IRRAS Studies on Chain Orientation in the Monolayers of Amino Acid

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J. Phys. Chem. B 2005, 109, 7428-7434

IRRAS Studies on Chain Orientation in the Monolayers of Amino Acid Amphiphiles at the Air-Water Interface Depending on Metal Complex and Hydrogen Bond Formation with the Headgroups Xuezhong Du,* Wangen Miao, and Yingqiu Liang Key Laboratory of Mesoscopic Chemistry (Nanjing UniVersity), Ministry of Education, and Department of Chemistry, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: December 22, 2004; In Final Form: February 5, 2005

Monolayers of N-octadecanoyl-L-alanine at the air-water interface on pure water and metal ion containing subphases have been studied using polarized infrared reflection-absorption spectroscopy (IRRAS). The metal complex and hydrogen bond formation with the headgroups give rise to a change in chain order depending on metal ion in the subphase. On pure water and Ag+-/Pb2+-containing subphase, the antisymmetric CH2 stretching band intensity [νa(CH2)] undergoes a slower increase than the symmetric one [νs(CH2)] below the Brewster angle, so the intensity ratios of νa(CH2)/νs(CH2) are less than 1 in the cases of Ag+ and Pb2+. Beyond the Brewster angle, the νa(CH2) band intensities are substantially reduced in comparison with the νs(CH2) ones in the cases of pure water and Ag+, but the νa(CH2) bands still remain negative-oriented in the presence of Pb2+. These unusual spectral features indicate that the alkyl chains take a preferential orientation with their C-C-C planes parallel to the water surface. The parallel packing of the alkyl chains results from the intermolecular hydrogen bonds CdO‚‚‚H-N between the neighboring amide groups, strengthened by the metal complex of covalent interaction. On the Ca2+-/Cu2+-containing subphase, the corresponding polarized spectra display a usual behavior. The alkyl chains are roughly estimated to be inclined around 35-40° from the surface normal on the assumption of chain segment orientation for the monolayers in the liquid-expanded phase. The chain conformation and tilt are closely related to the formation of intramolecular hydrogen bonds and the ionic interaction of the metal complex in the cases of Ca2+ and Cu2+.

Introduction Amino acid amphiphiles are of special interest in studying chiral discrimination effects. The studies of chirality-dependent intermolecular forces in two-dimensional self-assemblies are of tremendous importance in many biological processes. The effect of molecular chirality on the morphology of biomimetic Langmuir monolayers using the surface pressure (π)-area (A) isotherm, Brewster angle microscopy (BAM), fluorescence microscopy, and grazing incidence X-ray diffraction (GIXD) has been recently reviewed by Nandi and Vollhardt.1 It is known that thermodynamic measurements such as the π-A isotherm cannot reveal detailed microscopic information. BAM and fluorescence microscopy can directly visualize the morphologies of the monolayers composed of chiral molecules; however, these observations are confined to macroscopic and mesoscopic scales2,3 and can determine neither molecular characteristics such as conformation and packing of the alkyl chains nor structure and interaction pattern of the headgroups. The GIXD technique is a valuable tool to obtain direct structural information of crystalline films at the air-water interface on the subnanometer scale,4 but is limited by the low scattering intensity arising from the monolayers at the interface.5 Infrared reflection-absorption spectroscopy (IRRAS) has emerged as one of the leading methods for structural analyses of monolayers at the air-water interface over the past decades,6-17 since the early works by Dluhy and co-workers in 1985.18 The IRRAS technique not only * To whom correspondence should be addressed. Fax: 86-25-83317761. E-mail: [email protected].

