J. Phys. Chem. 1982, 86,5123-5127
A temperature of this magnitude would certainly thermally pyrolyze glycerol, but the lifetime of this high temperature is very short,1° which raises questions as to whether indeed thermal formation of free radicals is involved. A characteristic of a fast argon atom which may be of interest in this problem is that it has a high momentum as well as a high energy. Thus non-Franck-Condon processes can be effected, and it is suggested that free radicals may be produced by momentum and energy transferring collisions which would bring about the direct fragmentation of glycerol molecules. This comprises a novel and interesting kind of radiation chemistry. It would be of interest to compare the products obtained in this work with those obtained by the use of ionizing radiation in neat glycerol, but no information on this point has been found. Note Added in Proof. A further experiment was made wherein glycerol was bombarded with a beam consisting (10)McNeal, C. J. Anal. Chem. 1982,54,43A.
5123
of only fast argon atoms. This was obtained by applying a deflection voltage to the mixed beam of argon atoms and ions emerging from the Capillaritron gun. The behavior of the argon spectra was the same as that depicted in Figures 1-6.
Acknowledgment. This work was one of the activities of the Rockefeller University Mass SpectrometricResearch Resource, which is supported in part by the Division of Research Resources, NIH. Many of the experiments reported here and most of the computer programming were made by Louis Grace. Initial modifications of the mass spectrometer for FAB service were made by Dennis Underwood. The OH- NCI measurements were made by Aladar Bencsath and Camy Ng. Useful discussions were held with Brian Chait. I am grateful to them for their help. A computer search of the glycerol radiation chemistry literature was kindly provided by the Radiation Chemistry Data Center, Radiation Laboratory, University of Notre Dame.
X-ray Photoelectron Spectroscopy of Cadmlum Arachidate Monolayers on Various Metal Surfaces F. C. Burnst and J. D. Swalen' IBM Research Laboratory, San Jose, Cellfornkr 95193 (Recelwed: June 30, 1982)
X-ray photoelectron spectra were measured for a single cadmium arachidate (cadmium eicosonate) monolayer bonded via the carboxylate group to various metal and metal oxide surfaces. Two well-defined peaks were observed in the C 1s spectra of these systems: an intense peak due to ionizations from the CH2 and CH3carbons and a less intense peak due to ionizations from the CO; carbon. Differences between the binding energies of these two carbon peaks were found to vary with the surface to which arachidate was bonded and these differences are interpreted as arising from differences in the charge transfer between the various surfaces and the arachidate. In addition the carbon initially present on the metal/metal oxide surfaces and the effect of submerging the surfaces in the monolayer tank were studied and it was found that this source of carbon had a negligible effect on the positions of the arachidate carbon peaks.
Introduction When Langmuir monolayer assemblies of fatty acids and their salts are transferred to solid surfaces by the Blodgett technique, highly ordered organic layers coating the solid surfaces are created.' These monolayer assemblies are of interest because they offer a means of studying intermolecular interactions and energy transfers and because of their potential applications as protective overcoatings and in lithography. For the preceding reasons there has been vigorous activity directed toward understanding the interaction between Langmuir-Blodgett monolayers of fatty acid salts and various types of surfaces.2 Techniques as varied as X-ray photoelectron spectroscopy (XPS),= attenuated total reflectance (ATR) s p e c t r o ~ c o p yRaman ,~~~ spectroscopy; and reflection Fourier transform infrared spectroscopy (FT IR)6J0J1have been utilized successfully in studying monolayer systems. Anderson and Swalen3 have reported the C 1s XPS spectra for one, three, and five monolayers of cadmium arachidate (Cd(CH3(CH2),,C02),),henceforth abbreviated t Present address: I B M Development Laboratory, Endicott, NY 13760.
