J. Phys. Chem. 1990, 94, 2005-2010
2005
Surface-Enhanced Raman Spectroscopy of Surfactants on Silver Electrodes Soncheng Sun,+ Ronald L. Birke,* and John R. Lombardi* Department of Chemistry, City College, City University of New York, New York, New York 10031 (Received: June 30, 1989)
Surface-enhanced Raman spectroscopy (SERS) has been used to study different kinds of surfactants (cationic, anionic, and nonionic surfactants) adsorbed on a roughened Ag electrode. Spectral assignments are made for the SERS spectrum of cetylpyridinium chloride (CPC), and it is shown that the molecule is oriented with its pyridinium ring end-on at the electrode surface at potentials positive to the point of zero charge (pzc) on Ag. Another cationic surfactant, cetyltrimethylammonium bromide, shows a weaker SERS spectrum than CPC; however, its spectrum indicates a “solidlike” hydrocarbon chain region near the head-group layer for spectra measured at potentials positive to the pzc. The solidlike structure is a region of extended hydrocarbon chain which is in the ail-trans configuration. The interactions between the Ag surface and the various kinds of surfactants are found to depend on the charge and type of head group, however; both the cationic and nonionic surfactants studied have a head-on orientation at potentials positive to the pzc. It has also been found that these cationic and nonionic surfactants show an unexpected enhancement for C-H stretching modes at potentials more negative than ca. -0.8 V vs SCE with a concurrent development of two new SERS vibrational bands at 2710 and 2815 cm-’ in the C-H stretching region.
Introduction The investigation of surfactants adsorbed on metal surfaces is extremely important in a variety of fields such as adhesion, lubrication, detergency, and corrosion inhibition. Research advances in all these fields are closely related to an understanding of the interaction between the surfactant and the metal surface as well as the structure of the adsorbed species. In addition, monomolecular surfactant films on metal surfaces are a way of producing well-defined organic structures at interfaces which can be used as model systems in chemical and biophysical studies. Although surface-enhanced Raman spectroscopy (SERS) is an ideally suited in situ method for studying surfactant molecules adsorbed on metal surfaces, SERS studies of long-chain surfactant molecules have been rather sparse and mostly concentrated on metal island films and SO IS.^-^ There has been one SERS study of a surfactant adsorbed on a electrode surface;* however, this investigation was limited to a cationic surfactant on Cu and the potential dependence was not studied. The Purpose of this work is to utilize the SERS technique for a comparative study of the in situ adsorption characteristics of the various types of surfactants-cationic, anionic, and nonionic-on a Ag electrode surface. The surfactant films are formed by adsorption from solution which is a method for producing spontaneously organized molecular assemblies of monolayers or multilayers. Because the surfactant micelles are still in solution when the Raman studies are made, we have used depolarization measurements to verify the surface-specific nature of the observed spectra. We found, indeed, that the observed Raman spectra are surface specific and that cationic and nonionic surfactants give well-defined SERS spectra. In particular the cationic surfactants show, at potentials positive to the pzc (ca. 4 9 V vs SCE on Ag), spectra characteristic of a rigid hydrocarbon chain in the region near the head-group layer. Such a “solidlike” structure with an extended hydrocarbon chain in the predominantly all-trans configuration would be expected for an organized monolayer. In addition both cationic and nonionic surfactants show an unexpected intensity growth of the long-chain C H modes as the Ag electrode potential is moved into the region of hydrogen evolution. Experimental Section Cetyltrimethylammonium bromide, C T A B , ( C H 3 (CH2)1SN+(CH3)3Br-); cetylpyridinium chloride, CPC, (C,,H33N+CSH5C1-); sodium dodecyl sulfate, SDS, (CH3(CH2),,0S03-Na+); polyoxyethylene(23)dodecanol, Brij-35, (CH3(CH,), I(OCH2CH2)230H);and Triton X-100 (CH3(C‘Present address: Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 262.
