J. PhyS. Chem. 1902, 86, 4675-4678
4675
Infrared Refiection-Absorption Spectra of Ordered and Disordered Arachidate Monolayers on Aluminum Wllllam 0. Golden," IBM Instruments, Inc., San Jose, Californla 951 10
Clinton D. Snyder, and B. Smltht IBM Corporation, General Products Division, San Jose, California 95 193
Infrared reflection-absorption spectroscopy (IRW)has been used to obtain the vibrational spectra of arachidate single monolayers adsorbed on evaporated aluminum surfaces. The arachidate monolayers were established either by Langmuir-Blodgett (LB) deposition technique (using cadmium arachidate) or by adsorption of the acid from a M n-hexadecane solution. Surface density measurements, using a radiolabeling technique, show the solution-adsorbedmonolayer to be only as dense, at saturation coverage, as the Langmuir-Blodgett monolayer. The infrared spectra show the solution monolayer to be considerably more disordered than the Langmuir-Blodgett monolayer. Contrary to the current self-assembly model of solution-depositedfatty acids, these results are explained by concluding that the solution monolayer, unlike the Langmuix-Blodgett monolayer, has no net orientation of the polymethylene chain along the surface normal.
Introduction Oriented Langmuir-Blodgett (LB) monolayers have been studied a great deal since their discovery1+ and it is generally understood that these monolayers are characterized by a high degree of structural order in which the hydrocarbon backbone of these densely packed assemblies are oriented mostly along the surface normal. Because of this high degree of order, well-defined films of controlled thickness can be produced by varying either the chain length of the fatty acid molecule or the number of layers deposited. The LB method is not readily applied to routine production of insulating or lubricating films and there has been considerable effort devoted to producing similar films by adsorption from organic ~ o l u t i o n . ~ - ~ ~ Bigelow et alS7observed the formation of oleophobic films from organic solution in 1946. These initial experimental results indicated that the films were close-packed monolayers with a high degree of order of the aliphatic chain along the surface normal. Brockway and Karle8and others6J0used electron diffraction to establish that a variety of long-chain fatty acids adsorbed from solution yielded oriented films with the aliphatic axes tilted away from the surface normal by less than 25'. Bigelow and Brockway pointed out in 1955, however, that diffraction patterns should be used with care in estimating molecular tilt because of surface roughness effects. Polymeropoulos and Sagiv" have also shown that there are differences in the electrical conductivities of LB monolayers and solution monolayers. To date, the general consensus is that selfassembled solution monolayers are as densely packed and well ordered as LB monolayers.6 Infrared spectra of LB monolayer assemblies have been obtained by a number of workers,13-17and in one case it has been specificallyshown that the hydrocarbon backbone of the cadmium arachidate (CdAA) LB assemblies is tilted away from the surface normal by less than 50.13 To date no infrared data have been available for direct comparison of the structure of LB monolayers with solution monolayers. In this paper we report the vibrational spectra of LB CdAA monolayers and solution-adsorbed (self-assembled) Present address: 289B Perkins Road, Rochester, NY 14623. 0022-3654/02/2086-4675$01.25/0
monolayers of arachidic acid on smooth aluminum surfaces in order to clarify the structural differences between these two types of monolayers. The surfaces for both categories of monolayers were identical in preparation so that any spectroscopic differences observed between the two monolayers were independent of surface roughness effects. Any differences observed can then be considered as due to the method of monolayer deposition. Experimental Details Substrates were prepared by evaporating 1000 A of aluminum on top of 3000 A of thermally grown quartz on single-crystal silicon wafers. Immediately prior to monolayer adsorption these aluminum substrates were cleaned in an oxygen plasma for 30 s. All monolayers were prepared from arachidic acid radiolabeled at the carboxylate carbon (eicoaanoic-1J4C acid, 2.15 mCi/mmol, California Bionuclear Corp.). Monolayer coverages were then determined by measuring the surface radioactivity