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Jun 15, 2012 - The structure of thin films of 1- and 2-butylimidazoles adsorbed on copper and steel surfaces under air was examined using sum frequenc...
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Adsorption of 1- and 2-Butylimidazoles at the Copper/Air and Steel/ Air Interfaces Studied by Sum Frequency Generation Vibrational Spectroscopy Michael T. L. Casford* and Paul B. Davies Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom S Supporting Information *

ABSTRACT: The structure of thin films of 1- and 2butylimidazoles adsorbed on copper and steel surfaces under air was examined using sum frequency generation (SFG) vibrational spectroscopy in the ppp and ssp polarizations. Additionally, the SFG spectra of both isomers were recorded at 55 °C at the liquid imidazole/air interface for reference. Complementary bulk infrared, reflection−absorption infrared spectroscopy (RAIRS), and Raman spectra of both imidazoles were recorded for assignment purposes. The SFG spectra in the C−H stretching region at the liquid/air interface are dominated by resonances from the methyl end group of the butyl side chain of the imidazoles, indicating that they are aligned parallel or closely parallel to the surface normal. These are also the most prominent features in the SFG spectra on copper and steel. In addition, both the ppp and ssp spectra on copper show resonances from the C−H stretching modes of the imidazole ring for both isomers. The ring C−H resonances are completely absent from the spectra on steel and at the liquid/air interface. The relative intensities of the SFG spectra can be interpreted as showing that, on copper, under air, both butylimidazoles are adsorbed with their butyl side chains perpendicular to the interface and with the ring significantly inclined away from the surface plane and toward the surface normal. The SFG spectra of both imidazoles on steel indicate an orientation where the imidazole rings are parallel or nearly parallel to the surface. The weak C−H resonances from the ring at the liquid/air interface suggest that the tilt angle of the ring from the surface normal at this interface is significantly greater than it is on copper.

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INTRODUCTION It has long been recognized that azoles and their derivatives are effective corrosion inhibitors for metallic surfaces, in particular for copper and its alloys, specifically on electronic circuit boards, copper-lead epoxy joints,1 and electrochemical environments.2−4 They are also known inhibitors of corrosion of iron.5 Other applications of these compounds include their use to form complexes with metal ions in solution, leading to enhanced metal ore extraction, and as useful adducts for improving the adhesion properties of epoxy resins.6 Furthermore, the coordination behavior of imidazole is biologically important because it bonds to metal ions in metalloproteins. There has been considerable experimental effort to understand the structure of imidazole and its derivatives on surfaces, especially on copper and other metals. Numerous techniques have been used for this purpose, including infrared (IR) spectroscopy, 7,8 surface-enhanced Raman spectroscopy (SERS),9−11 and cyclic voltametry.7,11−17 This has, in turn, stimulated efforts to gain a better understanding of the vibrational spectra of imidazole. However, despite the extensive experimental effort, the structure of imidazole compounds on metal surfaces is incompletely understood. Recently, sum frequency generation (SFG) spectroscopy, an interface-specific nonlinear laser technique, has been used to examine the adsorption of imidazoles and their ionic liquid © 2012 American Chemical Society

derivatives at a variety of dielectric interfaces, specifically the quartz, hydrophobically modified quartz, and pure imidazole air interfaces.18−20 The SFG technique has also been applied to study imidazole adsorption at a platinum electrode in solution.21 SFG is uniquely suited to the study of adsorbates on metal surfaces because it provides information on the polar orientation, molecular conformation, and tilt angle of the adsorbed layer without any interfering contribution from the bulk surface or overlaying solvent. In this work, we have measured the SFG spectra of butylimidazole on copper and steel under air. The use of an alkyl-substituted derivative of imidazole potentially enables both the ring and alkyl side chain to be probed by SFG. Butylimidazole exists as two isomers (Figure 1), both of which have been examined here. The 2butyl compound exhibits extensive hydrogen bonding and is a crystalline solid at room temperature [melting point (mp) of 52 °C]. The lack of a N−H group in the imidazole ring of 1butylimidazole precludes intermolecular hydrogen bonding between adjacent imidazole rings, as seen in imidazole itself.15 Consequently, 1-butylimidazole is a liquid at room temperature. For completeness, the liquid/air interfaces of both compounds Received: April 2, 2012 Revised: June 15, 2012 Published: June 15, 2012 10741