0022-3654/90/2094-2005$02.50/0
TABLE I: Depolarization Ratio of CTAB and CPC
ILl4
freql cm-I
assignments
CTAB 2846 CHI sym str 2885 CHI asym str 2925 CH2 sym strand overtone of CH, scissoring 2960 CPC 2844 CH, sym str 2884 CH2 asym str 2922 CH, sym str and overtone 2956 of CH, scissoring 1026 sym and trigonal ring breathing 3076 ring CH str
normal SERS 0.08 0.30 0.12 0.20 0.07 0.18 0.12 0.31 0.04 0.13
0.40 0.40 0.38 0.38 0.51 0.43 0.46 0.41 0.60 0.43
H2)7C6H4(OCHzCH2)9,50H) are reagent grade (Fisher Scientific) and were used without further purification. It was determined that there was no observable difference in the SERS spectrum with recrystallized surfactant as compared to a surfactant which was used without further purification. The equipment for spectroscopic and electrochemical measurements has been described e l ~ e w h e r e . ~All of the potentials are referenced to the saturated calomel electrode (SCE). For simplicity, in the measurement of depolarization ratios, we changed the direction of polarization of the analyzer which was located between the sample and the entrance slit of the monochromator instead of changing the polarization direction of the incident light. The laser light was polarized parallel to the scattering plane, Le., p polarized. In this case the depolarization ratio is defined as the ratio of IL/Ill, where I , is the intensity of the scattered radiation polarized perpendicular to the scattering plane, and Ill is the intensity of scattered radiation polarized parallel to the scattering plane.
Results and Discussion Surface Origin of Spectra. There are several criteria which (1) Sandroff, C. J.; Garoff, S.; Leung, K.P. Chem. Phys. Lett. 1983,96(5), 547. (2) Garoff, S.; Sandroff,C. J. J . Phys. 1983, CIO,483. (3) Heard, S. M.; Grieser, F.; Barraclough, C. G. Chem. Phys. Lert. 1983, 95(2), 155. (4) Knoll, W.; Philpott, M. R.; Golden, W . G. J . Chem. Phys. 1982, 77(1), 219. ( 5 ) Girlando, A.; Gordon 11, J. G.; Heitmann, D.; Philpott, M. R.; Seki, H.; Swalen, J. D. Surf. Sci. 1980, 101, 417. (6) Knoll,W.; Philpott, M. R.; Swalen, J. D.; Girlando, A. J . Chem. Phys. 1982, 77(5), 2254. (7) Moskovits, M . ; Suh, J . S. J . A m . Chem. SOC.1985, 107, 6826. (8) Dendramis, A. L.; Schwinn, E. W.; Sperline, R. P. Surf. Sci. 1983, 134, 615. (9) Sun, S. C.; Bernard, 1.; Birke, R. L.; Lombardi, J. R. J . Electroanal. Chem. 1985, 196, 359.
0 1990 American Chemical Society
Sun et al.
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
I
2600
2900
3000
, 2800
2900
3000
(E)
(A)
Wavenumber ( c m - ' 1
Figure 1. Depolarization property of CTAB around the 2900-cm-' region. (A) normal Raman spectrum of 0.1 M CTAB aqueous solution; (B) SERS spectrum of 10 mM CTAB on a roughened Ag electrode at -0.3 V vs SCE. The upper spectra (a) are parallel polarized and the lower spectra (b) are perpendicular polarized. The notation m X n on the side of each spectra is the number of counts per second (cps) full scale with n being the exponent of the base 10, Le., 5 X 3 = 5000 cps.
have been used to verify that an observed Raman spectrum from an electrochemical interface originates from the molecules adsorbed on the metal surface, instead of from the free molecules in the solution phase. These criteria include an observable potential dependence, laser excitation frequency dependence, and a dependence on the roughness of the metal surface. In addition, the depolarization property of the Raman scattering offers possibly the most telling evidence for distinguishing between surface-enhanced Raman scattering and the normal Raman scattering for a solution phase. For the normal Raman spectrum of the molecules in the solution phase, the depolarization ratio of nontotally symmetric modes is close to 0.75, but for highly symmetric modes it is close to zero. However, in a SERS spectrum both symmetric and nonsymmetric modes for adsorbed molecules have similar values of the depolarization ratio (0.6-0.75). Figure 1 shows the normal Raman spectrum (A) and the surface Raman spectrum (B) of CTAB in the C-H stretching region. The upper spectrum is parallel polarized and the lower spectrum is perpendicular polarized. A similar set of spectra were found for CPC. For the SERS spectrum the depolarization ratio differs only slightly from one mode to another, but for the normal Raman spectrum different modes have very different depolarization ratios. Table I shows the depolarization ratios of the cationic surfactants CTAB and CPC in the C-H stretching region. Almost all of the modes in the SERS spectrum have similar depolarization ratios (between 0.4 and 0.6). However, in the normal Raman spectrum the highly symmetric modes, such as the symmetric C-H stretching around 2845 cm-I, have a very low depolarization ratio (close to zero) for both CTAB and CPC. The trigonal breathing for CPC at 1026 cm-I is also listed in Table I. It is a totally symmetric breathing mode and has the depolarization ratio of 0.04 in the normal Raman spectrum but about 0.6 in the SERS spectrum. Thus, data in Table I are good evidence that the Raman spectra observed from the electrode surface are the result of a SERS process from adsorbed molecules. Chemical quenching of active sites is another method for distinguishing SERS from a normal Raman spectrum. It has been found9J0that surface enhancement is related to the formation of Ag' adcluster complexes with halides or Raman scatterers. If an additional complexing reagent is added to form a new surface complex, which is readily soluble in water, the original Ag' center will be lost from the surface. For example, by adding S20,2-or SO,2-,which has a strong tendency to form water-soluble complexes with Ag', the active sites on the silver surface will be lost. This property has been discussed in our halide concentration (10) Watanabe, T.; Kawanami, 0.;Honda, K.; Pettinger, B. Chem. Phys. Lett. 1983, 102(6),565.
