Langmuir 1992,8, 2049-2063
2049
Determination of Alcohol Solvent Orientation and Bonding at Silver Electrodes Using Surface-Enhanced Raman Scattering: Methanol, Ethanol, 1-Propanol, and 1-Pentanol Raymond L. Sobocinski and Jeanne E. Pemberton* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received April 29, 1991. I n Final Form: April 24, 1992
Interfacial solvent structure at Ag electrodes in various straight-chain alcohol electrolyte systems has been studied using surface-enhancedRaman scattering (SERS). SERS provides detailed information regarding the nature of interfacial bonding and alkyl chain orientation. Methanol appears to be in a unique hydrogen-bonding environment at the interface, as indicated by a 10-cm-l decrease in v(C-0) frequency from its bulk solution value; Cz and longer chain alcohols at the interfaceshow no shift in v(C-0) frequency from bulk solution. The alkyl chains are largely parallel to the surface (i.e., greater than 4 5 O relative to the surface normal) and exist in a largely all-trans configuration at electrodepotentials positive of the PZC (potential of zero charge). These molecules are oriented with the hydroxyl group at the surface and the alkyl chain tilted slightly away from the surface. At more negative potentials near the PZC, the alkyl chain conformations become more disordered,and the alkyl chains “stand up”,with the methyl group directed into bulk solution. For methanol, two distinct orientations exist simultaneouslyat the interface in the vicinity of the PZC. The growth of a new methylene band for Cz, C3, and C5 alcohols at increasingly negative potentialssuggests significantinteractionof the methylene hydrogens with the surface. In contrast to the C3 and C5 alcohols, the C1 and Cz alcohols exhibit little bulk spectroscopic behavior, indicating distinct orientations at potentials negative of the PZC. All observations support the idea that the a-carbon is confined near the surface at all potentials for all four alcohols. The driving force for this behavior is proposed to be the formation of C-H-Ag agostic bonds. Introduction
Detailed information regarding organic solvent structure, such as bonding and orientation, at metal electrodes has been somewhat elusive. In fact, several have noted the lack of fundamental information regarding the nonaqueous electrochemical interface in general. The behavior of alcohols, in particular, has been of interest to electrochemists for many years. Methanol has received considerable attention4due to its importance in fuel cell applications. The adsorption behavior of more complex and longer chain alcohols from aqueous solutions has also been studied for ethanol,5-12 1-propanol,l3-l5 2-prop a n 0 1 , ~ ~l-butanol,18 J~ isobutanol,18 2-butanol,18 l-pen-
* To whom correspondence should be addressed.
(1)Borkowska, Z. J. Electroanal. Chem. 1988,244, 1. (2)Trasatti, S. Electrochim. Acta 1987,32,843. (3)Parsons, R. Electrochim. Acta 1976,21,681. (4)Parsons, R.;VanderNoot, T. J. Electroanal. Chem. 1988,257,9. This is an extensive review article with 182 references from 1981-1987, 44 of which refer to methanol oxidation. (5)Chang, S.-C.;Leung,L.-W. H.; Weaver, M. J.J.Phys. Chem. 1990, 94,6013. (6)Holze, R. J.Electroanal. Chem. 1988,246,449. (7)Beden, B.; Morin, M. C.; Hahn, F.; Lamy, C. J.Electroanal. Chem. 1987,229,353. (8)Willsau, J.; Heitbaum, J. Electrochim. Acta 1986,31,943. (9)Willsau. J.: Heitbaum. J. J.Electroanal. Chem. 1985,194,27. (10)Snell, K. D.; Keenan, A. G. Electrochim. Acta 1982,27,1683. (11)Snell, K. D.; Keenan, A. G. Electrochim. Acta 1981,26,1339. (12)Morin, M. C.; Lamy, C.; Leger, J. M.; Vasquez, J. L.; Aldez, A. J. Electroanal. Chem. 1990,283,287. (13)Goncalves, R.S.;Leger, J. M.; Lamy, C. Electrochim. Acta 1988, 33,1581. (14)Ocon, P.;Alonso, C.; Celdran, R.; Gonzalez-Velasco, J. J.Electroanal. Chem. 1986,206, 179. (15)Karolczak, M. Langmuir, 1990,6,863. (16)Nicoletti, J. W.; Whitesides, G. M. J. Phys. Chem. 1989,93,759. (17)DiCosimo, R.;Whitesides, G. M. J. Phys. Chem. 1989,93,768. (18)Takky, D.; Beden, B.;Leger, J. M.; Lamy,C. J.Electroanal. Chem. 1988,256,127.
