Determination of Chain Orientation in the Monolayers of Amino-Acid

Sep 29, 2007 - The chain orientation in the monolayers of amino-acid-derived Schiff base, 4-(4-dodecyloxy)-2-hydroxybenzylideneamino)benzoic acid (DSA...
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Langmuir 2007, 23, 11034-11041

Determination of Chain Orientation in the Monolayers of Amino-Acid-Derived Schiff Base at the Air-Water Interface Using in Situ Infrared Reflection Absorption Spectroscopy Huijin Liu, Wangen Miao, and Xuezhong Du* Key Laboratory of Mesoscopic Chemistry (Ministry of Education), State Key Laboratory of Coordination Chemistry, and School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed July 6, 2007. In Final Form: August 12, 2007 The chain orientation in the monolayers of amino-acid-derived Schiff base, 4-(4-dodecyloxy)-2-hydroxybenzylideneamino)benzoic acid (DSA), at the air-water interface has been determined using infrared reflection absorption spectroscopy (IRRAS). On pure water, a condensed monolayer is formed with the long axes of Schiff base segments almost perpendicular to the water surface. In the presence of metal ions (Ca2+, Co2+, Zn2+, Ni2+, and Cu2+) in the subphase, the monolayer is expanded and the long axes of the Schiff base segments are inclined with respect to the monolayer normal depending on metal ion. The monolayer thickness, which is an important parameter for quantitative determination of orientation of hydrocarbon chains, is composed of alkyl chains and salicylideneaniline portions for the DSA monolayers. The effective thickness of the Schiff base portions is roughly estimated in the combination of the IRRAS results and surface pressure-area isotherms for computer simulation, since the only two observable pand s-polarized reflectance-absorbance (RA) values can be obtained. The alkyl chains with almost all-trans conformations are oriented at an angle of about 10° for H2O, 15° for Ca2+, 30° for Co2+, 35°-40° for Zn2+, and 35°-40° for Ni2+ with respect to the monolayer normal. The chain segments linked with gauche conformers in the case of Cu2+ are estimated to be 40°-50° away from the normal.

Introduction Langmuir monolayers at the air-water interface have successfully been used not only as a simple model for biological membranes1 but also as a powerful means to control molecular orientation and packing necessary for molecular devices.2 In recent years, considerable progress has been made in the structural characterization of the monolayers at the air-water interface using the techniques such as Brewster angle microscopy (BAM), fluorescence microscopy, grazing incidence X-ray diffraction (GIXD), and infrared reflection absorption spectroscopy (IRRAS) as well as surface pressure-area (π-A) isotherms.1,3-9 The thermodynamic measurements cannot reveal detailed microscopic information. BAM and fluorescence microscopy can directly visualize the morphologies of the monolayers, but these observations are confined to macroscopic and mesoscopic scales4,5 and can determine neither conformation order of hydrocarbon chains nor structure of headgroups. The textures by BAM observations give a direct indication about the orientation of the average molecular orientation in the monolayers; however, characterization of orientation order in a condensed phase requires that the observed textures occur under well-defined conditions.3 The * To whom correspondence should be addressed. E-mail: xzdu@ nju.edu.cn. Fax: 86-25-83317761. (1) Leblanc, R. M. Curr. Opin. Chem. Biol. 2006, 10, 529. (2) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (3) Nandi, N.; Vollhardt, D. Chem. ReV. 2003, 103, 4033. (4) Mo¨hwald, H. Annu. ReV. Phys. Chem. 1990, 41, 441. (5) Knobler, C. M. AdV. Chem. Phys. 1990, 77, 397. (6) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. Chem. ReV. 2001, 101, 1659. (7) Blaudez, D.; Buffeteau, T.; Desbat, B.; Turlet, J. M. Curr. Opin. Colloid Interface Sci. 1999, 4, 265. (8) 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. (9) Mendelsohn, R.; Braunder, J. W.; Gericke, A. Annu. ReV. Phys. Chem. 1995, 46, 305.