2700 2800 2900 3000 Wavenumber
cm-1
Figure 2. SERS spectra of 10 mM Brij-35 in 0.01 M KBr solution around the 2900-cm-l region (a) before and (b) after an injection of
Na2S20,. dependence inve~tigation.~ We found that, at high concentration of chloride, the intensity of a pyridinium band decreases as the chloride concentration increases and this decrease is irreversible. This decrease can be attributed to the formation of Ag' cluster halide complexes at high concentrations of halides. Figure 2 shows the C-H stretching range around 2900 cm-' of the SERS spectrum of 10 mM Brij-35, a nonionic surfactant, at -0.3 V. The upper curve is the spectrum before addition of Na,S203. The lower spectrum is after injection of Na2S203.The spectrum disappeared immediately after the injection. This quenching by S20,2-gives further evidence of SERS. It is, therefore, safe to conclude that the observed spectrum is indeed a surface-enhanced spectrum. Comparison of Surfactants of Different Charge Types. On comparing the SERS spectrum of various surfactants, we found that the surfactant which has a nitrogen-containing heterocyclic head (CPC) gave the strongest SERS intensity in comparison with other kinds of surfactants. The SERS spectra of other cationic surfactants (CTAB) and nonionic surfactants (Brij-35 or Triton X-100) were also obtained, but the intensities are much weaker than that of the nitrogen-containing heterocyclic surfactants. All of these spectra can be shown by the methods previously discussed to be the surface-enhanced Raman spectra and not normal Raman spectra from the solution phase. Only a very weak signal was observed for anionic surfactants, such as SDS, under the same conditions as other surfactants. These differences indicate, as would be expected, that different kinds of surfactants have different interactions with the metal surface. The CPC has a nitrogen-containing heterocyclic head, however; it is different from pyridine, since the nitrogen atom is linked to an alkane chain, and there is no lone pair of electrons that is able to bond directly to the silver surface. One possible interaction between CPC and Ag active sites is the interaction through the conjugate a orbital of the pyridine ring, and another interaction involves ion pair formation with specifically adsorbed halide ions where the positively charged head group is able to form an ion pair with an adsorbed halide anion. This kind of interaction has been observed and discussed in detail for pyridinium9 and methyl pyridinium." In the case of CTAB a head group attached orientation has been previously suggested on a Cu electrode.* The weaker intensity of CTAB in comparison with CPC is probably due to the weaker interaction between the cationic surfactant and the specifically adsorbed halides. The interaction between halide and CPC (11) Bunding, K. A.; Bell, M. I.; Durst, R. A. Chem. Phys. Len. 1982, R9(1), 54.
SERS of Surfactants on Ag Electrodes
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 2007 TABLE 11: Raman Shift and Assignment of CPC
normal
Wilson's no. 16a
approx descripn of vibrn Ag-C1 stretch out-of-plane ring deformn
6b 11 10b 18a 17a
in-plane ring deformn out-of-plane C H deformn out-of-plane C H deformn in-plane C H deformn out-of-plane deformn
+
100
300
500
700
900
Wavenumber
1100 1300 1500 1700 [ c m-I )
Figure 3. (a) S E R S spectrum of 10 mM of CPC in 0.1 M KC1 aqueous solution on a Ag electrode at -0.2V vs S C E and (b) normal Raman spectrum of 0.5 M CPC solution around the 100-1700-cm-' region.