Cd(AA)2,on various oxide surfaces. These authors measured the difference between the carbon l s binding energies due to the aliphatic carbons and that due to the carboxylate carbon. Changes in the binding energy difference in going from one to several monolayers were interpreted in terms of charge transfer between the oxidized (1)G. L.Gaines, Jr., 'Insoluble Monolayers at LiquidGas Interfaces", Wiley-Interscience, New York, 1966,Chapter 8. (2)H. Kuhn, D.Mobius, and H. Biicher in "Physical Methods for Chemistry", Vol. I, Part IIIB, A. Weissberger and B. W. Rossiter, Eds., Wiley-Interscience, New York, 1972,Chapter VIII. (3) H. R. Anderson, Jr., and J. D. Swalen, J. Adhes., 9,197-211 (1978). (4)C. R. Brundle, H. Hopster, and J. D. Swalen, J. Chem. Phys., 70, 5190-6 (1979). (5) D. T. Clark, Y. C. T. Fok, and G. G. Roberts, J. Electron Spectrosc. Relat. Phenom., 72,173 (1981). (6)T. Ohnishi, A. Ishitani, H. Ishida, N. Yamamoto, and H. Tsubomura, J . Phys. Chem., 82, 1989-91 (1978). (7)J. G. Gordon, 11, and J. D. Swalen, Opt. Commun., 22,375 (1977). (8) I. Pockrand, J. D. Swalen, J. G. Gordon, 11, and M. R. Philpott, Surf. Sei., 74,237 (1978). (9)W. Knoll, M. R. Philpott, J. D. Swalen, and A. Girlando, J . Chem. Phys., 77,2254 (1982). (10)D.Allara and J. D. Swalen, J. Phys. Chem., 86, 2700 (1982). (11)J. F.Rabolt, F. C. Burns, N. E. Schlotter, and J. D. Swalen, J . Chem. Phys., in press.
0022-3654/82/2086-5123$01.25/00 1982 American Chemical Society
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Burns and Swalen
surface and the monolayer. This interpretation is very attractive since it can be used both to discuss the acid-base characteristics of the surfaces and to evaluate the importance of charge transfer in the bonding of monolayers to surfaces. We have conducted XPS studies for a single monolayer of Cd(AA), on various metal (Au, Ag, Cu, Ni, Pt) and metal oxide (Al,03, Bi203, Cr03, SnO,, h03) surfaces in order to extend and expand on the ideas proposed by Anderson and Swalen. In addition to these studies, untreated surfaces, surfaces treated only by submersion in the monolayer tank, surfaces coated with bilayers of Cd@V2, and surfaces with a monolayer of Cd(AA),deposited in the reverse direction so that the methyl group of the arachidate interacted with the surfaces were all studied with XPS.
Experimental Section With the exception of Bi,03 all metal/metal oxide surfaces were prepared by vapor depositing 1000 A of the appropriate metal onto clean (6 X 10) mm2 glass slides. Upon exposure to the atmosphere the more reactive of the metals, Al, In, Sn, and Cr, underwent air oxidation to produce an oxide layer. No treatment other than air oxidation was used to produce the oxide surfaces. A surface of Bi,O, was prepared by direct deposition of Bi203. After removal from the evaporator the metal-coated glass slides were transferred to a desiccator which was kept under a positive nitrogen pressure. During the transfer from the evaporator to the desiccator the metal surfaces were exposed to the atmosphere and oxidation occurred. The metallmetal oxide coated slides were stored in the desiccator until used. A Langmuir-Blodgett trough' containing doubly distilled water buffered to a pH of 6.8 with NaHC03 and with a4X M concentration of cadmium chloride was used to transfer the Cd(AA), monolayer(s) to the metal/metal oxide surfaces. For all of the transfers of Cd(AA)2,it was necessary first to position the trough's moving barrier so that, when the arachidic acid was placed on the surface of the water, the Cd(AA), produced at the surface would form an expanded phase. In order to transfer a monolayer of Cd(AA)2 with the carboxylate group bonded to the surface, we immersed the substrate to be coated with Cd(AA)2in the trough. A layer of arachidic acid was then spread on the water surface by a dropwise addition of a chloroform solution of arachidic acid. After the chloroform evaporated, the arachidic acid film on the water surface was compressed by using the moving barrier which was subjected to a lateral pressure of approximately 30 dyn/ cm3and the