Spectral Studies of Cadmium Arachidate Films
The Journal of Physical Chemistry, Vol. 82,No. 18, 1978 1989
X-ray Photoelectron Spectra and Fourier Transform Infrared Spectra of Mono- and Multilayer Films of Cadmium Arachidate T. Ohnlshl,” A. Ishitani,lb H. Ishida,lb N. Yamamoto,” and H. Tsubomura’l’ Department of Chemistty, Faculty of Engineering Science, Osaka Universlty, Toyonaka, Osaka 560, Japan, and Toray Research Center, Toray Industrles Incorporated, Sonoyama 3, Otso 520, Japan (Received May 23, 1978)
Mono- and multilayer films of cadmium arachidate deposited on glass plates by the Langmuir-Blodgett method were studied by X-ray photoelectron spectroscopy (XPS) and infrared attenuated total reflection spectroscopy (IR-ATR). The XPS results showed that the signal of the glass substrate became weaker with an increasing number of fatty acid layers. A very long escape depth (130-180 A) of electrons in the layers was calculated from this result. The Cd signal for the monolayer indicates the presence of an interaction of Cd2+with the glass. The IR-ATR spectra were measured by use of a Fourier transform spectrometer. The results showed a regular perpendicular alignment of the fatty acid chain for layers of various thicknesses and a strong interaction of the carboxylate group with the glass in the monolayer spectrum.
Introduction The structures of Langmuir molecular layers of organic molecules such as fatty acid salts deposited on solid surfaces have been investigated by means of infrared absorption spectro~copy,~-~ X-ray diffraction? and electron diffra~tion.~ The visible and ultraviolet spectra of fatty acid monomolecular layers mixed with organic dyes, such as carotenoid and cyanine dyes, were also reported.l0Jl No reliable infrared spectrum of a monolayer of a fatty acid salt has been obtained. X-ray photoelectron spectroscopy, XPS, has been attracting attention as a powerful method for structure analysis of solid surfaces. XPS studies of Langmuir films, in view of their uniform thicknesses on solid surfaces, are particularly interesting. While a few XPS spectra of thick Langmuir films of fatty acids such as bromostearic acid were reported,12 the XPS spectra of thin Langmuir films including a few molecular layers have been seldom reported.13J4 We have obtained XPS spectra and infrared attenuated total reflection (IR-ATR) spectra of mono- and multilayer films of an arachidic acid salt deposited on glass plates and discussed the structure of the layers, the results of which will be reported in this paper. Experimental Section The methods of the deposition of mono- and multilayer films of cadmium arachidate onto glass plates and the preparation of materials are the same as those described in a previous paper.ll Commercial microscope glass slides (37 X 13 X 1 mm) were used as glass substrates. They were cleaned carefully with a warm chromic acid mixture, followed by washing with water just before use. An mol/L cadmium aqueous solution containing 3 X sulfate and 2 X lo4 mol/L potassium bicarbonate was used as a subphase. For the preparation of crystalline cadmium arachidate, sodium arachidate was precipitated from an ethanol solution of arachidic acid by addition of sodium hydroxide. The sodium ion was then exchanged with the cadmium ion in an aqueous solution. The XPS spectra of the mono- and multilayer films of cadmium arachidate deposited on glass plates were obtained by use of a Kokusaidenki ES 200 spectrometer using A1 Kq,2 radiation at 10 kV and 20 mA. The pressure in Torr. As the samples the sample chamber was 5 X were sometimes colored by X-ray irradiation, all measurements were made at -10 OC in order to minimize 0022-3654/78/2082-1989$01 .OO/O
decomposition. It was confirmed by “wide scanning” that the composition of the sample was not changed by the measurements. The electron kinetic energies, KE, were calibrated using the carbon 1s electron peak of cadmium arachidate, set equal to 1202.0 eV, as an internal reference. The kinetic energies are related with the electron binding energies (BE) by the equation KE = 1486.6 - BE. Electrons emitted along the direction perpendicular to the incident X-ray were analyzed. In most of the measurements, the angle (0) between the line normal to the plane of molecular layers and the direction of the analyzed electron beam was 10’. Measurements at different angles were also made in order to see its effect on the electron intensities. Unless otherwise specified, the data for 0 = 10’ are described in the present paper. The IR-ATR spectra of the mono- and multilayer films were measured by use of a Digilab Model FTS-20 B spectrophotometer, equipped with a standard nichrome wire source, a TGS detector, and a Wilks Model 50 ATR attachment, together with a KRS-5 reflecting plate (50 X 20 X 2 mm) in contact with the layers as shown in Figure 1. In order to increase the signal-to-noise ratio, the ATR spectrum of each of the samples was scanned 625 times and the spectral data accumulated. The logarithm of the ratio, R, of the intensity of the light passing through a KRS plate to that passing through the KRS plate in contact with the sample plate, was recorded against the wave number. In the present work, the angle of incidence at the air-KRS plate interface was 45’ and the number of total reflections at the interface between the KRS and glass plates was 9. The resolution of the spectra was 4 cm-l.
