6518
Langmuir 1998, 14, 6518-6527
Surface Vibrational Spectroscopy of Lubricants Adsorbed at the Iron-Water Interface David C. Duffy, Adrian Friedmann,† Simon A. Boggis, and David Klenerman* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 3AP, United Kingdom Received June 2, 1998. In Final Form: August 18, 1998 Sum-frequency spectroscopy has been used to obtain the vibrational spectra of two molecules, potassium oleate and sodium octanoate, adsorbed at the iron-water interface. These two molecules reduce the friction of iron-steel contacts: the surface structures inferred from the vibrational spectra indicate how these molecules lubricate iron under aqueous solutions. Potassium oleate causes a reduction in the friction of iron surfaces at 10 mM; the strength and phase of the resonances in the vibrational spectra of oleate indicate that a bilayer was adsorbed at the iron surface at this concentation. Sodium octanoate reduces the friction of iron surfaces under anodic potentials. The vibrational spectra of adsorbed octanoate did not contain any resonances at any applied potential: this observation suggests that a completely disordered film was adsorbed at iron. The comparison between the coefficients of friction suggests that the more ordered films formed by oleate are more effective at lubricating than the disordered octanoate films.
Introduction The friction between two clean metal surfaces in contact is high: the dynamic coefficient of friction, µc, is typically around 1.1 The presence of a lubricating film between the metals can greatly reduce friction by physically separating asperities on the surfaces that hinder sliding. At low loads and high speeds, friction is reduced by a hydrodynamic mechanism whereby the load between the contacting surfaces is supported by a lubricant film of macroscopic thickness. At high loads and low sliding speeds, thick lubricant films break down and friction is reduced by a layer at the surface whose thickness is of molecular dimensions. The reduction of friction by microscopically thin films is called boundary layer lubrication, for which µc is typically between 0.05 and 0.25. We report the surface vibrational spectra of two molecules that are believed to act as boundary layer lubricants at iron surfaces in aqueous solution. From the vibrational spectra, we have inferred the structures formed at the iron-water interface and thereby gained insight into the mechanisms of lubrication at the molecular level. It is well-known that hydrocarbon molecules with terminal carboxylate groups can reduce the friction of metal surfaces. For example, adding a trace amount of stearic acid to oil results in a large reduction in the friction and wear of metal surfaces: this effect has been attributed to the repulsive interactions between adsorbed films 1 or 2 molecules thick.1a We have also observed that aqueous solutions of a soluble, long-chain carboxylic acid, potassium oleate, reduced the friction of the iron-steel contact.2 The mechanism by which carboxylate-terminated hydrocarbons lubricate metals in both aqueous solution and oil is thought to be as follows. A tenacious film of molecules † Current address: Novartis AG, R-1096.3.19, CH-4002 Basle, Switzerland.
(1) (a) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Oxford University Press: Oxford, U.K., 1964. (b) Williams, J. A. Engineering Tribology; Oxford University Press: Oxford, U.K., 1994. (c) Hutchings, I. M. Tribology; Edward Arnold: London, 1992. (2) Oleate has been used previously to reduce the friction of solid-oil interfaces: see Beltzer, M. J. Tribol. 1992, 114, 675. Wei, J.; Xue, J. Wear 1993, 160, 61. It is also used commonly as a flotation agent in mineral recovery, see: Rogan, K. R. Colloid Polym. Sci. 1994, 272, 82.
adsorbs at the metal surface by the formation of the metallic carboxylate soap. Lateral interactions between the hydrocarbon chains of the strongly bound molecules results in the formation of an oriented film. Repulsive interactions between oriented films on two surfaces reduces the area of contact at asperities and hence reduces friction and wear. This mechanism is plausible for molecules with long hydrocarbon chains (typically > 10 CH2 groups), such as stearate and oleate, where strong chain-chain interactions promote the formation of closepacked adsorbates. Close-packed structures are less likely to form, however, if the adsorbed molecules have short hydrocarbon chains: short-chain molecules are usually less effective lubricants. For example, in a study of the friction of iron-steel contacts, Brandon et al.3 found that for unpolarized surfaces, the coefficient of friction under an aqueous solution of sodium octanoate was the same as under pure water (µc ) 0.45). When anodic potentials were applied to the iron, however, µc was reduced to 0.25 under a solution of 60 mM octanoate; under water, friction increased and the surface corroded at anodic potentials. These authors attributed the reduction in µc to the formation of multilayers of octanoate. On the basis of FTIR measurements, Spikes et al.4 suggested that a thick layer of iron octanoate was formed at the surface; subsequent atomic force microscopy (AFM) measurements5 indicated that a much thinner, heterogeneous layer, consisting of islands of iron octanoate, was present at the metal surface. These studies illustrate that there is still ambiguity over the structures formed by carboxylate molecules at the surface of iron. We have taken the friction measurements of iron surfaces in the presence of solutions of oleate and octanoate as our starting points in using surface vibrational spectroscopy to examine the structure of lubricants adsorbed at the iron-water interface. Although boundary layer lubrication is of primary importance in most engineering applications,1b,6 relatively (3) Brandon, N. P.; Bonanos, N.; Fogarty, P. O.; Mahmood, M. N. J. Electrochem. Soc. 1992, 139, 3489. (4) Zhu, Y. Y.; Kelsall, G. H.; Spikes, H. A. Tribol. Trans. 1994, 37, 811. (5) Zhu, Y.; Kelsall, G. H.; Spikes, H. A. Tribol. Lett. 1996, 2, 287. (6) Spikes, H. A. Langmuir 1996, 12, 4567.
