202
Langmuir 1987, 3, 202-209
excess of 9.0 X mol/cm2 f 25%. This is fairly close to the averaged value calculated from ATR spectra, 5.7 X mol/cm2 f 14%. When one considers that the ZnSe powder was not cleaned by O2 plasma before the depletion experiments in the same way the IRE was cleaned, the agreement is quite good. It is possible that the CH2C12 solution afforded a measure of cleaning for the powder which could not be effected in water and thus gave better agreement with CH2C1, ATR results. The surface excess values determined here compare well with those determined by Maoz and Sagiv," who used ATR of dried films of several surfactants containing 20carbon alkyl chains, along with other methods. They calculated absorbtivities (by peak height) of 2.3 X absorbance units per reflection per CH2unit. The spectra reported here demonstrated absorbtivities (by peak height) of 1.6 X absorbance units per reflection per CH2unit from either water or CH2C12. Our surface excess values of ca. 6.6 X lo-'' mol/cm2 give specific areas near 0.25 nm2/molecule. Because our aqueous solutions were above the critical micelle concentration, near-maximal deposition was expected (at least packed monolayer density).14 Maoz and Sagiv attribute their spectra to closely packed monolayers of surfactant, deposited from bicyclohexyl solution, with specific areas of 0.20 nm2/molecule. From this we infer that all our films were very similar to packed monolayers. Upon comparison of the values of r in Table I for samples B and C, it appears that the presence of 1%acetonitrile had an insignificant effect on the adsorption of CPC from water. The values of sample absorbance and of r with and without acetonitrile were well within the expected error range of f7.8%. Similarly, upon comparison of samples A and B in Table I, the absorbances due to acetonitrile do not appear to have been influenced by the adsorbed film of CPC. Had some increased amount of acetonitrile been dissolved into the adsorbed film, the intensity of the band at 2255 cm-' would have increased. Another concern was that a compact adsorbed CPC film would exclude acetonitrile from the interface. This is the mathematical equivalent of a negative surface excess of 1% acetonitrile solution, which may be calculated using the right-most term of eq 2 above. Assuming the CPC film
to be 2.0 nm thick, the loss of intensity in the 2255-cm-' band would have been only 7 X absorbance units per reflection, which is insignificant. Conclusions (1)A convenient, reliable method for the study of ad-
sorption, i.e., determination of Gibb's surface excess (r) has been developed. The method has been validated for adsorption onto one available IRE material, ZnSe. With the availability of computers and advanced calculators, it is a simple matter to use the correct equations. We have used the cylindrical ATR device, but the method can be applied to flat-plate ATR devices. This method, with the proper theory,20could be extended to the in situ determination of adsorption onto various films which have been coated onto IRE'S.^^ Such in situ determinations are more meaningful in most cases than examining adsorption onto these surfaces after drying the IRE. The implications to quantitative ATR spectroscopy are currently under investigation. (2) This work demonstrates that spectrophotometers have increased in sensitivity to where C-H IR bands may be used analytically in aqueous solution. One is not limited, by proximity of the very intense 0-H stretching bands of water, to the use of bands of other functional groups, e.g., C=O and SO,. (3) Further work is planned concerning the adsorption behavior of hydrophobic chelating agents of the type used in extractive metallurgy. We are interested in whether the metal ion complexes of these extraction agents are surface active and how adsorbed complexes would influence the rates of phase transfer. Acknowledgment. The Strategic Metals Recovery Research Facility at the University of Arizona thanks the Perkin-Elmer Corp. for their academic relations gift of the Model 1800 Fourier transform IR spectrophotometer. We also thank the National Science Foundation for a grant in support of this research. Registry No: CPC, 123-03-5;ZnSe, 1315-09-9. J. O p t . SOC.A m . 1968,58, 380-390. (21) References contained in ref 6. (20) Hansen, W. N.
Vibrational Coupling in Carbon Monoxide Adlayers on a Platinum Electrode Mark W. Severson," Andrea Russell, Douglas Campbell, and Joel W. Russell Department of Chemistry, Oakland University, Rochester, Michigan 48063 Received August 6, 1986. I n Final Form: October 29, 1986
Infrared spectra of carbon monoxide on a platinum electrode in 1 M sulfuric acid solution at partial coverages are reported for pure l2C0 and for mixtures of l2C0 and 13C0. The results are interpreted in terms of a vibrational coupling model. It is shown that the effective coupling in this adlayer is about twice as large as is observed for carbon monoxide adsorbed on Pt(ll1) under ultrahigh vacuum conditions; the coupling does not appear to change with electrode potential. Evidence for a structure change of the partial coverage adlayer upon coadsorption of hydrogen is also presented. Introduction The infrared spectrum of carbon monoxide on platinum electrode surfaces has attracted both experimentalla and
* To whom correspondence should be addressed.
theoreti~al'-'~attention in recent years. The vibrational spectrum of CO adsorbed on metals has also been exten(1) Beden, B.; Bewick, A.; Kunimatsu, K.; Lamy, C. J . Electroanal Chem. 1982, 142, 345.