allows the characterization of chain conformation and headgroup structure but also provides qualitative/quantitative information about molecular orientation. Several IRRAS studies have determined chain orientation in ordered phospholipid and fatty acid monolayers on the assumption of uniaxial orientation of hydrocarbon chains of all-trans conformation.7,12,16,19,20 In general, the observed IRRAS results are consistent with those derived from X-ray scattering measurements. Hu¨hnerfuss and co-workers9-11 studied in detail the correlation between the chiral discrimination effect and the chain order/ headgroup structure of N-acyl amino acid monolayers at the air-water interface using the IRRAS technique, but some valuable information on headgroup structure and chain orientation is limited due to water vapor interference and unpolarized IR radiation, respectively. Recently, we reported the first observation of sharp NH stretching bands from N-octadecanoylL-alanine monolayers at the air-water interface using the technique and the remarkable change in type and strength of hydrogen bonds within the monolayers depending on the metal ion in the subphase.17 In the presence of metal ions, the condensation of the film-forming molecules is associated with an increase in intermolecular hydrogen-bonding interaction and chain order as well as a covalent interaction in the metal complex, and the expansion is related to the formation of intramolecular hydrogen bonds and the decrease of chain order together with an ionic interaction in the metal complex.17 In this paper, the orientation of the alkyl chains in the monolayers on pure water and metal ion containing subphases has been investigated in detail using the IRRAS technique to give insight

10.1021/jp0441700 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/17/2005

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into the correlation between the hydrophilic interactions (metal complex and hydrogen bond formation) and the hydrophobic interactions (chain order and chain orientation). In addition, the chain segment orientation model is developed to describe roughly the alkyl chains in the monolayers in the liquidexpanded phase. Experimental Section Materials. The synthesis of N-octadecanoyl-L-alanine sample was described previously.21 The chemical reagents used were of analytical grade, and the water used was double distilled after a deionized exchange. The pure water was adjusted to pH 3.0 by the addition of HCl, and the pH values of the ion-containing subphases (1 mM, AgNO3, PbCl2, CaCl2, and CuCl2) were not adjusted with HCl or NaOH. IRRAS Measurements. IRRAS measurements of N-octadecanoyl-L-alanine monolayers at the air-water interface were performed on an Equinox 55 FT-IR spectrometer (Bruker, Germany) equipped with an external narrow band mercurycadmium telluride (MCT) detector using a Bruker XA-511 external reflection attachment. A Langmuir trough was fixed on a shuttle device which allowed for switching the infrared beam between the sample (film-covered surface) and reference (film-free surface) troughs. The shuttle, driven by a computercontrolled stepper motor, allowed interferograms from the reference and sample sides to be collected in an alternating fashion. The temperature was set to 22 °C. The film-forming molecules were originally spread almost as gaseous monolayers from chloroform solution of desired volumes. A time of 20 min was allowed for solvent evaporation, and then the measurement system was enclosed for the equilibrium of humidity and the relaxation of the monolayers for 4 h prior to compression. The monolayers were then compressed discontinuously to the desired surface pressures from 0 mN/m. The spectra were recorded with a resolution of 8 cm-1 by coaddition of 1024 scans unless otherwise specified, and a relatively high signal-to-noise ratio could be obtained by carefully controlling the experimental conditions. The external reflection-absorption spectra of pure water (pH 3.0) and ion-containing solutions (1 mM) were used as references, respectively. All of the spectra were reproducible above the surface pressure of 5 mN/m (the sample spectra overlapped with water vapor bands around 0 mN/m sometimes). The IRRAS spectra were used without any processing or baseline correction. There are three advantages for the use of a shuttle system in the experimental setup:12 the first is a good water vapor match between the reference and sample, the second is the same subphase levels in the reference and sample compartments connected by the three small tubes at the bottom of the trough, and the third is the acquirement of the p- or s-polarized spectra of the same monolayer by switching the KRS-5 polarizer to p- or s-polarized lights. The polarizer efficiency was reported to be about 98-99.2% for the same accessory.22,23 Results and Discussion Chain Orientation in the Monolayers on Pure Water and Ag+-/Pb2+-Containing Subphase. Figures 1-3 show the polarized IRRAS spectra of N-octadecanoyl-L-alanine monolayers on pure water (pH 3.0) and the Ag+-/Pb2+-containing subphase at various angles of incidence. At the 30° incident angle, the corresponding p-polarized spectra have been studied in detail recently.17 On pure water (Figure 1a), the two bands at 2919 and 2851 cm-1 due to the antisymmetric and symmetric CH2 stretching modes [νa(CH2) and νs(CH2)] indicate that the

Figure 1. IRRAS spectra of N-octadecanoyl-L-alanine monolayers at the air-water interface on pure water (pH 3.0) at surface pressure 20 mN/m at 22 °C against different incidence angles: (a) p polarization; (b) s polarization.

