Polarization Modulation Infrared Reflection Absorption Spectroscopy

The νas(C-O-C) mode gives a negative band, implying ... Infrared transmission and reflection measurements of the transferred monolayer were applied t...
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Langmuir 2002, 18, 9422-9428

Polarization Modulation Infrared Reflection Absorption Spectroscopy of Gibbs Monolayer at the Air/Water Interface Md. Nazrul Islam, Yanzhi Ren, and Teiji Kato* Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan Received June 20, 2002. In Final Form: September 10, 2002 Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate Gibbs monolayers of ethylene glycol mono-n-dodecyl ether (EGDE), spontaneously adsorbed at the interface of air and aqueous EGDE subphase. Brewster angle microscopic images reveal that the EGDE monolayer forms circular domains in a condensed state. The PM-IRRAS measurements reveal a νas(CH2) band at 2918.2 cm-1 and a δ(CH2) band at 1469.2 cm-1, indicating respectively the planar trans zigzag conformation and crystalline packing of the -(CH2)11- chain. The νas(C-O-C) mode gives a negative band, implying that the ether linkage preferentially stands up in the Gibbs monolayer. The Gibbs monolayer was successfully transferred onto solid substrates such as Au and CaF2, using the conventional Langmuir-Blodgett technique. Infrared transmission and reflection measurements of the transferred monolayer were applied to obtain the anisotropic complex refractive indexes. Using these indexes, the PM-IRRAS spectrum of the EGDE monolayer at the air/subphase interface was simulated and compared to the experimental one. It is concluded that the -(CH2)11- chain is tilted at 21° with respect to the surface normal and twisted at 56° on the aqueous subphase. The physical act of LB transfer onto solid substrates has caused reorientation of the -(CH2)11- chain and the ether linkage as well as deterioration of the chain conformational order.

Introduction In recent years, there has been a growing interest in in situ characterization of Langmuir monolayers at the air/ water interface using infrared spectroscopy. 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 whereas others5-13 like to use polarization-modulated IRRAS (PM-IRRAS) at the air-water interface. As a surface sensitive technique, PM-IRRAS technique can * To whom correspondence should be addressed. Phone: +8128-689-6170. Fax: +81-28-689-6170. 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. 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. (13) Ren, Y. Z.; Iimura, K.; Kato, T. J. Phys. Chem. B 2002, 106, 1327.

provide details of molecular orientation and packing of the molecules in the monolayer. This mode involves the use of a photoelastic modulator (PEM), which modulates the polarization state of the incident electric field at a frequency of tens of thousands of hertz. The advantage over the ordinary IRRAS mode is that modulated reflectivity is independent of isotropic adsorption from gas or bulk water, so that the interfering effects of water vapor and carbon dioxide can fairly be eliminated. In this paper, we endeavor to extend the PM-IRRAS technique to Gibbs monolayers. Although Langmuir monolayers are known as spread monolayers, Gibbs monolayers are known as adsorbed monolayers. The fabrication of Langmuir monolayers involves dissolving amphiphilic molecules into an evaporative solvent and spreading it at the surface of an aqueous subphase with a microsyringe. On the other hand, the formation of Gibbs monolayers involves the spontaneous adsorption of the molecules dissolved in water onto the air/subphase interface. The spontaneous adsorption of the molecules at the interface results in an increase of surface pressure (π) with time (t) showing a dramatic change of morphological feature of the adsorbed molecules. After a definite period of time, the π-t adsorption isotherm experiences a conspicuous cusp point followed by a plateau region indicating a characteristic feature of a first-order phase transition. Using a Brewster angle microscope (BAM), several research groups14-16 including ours have characterized a wide variety of texture in the condensed phase domains in the adsorbed monolayers that appear just after (14) (a) He´non, S.; Meunier, J. Thin Solid Films 1992, 210/211, 121. (b) He´non, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148. (15) (a) Vollhardt, D.; Melzer, V. J. Phys. Chem. B 1997, 101, 3370. (b) Melzer, V.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 591. (c) Vollhardt, D.; Fainerman, V. B.; Emrich, G. J. Phys. Chem. B 2000, 104, 8536. (16) (a) Hossain, M. M.; Masaaki, Y.; Kato, T. Langmuir 2000, 16, 3345. (b) Hossain, M. M.; Kato, T. Langmuir 2000, 16, 10175. (C) Hossain, M. M.; Suzuki, T.; Kato, T. Langmuir 2000, 16, 9109.