0743-7463/87/2403-0202$01.50/0 0 1987 American Chemical Society
Vibrational Coupling in CO Adlayers
sively studied under ultrahigh vacuum (uhv) condit i o n ~ , allowing ~ ~ - ~ ~information obtained from these experiments to be applied to the more complicated electrochemical systems. When carbon monoxide is adsorbed on a platinum electrode from aqueous acid solution, a relatively intense band a t about 2080 cm-' is observed in the infrared spectrum, and by analogy with results obtained for CO adsorption under uhv conditions, this band has been assigned to the C-0 stretching motion of CO adsorbed in an on-top site, i.e., bound to a single platinum atom.' A second band a t about 1850 cm-' has also been observed and assigned to bridge-bonded CO;596the intensity of this band is much smaller than is observed under uhv conditions on Pt(ll1). The 2080-cm-I band appears to arise from the major part of the adsorbate formed under these conditions, and it has been the most extensively studied. The wavenumber of this band is potential-dependent; for a saturation coverage of CO on a platinum electrode in aqueous acid solution it shifts to higher wavenumber as the electrode potential is increased, and the shift is linear, with a slope of 30 ~ m - ' / V . ~ g ~ Vibrational spectra of CO adsorbed on metals under uhv conditions have shown that the wavenumber of the bands assigned to on-top and bridge-bonded CO are coveragedependent.'3J4J8J9 Both of these bands shift to higher wavenumber as the coverage is increased. Most analyses of this behavior have focused on the more intense band assigned to on-top CO. Infrared spectra due to adsorbates formed from mixtures of different isotopes of carbon monoxide on platinum single-crystal surfaces have demonstrated fairly convincingly that the wavenumber shift with coverage is due to dynamical coupling within the adsorbate layer.'4Js Although the origin of this coupling is not entirely clear, it has most often been described in terms of a dipole-dipole coupling m ~ d e l . ~ ~ - ~ l (2) Russell, J. W.; Overend, J.; Scanlon, K.; Severson, M.; Bewick, A. J. Phys. Chem. 1982,86, 3066. (3) Russell, J. W.; Severson, M.; Scanlon, K.; Overend, J.; Bewick, A. J. Phys. Chem. 1983,87, 293. (4) Golden, W. G.; Kunimatsu, K.; Seki, H.; J. Phys. Chem. 1984,88, 1275. (5) Kunimatsu, K.; Golden, W. G.; Seki, H.; Philpott, M. R. Langmuir 1985, 1, 245. (6) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G.; Philpott, M. R., submitted for publication. (7) Ray, N. K.; Anderson, A. B. J . Phys. Chem. 1982,86, 4851. (8) Hollaway, S.; Norskov, J. K. J.Electroanal. Chem. 1984,161, 193. (9) Lambert, D. K. Phys. Reo. Lett. 1983, 50, 2106. (10) Lambert, D. K. Solid State Commun. 1984,51, 297. (11) Lambert, D. K. J . Vac. Sci. Technol. B 1985, 3, 1429. (12) Korzeniewski, C.; Pons, B. S.; Schmidt, P. P.; Severson, M. W. J . Chem. Phys., in press. (13) Shigeishi, R. A.; King, D. A. Surf. Sei. 1976, 58, 379. (14) Crossley, A.; King, D. A. Surf. Sei. 1977,68,528; Surf. Sci. 1980, 95, 131. (15) Krebs, H. J.; Luth, H. Appl. Phys. 1977, 14, 337. (16) Horn, K.; Pritchard, J. J. Phys. 1977, 38, C4, 164. (17) Golden, W. G.; Dunn, D. S.; Overend, J. J . Phys. Chem. 1978,82, 843. (18) Severson, M. W.; Tornquist, W. J.; Overend, J. J . Phys. Chem. 1984, 88, 469. (19) Haydon, B. E.; Bradshaw, A. M. Surf. Sci. 1983, 125, 787. (20) Baker, M. D.; Chester, M. A. In Vibrations at Surfaces; Caudano, R., Gilles, J. M., Lucas, A. A., Eds.; Plenum Press: New York, 1983; p 289. (21) Froitzheim, H.; Hopster, H.; Ibach, H.; Lehwald, S. Appl. Phys. 1977, 13, 147. (22) Hopster, H.; Ibach, H. Surf. Sei. 1978, 77, 109. (23) Steiniger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 264. (24) Avery, N. R. J. Chem. Phys. 1981, 74, 4202. (25) Blyholder, G. J. Phys. Chem. 1964, 68, 2772. (26) Hammaker, R. M.; Francis, S. A,; Eischens, R. P. Spectrochim. Acta 1965,21, 1295.