GIXD technique is a valuable tool to obtain direct structural information of crystalline films at the interface on the subnanometer scale6 but is limited by the low scattering intensity arising from monolayers at the air-water interface.10 IRRAS has emerged as one of the leading structural analyses for monolayers at the air-water interface over the past decade7-9,11-22 since the early works by Dluhy and co-workers in the mid-1980s.23 The IRRAS technique not only allows for the characterization of chain conformation and headgroup structure but also provides quantitative information about molecular orientation. Gericke et al. first applied the Kuzmin and Michailov’s optical model24,25 to describe IRRAS band intensities and to fit theoretical calculations to experimental data for the quantitative determination of orientation angles.9,11,13,14 IRRAS studies on chain orientation have been so far limited to the monolayers of fatty acids/alkanols and phospholipids on the assumption of the uniaxial (10) Ren, Y.; Meuse, C. W.; Hsu, S. L. J. Phys. Chem. 1994, 98, 8424. (11) Gericke, A.; Michailov, A. V.; Hu¨hnerfuss, H. Vib. Spectrosc. 1993, 4, 335. (12) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027. (13) Flach, C. R.; Braunder, J. W.; Taylor, J. W.; Baldwin, R. C.; Mendelsohn, R. Biophys. J. 1994, 67, 402. (14) Flach, C. R.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. B 1997, 101, 58. (15) Huo, Q.; Dziri, L.; Desbat, B.; Russell, K. C.; Leblanc, R. M. J. Phys. Chem. B 1999, 103, 2929. (16) Ren, Y.; Kato, T. Langmuir 2002, 18, 6699. (17) Ren, Y.; Hossain, M. M.; Iimura, K.-I.; Kato, T. J. Phys. Chem. B 2001, 105, 7723. (18) Brauner, J. W.; Flach, C. R.; Xu, Z.; Bi, X.; Lewis, R. N. A. H.; McElhaney, R. N.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. B 2003, 107, 7202. (19) Du, X.; Miao, W.; Liang, Y. J. Phys. Chem. B 2005, 109, 7428. (20) Wang, Y.; Du, X.; Guo, L.; Liu, H. J. Chem. Phys. 2006, 124, 134706. (21) Wang, Y.; Du, X.; Miao, W.; Liang, Y. J. Phys. Chem. B 2006, 110, 4914. (22) Wang, C.; Zheng, J.; Oliveira, O. N., Jr.; Leblanc, R. M. J. Phys. Chem. C 2007, 111, 7826. (23) Dluhy, R. A.; Cornell, D. G. J. Phys. Chem. 1985, 89, 3195. (24) Kuzmin, V. L.; Michailov, A. V. Opt. Spectrosc. 1981, 51, 383. (25) Kuzmin, V. L.; Romanov, V. P.; Michailov, A. V. Opt. Spectrosc. 1992, 73, 1.

10.1021/la702017p CCC: $37.00 © 2007 American Chemical Society Published on Web 09/29/2007

