(Figure 2) attributed to sites on the external surface. These latter figures compare well with calculated amounts of surface hydroxyl groups which would about equal a maximum of 0.5 to 1exchange site per unit cell for crystals in the 5-15-pm size range. These 180-exchangemeasurements were supplemented by IR,TPD, and TGA studies in order to examine whether or not the one exchangeable oxygen per Al could be related to framework hydroxyl groups, and the additional exchange to silanol groups on the crystal surface. The results are summarized in Table I. IR measurements (based on integrated absorbance at 3600 cm-I), TPD experiments (based on cumulative NH3 evolution of temperature programmed desorption), and TGA (recording amount of chemical water formed between 500 and 1150 "C) all supply substantial evidence for a one-to-one relationship between Brcansted acid centers and Al content in highpurity ZSM-5. Reasonably good agreement can also be noted in Table I with respect to l80exchange if the data are corrected for surface exchange (by substracting 0.7 l8oeX per unit cell). Comparable l80-exchangeexperiments were also carried out with (ordinary) ZSM-5 samples containing some gel and with X-ray amorphous silica. The former were found to exchange over five times the amount of l80compared to gel-free samples of ZSM-5. Chromatography gel exchanged 39% and Aerosill5% of the oxygen in 2 days at
95 "C. This demonstrates the sensitivity of the 180-exchange method to detect gel impurities in zeolite samples. The reported 180-exchangestudies of ammonium ZSM-5 and water supply ample evidence that not only Si-0-A1 but also Si-0-Si bridges in zeolites are cleaved at measurable rates even under mild conditions at temperatures as low as 95 "C (see Figure 1). This finding is very significant for an improved understanding of reactions of zeolite frameworks at moderate temperatures and the role of water in such procewa. Under steaming conditions (600 "C and 1bar of water vapor) Al-free ZSM-5 was found to exchange 40% of its framework oxygen atoms in 1h, while 70% exchange was recorded in the case of a sample of ZSM-5 containing 4.1 A1 per unit cell. This throws new light on the reactivity of Si-0-Si bridging oxygens which (at least in zeolite chemistry) have all too frequently been assumed to be "inert". On the other hand, point defects (vacancies) which are bound to occur in silicate frameworks could well turn out to be crucial in determining the mobility of framework atoms.
Acknowledgment. We thank Professor P. Baertschi for his continued interest and practical support of this work. Thanks are also due to Mr. A. Rub for carrying out TGA analyses, and to Dr. P. A. Jacobs and Dr. Lynne McCusker for helpful discussions. This study was supported by funds from the Swiss National Science Foundation.
An Infrared Reflection Spectroscopy Study of Oriented Cadmium Arachidate Monolayer Films on Evaporated Silver D. L. Allara' Bell Laboratories,Murray Hili, New Jersey 07974
and J. D. Swalen IBM Research Laboratories. Sen Jose, California 95193 (Received: December 3, 1981)
The infrared spectra of one to ten monolayer assembliesof cadmium arachidate have been measured by glancing angle reflection spectroscopy. No frequency shifts were noted for the bands with increasing number of layers and the intensities scaled almost linearly. The observed and assigned bands exhibited a very different intensity pattern from calculated spectra based on bulk cadmium arachidate optical constants. We explain our results on the basis of oriented monolayer assemblies with the alphatic chains normal to the surface. Our data allow for a small angle of tilt of the chains away from the surface normal and suggest some twisting of the first few methylene groups adjacent to the carboxylate group.
Introduction Organized monolayer assemblies have been extensively studied since the first reports by Langmuir and Blodgett.'+ A variety of techniques has been used to characterize the structures of these films, in particular, assemblies of multilayers many monolayers thick. However, there are very few reports of characterization of films one to several monolayers in thickness. X-ray photoelectron spectroscopy (XPS) studies have been reported recently for single
monolayers of cadmium arachidate (Cd(CH3(CH2)&02H),)on silver! gold,' indium! and glass8 substrates. An infrared spectrum for a monolayer of cadmium arachidate on a glass substrate in contact with an internal reflection element has been reported.8 It is generally assumed from known values of the molecular packing density that the first monolayer of cadmium arachidate on silver is oriented with the alkyl chain axis perpendicular to the substrate surface plane. The XPS studies are consistent with this
(1)I. Langmuir, J. Am. Chem. Soc., 39,1848 (1917). (2)K.B. Blodgett, J. Am. Chem. SOC., 57, 1007 (1953). (3)K.B. Blodgett, Phys. Reu., 55,391 (1939). (4)I. Langmuir, Proc. R. SOC.(London),Ser. A , 170,15(1939). (5)See G. Gaines, 'Insoluble Monolayers at Liquid Gas Interfaces", Wiley, New York, 1966,for more references.