alkyl chains predominantly take an ordered conformation.24 The singlet peak at 1472 cm-1 assigned to the CH2 scissoring mode [δ(CH2)] indicates that the alkyl chains in the monolayers are in a triclinic subcell structure where adjacent C-C-C planes are packed in a parallel fashion.25 The 1705 cm-1 peak due to the CdO stretching vibration in the carboxylic acid is associated with the formation of out-of-plane cyclic dimers.17,21 The bands at 3326 (not shown), 1650, and 1539 cm-1 ascribed to amide A [ν(NH)], amide I [ν(CdO)], and amide II [δ(NH)] bands suggest the occurrence of the hydrogen-bonding interaction between the adjacent molecules through their amide groups.17 In the case of Ag+ (Figure 2a), a strong band at 1512 cm-1 and a very weak peak at 1402 cm-1 are attributed to the antisymmetric and symmetric carboxylate stretching vibrations [νa(COO) and νs(COO)], respectively. Compared with the monolayer on pure water, the amide A and amide I bands take a small shift to lower frequencies, suggesting that the intermolecular hydrogen-bonding interaction is slightly enhanced due to the metal complex, and the corresponding νa(CH2), νs(CH2), and δ(CH2) band positions remain unchanged, which suggests that both the conformation (almost all-trans) and the packing (triclinic subcell structure) of the alkyl chains are maintained in the case of Ag+. A bridging bidentate coordination is preferred for the metal complex on the basis of the binding modes of transition metals to amino acids26 and the experimental structures of metalloproteins (linear coordination geometry for silver ions).27 In the case of Pb2+ (Figure 3a), most of the bands are apparently doublet, especially those in the spectrum obtained at 4 cm-1 resolution.17 The two bands at 1639 and 1625 cm-1

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Figure 2. IRRAS spectra of N-octadecanoyl-L-alanine monolayers at the air-water interface on Ag+-containing subphase at surface pressure 20 mN/m at 22 °C against different incidence angles: (a) p polarization; (b) s polarization.

are due to the amide I band, and the two bands at 3310 and 3300 cm-1 (not shown) are assigned to the amide A band.17 It is clear that the intermolecular hydrogen-bonding interaction is substantially strengthened in this case. The broad band containing more than one component with a negative maximum at 1527 cm-1 is due to the νa(COO) vibration,17 and the doublet band at 1423 and 1410 cm-1 is preferentially assigned to the νs(COO) vibration mixed with the δ(CRH2) mode.17,28 It is suggested that the two types of metal complex, chelating and bridging bidentate coordination, are formed according to the studies of LangmuirBlodgett films of lead carboxylate in the literature.29,30 For s-polarized radiation, which is polarized perpendicular to the plane of incidence, the bands are always negative and the intensity decreases with increasing angle of incidence (parts b of Figures 1-3). For p-polarized radiation, which is polarized parallel to the plane of incidence, the bands are initially negative, 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 (parts a of Figures 1-3). The exact position of the Brewster angle φ depends on the wavelength of light and the optical properties of the substrate. For the airwater interface the Brewster angle can be estimated by calculating tan φ ) n2, where n2 is the real part of the H2O refractive index at a given wavenumber (φ ) 54.5° for the νa(CH2) band at 2920 cm-1 (n2 ) 1.402),31 and φ ) 54.2° for the νs(CH2) band at 2850 cm-1 (n2 ) 1.385)31). Beyond the Brewster angle the bands become positive and their intensities decrease upon further increase of the incident angle (except for the case of Pb2+).

Du et al.

Figure 3. IRRAS spectra of N-octadecanoyl-L-alanine monolayers at the air-water interface on Pb2+-containing subphase at 22 °C against different incidence angles: (a) p polarization; (b) s polarization.