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a clean copper or a clean mild steel reference surface recorded in air prior to modeling. Repeat SFG spectra of a minimum of three separate samples were recorded to ensure the reproducibility of the results. The spectra of the cleaned metal surfaces were recorded immediately after cleaning and also 30 min later. Because there was no significant indication of contamination within this period in the SFG (or RAIRS) spectra, it was concluded that the recorded spectra were due to the adsorbed layer rather than surface contamination from the ambient laboratory atmosphere. Once the imidazole had been spun onto the surface, no further cleaning was possible. The SFG (and RAIRS) spectra were recorded in enclosed sample chambers to limit surface contamination as much as possible. A complete theoretical description of SFG may be found elsewhere,24,25 and only a brief summary is given here. For a metalsurface only polarization combinations that contain p-polarized IR light give rise to significant SFG intensity because of the metal surface selection rule and the high reflectivity of metal surfaces in the IR region. Therefore, for metal surfaces, only the ssp and ppp polarization combinations are suitable. In the ssp polarization combination, only modes that have transition dipole moments perpendicular to the surface appear in the observed SFG spectra. This is because only a single component of the nonlinear susceptibility tensor χ(2)xxz is probed in this combination (where the subscript refers to the orientational alignment of the sum frequency signal, visible pump, and IR pump beams, respectively). Because the IR component resides solely in the z axis, only transition dipoles with a component in this orientation produce SFG photons. On the other hand, spectra in the ppp combination contain contributions from several susceptibility components, specifically χ(2)zzz, χ(2)xxz, χ(2)xzx, and χ(2)zxx. IR active vibrations in both the z and x axes are therefore potential sources of SFG signals in the ppp beam polarization; i.e., this polarization probes vibrational resonances with transition dipoles oriented both parallel and perpendicular to the surface. The modeling of the SFG spectra used a least-squares Levenberg−Marquardt algorithm to fit the resonances to Lorentzian line profiles derived from the SFG equation. The modeling allowed for the frequency, integrated intensity, and widths of the vibrational resonances to be determined, as well as the intensity and phase of the non-resonant susceptibilities. Further details can be found in part 1 of the Supporting Information. The intensity value derived from comparison of the ppp and ssp spectra can in theory be used to calculate the tilt angle of the SFG active species adsorbed on the metal surface. However, these calculations rely upon several assumptions, of which the thickness of the film is an important assumption. Previously, it has been shown that the intensity of the SFG spectrum in the ppp polarization is strongly influenced by the film thickness, while that in the ssp polarization is less so. Separation of the top surface of the spin-coated film from the metal substrate in the case of a gold substrate has been shown to produce a SFG intensity variation of 3 orders of magnitude for film thicknesses up to 200 nm.26,27 Even over the range of film thicknesses used in this work estimated to be between 0.5 and 6 nm, the variation in intensity between the spectral intensity recorded in the ppp polarization and that recorded in the ssp polarization would introduce large uncertainties. Because the film thickness effects on copper and steel have yet to be studied, no quantitative interpretation of the results was attempted in this study.

Figure 1. Molecular structures of 1- and 2-butylimidazoles.

have been examined by SFG. Complementary attenuated total reflection (ATR) IR spectroscopy, reflection−absorption infrared spectroscopy (RAIRS), and Raman spectroscopy of the butylimidazoles have been recorded and analyzed to aid in the assignment and interpretation of the SFG spectra.