Determination of Chain Orientation

orientation of hydrocarbon chains with the all-trans conformations.11,13,14,20,26-32 For the fatty acid/alkanol molecules, the sizes of the headgroups are relatively small, and the thickness of the monolayers, which is an important parameter for the theoretical calculation, is approximately related to the extended length and tilt angle of chains.11,14,20 For the phospholipid molecules, the sizes of the headgroups are relatively large, but the headgroups are inserted into the aqueous subphases, so the thickness of the monolayers is also related to the extended length and tilt angle of chains.26-29 In general, the observed IRRAS results are consistent with those derived from the X-ray scattering measurements. The interpretation of polarization modulation-IRRAS (PM-IRRAS) spectra, a differential IR reflectivity technique, in terms of molecular orientation relies on a specific surface selection rule which connects the intensity and the sense of bands (i.e., positive or negative) to the orientation of the corresponding transition moment relative to the surface.7,8,33 The orientation of much more different compounds, such as liquid crystal, ferroelectric, peptide, and protein, has been determined at the airwater interface with the PM-IRRAS method.33-35 However, few of the monolayers of complicated molecules at the airwater interface have been studied using the IRRAS technique,21,22,36 particularly for quantitative determination of molecular orientation. Schiff bases are one of the most important classes of organic compounds and have extensive applications in various fields. Schiff bases of salicylidene derivatives exhibit photochromism and/or thermochromism in the crystalline state,37,38 besides, they readily coordinate with transition metals.39 The corresponding metal complexes exhibit fascinating chemical, optical, electric, and catalytic properties. It has been shown that their stability and properties are greatly enhanced after the introduction of the Schiff base segments into the monolayers at the air-water interface.40 So far, Langmuir-Blodgett (LB) films of Schiff bases and their metal complexes have been investigated including different kinds of amphiphiles.40-47 Amino-acid-derived Schiff bases are a kind 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.38 (26) Gericke, A.; Flach, C. R.; Mendelsohn, R. Biophys. J. 1997, 73, 492. (27) Bi, X.; Taneva, S.; Keough, K. M. W.; Mendelsohn, R.; Flach, C. R. Biochemistry 2001, 40, 13659. (28) Dyck, M.; Kerth, A.; Blume, A.; Lo¨sche, M. J. Phys. Chem. B 2006, 110, 22152. (29) Du, X.; Wang, Y. J. Phys. Chem. B 2007, 111, 2347. (30) Gericke, A.; Hu¨hnerfuss, H.; Michailov, A. V. Proc. SPIE-Int. Soc. Opt. Eng. 1992, 1575, 554. (31) Gericke, A.; Hu¨hnerfuss, H. Proc. SPIE-Int. Soc. Opt. Eng. 1993, 2089, 570. (32) Gericke, A.; Hu¨hnerfuss, H. Langmuir 1995, 11, 225. (33) Blaudez, D.; Turlet, J. M.; Dufourcq, J.; Bard, D.; Buffeteau, T.; Desbat, B. J. Chem. Soc., Faraday Trans. 1996, 92, 525. (34) Cornut, I.; Desbat, B.; Turlet, J. M. Biophys. J. 1996, 70, 305. (35) Blaudez, D.; Boucher, F.; Buffeteau, T.; Desbat, B.; Grandbois, M.; Salesse, C. Appl. Spectrosc. 1999, 53, 1299. (36) Miao, W.; Du, X.; Liang, Y. J. Phys. Chem. B 2003, 107, 13636. (37) Hodjoudis, E.; Mavridis, I. M. Chem. Soc. ReV. 2004, 33, 579. (38) 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. (39) Garnovskii, A. D.; Nivorozhkin, A. L.; Minkin, V. I. Coord. Chem. ReV. 1993, 126, 1. (40) Nagel, J.; Oertel, U.; Friedel, P.; Komber, H.; Mo¨bius, D. Langmuir 1997, 13, 4693. (41) Chen, X.; Xue, Q.-B.; Yang, K.-Z.; Zhang, Q.-Z. Langmuir 1995, 11, 4082. (42) Sundari, S. S.; Dhathathreyan, A.; Kanthimathi, M.; Nair, B. U. Langmuir 1997, 13, 4923. (43) Liu, M.; Xu, G.; Liu, Y.; Chen, Q. Langmuir 2001, 17, 427. (44) Hemakanthi, G.; Dhathathreyan, A.; Mo¨bius, D. Colloids Surf. A 2002, 198-200, 443. (45) Pang, S.; Liang, Y. J. Colloid Interface Sci. 2003, 231, 59. (46) Jiao, T.; Liu, M. Langmuir 2006, 22, 5005. (47) Jiao, T.; Zhang, G.; Liu, M. J. Phys. Chem. B 2007, 111, 3090.

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Saito and co-workers investigated the photochromism of the LB films of amino-acid-derived Schiff bases in the case of Ba2+.48 In this paper, the IRRAS technique is used to determine chain orientation in the monolayers of 4-(4-dodecyloxy)-2-hydroxybenzylideneamino)benzoic acid (DSA), a nonlinear molecule with a bending angle between the Schiff base segment and alkyl chain connected with an -O- linkage, on aqueous subphases containing metal ions. Experimental Section Materials. The compound 4-(4-dodecyloxy)-2-hydroxybenzylideneamino)benzoic acid (DSA) was synthesized in our laboratory. The chemical reagents used were of analytical grade. Water used was double distilled (pH 5.6) after a deionized exchange. The pH of metal ion-containing solutions (1 mmol/L) was as follows: pH 7.5 for CaCl2, pH 8.2 for CoCl2, pH 6.8 for ZnCl2, pH 8.0 for NiSO4, and pH 5.3 for CuCl2. The aqueous ZnCl2 and CuCl2 solutions were not adjusted with HCl or NaOH.