molecules can involve both an electrostatic attraction and also the overlap of the electron density between the halide anions and the x orbital of the pyridine ring. The weaker interaction between the halide and the CTAB molecules is probably due to the stereo effect of three methyl groups on the head of CTAB. For nonionic surfactants, such as Brij-35 or Triton X-100, a SERS intensity weaker than CPC but similar to CTAB was observed, again indicating a weaker interaction between the Ag surface and the nonionic surfactant than with CPC. The nonionic surfactant has a poly(ethy1ene glycol) moiety (-OCH2CH2),0H, which, as a Lewis base, interacts with the metal surface by the lone pairs of electrons on the oxygen atoms. The evidence for this interaction is the pronounced shift of the SERS bands of the poly(ethy1ene glycol) moieties upon adsorption.12 It has also been suggested that the benzene ring moiety in Triton X-100 may be adsorbed via a x-electron interaction with the surface based on the observation of relatively higher enhancement of the benzene ring modes than other modes.I2 It is also instructive to consider the alkane chain moiety in these surfactants. The alkane chains should be oriented away from the surface due to the hydrophilic property of the Ag surface. This can be seen from the SERS spectra of Brij-35 in the C-H stretching range (2700-3000 cm-I) in Figure 2. The predominant bands around 2840,2875,2920, and 2950 cm-I correspond to the C-H stretching modes of poly(ethy1ene glycol) moieties. The relative intensities of these modes are very different from those of the dodecanyl alkane chain. For poly(ethy1ene glycol) moieties the relative intensities of 2920 and 2950 cm-' are much higher than the 2840- and 2875-cm-I bands. But, for the alkane chain the 2845-cm-l band has the highest intensity compared with other C-H stretching bands. The very weak signal for anionic surfactants, such as SDS is probably due to the weak interaction between the negatively charged head group and the surface. The spectrum is very weak not only in the presence of halides but also in the absence of halides. The former result is presumably due to the competition between halides and surfactant. This kind of competition has been observed in the SERS measurement of p-nitrobenzoate anions. The halide ions were found to quench the SERS intensity of p-nitroben~oate.~~ The SERS spectra of mono- and bicarboxylic acid adsorbed on silver surface has also been reported7 and it has been found that the SERS spectra of monocarboxylic acids became undetectable when chain lengths beyond dodecanoic acid were (12) Mengoli, G.; Musiani, M. M.; Pelli, B.; Fleischmann, M.; Hill, I . R. Efecfrochim.Acra 1983, 28(12), 1733. (13) Sun, S.C.; Birke, R. L.; Lombardi, J. R. J . Phys. Chem. 1988, 92, 5965.
1 12 18b 15 9a
14
sym and trigonal ring breathing in-plane C H deformn in-plane C H deformn in-plane C H deformn C H 2 twistc ring stretch C H 2 wagC C H 2 scissorsc C H I scissorsc
8b 8a
ring stretch ring stretch
C H 2 sym stretchC CH, asym stretchC C H I sym stretch overtones of CH, scissoring 2, 7b, 13 C H stretch 20a, 20b
S E R S of CPC" freq" re1 224 57 400 2 594 2 642 8 680 1 770 3 812 12 860 5 946 3 976 2 1024 100 2 1050 1168 11 1208 5 1292 1 1318 2 1 1370 1390 1 1438 7 1480 2 1494 2 1574 10 1628 93 2718 2 17 2844 13 2884 2922c 11 7 2956 3076 31
Ramanb spectra of CPC freqb re1
594 642
4 13
770 814 865
6 5
5
1026 100 1056 6 1170 12 1214 5 1298 11
1434
22
1580 1631
7 13
1846 2880 2922 2962 3092
86 63 56 16 33
'SERS spectrum of IO mM of C P C in 0.1 M KCI solution on Ag electrode. Double-potential pretreatment -0.2 to +0.3 to -0.2 V; 1-s pule was used. *Normal Raman spectrum of 0.5 M C P C aqueous solution. Modes represent tail group.
used. This is presumably due to the competition between micelles and the solid-liquid interface. The surfactant molecules with a longer hydrophobic chain prefer to stay in micelles rather than on the solid-liquid interface. Hence, the SERS spectrum for long chain anionic surfactant is hardly observed. Assignment of the SERS Spectrum of CPC. We report on the details of the CPC S E W spectrum because it was the most intense of the surfactants studied. In order to utilize SERS spectra to study surfactant surface interactions, it is necessary to have reasonably good assignments. The SERS spectrum of 10 mM CPC on a Ag electrode and the normal Raman spectrum of 0.5 M CPC in aqueous solution in the 100-1700-~m-~region and in the 2800-3100-cm-1 region are shown in Figures 3 and 4, respectively. The assignment of the CPC spectrum is listed in Table 11. This assignment is based on a comparative analysis of the normal Raman spectra and the SERS spectra for a series of surfactants with different head g r o ~ p s ~ ,and ' - ~on the earlier assignments of SERS spectrum for N-methylpyridinium ion on a Ag electrode." Comparing the relative intensities of the SERS spectrum and the normal Raman spectrum in Table 11, we can clearly see that the enhancement of the head group (pyridinium ring) and the tail group (alkane chain) is quite different. In the SERS spectrum the average intensity of the modes f r o m t h e tail group is about 5 times less enhanced than that from the head group at -0.2 V. On the basis of many of the theoretical SERS models which have predicted stronger enhancement for bands associated with molecules that are close to the surface,14 we can conclude that the pyridinium head group is attached to the surface, leaving the long tail directed away from the surface. Similarly, for CTAB molecules adsorbed on a Cu electrode surface a preferential en(14) Furtak, T. E.; Reyes, J. Surf. Sci. 1980, 93, 351.
