4d), the ν(CdO) band completely disappears, and the 1518 cm-1 band due to the νa(COO) mode is present. In the vicinity of 0 mN/m, the alkyl chains take on almost alltrans conformations, and the intensities of the bands indicate the (59) Gericke, A.; Hu¨hnerfuss, H. Thin Solid Films 1994, 245, 74. (60) Ren, Y.; Iimura, K.-I.; Kato, T. Langmuir 2001, 17, 2688.

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Figure 6. IRRAS spectra of the three-dimensional structures of the compressed HBA monolayer on the aqueous Ba2+ subphase (pH 12) at a surface pressure of 20 mN/m against different angles of incidence at 22 °C: (a) p-polarization; (b) s-polarization.

formation of a monolayer in comparison with panels a-c of Figure 4. However, all of the bands are considerably intensified with increasing surface pressure, which suggests the formation of the three-dimensional structures of the compressed monolayer. The corresponding spectra of the OH stretching bands on the ion-containing subphases are presented in Figure S1 in the Supporting Information. The magnitude of the change in the OH stretching band intensity with surface pressure is nearly identical to those of the two CH2 stretching band intensities. A strong positive band at 1433 cm-1 below the Brewster angle due to the νs(COO) vibration is clearly observed even at 0 mN/m and becomes negative above the Brewster angle (Figure 5). Such spectral behaviors have never been observed before. They present an excellent example for the selection rule of IRRAS spectra at the air-water interface for p-polarization with the transition moments perpendicular to the water surface. The spectral features suggest that the νs(COO) transition moments are oriented perpendicular to the water surface and that the long axes of the Schiff base segments are oriented preferentially vertical to the surface. In addition, the three peaks at 1308, 1257, and 1174 cm-1 are negative above the Brewster angle (Figure 5). The 1308 cm-1 band is assigned to the mixed modes of the ν(CdC) and β()C-H) vibrations.51 The spectral features of the three peaks indicate that their transition moments are oriented along the surface normal, which supports the vertical orientation of the long axes of the Schiff base segments. Figure 6 shows p- and s-polarized IRRAS spectra in the case of Ba2+ (pH 12),

BehaVior of HBA Monolayers at the Air-Water Interface

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Figure 7. Schematic representation of the metal coordination and orientation of the Schiff base segments at the interface in the case of Ba2+ (pH 12).

Figure 9. Comparison of the simulated (lines) and measured (symbols) RA values of the νa(CH2) bands from the HBA monolayers on the ion-containing subphases at 20 mN/m for p- and s-polarization. The primary surface film parameters for the simulation are Lchain ) 2.07 nm and Γ (degree of polarization) ) 0.009: (a) Cu2+ (pH 6.1), dsc ) 1.02 nm, θ ) 30°, and kmax ) 0.46; (b) Ca2+ (pH 11), dsc ) 1.10 nm, θ ) 20°, and kmax ) 0.52; (c) Ba2+ (pH 10), dsc ) 1.10 nm, θ ) 23°, and kmax ) 0.52. Figure 8. IRRAS spectra of the collapsed monolayer of HBA on pure water at the surface pressure of 30 mN/m against different angles of incidence at 22 °C: (a) p-polarization; (b) s-polarization.

respectively. The changes in sign of the νs(COO) and ν(C-O) bands for p-polarization with incidence angle are very clear. The metal coordination and orientation of the Schiff base segments at the interface are schematically presented in Figure 7. Chain Orientation in the Monolayers. Figure 8 shows IRRAS spectra of the HBA monolayers on pure water at a surface pressure of 30 mN/m at different incidence angles for p- and s-polarization, respectively. The ratio νa(CH2)νs(CH2) for s-polarization is larger than 1 and remains almost constant with angle of incidence,

while the ratio takes an obvious change for p-polarization. Below the Brewster angle, the magnitude of the increase in the νs(CH2) intensity is more than that in the νa(CH2) one, so that the ratio is reduced to be smaller than 1 up to the incidence angle of 45°. Above the Brewster angle, the two bands become positive, and the νa(CH2) intensity is further reduced in comparison with the νs(CH2) one. It is known that both CH2 transition moment directions are perpendicular to the chain axis with the νs(CH2) transition moment along the bisector of the H-C-H bond angle, while the νa(CH2) transition moment is perpendicular to the bisector. The spectral features indicate that the C-C-C planes