Langmuir, Vol. 3, No. 2, 1987 203
Because of the similarities between the spectra of CO adsorbed on Pt under uhv conditions and adsorbed on platinum electrodes, one might reasonably expect that intermolecular coupling between nearby adsorbed CO molecules should be an important effect for CO adsorbed on a platinum electrode in solution. In order to investigate the role of intermolecular coupling in adsorbates formed from CO on Pt electrodes in electrochemical systems, we decided to study the coverage dependence of the infrared spectrum and the spectra due to isotope mixtures. Experimental Section T h e electrochemical cell which was used for the infrared measurements has been described p r e v i ~ u s l y . ~Prior t o each experiment, the 8-mm diameter platinum working electrode was polished successively with 1.0, 0.3, and 0.05-pm alumina on a polishing cloth. Acid solutions, 1.0 M, were prepared from organic-free water and sulfuric acid (Baker Ultrex). The reference electrode was a saturated calomel electrode (SCE); all potentials reported here are with reference t o this electrode. After t h e solution was deaerated with nitrogen, the platinum electrode was subjected to cathodic-anodic potential sweeps between -0.25 and +1.3 V until a cyclic voltammogram characteristic of a clean platinum electrode in acidic solution was obtained. Carbon monoxide was then bubbled through t h e solution for 5 min for t h e experiments with pure l2C0; preparation of solutions containing mixtures of "CO and I3CO is described below, as is t h e procedure used to form the adlayers. The infrared spectra reported here were measured using an IBM Instruments Inc. IR-98 Fourier transform infrared spectrometer. The procedure used to obtain t h e spectra was simply to record a 128-scan spectrum a t the potential of interest and then to sweep the potential to +1 V, a t which CO is not adsorbed, and record a 128-scan reference spectrum. Thus, all the spectra reported here are absorbance spectra with the spectrum a t +1 V used as reference. All spectra were obtained with 4-cm-' resolution, using a liquid nitrogen cooled HgCdTe detector (Infrared Associates). Acquisition of each 128-scan spectrum required about 1 min.
Results Carbon monoxide adlayers on the platinum electrode were established by removing the electrode from the window a t a potential of +1 V and then changing the potential to that desired for adsorption. Typically, about 15 s were required for saturation coverage to be achieved, and so partial coverage adlayers could be established by leaving the electrode in the bulk solution for shorter periods of time. The concentration of CO in the thin layer between the electrode and window was low enough and diffusion of carbon monoxide into the thin layer slow enough that no significant increase in coverage was observed in the time required to obtain the infrared spectra, typically 1-4 min. After the spectrum due to the adlayer was obtained, the potential was ramped to +1 V at a rate of 100 mV/s, and the linear-sweep voltammogram (LSV) was recorded. The area under the peak in the LSV due to oxidation of CO to COPwas measured, and used as a measure of the coverage of CO on the electrode surface. Coverages reported here are relative to saturation. Some difficulties were encountered with this method of measuring the coverage, due to the fact that the CO oxidation peak i n the LSV is superimposed on the current due to formation of the platinum oxide layer, requiring estimation of a base line for the CO oxidation peak. As a result, the measured coverages are uncertain by about 10%. A series (27) Mahan, G. K.; Lucas, A. A. J . Chem. Phys. 1978, 68, 1344. (28) Persson, B. N. J.; Ryberg, R. Phys. Reu. B 1981, 24, 6954. (29) Scheffler, M. Surf. Sei. 1979, 81, 562. (30) Persson, B. N. J. Surf. Sei. 1982, 116, 585. (31) Moskovits, M.; Hulse, J. E. Surf. Sei. 1978, 78, 397.
204 Langmuir, Vol. 3, No. 2, 1987
Severson et al.
L 7elative
:overage
1 .o
1.9
1.71
1.58 1
I
I
1
I
0 1
0 2
a?
01
0 5
I 0 6
0 7
1 O B
0 9
3.37 Relative Coverage
).22
1.12
Figure 2. Dependence of the wavenumber of the band due to adsorbed CO on the coverage relative to saturation at three different electrode potentials: ( 0 )+200 mV (SCE); (X) 0 mV (SCE);(0)-200 mV (SCE).
0.07
2150
2100
2050
2000
1950
Wavenumber.em-'
Figure 1. Infrared spectra for the indicated partial coverages of CO on the platinum electrode at a potential of +200 mV (SCE).