10.1021/la980641w CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998
Vibrational Spectra of Lubricants Adsorbed at Fe-H2O
few techniques are available that probe the surface molecular structure of lubricants in situ, i.e., when the solid surface is in contact with solution. The most successful of these techniques have been AFM, which has been used to perform nanotribology on monolayer films,7,8 and interferometry, which has been used to determine the thickness of lubricant films at steel-glass contacts.6 Sum-frequency spectroscopy (SFS) is a technique based on a nonlinear optical effect that has been used extensively to study the structure of hydrocarbon films at a range of surfaces (for review see ref 9). The only published SFS studies on friction modification, however, have used model lubricants that were either chemically anchored to the surface or deposited using the Langmuir-Blodgett (LB) technique.10,11 We report the vibrational spectra obtained by SFS of molecules that adsorb spontaneously at iron from aqueous solutions. In SFS,9 two pulsed laser beams of visible frequency (ωvis) and mid-IR frequency (ωIR) are overlapped at a surface. The nonlinear optical effect of sum-frequency generation (SFG) results in the emission of light at ωsum ) ωvis + ωIR. The intensity of the SFG light (Isum) is proportional to |χ(2)|2, where χ(2) is the second-order nonlinear susceptibility.9 χ(2) is zero in centrosymmetric environments; so, in experiments on iron surfaces under aqueous solutions, SFG light is only emitted from the ironwater interface. SFS is therefore a surface-specific technique. There are two contributions to the surface nonlinear susceptibility that determine Isum. First, there is a contribution from molecules adsorbed at the surface, (2) χijk,R , that is determined by the average of the hyperpolarizability of the molecules (βlmn) over their orientations (2) ) N〈βlmn〉/o. at the surface (number density, N), i.e., χijk,R (2) is given Under the conditions of the SFS experiment, χijk,R by9 (2) χijk,R )
N〈MlmAn〉 2po(ωv - ωIR - iΓv)
ijk ) xyz; lmn ) abc (1)
where ων is the frequency of a vibrational resonance of the is the vibrational relaxation time, and molecules, Γ-1 v 〈MlmAn〉 is the product of the Raman and IR transition moments of the resonance averaged over molecular orientations at the surface. The ijk and lmn indices refer to the axes of the surface and molecule-fixed Cartesian coordinates, respectively.12 From eq 1, it is clear that when the frequency of the IR laser coincides with that of a vibrational resonance of the molecules at the surface, 13 and hence there will be a change then χ(2) R will change in Isum. By scanning the IR laser and measuring the intensity of the emitted light, a vibrational spectrum of (7) For a recent review on general aspects of nanotribology and the use of scanning probe techniques in this field, see: Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (8) (a) Liu, Y.; Evans, D. F.; Song, Q.; Grainger, D. W. Langmuir 1996, 12, 2, 1235; (b) Tsukruk, V. V.; Bliznyuk, V. N.; Hazel, J.; Visser, D.; Everson, M. P. Langmuir 1996, 12, 4840. (9) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281. (10) (a) Du, Q.; Xiao, X.; Charych, D.; Wolf, F.; Frantz, P.; Shen, Y. R.; Salmeron, M. Phys. Rev. B 1995, 51, 7456. (b) Huang, J. Y.; Song, K. J.; Lagoutchev, A.; Yang, P. K.; Chuang, T. J. Langmuir 1997, 13, 3, 58. (11) Fraenkel, R.; Butterworth, G. E.; Bain, C. D. J Am. Chem. Soc. 1998, 120, 203. (12) In the surface Cartesian coordinate system, the z-axis is the surface normal, and the x- and y-axes lie in the plane of the surface, with the x-axis also in the plane of incidence. In the molecule-fixed Cartesian coordinates, the c-axis is parallel to the main symmetry axis. (13) The ijk indices are omitted henceforth for clarity.
Langmuir, Vol. 14, No. 22, 1998 6519
the molecules at the surface is obtained. For χ(2) R to be nonzero, the molecules must have a noncentrosymmetric arrangement at the surface: isotropic or completely disordered adsorbates do not produce a sum-frequency (SF) spectrum. The second contribution to χ(2) arises from the truncation of the bulk states of the metal at the surface. This term varies little with ωIR and is consequently referred to as the nonresonant susceptibility, χ(2) NR. The total intensity of the SFG light is given by14 (2) 2 Isum ∝ |χ(2) NR + χR |
(2)
(2) Both χ(2) R and χNR are complex quantities, and hence the appearance of resonances in SF spectra depends on the relative size and phase of the resonant and nonresonant susceptibilities.9 SF spectra yield direct structural information as well as providing a “fingerprint” of the molecules at the surface. First, the phase of the resonances depends on the net polar orientation of the adsorbed molecules;9 the phase of χ(2) R can be measured against the internal phase reference conveniently provided by the nonresonant signal.14,15 Second, the relative strength of resonances in SF spectra acquired with different combinations of the polarizations of the laser beams can yield the average tilt of the adsorbed molecules.16 Third, comparisons between spectra acquired with the same laser polarizations indicate qualitatively the degree of orientational order of adsorbates: highly oriented adsorbates generally produce SF spectra containing strong resonances; less well ordered adsorbates have weaker SF spectra. In this paper, we describe first the experimental arrangement used to obtain SF spectra of the iron-water interface. We then present SF vibrational spectra of iron surfaces in contact with aqueous solutions of oleate and octanoate; we also describe friction and ac impedance measurements. Finally, we propose structures for the adsorbed films and discuss how they help us to understand the mechanisms of lubrication.