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EXPERIMENTAL SECTION

Copper films were prepared by thermal evaporation on silicon wafer substrates. Average film thicknesses determined by scanning electron microscopy (SEM) imaging were 200 nm. Copper samples for SFG were 1 cm2 in size. The steel sample consisted of a 20 × 20 × 3 mm plate [specification, 1.0711 (En1A); C %, 0.07/0.15; Si %, 0.1 maximum; Mn %, 0.80/1.20; S %, 0.20/0.30; and P %, 0.07 maximum; free machining mild steel], which was freshly polished with diamond paste (45−0.1 μm) and sonicated in acetone before use. 1-Butylimidazole from Aldrich (99%) and 2-butylimidazole from Molekula were used as received. All glassware, O-rings, and stainlesssteel components were cleaned following standard procedures,22,23 rinsed 20 times with 18.2 MΩ cm−1 Millipore water, and dried under a nitrogen stream before use. Copper samples were rinsed in highperformance liquid chromatography (HPLC)-grade methanol upon removal from the evaporator chamber and then rinsed in ultrapure water (Millipore, 18.2 MΩ cm−1) before storage in sealed glass vials under HPLC-grade methanol until used. Cast films on copper and steel were produced by spin coating from HPLC methanol solutions at 2000 rpm for 10 min (Laurell Technologies WS-400B-6NPP/lite spin coater). RAIRS spectra of the films in air were recorded on a Perkin-Elmer spectrum 100 FTIR spectrometer equipped with a liquid nitrogencooled mercury cadmium telluride (MCT) detector covering the range 400−4000 cm−1. A total of 512 scans at a resolution of 4 cm−1 were co-added to achieve the final spectra. All spectra were recorded at least 3 times on an individual sample, and a minimum of 3 samples were used. Samples for the RAIRS spectra were 75 × 10 mm in size. Generally, the same sample was first recorded by RAIRS before measurement on the SFG spectrometer. Bulk IR spectra were recorded using ATR of thick films deposited by solvent evaporation. SFG spectra were recorded on an EKSPLA picosecond spectrometer (30 ps pulses at 20 Hz) in both ppp and ssp polarization combinations (sum frequency, visible, and IR) with a co-propagating beam geometry. The spectra were normalized to the product of the IR and visible beam intensities using the EKSPLA normalization facility. The IR wavelength was tuned across the C−H and N−H stretching regions, and the visible beam wavelength was fixed at 532 nm for all samples. Solid samples were mounted on a vacuum chuck held on a 6 axis micrometer-controlled mount. Liquid/air samples were contained in a cleaned glass Petri dish mounted on a thermostatted steel heating plate with the temperature controlled to ±1 °C. The input beams of the IR and visible lasers were set at angles of 53° and 60°, respectively, to the surface normal. Separation of the SFG output from reflected 532 nm radiation was achieved through a 532 nm holographic notch filter and a SOLAR TII monochromator. A total of 200 shots per point were co-added, and the spectra were scanned at intervals of 2 cm−1 between 2800 and 3400 cm−1. The SFG spectra of interest here lie in the C−H and N−H stretching region from the 2800 to 3200 cm−1 range. Spectra recorded on metal substrates were normalized to either

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RESULTS IR and Raman Spectra of 1- and 2-Butylimidazoles. Because the SFG intensity of a spectral mode is determined by both its IR and Raman transition moments, along with the coincidence or near coincidence of their respective frequencies, the IR and Raman spectra of 1- and 2-butylimidazoles were recorded and assigned. The IR survey spectra of both butylimidazoles recorded in the bulk by ATR are shown in Figure 2. Significant differences in the spectral profiles can be seen across the spectra, although those most relevant to this 10742

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Figure 3. Survey depolarized Raman spectra of 1-butylimidazole (blue) and 2-butylimidazole (red) (spectra offset for clarity). Figure 2. Survey IR spectra of thick films of 1-butylimidazole (blue) and 2-butylimidazole (red) recorded by ATR (spectra offset for clarity). Bands indicated by a and b in 2-butylimidazole denote the positions of the N−H bending modes at 1154 and 1565 cm−1. The positions of these bands in imidazole itself have been reported at 1148 and 1543 cm−1.38 (Inset) Enlargement of the C−H stretching region of the imidazole ring.