of infrared spectra was measured for partial coverages of CO at different potentials. Adsorption was carried out at +200 mV, and infrared spectra at +200,0, and -200 mV were then recorded without removing the electrode from the window. The potential was then swept to +1V for the LSV measurement and reference infrared spectrum. This procedure ensured that the coverage for the spectra obtained at these three potentials was constant, although the value for the coverage obtained is somewhat uncertain. Infrared spectra in the CO stretching region for increasing coverages of carbon monoxide on the platinum electrode at +200 mV in aqueous H2S04are shown in Figure 1. At a fractional coverage of 0.07, a band is observed at 2044 cm-', and this band shifts to higher wavenumber with increasing surface coverage, appearing at 2084 cm-' a t saturation coverage. Spectra were also recorded for other electrode potentials, and similar results were obtained. The wavenumber of the CO stretching vibration at +200,0, and -200 mV as a function of surface coverage is plotted in Figure 2. It can be seen that there is a relatively large initial increase in wavenumber as the coverage is increased and a smaller increase a t higher coverage. Results similar to this have been obtained for CO adsorbed on single-crystal platinum surfaces under ultrahigh vacuum (uhv) conditions.13 For any given coverage, the wavenumber of the CO stretching vibration band increases with potential, as has been reported previously for saturation coverages of CO on platinum electrodes; from Figure 2 it is seen that this shift with potential is largest at low surface coverage, about 50 cm-I/V, as compared to a 30 cm-'/V shift a t saturation coverage. The integrated intensities of the bands observed at +200,0, and -200 mV are plotted as a function of coverage in Figure 3. The band intensity shows an initial large increase with coverage and a smaller increase at the higher coverages. Qualitatively similar results have been obtained for CO adsorbed on single-crystal Pt under uhv conditions, with
'O
tI
I
I
I
I
I
1
I
I
I
0 1
0 2
0 3
0 4
0 5
0 8
0 7
0 8
0 9
Relative Coverage
Figure 3. Dependence of the integrated intensity of the band due to adsorbed CO on the coverage relative to saturation at three different electrode potentials: ( 0 )+200 mV (SCE);( X ) 0 mV (SCE); (0) -200 mV (SCE).
some dependence on which crystal face is For a given coverage, the integrated intensities of the bands are seen from Figure 3 to be approximately constant for the three potentials. During the course of these experiments, we noticed that for a given partial coverage of CO, when the electrode potential was changed from +200 to -200 mV, there was, in addition to the decrease in wavenumber, a significant increase in the bandwidth as well We also noticed that the change in both wavenumber and band shape was not reversible; when the potential was returned to +200 mV, the CO band shifted to a higher wavenumber, but not as high as the initial value, and it retained about the same bandwidth as was observed at -200 mV. These results are shown in Figure 4 for three partial coverages; similar results were obtained for all partial coverages observed. From Figure 4 it is seen that this irreversible behavior is most pronounced for the lowest coverages; it becomes less important at higher coverages, and at saturation, reversible results are obtained. The infrared spectra of adlayers formed from isotopic mixtures of l2C0 and I3CO at saturation coverage at +200 and -200 mV are shown in Figures 5 and 6. Solutions of
Langmuir, Vol. 3, No. 2, 1987 205
Vibrational Coupling in CO Adlayers
i
I
IIC
lllb
llla
IIC
Ib
Ila
IC
Ib
la
t
2150
I
2100
I
2050
2
P
0
I
2100
2 0'5 0
2obo
I
1950
0
Wavenumber, cm-'
Figure 4. Infrared spectra obtained for potential step experiments for partial coverages of CO. In each case, spectrum a was recorded with the electrode potential at +200 mV (SCE)following adsorption at this potential, spectrum b at -200 mV (SCE) following spectrum a, and spectrum c at +200 mV (SCE) following spectrum b. Relative coverages: (I) 0.3; (11) 0.6; (111) 0.75.
the isotopic mixtures were prepared by degassing the aqueous acid solution on a vacuum line and then admitting a mixture of the isotopes at a pressure of 200 torr. An equilibrium concentration of the gas mixture in solution was attained by shaking the solution in the presence of the gas for a minimum of 12 h. This resulted in a solution of rather low CO concentration, as was evidenced by the fact that about 10 min were required for saturation coverage to be achieved on the electrode in bulk solution. Furthermore, it appears that the saturation coverages achieved with this procedure were somewhat lower than were achieved with the higher solution-phase concentrations used for the pure l2C0 adsorption experiments. For all of the isotope mixture experiments, adsorption was carried out at +200 mV. For the spectra at +200 mV, the band for pure I3CO appears at 2026 cm-', and that for pure l2C0 at 2083 cm-l; for mixtures of the two isotopes, a single band at wavenumber between the pure isotopes a single band at wavenumber between the pure isotope wavenumbers is observed. For the same adlayers at -200 mV, essentially identical spectra are observed, except that all the bands are shifted to lower wavenumber by about 1 2 cm-'. The coverage dependence of the spectra due to the isotopic mixtures was also studied. A series of spectra for increasing coverages of a 10% l2C0 mixture at +200 and -200 mV are shown in Figures 7 and 8, and a series of spectra for a 50% l2C0 mixture at these two potentials is shown in Figures 9 and 10. For the 10% l2CO mixture at +200 mV, a single broad band at about 1995 cm-l is observed at the lowest coverage. As the coverage is increased, the spectrum appears to be made up of two
Wavenumber, c m - l
Figure 5. Infrared s ectra for saturation-coverage isotopic mixtures of l2C0 and ! CO at +200 mV (SCE).
overlapping bands, one at about 2000 cm-' and a second band at about 2027 cm-I. With further increases in coverage, both bands increase in intensity and shift to higher wavenumber; the intensity of the higher wavenumber band increases more rapidly than does that of the lower wavenumber band, until at saturation coverage it is the major band in the spectrum. Similar results are observed at -200 mV for the same coverages; in this case the two bands appear at significantly lower wavenumber than at +200 cm-'. For the 50% l2C0mixture, a single band is observed at all coverages for both +200 and -200 mV. This band increases in intensity and shifts to higher wavenumber with increasing coverage.