10.1021/la0205708 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/30/2002

PM-IRRAS at the Air/Water Interface

the appearance of the cusp point in the adsorption isotherms. Although amphiphile molecules dissolved in the aqueous media undergo spontaneous adsorption at the interface showing a gradual increase of surface pressure, in addition to temperature and dipolar interactions, van der Waals interactions between the alkyl chains govern the monolayer state. Thus, at a definite temperature, condensed domain formation is possible only when the van der Waals interactions are dominant over the dipolar repulsive interactions between the surfactant heads. Of crucial importance to the PM-IRRAS spectroscopy of Gibbs monolayers 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 inplane components in the sense parallel to the subphase surface, and z stands for the out-of-plane component in the sense perpendicular to the subphase surface. We transferred the Gibbs monolayer onto solid substrates using the conventional Langmuir-Blodgett (LB) technique. The in-plane complex refractive indexes can be obtained from normal transmission measurements of the deposited monolayer on transparent substrates such as CaF2, ZnSe, or Si. In the normal transmission mode, the electric vector of the incident light interacts solely with the in-plane component of the optical constant. The out-of-plane complex refractive indexes can be obtained from the grazing incidence reflection (GIR) measurements of the deposited monolayer on reflective substrates such as Au or Al. In the GIR mode, the electric vector interacts almost exclusively with the out-of-plane component of the optical constant. On the basis of these optical constants, the PM-IRRAS spectrum of the Gibbs monolayer on the aqueous subphase can be simulated. Comparing the simulated spectrum with the experimental one, the molecular orientation on the subphase can be predicted. In the present study, we use film balance and Brewster angle microscopy to understand the adsorption kinetics and surface morphology of the adsorbed monolayer of EGDE at different concentrations. This study allows us to know the nature of the adsorbed monolayer at the airwater interface. To obtain more information about the molecular packing and orientation of the adsorbed molecules in a condensed phase at the air/water interface, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) has been used. The experiments on PM-IRRAS were performed after the surface pressure had attained the equilibrium value. Simulation of the IR spectra based on the optical constant of the monolayer allowed the interpretation of the PM-IRRAS spectra and effect of transfer of monolayer to solid substrates. Materials and Methods Adsorption Kinetics and Surface Morphology. The amphiphile EGDE (CH3(CH2)11OCH2CH2OH) was supplied by Nikko Chemicals Co. Ltd. Tokyo, Japan with a purity of >99% and was used as received. The experimental setup for the adsorption kinetics and surface morphology study was equipped with a home-built Langmuir trough of dimension 41 cm × 16 cm × 2 mm above which a Brewster angle microscope (BAM) was mounted. The surface pressure was measured by the Wilhelmey method using a small rectangular glass plate. The BAM is composed of a 20 mW semiconductor laser, a GlanThompson polarizer, an analyzer, a zooming microscope with a CCD camera of high sensitivity, a TV monitor, and