work are in the C−H stretching region between 2800 and 3200 cm−1. The assignment of the bands in this region follows from published IR and Raman spectra. For 1-butylimidazole, the assignment relies closely on the procedure by Romero and Baldelli,18 who used polarized and depolarized Raman spectroscopy to determine the symmetric and asymmetric modes of 1-butylimidazole. The bands of 2-butylimidazole can be assigned from analyzing the depolarization ratios and by comparison to the bands observed in the 1-butyl compound and imidazole itself as described by Kirin et al.28,29 The presence of the N−H group in the 2-butylimidazole ring leads to extensive hydrogen bonding, which manifests itself as a broad spectral feature between 2600 and 3200 cm−1, which is absent in the 1-butyl compound. This enables the C−H stretching modes of the alkyl chain between 2800 and 3000 cm−1 in the latter to be identified along with the H−CC−H asymmetric and symmetric ring stretching modes at 3106 and 3133 cm−1 (present as a shoulder). In contrast, hydrogen bonding obscures these modes in 2-butylimidazole. Furthermore, the presence of the ring N−H group gives rise to the N− H bending bands at 1154 and 1565 cm−1, which are absent in the 1-butyl compound (Figure 2). In contrast with the differences in the two room-temperature IR survey spectra presented in Figure 2, the depolarized Raman survey spectra of 1- and 2-butylimidazoles (Figure 3) are closely similar in appearance. Furthermore, the spectra in the C−H stretching region are now more prominent features of the overall spectrum than they were in the IR spectra. The most noticeable differences between the two Raman spectra in this region are the relative intensities of the symmetric and asymmetric ring C−H alkene stretching bands. Differences in the intensity of these bands are most noticeable in the polarized spectra, which are shown specifically in Figure 4. The assignments of the C−H stretching bands of both compounds are collected in Table 1. Thermal Changes in IR Spectra. The RAIRS of the two compounds adsorbed as thin films on copper surfaces under air respond differently to changes in temperatures. The spectrum of 1-butylimidazole shows little change upon heating to 100 °C,

Figure 4. Polarized Raman spectra of 1-butylimidazole (left-hand panel) and 2-butylimidazole (right-hand panel) in the C−H stretching region. Red, parallel polarization; blue, perpendicular polarization.

Table 1. Assignment of the IR and Raman Spectra of 1- and 2-Butylimidazoles in the C−H Stretching Region band positions (cm−1) IR 2863 2874 2902 2933 2959 3106 3133 2862 2873 2896 2930 2959 2987 3045 3110 3132 3147

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assignment

1-Butylimidazole 2863 (shoulder) CH2 symmetric stretch (d+) 2874 CH3 symmetric stretch (r+) 2911 CH2 asymmetric stretch (d−) 2938 CH3 symmetric FR (r+FR) 2962 CH3 asymmetric stretch (r−) 3110 CHring asymmetric stretch 3135 CHring symmetric stretch 2-Butylimidazole 2863 CH2 symmetric stretch (d+) 2876 CH3 symmetric stretch (r+) 2904 2916 CH2 asymmetric stretch (d−) CH3 symmetric FR (r+FR) 2962 CH3 asymmetric stretch (r−)

3113 3141

CHring asymmetric stretch CHring symmetric stretch

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resonance. However, because the overlap between the IR and Raman peak positions is poor and this mode is often absent in SFG spectra, this peak is tentatively assigned instead to the d+ Fermi resonance, which conversely is often strongly apparent in the SFG spectra of alkyl chains. Importantly, this resonance is not used in the interpretation of the results here because of this assignment ambiguity. In the ssp polarization, the two strong peaks, modeled at 2880 and 2942 cm−1, are assigned to the methyl r+ and r+ Fermi resonance modes. Both of the features in the ssp polarization are asymmetric in profile, which may indicate the presence of small underlying methylene resonances. These were therefore included in the modeled spectra, which gave positions at 2862 cm−1 (d+) and 2920 cm−1 (d−/d+FR). Lastly and significantly, both of the ssp spectra show small peaks at 3113 and 3140 cm−1, which have been assigned to the asymmetric and symmetric C−H ring stretching modes, respectively. Because both resonances appear solely in the ssp spectra and no modes are apparent in the ppp spectra, this gives rise to the possibility that both resonances in the ssp spectra are due to the symmetric ring C−H stretch. The absence of a strong asymmetric C−H stretch in the ppp spectra however argues against this assignment, because this mode should be strong in the ppp spectrum according to the polarization selection rules given by Lu et al.30,31 and Wang et al.32 Because of their close proximity to the peak positions observed in the Raman and IR spectra, these resonances are therefore assigned to the asymmetric and symmetric ring stretches, as mentioned above. SFG Spectra of Butylimidazoles on Copper under Air. The SFG spectra of 1- and 2-butylimidazoles on copper under air at room temperature are shown in Figure 6. The profiles of