(6)C.R. Brundle. H. Howter, and J. D. Swalen, J. Chem. Phys., 70, 5190 (1979). (7)D.T.Clark.and Y. C. T. Fok. J. Electron Spectrosc. Relat. Phenon., 22, 173 (1981). (8)T.Ohnisi, A. Iahitani, H. Ishida, N. Yamamoto, and H. Tsubomum, J. Phys. Chem., 82, 1989 (1978).
aa22-3654182i2a86-27aa~a~ .25/0
0 1982 American Chemical Society
I R Study of Cadmium Arachldate Films
assumption although they cannot define the exact structure and chain tilt angles. In the case of the infrared report8 the spectra in the CH2 bending region were interpreted qualitatively in terms of a perpendicular alignment. These authors8 also pointed out that there appear to be some qualitative differences between a monolayer spectrum and a nine-layer spectrum in the 14W1600-cm-' region and suggest that there is a strong interaction of the first layer with the substrate. The present study was undertaken to provide information on the structure of cadmium arachidate films from one to ten monolayers thick. Silver was chosen as the substrate in order to allow comparisons with the previous XPS s t ~ d y .The ~ method of external reflection spectroscopy (Em)in the infrared region, also called reflection absorption infrared spectroscopyB(RAIR), was used to obtain vibrational spectra of the films. This technique has sufficient sensitivity to see monolayers. In addition, the well-known anisotropy of the surface electric fieldgin ERS should allow conclusions to be drawn as regards the orientation of active oscillators and thus the adsorbed molecules. It was also a purpose of the study to determine how useful the latter approach would be since there have been no reports of its quantitative applications to orientation measurements on well-defined organic monolayers. Experimental Section The preparation of the cadmium arachidate films on silver has been described elsewhere.6 The films were prepared by dipping a 2.5-cm square glass substrate coated with >700-A evaporated silver through a compressed monolayer assembly on a water surface. Several drops of a 6 X M solution of arachidic acid (Analabs, No. Haven, CT) dissolved in distilled chloroform were placed on the surface of water doubly distilled from deionized water and treated with potassium permangnate to oxidize any organic material. In addition, the water contained 2.5 X M CdClz and was buffered with NaHC03 to a pH of 6.8. With evaporation of the chloroform the arachidic acid was compressed on the surface with a linear pressure of approximately 30 dyn/cm. A sample of cadmium arachidate was prepared by refluxing for 20 h a mixture of 1 mmol of potassium arachidate in 20 mL of water containing dissolved cadmium acetate (-0.1 M). After suction filtration and repeated water washes the product was dried in vacuo at 70 O C for 1h. Elemental analyses gave the following results: Calcd C, 65.27; H, 10.69; Cd, 15.28. Obsd: C, 65.12; H, 10.73; Cd, 15.42. Spectra were taken with a Fourier transform infrared spectrometer (Digilab Model 15-B). For transmission spectra a KBr pellet of cadmium arachidate was prepared and measured in a standard way. For reflection spectra the optics included a variable 1-2-cm aperture just outside the interferometer exit to give an approximately f/30 beam which was focused to a 2-mm spot on the sample. The remainder of the optics have been described elsewhere.lOJ' The polarized (Perkin-Elmer wire grid polarizer) incident beam was reflected off the sample surface a t a glancing angle of incidence of 86" and returned via the detector optics to a liquid-nitrogen cooled mercury cadmium tel(9) R. G. Greenler, J. Chem. Phys., 44, 310 (1966). (10) D. L. Allara, A. B a a , and C. A. Pryde, Macromolecules, 11,1215 (1978). There are two typographical errors in this reference: The lower left-hand matrix element for Cj should be rjdi6j-1)and the phase angle 6j should be (27r/A)E,dj COB 6 ) (11) D. L. Allara in "Vibrational Spectroscopies for Adsorbed Species", A. T. Bell and M. L. Hair, Ed., ACS Symp. Ser., No. 137, Chapter 3 (1980).
