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Polarization Modulation Infrared Reflection Absorption Spectroscopy of a Cadmium Arachidate Monolayer at the Air/Water Interface Yanzhi Ren and Teiji Kato* Satellite Venture Business Laboratory, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan Received February 20, 2002. In Final Form: July 10, 2002 A polarization modulation infrared reflection absorption spectrum (PM-IRRAS) of a Langmuir monolayer on the water surface was constructed. The optimal incidence angle, the surface selection rule, and the frequency-orientation correlation were determined. The validity of this theory depends on how the anisotropic complex refractive indexes of the Langmuir monolayer are obtained. It is recommended that these optical constants should be calculated from the infrared spectra of the deposited monolayer on solid substrates. Using the cadmium arachidate (CdA) monolayer on the aqueous Cd2+ subphase as an example, we demonstrated why the optical constants should be obtained from the single monolayer rather than the multilayer deposited on solid substrates. The simulated and experimental PM-IRRAS spectra of the CdA monolayer on the aqueous Cd2+ subphase had similar intensities of the νas(CH2), νs(CH2), νas(COO), and δ(CH2) bands, justifying the optical constants’ determination procedure. They were dissimilar in the νas(CH2) and νas(COO) frequencies, and the δ(CH2) bandwidth, indicating the problem of LB transfer artifacts. Finally, the present simulation procedure was compared to the previous one in which the optical constants were determined from multilayer spectra. The interlayer headgroup interaction is found to be responsible for the anomalous carboxylate stretching intensities in the previously simulated PM-IRRAS spectrum.
Introduction In recent years, there has been a growing trend of using infrared spectroscopy to characterize a Langmuir monolayer at the air/water interface in-situ. The infrared spectroscopy used in the external reflection absorption mode is known as IRRAS. Some spectroscopists prefer to use s-polarized, p-polarized, or unpolarized IRRAS,1-4 while others5-12 like to use polarization-modulated IRRAS (PM-IRRAS) at the air/water interface. The PM-IRRAS mode involves the use of a photoelastic modulator (PEM), which modulates the polarization state of the incident * To whom correspondence should be addressed. Fax: +81-28689-6179. E-mail:
[email protected]. (1) (a) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373. (b) Hunt, R. D.; Mitchell, M. L.; Dluhy, R. A. J. Mol. Struct. 1989, 214, 93. (c) Dluhy, R.; Stephens, S. M.; Widayati, S.; Williams, A. D. Spectrochim. Acta A 1995, 51, 1413. (2) (a) Buontempo, J. T.; Rice, S. A. Appl. Spectrosc. 1992, 46, 725. (b) Buontempo, J. T.; Rice, S. A. J. Chem. Phys. 1993, 98, 5835. (3) (a) Gericke, A.; Michailov, A.; Hu¨hnerfuss, H. Vib. Spectrosc. 1993, 4, 335. (b) Flach, C. R.; Brauner, J. W.; Mendelsohn, R. Biophys. J. 1993, 65, 1994. (c) Mendelsohn, R.; Brauner, J. W.; Gericke, A. Annu. Rev. Phys. Chem. 1995, 46, 305. (d) Flach, C. R.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. B 1997, 101, 58. (4) Sakai, H.; Umemura, J. Langmuir 1998, 14, 6249. (5) Blaudez, D.; Buffeteau, T.; Cornut, J.-C.; Desbat, B.; Escafre, N.; Pezolet, M.; Turlet, J.-M. Appl. Spectrosc. 1993, 47, 869. (6) Blaudez, D.; Turlet, J.-M.; Dufourcq, J.; Bard, D.; Buffeteau, T.; Desbat, B. J. Chem. Soc., Faraday Trans. 1996, 92, 525. (7) Buffeteau, T.; Blaudez, D.; Pe´re´, E.; Desbat, B. J. Phys. Chem. B 1999, 103, 5020. (8) (a) Ren, Y. Z.; Hossain, M.; Iimura, K.; Kato, T. Chem. Phys. Lett. 2000, 325, 503. (b) Ren, Y. Z.; Iimura, K.; Kato, T. Chem. Phys. Lett. 2000, 332, 339. (9) (a) Ren, Y. Z.; Iimura, K.-I.; Kato, T. J. Chem. Phys. 2001, 114, 1949. (b) Ren, Y. Z.; Iimura, K.-I.; Kato, T. J. Chem. Phys. 2001, 114, 6502. (10) Ren, Y. Z.; Iimura, K.-I.; Kato, T. Langmuir 2001, 17, 2688. (11) Ren, Y. Z.; Iimura, K.; Ogawa, A.; Kato, T. J. Phys. Chem. B 2001, 105, 4305. (12) Ren, Y. Z.; Hossain, Md. M.; Iimura, K.; Kato, T. J. Phys. Chem. B 2001, 105, 7723.