relative to a calibration standard with a (1)I. Langmuir, J. Am. Chem. SOC., 39, 1848 (1917). (2)K. B. Blodgett, J. Am. Chem. SOC., 57, 1007 (1953). (3)L. H.Germer and K. H. Storks, J. Chem. Phys., 6 , 280 (1938). (4)K. B. Blodgett, Phya. Reo., 55, 391 (1939). (5)J. Langmuir, Proc. R. SOC. London, Ser. A , 170,15 (1939). (6)Additional references can be found in: (a) G. Gaines, 'Insoluble Monolayers at Liquid-Gas Interfaces", Wiley, New York, 1966; (b) E. Kay and P. S.Bagus, E&., "Topicsin Surface Chemistry", Plenum Press, New York, 1978. (7)W. C.Bigelow, D. L. Pickett, and W. A. Zisman, J. Colloid Sci., 1, 513 (1946). (8)L. 0. Brockway and J. Karle, J. Colloid Sci., 2,277 (1947). (9)W.C. Bigelow and L. 0. Brockway, J.Colloid Sci., 11,60(1946). (10)L. S.Bartell and R. J. Ruch, J. Phys. Chem., 60, 1231 (1956);63, 1045 (1959). (11)E. E. Polymeropoulos and J. Sagiv, J. Chem. Phys., 69, 1836 (1978). (12)J. Sagiv, J.Am. Chem. Soc., 102,92 (1980). (13)D.L. Allara and J. D. Swalen, J. Phys. Chem., 86,2700(1982). (14)J. D.Swalen, private communication. (15)T. Ohnisi, A. Ishitani, H. Ishida, N. Yamamoto, and H. Tsubomura, J. Phys. Chem., 82,1989 (1978). (16)J. F. Blanke, S. E. Vincent, and John Overend, Spectrochim. Acta, Part A , 32, 196 (1976). (17)J. F. Rabolt, F. C. Burns, N. E. Schlotter, and J. D. Swalen, to be submitted for publication.
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 24, 1982
4676
Golden et ai.
TABLE I: Infrared Vibrational Modes of Methylene and Methyl Groups of Extended Methylene Chains designation
CH, modes
CH3 in n-alkanes a
mode
species4
d-(n)
BZU B3U B3u B2U
{;ill
U,
cm-’
description
2920 2850 1467 720 2962-2964 2952-2953 2871-2873
asym C-H stretch sym C-H stretch HCH scissor CH, rock asym C-H stretch ( / I to skeletal plane) asym C-H stretch (I t o skeletal plane) sym C-H stretch
Assuming D,, symmetry
gas-flow proportional counting system (Nuclear Chicago, Model 470). The calibration standard was an LB monolayer of radiolabeled CdAA on single-crystal silicon, the surface density of this standard being taken as 5 X 1014 molecules/cm2.14 The single LB monolayer of CdAA on aluminum was prepared by the Langmuir-Blodgett monolayer deposition technique6 such that the carboxylate group was closest to the substrate. The solution monolayer was deposited by dipping the substrate into a 1.0 X M solution of arachidic acid in n-hexadecane until saturation coverage was obtained. This was followed by thorough rinsing of the surface in neat n-hexadecane to remove unbound arachidic acid; residual hexadecane was then removed by a pentane wash. This method is from Polymeropoulos and Sagiv.l’ A transmission infrared spectrum of CdAA in KBr was obtained with an IBM Instruments, Inc., IR/98 Fourier transform infrared spectrometer in order to check purity and provide a reference spectrum of the bulk sample. Single-reflection, grazing-incidence infrared spectra of these monolayers in the C-H stretching-mode region were obtained by infrared reflection-absorption spectroscopy (IRRAS).18
Vibrational Spectra of Long 11 -Alkyl Groups in the C-H Stretch Region Figure 1 shows the transmission infrared spectrum of the C-H stretching modes for CdAA in KBr. We adopt the notation and spectral assignments of Snyder and coworker~ in ~the ~ ~following ~~ discussion (viz., Table I). The C-H stretching region of the CdAA infrared spectrum is complicated by Fermi resonance interactions between the C-H modes (d) and the overtones and combinations of the CH2 scissor modes (6 at ca. 1467 cm-’). Table I summarizes the assignments, symmetries, and band positions of the infrared-active CH2 modes for the methyl groups and extended methylene chains for this type of molecule.21 The most intense feature in the spectrum in Figure 1 is due to the asymmetric methylene stretch, d-(a) (2920 em-’). The broad base-line feature beneath this and the other bands in this region is due to the Fermi resonance system of the d+(a)symmetric stretch (2850 cm-l) with the overtone and combination levels of the CH2 scissor mode The breadth of this feature is due to the “dispersion”of 6(a) due to the length of the polymethylene chain. The intensity of the symmetric methylene stretching mode d + ( r ) a t 2850 cm-’ is consequently weakened by intensity transfer to the 6(a) overtone^.^^^^^ It is the perturbation of the absolute intensities of the C-H modes which makes it difficult to make much more 6
(
~
)
.