For the s polarization (parts b of Figures 1-3), the ratio of the νa(CH2) band intensity to the νs(CH2) one [νa(CH2)/νs(CH2)] is greater than 1 and remains almost unchanged with angle of incidence on various kinds of subphases; however, the ratio takes an obvious change for the p polarization. Below the Brewster angle, the extent of the increase in the νs(CH2) band intensity is more than that in the νa(CH2) one in the case of pure water, so the ratio is reduced to be comparable to 1 at the incident angles of 45° and 48°. Beyond the Brewster angle, the two bands are positive-oriented, but the νa(CH2) band intensity is considerably reduced in comparison with the νs(CH2) one. In the presence of Ag+, the ratio is further lowered to be less than 1 up to the incident angle of 45° below the Brewster angle, and beyond the Brewster angle the case is similar to that on pure water. In the presence of Pb2+, the νa(CH2) band intensities are almost smaller than the νs(CH2) ones in the range of the incident angles investigated below the Brewster angle. Beyond the Brewster angle the νs(CH2) bands become positive, but the νa(CH2) bands still remain negative-oriented with weak intensities. The above unusual cases are scarcely encountered in classical IRRAS spectra before (excluding polarization modulation (PM)IRRAS due to a different surface selection rule), particularly for the case of the presence of Pb2+. It is known that both methylene transition moment directions are perpendicular to the alkyl chains, with the νs(CH2) transition moment being oriented along the bisector of the methylene H-C-H bond angle, while the νa(CH2) transition moment is perpendicular to this. For s polarization, a methylene stretching band intensity will become maximal, when all transition moments are oriented horizontally; i.e., the alkyl chains are perpendicular to the water surface. For

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Figure 4. Schematic representation of chain orientation of monolayers at the air-water interface on pure water and Ag+-/Pb2+-containing subphase with their C-C-C planes parallel to the water surface (a) in comparison with the planes perpendicular to the surface (b).

s polarization of the IR beam, bands are always negative and the intensity decreases with increasing angles of incidence. For p polarization of the IR beam, the bands are initially negative and the band intensities increase with increasing angle of incidence until the Brewster angle is reached. Beyond the Brewster angle the band becomes positive and the band intensities decrease upon further increase of the angle of incidence. For a vibration with a transition moment along the surface normal (perpendicular to the water surface), the situation is reversed: for incident angles smaller than the Brewster angle a positive band will be observed, while for larger angles a negative band will be found.32 The spectral changes in Figures 1-3 indicate that the C-C-C planes of the alkyl chains are preferentially oriented parallel to the water surface, with the νs(CH2) transition dipole moments parallel to the water surface and the νa(CH2) ones perpendicular to the surface normal more or less, which is schematically illustrated in Figure 4. The apparent reduction in the νa(CH2) intensity results from the opposite signs of the bands with the directional transition dipole moments oriented parallel to the surface normal. It is obvious that the C-C-C planes of the alkyl chains in the case of Pb2+ are inclined to the water surface more than those on pure water and the Ag+-containing subphase. Blume et al.33 recently presented the simulation of IRRAS spectra of different orientations of a β-sheet secondary structure on water, but they only observed the β-sheets of different isomers aligned almost parallel to the water surface (parallel orientation). The δ(CH2) bands at 1472 cm-1 in the cases of pure water and Ag+ also indicate that the C-C-C planes of the alkyl chains are packed in a parallel fashion.25 The δ(CH2) bands in the case of Pb2+ at 4 cm-1 resolution appear at 1471 cm-1.17 On the other hand, the formation of the intermolecular hydrogen bonds CdO‚‚‚H-N between the adjacent amide groups would result in the parallel packing of the C-C-C planes of the corresponding chains. It is obvious that the preferential orientation of the alkyl chains is closely related to the intermolecular hydrogen bond and metal complex via covalent bonds within the headgroups. The surface selection rule for the air-water interface is different from that for the metal surfaces, because water is a low absorbing substrate and the intensity of the reflected beam is much lower than the intensity of the incoming beam. For the metal surfaces, the mean square electric field intensities parallel to the surface are negligibly small, and for the electric field component perpendicular to the surface, the mean square electric field intensity is large enough to sample monolayer-associated normal modes that are oriented vertical to the surface. The

Figure 5. IRRAS spectra of N-octadecanoyl-L-alanine monolayers at the air-water interface on Ca2+-containing subphase at surface pressure 20 mN/m at 22 °C against different incidence angles: (a) p polarization; (b) s polarization.