electric field at a frequency of tens of thousands of hertz. The PM-IRRAS mode can enhance the absorption feature due to the surface species and eliminate that due to the isotropic environment. Consequently, the signal-to-noise ratio is much improved. However, quantitative analysis of the PM-IRRAS spectra has always been a problem, since the spectral intensity depends on the way the polarization demodulation is done. When a lock-in amplifier is used, the PMIRRAS intensity is to be multiplied by a constant,13 which depends on the gain and filter of the electronic processing. Moreover, the PM-IRRAS intensity suffers from attenuation when the time constant of the lock-in amplifier is high or the mirror velocity of the Michelson interferometer is high. In this respect, the synchronous sampling demodulation (SSD) electronics guarantees that the PMIRRAS intensity has the correct physical meaning. We shall apply the SSD to the air/water interface and construct a quantitative PM-IRRAS spectrum. For this task the first thing is to obtain the anisotropic complex refractive indexes of the monolayer being investigated. The anisotropic complex refractive indexes include the real refractive indexes nx, ny, and nz and the extinction coefficients kx, ky, and kz. Here x and y refer to the in-plane components in the sense parallel to the water surface while z stands for the out-of-plane component perpendicular to the water surface. In this connection, Buffeteau et al. have suggested to deposit the film material into LangmuirBlodgett (LB) multilayers.7 They obtained the in-plane and the out-of-plane optical constants from normalized transmission spectra and grazing incidence reflection spectra of multilayers of CdA, respectively, to simulate the PM-IRRAS spectra of a CdA monolayer on the aqueous Cd2+ subphase. They observed that the simulated and (13) Buffeteau, T.; Desbat, B.; Blaudez, D.; Turlet, J. M. Appl. Spectrosc. 2000, 54, 1646.
10.1021/la020184q CCC: $22.00 © 2002 American Chemical Society Published on Web 09/21/2002
Cadmium Arachidate Monolayer at the Air/Water Interface
experimental PM-IRRAS spectra are similar in the ν(CH2) region but the simulated spectrum is abnormally strong in the ν(COO) region. Our reasoning is that the complex refractive indexes of a single monolayer deposited on solid substrates rather than a multilayer should be used in the PM-IRRAS simulation. During the accumulation of LB multilayers, the interlayer headgroup interaction helps to reorganize the molecules. As a consequence, the molecular orientation and crystal lattice in a multilayer usually differ from those of a single monolayer, particularly for the CdA system. Theory Abele´s and Hansen’s matrix formalism can be simplified to a three-phase model when treating the air/monolayer/ substrate system.14 The first phase is the semiinfinite air with a complex refractive index of 1 + i × 0, where i2 ) -1. The second phase is an uniaxial monolayer with thickness d and complex refractive indexes nx + i × kx ) ny + i × ky and nz + i × kz. Here the uniaxial symmetry is to be understood in the general sense that nx + i × kx ) ny + i × ky * nz + i × kz. It is not to be understood in the special sense that the molecules have a uniaxial distribution around the z-axis and the transition dipole moment (TDM) rotates freely around the molecular axis. The third phase is the semiinfinite substrate or subphase with a complex refractive index N + i × K. The N and K are to be obtained from ref 15, followed by interpolation. Let θ1 and θ3 denote the complex incidence angles in the first and third phases, respectively. According to Snell’s law, we have