immersed substrate was slowly drawn through the arachidic acid film with the moving barrier maintaining constant pressure on the film. Under these conditions of transfer it is known that the arachidic acid will transfer as the cadmium salt and that the measured area and volume per molecule show that the molecule is oriented approximately normal to the surface. A monolayer with the carboxylate group bonded to the surface will be referred to as a head-on situation. Bilayers of Cd(AA), are transferred to the surfaces in a slightly different manner. As described in the previous paragraph, the trough's moving barrier was placed in the appropriate position. Next, a layer of arachidic acid was placed on the surface of the water and compressed as previously described. At this point the substrate was pushed completely through the arachidic acid film and then drawn back through the acid film. Again it is the cadmium salt which is deposited and the bilayer is oriented approximately normal to the surface. The methyl group of the first transferred Cd(AA), layer is next to the surface
Flgure 1. Representation of the various monolayers. The 0's represent the carboxylate group and the 0's represent the aliphatic chains. (a)Single monolayer head on the surface. (b) Single monolayer tail on t h s surface. (c)Bilayer on the surface.
while the methyl group of the second transferred Cd(AA)2 layer is exposed to the air. If, instead of pulling the substrate back through the arachidic acid film the arachidic acid film was removed from the surface before the substrate was withdrawn, then only a single monolayer would be transferred. This monolayer should have the methyl group next to the surface and the carboxylate group sticking up in the air. Such a monolayer will be referred to as the tail-on situation. Thermodynamically this situation is, however, unstable so some reversal might be expected either during drawing (most likely) or subsequently in the film. In Figure 1the idealized head-on monolayer, tail-on monolayer, and bilayer are drawn. A final set of surfaces were prepared by immersing metal/metal oxide coated glass slides in the LangmuirBlodgett trough. The substrates remained submerged for a period equal to the time needed to spread and compress an arachidic acid layer on the water, before the substrate was withdrawn. X P S spectra were collected on a dispenion-compensated monochromatized spectrometer (Hewlett-Packard Model 5950B) using A1 K a radiation. The geometrical arrangement of the sample with respect to the incident radiation and the angle of observation of the emitted electrons has been described previ~usly.~ A flood gun current of 0.04 mA was found to be sufficient to prevent charging on any of the samples. To ensure that there was no desorption of the monolayer during data collection, we followed two procedures. First, for all of the samples, the first and last spectra measured were the high-resolution spectra of the most intense metal ionization. The variations between the first and last metal spectra were the same as variations in the spectra of a gold standard which was measured before and after the sample. Second, for each surface with a monolayer transferred to it, spectra were recorded at different temperatures. Each surface (for different samples) has spectra collected at 40 and 15 "C and several spectra were collected for samples maintained at 0 and -30 "C. No significant variation in the spectra with temperature was observed.
The Journal of Physical Chemistry, Vol. 86, No. 26, 1982 5125
Cadmium Arachidate Monolayers on Metal Surfaces
’”c
n
t
0.8
A Ebe
in Bi \
0 0.2 4[
0.0 280
TABLE I: Differences in Binding Energies (AEbe )” between the Carboxylate and Aliphatic Carbons of Cadmium Arachidate, Arachidic Acid, and Bilayers of Cadmium Arachidate on Silver
284
282
286
288
290
Binding Energy ieVl
Figure 2. XPS spectrum of bulk arachidic acid. The 0’s are the experimental points and the solid line Is the sum of the two Gaussians used to ftt the experimental points. The experimental points have been scaled to fall between 0.025 and 1.025.