tanol,lg propanediol,20and ethylene g l y c 0 1 . ~ ~Studies -~~ involving a series of a l c ~ h o l s lhave ~ ~ ~been ~ - particularly ~~ useful in understanding the effect of molecular structure on adsorption. Toward this end, we have chosen to investigate the orientation and bonding of a homologous series of straight-chain alcohols,methanol, ethanol, l-propanol, and 1-pentanol, at Ag electrodes. These alcohols are used as solvents in the presence of supporting electrolyte,and thus represent nonaqueous electrochemical systems. Double-layer capacitance measurements have been widely used for the investigation of organic solvent structure at metal electrodes. However, this approach has yielded little conclusive evidence for specific bond orientations of solvent molecules in the double layer. For example, Grahame28 observed a positive shift in the potential of zero charge (PZC) of Hg in methanol-water mixtures with greater concentrations of methanol and concluded that the methanol dipole points away from the surface. However, Garnish and Parsons28concluded from the temperature dependence of the double-layer capac(19)Povov, A.;Velev, 0.;Vitanov, T. J.Electroanal. Chem. 1988,256, 405. (20)Ocon, P.; Beden, B.; Lamy, C. Electrochim. Acta 1987,32,1095. (21)Christensen, P. A.;Hammet, A. J.Electroanal. Chem. 1989,260, 347.
(22)Beden, B.; Kadirgan, F.; Kahyaoglu, A.; Lamy, C. J.Electroanal. Chem. 1982,135,329. (23)Kokkinidis. G.; Jannakoudakis, D. J. Electroanal. Chem. 1982, 133,307. (24)Holze, R.;Schneider, J.; Hamann, C. H. Ber. Bunsen-Ges.Phys. Chem. 1988,92,1319. (25)Holze, R.;Beltowska-Brzezinska, M. Electrochim. Acta 1985,30, 937. (26)Holze, R.;Beltowska-Brzezinska, M. J. Electroanal. Chem. 1986, 201,387. (27)Kokkinidis, G.; Jannakoudakis, D. J. Electroanal. Chem. 1983, 153,185. (28)Payne, R.In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Tobias, C. W., Eds.; Interscience Publishers: New York, 1970;Vol. 7 p 10.
1992 American Chemical Society
2050 Langmuir, Vol. 8, No.8,1992
itance that the reverse orientation occurs. More recently, FawcetP has suggestedthat the minimum in capacitance as a function of potential is due to methanol dipoles oriented parallel to the Hg electrode surface. In another case, Dutkiewicz and Lamperskim ascribed the minimum to a low polarizability of molecules at the interface caused by cluster formation. Spectroscopic methods such as surface-enhanced Raman scattering(SERS) and infrared reflection-absorption spectroscopy (IRRAS) have provided the most detailed information about nonaqueous solvent orientation at electrode surfaces. The utility of SERS has been demproonstrated in the study of methanol,3l pylene carbonateF3and pyridine" at Ag electrodes, while IRRAS has been used to evaluate the orientation of acetonitriles at Pt electrodes. In most cases, molecular orientation has been inferred from shifts in vibrational frequencies relative to bulk solution. However, more detailed orientationalinformationcan be obtained through the use of surface selection rules at rough surfaces. These rules suggest that, for a selected excitation wavelength region, vibrational modes possessing polarizability tensors along the surface normal will experience the greatest intensity enhancement, while those possessing tensors along the plane parallel to the surface will experience the least intensity enhancement. Consequently, one should be able to evaluate interfacial solvent orientation from relative intensities in the SERS spectrum of interfacial solvent molecules. In this report, we describe the use of SERS to evaluate the potential-dependent bonding and orientation of a homologous series of primary alcohols at Ag electrodes. One of the goals of this work is to reveal some of the factors which control alcohol solvent orientation in the electrochemical double layer. A second goal is to determine the usefulness,and self-consistency,of S E W surfaceselection rules for these molecules at roughened Ag electrodes. Toward this end, the potential dependences of different vibrationalbands are consideredwith respect to each other, and with respect to relative intensities from bulk solution.