sin θ1 ) (N + i × K) sin θ3
(1)
Let the subscripts s and p denote the cases of s- and p-polarized incidence, respectively. Let θ2s and θ2p denote the complex incidence angles in the second phase. We have
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The p-polarized transmittance Tp of the three-phase system is given by
Tp ) 4 Re q3p cos θ1/|{Mp[1,1] cos θ1 + Mp[1,2]q3p cos θ1 + Mp[2,1] + Mp[2,2]q3p|2} (6) where Re q3p refers to the real part of q3p and Mp[i,j] refers to the ijth element of Mp. The p-polarized reflectance Rp of the three-phase system is given by
Rp ) |(Mp[1,1] + Mp[1,2]q3p) cos θ1 - Mp[2,1] Mp[2,2]q3p|2/{|(Mp[1,1] + Mp[1,2]q3p) cos θ1 + Mp[2,1] + Mp[2,2]q3p|2} (7) The s-polarized reflectance, Rs, of the three-phase system is given by
Rs ) |(Ms[1,1] + Ms[1,2]q3s) cos θ1 - Ms[2,1] Ms[2,2]q3s|2/{|(Ms[1,1] + Ms[1,2]q3s) cos θ1 + Ms[2,1] + Ms[2,2]q3s|2} (8) The p-polarized transmission absorbance, AT, is defined as
AT ) -log(Tp/Tp′)
where Tp′ is evaluated in the same way as Tp except that its kx, ky, and kz values are taken to be zero. The p-polarized reflection absorbance AR is defined as
AR ) -log(Rp/Rp′)
S)
q2s ) cos θ2s(nx + i × kx), q2p ) cos θ2p/(nx + i × kx), q3s ) cos θ3(N + i × K), and q3p ) cos θ3/(N + i × K) (3) The phase thicknesses βs and βp of the single monolayer are given by
βs ) 2πνd(nx + i × kx) cos θ2s and βp ) 2πνd(nx + i × kx) cos θ2p (4) The characteristic matrix of the three-phase system is defined as
(
cos β Ms ) -iq ssin β 2s s
-i cos βs/q2s cos βs
(
cos β Mp ) -iq psin β 2p p
)
and -i sin βp/q2p cos βp
)
(5)
(14) (a) Abele´s, F. Ann. Phys. 1948, 3, 504. (b) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380. (15) Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: Orlando, FL, 1985.
(10)
where Rp′ is evaluated in the same way as Rp except that its kx, ky, and kz values are taken to be zero. Considering only the second harmony, the PM-IRRAS signal S is defined as16
sin θ1 ) (nx + i × kx) sin θ2s ) (nz + i × kz) sin θ2p (2) It is convenient to define
(9)
(Rp - Rs)J2(νπ/νretard) (Rp + Rs) ( (Rp - Rs)J0(φ)
(11)
where νretard is the half-wave retardation frequency, J2 and J0 are the second- and zeroth-order Bessel functions, respectively, and the “+” and “-” parts of the symbol “(” correspond to the p- and s-polarized incidences before the PEM, respectively. It is a convention to present the PM-IRRAS spectra as S/(S′ - 1), where S′ is evaluated in the same way as S except that its kx, ky, and kz values are taken to be zero. To determine the orientation angle φ of a TDM relative to the z-axis on metallic substrates from the experimental AR, a hypothetical isotropic monolayer with a complex refractive index
niso + i × kiso ) (nx + ny + nz)/3 + i × (kx + ky + kz)/3 (12) is constructed and its p-polarized reflection absorbance Aiso is evaluated by substituting nx + i × kx ) ny + i × ky everywhere with niso + i × kiso in eqs 1-10. φ is determined through
3 cos2 φ ) AR/Aiso
(13)
(16) Hipps, K. W.; Crosby, G. A. J. Phys. Chem. 1979, 83, 555.
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Figure 1. PM-IRRAS intensities of a hypothetical uniaxial monolayer at the air/water interface versus the TDM orientation angle φ with n∞ set at 1.5 (a) and versus n∞ with φ set at 90° (b). d ) 2.7 nm.