normal transmission IR spectrum (electric field vector parallel to the substrate surface) and the grazing incidence reflection (GIR) IR spectrum (electric field vector perpendicular to the substrate surface) of the N-octadecanoyl-L-alanine LangmuirBlodgett (LB) films on the CaF2 and silver-coated quartz substrates were comparred.34 A remarkable decrease in the methylene stretching band intensities in the GIR spectrum indicated that the long hydrocarbon chains were standing up from the surface in the LB films.34 In addition, the intensity ratio of νa(CH2)/νs(CH2) was obviously different between the two infrared modes, in which the νs(CH2) band was substantially reduced in intensity in the GIR IR spectrum.34 This suggested that the C-C-C planes of the alkyl chains were oriented parallel to the substrate surface and did not take on all possible orientations about the chain axis. In this way the νs(CH2) vibration had a large component parallel to the substrate and would therefore absorb strongly in the transmission mode and weakly in the GIR mode. The features of chain orientation in the LB films are consistent with those in the monolayers at the air-water interface. Obviously, the chain orientation in the monolayers at the air-water interface is almost maintained after the monolayers are transferred onto the solid substrate surfaces.35 Chain Orientation in the Monolayers on Ca2+-/Cu2+Containing Subphase. Figures 5 and 6 show the polarized IRRAS spectra of N-octadecanoyl-L-alanine monolayers on Ca2+-/Cu2+-containing subphase at various angles of incidence. Similarly, the p-polarized IRRAS spectra at the 30° incident angle were reported recently.17 The νa(CH2) and νs(CH2) bands appearing at 2922 and 2853 cm-1, respectively, are indicative of the appearance of gauche conformers in the chains. The bands

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Figure 6. IRRAS spectra of N-octadecanoyl-L-alanine monolayers at the air-water interface on Cu2+-containing subphase at surface pressure 20 mN/m at 22 °C against different incidence angles: (a) p polarization; (b) s polarization.

in the 1600-1500 cm-1 region due to the νa(COO) mode are very weak and broad. Such weak features have been attributed to the ionic interaction between the metal ions and the carboxylate groups.10,15 The amide A bands are substantially weakened and broadened around that region due to the formation of intramolecular hydrogen bonds.17 The metal complex and hydrogen bond formation with the headgroups give rise to an alteration in chain conformation. With the increase of incident angle, the s-polarized IRRAS spectra (parts b of Figures 5 and 6) have a change similar to those on pure water and on the Ag+-/Pb2+-containing subphase (parts b of Figures 1-3), and the intensity ratios of νa(CH2)/ νs(CH2) remain nearly constant. The p-polarized spectra (parts a of Figures 5 and 6) display a usual change below and beyond the Brewster angle in contrast with those in the cases of pure water and Ag+-/Pb2+-containing subphase, and the intensity ratio is always greater than 1 and basically remains unchanged except for a little fluctuation around the Brewster angle. These spectral features indicate an isotropic orientation of the hydrocarbon chains. The νa(CH2) and νs(CH2) bands are usually found around 2917 and 2849 cm-1 for an all-trans hydrocarbon chain and shift to 2926 and 2855 cm-1 for a disordered chain (liquidcrystalline state). The alkyl chains of the monolayers on the Ca2+-/Cu2+-containing subphase are not as disordered as in the liquid-crystalline phase of the bulk dispersion.36 Taking the facts mentioned above into account, the alkyl chains are conjectured to be composed of a few small all-trans chain segments connected by several kinks of gauche conformers. Although the hydrocarbon chains with all-trans conformation are depicted in