Experimental Section Sum-Frequency Spectroscopy (SFS). SFS experiments were carried out using a laser system first reported by Shen et al.17 and described in detail elsewhere.18 The fundamental beam (1064 nm; 35 ps; 20 Hz) from a Nd:YAG laser (Continuum, YG601) was frequency-doubled in KD*P to provide the pump beam (532 nm; ∼25 ps; 20 mJ/pulse) for optical parametric generation and amplification (OPG/OPA)17 in two β-barium borate (BBO) crystals (type I, θ ) 31°). OPG/OPA yielded pulses of near-IR light (1.1-2.4 µm; ∼20 ps; 200-400 µJ/pulse); the frequency of the near-IR light was selected by a diffraction grating (JobinYvon, 25 × 25 mm, 600 lines/mm, blazed at 1.5 µm) that also served to reduce the bandwidth of the light.17 OPA in LiNbO3 between the tunable near-IR and fundamental (1064 nm) beams yielded pulses of tunable mid-IR light (2500-4000 cm-1; ∼20 ps; ∼100 µJ/pulse). The bandwidth of the mid-IR pulses was estimated to be 5-8 cm-1.18 The wavelength of the mid-IR laser was tuned by setting the wavelength of the tunable near-IR light using the diffraction grating, and phase-matching all of the nonlinear processes: the grating and nonlinear crystals were mounted on computer-controlled rotation stages (Time and (14) Bain, C. D.; Ong, T. H.; Ward, R. N.; Davies, P. B.; Brown, M. A. Langmuir 1991, 7, 1563. (15) Ward, R. N.; Duffy, D. C.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1994, 98, 8536. (16) Bell, G. R.; Ward, R. N.; Bain, C. D. J. Chem. Soc., Faraday Trans. 1996, 92, 515. (17) Zhang, J. Y.; Huang, J. Y.; Shen, Y. R.; Chen, C. J. Opt. Soc. Am. B 1993, 10, 1758. (18) Duffy, D. C. Ph.D dissertation, University of Cambridge, 1996.
6520 Langmuir, Vol. 14, No. 22, 1998 Precision, SN57-51) to facilitate tuning of the wavelength of the mid-IR pulses. SF spectra of iron surfaces were acquired in the following way. Mid-IR and visible (532 nm) laser pulses (both p-polarized) were overlapped at an iron surface in a co-propagating geometry with incident angles of 55° and 60°, respectively. The emitted SFG light (p-polarized) was selected by a grating spectrometer (Acton Research Corp., SpectraPro-500) and detected by a gated diode array (Princeton Instruments Inc., IRY-700). The integrated SFG signal was normalized to the average signal from a PbSe photodiode that measured the energy of a fraction of the mid-IR beam split-off using a CaF2 plate. The mid-IR frequency was calibrated by simultaneously measuring the absorption spectrum of a thin sample of polystyrene using a fraction of the mid-IR beam. To acquire a SF spectrum, the SFG, normalization, and calibration signals were typically averaged over 2000 pulses at 2 cm-1 increments between 2800 and 3000 cm-1. Iron surfaces were housed in a standard liquid cell.15 Iron samples (99%, Advent) were polished to a mirror finish and inset in a Kel-F shaft that was attached to a micrometer. A phosphor bronze spring ensured electrical conductivity between the sample and the body of the micrometer. The samples were housed in a sealed PTFE liquid cell body (capacity ) 4.9 cm3) that had an equilateral CaF2 prism (15 × 15 × 15 mm) attached to the front. The cell body also had ports for a Ag/AgCl reference electrode, for a Pt counter electrode, and for loading and drainage of solutions. Prior to acquisition of a SF spectrum, the iron sample was positioned approximately 5 mm from the prism and left to adsorb under a solution of either potassium oleate or sodium octanoate.19 Under solutions of sodium octanoate, the potential of the iron surface was controlled by a potentiostat (ACM Instruments, Electrochemical Interface). The iron sample was then moved up to the prism, trapping a thin film (∼1-2 µm) of solution. The mid-IR and visible laser beams were then overlapped onto the iron surface to within an area of about 1 mm2. The energy of the visible beam (∼3 mm diameter) was restricted to 15 µJ/pulse to avoid laser damage to the iron surfaces. Control experiments (as described elsewhere)15 indicated that the SF signal arose solely from the iron surfaces and not from molecules at the prism surface. The SF spectra contained a weak nonresonant background that varied slowly with IR wavenumber due to dispersion and absorption of the IR light by the prism and water (see Figure 6); to produce flat baselines for fitting and presentation, the spectra were divided by a smooth curve fitted to the nonresonant background. The position, strength, and phase of the vibrational resonances were determined by fits to the SF spectra.14 The SF intensity was modeled as a function given by |∑vfv + SNRei|2. SNR and are the strength and phase of the nonresonant SF signal, respectively. fν describes a Voigt profile for each resonance and is given by a convolution of a Lorentzian function, SR/(ω - ωIR - iΓν), with a Gaussian distribution of resonant frequencies (ω), about the central frequency ων, of width σν. SR is the resonant line strength and is proportional to χ(2) R . Fits to this function yielded