2008
Sun et al.
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
I
c 0
1000
1100
1200
Wavenumber ( c m - ' 1
Figure 5. (a) Normal Raman spectrum and (b) S E R S spectrum of CTAB in the C-C stretching region around 1000-1200 cm-I. G represents the gauche conformation and T represents the trans conformation.
2800
3000
3200
Wavenumber (cm-') Figure 4. (a) SERS spectrum of 10 mM of CPC in 0.1 M KCI aqueous solution on a Ag electrode at -0.2V vs SCE and (b) normal Raman spectrum of 0.5 M CPC solution around the 2800-3100-~m-~region.
hancement of the vibration bands associated with the (CHJ3N+ group has been observed.8 The SERS spectral data can also be used to tell whether the head group is adsorbed in a flat or end-on orientation. Figures 3 and 4 show that the two bands at 1628 and 3076 cm-' are more enhanced relative to other bands in the SERS spectra. These bands are due to the in-plane vibrations of the ring stretch, 1628 cm-l, and the C-H stretch, 3076 cm-l, Table 11. Moskovits and Suhi5have pointed out that such a relative enhancement occurs when a planar molecule is standing up on the surface since then there is a strong coupling of the in-plane modes with the electric field normal to the surface, the normal electric field component being strongly enhanced by plasmon excitations in metallic microstructures. Thus the pyridinium head group is in a standing up configuration at the electrode surface. There are five main bands in the 2500-3100-~m-~region. As mentioned, the band at 3076 cm-I has been assigned to the C-H stretching of the pyridine ring." The bands at 2846 and 2880 cm-' have been assigned to C H 2 symmetric and antisymmetric stretching modes respectively for many long-chain surfactants with different head g r o ~ p s . ~ This - ~ Jassignment ~~~ coincides with the depolarization property of CPC in the normal Raman spectrum as shown in Figure 1 and Table I. The assignments of the 2922and 2962-cm-l bands are less certain since there have been differences among previous assignments. Our observations support the assignment given by Snyder and Schererl9 for 2922 cm-I as the combination band of CH2 stretching and 2962-cm-I band as the overtone of CH, scissoring. Based on the head-on orientation for CPC discussed previously, the hydrophobic CH3 terminal must be away from the surface. However, from Figure 4 we can see that the shoulder at 2956 cm-' is more enhanced than the CHI stretching bands at 2844 and 2884 cm-I. Thus it is less reasonable to assign the 2956-cr$ band to the terminal CH, stretching mode as given in ref 7, 8, and 18. In addition, the depolarization ratio of the 2922-cm-' band (0.12) is smaller than that of 2884-cm-' band (0.18) for CPC, and the ratio of the depolarization ratios of these bands is even smaller for CTAB where the 2925-cm-' band depolarization ratio is 0.12 and that for the 2885-cm-] band (15) Moskovits, M.; Suh, J. S. J . Phys. Chem. 1988, 92. 6327. (16) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (17) Kamogawa, K.; Tajima, K.; Hayakawa, K.; Kitagawa, T. J . Phys. Chem. 1984,88, 2494. (18) Okabayashi, H.; Kitagawa, T. J . Phys. Chem. 1978.82, 1830. (19) Snyder, R. G.; Scherer, J . R. J . Chem. Phys. 1979, 71. 3221.