implying that the structure of the molecule is quite thermally stable on the copper surface up to this temperature. However, the spectrum of the 2-butyl compound shows marked changes upon heating to 100 °C, namely, the loss of the spectral features associated with hydrogen bonding (see part 2 of the Supporting Information), with the spectrum becoming similar to the 1-butylimidazole spectrum at this temperature. Similar RAIRS results were obtained when both compounds were heated on steel surfaces; however, after mild heating, the spectra show little or no evidence of the ring stretching modes and show a rapid decrease in intensity with time on the steel surface. In contrast, in the bulk phase, the IR transmission spectrum of 2-butylimidazole at the elevated temperature remains the same as that at room temperature. This infers that the intermolecular hydrogen bonding is less strong than adsorption to the metal surface. SFG Spectra of Butylimidazoles at the Liquid/Air Interface. The SFG spectra of pure liquid 1- and 2butylimidazoles in contact with air at 55 °C are shown in Figure 5. This temperature was chosen because the bulk 2-butyl

Figure 5. SFG spectra of the butylimidazoles at the liquid/air interface recorded at 55 °C in ppp and ssp polarizations. Spectra are normalized to the laser power. Left-hand spectra, 1-butylimidazole; right-hand spectra, 2-butylimidazole. Circles are the experimental data points, and solid lines are the calculated fits.

isomer is a crystalline solid at room temperature and its spectrum recorded at this temperature is irreproducible, most likely because the laser beams sample different crystalline faces. The room-temperature spectra of the 1-butyl compound are essentially the same as those shown in Figure 5. The spectra of both 1- and 2-butylimidazoles shown in Figure 5 are virtually identical between 2800 and 3000 cm−1, and it is safe to assume identical assignments apply. In the ppp polarization, the modeled peak positions were 2881, 2914, and 2968 cm−1, which can be assigned to the methyl r+, methylene d+FR/d−, and methyl r− resonances, respectively. The calculated phase of the observed resonances is close to 90°. The assignment of the 2914 cm−1 peak to methylene d+FR is different from that implied by the Raman and IR spectra in Table 1. In the IR and Raman spectra, the asymmetric stretching mode d− occurs at 2902 and 2911 cm−1, respectively. Previous SFG studies by Lu et al.30,31 have shown that, on dielectric surfaces, the d− mode is strong in the ppp spectra and generally weak or absent in the ssp polarization, which would imply an assignment of d− to this

Figure 6. SFG spectra of the butylimidazoles adsorbed on freshly cleaned copper under air in ppp and ssp polarizations. Left-hand spectra, 1-butylimidazole; right-hand spectra, 2-butylimidazole. Spectra are normalized to the power and clean copper non-resonant background.

some of the ssp spectral features of 1-butylimidazole are significantly different from those of their ppp counterparts. The non-resonant phases, which gave the best spectral fits, were −90° for the ppp resonances and −22° for the ssp resonances (see part 3 of the Supporting Information). In the ppp polarization, the 1-butyl spectrum shows three resonances present as dips at 2886, 2940, and 2975 cm−1, which can be assigned to methyl r+, methyl r+ Fermi resonance, and methyl 10744

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r−. Additionally, three resonances are apparent at 3100, 3116, and 3162 cm−1, the latter two of which are assigned to the ring C−H asymmetric and symmetric stretching modes, respectively.18,19 The peak at 3100 cm−1 is unassigned. Magnified spectra between 3050 and 3200 cm−1 are shown in Figure 7.