Isotherm Measurements. Surface pressure-area (π-A) isotherms of DSA monolayers at the air-water interface 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 barriers compressed or expanded symmetrically at the same velocity. Chloroform/ether (1:1, v/v) solutions of desired volumes were spread on pure water and ioncontaining subphases, and then 15 min was allowed for solvent evaporation. The two barriers compressed symmetrically at a rate of 8 mm/min. The subphase temperature was kept at 22 °C. 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 and a liquidnitrogen-cooled MCT detector. 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 perpendicularly polarized beams, and efficiency of the polarizer was determined to be about 99.2%.29 These experiments were carried out at 22 °C. The film-forming molecules were spread from a chloroform/ether (1:1, v/v) solution 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 compressed discontinuously to the desired surface pressure of 20 mN/m from ∼0 mN/m. After 30 min of relaxation, the two moving barriers were stopped and the monolayer areas were kept constant. Surface pressures decreased slightly during the acquisition of IRRAS spectra. The external reflection absorption spectra of pure water and ion-containing solutions were used as references, respectively. 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 beam followed by data collection using s-polarized one. The IRRAS spectra were presented without smoothing or baseline correction.

Results and Discussion Isotherms of Monolayers at the Air-Water Interface. Figure 1 shows π-A isotherms of DSA monolayers on pure water and ion-containing subphases, respectively. On pure water, DSA molecules form a condensed monolayer. The linear part of (48) Kawamura, S.; Tsutsui, T.; Saito, S.; Murao, Y.; Kina, K. J. Am. Chem. Soc. 1988, 110, 509.

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Figure 1. π-A isotherms of DSA monolayers on pure water and ion-containing subphases (Ca2+, Co2+, Zn2+, Ni2+, and Cu2+), respectively: compression rate, 8 mm/min; temperature, 22 °C.

Figure 2. Schematic illustration of the principle of p- and s-polarized IRRAS.

the isotherm is extrapolated to zero surface pressure to obtain a limiting area of 0.254 nm2, which is very close to the crosssectional area of the long axis of the salicylideneaniline nucleus.38,48 This means that the long axis of the Schiff base segment is oriented almost perpendicular to the water surface. The crystal structure of methyl 4-(2,4-dihydroxybenzylideneamino)benzoate, very similar to the chromophore in the amphiphile studied here, was recently determined by X-ray diffraction.38 The compound shows a nonplanar conformation with a dihedral angle of 47.69° between the planes of the N- and C-phenyl rings.38 Three basic cross-sectional areas of the Schiff base segments are estimated to be about 0.214 nm2 along the short axis, 0.518 nm2 along the long axis, and 0.887 nm2 in the plane of the N-phenyl ring. The presence of metal ions in the subphases gives rise to expansion of the monolayer to different extents. The limiting area is 0.288 nm2 for Ca2+, 0.305 nm2 for Co2+, 0.333 nm2 for Zn2+, and 0.378 nm2 for Ni2+. However, in the presence of Cu2+, a liquid-expanded monolayer is formed with a limiting area of 0.427 nm2. The above limiting areas are larger than the cross section of the vertically oriented hydrocarbon chain (ca. 0.2 nm2) and should reflect the dimension of the Schiff base segment. The discrepancies in isotherm and limiting area suggest that different molecular orientation and/or metal coordination occur. IRRAS Spectra of the Monolayers at the Air-Water Interface. Figure 2 presents schematic illustration of the principle of p- and s-polarized IRRAS. The electric field vector of the s-polarized IR beam is perpendicular to the plane of incidence

Figure 3. IRRAS spectra of a DSA monolayer on pure water at the surface pressure of 20 mN/m against different angles of incidence at 22 °C: (a) s polarization; (b) p polarization.

(parallel to the y direction, i.e., the direction of barrier compression), the bands are always negative and their intensities decrease with increasing angle of incidence. The electric field vector of the p-polarized IR beam is parallel to the plane of incidence. For the vibrations with their transition moments parallel to the x direction, i.e., along the barriers, the bands are initially negative and their intensities increase with increasing angle of incidence and reach a maximum, and then a minimum in the reflectivity is found at the Brewster angle.9 The exact position of the Brewster angle, φ, depends on the wavelength of light and the optical properties of the substrate. For the air-water interface, the Brewster angle can be estimated by calculating tan φ ) n2/n1, where n1 and n2 are the real parts of refractive indices of air and H2O at a given wavenumber, respectively (φ ) 54.5° for the νa(CH2) band at 2920 cm-1 and φ ) 54.2° for the νs(CH2) band at 2850 cm-1). Above the Brewster angle, the bands become positive and their intensities decrease upon further increase of incidence angle.9 For the vibrations with their transition moments along the surface normal (the z direction, i.e., perpendicular to the water surface), the situation is reversed: below the Brewster angle, positive bands will be observed, whereas above the Brewster angle, negative bands will be found.9,16,19,22,26 The p-polarized beam provides most of the useful information on orientation angle. Figures 3-8 show IRRAS spectra of the DSA monolayers at the air-water interface and ion-containing subphases at the surface pressure of 20 mN/m against angles of incidence for sand p-polarization, respectively. The p-polarized IRRAS spectrum of the monolayer on pure water at an incidence angle of 60° is