To determine the orientation angle φ of a TDM from the experimental PM-IRRAS spectra, we need to substitute nx + i × kx ) ny + i × ky with 1.5niso sin2 φ + 0.5n∞(3 cos2 φ - 1) + i × 1.5kiso sin2 φ and nz + i × kz with n∞ + 3(niso - n∞) cos2 φ + i × 3kiso cos2 φ everywhere in eqs 1-11. Here n∞ is the real refractive index in the dispersion tail of the band in question, where no characteristic absorption occurs. The out-of-plane n∞ is taken equal to the in-plane n∞. Experimental Section The arachidic acid was a gift from Nippon Fat and Oil Co. with a guaranteed purity of 99.9%. The subphase was an aqueous cadmium acetate solution of concentration 5 × 10-4 M and pH ) 6.7, which ensured total deprotonation of arachidic acid. More details about the Langmuir-Blodgett deposition of the single monolayer as well as infrared measurements of the deposited monolayer were to be seen in the previous paper.12 It is stressed again that the Langmuir monolayer was deposited only once. A surface pressure of 20 mN m-1 and a subphase temperature of 293 ( 1 K were maintained during the deposition as well as the PM-IRRAS measurement. The Nicolet Magna 860 infrared spectrometer was equipped with a polarization modulation part. A synchronous sampling demodulator (GWC Instruments, SSD-100) was used to record the PM-IRRAS spectra. The beam was initially p-polarized before the photoelastic modulator, which oscillated at a fixed frequency of 100 kHz. In the PM-IRRAS experiment the incidence angle was 80°, the half-wave-retardation frequency was 1408 cm-1, the mirror speed was 0.9494 cm/s, the resolution was 8 cm-1, and the accumulation number was 3000. The PM-IRRAS spectra were reported as S/S′ - 1, where S and S′ represent the monolayer-covered and uncovered subphase, respectively. All the PM-IRRAS simulations in the present work were carried out at the half-wave-retardation frequency of 1408 cm-1, the incidence angle of 80°, and the initial p-polarization before the PEM. The peak frequencies were determined by the “center of mass” method. Unless otherwise stated, the allowed inaccuracy was (0.1 cm-1.
Results and Discussion Optimal Incidence Angle and Selection Rule. The above three-phase model is first applied to determine the optimal incidence angle in PM-IRRAS experiments at the air/water interface. An isotropic material is constructed that has a Lorentzian band with νc ) 2920 cm-1, kc ) 0.3, and Λ ) 12 cm-1. νc is the frequency corresponding to the maximum extinction coefficient kc, and Λ is the full-widthat-half-maximum (fwhm). Figure 1a shows the evolution of the PM-IRRAS spectra with the TDM orientation angle φ, where the incidence angle is set at 80°. It is seen that, with φ decreasing from 90° to 0°, the PM-IRRAS intensity decreases from the positive maximum to the negative maximum. Either in the region 80° < φ < 90° or in the region 0° < φ < 10°, the PM-IRRAS intensity changes hardly at all with φ.
Ren and Kato
Figure 2. Experimental (a) and simulated (b) transmission spectra of one-layer cadmium arachidate deposited at 293 K and 20 mN/m onto a CaF2 substrate at normal incidence. n∞ ) 1.5, and d ) 2.64 nm.