Data Analysis Output from our spectrometer consists of a point-bypoint plot of electron counts vs. binding energy. Each spectrum was digitized by using an electronic digitizing pad and the digitized data were then transferred to an APL work space for analysis as Gaussian curves. A linear base line was determined by using the first and last 10 points to be fitted and the base line was subtracted from the data The resulting data points were then fitted to a function, g, which had the form g=
m
n
WjAie-Bi2(E,-Eid2
j=1 i = l
where n = number of Gaussians used in the fitting, m = number of experimental points, Wj = weight assigned to point j , Ai = amplitude of Gaussian i, Bi = exponent of Gaussian i, E. = energy of point j , and Eio= center of Gaussian i. d e took Ej, to be a measure of the binding energy associated with peak i. A multiple nonlinear regression routine based on the method of steepest ascent was used to simultaneously vary any or all of the Ai, Bi, and Ei, until convergence was achieved in a least-squares sense as measured by the square of the difference between g and the experimental counts.12 In Figure 2 the carbon 1s XPS spectrum of bulk arachidic acid (the open circles) is reproduced along with the sum of the two Gaussians (the solid line) used in fitting this spectrum. Very good agreement with the experimental spectra is obtained about the maximum of the two clearly defined peaks.
Results and Discussion Bulk Arachidic Acid, Bulk Cadmium Arachidate, and a Bilayer of Cadmium Arachidate on Metal Surfaces. The carbon 1s XPS spectrum of bulk arachidic acid which is reproduced in Figure 2 is typical of all the XPS spectra which have been obtained for the arachidate systems reported in this paper. There are clearly two peaks in the carbon 1s spectra; a very intense peak centered at approximately 283.6 eV and another much less intense peak at higher binding energy. The intense peak is assigned to ionizations arising from the CH2 and CH3carbons and the higher binding energy peak is assigned to ionizations arising from the carbon of the carboxylate group. It is the difference between the carbon 1s binding energies of the aliphatic carbons and the carboxylate carbon which is of interest in this study. This difference will be denoted as (12)R. H. Lacombe, IBM Technical Report TR22.2319,IBM East Fishkill Facility, Hopwell Junction, NY 12533.
Cd(AA),
1 2 3 4 5
3.57 3.59 3.56 3.57 3.62
4.32 4.36
3.61 3.58 3.55
3.58
4.34
3.58
av
arachidic acid
bilayer of Cd(AA), o n silver
run no.
a AEbe = Eio(carboxylate) - Eio(aliphatic) and is m e a s ured in electron volts.
AEbs (measured in electron volts) and the centers of the fitted Gaussians will be used as a measure of the binding energies of the two types of carbons; thus hEbe
= Ei,(carboxylate) - Ei,(aliphatic)
If the difference between the experimental and fitted spectra in Figure 2 is taken, then a third ionization can be resolved in the 285.5-eV region. This third ionization (not fitted in Figure 2) has been observed before and is assigned to an ionization arising from the CH2 carbon bonded to the C02- group.12 Since this third ionization did not affect h E b e , this ionization was not investigated further. In Table I the AEbe’sare presented for five, two, and three different samples of Cd(AA)2,arachidic acid, and a bilayer of Cd(AA)2 on silver, respectively. Since the electron counts were measured at intervals of 0.08 eV, the reproducibility of these experiments is very satisfactory. In general (the exceptions will be discussed later), the reproducibility of the results presented in Table I is typical of the reproducibility of the results which have been obtained for the arachidate systems. The differences between the hEbsfor arachidic acid and Cd(AA)2illustrate how a change in the bonding environment experienced by an atom (in this case the carbon of the carboxylate group) can change the binding energies of that atom’s core electrons. In going from the neutral acid to the cadmium salt, the carbon in the carboxyl group acquires a negative charge which is delocalized over all three atoms of what is now a carboxylate group. This delocalization of the negative charge results in the carboxylate carbon Is electrons experiencing a more negative potential, which in turn destabilizes the carbon 1s electrons in the carboxylate group relative to the carboxyl group. The destabilization of the carboxylate carbon 1s electrons in Cd(AA)2decreases the binding energy of those 1s electrons relative to the corresponding electrons in arachidic acid13 and the measured AEbeshows a corresponding decrease. The agreement between the hEb:S for Cd(AA)2and a bilayer of Cd(AA)2on a silver surface is not surprising. Both are Cd(AA)2systems and in the bilayer the carboxylate group does not interact with the surface. Similar results are obtained for bilayers on other surfaces. Because of the agreement between the AEbe’sfor bulk Cd(AA), and bilayer of Cd(AA)2and because single monolayers are also (13)Formally, the binding energies of the carbon 1s electrons are negative and the destabilization of the carboxylate carbon 1s electrons results in their binding energy increasing (binding energy becoming less negative). However, the XPS experiment measures the absolute value of the binding energy which decreases when the binding energy increases. The discussion in the text is based on the measured absolute value.