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a video recording system. Images recorded were treated with an image processing software to maximize the contrast and to correct the distortion of the images caused by the oblique glancing of the microscope. The experiments were carried out pouring a definite amount of aqueous solution into the trough. To attain equilibrium with desired experimental temperature, the solution was allowed to stand for about 25 min before starting the experiment. The molecules already adsorbed at the solution surface during this span of time were removed by sweeping the surface by the movable Teflon barriers. Under these conditions, the surface concentration of the amphiphile molecules can be considered to be zero, i.e., π ) 0 at t ) 0. The increase of surface pressure was then followed with time, and simultaneously, the surface of the aqueous solution was observed by BAM. PM-IRRAS Measurements and LB Deposition. A small Langmuir trough of dimensions 10 cm × 8.5 cm × 2 mm was used for PM-IRRAS experiments. The trough was placed in a sealed box. The subphase temperature was set to 5 °C and was detected by a platinum wire temperature sensor. For carbon dioxide and water vapor to be reduced, the assembly was continuously purged at a constant speed from a Whatmann 75-52 JA purge gas generator. 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 2 × 50 ) 100 kHz. In the PM-IRRAS experiment, the incidence angle was 75°, the half-wave-retardation frequency was 1408 cm-1, the mirror speed was 0.6328 cm/s, the resolution was 8 cm-1, and the scan number was 1500. The PM-IRRAS spectra were reported as S/S′ - 1, where S and S′ represent the monolayer-covered and uncovered subphase, respectively. LB transfer of the Gibbs monolayer was started after the equilibrium adsorption was reached, using the horizontal scooping-up method.17 We applied this technique to transfer the Gibbs monolayer onto solid substrates considering the fact that, at equilibrium surface pressure of 49.7 mN/m, the surface is fully covered by the condensed adsorbed films and the molecules are uniformly distributed at the air-water interface. The subphase temperature was 278 K, and the deposition speed was 0.5 mm/min. The monolayer was deposited only one time. Infrared spectra of the deposited monolayer were measured at a resolution of 4 cm-1 with 500 scans, using the Nicolet Magna 860 infrared spectrometer and a mercurycadmium-telluride detector. The normal transmission spectra were measured for a single monolayer deposited on both sides of a CaF2 substrate and the transmission absorbance was divided by 2. The grazing incidence reflection spectra were measured for a single monolayer deposited on Au-coated glass substrates with p-polarized light incident at 85°, using the Series 501 reflection accessory (Spectra-Tech, Inc.). The GIR spectra were reported as the reflection absorbance -AR ) -log(Rp/Rp′), where Rp is the reflectance of the monolayer-covered Au substrate and Rp′ is that of the bare Au substrate. The peak frequencies were determined by the “center of mass” method, using the software Grams/386. Otherwise explicitly stated, we claim an accuracy of (0.1 cm-1. The orientation angles have an accuracy of (0.5°, if they are to be compared within the scope of this paper. (17) Kato T. Jpn. J. Appl. Phys. 2002, 27, L1358.

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Figure 2. Two-dimensional plot of the PM-IRRAS spectra in Figure 2 at (a) θ1 ) 75° and φ ) 90°; (b) θ1 ) 25° and φ ) 90°; (c) θ1 ) 75° and φ ) 0°; and (d) θ1 ) 25° and φ ) 0°. n∞ ) 1.4, d ) 2.0 nm, and νretard ) 1408 cm-1.

Figure 1. (a) π-t adsorption kinetics of aqueous solution of EGDE at different concentrations: (I) 1.0 × 10-5 M, (II) 1.3 × 10-5 M, and (III) 2.0 × 10-5 M at 278 K. (b) BAM images of Gibbs monolayer adsorbed at the interface between air and aqueous EGDE solution of concentration 1.3 × 10-5 M at 278 K.

Results and Discussion Adsorption Kinetics and Surface Morphology. Figure 1a shows the π-t adsorption isotherms of EGDE at different concentrations at 5 °C. With time, the spontaneous adsorption of the EGDE molecules at the solution surface results in a gradual increase of surface pressure. When the surface pressure attains a definite value at a definite temperature, the π-t adsorption isotherms show a conspicuous cusp point followed by a pronounced plateau region. Such a feature is characteristic of a first-order phase transition from a lower density liquid expanded (LE) phase to a higher density liquid condensed (LC) phase.14-16 The cusp point moves to early times with increasing the concentration of the solution. After the plateau region, the surface pressure experiences a second rise of the values that leads to global equilibrium. For PM-IRRAS measurement, we used a 3.0 × 10-5 M solution so that the system attains equilibrium surface pressure rapidly. The collection of PM-IRRAS spectra and LB deposition of the monolayer onto solid substrates were performed after the solution had reached the equilibrium surface pressure to ensure the homogeneity of the film. At this concentration, the higher rate of adsorption (data not shown) makes it rather difficult to measure the adsorption kinetics accurately. However, measurement of equilibrium surface pressure reveals that 30 min is sufficient to reach the global equilibrium value at this concentration. The condensed phase domains observed by BAM further confirm the existence of the first-order phase transition in the adsorbed layers of the amphiphile. Figure 1b shows the pattern of condensed domains surrounded by the liquid-expanded phase formed with a 1.3 × 10-5 M solution at 5 °C. The structures formed in the initial stage are found to be irregularly shaped (image A) at this temperature, which undergo relaxation to a circular shape (image B) with time. Such a feature is obviously due to the existence of a competition between the growth rate instability and the relaxation rate governed by the line tension that favors circular shape to minimize the perimeter of a domain. With higher concentration of