Discussion The results reported here and by other workers'+ show that there are three effects which cause changes in the spectrum of carbon monoxide adsorbed on a platinum electrode in aqueous acid solution. The observed spectrum depends on the surface coverage, the electrode potential, and the potential at which the adlayer was formed. The shift of the C-0 stretching band with electrode potential has been the most extensively studied effect. At saturation coverage in aqueous acid solution, the band increases linearly in wavenumber with potential with a slope of 30 c ~ - ' / V . ~ This - ~ shift was first described in terms of a bonding change between carbon and oxygen with changes in potential. In this interpretation, as the electrode potential becomes more positive, electrons are withdrawn from a molecular orbital which is primarily antibonding between carbon and oxygen, resulting in an increase in the carbon-oxygen bond strength and an increase in vibrational wa~enumber.~,'-~ Lambert4-11has proposed an alternate model for the shift. He has shown
Severson et al.
206 Langmuir, Vol. 3, No. 2, 1987
3elative 12
overage
co
fraction
0
70
1 .o
-
I
59 1980 47
39
37
I
2100
I
2150
I
I
I
2000
2050
1950
1
Wavenumber. c m - ’
2 0’5 0
2 160
2 010 0
1950
Figure 8. Infrared spectra for partial coverages of a 1:9 mixture of l 2 C 0 and 13C0 a t -200 mV (SCE).
Warenumber.cm-l
Figure 6. Infrared s ectra for saturation-coverage isotopic mixtures of l 2 C 0 and CO a t -200 mV (SCE).
Y
Relative coverage
Relative coverage
I .o
1 0
3.54
0.78
1.44
0 59
1.34
I
I 0 47
1.2
2a33 0.39
--I
1.15
0.37
P
2100
0.37
2 0’5 0
2 0100
I
1950
I
1900
2
0
I
2050
I
2000
I
1950
1
0
Wavenumber, c m - ’
Figure 9. Infrared spectra for partial coverages of a 1:l mixture of l 2 C 0 and 13C0 a t +200 mV (SCE).
Wavenumber, Cm-’
Figure 7. Infrared spectra for partial coverages of a 1:9 mixture of W O and 13C0 a t +200 mV (SCE).
that a coupling of the carbon-oxygen vibration with the intense electric field present at the surface of an electrode can result in a change in frequency due to the first-order Stark effect. From the results of an ab initio study of CO
on a metal cluster, Kunimatsu et al.5 have stated that changes in the carbon-oxygen bond with potential are quite small and that the shift with potential is due entirely to the first-order Stark effect. Most recently, Korzienewski et a1.12 have shown that changes in the binding energy between the adsorbed CO molecule and the surface can result in changes in the CO stretching frequency on the
Langmuir, Vol. 3, No. 2, 1987 207
Vibrational Coupling in CO Adlayers
I
I
I.
2
4
6
a/
I
I
2024
8
10
s
Figure 11. Calculated dependence of the relative intensity in the high-frequency component of the isotopic doublet on the relative magnitudes of the effective coupling force constant and the separation of the uncoupled frequencies. The calculation is for a 1:1 mixture of two isotopes and a positive effective coupling force constant.
Zo.. I
2100
1
2050
1 2000
I
1950
I
1900
Figure 10. Infrared spectra for partial coverages of a 1:l mixture
of "%O and 13C0 at -200 mV (SCE).