Results and Discussion X P S Spectra. Figure 2 shows the XPS spectra measured for the mono- and multilayer films deposited on glass plates. No sodium ion signal was observed. The Cd and C peaks of the multilayer films in general agree with the signals of bulk cadmium arachidate. The Si and 0 peaks observed are mainly attributed to the glass. They became weaker and those due to cadmium arachidate layers became stronger with increasing number of layers as shown in Figure 2 and Table I. However, the signal due to the glass persisted rather strongly even for the case of the nine-layer film. According to the results of the present IR-ATR spectra (vide infra) and the electric capacitance for the multilayer films,11J6cadmium arachidate molecules are thought to align perpendicularly to the glass surface in the multilayer films and therefore the thickness of a 0 1978 American Chemical Society
1990
The Journal of Physical Chemistry, Vol. 82,No. 18, 1978
Tsubomura et al.
45 ..
Flgure 1. The geometry of the sample for the IR-ATR spectral measurements. 5 layers 015
Cl5 Cd
950 Bulk
I
1
3 layers
1000 1500 Kinetic energy (eV)
955
c 1s
960 1075 1080 Kinetic energy(eV)
Figure 3. Narrow scan XPS spectra of the cadmium arachidate layers on glass.
(b)
1000 1500 Kinetic energy(eV)
Flgurr 2. Wide scan XPS spectra of cadmium arachidate layers deposited on glass.
TABLE I: Kinetic Energy and Relative Band Area of 0 1s. Si 20. and Cd 3d.1, Signalsa 0 Is
KE,eV 1 layer 954.1 5layers 954.0 7 layers 954.2 9 layers 954.2 bulk 955.4 glass 954.2
r 2.35 0.79 0.46 0.40 0.16
Si 2p KE,eV r 1383.5 0.73 1383.5 0.20 1383.5 0.15 1383.6 0.11
Cd 3d,/, KE,eV r 1080.7 0.26 1081.2 0.27 1081.2 0.22 1081.3 0.26 1081.1 0.23
1383.7
KE is the kinetic energy, r is the ratio of the band area of the signal to that of the C 1s signal in the same sample, a
multilayer film is regarded to be the product of the thickness of the constituent monomolecular layer and the number of the layers. The escape depth of electrons in the layers was estimated to be 130-180 A. There are somewhat controversial reports on the escape depth of electrons in solid organic materials either in bulk or in molecular films. For example, Siegbahn et a1.12 and HenkeI3J4 reported values of the order of 100 A, while Powell16 and Clark et alaL7 gave values of 20-30 A. Our result is nearly in agreement with the former but a little larger than them. This somewhat high escape depth obtained by us might be due to free interstitial spaces likely to be present between adjacent hydrocarbon chains owing to the larger cross sectional area of the COz- group than that of the hydrocarbon chain. The spectra given in Figure 3 show that the 0 1s signal of the glass is mixed with the carboxylate oxygen signal with increasing number of layers. The peak positions of the Cd signals for the multilayer films are the same as those for the spectrum of the bulk arachidate, but the peaks of the monolayer are shifted to lower kinetic energy by ca. 0.5 eV. This result indicates the existence of interactions of the Cd2+ion with the glass surface. The 0 Is and Si 2p signals for the glass became weaker by increasing the angle of emitting electrons (8) (Table 11). Similarly, the intensities of the Cd signals also decrease
9
1 layer
I/
I
Glass
3000
2000
1500
Wave number (cm-')
Figure 4. (a) Infrared absorption spectrum of cadmium arachidate in a KBr disk. (b) Infrared ATR spectra for mono- and multilayer films of cadmium arachidate.
TABLE 11: Dependence of Relative Band Area to e for the Five-Layer Film deg 10 30 45 60
0 1s
Si 2p
Cd 3d,,,
0.79 0.70 0.54 0.43
0.20 0.17 0.09 0.04
0.27 0.23 0.19 0.12
a 6 is the angle between the line normal to the plane of the molecular layer and the direction of the analyzed electron beam.