Figure 8. SFG spectra of the butylimidazoles adsorbed on steel under air in ppp and ssp polarizations. Left-hand spectra, 1-butylimidazole; right-hand spectra, 2-butylimidazole. Spectra are normalized to the power and clean steel non-resonant background.

of the ppp spectrum were, in fact, higher than those for the ppp spectra on copper. Both ssp spectra show two broad and poorly defined features, which required careful modeling to determine their band centers. The best modeling fit (phase 15°) yielded frequencies of 2880 and 2940 cm−1. These resonances are tentatively assigned to the methyl r+ and methyl r+ Fermi resonance modes, respectively. Resonances from the ring C−H modes again appear to be absent in the ssp spectra, but this may be a result of the considerably lower S/N ratios in this polarization rather than a particular surface orientation of the molecules.

Figure 7. SFG spectra from 3050 to 3200 cm−1 expanded from Figure 6 of the butylimidazoles adsorbed on freshly cleaned copper under air. Left-hand spectra, 1-butylimidazole; right-hand spectra, 2-butylimidazole. Dashed lines indicate the positions of the asymmetric and symmetric ring C−H resonances.

The modeled frequencies of the ssp resonances are 2885, 2940, and 3120 cm−1, with a very weak shoulder at 2980 cm−1. These positions correspond closely to the resonance positions found in the ppp spectrum. Any resonance from the symmetric ring stretching mode appears to be absent in the ssp spectrum of 1butylimidazole. The spectra of 2-butylimidazole, both showing a modeled phase of 90°, are significantly more intense than the spectra of the 1-butyl isomer. The ppp spectrum again exhibits six welldefined dips, which, when modeled, give the same frequencies as the ppp spectrum of the 1-butyl isomer. The ssp spectrum shows three dips, which coincide with frequencies in the ppp spectrum; the 2975 cm−1 resonance in the ppp spectrum is absent in the ssp polarization. The spectral profiles in the ssp polarization are noticeably different for the two butylimidazoles. SFG Spectra of Butylimidazoles on Steel under Air. The normalized SFG spectra of the 1- and 2-butylimidazoles on steel under air at room temperature are shown in Figure 8. The spectra are noticeably different from those on copper, generally appearing as peaks rather than dips, with a modeled phase angle of around 48°. Prior to normalization, the absolute spectral intensity of the signal from the steel surface is nearly 2 orders of magnitude smaller than that observed for copper. Both ppp and ssp spectra on steel are now closely similar to each other in intensity, although there are some differences in relative intensities of the butyl chain C−H stretching modes in the two ppp spectra. The ppp spectra of the 1- and 2-butyl isomers show the same three methyl resonances already noted in the spectra on copper. In addition, there are two shoulders on the methyl resonances, at 2855 and 2920 cm−1 which can be assigned to the methylene d+ and d+FR/d− modes. Most importantly, no evidence was found for the C−H ring stretching modes, even though the S/N ratios in this region

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DISCUSSION A comparison of the SFG spectra presented in Figures 5−8 reveals that there are noticeable differences in the orientation of the imidazole ring on the different surfaces investigated. At the liquid/air interface, the ring C−H resonances are absent in the ppp polarization and are rather weak in the ssp polarization for both isomers in comparison to the butyl C−H methyl resonances. This is in marked contrast to their slightly greater intensity for 1-butylimidazole on the hydrophobic quartz interface reported by Romero et al.18 The implication is that the orientation of the butylimidazole ring at the liquid/air interface is either more disordered than when butylimidazole is adsorbed onto a solid hydrophobic surface or that the ring is lying predominantly in the plane of the interfaces and is free to rotate, giving rise to an effectively isotropic distribution of the ring C−H stretching modes. The butyl chain of both 1- and 2butylimidazoles however appears strongly oriented along the surface normal at all of the interfaces investigated, as shown by the almost complete absence of the r− methyl asymmetric stretch in the ssp spectrum compared to the high intensity of this resonance in the ppp spectra. The very close similarity between the 1- and 2-butylimidazole spectra shown in Figure 5 strongly suggests that, at the liquid/air interface at 55 °C, the ordering, orientation, and conformation of imidazole is independent of the molecular structure of the specific isomer. The phase of the SFG spectra of butylimidazole on copper and steel is determined by the interference of the resonant susceptibility of the adsorbate and non-resonant susceptibility arising from the electronic structure of the metal/metal oxide 10745