order of that observed, even in the absence of a change in the carbon-oxygen bonding parameters. It is possible that a combination of these effects contributes to the shift of the band with potential. The changes in the spectrum of adsorbed CO with coverage, and the results for isotope mixtures, are similar to the results obtained for CO adsorbed on platinum under uhv condition^.'^-^^ The uhv experiments have been the object of extensive interpretation,2531and there is general agreement that intermolecular coupling in the adsorbate layer is the source of the coverage-dependentwavenumber shifts. Dipole-dipole coupling has been the model most often used to describe the interaction, though other effects may contribute as well. It is convenient to use a simple effective force constant model which has been described previously18to draw some conclusions about the magnitude of the coupling in the adlayer formed under the conditions of the electrochemical experiment. In this model we assume that the CO-stretching mode may be separated from the other vibrational modes of the adsorbate-substrate system, that the bond in an adsorbed CO molecule may be described by an effective principal force constant f,, and that intermolecular interactions in the adsorbed layer may be represented by an effective interaction force constant f,,,. For saturation coverages of isotope mixtures, this model gives essentially the same results as more elaborate dipole-dipole coupling models, without assuming an explicit type of intermolecular interaction. For a mixture of isotopes, the effect of vibrational coupling on the observed spectrum is determined by the magnitude of the effective interaction force constant and the separation of the frequencies due to the isotopes in the absence of coupling. With the notation of ref 18, the uncoupled frequencies are calculated from X = Xo + ( M / N ) k and A' = Ad [ ( N- M)/N]k', where Xo and Ad are the effective principal force constants for the two isotopes, k and k' are the effective interaction force constants, ( M l N ) is the fraction of the first isotope, and ( N - M ) / N is the fraction of the second isotope. The effective coupling between isotopes is given by a = ( [ M ( N- M)]1/2/N)k".32
+
The dependence of the relative intensities on the ratio a / 8 , where 8 = (A - A')/2, is plotted in Figure 11 for a 1:l mixture of two isotopes, using a positive coupling force constant. This was calculated from eq 11 of ref 18, assuming that the frequencies for the mixture in the absence of coupling between isotopes are 2080 and 2030 cm-'. From this, it can be seen that the effect of coupling on the spectrum due to isotopic mixtures is for the high-wavenumber component of the isotopic doublet to have a larger intensity than would be expected for such a mixture in the absence of coupling. For quite large coupling, i.e., for values of a16 greater than about 1.5, only a single band is expected to be observed in the spectra due to isotopic mixtures; the wavenumber of this band depends on the isotopic composition and appears at a position between the two pure isotope bands. The observation of a single band, intermediate in wavenumber to the pure isotope bands, for saturation coverage of mixtures of l2C0 and 13C0 on the platinum electrode in acid solution, shown in Figures 5 and 6, indicates that strong coupling exists in this adlayer. The fact that there is no significant change in the intensities of the bands due to the isotope mixtures at any of the coverages studied, cf. Figures 5-10, when the potential is changed indicates that there is no large change of coupling with potential. Values for the parameters in the effective force constant model which apply at saturation coverage can be derived from two experimental observations. The appearance of a single band in the isotopic mixture spectra at saturation coverage requires that the ratio of the effective coupling force constant to the effective principal force constant be greater than or on the order of 0.07; this will result in relative intensities of the two bands in the calculated spectrum having a ratio of about 10:l. Second, the sum of the two force constants must result in the observed saturation coverage wavenumber for a single isotope. Using the observed wavenumber of 2079 cm-l for saturation coverage l2COat 0 V, we obtain an effective principal force constant f, = 16.36 mdynA at 0 V, and an effective coupling force constant f,,( = 1.1mdyn/A. The intermolecular coupling does not appear to change with potential, so we may assume that the potential dependence of the spectrum is entirely reflected in a change in effective (32) For different isotopes of the same molecule, the various force constants are related by ratios of the reduced maas, i.e., iff, is the effective principal force constant for adsorbed CO, Xo = fJp, and Xo = fJd, where p and p' are the reduced masses for the two isotopes, e.g. l / p = 1/12 + If the effective coupling force constant appropriate for CO adlayers is f,,., then k = f,, / p , k' = frr,/p', and k" = frr./(pp')'''.
208 Langmuir, Vol. 3, No. 2, 1987
Severson et al.
Table I. Calculated Wavenumbers (em-') and Relative Intensities and Observed Wavenumbers for Saturation Coverage Mixtures of I2CO and I3CO +200 mV -200 mV 'TO calcd calcd obsd calcd calcd obsd fraction wavenumber intensity wavenumber wavenumber intensity wavenumber 0 2039 1 2026 2027 1 2016 0.87 2038 2037 0.87 2025 0.1 2049 2009 0.13 0.13 1997 0.88 2042 0.88 2055 2048 0.3 2060 1997 0.12 0.12 1985 0.92 2046 0.92 2061 2057 0.5 2069 0.08 0.08 1977 1989 1 2072 1.0 2085 1 2085 2073