is 0.30 (Table I), indicating that these two bands are unlikely to have the same assignment as a CH2 asymmetric stretching mode as given in ref 20. Conformation of CTAB Surfactant at the Interface. The 1050-1 150-cm-' region of the Raman spectrum of surfactants contains the skeletal vibration of the C-C stretching modes which are sensitive to the conformation of the hydrocarbon chain. In the normal Raman spectrum of CTAB, for example, there are three bands located around 1065, 1090, and 1121 cm-' (Figure 5a). The bands at 1065 and 1121 cm-' have been assigned to C-C symmetric and antisymmetric stretching modes, respectively. The 1065- and 1121-cm-I bands represent the trans (T) conformation and the 1090-cm-' band represents the gauche (G) conformation.is2%2'In Figure 5a the normal Raman spectrum of 0.1 M CTAB shows a relatively high intensity for the ca. 1090-cm-I band and low intensities for the 1065- and 1121-~m-~ bands. Since the critical micelle concentration, cmc, of CTAB is 9.2 X M,22 the concentration of 0.1 M CTAB solution is much higher than the cmc. The contribution to the normal Raman spectrum from the free molecules in the solution phase is negligible; that is, the spectrum represents only CTAB micelles. The larger contribution of gauche conformers indicates that the hydrocarbon chain in the micelle core remains "fluid" (kinked) with several gauche isomers present. This spectrum is very similar to the previously reported spectrum of CTAB at concentrations above the In Figure 5b the SERS spectrum of 0.01 M CTAB on a Ag electrode at -0.6 V shows the G band around 1090 cm-' to be weaker than the T bands at 1065 and 1121 cm-l, indicating that near the head-group layer there is a layer of solidlike hydrocarbon chain. Comparison of Figure 5b with the same spectral region of the SERS of 1-hexadecanethiol on silver island films shows a remarkable similarity.'J Thus the adsorbed film of CTAB at the Ag-electrolyte interface has the same conformation as the solidlike phase of a monolayer of 1-hexadecanethiol on a silver island film, even though the former interphase is in contact with overlying bulk water. Other features of the SERS spectrum of CTAB require comment. A comparison of the SERS spectrum of 0.01 M CTAB recorded on Ag at -0.3 V with 488-nm excitation with that of CTAB on a copper substrate* taken with 661-nm excitation shows similar features. In both cases the band at ca. 1306 cm-l due to methylene twisting is much lower in relative intensity in the SERS spectrum compared to the normal Raman spectrum of the micelle solution. Furthermore, the band at 2844 cm-I in the C-H stretching region predominates over the 2884-cm-I band. This situation usually implies a disordered hydrocarbon chain; i.e., the 2884-cm-l band is normally the strongest in the C-H stretching region for a solidlike surfactant layer.23 Using this criterion, an (20) Larsson, K.; Rand, R. P. Biochim. Biophys. Acta 1973, 326, 245. (21) Gaber, B. P.; Yager, P.; Peticolas, W. L. Biophys. J . 1978, 21, 161. (22) Berg, R. W. Spectrochim. Acta 1978, 34A, 655. (23) Kalyanasundaram, K.; Thomas, J. K. J . Phys. Chem. 1976.80, 1462.
SERS of Surfactants on Ag Electrodes
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 2009
1 ---
-08 -0 7
-08 -0 I
__ycI/
L
A
v -0.4
-0.3
_c_
-
9
-0.2 -0.1
2700
2900
3100
wavenumber
zwo
2800
3000
r o v c m h r (cm-1)
(cm -'I
Figure 6. SERS spectra of 10 m M Brij-35 on a Ag electrode around the 2700-3 100-cm-' region at different potentials.
interpretation of the data from the C-H stretching region based on the relative intensity data would be contradictory to the evidence for an extended trans chain with crystalline properties as deduced above from the C-C stretching region. However, Dendramis et aL8 have argued that the band at ca. 2850 cm-' in the SERS spectrum of CTAB contains a contribution from the methyl groups of the surfactant head group which are preferentially enhanced at the surface. Thus conclusions regarding conformational properties of the tail group cannot be made as usual from the ratio of intensities of the 2880-2850-cm-I bands. Also consistent with an ordered hydrocarbon chain on the electrode surface is the loss of intensity in the 1306-cm-' band which can unequivocally be assigned to the methylene twisting motions in the tail group. The picture of the conformation of CTAB at the metal surface at potentials more positive than -0.9 V which emerges from the spectral analysis is a surface-attached head group with an extended chain, close-packed tail region. It is not possible to tell whether a monolayer or bilayer exists at the surface from this spectral evidence. A more detailed geometric picture of the interface such as the one given from IR reflectance spectroscopy in the work of Allara and N ~ z z on o ~oriented ~ monolayers of n-alkanoic acids on an aluminum substrate was not attempted for the more complicated SERS activated surface. Potential Dependence. One interesting feature of the SERS spectra of surfactants on a Ag electrode is the potential dependence which shows a marked change at very negative potentials. Figures 6 and 7 show the SERS spectra of Brij-35 and CTAB in the C-H stretching range around 2900 cm-' at different potentials. There are two intensity maxima of the C-H stretching modes in the potential range from -0.1 to -1.7 V. The first maximum is around -0.5 V. This maximum, as with most SERS scatterers, can be attributed to a resonant charge-transfer mechanism which has been discussed in detail previously.25,26 When the potential is
Figure 7. SERS spectra of 10 m M CTAB on a Ag electrode around the 2700-3 lOO-cm-' region at different potentials.