However, in the region 30° < φ < 70° the intensity changes rapidly with φ and the PM-IRRAS technique is able to determine the orientation angles in this region. The effect of n∞ on the simulated PM-IRRAS spectra is shown in Figure 1b for the incidence angle 80° and the TDM orientation angle 90°. The PM-IRRAS intensity at 2920 cm-1 increases linearly from 0.0118 to 0.0154 with increasing n∞ from 1.3 to 1.7. It is acceptable to adopt some approximate values of n∞ from the literature for organic materials whose 3000-2800 cm-1 region is dominated by ν(CH2) absorption, which are estimated to be within 1.5 ( 0.05, which corresponds to an error of (3% in the PM-IRRAS intensity. Here, we have used n∞ ) 1.50 for ν(CH2) modes.17 In addition, Buffeteau et al. have used an out-of-plane n∞ of 1.55 and an in-plane n∞ of 1.49 for the CdA system. Optimal Deposition Conditions. Figure 2a shows the normal transmission spectrum of one-layer CdA deposited at 293 K and 20 mN/m onto a CaF2 substrate. The νas(CH2) and νs(CH2) bands are found at 2917.9 and 2850.1 cm-1 with peak heights of 0.0028 and 0.0019, respectively. The normal transmission spectrum of onelayer CdA deposited at 283 K and 20 mN/m is to be seen in Figure 6 of ref 12, where the νas(CH2) and νs(CH2) bands have peak heights of 0.0021 and 0.0014, respectively. The CdA monolayer is orthorhombic on the aqueous Cd2+ subphase at 283 K. After deposition onto the CaF2 substrate, it exhibits a hexagonal subcell packing, as indicated in the δ(CH2) band at 1469.0 cm-1 with an fwhm of 5.5 cm-1.18 In contrast, the CdA monolayer is hexagonal on the aqueous Cd2+ subphase at 293 K. After deposition onto solid substrates, it remains hexagonal, according to the electron diffraction data.19 Therefore, the CdA monolayer should be deposited at 293 K rather than at 283 K from the aqueous subphase. Moreover, Au substrates rather than Al substrates should be used to deposit the CdA monolayer. It is known that fatty acids spontaneously form chemical bonds with the aluminum surface during the LB transfer from the aqueous Cd2+ subphase, while no chemical bonds occur with the gold surface.20 (17) Ren, Y. Z.; Iimura, K.; Kato, T. J. Phys. Chem. B 2002, 106, 1327. (18) Weers, J. G.; Scheuing, D. R. In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.; American Chemical Society: Boston, 1991. Page 91 about the triclinic (a strong, sharp, and narrow singlet δ(CH2) band at 1474-1470 cm-1), hexagonal (a relatively broader singlet at 1469-1468 cm-1), amorphous (a broad band around 1465 cm-1), and orthorhombic packing (splitting to two bands at approximately 1473 and 1462 cm-1) of hydrocarbon chains. (19) Schwartz, D. K.; Garnaes, J.; Viswanathan, R.; Zasadzinski, J. A. N. Science 1992, 257, 508. (20) Ren, Y. Z.; Asanuma, M.; Iimura, K.-I.; Kato, T. J. Chem. Phys. 2001, 114, 923.
Cadmium Arachidate Monolayer at the Air/Water Interface
Figure 3. In-plane complex refractive indexes of one-layer cadmium arachidate deposited at 293 K and 20 mN/m onto a CaF2 substrate. n∞ ) 1.5, and d ) 2.64 nm.
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Figure 5. Experimental (a) and second-time simulated (b) GIR spectra of one-layer cadmium arachidate deposited at 293 K and 20 mN/m onto a gold substrate. Spectrum c is the GIR spectrum of a hypothetical isotropic monolayer of cadmium arachidate on gold. The simulation has used the complex refractive indexes in Figures 4 and 7. n∞ ) 1.5, d ) 2.64 nm, and θ1 ) 85°.
Figure 4. Experimental (a) and first-time simulated (b) GIR spectra of one-layer cadmium arachidate deposited at 293 K and 20 mN/m onto a gold substrate. n∞ ) 1.5, d ) 2.64 nm, and θ1 ) 85°.