principal force constant, i.e., f, = fo + f 'V, where fo = 16.36 mdyn/A, V is the electrode potential relative to SCE,and f = 0.5 mdyn A-' V-' is obtained from the 30 cm-l/V shift at saturation coverage. Wavenumbers and relative intensities calculated for saturation coverage isotope mixtures by using these values are given in Table I, along with the observed values. The observed values of all but the pure l2CO band are about 10 cm-' lower than the calculated values; this is most likely due to a somewhat lower saturation coverage formed under the conditions of the isotope experiments. Assuming this to be the case, the calculated wavenumber of the higher wavenumber component of the isotope mixture doublet agrees well with the observed wavenumber, and the calculated relative intensities agree well with the fact that only a single band is observed in the experimental spectra. The effective interaction force constant calculated in this way gives a lower limit for the magnitude of the coupling. If larger values of the interaction force constant are used, there is a relatively small effect on the calculated wavenumbers and the relative intensity of the high-wavenumber component becomes larger than shown in Table I, and this would match the experimental results equally well. For CO adsorbed on Pt(ll1) at 300 K under uhv conditions, an interaction force constant of 0.54 mdyn/A was used,l8 so the effective coupling for saturation coverage CO on platinum in sulfuric acid solution is at least twice as large as observed under uhv conditions. A model for the vibrational coupling which is valid over all ranges of coverage and electrode potential studied is more difficult to derive. We have no knowledge of the underlying surface structure or of the structure of the adsorbate under these conditions; these structures most likely show significant heterogeneity and may well change significantly with coverage, potential, or both. Indeed, there is evidence, discussed below, for different structures which depend on the potential of adsorption. Thus, both the effective principal and interaction force constants could, in general, be functions of both coverage and potential, and it may be impossible to express these parameters as simple functions of these variables. For these reasons, we do not believe that it is appropriate to assume an explicit physical origin, such as dipole-dipole coupling, for the intermolecular coupling. The coupling is strong, and it is likely that more than one physical effect contributes to the interaction; assuming roughly the same molecular parameters for adsorbed CO on a platinum electrode in solution as has been assumed for adsorbed CO under uhv conditions, dipoledipole coupling would predict such large coupling for high densities, but at such small intermolecular separations, other effects could also make a significant c o n t r i b ~ t i o n . ~ ~ The final effect to be considered is the dependence of the infrared spectrum on the potential of adsorption. Kunimatsu et a1.6 have interpreted their results for CO
adsorbed at saturation coverage in and out of the hydrogen region as being due to different adsorbate structures. Our results may be interpreted in the same way. The observation of different spectra at +200 mV at partial coverage before and after the potential is stepped to -200 mV must be due to an irreversible change in the structure of the adsorbate which occurs when hydrogen is coadsorbed. The larger shift of the CO-stretching band with potential at partial coverages is probably due to this structure change. We can say little about these structures at this time. The appearance of two bands at low coverage in the spectra due to the 10% l2C0 mixture, as well as the increase in wavenumber of the band due to pure l2C0 with coverage, indicates that adsorption in the double-layer region proceeds randomly; i.e., island formation does not occur. It is interesting that for a saturation-coverage adlayer adsorbed in the double-layer region, the results of Kunimatsu et al. appear to indicate that subsequent oxidation occurs at the edges of island^.^ Kunimatsu et a1.6have measured the infrared spectrum of saturation coverage CO adlayers formed at potentials in the hydrogen region. They observed two bands in the CO-stretching region, at about 2080 and 1860 cm-l, which are assigned to on-top and bridge-bonded carbon monoxide, respectively. They found that although the intensity of the band due to bridge-bonded CO is much smaller than the band due to CO in an on-top site, it was more intense than when adsorption of the adlayer was carried out at potentials in the double-layer region. There are differences in the behavior of the spectrum at potentials at which carbon monoxide oxidation occurs, depending on the potential at which adsorption occurred. Kunimatsu et al. observed from their infrared spectra that following adsorption in the hydrogen region, the first CO species oxidized is the bridging species and that the wavenumber of the on-top-bound CO decreased at these potentials, prior to oxidation of the on-top species. They postulated that upon removal of the bridging species, the remaining ontop-bound CO occupied the newly vacated sites, resulting in a larger average intermolecular distance and smaller coupling. It may also be that coupling of the on-top to the bridge-bound molecules plays a role;'* because of the large difference in wavenumber, about 200 cm-l, the effects of such coupling should be relatively small, but if the molecules are relatively close to one another, the effect on the vibrational wavenumber could be large enough to explain part or all of the observed decrease in wavenumber of the band due to the on-top bound CO upon removal of the bridging species. Summary From the results reported here we have shown that vibrational coupling exists in adlayers formed from carbon monoxide on a platinum electrode in acidic solution and that the effective coupling at saturation coverage is at least
Langmuir 1987, 3, 209-214 twice as large as is observed for CO on Pt(ll1) in vacuum. We have also shown that the adsorption of hydrogen results in a change in the infrared spectrum of partial-coverage CO adlayers which is irreversible on the time scale of our experiments; we have attributed this to a change in the structure of the adlayer. More information on the structure of adlayers formed from carbon monoxide on platinum electrodes is required to develop a model for the physical origin of the vibrational coupling.
209
Acknowledgment. The infrared spectra were obtained on an IBM Instruments Inc. IR-98 FTIR spectrometer which was purchased with the assistance of a University Instrumentation Grant from the Department of Defense, administered through the Office of Naval Research. We thank Dr. W. G. Golden for supplying a preprint of (see ref 6 this paper). Registry No. Pt, 7440-06-4; CO, 630-08-0; H2,1333-74-0.