shifted to negative values greater than -0.8 V, the intensities of the C-H stretching bands increase dramatically until they reach a second maximum around -1.3 V. The intensity of the second maximum is much higher (about 2-20 times) than that of the first maximum, indicating an additional enhancement at very negative potentials. As the intensity increases at these negative potentials, there are two new bands at around 2710 and 2815 cm-' which increase at a relatively higher rate than other C-H stretching bands as the potential is shifted beyond -1.3 V. In order to interpret this phenomenon, the following effects should be considered. Firstly, we must consider the change of the hydrophilic property of the metal surface at very negative potentials. As discussed previo~sly,~ the adsorption of pyridinium ions is closely related to an ion pair formed with specifically adsorbed halide ions. The Raman intensity depends on the amount of adsorbed halide. Similar to pyridinium, the cation surfactants would be expected to also be adsorbed via specifically adsorbed halide ions. At negative potentials to the point of zero charge (pzc), halide desorbs from the surface because of the electrostatic repulsion. The loss of the halide ions from the surface would cause the desorption of the surfactant; however, the head group of a cationic surfactant would be expected to be attracted to a negatively charged surface. Then as the potential is moved into the region of hydrogen evolution, the interface becomes more hydrophobic, possibly causing the hydrophobic tail to adsorb on to the surface. The hydrogen ion can be reduced to atomic hydrogen or hydrogen molecules which may form an adlayer between the solid and liquid phases. This will greatly increase the hydrophobic property of the surface, possibly causing a change in the orientation of the surfactant. This orientation change appears to be indicated by the observation that as the intensities of the tail group increase the intensities of the head group decrease and that both cationic and nonionic surfactants give the same spectrum in the C-H D.Phys. Reu. Lett. 1983,50(17), 1301. ( 2 6 ) Lombardi, J. R.; Birke, R. L.; Lu, T. H.; Xu,J. J. Chem. Phys. 1986, 84(8), 4174. ( 2 5 ) Furtak, T. E.; Roy,
(24) Allara, D. L.; Nuzzo, R. G.Langmuir 1985, I , 52.
2010
J. Phys. Chem. 1990, 94, 2010-2013
stretching region at potentials more negative than -1.1 V (see Figures 6 and 7). The large enhancement of the intensity at the second maximum might be attributed to the formation of adsorbed hydrogen. An additional enhancement is also observed when the electrode potential is stepped to the region of rapid hydrogen evolution prior to performing an oxidation-reduction cycle for any SERS mea~ u r e m e n t . ~ ’This additional enhancement has been accounted for by a calculation of the increase of the SERS enhancement when the dielectric constant is changed from an aqueous to a hydrogen film.** The rapid decrease of the SERS intensity at a potential more negative than -1.3 V is probably due to the evolution of a large amount of hydrogen gas which forms bubbles and destroys the interface. On the other hand, another possible mechanism for producing a second maximum in the intensity vs potential data would be a resonance from a second charge-transfer state. However, there is no independent evidence for such a process. With CPC an additional factor to consider is a reduction reaction at the electrode which occurs at a potential around -0.9 V. However, the same potential dependence is still observed as with the other kinds of surfactants, such as CTAB, Triton X-100, or Brij-35 which do not undergo any redox reaction in the potential range used for a silver electrode (0 to -1.7 V vs SCE). The first intensity maximum is around -0.5 V for both the head- and tail-group SERS modes. However, all of the modes from the head group, including the C-H stretching mode of the pyridine ring around 3070 cm-I, disappear at a potential more negative than -0.9 V. At the second maximum only the modes from the alkane chain can be observed. The SERS spectrum of the C-H stretching modes at the second intensity maximum for Brij-35 is very diffferent from the spectrum (27) Barz, F.; Gordon 11, J . G.;Philpott, M. R.; Weaver, M. J . Chem. Phys. Lett. 1983, 91, 168. (28) Kerker, M.; Wang, D.-S. Chem. Phys. Left. 1984, 1 0 4 ( 5 ) , 516.