Sobocinski a n d Pemberton
trically cooled to -25 "C. The spectral band-paeewas maintained at 7.0 cm-1. Solution spectra were acquired at 1-cm-' incrementa over a0.5-s integration period, while SERS spectrawere acquired at 2-cm-' incrementa over a 1-8integration period. For potentialdependent surface Raman spectra (and corresponding reference bulk spectra), a triple monochromator/chargacoupleddevice (CCD) detector Raman system was used. Scattered radiation was collected with a Nikon f/1.4 camera lens (50" focal length) and focused onto the entrance slits of a Spex 1877 Triplemate. The gratings in the filter stage were 600 gdmm, and the grating used in the spectrograph stage was 1200 gr/mm. Slit widths were typically 0.3 mm/4 mm/0.3 mm. Approximately 50% of the circular image at the entrance slit passed into the spectrometer. Detection was accomplished with a Photometrics (Tucson, AZ) PM512 frontaide-illuminated charge-coupleddevice (CCD). This device was cooled to between -100 and -110 "C with liquid nitrogen. The CCD images were processed with a Photometrics RDS200 system equipped with a custom version of SpectraCalc. Electrochemical Conditions a n d Instrumentation. The working electrode consisted of a polycrystalline Ag disk (99.9%, Johnson Matthey) which was mechanically polished to a mirror finishusing0.3-rmalumina,rinsedwithdistilled,deionized water, and then sonicated for 2 min in distilled, deionized water to remove any trapped alumina. A Pt wire served as the auxiliary electrode. Potential-dependent spectra are reported versus a Ag/Ag+referenceelectrodein methanolcontaining 0.01 M AgNOs and 0.1 M tetrabutylammonium perchlorate. A Pt wire/glass junction salt bridge containing saturated LiBr in methanol was also used to preserve the integrity of the reference electrode. Electrode potentials were controlled with an IBM Model EC/ 225 voltammetric analyzer. Linear potential ramps were performed using a triangle wave format. Total charge passed was monitored with a Princeton Applied Research Model 379 digital coulometer. Materials. Methanol was purchased as anhydrous AR grade from Mallinckrodt with water labeled as 0.006%. Ethanol (absolute) was obtained from Midwest Grain Co. of Illinois (Pekin, IL). l-Propanol was purchased as high puritygrade (0.012% water) from Burdick and Jackson. 1-Pentanol was purchased from Fischer. All solventa were used as received. It is notable that preliminary SERS results indicate that slight increases in water content (up to 1mol %) do not necessarily lead to a corresponding increase in surface water concentration. Aqueous solutions were made from distilled, deionized water. LiBr (Aldrich, anhydrous 99+%) was dried at 160 "C under a 2 X 10-l Torr vacuum for 24 h. Experimental Section Electrochemical Roughening and Emermion Procedures. In order to obtain significantly enhanced Raman signals from Spectroscopic Conditions and Instrumentation. Excispecies at the electrode-solution interface, it is necessary to tation was providedexclusivelyby the 514.5-nm line of a Coherent roughen the surface. This