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stretching mode oriented in the plane of the interface and the asymmetric stretch oriented vertically. This configuration would be consistent with the observed SFG spectra reported here and may also account for the shift in position of the ring C−H resonance relative to that observed at the liquid/air interface. In contrast, the end on adsorption would require the symmetric stretch to be oriented vertically, with the asymmetric stretch in the plane of the interface, which is not observed in these results. Moreover, this mode of adsorption has also been suggested by Mousavi et al.36 using a quantum mechanical cluster model calculation for 2-hexylimidazole on a nonoxidized mild steel surface. It seems reasonable to conclude, in the light of the RAIRS and SFG results and previously mentioned SERS studies, that the butylimidazoles bind to the copper surface through the pyridine-type nitrogen, thus orienting the imidazole ring perpendicular to the copper surface. However, it is equally apparent that the surface oxidation state of the copper is critical to the observed orientation of the adsorbed film. It is therefore likely that the degree of oxidation of the steel surface is also a factor, meriting further study. The phase and absolute SFG intensity of the non-resonant SFG signal on mild steel are considerably different from those observed experimentally for copper. The SFG spectra of both imidazoles on steel comprise of resonances that are peaks in the ppp polarization and a combination of a weak dip and strong peak in the ssp polarization, consistent with the spectra reported by Zhang et al.37 for an octadecane thiol monolayer on mild steel. Their reported fitted non-resonant phase angles were 45° (ppp) and 15° (ssp), which are in close agreement with those found in the present work. It can therefore be concluded that, on steel, the polar orientation of the butyl chain in both imidazoles is away from the surface, similar to that observed on copper. Modeling of the SFG spectra showed that, in addition to the strong methyl resonances, there is a significant contribution in both the ssp and ppp spectra from the methylene symmetric stretching modes. This contrasts with the spectra on copper, where the methylene resonances were absent or extremely weak. An increase in the methylene/methyl intensity ratio is often taken to imply an increase in conformational disorder. However, because the butyl chain is so short, the majority of the methylene groups are inherently non-centrosymmetric. The increase in the methylene intensity may therefore be due to either an increase in the conformational order or a change in the tilt angle of the butyl chain. Additionally, the asymmetric methyl resonance (r−) is apparent in both ssp and ppp polarizations, suggesting that the methyl group has a greater tilt angle from the surface normal than it has on copper. In all of the spectra recorded on steel, the ring stretching resonances are entirely absent in the ppp spectra and are absent (or extremely weak) in the ssp spectra. The absence/ weakness of the ring stretches in both polarizations and the presence of the r− mode in the ssp spectrum strongly implies the randomization of the imidazole ring orientation along with significant tilting and possibly some conformational disordering of the alkyl chain. This is likely to occur if the imidazole ring lies principally parallel to the surface. This orientation is suggested by Wang et al.14 at low anodic potentials for the adsorption of imidazole on pure iron.