I
14FZ
A
1800
4600
1400
1200
1100
WAVENUMBERS, cm-'
Figure 1. Infrared reflection spectra of layers of cadmium arachidate on evaporated silver for the region 1100-1800 cm-'. S o l i lines are observed spectra and the dashed line is a spectrum calculated for a ten monolayer structure obtalned by using bulk optical constants corrected for the increased packing density of monolayer films.
-
0
P
9 0.025 0 -rJI
0.0
3100
2900 2800 WAVENUMBERS
3000
2700
Flgure 2. Infrared reflection spectra as in Figure 1 but showing the region 2700-3100 cm-'.
luride detector which was sensitive between 700 and 3000 cm-'. Spectra were taken at 2-cm-' resolution and generally several hundred scans were accumulated for acceptable signal/noise. Untreated silver films were used to obtain reference spectra. Results Typical spectra are shown in Figures 1 and 2. For all spectra reported here the units of intensity are -log (R/Ro) where R and Ro are the reflectivities of the sample and untreated silver films, respectively. Figure 1 shows the region from 1100 to 1800 cm-l. Below 1100 cm-l until the detector cutoff and between 1800 and 2700 cm-' the spectra show no significant features. Figure 2 shows spectra from 2700 to 3100 cm-l, the upper limit of detector sensitivity. The major peaks are assigned as follows. The sharp bands between 1200 and 1350 cm-' are the chain modes w(CH,) associated with the rocking and twisting of the CH2groups and are observed in long-chain fatty acids and derivatives in the crystalline state.12 The band at 1400 cm-' might be associated with the a-CH2 group and could be a deformation mode perturbed by the adjacent carboxylate g r 0 ~ p . l ~ However, no firm assignment can be (12) R. A. Meiklejohn, R. J. Meyer, S. M. Aronovic, H. A. Schuette, and V. W. Meloch, Anal. Chem., 29,329 (1957).
2702
The Journal of Physical Chemistv, Vol. 86, No. 14, 1982
-
made for this mode presently. The weak shoulder at 1465 cm-' is most likely the CH2 deformation14 (6(CH2)). The strong peak at 1432 cm-l is the symmetric carboxylate stretching model4 (v,(COJ) and the weak peak at 1543 cm-l should be the corresponding asymmetric model4 (va(C02)). The above assignments also are quite consistent with those reported15for the spectrum of adsorbed hexanoate ion on aluminum oxide measured by elastic electron tunneling spectroscopy. The spectra shown in Figure 2 correspond to the C-H stretching vibrations16J7(positions measured for the ten layer sample). These are the CH, asymmetric vibration (v,(CH,)) at 2962 cm-', the CH2 asymmetric vibration (va(CH2))exhibiting a doublet at 2931 and 2920 cm-', the CH, symetric vibration (v,(CH,)) at 2875 cm-', and the CH2symmetric vibration at (v,(CH2))at 2851 cm-'. The apparent shoulder on the CH, asymmetric stretch in Figure 2 is actually an artifact of the noise and is not present in spectra of other samples. However, the CH2 asymmetric splitting is real. Ohnishi and co-workers*report for the internal reflection IR spectra of cadmium arachidate films on glass (presumably for a nine layer sample) the ua(CH2),ua(C02),and u,(C02) vibrations at 2920,1545, and 1430 cm-l, respectively, in good agreement with our respective valus of 2920, 1543, and 1432 cm-l. However, they reports v,(CH2) at 2860 cm-l, a 9-cm-' shift from our value of 2851 cm-'. The reason for this is not clear but both values fall within the normal range14 for u,(CH,) which is 2853 A10 cm-l. They reported no values for the CH, stretching modes. However, in internal reflection the methyl modes will contribute much less intensity to the total C-H mode spectrum, approximately in the ratio 1:12 for the number of C-H bonds in the methyl group to those in the methylene group. On the other hand, in external reflection the surface E field which is oriented almost perpendicularly to the surface cannot excite the methylene stretching modes very effectively because these groups are oriented almost parallel to the surface (see Discussion). Here the methyl and methylene vibrations appear at comparable intensity.