~
~
9
~
~
(18)W.G. Golden, D. S. Dunn, and J. Overend, J. Catal., 71, 395 (1981). (19)R. G.Snyder, S. L. Hsu, and S. Krimm, Spectrochim. Acta, Part A, 34, 395 (1978). (20) R. G. Snyder and J. R. Scherer, J. Chem. Phys., 71,3221 (1979). (21) W. Knoll, M. R. Philpott, and W. G. Golden, J. Chen. Phys., 77, 219 (1982).
I .vu
0 75
0.50
0.25
3000
2950
2900 2850 wave number ( c m - l )
2800
Figure 1. Infrared spectrum of CdAA powder in a compressed KBr pellet.
L
Y
1
-0
E 0.3 ” % +
D
m L-
0.2
m C m -1
,”
0.1
0
t
P
t.
u
F -
0
2
0
1
I
1
I
1
I
20 40 60 time in solution (minutes)
1
I 80
Figure 2. Adsorption of arachidic acid on aluminum a s a function of M arachidic acid in n-hexadecane; T = substrate time in 1.0 X 300 K.
than a qualitative argument concerning their relative intensities. However, the relative intensity of d-(a) with respect to r;, the asymmetric methyl stretch, will be indicative of the larger number of surface infrared-active
The Journal of Physical Chemistry, Vol. 86, No. 24, 1982 4877
Arachdate Monolayers on Aluminum
loo
-+
I
g 80
I
I
2
-8m .-
=
._
60
3000
2900
2800
2900
2800
L
0 Y
oi c
a 40
vi m
m
0)
I
2u
20
t
01 0
1 1
10
I 20 time (hours)
I
30
'
Flgure 3. Thermal desorption in air of solutiondeposited monolayers of arachMlc acM adsorbed on (a) aluminum (A)and (b) silicon (solid); T = 473 K.
methylene groups in the monolayers.
Experimental Results Figure 2 shows the adsorbate density of solution-adsorbed arachidic acid on evaporated aluminum as a function of substrate time in the solution. From the figure, saturation coverages appear to occur at 0.36 of the standard LB monolayer (Le., 1.8 X lOI4 molecules/cm2). Exposure times of several hours did not significantly increase this value. The extent to which the arachidic acid is bound to the aluminum substrate is illustrated in Figure 3. Thermal desorption curves are shown for arachidic acid adsorbed from solution on aluminum and silicon. Since the carboxylic acid group should have no affinity for the silica surface, the desorption curve for it is representative of an unbound and easily desorbed surface layer. For the aluminum substrate, it is clear that the arachidic acid is f i i y attached to the surface, presumably through formation of some degree of ionic bonding of the carboxylate group with the surface. Figure 4a shows the IRRAS spectrum of the LB monolayer of CdAA on aluminum. In this spectrum the relative intensity of the d-(?r) mode (CH2asymmetric stretch) with respect to the r; mode (asymmetric methyl C-H stretch) is strikingly different from that shown in the spectrum of Figure 1. Figure 4b, the IRRAS spectrum of the solution monolayer, shows that the relative intensities of these two modes, in this case, are very similar to those of the bulk sample of CdAA (viz., Figure 1). On the left-hand side of Figure 4 is depicted schematically the proposed structure whose spectrum is shown on the right. The band positions in both a and b of Figure 4 within experimental error are not significantly different from those measured in Figure 1. The differences between spectra 4a and 4b and the similarity of spectra 4b and 1 can be explained in terms of the surface selection rules for infrared reflection-absorption s p e c t r ~ c o p y .Since ~ ~ upon reflection at grazing (22) A. Francis and A. H. Ellison, J. Opt. SOC.Am., 49, 130 (1959). (23) R. G. Greenler, J. Chem. Phys., 44, 310 (1965);SO, 1963 (1968).