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The Journal of Physical Chemistry, Vol. 86,No. 26, 1982
Cd(AAI2monolayers, 3.58 eV will be used as a reference
Gx* Immersion into the Monolayer Tank and Monolayers of Cd(AA), Tail-On. Previous studies3,*have shown there to be carbon present on metal/metal oxide surfaces which have been exposed to the atmosphere. In addition the monolayer tank contained HC03- as a buffering agent and as a potential source of extraneous carbon. For the above reasons XPS studies of the carbon on the metal/metal oxide surfaces before and after immersion in the monolayer tank were undertaken in order to determine what effect, if any, these sources of background carbon had on the measured AEb of a single monolayer head on the various surfaces. It was found that the carbon XPS spectra before and after immersing the metal/metal oxide surfaces exhibited two peaks; however, the variation in &!?be for these for a carbon peaks was 4-5 times the variation in &&!e single monolayer of Cd(,W2 head on surfaces. Also, it was found that a variation of 1-14 days between when the metal surface was deposited and when the XPS spectra were collected resulted in the carbon-to-metal intensity ratio increasing by a factor of about 4. While the variations in the amount of carbon on the surface and in m b e were found to be approximately the same before and after immersion in the monolayer tank, the actual values of AEbe changed after immersion. This change in &!?be upon immersion was due to a change in E,, of the higher-energy peak. Whatever gave rise to the high-energy carbon peak in the metal/metal oxide surfaces, probably COP, was dissolved in it or reacted with the monolayer solution presumably replacing the original carbon species. It would have been ideal if these studies could have been conducted in such a manner that there was no source of carbon other than Cd(AA),. However, since it was not possible to exclude background carbon from the surfaces, the opposite experiment was performed; the XPS spectra of the Cd(AA), monolayer head on each metal/metal surface was obtained at least twice and under conditions where the amount of background carbon was found to vary significantly. The variation in AEb for a monolayer of Cd(AA)2 head on the surfaces was found to be significantly less than the variation in either the amount of background carbon or the a b e of the background carbon; for this reason we believe that the background carbon has little influence on the A&,, of single monolayers head on the surfaces. Investigations of a single monolayer of Cd(AA), tail on the metal/metal oxide surfaces were limited to silver, since it was found that the carbon 1s XPS spectra of a single Cd(AA)2monolayer tail on a siluer surface were not reproducible. The initial motivation behind studying a tail-on monolayer was to confirm that the of bulk Cd(AA)2was the appropriate reference for the head-on monolayer systems. Bilayers of Cd(AA), eventually served this purpose. In retrospect it is not surprising that the tail-on monolayer was not reproducible since the interaction between the silver surface and the methyl group would not be expected to be great and drawing the hydrophilic surface of a monolayer through water would be expected to cause rearrangement of the monolayer and loss of the monolayer material to the water surface. In addition, wetting angle measurements on monolayers formed so that the hydrophilic end of the monolayer should be at the air interface indicate that such a surface is not completely hydrophilic; i.e., there has been some rearrangement in the monolayer so that some hydrophobic parts of the monolayer are at the air interface. The nonreproducibility of the tail-on monolayer is consistent with varying amounts of rearrangement within the monolayer so that the car-