Discussion When the spectra for our films of different thicknesses are compared no frequency shifts or line width changes are observed between various films within the limits of resolution (2 cm-') and noise. The most accurate comparison between one layer and multilayer samples can be made for the strong symmetric carboxylate stretch, u,(CO,), band. Digital subtraction with appropriate scaling shows no significant differences. This is in contrast to the study of Ohnisi and co-workerss in which considerable differences were reported in the 1400-1600-~m-~ region between one and nine monolayers on glass. Although the exact nature of these differences was not given the authors attributed them, at least in part, to differences in the experimental procedure for making these two films and differences in the procedures for obtaining spectra. Further examination of carboxylate and CH stretching mode intensities, however, reveals several other interesting features. A theory of infrared reflection developed by (13)R.G.Sinclair, A. F. McKay, and R. N. Jones, J. Am. Chem. SOC., 74,2570(1952);R.G.Sinclair, A. F. McKay, G. S. Myers, and R. N. Jones, ibid., 74, 2578 (1952). (14)L. J. BeUamy, Ed.,"The Infrared Spectra of Complex Molecules", Wiley, New York, 1975,Chapter 2. (15)D. A. Cass, H. L. Strauss, and P. K. Hansma, Science, 192, 1128 (1976). The assignmenta for the asymmetric and symmetric carboxylate stretching modes were reversed in this paper. (16)R. G.Snyder, J. Mol. Spectrosc., 36, 222 (1970). (17)J. P.Hendra, H. P. Jobic, E. P. Marsden, and D. Bloor, Spectrochirn. Acta, Part A , 33, 445 (1977).
Allara and Swalen
2 NUMBER OF MONOLAYERS
Figure 3. Observed absorption intensity vs. number of monolayers for the symmetric carboxylate stretching fnode at 1432 cm-' of cadmium arachidate films on silver.
Greenlerg and later adapted to thin filmslo shows that an approximately linear relationship between band intensity and film thickness should apply to films several hundred angstroms thick on typical metal surfaces, such as silver, and containing medium strength oscillators such as carboxylate group. In Figure 3 a plot of the intensity of the u,(C02) band against number of cadmium arachidate layers is given. Each layer is known to be 26.8 A t h i ~ k ; ~ J the 'J~ thickness range in this plot is thus 295 A. The error bars shown represent the scatter from several samples. Above four layers (107 A) the plot is quite linear but below four layers the points deviate slightly from the line. The deviation at one monolayer is equivalent to a decrease in the absorption index, K , of the band by about a factor of two where K is the imaginary part of the complex refractive index, A. The reason for this decrease is not clear but probably is not due to any change of the optical constants of the surface carboxylate layer alone. We might expect a %aw tooth" plot with an odd-even variation from the structure of these films. Odd-numbered layered samples are deposited with the carboxylate group in contact with the substrate and interlayers alternating between CH3-CH3 and C02-C02 contacts. Even-numbered layer samples in contrast have hydrocarbon tails touching the substrate. However, no such odd-even variations were observed. The presence of increasing number of surface patches in the thinner films could give the observed deviations in Figure 1. For example, patches in cadmium arachidate films on silver previously have been invoked to account for variations in core level photoemission signah6 The CH stretching mode intensities also followed, within the scatter of the data, linearly with the number of layers but had lower signal-to-noise ratios than the strong symmetric carboxylate band. Further, the slopes of the individual modes appear to vary slightly from one to another. On the basis of conventional 0ptica1,~J~ optical wave guide,lg and infrared internal reflection8 studies, films of cadmium arachidate have been thought to be oriented with their alkyl chains in a zigzag trans conformation with the chain axis normal to the substrate surface. Analysis of the mode intensities show striking differences between our reflection spectra of films and transmission spectra of bulk (18)K. H. Drexhage, "Interaction of Light with Molecular Dye Layers", in "Progress in Optics", Vol. XII, E. Wolf, Ed., North Holland, Amsterdam, 1974,Chapter IV. (19)J. D. Swalen, K. E. Rieckhoft, and M. Tacke, Opt. Comrnun., 24, 146 (1978).