Du et al. the theoretical model,6 the actual molecules may contain some gauche conformers in the chains. The chain orientation could be roughly estimated to describe the monolayers at the airwater interface in the liquid-expanded phase on the basis of the model of chain segment orientation, which described well the chain orientation distribution in black soap films consisting of a thin layer of aqueous core enclosed between two surfactant monolayers in the liquid-crystalline phase and interpreted the long-range interaction in the surfactant monolayers and film stability.37 It is well-known that the IRRAS data are defined as plots of reflectance-absorbance (RA) vs wavenumber. RA is defined as -log(R/R0), where R and R0 are the reflectivities of the filmcovered and film-free surfaces, respectively. Three different theoretical optical models have been developed to simulate RA by Schopper,38 Kuzmin and Michailov,39,40 and Yamamato and Ishida,41 and similar results were obtained from the three formulations upon computer simulation. In this paper, we use Kuzmin and Michailov’s model39,40 to describe IRRAS band intensities.6,7,12 Herein, the νs(CH2) bands are used for the evaluation of tilt angle of the chains since there is no interference from other bands in this region. The following parameters are required to calculate an RA value using Kuzmin and Michailov’s formulation:39,40 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;31 angle between dipole moment vector and chain axis, R ) 90° for the νs(CH2) mode; film thickness, d, obtained by taking into account the tilt angle of chains θ and extended length of a molecule L; ordinary and extraordinary refractive indices of hydrocarbon chains in the mid-IR region, nord ) 1.46 and next ) 1.57,6 and corresponding directional refractive indices of the film, nx ) ny and nz, obtained from nord, next, and θ; 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. Here only two unknowns remain, kmax and θ. We use a series of kmax values by computer simulation to calculate RA values to fit the measured RA data. The tilt angle θ should be obtained subsequently by the best fit. Parts a and b of Figure 7 show the theoretical RA (solid lines) and measured data (square symbols) of the νs(CH2) bands of the N-octadecanoyl-L-alanine monolayers in the cases of Ca2+ and Cu2+, respectively, against incident angle at various orientation angles by using the obtained kmax value of 0.2. The chain segments in the two monolayers are evaluated to be inclined at 40° and 35° from the surface normal on the Ca2+and Cu2+-containing subphases, respectively, which is schematically represented in Figure 8. The large tilt angle of the chains in the cases of Ca2+ and Cu2+ is due to the expansion of the monolayers caused by the ionic interaction in the metal complex and intramolecular hydrogen bond formation. Thomas et al.42 investigated the relative positions of chains in the cetyltrimethylammonium bromide (CTAB) monolayer at the air-water interface at 30 °C using neutron reflectivity. They found that the chains were strongly tilted away from the surface normal and that the tilt became greater as the surface concentration was reduced. The above results indicate that the chain orientation in the monolayers is strongly dependent on metal complexing and hydrogen bonding with the headgroups. The hydrophilic interactions between the headgroups, such as metal complexing and hydrogen bonding, have a strong influence not only on chain conformation but also on chain orientation to reach a new interaction balance between the headgroups and alkyl chains.

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J. Phys. Chem. B, Vol. 109, No. 15, 2005 7433 Brewster angle, the νa(CH2) band intensities are substantially reduced in comparison with the νs(CH2) ones in the cases of pure water and Ag+, but the νa(CH2) bands still remain negativeoriented in the case of Pb2+. The unusual spectral features indicate that the alkyl chains take a preferential orientation with their C-C-C planes parallel to the water surface. The parallel packing of the alkyl chains results from the intermolecular hydrogen bonds CdO‚‚‚H-N between the neighboring amide groups, strengthened by the metal complex of covalent interaction. On the Ca2+-/Cu2+-containing subphase, the corresponding polarized spectra display a usual behavior. The alkyl chains are roughly estimated to be inclined around 35-40° from the surface normal with the assumption of chain segment orientation for the monolayers in the liquid-expanded phase. The chain conformation and chain tilt are closely related with the formation of intramolecular hydrogen bonds and the ionic interaction of the metal complex in the two cases. Acknowledgment. This work was financially supported by the Natural Science Foundation of China (Grant 20303008), Ministry of Education, and Nanjing University. References and Notes

Figure 7. Comparison of simulated (solid lines) and measured (square symbols) νs(CH2) band intensities for p and s polarization. The surface film parameters for the simulation are kmax ) 0.2, Γ (the degree of polarization) ) 0.020, L ) 2.8 nm, R ) 90°: (a) θ ≈ 40° on Ca2+containing subphase; (b) θ ≈ 35° on Cu2+-containing subphase.

Figure 8. Schematic illustration of chain segment orientation of monolayers at the air-water interface on the Ca2+-/Cu2+-containing subphase in the liquid-expanded phase.

Conclusions The metal complex and hydrogen bond formation with the headgroups of the N-octadecanoyl-L-alanine monolayers at the air-water interface on pure water and ion-containing subphases give rise to a change in chain order depending on metal ion in the subphase. On pure water and Ag+-/Pb2+-containing subphase, the antisymmetric CH2 stretching band intensity [νa(CH2)] takes a slower increase than the symmetric one [νs(CH2)] below the Brewster angle, so their intensity ratios of νa(CH2)/νs(CH2) are less than 1 in the cases of Ag+ and Pb2+. Beyond the

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