solution, the formation of condensed domains occurs very rapidly. The domains formed under this condition do not get enough time to be relaxed to circular shape and the surface becomes fully covered by the densely packed condensed domains. Details of the morphological feature of the EGDE monolayers are found to be elsewhere.18 PM-IRRAS Spectroscopy. Optimal Incidence Angle. The three-phase model described in our previous paper19 was applied to treat the PM-IRRAS spectra of EGDE monolayer at the air-water interface. This model describes that during the accumulation of LB multiplayers the interlayer interaction can result in a reorganization of the molecules. As a consequence, the molecular orientation and crystal lattice in a multiplayer can differ from those of a single monolayer. Therefore, the complex refractive indexes of a single monolayer deposited on a solid substrate rather than a multiplayer should be used in the PM-IRRAS simulation. An isotropic material is constructed that has a Lorentzian band with νc ) 1122 cm-1, kc ) 0.2, and Λ ) 26 cm-1. νc is the frequency corresponding to the maximum extinction coefficient kc, and Λ is the full-width-at-half-maximum (fwhm). This band has been defined to imitate the antisymmetric C-O-C stretching band of the ether linkage of EGDE. According to Ohta and Ishida,20 the complex refractive indexes of this isotropic material at ν > 0 can be calculated through

kiso )

kc(Λ/2)2 (ν - νc)2 + (Λ/2)2

(1)

and

niso ) n∞ -

kc(Λ/2ν - νc) (ν - νc)2 + (Λ/2)2

(2)

Here, n∞ is the real refractive index in the dispersion tail of the Lorentzian band and is assumed to be 1.4. Figure 2 shows the two-dimensional plot of the simulated PM-IRRAS spectra of spectra with variable orientation angle φ of the transition dipole moment (TDM). The peak frequencies have an inaccuracy of (0.005 cm-1. For the same TDM orientation angle of φ ) 90°, the peak frequency is 1120.73 cm-1 in Figure 2a at θ1 ) 75° and (18) Islam, M. N.; Kato, T. J. Colloid Interface Sci. 2002, 252, 365. (19) Ren, Y.; Kato, T. Langmuir, accepted. (20) Ohta, K.; Ishida, H. Appl. Spectrosc. 1988, 42, 952.