Anisotropic Optical Constants. The monolayer thickness d of CdA is known to be 2.64 nm from Buffeteau et al.7,13 As demonstrated in the previous paper,21 the inplane complex refractive indexes can be obtained from the normal transmission spectrum of the single monolayer. Figure 3 shows the nx and kx for one-layer CdA at 20 mN/ m. Using these nx and kx, the normal transmission spectrum of one-layer CdA is simulated and shown in Figure 2b. The exact superimposition of Figure 2a and b suggests that we have obtained the correct nx and kx. Parts a and b of Figure 4 show the experimental (AR(exp)) and first-time simulated (AR(first)) GIR spectra of one-layer CdA deposited on a gold substrate at 293 K and 20 mN/m, respectively. Parts a and b of Figure 5 show the experimental and second-time simulated GIR spectra, respectively. It is seen that the second-time calculated spectrum coincides well with the experimental spectrum. Figure 6 shows the finally obtained out-of-plane complex refractive indexes. Molecular Orientation in the Deposited Monolayer. Using the complex refractive indexes in Figures 3 and 6, a hypothetical isotropic monolayer of CdA is constructed through eq 12. Figure 5c shows the simulated spectrum of this monolayer on a gold substrate. The molecular orientation of CdA in the deposited monolayer is determined through eq 13, where the integrated intensities of the band in Figure 5a and c are used. The orientation angles φas(CH2) and φs(CH2) of the νas(CH2) and νs(CH2) TDMs with respect to the surface normal are calculated to be 77° and 78°, respectively. The tilt angle R of the hydrocarbon chain relative to the surface normal is calculated to be 17° through cos2 R + cos2 φas(CH2) + cos2 φs(CH2) ) 1. (21) Ren, Y. Z.; Kato, T. Langmuir, submitted.
Figure 6. Out-of-plane complex refractive indexes of one-layer cadmium arachidate deposited at 293 K and 20 mN/m onto a gold substrate. n∞ ) 1.5, and d ) 2.64 nm.
The 17° is appreciably larger than the 12° of cadmium arachidate multilayers (7-10 layers) reported by Buffeteau et al. It falls outside the range 3-13°22 reported by other investigators for multilayers of cadmium salts of long chain fatty acids. Thus, reorientation of the hydrocarbon chains has taken place during the LB transfer from one layer to multilayers. This reorientation should have been caused by the unstructured-to-orthorhombic crystallization in CdA multilayers. According to the atomic force microscopic images,19 multilayers of CdA possessing a head-to-head connection generally pack in an orthorhombic way. On close examination of the transmission spectra reported by Buffeteau et al., we observe that the δ(CH2) mode splits, indicating that the CdA multilayer is indeed orthorhombic. The above conclusion can also be drawn directly by comparing the intensity ratio of ν(CH2)/ν(CH3) in the GIR spectra on gold. In Figure 4a of the monolayer spectrum the νas(CH2) band is about four times stronger than the νas(CH3) band and the νs(CH2) band is about two times stronger than the νs(CH3) band. In the multilayer spectrum reported by Buffeteau et al., the νas(CH2) intensity is similar to the νas(CH3) intensity and the νs(CH2) intensity is considerably smaller than the νs(CH3) intensity. (22) (a) Duschl, C.; Knoll, W. J. Chem. Phys. 1988, 88, 4062. (b) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62.
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Table 1. Maximum Extinction Coefficients for Several Infrared Modes in Cadmium Arachidate Monolayers and Multilayers Deposited at 293 K onto Solid Substrates kc values in this paper of a monolayer
kc values in ref 7 of a multilayer
k
νas(CH2)
νs(CH2)
νas(COO)
νs(COO)
νas(CH2)
νs(CH2)
νas(COO)
νs(COO)
kx(max) kz(max)
0.501 0.043
0.364 0.022
0.290 0.020
0.064 0.127
0.483 0.022
0.338 0.013
0.418 0.025
0.068 0.363