was accomplished through a linear Innova 90-5 Ar+ laser. All spectra were acquired with a laser potential sweep oxidation-reduction cycle (ORC). All Ag surfaces power of 120-130 mW focused to a spot of ca. 50-rm diameter were roughened ex situ in 0.4 M LiBr aqueous solutions. at the surface. The angle of incidence was ca. 60" with respect Typically, the electrode is initially poised at -0.40 V versus a to the surface normal. Potential-dependent SERS spectra were AgIAgC1 reference electrode, and ramped to a final potential of obtained with incident light polarized parallel with respect to ca. -0.07 V at a scan rate of 5 mV/s. The fiial potential is chosen the plane of incidence. The reference bulk spectra for these such that 30 mC of oxidative charge is passed at the electrode studies were obtained from a 2-3." layer of solution between of geometric area 0.385 cm2. Since the actual area of the rough a smooth, polished polycrystalline Ag electrode and the quartz surface is ca. twice that of the geometric area, the total oxidative window of the spectroelectrochemical cell. All survey spectra charge passed is ca. 40 mC/cm2. were acquired with incident light polarized perpendicular to the Emersion from the aqueous 0.4 M LiBr solution was carried plane of incidence. out at open circuit potential. The emersed interface was rinsed T w o Raman spectrometers were used in these experiments. with the alcohol solvent of study and placed into an air-tight For survey spectra, scattered light was collectedwith an elliptical spectroelectrochemical cell which was filled with a solution of mirror and focused into a Spex 1403double monochromator with 0.4 M LiBr in neat alcohol. The transfer, rinse, and fiiing 1800 gr/mm holographically ruled gratings. Detection in this procedure required only 20-30 8. The electrode was then placed case was accomplished with a high-sensitivity, GaAs photocathclose to the window such that a small portion of it actuallytouched ode, RCA C31034A photomultiplier tube which was thermoelecthe quartz window. The surface was sampled at another portion of the electrode surface. This configuration minimized bulk (29)Fawcett, W.R. J. Phys. Chem. 1978,82,1385. solvent contributions to the spectra. (30)Dutkiewicz, E.;Lamperski, S. J. Electroanul. Chem. 1988,247, Cyclic Voltammetry. Survey cyclic voltammograms of 0.4 57. (31)Sobocinski, R.L.;Pemberton, J. E. Langmuir,1990,6,43. M LiBr/alcohol solutions at smooth Ag are shown in Figure 1. (32)Irish,D.E.;Hill,I.R.;Archambault,P.;Atkineon,G.F.J.Solution The potentialwindowinmethanolwithinwhichminimumcurrent Chem. 1986,14,221. flows is found to be between -0.4 V (ca.the open circuit potential (33)Hill, I. R.;Irish, D. E.; Atkinson, G.F. Langmuir 1986,2,752. of this system) and-1.6 V, essentiallya 1.2-V window. The cyclic (34)Pemberton, J. E. Chem. Phya. Lett. 1986,115,321. voltammograms for the other alcoholsare largelyfeatureless and (35)David", T.;Pons, B. S.; Bewick, A.; Schmidt, P. P. J. Electroanal. Chem. 1981,125,237. indicate similar potential windows from ca. -0.4 to -1.8 V.