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of the alkyl chains are preferentially oriented parallel to the water surface, with the νs(CH2) transition moments parallel to the water surface and the νa(CH2) ones along the surface normal more or less.30,50 The apparent reduction in the νa(CH2) intensity results from the opposite increment of the band with the directional transition moment along the surface normal. For the monolayer at 20 mN/m (Figure S2), the experimental data cannot be well simulated by the Kuzmin and Michailov’s optical model61,62 on the assumption of uniaxial orientation (see the following). This can be understood from the spectral behaviors of the collapsed monolayer at different incidence angles at 30 mN/m. A favorable orientation of the C-C-C planes of the alkyl chains in the monolayer also occurs more or less at 20 mN/m. For the HBA monolayers on the ion-containing subphases (Cu2+, pH 6.1; Ca2+, pH 11; Ba2+, pH 10) at 20 mN/m (Figures S3-S5), the ratio νa(CH2)/νs(CH2) for p-polarization remains almost unchanged with angle of incidence, which assumes a uniaxial orientation of all-trans hydrocarbon chains with free rotation around their chain axes. Gericke et al. first applied the Kuzmin and Michailov’s optical model61,62 to describe IRRAS band intensities.21,22,26,27 Orientation angles are determined by the simulation of theoretical calculations in the three-phase system (air, anisotropic monolayer, and isotropic liquid substrate) to experimental data.26 The film thickness is an important parameter for the fit of theoretical calculation to experimental data. On the basis of the method developed recently for the DSA monolayers,20 the film thickness of the HBA monolayer is regarded to be composed of the sublayers of hydrocarbon chains and Schiff base segments. The film thickness (d) is related to the tilt angle of the hydrocarbon chains (θchain), extended lengths of chains (Lchain), and effective thickness of Schiff base segments (dsc) by the relationship d ) Lchain cos θ + dsc.20 In the combination of the IRRAS results and π-A isotherms, the effective thickness of Schiff base segments can be roughly estimated. In the case of Cu2+ (pH 6.1), the long axes of the Schiff base segments are inclined with respect to the surface normal. Taking into account the limiting area of 0.301 nm2 and the cross-sectional areas of the Schiff base segment along the short and long axes (0.214 and 0.518 nm2),18 the Schiff base segments are roughly oriented at about 25° away from the normal. Similarly, in the cases of Ca2+ (pH 11) and Ba2+ (pH 10), the long axes of the Schiff base segments are nearly vertical to the water surface with a tilt angle of about 10°. The simulated (lines) and measured (symbols) RA values of the νa(CH2) bands in the HBA monolayers on the ion-containing subphases against angle of incidence are shown in panels a-c of Figure 9, respectively. The intensities of the measured bands are obtained by evaluation of the peak (61) Kuzmin, V. L.; Michailov, A. V. Opt. Spectrosc. 1981, 51, 383. (62) Kuzmin, V. L.; Romanov, V. P.; Michailov, A. V. Opt. Spectrosc. 1992, 73, 1.

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heights after the bands in the region 3000-2800 cm-1 are baseline corrected. The tilt angles of the alkyl chains are evaluated to be 30° for Cu2+ (pH 6.1), 20° for Ca2+ (pH 11), and 23° for Ba2+ (pH 10) by the best fit.

Conclusions The interaction of the carboxyl groups in the HBA monolayers with metal ions (Cu2+, Ca2+, and Ba2+) depends closely on the type of metal ions and the pH of the aqueous subphase. Cu2+ ions can completely interreact with the carboxyl groups in the monolayers at pH 6.1, while Ca2+ and Ba2+ ions react with them only when pH of aqueous solutions is considerably increased. However, the presence of metal ions gives rise to condensation of the monolayer in the cases of Cu2+ (pH 6.1), Ca2+ (pH 11), and Ba2+ (pH 10), even leading to the formation of the threedimensional structure of the compressed monolayer in the case of Ba2+ (pH 12). The Schiff base segments undergo a gradual decrease in orientation angle relative to the surface normal with increasing surface pressure except for the case of Ca2+ (pH 11), where their orientations remain basically unchanged with surface pressure. The spectral behaviors of the Schiff base segments with incidence angle in the case of Ba2+ (pH 12) have so far presented an excellent example for the selection rule of IRRAS at the air-water interface for p-polarization with vibrational transition moments perpendicular to the water surface. The orientations of the alkyl chains in the HBA monolayers at the air-water interface are quantitatively determined on the assumption that the thicknessness of the HBA monolayers at the air-water interface are composed of the sublayers of alkyl chains and Schiff base segments. At 20 mN/m, the alkyl chains are uniaxially oriented at 30° for Cu2+ (pH 6.1), 20° for Ca2+ (pH 11), and 23° for Ba2+ (pH 10) with respect to the surface normal, in contrast to a favorable orientation of the alkyl chains with the C-C-C planes parallel to the water surface on pure water, particularly for the collapsed monolayer at 30 mN/m. Acknowledgment. The work was supported by the National Natural Science Foundation of China (Grant Nos. 20673051, 20635020, and 20873062), the Natural Science Foundation of Jiangsu Province (Grant No. BK2007519), and the program for New Century Excellent Talents in University (NCET-07-0412). Supporting Information Available: p-Polarized IRRAS spectra of the OH stretching bands with surface pressure on the ion-containing subphases and p- and s-polarized IRRAS spectra of the HBA monolayers on pure water and ion-containing (Cu2+, pH 6.1; Ca2+, pH 11; Ba2+, pH 10) subphases at a surface pressure of 20 mN/m against different angles of incidence. This material is available free of charge via the Internet at http://pubs.acs.org. LA803976C