PM-IRRAS at the Air/Water Interface

1120.76 cm-1 in Figure 2b at θ1 )25°. Therefore, the peak frequency shift, caused by the incidence angle change, is negligible. This conclusion can also be drawn by comparing parts c and d of Figure 2, which give peak frequencies at 1130.04 and 1130.25 cm-1, respectively. On the other hand, the peak frequency has significant shifts with the variation of the TDM orientation angle. From parts a and c of Figure 2, the peak frequency shifts from 1120.73 to 1130.04 cm-1 with φ changing from 90° to 0°. The spectrum in Figure 2c is distinguished from other spectra and is specially drawn by a dashed line. Its negative band sign and higher frequency shift correspond to the spectral feature exhibited by the νas(C-O-C) band in the Gibbs monolayer of EGDE (see below). Separating -(CH2)11- and -(CH2)2-. The surfactant EGDE has a long hydrophobic part and a bulky hydrophilic part. The former contains a -(CH2)11- chain and the latter contains a -(CH2)2- moiety. The -(CH2)11- chain is expected to be orderly packed and well oriented in the adsorbed monolayer, considering the favorable van der Waals interactions between long hydrocarbon chains. On the other hand, the -(CH2)2- moiety most probably exists in an amorphous state. Previous infrared studies21 have shown that the peak frequencies of νas(CH2), νs(CH2), and δ(CH2) bands are sensitive indicators for the chain conformation and crystalline packing. The νas(CH2) and νs(CH2) modes of a planar trans zigzag polymethylene chain give lower frequencies (below 2920 and 2851 cm-1, respectively), whereas those of an amorphous chain in the liquid state give higher frequencies (∼2928 and ∼2856 cm-1, respectively). The δ(CH2) mode of a trans zigzag polymethylene chain depends on the crystal lattice, whereas that of an amorphous chain gives a broad band around 1465 cm-1. 22 We shall assume that the vibrational features of the -(CH2)2- moiety resemble those of an amorphous hydrocarbon chain both in the adsorbed monolayer and in the deposited monolayer. It has three Lorentzian bands at 2928, 2856, and 1465 cm-1 with fwhm of 20, 15, and 15 cm-1 and maximum extinction coefficients of 0.04, 0.03, and 0.01, respectively. The thickness d of the EGDE monolayer is estimated to be approximately 2.0 nm, i.e., 1.5 nm of the hydrophobic part plus 0.5 nm of the hydrophilic part. Using these parameters, the transmission spectrum arising from the -(CH2)2- moiety alone on a CaF2 substrate is simulated and shown in Figure 3c. Figure 3a shows the experimental spectrum of the whole monolayer on a CaF2 substrate. Figure 3b is obtained by (21) Weers, J. G.;Scheuing, D. R. In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.; American Chemical Society: Washington, DC, 1991; p 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]. (22) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press, Inc.: San Diego, CA, 1991; p 63 (about aliphatic ethers). (23) This error has been estimated for the normal transmission spectra of monolayers of common organic materials with real refractive indexes of 1.1∼1.7 and extinction coefficients of 0∼0.5. The CaF2 substrate is 1 mm thick, and the spectral resolution is 4 cm-1. According to K. Ohta and H. Ishida (Appl. Opt. 1990, 29, 2466), the multiple reflections inside such a CaF2 substrate do “not interfere” with each other. The light beam that can finally arrive at the detector should experience even (0, 2, 4, 6, ...) times of reflection inside the CaF2 substrate. Each reflection at the monolayer/CaF2 interface reduces the light intensity to at most 1/20 and double reflection leaves a ratio at most 1/400. Vandevyver et al. have also argued that the multiple reflections inside a transparent substrate are negligible when the substrate refractive index is not great. See: Vandevyver, M.; Barraud, A.; RaudelTeixier; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571.

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Figure 3. Normal transmission spectra of one-layer EGDE deposited from the aqueous EGDE solution of concentration 3.0 × 10-5 M at 278 K onto a CaF2 substrate. (a) Experimental spectrum of the whole monolayer; (b) subtracted spectrum; (c) subtrahend spectrum of the -(CH2)2- part. Table 1. Assignment of the Bands in Figure 4b for the CH3(CH2)11-O-(CH2)2-OH Monolayer without the -(CH2)2- Part modea

frequency (cm-1)

νas(CH3) νas(CH2) νs(CH2) δ(CH2) δas(CH3) δs(CH3) νas(C-O-C) νas(C-C-OH)

2956 2922 2852 1467 1458 1378 1121 1068

polarizationb ⊥ the -(CH2)11- chain axis ⊥ the -(CH2)11- chain axis ⊥ the -(CH2)11- chain axis along the ether linkage

a ν , symmetric stretch; ν , antisymmetric stretch; δ, deformation. s as ⊥, perpendicular to. The ether linkage is defined as the image line joining the two carbon atoms of C-O-C.