values of ων, σν, and SR for each resonance. Friction and ac Impedance Measurements. Friction measurements were made between 9.5-mm diameter polished iron spheres and mild steel flats, in the presence of solutions of potassium oleate.20 Both the iron spheres and flats were polished to a 1 µm finish. In this sphere-flat configuration, a mild steel strip was passed multiple times over a stationary iron sphere at a constant speed (0.4 mm/s) in a reciprocating motion that had a magnitude of 2.4 cm. The contact region was fully flooded with the solutions. Coefficients of friction were obtained from linear fits to plots of friction force against normal load (1-400 mN). Ac impedance measurements (Schlumberger, Solatron 1286 electrochemical interface) were carried out in the same liquid (19) Solutions of sodium octanoate (Sigma, 99+%) and potassium oleate (Aldrich, supplied as a paste 40% by weight in H2O) were prepared in either D2O (Fluorochem Ltd., 99.9 atom % D) for SFS experiments or H2O (Milli-Q Plus, resistivity >18 MΩ cm) for friction and ac impedance measurements. The pH of the solutions was adjusted by adding 0.1 M NaOH. Solutions of potassium oleate were filtered to remove small amounts of precipitated acid. Solutions were discarded within 3 days of preparation. (20) The mild steel-iron contact allows us to compare our results with previously published data (ref 3).
Duffy et al. Table 1. Values of Reciprocal Capacitance for Iron Surfaces under Solutions of Potassium Oleate (pH 10) concn of potassium oleate (mM) 0.5 1 2 5 10 under D2O (after adsorption for 2 h in a 10 mM solution of oleate)
1/c (µF-1) 0.31 0.36 0.30 0.38 0.47 0.21
cell as was used for SFS experiments. The real (Z′) and imaginary (Z′′) components of the impedance of iron surfaces under solutions of potassium oleate were recorded as a function of the frequency of the driving ac field (ω ) 0.025-500 Hz). From the relationship Z′′ ) -1/ωC, the slope of a plot of Z′′ against 1/ω yields the reciprocal capacitance (1/C) of the interfacial region of the iron surface.21 To a first approximation, 1/C is proportional to the thickness of the film at the surface.21 Under solutions of sodium octanoate, 1/C was measured between -800 and +800 mV vs SHE22 at 100 mV intervals in dc voltage; for potassium oleate, measurements at the rest potential were made as a function of concentration. In both cases, the driving ac voltage had an amplitude of 10 mV. It was not possible to determine 1/C under pure water at the rest potential due to corrosion of the iron surface.
Results Friction and Capacitance Measurements. The coefficient of friction of the mild steel-iron contact was measured in the presence of water and a solution containing 10 mM potassium oleate. The coefficient of friction under water (pH 10) was 0.45; under 10 mM potassium oleate, µc was reduced to 0.09. Experiments where the surface was removed from 10 mM solutions of oleate, rinsed in water, and the friction measurement repeated under water at the same contact point gave irreproducible results, but generally we observed an increase in friction on washing. We found it difficult to measure the friction of the steel-iron contact at lower oleate concentrations ( 0.5 mM. Table 1 shows the values of 1/C for iron surfaces under solutions of oleate. These data are difficult to interpret because the surface impedance depended on both the thickness and dielectric constant of the adsorbate as the concentration of oleate was changed. The value of 1/C should, however, provide a guide to the thickness of the oleate layer.21 The values of 1/C indicate that the oleate film formed at 10 mM was significantly thicker than the film formed at 0.5 mM. Furthermore, the thickness of the surface layer was approximately halved when the sample was removed from a 10 mM solution of oleate and replaced in water. Figure 1 shows 1/C plotted against the potential applied to an iron surface under an aqueous solution containing 60 mM sodium octanoate.23 Figure 1 also shows the coefficient of friction of iron-steel contacts under a solution of sodium octanoate at several potentials.3 The values of 1/C reproduce those made by previous workers3 and show that a reduction in friction is accompanied by the adsorption of octanoate at the iron surface. We note that (21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980. (22) Although the potentials were measured against a Ag/AgCl reference electrode, we report the values as referenced against a standard hydrogen electrode (SHE) to be consistent with previous work (ref 5). (23) This concentration corresponds to a solution containing 1 wt % sodium octanoate, which was used in experiments described in ref 3.
Vibrational Spectra of Lubricants Adsorbed at Fe-H2O
Langmuir, Vol. 14, No. 22, 1998 6521
Figure 1. Plot of 1/C against applied potential for an iron surface under a solution (pH 10) containing 60 mM sodium octanoate. Coefficients of friction at three potentials, reported in ref 3, are shown.