I R Study of Cadmium Arachdate Films
The Journal of Physlcal Chemistry, Vol. 86, No. 14, 1982 2703
TABLE I: Geometries of Selected Vibrational Modes in a Cadmium Arachidate Film direction of 3, relative to surface normal obsd direction of 2, mode v , cm- ' molecular coordinates tilt in XZ plane" tilt in YZ planeb averageCof a range of Va(CH3) 2962 I to C-CH, bond averageCof a range of a12 t o 90" (@+ a / 2 ) to 90" cos e = sin (a/2) cos @ ' 90" - (@+ a / 2 ) VACH,) 2875 II to C-CH, bond varies from 0 to 90" 1 va(CH, 1 2920 1 to C-C-C chain plane 1 90" -6 VdCH,) 2851 II to H-C-H plane, bisecting HCH angle Va(C0,) 1543 1 to C-CO, bond depends on rotational position depends on rotational position around (0.17c) C-CO, bond; around C-CO, bond; range o f a / 2 to 90" range of (@t a / 2 ) to 90" cos e = sin ( a / 2 ) cos @' 90" - (@ + a / 2 ) VS(C0,) 1432 I/ t o C-CO, bond averageCof a range of averageCof a range of 0 / 2 to 90" Sa(CH3) 1 to C-CH, bond (@t a / 2 ) t o 90" cos e = sin ( a / 2 ) cos @' II C-CH, bond 90" - (6 + a / 2 ) 6 S(CH3) 1 1465 II to H-C-H plane, 90" - @ 6 (CH,) bisecting HCH angle I
" @ is the tilt of the trans carbon chain axis from the surface normal, Z direction, with rotation in the XZ plane and a is the sp3 tetrahedral dihedral angle, nominally 109" (see Figure 4). @' is the tilt of the chain away from the Z direction in For free rotation of the -CH, group. See ref 20 and text. the YZ plane (see Figure 4). cadmium arachidate in KBr. To check this difference quantitatively values of K were determined from the transmission measurement. Then n was evaluated from a Kramers-Kronig transform and reflection spectra for a hypothetical ten layer film assembly on silver were generated'O for the experimental conditions used in this study. The increased packing density of the monolayer film relative to the bulk (a factor of 2.8) is built into the calculation. The results are shown in Figures l and 2 as dashed line spectra. Figures 1 and 2 show that the surface film spectra exhibit suppressed intensities of the normally strong modes of the bulk crystallite spectra, to give spectra generally more featureless and weaker than we typically observe for monolayer samples." We interpret these effects in terms of orientation. Since the molecules in both the experimental transmission and reflection spectra are identical the differences in relative peak intensities then are due to the anisotropy of both the E field at the silver surface and the monolayer assemblies. The surface E field effectively has only a normal component (2 direction in Figure 4). The intensity of the ith mode is proportional to the square of scalar product of the E field and the vibration direction
li a lMJI12 (1) where Mi = aiii/aqi is the dipole moment derivative with respect to the ith normal coordinate. The relative effect of Orientation on the surface can be calculated for different models of the arachidate structure by evaluating the expression
ai
where 8 is the angle between and the surface normal and Z is a unit vector in the Z direction. Table I shows the effects of orientation for a zigzag trans chain oriented a t various angles to the surface. The coordinate system is defined in Figure 4 where 4 is the angle between the normal to the surface and the chain axis and rotation is the XZ plane. Calculations were also done for tilting in the YZ plane where the tilt angle is 4'. The CH3 group is assumed to have sufficiently fast rotationz0 a t room temperatye to give a single intensity with an averaged value of M over the CH3 rotation. Two possibilities are (20) R.A. MacPhail, R. G.Snyder, and H. L. Straws, J.Am. Chem. SOC., 102, 3976 (1980).
"
/"?p-
Flgurs 4. Representation of the model used in calculation of mode intensities as functions of chain orientation relative to the substrate surface.
probable for the C02group. It may undergo slow rotation which would lead to some averaged value of individual peak intensities determined by averaging cos2 0 over all rotational positions of the C02 group. Alternatively the COPgroup may not rotate and the intensity will be the average of intensities for a variety of rotational positions whose distribution is determined by specific surface interactions. The descriptions of the molecular and surface geometries for the considered modes are given in Table I. Calculationsof the effects of orientation for these modes are given in Tables I1 and 111. Values of cos28 for each mode are given for several values of the tilting angle 4 of the C-C