b

subtracting Figure 3c from Figure 3a. It is deemed to represent the transmission spectrum of the EGDE monolayer without the -(CH2)2- part. Table 1 lists the assignment of the bands in Figure 3b, based on Lin-Vien et al.’s book.23 The shoulder band at 1458 cm-1 is unambiguously assigned to the asymmetric CH3 deformation mode, based on a comparison with the infrared spectrum of C12H25OH. In-Plane Optical Constants. The three-phase model described in the previous paper19 takes into account the multiple reflections inside the monolayer and neglects the multiple reflections inside the CaF2 substrate. The resulting error is estimated to be about 1/400, which is negligible.24 Under the thin film approximation (d , λ), a first estimation of the imaginary part Imx of x can be obtained from an approximate equation:7

Imx ≈

(1 + N)(1 - 10-A T ) 4πνd

(3)

Here AT is the transmission absorbance in Figure 3b. The real part Rex of x can be derived from Kramers-Kronig transformation of the associated imaginary part:

Rex ) n∞2 +

2 π

∫νν(max) + 0.01

(

ν0Imx

)

ν02 - ν2

dν0 +

ν-0.01 ∫ν(min)

2 π

(

ν0Imx

)

ν02 - ν2

dν0 (4)

where ν(max) and ν(min) are the upper and lower limits of the spectral region of interest. An interval of 0.02 cm-1 (24) Ren, Y.; Kato T. Langmuir 2002, 18, 6699.

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Figure 4. In-plane complex refractive indexes of one-layer EGDE without the -(CH2)2- part.

Figure 6. Out-of-plane complex refractive indexes of one-layer EGDE without the -(CH2)2- part.

where n∞ ) 1.4 and θ1 ) 85°. The real part Rez of z is derived from the Kramers-Kronig transformation of Imz. The out-of-plane complex refractive index is computed through

nz + i kz ) (Rez + i Imz)1/2

Figure 5. GIR spectra of one-layer EGDE deposited from the aqueous EGDE solution of concentration 3.0 × 10-5 M at 278 K onto a gold substrate. (a) Experimental spectrum of the whole monolayer; (b) subtracted spectrum; (c) subtrahend spectrum of the -(CH2)2- part. The incidence angle was 85°.

is excluded from the integration in order to avoid the singularity at the pole ν ) ν0. n∞ is chosen to be 1.4, according to infrared ellipsometric data.25 The in-plane complex refractive index is computed through

nx + i kx ) (Rex + i Imx)1/2

(5)

Using these nx and kx, the normal transmission spectrum of one-layer EGDE without the -(CH2)2- part is simulated and compared to Figure 3b. They superimpose on each other exactly (not shown), indicating that we have obtained the correct nx and kx. Figure 4 shows the nx and kx. Out-of-Plane Optical Constants. Figure 5a shows the experimental GIR spectrum of the EGDE monolayer deposited on a gold substrate. Figure 5c shows the simulated GIR spectrum of the -(CH2)2- part alone. Figure 5b is obtained by subtracting Figure 5c from Figure 5a. It should represent the GIR spectrum of the EGDE monolayer without the -(CH2)2- part. The out-of-plane dielectric constant z can be obtained from the reflection absorbance AR in Figure 5b. Under the thin film approximation (d , λ), a first estimation of the imaginary part Imz of z is calculated through7

Imz ≈

n∞4 cos θ1 (1 - 10-AR) 8πνd sin2 θ1

(6)

(25) Flach, C. R.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. B 1997, 101, 58.

(7)

Using these nz and kz, the GIR spectrum of one-layer EGDE without the -(CH2)2- part is simulated and compared to Figure 5b. They are almost the same (not shown) with minor mismatch, indicating that the kz and nz are basically correct. The extinction coefficient is further calibrated until the minor mismatch disappears. Figure 6 shows the finally obtained complex refractive indexes. Using the complex refractive indexes in Figures 4 and 6, a hypothetical isotropic monolayer of EGDE without the -(CH2)2- part with a complex refractive index

niso + i kiso ) (nx + ny + nz)/3 + i (kx + ky + kz)/3

(8)

is constructed, and its p-polarized reflection absorbance Aiso is evaluated. The GIR spectrum of this isotropic monolayer on a gold substrate is then simulated (not shown) and compared to Figure 5b. From the experimental AR, the orientation angle φ of a TDM relative to the z axis is determined from the following equation:

3 cos2 φ ) AR/Aiso

(9)

where the integrated intensities of the band in the isotropic spectrum and Figure 5b 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 69° and 70°, respectively. The tilt angle R of the -(CH2)11chain relative to the surface normal is calculated to be 30° through

cos2 R + cos2 φas(CH2) + cos2 φs(CH2) ) 1 (10) The TDM orientation angle of the νas(C-O-C) mode is calculated to be 67° with respect to the surface normal. Apparent PM-IRRAS Simulation. Figure 7a shows the experimental PM-IRRAS spectrum of the Gibbs monolayer on the aqueous EGDE subphase. Figure 7c shows the simulated PM-IRRAS spectrum of the -(CH2)2- part alone. Figure 7b is obtained by subtracting Figure 7c from Figure 7a. It should represent the PM-IRRAS spectrum of the EGDE monolayer without the -(CH2)2- part. The νas(CH2) and νs(CH2) bands are found at 2918.2 and 2850.4 cm-1, respectively, in Figure 7b. In the abundant litera-

PM-IRRAS at the Air/Water Interface

Figure 7. PM-IRRAS spectra of the Gibbs monolayer adsorbed at the interface between air and the aqueous EGDE solution of concentration 3.0 × 10-5 M at 278 K. (a) Experimental spectrum of the whole monolayer; (b) subtracted spectrum; (c) subtrahend spectrum of the -(CH2)2- part. The incidence angle was 75°.

Figure 8. Experimental and simulated PM-IRRAS spectra of one-layer EGDE without the -(CH2)2- part at the air/water interface. Spectrum a, solid line, shows Figure 7b again. Spectrum b, dashed line, has been simulated by using the complex refractive indexes displayed in Figures 4 and 6. n∞ ) 1.4, d ) 2.0 nm, θ1 ) 75°, and νretard ) 1408 cm-1.

tures about the infrared spectra of hydrocarbon chains,26 the νas(CH2) frequency (not the νs(CH2) frequency) is regarded as an indicator of the trans/gauche ratio of the hydrocarbon chain. The frequency of 2918.2 cm-1 should imply a planar trans zigzag conformation of the -(CH2)11chain in the Gibbs monolayer. The δ(CH2) band has a high frequency of 1469.2 cm-1, which immediately rejects any possibility of random packing of the -(CH2)11- chain. Therefore, the -(CH2)11- chain is well organized and orderly packed in the Gibbs monolayer. Figure 8 compares this spectrum (solid line) with the simulated one (dashed line). The simulation has used the complex refractive indexes displayed in Figures 5 and 7. This action is equivalent to assume that the molecular orientation of EGDE at the air/subphase interface is the same as that deposited on solid substrates. The first impression is that the νas(C-O-C) band around 1130 cm-1 has opposite signs in parts a and b of Figures 8. The negative band sign in Figure 8b immediately rejects any possibility of random orientation of the ether linkage. According to the PM-IRRAS selection rule, the negative band sign in Figure 8a should imply that the νas(C-O-C) TDM is oriented preferentially along the normal to the subphase surface. The νas(C-O-C) band is polarized along the image line joining the two carbon atoms of the ether linkage. (The C-O-C angle is 112° rather than 180°.) Thus, the ether linkage is oriented preferentially upward on the subphase surface. This feature is to be compared with the preferential parallel orientation (67° with respect to the surface normal) of the νas(C-O-C) TDM after deposition onto solid substrates. (26) For example, see: Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56.

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Figure 9. Angle-dependent simulation of the PM-IRRAS spectra of one-layer EGDE without the -(CH2)2- part at the air/water interface. Spectrum a, solid line, shows Figure 7b again. Spectra b-f have been simulated by assuming TDM orientation angles of 81, 75, 69, 39, and 35°, respectively. n∞ ) 1.4, d ) 2.0 nm, θ1 ) 75°, and νretard ) 1408 cm-1.