Alcohol Solvent Orientation and Bonding
Langmuir, Vol. 8, No. 8,1992 2051 /CH$H20H
-
c
0.0
-0.4
-1.2 -1.6 E (Volts vs. Ag/Ag+)
-0.8
-2.0
I
8
N
-
GGxET&&
600 1 4 0 0 2200 3000 3800
WAVENUMBER (cm-9
WAVENUMBER (cm-1)
-2.4
Figure 1. Cyclic voltammetry of a Ag electrode in 0.4 M LiBr solutions of (A) methanol, (B) ethanol, (C) 1-propanol, and (D) 1-pentanol. Table I. Raman Frequencies (cm-’) and Assignments for Methanol-Electrolyte Solutions in the Liquid Phase 0.4 M 0.4 M LiBrLiBrmethanol assignmento ref methanol assignmento ref 1036 u(C-0) 37 2840 vn(CH3)b 38 1112 CH3rocka’ 37 2920 bn(CH3)ot 38 1160 CH3rocka” 37 2947 FR(vn(CH3)+ 38 6,(CH3) 37 b(CH3)0 t ) b 1454 ba(CH3) 1471 37 2990 v,(CH3) 38 3358 ~(0-H) 43 Oot = overtone, Y = stretch, b = bend, FR = Fermi resonance. The bands at 2840 and 2947 cm-l are Fermi resonance bands of unequal intensity; consequently, the mixing of character should be small, and the band at 2840 cm-l primarily from vn(CH3). Curve Fitting. Spectral decomposition was accomplished with a curve fitting program available in SpectraCalc. The program works by first asking the user to make an initial guess of the number of bands present, their peak positions, peak widths, and whether they are Gaussian, Lorentzian, or a mixture. The program uses these initial guesses to find the combination of band heights, positions, and widths which best fit the spectrum. Each parameter may also be independently constrained for the actual analysis. The x 2 fit error (residual) is shown after each pass, indicating the quality of the fit. Initial guesses are based on assignments in the literature shown in Table I. If the s u m of the calculated peaks is not within experimental error of the actual spectrum, an additional peak is added in some cases to account for unique surface species. Prior to curve fitting, each spectrum was background subtracted using a multiple point polynomial baseline estimate. In general, no parameters were constrained, except for some small broad bands which, without peak frequency constraint, would produce an extremely broad band of unreasonable width. Surface peaks were decomposed using a 20 % Lorentzian/BO% Gaussian peak shape, which was determined to give the smallest residual for a number of fits of a well-resolvedsurface peak. Peaks in the bulk spectrum were decomposed assuming a 50% Lorentzian peak shape, which gave the smallest residual for a well-resolved bulk solution peak. Three criteria were applied for a successful curve fit: (1)the change in x 2 should be less than 2-376 of the previous iteration, (2) the full widths at half-maximum (fwhm) of all resulting peaks should be similar to each other (within a factor of ca. 2), and (3) the sum of the calculated peaks should be within the experimental error of the actual spectrum. In general,the averagestandard deviation of decomposedbulk peaks is less than 1% of the average value, while the average standard deviation of decomposed surface peaks is 10-15%. These integrated band strengths were employed to quantify relative intensities.
I
6LO 1 4 0 0 22’00 3000 3800 WAVENUMBER (cm-9
600
1400 2200 3000 3800 WAVENUMBER (cm-1)
Figure 2. Raman spectra of alcohols containing 0.4 M LiBr (A) in bulk solution and (B) a t roughened Ag electrode a t open circuit potential (ca. -0.40 V versus Ag/Ag+). Table 11. Raman Frequencies (cm-’) and Assignments for Ethanol-Electrolyte Solutions in the Liauid Phase 0.4 M 0.4 M LiBrLiBrethanol assignment’ ref ethanol assignmento ref 885 ~ ~ ( c - C - 0 ) 39,40 2880 v ~ ( C H ~ ) ~ 45 1054 v,(C-C-O) 40 2900 Va(CHd 46 1096 CH3rock 41 2930 FR(v,(CH~) + 46 1278 CH2 twist 42 WH3) ot) 1457 b,(CH3) 43 2974 v,(CH3) 41 1482 b(CH2) scissor 41,43 3366 ~(0-H) 43 2847 Y.(CH~) 44,45 ot = overtone, Y = stretch, 6 = bend, FR = Fermi resonance. b T ~ bands o are associated with this mode; the second band is designatedby Snyder and co-workers46~~ as a FR band. The notation of Snyder and co-workers is used here. ~~
Results Bulk Solution Spectra. The solution spectra for methanol, ethanol, propanol, and pentanol in the presence of 0.4 M LiBr are shown in Figure 2A. Consistent assignments for these vibrational bands have not been published heretofore and are shown in Tables I-IV. A more comprehensive presentation and detailed discussion of assignments will be presented elsewhere.36 Most of the assignments are taken from refs 37-51, while others are confirmed from polarization and temperature studies in (36) Joa, S. L.; Sobocinski, R. L.; Pemberton, J. E. J. Phys. Chem., submitted for publication. (37) Falk, M.; Whalley, E. J. Chem. Phys. 1961, 34, 1554. (38) Seifert, F.; Dehme, K.; Holzer, W. J. Phys. Chem. 1990,94,4778. (39) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; Wiley: New York, 1974. (40) Sheppard, N.; Simpson, D. M. Q.Reu. 1953, 7, 19. (41) Mikawa, Y.; Brasch, J. W.; Jakobsen, R. J. Spectrochim. Acta 1971,27A, 529. (42) Sheppard, N. J. Chem. Phys. 1948, 16, 690.