Determination of Chain Orientation

presented in comparison with the FTIR transmission spectrum of the bulk DSA compound in the Supporting Information. On pure water (Figure 3), below the Brewster angle, the two negative bands at 2920 and 2851 cm-1 due to the antisymmetric and symmetric CH2 stretching vibrations [νa(CH2) and νs(CH2)] indicate that the hydrocarbon chains take almost all-trans conformations. A band at 1687 cm-1 is due to the CdO stretching vibration of the carboxylic groups [ν(CdO)].49,50 The bands at 1595, 1570, and 1402 cm-1 are due to the skeleton stretching vibrations of the aromatic rings [ν(CdC)].49-54 A band at 1290 cm-1 is assigned to the ν(C-O) vibration in the phenols and phenyl-O-C groups mixed with the ν(C-O)/δ(OH) modes in the carboxylic groups.49,53,55 Above the Brewster angle (Figure 3b), a positive peak at 1616 cm-1 is assigned to the ν(CdN) mode. Two negative dips in the vicinity of the ν(CdN) band result from the negative-oriented baseline distortion above the Brewster angle due to the altered structure of the water adjacent to headgroups of the film constituents (in contrast with the positive-oriented one below the Brewster angle) overlapped with the positive-oriented ν(CdN) band. A negative peak around 1434 cm-1 is due to the ν(CdC) vibration,50,52 and the other negative peak at 1201 cm-1 to the ν(C-N) mode,51,52 which suggests that the long axes of the salicylideneaniline portions are oriented preferentially perpendicular to the water surface. On the ion-containing subphases, the ν(CdO) bands cannot be observed, which indicates that the metal ions coordinate with the carboxylic groups. The νa(CH2) and νs(CH2) band intensities are reduced depending on metal ion at the same surface pressures, which is consistent with the expansion features of the corresponding isotherms as well as molecular orientation. In the presence of Ca2+ (Figure 4), the νa(COO) bands are present around 1551 and 1514 cm-1. Above the Brewster angle (Figure 4b), a sharp strong negative peak is clearly observed at 1451 cm-1 due to the νs(COO) mode, which indicates that the νs(COO) transition moment is nearly perpendicular to the water surface. This means that the salicylideneaniline portions are oriented almost vertical to the water surface. The ν(CdN) band appears at 1614 cm-1, implying that the imino nitrogen atoms do not coordinate to Ca2+ ions. Similarly, in the presence of Co2+ and Zn2+, the νa(COO) bands appear around 1535 and 1530 cm-1, respectively, and the corresponding negative peaks at 1444 and 1427 cm-1 above the Brewster angle (part b of Figures 5 and 6) are due to the νs(COO) vibration. These suggest that the νs(COO) transition moments are preferentially perpendicular to the water surface and that the salicylideneaniline portions are almost oriented vertical to the water surface. The ν(CdN) bands around 1612 cm-1 also suggest that the imino nitrogen atoms do not coordinate to Co2+ and Zn2+ ions. In the cases of Ca2+, Co2+, and Zn2+, above the Brewster angle a negative peak at about 1200 cm-1 due to the ν(C-N) vibration51,52 is clearly observed, which means that the C-N bonds take an orientation preferentially along with the monolayer normal. This provides another evidence for the orientation of the long axes of the salicylideneaniline portions. In the presence of Cu2+ (Figure 7), the νa(CH2) and νs(CH2) bands shift respectively up to 2923 and 2853 cm-1 and decrease (49) Kutsumizu, S.; Kato, R.; Yamada, M.; Yano, S. J. Phys. Chem. B 1997, 101, 10666. (50) Tao, Y.-T.; Lin, W.-L.; Hietpas, G. D.; Allara, D. L. J. Phys. Chem. B 1997, 101, 9732. (51) Meic´, Z.; Baranovic´, G. Pure Appl. Chem. 1989, 61, 2129. (52) Kozhevina, L. I.; Prokopenko, E. B.; Rybachenko, V. I.; Titov, E. V. J. Mol. Struct. 1993, 295, 53. (53) Teyssie, P.; Charette, J. J. Spectrochim. Acta 1963, 19, 1407. (54) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (55) Green, J. H. S. Spectrochim. Acta, Part A 1977, 33, 575.