TABLE 11: Differences in Binding Energies (AEb, p between the Carboxylate and Aliphatic Carbons for Cadmium Arachidate Monolayers on Metal and Metal Oxide Surfaces surfaceb
AEbe, eV
AgC Ptd A uc snd
3.33 3.51 3.57 3.58 3.59
N id
surfaceb cue C re Bi,O I nd AIC
3e
AEb,, e~
3.59 3.60 3.64 3.70 3.81
The surAEb, = Eio(carboxylate) - Eio(aliphatic). face is designated according to the material which was deposited o n glass slides. Ag, Au, and Pt showed no signs of oxidation. Sn, Cr, In, and AI were heavily oxidized while Ni and Cu were only slightly oxidized a t most. Average Average o f four values. e Average of two of five values. values. a
boxylate group experiences different bonding environments in different samples. For the bilayers, the interaction between the two layers prevents rearrangement and reproducible results are obtained. Monolayer of C d ( A A ) 2Head on MetallMetal Oxide Surfaces. In Table I1 the AEbe’s for a single monolayer of Cd(AA), head on various metal/metal oxide surfaces are presented. Each surface is designed according to the material deposited on the glass slide; the Al, In, Cr, and Sn surfaces were heavily oxidized while the Cu and Ni surfaces were slightly oxidized at most. All of the AEb’s presented in Table I1 are averages for at least two different samples and, except for Au and Pt, the reproducibility of the results is comparable to the reproducibility of the arachidic acid, Cd(AA)2,and bilayer results reported in Table I. For purposes of discussion the surfaces fall into three groups: (1)Al, Ag, and In; (2) Sn, Ni, Cu, Cr, and Bi203;and (3) Au and Pt. The extreme values of U b e found in Table 11, &!?be = 3.33 eV for Ag, AEb = 3.70 eV for In, and AEh = 3.81 eV for Al, illustrate that it is possible to obtain values of both greater than and less than the reference h E b e , 3.58 eV for bulk Cd(AA)2. If one uses the same reasoning as was applied in comparing the AEb’s for bulk Cd(AA)2and bulk arachidic acid, the decrease in A& for the Cd(AA)2 monolayer head on the Ag surface indicates that the electron density is being transferred from the silver surface onto the carboxylate group. Similarly, the increase in aEb for the Cd(AA)2monolayer head on the aluminum oxide and indium oxide surfaces indicates that electron density is being transferred from the carboxylate group onto the metal oxide surface. In the Lewis sense the Ag is acting as a base toward the carboxylate group while aluminum oxide and indium oxide surfaces are acting as acids toward the carboxylate group. A further conclusion which can be drawn from these three values of &?$,e is that there is a mechanism by which electron density can be transferred from the surface to the carboxylate group and vice versa. It is highly probable that both mechanisms are operative for all the systems studied and the extreme values of AEb represent instances where one or the other mechanism dominates. The simultaneous transfer of electron density in opposite directions by two different mechanisms has been well documented in organometallic ~hemistry.’~,’~ A situation exactly analogous to that found in organometallic chem(14) P. S. Bagus and B. 0. Roos, J . Chem. Phys., 75, 5961-2 (1981). (1.5) T. F. Block and R. F. Tenske, J. Am. Chem. SOC.,99,4321 (1977), and references therein; R. F. Fenske in “Progressin Inorganic Chemistry”, Vol. 21, S. Lippard, Ed., Wiley, New York, 1976, p 179, and references therein.