According to the surface selection rule of p-polarized GIR measurements on metal substrates, it can be empirically concluded that the νas(CH2) and νs(CH2) TDMs have smaller orientation angles in the monolayer than in the multilayer. The hydrocarbon chain axis, being perpendicular to both the νas(CH2) TDM and the νs(CH2) TDM, must have a larger tilt angle in the CdA monolayer than in the multilayer. The orientation angle of the νas(COO) TDM with respect to the surface normal is calculated to be 79°. The νas(COO) TDM is polarized along the image line joining the two oxygen atoms. It is imaginable that this line lies preferentially flat on the substrate surface. The orientation determination of the νs(COO) TDM is hampered by the multiplicity of this mode. In the GIR spectrum of Figure 4a there is only one band at 1431.2 cm-1 assignable to the νs(COO) mode. In the transmission spectrum of Figure 2, there are two bands assignable to the νs(COO) mode around 1410 and 1440 cm-1. Using the rough equation 3 cos2 φ ≈ kz/kiso, the orientation angles of all the νs(COO) TDMs fall in the safe range 35°-65°. The νs(COO) TDM is polarized along the image line bisecting the O-C-O angle. It is imaginable that this bisecting line has no preferential orientation in the CdA monolayer. The same conclusion also holds for one-layer CdA deposited at 283 K onto gold substrates. On the other hand, it is known from Buffeteau et al. that the COO group is symmetrically anchored in the CdA multilayer, with the orientation angle of the νs(COO) TDM near 0°. Thus, reorientation of the carboxylate group has taken place during the LB deposition from one layer to multilayers. The strong interlayer headgroup interaction should be responsible for this reorientation. PM-IRRAS Simulation on the Aqueous Subphase. Parts a and b of Figure 7 show the experimental and simulated PM-IRRAS spectra of the CdA monolayer on the aqueous Cd2+ subphase at 293 K and 20 mN/m, respectively. The band intensities have no big change between the two spectra, in either the methylene or carboxylate stretching region. On the other hand, Buffeteau et al. observed significant discrepancy in the 14001600 cm-1 region between the experimental and simulated PM-IRRAS spectra. Most probably, this discrepancy results from the large extinction coefficients of the νas(COO) and νs(COO) modes used in their simulation procedure. Table 1 compares the maximum extinction coefficients determined from the CdA monolayer with those from the multilayer. On close examination of the C-H stretching region of Figure 7, it is seen that the νas(CH2) band has a frequency shift from 2917.9 cm-1 in the simulated spectrum to 2916.3 cm-1 in the experimental one (Table 2) implying that some gauche conformers have appeared after the LB deposition. The δ(CH2) fwhm is indicative of the interchain packing order. As listed in Table 2, the δ(CH2) band has an fwhm of 11 ( 0.5 cm-1 in the simulated spectrum and 4 ( 0.5 cm-1 in the experimental spectrum. Thus, the LB transfer process has caused the hexagonal subcell packing on the aqueous subphase to deteriorate into an unstructured state. This deterioration is expected to cause the hydrocarbon chain to be more inclined. Since the νas(CH2) and νs(CH2) TDMs are oriented at 77°-78° on the solid substrate, their
Figure 7. Experimental (a, solid line) and simulated (b, dashed line) PM-IRRAS spectra of a cadmium arachidate monolayer on the aqueous Cd2+ subphase at 293 K and 20 mN/m. Spectrum b has been simulated using the complex refractive indexes displayed in Figures 4 and 7. n∞ ) 1.5, d ) 2.64 nm, and θ1 ) 80°. Table 2. PM-IRRAS Frequencies of Several Infrared Modes for the Cadmium Arachidate Monolayer on the Aqueous Cd2+ Subphase at 20 mN/m and 293 Ka frequency mode νas(CH2) νs(CH2) δ(CH2) δ(CH2) fwhm νas(COO)
simulated
(cm-1)
2917.9 2850.1 1468.1 11 ( 0.5 1543 ( 0.5
experimental (cm-1) 2916.3 2850.1 1468.7 4 ( 0.5 1534 ( 0.5
a Unless otherwise explicitly stated, we allow an inaccuracy of (0.1 cm-1.