Figure 2. SF spectrum of a LB film of a monolayer of the cadmium salt of behenic acid (CH3(CH2)20COOH) at iron. The spectrum was recorded under air. The SF, visible, and IR beams were p-polarized. The three strong resonances arise from modes of the terminal methyl groups (see text).
the maximum value of 1/C of an iron surface under 60 mM octanoate (Figure 1) is approximately half that of iron under 10 mM oleate (Table 1). Sum-Frequency Spectra. (a) LB Film of Cadmium Behenate at Iron. To facilitate the analysis of the SF spectra of oleate and octanoate adsorbed at iron, we recorded the SF spectrum of a carboxylate-terminated hydrocarbon molecule whose surface structure is known. Figure 2 shows the SF spectrum of a monolayer of the cadmium salt of behenic acid (CH3(CH2)20COOH) deposited at an iron surface using the LB technique. The LB film was prepared with an area per behenate anion of 20 Å2: at this packing density the hydrocarbon chains are highly ordered.24 The three strongest peaks in the SF spectrum are assigned to modes of the terminal CH3 group of the behenate anion. Resonances at 2878, 2935, and 2960 cm-1 are assigned to the symmetric methyl stretching mode (r+), to its Fermi resonance with an overtone of a bending mode (r+ FR), and to the antisymmetric stretching mode (r-), respectively.15 Resonances from the CH2 modes of the hydrocarbon chains have been observed previously in SF spectra at 2850 cm-1 (symmetric stretching mode, d+), and in the range 2900-2930 cm-1 (Fermi resonance 15 of the symmetric stretch, d+ Only weak resonances FR). (24) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991.
Figure 3. SF spectra of an iron surface under solutions (pH 10) containing (a) 10 × 10-3, (b) 5 × 10-3, (c) 2 × 10-3, and (d) 0.5 × 10-3 mol/dm3 potassium oleate. The baseline of the nonresonant SF signal corresponds to spectrum (d); the other spectra are offset vertically for clarity. The SF signal was 2 point adjacent averaged. All laser beams were p-polarized.
from the d+ and d+ FR modes appear in the spectrum of cadmium behenate. Other densely packed monolayers yield similar SF spectra.14,25 Potassium Oleate at the Iron-Water Interface. Figure 3 shows the SF spectra of iron surfaces under solutions of potassium oleate at four concentrations. The SF spectra at 1 mM were not reproducible and showed some variation: two typical spectra are shown in Figure 4. Solutions were adsorbed for 20 min before the SF spectra were acquired; an increase in the adsorption time of up to 10 h did not affect the appearance of the spectra. Figure 5 shows the SF spectrum of an iron surface recorded under D2O after the iron had been placed in a solution containing 10 mM potassium oleate for 2 h, rinsed in D2O, and then replaced in the liquid cell. We note first some general trends in the appearance of the SF spectra in Figures 3-5. First, the SF spectra of oleate at the iron-water interface differ markedly from the spectrum of the LB film (Figure 2). The molecular signal from the adsorbed oleate molecules is much smaller than from the monolayer of behenate ions: resonances from oleate appear as a superposition against the weak nonresonant signal,26 whereas the resonances in the SF spectrum of the LB film are very strong. We infer immediately that oleate does not form densely packed (25) (a) Akamatsu, N.; Domen, K.; Hirose, C. J. Phys. Chem. 1993, 97, 10070. (b) Maechling, C. R.; Kliner, D. A. V.; Klenerman, D. Appl. Spectrosc. 1993, 47, 167.
6522 Langmuir, Vol. 14, No. 22, 1998
Figure 4. Two representative SF spectra of iron surfaces under solutions (pH 10) containing 1 × 10-3 mol/dm3 potassium oleate. The baseline for the nonresonant SF signal corresponds to the lower spectrum. The SF signal was 2 point adjacent averaged. All laser beams were p-polarized.
Figure 5. SF spectrum of an iron surface under pure D2O. Prior to acquisition of the spectrum, the iron sample had been left in a solution containing 10 × 10-3 mol/dm3 potassium oleate for 2 h and then rinsed in D2O. All laser beams were p-polarized. The solid line is a fit to the experimental spectrum (see text).
monolayers at the surface of iron. Furthermore, unlike in the spectrum of behenate, there are resonances in the SF spectra of oleate (at about 2860 and 2890-2930 cm-1) that arise from the CH2 groups of the hydrocarbon chains. Second, differences between the spectra of oleate show that the surface structure changed with concentration. At 0.5 mM the resonances in the spectrum are weak; the spectra at 2, 5, and 10 mM have a similar appearance in which several resonances appear as stronger dips against the background SF signal. We infer from the difference between the spectra at 0.5 mM and g2 mM, and from the variety of spectra observed at 1 mM (Figure 4), that there is a transition in the surface structure of oleate at about 1 mM. This structural transition coincides with the critical micelle concentration (cmc) of oleate.27 Third, the resonances in the SF spectrum of the iron surface after it had (26) The nonresonant signals from iron surfaces were much lower than from another metal, gold, that has been used extensively in SFS to study the adsorption of hydrocarbons (see ref 15). At the laser energies used in these experiments, the nonresonant signal from iron was typically 1 photon/pulse compared to >100 photons/pulse from gold. (27) Mukerjee, P.; Mysels, K. J. NSRDS-NBS 36, 1970.