1
3000
1
wave number (cm-1)
Flgure 4. IRRAS spectra of arachidate monolayers on aluminum: (a) LB monolayer; (b) solutiondeposited monolayer. On the left-hand side Is deplcted schematlcally the proposed monolayer assembly associated with the spectrum shown on the right. (R is the reflectivity.)
incidence from a metal surface there is no surface electric vector amplitude parallel to the surface, there will be no infrared radiation absorbed for dipole transition moments oriented in that direction. The electric vector perpendicular to the surface, upon reflection, is nonzero, however, so dipole transition moments oriented along the surface normal will be allowed. For an LB monolayer, with its aliphatic chain oriented essentially normal to the surface, the C-H dipole transition moment for the methylene groups will be mostly in the plane of the surface and not absorb infrared radiation to any large degree. Increasing the tilt of the long-chain axis of the molecule produces more out-of-plane methylene C-H transition moments and increases their surface infrared activity. Thus, as described by Allara and Swalen,13 spectrum 4a, showing substantial attenuation of the d-(?r) mode relative to the r; mode, is illustrative of a well-ordered CdAA monolayer. Figure 1 can be considered representative of a truly disordered bulk sample of CdAA in which no orientation of the C-H transition moments is preferred over another. Since the relative intensities of the d-(?r) to r; modes in the spectrum of the solution monolayer are similar to those in Figure 1, we conclude that the solution monolayer is essentially disordered; Le., on the average the methylene C-H dipole transition moments are not oriented along the surface normal. The structural model proposed here for these monolayers is one of an assembly of loosely packed molecules arranged predominately with the hydrocarbon axes in the plane of the surface and yet with the carboxyl-aluminum bond still intact.
Conclusion Surface coverage measurements given here show that the solution monolayer density of arachidic acid on aluminum is 0.36 of the LB monolayer density for the same substrate. This indicates that the solution monolayer is (24) R. G. Greenler, R. R. Rahn, and J. P. Schwartz, J. CataE.,23,42 (1971). (25)J. D. E. McIntyre and D. E. Aspnes, Surf. Sci., 24, 417 (1971). (26)M. J. Dignam, B. Rao, M. Moskovits, and R. W. Stobie, Can. J. Chem., 49, 1115 (1971). (27) W. Hansen, J. Opt. SOC.Am., 58, 380 (1968).
4678
J. phys. Chem. 1902, 86, 4678-4683
not close packed, but randomly dispersed on the surface with the hydrocarbon backbone nonlinear and not oriented preferentially along the surface normal. The infrared spectra of these two systems, by comparison of the relative intensities of the asymmetric methylene C-H stretch, d-(.lr), to the asymmetric methyl C-H stretch, ra-, support the conclusion that the solution monolayers are a thinner assembly on the average,mwell bound to the
surface through the carboxylate group (viz., Figure 3), but with the majority of the methylene chain of each molecule lying in the plane of the surface. We conclude from these data that solution-deposited fatty acid monolayers do not adsorb to form highly ordered structures; Le., solution monolayers are much less dense and much more disordered than LB monolayers.
(28)The shift in the position of the surface plasmon resonance angle reflection minimum obsked for an LB monoliyer ie much greater t h k that for solution monolayers. This implies that the latter monolayer is proportionally thinner and/or patchier. J. D.Swalen and M.R. philpott, private communication.
Acknowledgment. We thank the followine individuals for helpful teihnical assistance and discus~ons: Ma R. Phibott, D. J. Pocker, - . J. F. Rabolt, N. Schlotter, J. Sunzeri, and J. D. Swalen.
Kinetics of the Reactions of CF, with O(,P) and O2 at 295 K K. R. Ryan' and 1. C. Plumb CSIRO Dlvlslon of Applied Mysics, Sydney, Australia 2070 (Received:June 1, 1982)
The reactions of CF3with O(3P)and 02 have been studied at 295 K by using discharge flow methods with helium as the bath gas. In one set of experiments two microwave discharges were used to produce independently sources of 0 atoms and CF3radicals. Under the conditions employed negligible consumption of 0 atoms occurred by reaction with CF3,thus allowing the rate coefficient to be obtained by observing the pseudo-first-order decay of CF3. These measurements gave a rate coefficient of (3.1 f 0.8) X lo-" cm3s-l for the reaction CF3+ 0 CF20 + F at 295 K. The rate coefficient for the reaction between O2and CF3was found to increase from 2.2 X to 1.24 X cm3s-l as the [He] increased from 1.6 X 1018to 2.7 X 10'' cmd3. The possibility of two pathways was examined, namely, CF3 + O2 CF3O2 and CF3 + O2 CF20 + FO. It was found that