Cadmium Arachidate Monolayers on Metal Surfaces
istry is found in the surface-monolayer systems studied in this work, and, drawing on the theory developed for the organometallics, one can propose mechanisms for electron transfer in the surface-monolayer systems. Transfer of electron density from the carboxylate group to the surface is probably accomplished from a C-type molecular orbital (MO) which is delocalized over all three atoms of the carboxylate group. This u MO is probably derived from the 0-H bonding MO which is destroyed upon formation of the anion. Also this u MO is probably the MO of the carboxylate group which contains the negative charge and is probably the highest occupied MO. The exact nature of this MO, bonding, nonbonding, or antibonding, cannot be determined from the XPS data. Theoretical calculations on the anion and infrared spectra of the surfacemonolayer systems could determine the nature of this MO. Electron transfer onto the carboxylate group is probably accomplished by transferring electron density from the surface into an empty a antibonding MO which is delocalized over the carboxylate group. This a MO is the antibonding counterpart to the delocalized a bonding MO which is formed when the anion is created. From the viewpoint of the surface, electron density would be transferred out of the Fermi level in the metals or the highest occupied bond of the oxides and into the conduction band for the conducting surfaces or the lowest unoccupied band of the oxides. The second group of surfaces, Sn, Ni, Cu, Cr, and Bi203, all have average hEb’s which are the same within the reproducibility of the data and which are the same as the average AEbefor bulk Cd(AA),. With respect to the model of s~rface-Cd(AA)~ head-on monolayer interaction proposed in the preceding paragraph, these values of h E b , indicate that the effect of electron density transfer in one direction is cancelled by transfer of electron density in the other direction. In effect, these values of AEb show that, if electron density can be transferred in both directions, then a leveling effect could occur analogous to the leveling effect which water has no aqueous acids and bases. For these five surfaces, Cd(AA), conceivably may not be the appropriate monolayer for distinguishing between these surfaces with respect to acid-base properties. A different monolayer for which one of the mechanisms of electron transfer is not operative may differentiate these five surfaces with respect to acid-base properties. So far, attempts at using arachidic alcohol or amine to distinguish between the acid-base properties of these five surfaces have been unsuccessful due to the inability to determine the conditions necessary for quantitative transfer of either monolayer to the surfaces.
The Journal of Physical Chemistry, Vol. 86, No. 26, 1982 5127
The final group of surfaces, Au and Pt, is different from the other two groups of surfaces in that the average for Au and Pt are averages of results which were not reproducible. It would be expected that the hEbefor Au and Pt should be similar to the a b e for an Ag surface since all three surfaces are noble metals with no oxidation. However, values of aEberanging from 3.33 to 3.69 eV were obtained for the Pt surfaces and aEb values ranging from 3.45 to 3.67 eV were obtained for the Au surfaces. These ranges of values for A& are similar to the range of values found for a monolayer of Cd(AA)2tail on a silver surface and indicate that various amounts of rearrangement had occurred in the monolayers on the Au and Pt surfaces. Of the 10 surfaces studied, the Au and Pt are the most hydrophobic and the surface-carboxylate interaction apparently is not sufficient to keep the monolayer from rearranging on these two surfaces. While the lower end of each range may be more representative of the “true” AEbe for a head-on monolayer on each surface, there is no way of determining how much rearrangement may have occurred in those monolayers on which the lower values of A&, were measured. XPS studies of a single monolayer of Cd(AA)2on metal/metal oxide surfaces have demonstrated the feasibility of using such measurements to investigate the acid-base properties of the surfaces. Examples of electron transfer from the surface to the carboxylate group (the surface acting as a base) and vice versa (the surface acting as an acid) were found. In addition, an effect analogous to the leveling effect that water has on aqueous acids and bases was observed on several of the surfaces. Mechanisms were proposed to explain how electron density was transferred in either direction and why the leveling effect may occur. Future work in this area should be directed toward finding a monolayer system in which only one mechanism of electron transfer was operative (the u donation to the surface) and where transfer was effective to the surfaces. Such a monolayer system should be able to discriminate between those surfaces on which a leveling effect was occurring and those surfaces where electron transfer was occurring. Infrared spectroscopy would provide valuable additional information about these systems, especially how the symmetric COz stretching frequency changes with change in surface.
Acknowledgment. We thank C.R. Brundle and W. Knoll of IBM Research San Jose and K. Mittal of IBM East Fishkill for helpful and informative discussions about this work. F.C.B. thanks IBM for providing a fellowship in 1980-81.