Duffy et al.
been adsorbed under a solution of oleate, removed, washed, and replaced in pure water (Figure 5) are clearly stronger than those of the iron surface under solutions of oleate (Figure 3). To understand more fully the changes in the SF spectra, we determined the strength and phase of the resonances from fits to the SF spectra; a sample fit is shown in Figure 5. The spectra at 0.5 mM were fitted by three weak resonances at 2879, 2954, and 2900 cm-1, which can be 28 assigned to the r+, r-, and d+ FR modes, respectively. The fits to the spectra at concentrations between 2 and 10 mM included a resonance with a negative line strength (SR < 0) at around 2860 cm-1 and a broad negative feature between 2890 and 2930 cm-1. These resonances can be assigned to the d+ and d+ FR modes, respectively. The fits to these spectra also included a resonance with negative line strength at about 2950 cm-1 that can be assigned to the r- mode; only weak resonances at 2875 and 2935 cm-1, where the r+ and r+ FR modes are expected to appear, were needed in the fits. The spectrum of iron under D2O after adsorption in a 10 mM solution of oleate (Figure 5), was also fitted by resonances of negative line strength arising from the CH2 modes. This fit, however, also included three resonances with positive line strengths (SR > 0) from the r + , r+ FR, and r modes. We also attempted to fit the methyl modes in this spectrum with negative line strengths: negative values of SR with a magnitude greater than SNR can give rise to peaks. Such an approach, however, gave poor fits to the spectra; satisfactory fits could only be obtained if the CH3 and CH2 modes had line strengths of opposite sign. The fitting parameters of the spectra in Figures 3-5 are given in Table 2. The strength of the methyl modes are also plotted as a function of concentration in Figure 6. (c) Sodium Octanoate at the Iron-Water Interface. Figure 7 shows SF spectra of iron under solutions containing 60 mM sodium octanoate at several potentials. A previous study has shown that the friction of iron surfaces under solutions of octanoate is reduced at these potentials.3 Although there was an increase in the nonresonant SF signal as the potential increased,29 no resonant signal from the hydrocarbon chains of the adsorbed octanoate molecules was observed. Featureless spectra were also recorded at lower potentials (-800 to +400 mV) at which no reduction in friction is observed.3 We conclude that the octanoate molecules adsorbed at iron did not have a net polar orientation. Discussion In this section we propose structures for the adsorbed oleate and octanoate films that are consistent with the SF spectra and measurements of film thickness. We then consider how these structures are related to the friction measurements. Surface Structure of Potassium Oleate. The SF spectra of oleate (Figures 3-5) take three general forms. Below 1 mM the spectra contain weak resonances. Between 2 and 10 mM, the spectra are stronger, with dips from the CH2 modes dominant. A dip from the antisymmetric methyl stretch (r-) is observed; the symmetric (28) Although we do not believe that the CH2 resonances arise from modes delocalized along the hydrocarbon chain (see Discussion), for simplicity, we have retained the notation for the CH2 modes (d+ and d+ FR) that are adopted for delocalized modes. (29) An increase in the nonlinear surface signal has also been observed by second harmonic generation (SHG): the increase in SHG signal was attributed to the greater polarizability of the iron surface at anodic potentials (Klenerman, D. Unpublished results).
Vibrational Spectra of Lubricants Adsorbed at Fe-H2O
Langmuir, Vol. 14, No. 22, 1998 6523
Table 2. Frequencies, Strengths, and Assignment of Resonances Determined from Fits to the SF Spectra of Potassium Oleate Adsorbed at Iron Surfacesa,b concn of potassium oleate (mM) 0.5 2
5
10
under D2O after adsorption in 10 mM oleate solution
resonant frequency ωv (cm-1)c
resonant strength normalized to nonresonant strength (SR/SNR)
gaussian hwhm σv (cm-1)d
2879 2900 2954 2860 2873 2926 2960 2862 2875 2913 2948 2861 2878 2911 2949 2860 2874 2920 2932 2962
1.4(1) -5.2(5) -0.9(1) -3.4(13) 0.7(7) -6.6(2) -3.1(2) -2.3(7) 0.6(4) -5.1(2) -1.1(1) -3.4(8) 1.3(4) -6.4(4) -2.3(2) -6.9(9) 2.7(1) -15.6(6) 1.9(1) 2.8(1)
6.9(4) 42.8(31) 6.5(7) 9.0(19) 6.8(28) 18.5(6) 8.8(4) 9.6(17) 6.1(18) 18.5(6) 5.1(4) 9.4(18) 6.1(16) 16.5(11) 5.2(5) 8.7(5) 5.4(1) 26.6(5) 4.6(2) 6.8(2)
assignment r+ d+ FR rd+ r+ d+ FR rd+ r+ d+ FR rd+ r+ d+ FR rd+ r+ d+ FR r+ FR r-
a All spectra were fitted with a nonresonant phase () of π/2. b All errors are quoted at the 1σ level and represent the error in the last digit of each value. A second set of spectra produced similar fitting parameters. c Absolute error in wavenumber values is (2 cm-1. d All resonances were fitted with a fixed Lorentzian hwhm of Γν ) 2.0 cm-1.