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Figure 4. IRRAS spectra of a DSA monolayer on aqueous Ca2+ solution at the surface pressure of 20 mN/m against different angles of incidence at 22 °C: (a) s polarization; (b) p polarization.

markedly in intensity. The band at 1584 cm-1 is significantly intensified; obviously, the two bands at 1584 and 1523 cm-1 are due to the νa(COO) vibration and the two bands at 1423 and 1378 cm-1 to the νs(COO) mode. In the monolayer of stearic acid (octadecanoic acid) on the Cu2+-containing subphase, the νa(COO) bands are located at 1585 and 1530 cm-1 and the νs(COO) band at 1408 cm-1.12 In contrast with the spectra in the cases of Ca2+, Co2+, and Zn2+, the νs(COO) bands are negative and strong below the Brewster angle and become positive above the Brewster angle in the presence of Cu2+, which indicates that the whole Schiff base portions take an orientation preferentially parallel to the water surface. The ν(CdN) band is shifted to 1607 cm-1 and the ν(C-O) band related to the phenolates53,55 to 1309 cm-1, which indicates that Cu2+ ions coordinate with the imino nitrogen and phenolate oxygen atoms. In the presence of Ni2+ (Figure 8), the spectrum is intermediate between the cases of Cu2+ and other ions. the νa(CH2) and νs(CH2) bands are considerably reduced but basically remain ordered conformations. The ν(CdN) and ν(C-O) bands take a small shift to lower and higher wavenumbers, respectively, which indicates that Ni2+ ions might partially coordinate with the imino nitrogen and phenolate oxygen atoms. Chain Orientation in the Monolayers. It is well-known that the 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. Gericke et al. first applied the Kuzmin and Michailov’s optical model24,25 to describe IRRAS band intensities.9,11,14,26 The theoretical calculations on the basis of

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Figure 5. IRRAS spectra of a DSA monolayer on aqueous Co2+ solution at the surface pressure 20 mN/m against different angles of incidence at 22 °C: (a) s polarization; (b) p polarization.

Figure 6. IRRAS spectra of a DSA monolayer on aqueous Zn2+ solution at the surface pressure of 20 mN/m against different angles of incidence at 22 °C: (a) s polarization; (b) p polarization.

the three-phase system (air, anisotropic monolayer, and isotropic liquid substrate)14 fit experimental data to determine orientation angles. The following parameters are required to calculate a RA value using the Kuzmin and Michailov’s formulation:24,25 refractive index and extinction coefficient of air, n1 ) 1 and k1 ) 0; refractive index and extinction coefficient of water (H2O), n2 and k2, obtained from the literature;56 angle between dipole moment vector and chain axis, R ) 90° for the νa(CH2) vibration; ordinary and extraordinary refractive indices of hydrocarbon chains in the mid-IR region, nord ) next ) 1.41,14 and corresponding directional refractive indices of the film, nx ) ny and nz, obtained from nord, next, and the given tilt angle; directional extinction coefficients of the film, kx ) ky and kz, obtained for the given tilt angle and transition moment direction when the film extinction coefficient, kmax, is known; film thickness, d, composed of hydrocarbon chains and Schiff base segments in this paper and obtained by taking into account the tilt angles of hydrocarbon chains (θ) and Schiff base segments (θsc) and extended lengths of chains (Lchain) and Schiff base segments (Lsc), i.e., d ) Lchain cos θ + Lsc cos θsc. There are three unknowns, kmax, θ, and θsc, provided that the degree of polarization, Γ, is determined, with only two observable RA intensities. In the combination of the IRRAS results and π-A isotherms, the orientation of the Schiff base segments can be roughly estimated. The film thickness d ) Lchain cos θ + dsc, where dsc is the effective thickness of the Schiff base segments depending on metal ion