surface. In this regard, copper can be considered to be a noble metal because all of the d orbitals are filled. In an analogous manner to the interband transitions in gold, copper has an electronic transition in the visible region;33 this gives rise to significant enhancement of the electric field at the metal surface because of excitation of the interband transition by the 532 nm photons and, in large part, explains the difference in signal intensities observed between the copper and steel surfaces. The enhancement observed on copper is similar to that observed on noble metal surfaces by Dreesen et al.34 For butylimidazole adsorbed on a freshly cleaned copper surface, the destructive interference between these two susceptibilities gives rise to dips in the recorded spectra in both the ppp and ssp polarizations. The appearance of the resonances as dips in the ssp polarization provides an absolute polar orientation for the butyl chain by comparing the phase observed here to that for an octadecane thiol self-assembled monolayer (see part 3 of the Supporting Information). Furthermore, reference to the spectra recorded in the literature for dodecane thiol on copper in air, where the orientation of the monolayer is known, shows that the methyl termination of the chain is oriented away from the metal surface into the air. The C−H butyl bands are dominated by the methyl symmetric stretches in both the ppp and ssp spectra, with only a minor contribution from the methylene modes. For long alkyl chains, such as octadecane thiol, the ratio of the methyl to methylene symmetric stretch is often used to qualitatively estimate the degree of chain conformational ordering. For the short butyl chain considered here, this approach may be unreliable because two of the three methylene groups are in inherently non-centrosymmetric local environments. Consequently, an increase in methylene resonance intensity may indicate either a change in the chain tilt angle or an increase in conformational disorder. On copper, the asymmetric C−H ring stretching resonance of butylimidazole is apparent in both the ssp and ppp spectra at circa 3120 cm−1 for both isomers and is considerably stronger relative to the C−H stretches than those observed at the liquid/ air interface. In 1-butylimidazole, the integrated intensity of the asymmetric C−H stretching resonance is essentially unchanged between the ppp and ssp spectra, while for 2-butylimidazole, its intensity in the ppp polarization is approximately 3 times that observed in the ssp polarization. Romero and Baldelli18,19 concluded that, when adsorbed on the dielectric quartz surface, the intensities of the symmetric and asymmetric resonances can be interpreted to show that the transition dipole of the asymmetric ring stretching vibration is rotated away from the surface. Additionally, the absence of the symmetric stretching resonance in the ssp polarization (Figures 6 and 7) means that its transition dipole must be parallel to the surface plane. The loss of the hydrogen bonding in the RAIRS spectra of 2butylimidazole on copper after moderate heating strongly suggests the formation of an imidazole copper complex via the pyridine-type nitrogen, at least for mildly elevated temperatures. This explanation is analogous to that proposed by Carter and Pemberton35 for adsorption of imidazole on silver but is at variance with the allylperoxide structure suggested by Yoshida et al.8 for undecylimidazole on a strongly oxidized copper surface (Cu2O), where the imidazole ring coordinates to the oxide film. In contrast, Yoshida et al. also note that, for a freshly polished copper surface where a CuO oxide layer is prevalent, there is no ordered adsorption. An imidazole copper bond via the pyridine-type nitrogen would require the ring to be standing upright on the surface with the symmetric ring

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CONCLUSION The orientation of thin films of 1- and 2-butylimidazoles on copper and steel surfaces under air and at the liquid imidazole/ 10746

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air interface have been investigated by SFG vibrational spectroscopy. For both isomers, the spectra show unambiguously that, in every case, the methyl group of the butyl side chain is orientated away from the interface and into air. The presence of weak methylene resonances in the spectra on steel and their absence in the spectra on copper and at the liquid/air interface infer lower conformational order or an increased tilt angle in the butyl chains when the imidazoles are adsorbed on steel. This implies that, for adsorption on steel, both 1- and 2butylimidazoles form less closely packed films. Additionally, the spectra on copper and at the liquid/air interface show resonances from the C−H stretching modes of the imidazole ring, suggesting that, at these interfaces, the imidazole ring is tilted away from the surface and aligned along the surface normal. Relative to the respective methyl resonances from the butyl chain, the C−H ring stretching modes are stronger on the copper surface than at the liquid/air interface, suggesting that the plane of the ring is aligned closer to the surface normal when the imidazoles are adsorbed on copper. In both cases, this orientation of the rings would facilitate close packing of the chains, which, in turn, would account for the absence of methylene resonances in their spectra. Conversely, the absence of resonances from the rings in the spectra on steel implies the rings are in the surface plane or close to it. As a consequence, the butyl side chains are less closely packed, accounting for the presence of methylene resonances in the spectra on steel. The conclusion that, on copper, the molecule as a whole is oriented along or close to the surface normal, confirmed by the complementary linear IR spectroscopy, strongly implies chemical bonding to the surface via the pyridine-type nitrogen atom of the ring. This conclusion is in accordance with the results of other IR studies and SERS investigations.

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ASSOCIATED CONTENT

* Supporting Information S

SFG spectral fitting, RAIRS spectra of 2-butylimidazole as a function of temperature, and SFG spectra of self-assembled monolayers (SAMs) of octadecanethiol (ODT) on copper to determine the phases of the spectra in the ppp and ssp polarizations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +44-1223-336526. Fax: +44-1223-336362. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank BP Castrol for providing financial support for this work.

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