Angle-Dependent PM-IRRAS Simulation. To determine the orientation angle of EGDE on the aqueous subphase, we need to construct a hypothetical EGDE monolayer with the orientation angle as a variable and with the -(CH2)2part as omitted. Figure 9a (solid line) is the same as Figure 8a and Figure 7b but displayed for ease of comparison. Parts b and c of Figure 9 show the simulated νs(CH2) band of a hypothetical EGDE monolayer with a TDM orientation angle of 81° and 75°, respectively. The νs(CH2) intensity in parts b and c of Figure 9 is respectively larger and smaller than that in the solid line of Figure 9a. Therefore, the actual orientation angle of the νs(CH2) TDM on the aqueous subphase should be somewhere between 75° and 81°. It is then determined to be 78°. In the same way, the orientation angle of the νas(CH2) TDM is determined to be 72° by comparing parts a, c, and d of Figure 9. The tilt angle R of the -(CH2)11- chain relative to the surface normal is calculated to be 21°. The different orientation angles φas(CH2) ) 72° and φs(CH2) ) 78° suggest that the hydrophobic chain has a twist orientation in the Gibbs monolayer. The twist angle β of the planar trans zigzag hydrocarbon chains is defined as the angle between the C-C-C plane and the plane formed by the chain axis and the surface normal. It is calculated to be 56° through tan β ) cos φas(CH2)/cos φs(CH2). On the other hand, the twist angle is calculated to be 46° for the deposited monolayer on solid substrates. The angle of 46° is close to the trivial twist angle of 45° and actually implies no twist orientation. Therefore, after the LB transfer, the twist orientation has disappeared from the EGDE monolayer. On close examination of the methylene stretching bands in Figure 9, it is seen that the νas(CH2) and νs(CH2) bands shift to higher frequencies after the LB deposition. The νas(CH2) and νs(CH2) bands are found at 2918.2 and 2850.4 cm-1, respectively, in Figure 9a. They are found at 2921.8 and 2852.7 cm-1, respectively, in the spectra (not shown) simulated by assuming a νas(CH2) TDM orientation angle of 72° and a νs(CH2) TDM orientation angle of 78°. The νas(CH2) frequency of 2921.8 cm-1 is generally regarded as indicative of a chain conformation somewhere between fully amorphous and preferentially trans zigzag. The frequency shift from 2918.2 to 2921.8 cm-1 should imply that some gauche conformers have appeared after the LB deposition and the -(CH2)11- conformational order has deteriorated on the solid substrate. The orientation angle of the νas(C-O-C) TDM is determined to be 37° by comparing parts a, e, and f of Figure 9. The νas(C-O-C) band is found at 1134.3 cm-1 in Figure 9a and at 1129.0 cm-1 in the spectrum (not shown) simulated for the TDM orientation angle of 37°.

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This difference may be explained by the different environment of the hydrophilic headgroup in the Gibbs monolayer and in the deposited monolayer. Conclusions PM-IRRAS spectroscopy has been applied to investigate in situ the Gibbs monolayer of EGDE at the air/subphase interface. The PM-IRRAS spectra reveal that both the hydrophobic chain and the hydrophilic headgroup are well organized and meaningfully oriented, consistent with the BAM observation. The two CH2 groups in the hydrophilic head are carefully separated from those in the hydrophobic chain. The anisotropic optical constants of the Gibbs monolayer are obtained from infrared measurements of the transferred monolayer on solid substrates. Using these

Islam et al.

constants, angle-dependent simulation of the PM-IRRAS spectra is carried out. It is found that the hydrophobic chain is tilted at 21° and twisted at 56° in the Gibbs monolayer. After LB transfer onto solid substrates, the hydrophobic chain reorients to 30° and loses the twist orientation. The chain conformational order simultaneously deteriorates. The TDM of the antisymmetric ether stretching mode is oriented at 37° on the aqueous subphase and relaxes to 67° on the solid substrate. Acknowledgment. The authors thank Associate Professor N. Suzuki for helpful discussion. Part of this work was supported by SVBL of Utsunomiya University. LA0205708