in the subphase, so that only two unknowns, kmax and θ, remain. A set of different combinations of kmax and θ are used by computer simulation to calculate RA values to fit measured data. In the case of pure water, the long axes of the Schiff base segments are almost perpendicular to the water surface, and in the cases of the Ca2+, Co2+, and Zn2+, the νs(COO) transition moments are nearly vertical to the water surface, so that the values dsc can be estimated. In the case of Ni2+, the chain order is basically retained, but the long axes of the Schiff base segments are obviously inclined with respect to the monolayer normal. Taking into account the limiting area of 0.378 nm2 and the crosssectional area (0.518 nm2) of the Schiff base segment along the long axis, the Schiff base segments are roughly oriented at about 45° away from the normal. In the case of Cu2+, the alkyl chains are disordered, and the long axes of the Schiff base segments are significantly tilted. Similarly, the long axes of the Schiff base segments are roughly estimated to be oriented at about 55° away from the normal in this case. Figure 9 shows theoretical RA values (lines) of the νa(CH2) bands for the DSA monolayer on pure water against the angle of incidence as well as the experimental data (symbols). The tilt angle of the alkyl chains is evaluated to be about 10°, and the kmax value is 0.60 by the best fit. The obtained kmax value seems to be reasonable in comparison with kmax ) 1.07 for the νa(CH2) band of hydrocarbon chains in the monolayers of behenic acid methyl ester (methyl docosanoate) with a chain tilt angle of 0°,14 taking into account chain length (11 methylene units in each chain for the former and 20 ones for the latter). For the stearic

(56) Bertie, J. E.; Ahmed, M. K.; Eysel, H. H. J. Phys. Chem. 1989, 93, 2210.

Determination of Chain Orientation

Figure 7. IRRAS spectra of a DSA monolayer on aqueous Cu2+ solution at the surface pressure of 20 mN/m against different angles of incidence at 22 °C: (a) s polarization; (b) p polarization.

acid monolayers on pure water and ion-containing subphases, the kmax values simulated for the νa(CH2) band of hydrocarbon chains (16 methylene units in each chain) are in the range of 0.75-0.85 with the orientation angle 0°-30°.20 The kmax value is related to the extinction coefficient for the vibrational mode and the density of the film-forming molecules at the air-water interface.27 The relative magnitude of the molecular density can be obtained from the appropriate π-A isotherms. The kmax value for the νa(CH2) bands in the ioncontaining (Ca2+, Co2+, Zn2+, and Ni2+) subphases are roughly estimated taking into account their molecular areas at 20 mN/m and the kmax value obtained on the pure water. From the s- and p-polarized IRRAS spectra of the DSA monolayers on the ioncontaining (Ca2+, Co2+, Zn2+, and Ni2+) subphases, the tilt angles of the hydrocarbon chains are readily evaluated by the fit of theoretical calculations to the experimental data (Figure 10ad). The tilt angles of the alkyl chains are about 15° for Ca2+, 30° for Co2+, 35°-40° for Zn2+, and 35°-40° for Ni2+ with respect to the normal of the monolayers, respectively. The chain tilt in the case of Ca2+ is consistent with previous small-angle X-ray diffraction measurements of the LB films of DSA from the Ba2+containing subphase, which yielded about 20° for the tilt angle of alkyl chains away from the perpendicular direction of a layer plane.48 Although the hydrocarbon chains with the all-trans conformations are depicted in the theoretical model,9 the actual molecules may contain some gauche conformers in the chains. The νa(CH2) and νs(CH2) bands are usually found around 2918 and 2850

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Figure 8. IRRAS spectra of a DSA monolayer on aqueous Ni2+ solution at the surface pressure of 20 mN/m against different angles of incidence at 22 °C: (a) s polarization; (b) p polarization.

Figure 9. Comparison of the simulated (lines) and measured (symbols) RA values of the νa(CH2) bands for the DSA monolayer on the pure water at 20 mN/m for p- and s-polarization. The surface film parameters for the simulation are Lchain ) 1.56 nm, R ) 90°, Γ ) 0.008, dsc ) 1.12 nm, kmax ) 0.60, and θ ≈ 10°.

cm-1 for the ordered chains and shift to 2925 and 2855 cm-1 for the disordered chains. The alkyl chains in the monolayers on the Cu2+-containing subphase are not as disordered as in the liquid-crystalline phase of the bulk dispersion.57 Taking the facts mentioned above into account, the alkyl chains are conjectured (57) Dicko, A.; Bourque, H.; Pe´zolet, M. Chem. Phys. Lipids 1998, 96, 125.