at potentials more positive than -0.8 V (Figure 6). However, all of the different kinds of cationic and anionic surfactants studied give spectra which are identical at the second maximum with those of Brij-35; cf. Figures 6 and 7. This fact indicates that the spectra at the second maximum correspond to the alkane moiety, which is the common feature of the surfactants. The two new bands at around 2710 and 2815 cm-I, which appear at a potential more negative than -0.8 V, are not found in the previous assignments of the SERS spectrum of a hydrocarbon chain. A possible explanation is that these two modes represent a splitting of the C-H stretching modes of the alkane chain, which are shifted by the very negative potential to lower energy. These new bands may represent a new structure at the interface which is a highly organized monolayer. This latter conclusion is supported by interfacial pressure vs area curves made from surrface tension vs concentration studies of a cationic surfactant, myristyltrimethylammonium bromide (similar to CTAB), as a mercury/electrolyte interface studied as a function of electrode potential.29 At potentials positive to the point of zero charge (pzc) on mercury the curves have a parabolic shape representative of a condensed liquid film, whereas, at potentials more negative than the pzc at ca. -0.8 V, the shape of the curves is more linear, indicating a solid film which is very rigid and of very low compressibility. However, a more detailed explanation in terms of adsorbate structure of the new SERS modes found in the C-H stretching region at very negative potentials must await further investigation.
Acknowledgment. R.L.B. and J.R.L. are indebted to the National Science Foundation (CHE-8711638), the PSC-BHE Research award program of the City University of New York (66367 and 667261), and the National Institutes of Health MBRS program (RR-08 168) for financial assistance. (29) Chan, S. Y.-Y. Ph.D. Dissertation, The City University of New York, 1988.
Interchromophoric Interactions In Ionic 1,n-Dicarbazolylalkanes John Masnovi,* Randolph B. Krafcik, Ronald J. Baker,**+and Robert L. R. Towns Department of Chemistry, Cleveland State University, Cleveland, Ohio 441 15 (Received: June 30, 1989)
The radical cations of 1,n-di-N-carbazolylalkanes 1-5 ( n = 1-5) were generated by flash photolysis of electron donor-acceptor complexes of tetranitromethane and 1-5. The carbazole radical cations combine in a second-order process with nitrogen dioxide released by dissociative electron capture by tetranitromethane. The kinetics and transient absorption spectra of the radical cations indicate appreciable interaction between two pendent carbazole groups separated by n = I , 3, or 4 methylene units, supported in the case of 1 by a single-crystal X-ray determination.
Introduction Association is enhanced in excited states of aromatic hydrocarbons, many of which form excimers (Ar:Ar*) following exElectronic delocalization provides the “driving force” for association in such species. Arene radical cations also may associate to form dimer ions (Ar:Ar’+).3,4 The dimers, which differ from the monomers in physical properties, typically can be distinguished by optical absorption spectroscopy. For example, recently we observed that anthracene forms dimer radical cations4 that are less reactive than monomer radical cations toward anionic n~cleophiles.~ Delocalization of charge between the two anthracenes constituting the dimer likely is responsible for the decrease in reactivity. ‘Author to whom correspondence concerning crystallographic information should be addressed.
0022-3654/90/2094-2010$02.50/0
We now report results that address the formation and reactivity of dimer radical cations derived from carbazole derivatives. Carbazole is a heterocycle that has the same number of ?r electrons as anthracene. Interest in carbazole radical cations stems from use of poly(vinylcarbazo1e) as a conducting organic polymer.6 (1) Birks, J. B. Photophysics of Aromafic Molecules; Wiley-Interscience: London, 1970. Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd 4.; Academic Press: New York, 1971. (2) Locke, R. J.; Lim, E. C. Chem. Phys. Lett. 1987, 134, 107. (3) Kira, A,; Nakamura, T.; Imamura, M. J . Phys. Chem. 1977,81, 51 1 . Kira, A.; Arai, S.; Imamura, M. Zbid. 1972, 76, 1 1 19. (4) Badger, B.; Brocklehurst, B. Trans. Faraday SOC.1970, 66, 2939. ( 5 ) Masnovi. J. M.: Kochi. J. K. J . Phvs. Chem. 1987. 91. 1878. (6) Masuhara, H.; Ohwada, S.; Mataga, N.; Itaya, A,; Okamoto, K.; Kusabayashi, S. Chem. Phys. Lett. 1978,59, 188. Pearson, J. M. Pure Appl. Chem. 1977, 49, 463.
0 1990 American Chemical Society