with 0. These results indicate the uniformity of the fatty acid molecular layers. At least, the presence of islands and bare surfaces having the size of a few score of molecules can safely be denied from these results. IR-ATR Spectra. Figure 4 shows the IR-ATR spectra of mono- and multilayer films of the arachidate deposited on glass. They are generally in good agreement with the absorption spectrum of crystalline cadmium arachidate measured in a KBr disk. The absorption spectra of the films in the wavenumber region below 1200 cm-l could not be obtained because of the strong absorption of glass. The drop at 2360 cm-' is attributed to the absorption by C02 in the light path. The assignment of the bands of the arachidate is given in Figure 4 and in Table 111."' Further details of the bands are shown in Figure 5 which was obtained by taking the difference between the mono- or nine-layer spectrum and the glass spectrum. The difference spectra for the seven-, five-, and three-layer films are essentially the same as that for the nine-layer, and they
The Journal of Physical Chemistry, Vol. 82, No. 18, 1978
Spectral Studies of Cadmium Arachidate Films
TABLE 111: Assignment of t h e Bands of C a d m i u m Arachidate v , cm-'
assignment
CH, a n t i s y m stretch CH, s y m stretch C0,-a n t i s y m stretch C0,-s y m stretch CH, scissoring CH, wagging'
2920 2860 1545 1430
1470 1350-1200
UCH,' VCH,Sa UCO,-~ uc0,CH, WCH,
1991
spectra of multilayer films, associated with CH2wagging and twisting vibrations (wCHJ, suggests that the hydrocarbon chains of cadmium arachidate molecules are in a planar trans-zigzag configuration in a way similar to that in the crystalline state. The same progression is observed in the spectra of multilayer films measured in the present work. The peak-height ratio of the UCH, band to the ~ C band for the nine-layer film is 0.26, somewhat smaller than that for the powder in the KBr disk (0.33). The transition moment of the aCHzband is thought to be perpendicular to the chain axis of the arachidic acid molecule, while that of wCHzis parallel to the chain axis. Since the component of the light perpendicular to the plane of incidence becomes stronger as the light passes through the ATR plate,'g the above small value for the ratio suggests that the cadmium arachidate molecules in the multilayer films are oriented along a direction more or less perpendicular to the glass surface.
References and Notes
I
3000
V
I
I
2000
1500
Wave number
(ern-')
Figure 5. IR difference spectra for monolayer (a) and nine-layer films (b).
are all in good agreement with the crystalline spectrum of cadmium arachidate. As Figure 4 shows, the intensities of the arachidate bands for multilayer films are nearly proportional to the number of layers. The present result for monolayer may be the first infrared absorption spectra of fatty acid monolayers. The spectrum of the monolayer is in good agreement with those of multilayer films in the CH region, but considerably different in the 1400-1600-~m-~ region. The spectrum of the monolayer in this region is changed by the Cd2+ concentration of the solution used in the preparation of the film and the direction of infrared light for spectral measurement. These results suggest that the carboxylate group interacts strongly with the glass surface. Takenaka et a1.6 concluded that the appearance of a band progression between 1350 and 1200 cm-l in the ATR
(1) (a) Osaka University. (b) Toray Industries Inc. (2) L. H. Sharpe, Proc. Chem. Soc., 461 (1961). (3) J..Bagg, M. B. Abramson, M. Fichman, M. D. Haber, and H. P. Gregor, J . Am. Chem. Soc., 86, 2759 (1964). (4) G. I. Loeb, J. ColloidInterfaceSci., 28, 236 (1968); G. I. Loeb and R. E. Baier, ibld., 27, 38 (1968). (5) H. Muller, S. Friberg, and H. H. Bruun, Acta Chem. Scand., 23, 515 (1969). (6) T. Takenaka, K. Nogami, H. Gotoh, and R. Gotoh, J. Colloid Interface Sci., 35, 395 (1971); T. Takenaka, K. Nogaml, and H. Gotoh, IbM., 40, 409 (1972); T. Takenaka and K. Nogami, Bull. Chem. Soc. Jpn., 45, 2367 (1972). (7) C. ChaDados and R. M. Leblanc, Chem. Phvs. Lett., 49, 180 (1977). (8) G. L. Ciark and P. W. Leppla, J. Am. Cheh. Soc.,58, 2199 (1936). (9) L. H. Germer and K. H. Storks, J . Chem. Phys., 6 , 280 (1938). (10) H. Kuhn, D. Moblus, and H. Bucher in "Techniques of Chemistry", A. Weissberger and B. W. Rossitor, Ed., Vol. 1, Part 3b, Wiley, New York, N.Y., 1972. (1 1) T. Ohnishi, M. Hatakeyama, N. Yamamoto, and H. Tsubomura, to be publlshed. (12) K. Larsson, C. Nordling, K. Siegbahn, and E. Stenhagen, Acta Chem. Scand., 20, 2880 (1966). (13) B. L. Henke, J . Phys. (Paris), C4, 115 (1971). (14) B. L. Henke, Adv. X-ray Anal. 13, 1 (1971). (15) B. Mann and H. Kuhn, J. Appl. Phys., 42, 4398 (1971). (16) C. J. Powell, Surface Sci., 44, 29 (1974). (17) D. T. Clark and H. R. Thomas, J . Polym. Sci.: Polym. Chem. Ed., 15, 2843 (1977). (18) L. J. Bellamy, "The Infra-red Spectra of Complex Molecules", Wiley, New York, N.Y., 1954. (19) It was confirmed from the result for the extended polypropylene films that the light passing through the ATR plate was highly polarized.
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