Sobocinski and Pemberton
2052 Langmuir, Vol. 8, No. 8, 1992 Table 111. Raman Frequencies (cm-') and Assignments for Propanol-Electrolyte Solutions in the Liquid Phase 0.4 M LiBrl-DrODanOi assignmenta ref 860 CH3 rock + u(C-C) 45 888 CH3 rock + v(C-C) 45 970
1060 1073 1104 1278 1300 1458 1482 2861 2880 2900 2914 2938 2964 3371
u(C-c-c-0) u(C4-c-0) u(C-C) gauche u(C-C)trans CHz twist CHz wag &(C&) 8(CHz) scissor
dCHz) Y.(CH~)~ Va(CHd FR(u,(CHz) + 6(CHz)ot) FR(u,(CH3) + b(CH3)ot)
va(CH3)
~(0-H)
39 39 36,48,49 36,48,49 42 42,47 43 43 45 45 46 46 46 45 43
not = overtone, Y = stretch, 6 = bend, FR = Fermi resonance. Two bands are associated with this mode; the second band is designated by Snyder and co-workersab&as a FR band. The notation of Snyder and co-workers is used here. b
Table IV. Raman Frequencies (cm-1) and Assignments for Pentanol-Electrolyte Solutions in the Liquid Phase 0.4 M LiBr1-pentanol assignmentn ref 840 856 888 982 1008 1058 1076 1118 1302 1442 1455 1467 2863 2876 2899 2914 2938 2960 3384
CH3 rock + v(C-C) CH3 rock + v(C-C)
u,(C-C-c-c-c-o)
50 45 45
v.(C-c-c-C-C-O)
39 36,48,49 36,48,49 42,47
v(C-C)gauche v(CC)trans CHz wag 6,(CHz) (9 6a(CHd
b(CH2)scissor vdCHdb v,(CHdb Va(CH2)
FR(u,(CHz) + 6(CHz) ot) FR(u,(CH3) + b(CH3) ot)
va(CH3) v(0-H)
39,51 43 45 45 46 46 46 45 43
ot = overtone, Y = stretch, 6 = bend, FR = Fermi resonance. bTWo bands are associated with this mode; the second band is designated by Snyder and c o - w o r k e r ~ ~a ~FR ~ band. ~ a s The notation of Snyder and co-workers is used here.
this laboratory. These assignments are essential for the determination of solvent orientation from SERS spectra. Alcohol Orientation at Open Circuit Potential. As an introduction, it is useful to compare survey spectra of adsorbed species with those in bulk solution. As shown in Figure 2B,significant changes between the bulk solution and SERS spectra at open circuit potential (ca. -0.4 V in these media) are observed. (43)Silvelstein, R. M.;Baasler, G. C.; Morrill,T.C . Spectrometric Identification of Organic Compounds, 4th ed.; Wiley: New York, 1981. (44)Sun, S.;Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1990,94, 2005. (45)Schachtachneider, J. H.; Snyder,R. G. Spectrochim. Acta 1963, 19, 117. (46)Snyder, R.G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86,5145. (47)Fukushima, K.;Zwolinski, B. J. J. Mol. Spectrosc. 1968,26,368. (48)Spiker, R.C.;Levin, I. R. Biochim. Biophys. Acta 1976,433,457. (49)Bryant, M.A.;Pemberton, J. E. J. Am. Chem. SOC.1991, IZ3, 8284. (50) Simanouti, T.; Mizushima, S. J. Chem. Phys. 1949,17,1102. (51)Wallach, D.F. H.; Verma, S. P.; Fookson, J. Biochim. Biophys. Acta 1979,559,153.