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Figure 10. Comparison of the simulated (lines) and measured (symbols) RA values of the νa(CH2) bands of the DSA monolayers on the ion-containing subphases at 20 mN/m for p- and s-polarization with the parameters Lchain ) 1.56 nm, R ) 90°, and Γ ) 0.008: (a) Ca2+, dsc ) 1.09 nm, kmax ) 0.54, and θ ≈ 15°; (b) Co2+, dsc ) 1.09 nm, kmax ) 0.48, and θ ≈ 30°; (c) Zn2+, dsc ) 1.09 nm, kmax ) 0.44, and θ ) 35°-40°; (d) Ni2+, dsc ) 0.79 nm, kmax ) 0.34, and θ ) 35°-40°.

to be composed of a few small chain segments of all-trans conformation connected by several kinks of gauche conformers. The chain orientation could be roughly estimated to describe the monolayers at the air-water interface in the liquid-expanded phase on the basis of the model of chain segment orientation, which described well the chain orientation distribution in the black soap films consisting of a thin layer of aqueous core enclosed between two surfactant monolayers in the liquid-crystalline phase58 and was recently applied to the monolayers at the airwater interface.19 Figure 11 shows the theoretical RA values (solid lines) and measured data (square symbols) of the νa(CH2) band of the DSA monolayer in the case of Cu2+ against the angle of incidence at different orientation angles using the obtained kmax value of 0.33. The chain segments in the monolayer are evaluated to be inclined at 40°-50° from the surface normal on the Cu2+-containing subphase, which is schematically represented in Figure 12. The large tilt angle of the alkyl chains in the case of Cu2+ is due to the expansion of the monolayer caused by the preferential orientation of the Schiff base segments parallel to the water surface with the coordination of Cu2+ ions not only to the salicylidene moieties but also to the carboxylate groups. For the other metal ions (Ca2+, Co2+, and Zn2+) which coordinate only to the carboxylate groups, the long axes of the Schiff base segments are preferentially vertical to the water surface, and the (58) Du, X.; Lu, Y.; Liang, Y. J. Colloid Interface Sci. 1998, 207, 106.

Figure 11. Comparison of the simulated (lines) and measured (symbols) RA values of the νa(CH2) bands of the DSA monolayer on aqueous Cu2+ solution at 20 mN/m for p- and s-polarization. The surface film parameters for the simulation are Lchain ) 1.56 nm, R ) 90°, Γ ) 0.008, dsc ) 0.64 nm, kmax ) 0.33, and θ ) 40°-50°.

chain tilt is related to the twist angle of the salicylideneaniline conformation (i.e., planar or nonplanar) because the Schiff base derivatives with a nonplanar conformation are very tightly packed.38 It is obvious that the orientation change of the alkyl

Determination of Chain Orientation

Figure 12. Schematic illustration of chain segment orientation of the DSA monolayer at the air-water interface on aqueous Cu2+ solution in the liquid-expanded phase.

chains is associated with coordination fashion and salicylideneaniline conformation depending on the metal ion.

Conclusions DSA is a nonlinear molecule with a bending angle between the Schiff base segment and alkyl chain connected with an -Olinkage. A condensed monolayer is formed on pure water with the long axes of Schiff base segments almost perpendicular to the water surface. In the presence of metal ions (Ca2+, Co2+, Zn2+, Ni2+, and Cu2+) in the subphase, the monolayer is expanded and the long axes of the Schiff base segments are inclined with

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respect to the monolayer normal depending on metal ion. It is obvious that the thickness of the DSA monolayer at the airwater interface is composed of hydrocarbon chains and salicylideneaniline portions. The monolayer thickness is an important parameter for the quantitative determination of hydrocarbon chains. The effective thickness of the Schiff base portions is first estimated in the combination of the IRRAS results and surface pressure-area isotherms, since the only two observable p- and s-polarized reflectance-absorbance (RA) values can be obtained. The alkyl chains with almost all-trans conformations are oriented at an angle of about 10° for H2O, 15° for Ca2+, 30° for Co2+, 35°-40° for Zn2+, and 35°-40° for Ni2+ with respect to the monolayer normal. In the presence of Cu2+, the chain segments linked with gauche conformers are estimated to be 40°-50° away from the normal. The method for the quantitative determination of hydrocarbon chains in the monolayers of complicated molecules at the air-water interface using the IRRAS technique is developed. Acknowledgment. The work was supported by the National Natural Science Foundation of China (Grant No. 20673051 and 20635020) and the Natural Science Foundation of Jiangsu Province (Grant No. BK2007519). Supporting Information Available: FTIR transmission spectrum of the bulk DSA compound in comparison with the p-polarized IRRAS spectrum of the DSA monolayer on pure water at the surface pressure 20 mN/m at an incidence angle of 60° together with band assignment. This material is available free of charge via the Internet at http://pub.acs.org. LA702017P