Figure 6. Plots of the resonant strengths of the symmetric and antisymmetric methyl modes (r+ and r-, respectively) in the SF spectra of oleate at the iron-water interface, as a function of concentration and after washing of the iron sample. The magnitudes of both modes increase after washing; the strength of the r- mode also changes sign. The errors in these values are given in Table 2.
methyl modes (r+ and r+ FR) are weaker. When iron samples were removed from 10 mM solutions of oleate and the spectra recorded under D2O, the dips from the CH2 modes were still present but the r- mode changed phase and the symmetric methyl modes appeared as strong peaks (Figure 6). We believe the SF spectra are consistent with the formation of bilayers at the surface of iron at concentrations of oleate between 2 and 10 mM. We propose that the bilayer is composed of a lower layer of oleate molecules that are bound through their carboxylate groups, and an upper layer in which the molecules are physisorbed through hydrophobic interactions with the lower layer, which orients the carboxylate groups toward the aqueous phase. A bilayer structure is supported by several facts. First, the resonances in the SF spectra under solutions of oleate are very weak compared to that of an oriented monolayer: this fact suggests that oleate forms an adsorbed film that has a high degree of centrosymmetry.30 A perfect bilayer has a centrosymmetric structure: the SF signal from the methyl groups in the lower layer, which point away from the surface, will cancel with the signal from the methyl groups in the upper layer, which point toward the surface. As the adsorbed oleate molecules give
Figure 7. SF spectra of an iron surface polarized at (a) +1000, (b) +800, and (c) +600 mV vs SHE, under solutions containing 60 mM sodium octanoate. All laser beams were p-polarized. These spectra were not corrected to remove the effects of dispersion and absorption of the IR beam by the prism and water and therefore have a curved baseline. The spectrum recorded at +1000 mV has been offset vertically by 100 arbitrary units for clarity of presentation.
rise to some resonant SF signal, however, we suggest that the bilayer formed by oleate is not perfectly centrosymmetric. Second, when the iron samples were transferred from a solution of oleate into D2O, there was an increase in the intensity of both the symmetric methyl and methylene modes. This observation is consistent with the partial desorption or disordering of the weakly bound upper layer of the bilayer upon removal of the iron from the solution of oleate, which reduces the centrosymmetry
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Figure 8. Schematic diagram of the bilayer formed by potassium oleate at the surface of iron. When the iron sample was removed from the solution of oleate and rinsed in D2O, the SF spectra indicated that the top layer of the bilayer was partially removed or disordered. The oleate molecules are presented as rods; the packing density and conformational order of the hydrocarbon chains cannot be inferred from the SF spectra, however.
of the adsorbate and gives rise to a greater net orientation of the CH3 and CH2 groups. Figure 8 shows schematically the structural change that we believe occurred when the iron sample was removed from a solution of oleate. In an unpublished SFS study of the adsorption of surfactants at the silica-water interface, Bain et al.31 also observed a large increase in the strength of C-H stretching resonances when a charged silica surface was removed from a solution of surfactant and the SF spectrum rerecorded under water: the change in the spectra was also attributed to the removal of the upper layer of a bilayer. Third, the sign of the r- mode changed from negative to positive when the iron surface was removed from a solution of oleate (Figure 6). The line strength of resonances in SF spectra in which the SF, visible, and IR beams are all p-polarized (Sv,ppp) is given by16 (2) (2) (2) Sv,ppp ∝ |F(zzz)χzzz,R + F(xxz)χxxz,R + F(xzx)χxzx,R + (2) F(zxx)χzxx,R | (3)
F(ijk) are parameters that incorporate the Fresnel factors of the visible and IR beams, and factors that relate the polarization generated at the sum-frequency to the electric field of the emitted light.32,33 Expressions that relate (2) χijk,R to the hyperpolarizability (βlmn) of the methyl groups and their average tilt with respect to the surface normal, θ, are reported in ref 33. Using these expressions, values for βlmn determined from Raman spectroscopy,16 calculated values of F(ijk),32 and eq 3, we determined relationships between θ and the line strengths of the methyl resonances in ppp-polarized spectra. These calculations showed that (29) It is difficult to quantify accurately the relative resonant strengths of the methyl stretching modes in the spectra of the LB film and the oleate adsorbate because of differences in the overlap of the laser beams and Fresnel factors. From fits to the spectra and calculated values of F(ijk), however, we estimate that the r- and r+ modes are approximately 7 and 10 times stronger, respectively, in the spectrum of the LB film than in the spectrum of oleate. (30) Bain, C. D.; Ward, R. N. Unpublished results. (31) F(ijk) for the iron-water interface was calculated using expressions given in Braun, R.; Casson, B. D.; Bain, C. D. Chem. Phys. Lett. 1995, 245, 326. The local refractive index experienced by the methyl groups was taken as 1.40. Refractive indices for iron were taken from The Handbook of Chemistry and Physics (66th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1985). (32) Hirose, C.; Akamatsu, N.; Domen, K. Appl. Spectrosc. 1992, 46, 1051.