The SERS behavior for methanol at Ag electrodes has been previously rep0rted.3~ However, the important features are reported here for comparative purposes. Differencesbetween the SERS and bulk methanolspectra are observed in the v(C-0), 6(C-H), v(C-H), and u(0-H) regions. For instance, the ratio I[v~(CH~)]/I[V,(CH~)] is 0.87 f 0.15for interfacial species as compared to the bulk value of 0.18f 0.02,which is a factor of 4.8 smaller. More easily observed, however, is the dramatic change in the I[U,(CH~)FR]/I[V,(CH~)~ ratio from bulk solution. These are the two most intense bands in this region and have peaks at 2947 and 2840 cm-l, respectively. This ratio is 0.51 f 0.02 in bulk solution and 1.98 f 0.18 for surface species, which is a factor of 3.9 larger. One explanation for these differences is that methanol in the interface has a preferred orientation with its symmetric methyl dipole more parallel to the surface than the asymmetric dipole. Given the orthogonal nature of the symmetric and asymmetric modes, one would predict that for a 45O tilt of the C-0 bond from the surface normal, these modes would couple with the normal and tangential surface electric fields to an equal extenteS2This is precisely the condition of these modes in the isotropic bulk solution where they interact equally with the electric field. Consequently, it is proposed that the axis along which the v,(CH3) occurs is more parallel to the surface than that of the va(CH3) and that this axis forms an angle of greater than 45O from the surface normal. The second important feature in the S E W spectrum of methanol at Ag is the shift of the v(C-0) frequency to lower energies, going from 1036 cm-l in the bulk to 1028 cm-l at the interface. In addition, the ratio I[v(C-O)]/ I(va(CH3)I decreases by a factor of 10on the surface, going from 1.33 f 0.15 in the bulk to 0.13 f 0.02 on the surface. Also notable in Figure 2 is a factor of 10 decrease in the v(C-0) intensity relative to the v,(CH~)FR at 2947 cm-l. These data imply that the va(CH3)couples with the surface electric field better than the v(C-01, thereby corroborating the orientation proposed above. The v(O-H) band from interfacial water is observed at 3520 cm-l, while the v(0-H) band from alcohol is surprisingly weak (given the quantity at the interface) and difficult to observe using PMT detection. This suggests that the alcohol 0-H bond is largely parallel to the surface at open circuit potential. This picture is generallyconsistent with the Raman results published previously for methanol adsorbed at potentials positive of the PZC.3l A survey SERS spectrumfrom ethanol at Ag is compared to the spectrum of bulk ethanol solution in Figure 2.The I[v~(CH~)]/I[V,(CH~)] ratio is 0.98 f 0.15 for interfacial ethanol compared to 0.88 f 0.01 in the bulk. Similarly, I[v~(CH~)]/I[~,(CH~)] does not change much going from 3.04f 0.01 in the bulk to 2.95 f 0.39 on the surface. These data suggest that both the u,(CH3) and v,(CH2) dipoles are at a tilt of ca. 45O from the surface normal. It is interesting to note that the 6a(CH3) mode at 1457 cm-l, which has a change in dipole moment similar in direction to the -.Y (CH3) mode, remains strong relative to both the v,(C-C0)and va(C-C-0)modes at 885and 1054cm-', respectively. This further supports a CH2 orientation of ca. 45O from the surface normal. No detectable shift in the v(C-C-0) bands is observed for interfacial species, indicating a chemical environment similar to that in bulk solution. Water, which inevitably exists in these solvents as a trace impurity, is detected at the interface as a band at 3500 cm-', similar to the other alcohols studied here. (52)Pemberton,J. E.; Bryant, M. A.; Sobocinski, R. L.; Joa, S.L. J. Phys. Chem. 1992,96,3776.
q
Langmuir, Vol. 8, No. 8,1992 2053
Alcohol Solvent Orientation and Bonding
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