orientation angles are expected to be in the range 77°-90° on the aqueous subphase. The νas(COO) frequency is found at 1534 ( 0.5 cm-1 in Figure 7a and 1543 ( 0.5 cm-1 in Figure 7b. This observation is consistent with our previous report.12 Before the compression-induced collapse, the PM-IRRAS frequency of the νas(COO) mode is always at 1534 cm-1 for the monolayer of cadmium behenate, cadmium arachidate, and cadmium stearate in the temperature range 293274 K. After the monolayer collapse, the νas(COO) frequency shifts to 1543 cm-1, indicating that the carboxylate headgroup has left the aqueous environment. To determine the orientation angle of the νas(COO) TDM on the aqueous subphase, we construct a hypothetical CdA monolayer with orientation angle as a variable. Figure 8a is the same as Figure 7a and is shown for ease of comparison. Parts b and c of Figure 8 show the simulated νas(COO) band of a hypothetical CdA monolayer with a TDM orientation angle of 90° and 70°, respectively. The νas(COO) intensities in parts b and c of Figure 8 are respectively larger and smaller than that in the experimental spectrum of Figure 8a. Therefore, the actual orientation angle of the νas(COO) TDM on the aqueous subphase should be somewhere between 70° and 90°. The spectral region of 1400-1450 cm-1 in Figure 7 is mainly due to various νs(COO) modes. Parts a and b of Figure 7 have similar νs(COO) intensities near 1410 cm-1, indicating that the TDM orientation here is basically preserved after the LB transfer. However, a downward (negative) band around 1445 cm-1 appears in the experimental spectrum, indicating that the TDM is more vertically aligned on the aqueous subphase. Parts d and e of Figure 8 show the simulated νs(COO) band by assuming a TDM
Cadmium Arachidate Monolayer at the Air/Water Interface
Langmuir, Vol. 18, No. 22, 2002 8565
νas(COO) band at 1534 cm-1 and the νs(COO) band around 1445 cm-1 is 1534-1445 ) 89 cm-1,24 corresponding to the chelating bidentate coordination. On the other hand, the ∆ for the νs(COO) band around 1410 cm-1 is 15341410 ) 124 cm-1, obviously not assignable to the chelating bidentate interaction. Therefore, multiple coordination modes exist between the cadmium ion and the carboxylate group. In the chelating bidentate coordination mode, the carboxylate group is nearly symmetrically anchored on the aqueous subphase and undergoes a large reorientation after the LB deposition. Figure 8. Experimental (a, solid line) and simulated (b-e, dashed line) PM-IRRAS spectra of a cadmium arachidate monolayer on the aqueous Cd2+ subphase at 293 K and 20 mN/ m. Spectra b and c have been simulated for the νas(COO) band alone by assuming a TDM orientation angle of 90° and 70°, respectively. Spectra d and e have been simulated for the region 1380-1450 cm-1 by assuming a TDM orientation angle of 65° and 0°, respectively. n∞ ) 1.5, d ) 2.64 nm, and θ1 ) 80°.
orientation angle of 65° and 0°, respectively. Parts d and e of Figure 8 resemble Figure 8a around 1410 and 1445 cm-1, respectively. Therefore, the νs(COO) TDM has an orientation angle close to 65° around 1410 cm-1 and close to 0° around 1445 cm-1. According to Nakamoto, 23 the frequency separation ∆ between the νas(COO) and νs(COO) bands can be used as a diagnostic tool to determine the interaction type between the carboxylate head and the metal ion. In general, the ∆ values are 80-110, 135-139, 130-160, and 150-200 cm-1, respectively, for chelating bidentate, ionic, monodentate, and bridging bidentate interactions in an aqueous environment. In the present case, the ∆ between the (23) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1986.
Conclusion The PM-IRRAS theory of a Langmuir monolayer has been presented. An important theme of this theory is how to obtain the anisotropic complex refractive indexes of the monolayer. Using the CdA system as an example, we have demonstrated that optical constants should be determined from the single monolayer rather than the multilayer for the purpose of PM-IRRAS simulation. The experimental PM-IRRAS spectrum was acquired with the aid of a synchronus sampling demodulator (SSD) that guarantees the quantitative aspect of the spectrum. Comparing the experimental spectrum with the simulated one, we have identified some LB transfer artifacts that caused the change in molecular orientation. Acknowledgment. The authors appreciate much the financial support from the Venture Business Laboratory of Utsunomiya University. LA020184Q (24) In Nakamoto’s rule, transmission frequency rather than PMIRRAS frequency of a vibrational mode should be used. The orientationinduced frequency increase of the PM-IRRAS band around 1445 cm-1 is estimated to be less than 5 cm-1.