Parameters Affecting Aqueous Micelles of CTAC, TTAC, and DTAC Probed by Fluorescence Quenching E. Roelants and F. C. De Schryver* Department of Chemistry, Laboratory f o r Molecular Dynamics and Spectroscopy, University of Leuven, Celestijnenlaan 200 F, B-3030 Heverlee, Belgium Received August 6,1986. I n Final Form: November 20, 1986 The quenching of the 1-methylpyrene (1-MePy) fluorescence by mobile (iodide ion, m-dicyanobenzene) as well as immobile (N-methyl-N-decylaniline, tetradecylpyridinium chloride) quenchers has been used to study the aqueous micellar systems of cetyltrimethylammonium chloride (CTAC), tetradecyltrimethylammonium chloride (TTAC), and dodecyltrimethylammonium chloride (DTAC). Fluorescence quenching allows the determination of the aggregation number, as one of the important parameters of the micelle. This study evaluates the influence of structural factors (chain length, headgroup) as well as environmental factors (detergent concentration such as addition of electrolyte, temperature, and counterion) on micellization.
Introduction Fluorescence quenching of added probes to micellar systems, nonionic as well as ionic ones, is a frequently used technique to obtain more information about micellar solution~.’-~The method consists in the solubilization of a probe (preferably a water-insoluble one) in the micellar phase, addition of a fluorescence quencher (mobile or immobile) to the system, and measurement of the time profile of the fluorescence intensity by the time-correlated single photon counting technique (SPC). The kinetic expression for fluorescence quenching in micellar systems has been developed by T a ~ h i y aInfelta ,~ et 81.: Dederen, Van der Auweraer et al.,7 and Almgren et ala8 The time depen(1)Croonen, Y.Ph.D. Thesis, K.U., Leuven, 1984. (2)(a) Croonen, Y.; Gelade, E.; Van den Zegel, M.; Van der Auweraer, M.; Vandendriessche, H.; De Schryver, F. C.; Almgren, M. J. Phys. Chem. 1983,87,1426.(b) De Schryver, F. C.; Croonen, Y.; Gelade, E.; Van der Auweraer, M.; Dederen, J. C.; Roelants, E.; Boens, N. In Proceedings of the International Symposium on Surfactants in Solution (Lund);Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984. (3) (a) Roelants, E. Ph.D. Thesis, in preparation. (b) Roelanta, E. Master Thesis, University of Leuven, 1982. (4)(a) Roelants, E.; Geltrde, E.; Van der Auweraer, M.; Croonen, Y.; De Schryver, F. C. J. Colloid Interface Sci. 1983,96,288.(b) Roelants, E.;Gelad&,E.; Smid, J.; De Schryver, F. C. J. Colloid.Interface Sci. 1985, 107, 337. (5)(a) Tachiya, M. Chem. Phys. Lett. 1975,33,289.(b) Tachiya, M. J. Chem. Phys. 1983,78, 5282. (6) (a) Infelta, P. P.; Gratzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78,190. (b) Infelta, P.P.; Gratzel, M. J. Chem. Phys. 1983,78, 5280. (7) (a) Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. Lett. 1979,68,451.(b) Dederen, J. C. Ph.D. Thesis K.U., Leuven, 1981. (c) Van der Auweraer, M. Ph.D. Thesis, K.U., Leuven, 1981. (d) Van der Auweraer, M.; Dederen, J. C.; Palmans-Windels, C.; De Schryver, F. C. J. Am. Chem. SOC.1982,104,1800. (e) Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. J. Phys. Chem. 1981,85,1198.
dence of the fluorescence intensity can be given by relation 1.5a,6a,7,9 The boundary conditions necessary to apply the I(t) = AI exp(-A2t - A3[l - exp(A4t)])
(1)
proposed kinetic expression are given in previous papers,2 as well as the meaning of the different parameters in eq ie2-4,5a,6a
A theoretical model for the intramicellar first-order quenching rate constant k,, was developed by Van der Auweraer et a1.9 and Hatlee et a1.I0 The systems studied are alkyltrimethylammonium chloride surfactants CH3-( CH2).-N (CH3) C1DTAC: m =3; n = 11 TTAC: n = 1 3 CTAC: n = 15 CDMAC: m = 2; n = 15 in which 1-methylpyrene (1-MePy) is solubilized as probe. As quenchers the mobile (ionic) iodide ion (I-) and (nonionic) m-dicyanobenzene (m-DCB) and the immobile N-methyl-N-decylaniline(MDA) and tetradecylpyridinium chloride (TPyC1) were used. The reason for the so-called “immobility”of the two latter can be ascribed to their long lipophilic tail: they will reside in the micelle during the lifetime of the excited probe; this means that the k- (exit rate constant for the quencher from the micelle) is negligible compared with kqm,simplifying the kinetic scheme. (8)Almgren, M.; Linse, P.; Van der Auweraer, M.; De Schryver, F. C. J . Phys. Chem. 1984,88,289. (9)Van der Auweraer, M.; Dederen, J. C.; Gelade, E.; De Schryver, F. C. J. Chem. Phys. 1981,74, 1140. (10)Hatlee, M. D.; Kozak, J. J.; Gratzel, M. Ber. Bunsenges. Phys. Chem. 1982,86,157.
0743-7463/ 87/ 2403-0209$01.50/0 0 1987 American Chemical Society