inverting the average orientation of the molecules at the surface, i.e., performing the transformation θ f π-θ, changes the sign of Sr-,ppp. The observed change in the sign of the r- mode shows that the methyl groups giving rise to the net SF signal have opposite polar orientation under water and under a solution of oleate. We rationalize the inversion of the net polar orientation by suggesting that the peak from the r- mode in the spectra under D2O arises from the molecules in the layer directly bound to the iron surface, and the dip from the r- mode in the spectra under a solution of oleate arises from molecules in the upper layer of the bilayer. Ac impedance measurements help to confirm the hypothesis of a bilayer structure. We observed that the value of 1/C was halved when the iron sample was removed from a 10 mM solution of oleate and replaced in water (Table 1). This observation is consistent with a 50% reduction in the thickness of the adsorbate that would result from the removal of the upper layer of the bilayer. Given the weight of evidence from SFS and ac impedance measurements, we will refer to the structure formed under solutions of oleate as the bilayer, and the structure under water as the monolayer. The hypothesis of the formation of a bilayer must be reconciled with several unusual aspects of the SF spectra. First, why do the CH3 and CH2 resonances have opposite phases in the spectra of the monolayer (Figure 5), whereas in all other SF spectra of monolayers of hydrocarbon chains the CH3 and CH2 resonances have the same phase?14-16 Second, why do the CH2 modes of oleate have the same phase in the bilayer (Figure 3) and in the monolayer (Figure 5), whereas the r- mode changes phase? Third, why are the r+ and r+ FR modes significantly weaker than and have the sign opposite that of the r- mode in the spectra of iron under solutions containing 2-10 mM oleate? To explain the relative phases of the CH3 and CH2 modes in the SF spectra we must consider the local symmetry of the hydrocarbon chain of oleate and the ensemble arrangement of the molecules at the surface. The intensity of CH2 modes in the SF spectra of saturated hydrocarbons is usually rationalized by the local symmetry of the chains.9 For instance, in an all-trans chain, adjacent CH2 groups have opposite orientations and give zero net SF signal. A gauche defect in a chain places a CH2 group in a noncentrosymmetric environment with the same polar orientation as the terminal methyl group: CH3 and CH2 resonances therefore usually have the same phase in SF spectra. As the CH3 and CH2 resonances in the monolayer of oleate (Figure 5) have line strengths with opposite signs, however, we must conclude that the terminal methyl groups and the methylene groups giving rise to the SF signal have opposite orientations. The opposite phase of the CH3 and CH2 resonances in the spectra of oleate at iron cannot be rationalized by the orientation of CH2 groups at gauche defects; we must consider the orientation of the CH2 groups on either side of the cis double bond.34 If we consider the geometry of an oleate molecule bound directly to the iron surface, it is clear that if the molecule has no gauche defects and is bound through the carboxylate group with one of the C-O bonds oriented close to the surface normal (as shown in Figure 9a), then the CH2 group above the double bond will be oriented close to the surface normal and the CH2 group below the double bond close to the plane of the surface. The resonant strength in ppp-polarized SF spectra of iron surfaces is proportional to the dipole (33) NMR indicated that the sample of potassium oleate was composed of 100% of the cis isomer: this fact was confirmed by the manufacturer.
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Figure 9. Schematic diagram showing two possible orientations of oleate molecules adsorbed at the surface of iron.
moment parallel to the surface normal (µz): the orientation of oleate shown in Figure 9a will give rise to a SF signal that is dominated by the CH2 group above the double bond. This CH2 group points toward the iron surface, i.e., in the direction opposite to the terminal methyl group; the methyl and methylene resonances in SF spectra will therefore have opposite phases, as we observe. This argument relies on one of the C-O bonds of the carboxylate groups being oriented close to the surface normal: we make two observations to justify this assumption. First, FTIR spectra of LB films of stearic acid indicate that the carboxylate molecules adopt such an orientation at the surface of germanium.35 Second, if the oleate molecules in the monolayer were oriented with one of the C-O bonds tilted away from the surface normal (Figure 9b), then the two CH2 groups adjacent to the cis double bond would have equal and opposite orientations and therefore produce no net resonant SF signal. In light of these observations, we assert that the methylene resonances in the SF spectra of the oleate monolayer (Figure 5) are dominated by the CH2 groups in asymmetric positions adjacent to the double bond rather than by CH2 groups at gauche defects. This assertion is supported by the blue-shift of ∼10 cm-1 in the vibrational frequency of the symmetric CH2 stretching mode of oleate compared to that of molecules containing saturated hydrocarbon chains:15 the frequency of the symmetric CH2 stretching mode in the IR spectrum of 1-butene is also blue-shifted compared to that of saturated hydrocarbons.36 The rationalization of the phases of the CH3 and CH2 stretching modes also explains why the CH2 modes appear as dips in the spectra of both the monolayer and bilayer. In both structures, the signal is dominated by the CH2 groups that are adjacent to the cis double bonds of oleate molecules in the layer closest to the iron surface. (34) (a) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. (b) Umemura, J.; Kamata, T.; Kawai, T.; Takeaka, T. J. Phys. Chem. 1990, 94, 62. (35) (a) Gallinella, E.; Cadioli, B. Vibr. Spectrosc. 1997, 12, 163. (b) Peng, J.; Mina-Camilde, N.; Manzanares, C. I. Vibr. Spectrosc. 1995, 8, 319.
Figure 10. Plots of the resonant strength of (a) the r+ mode and (b) the r- mode in ppp-polarized SF spectra as a function of the average tilt (θ) of the methyl groups to the surface normal.
We provide an explanation for why the symmetric methyl stretching modes are significantly weaker than the r- mode in the spectra of bilayers of oleate by considering the dependence of the resonant strengths on the tilt of the methyl groups at the surface. Figure 10 shows plots of the line strength of the r+ and r- modes as a function of the tilt of the methyl groups, θ, calculated from eq 3. If the oleate molecules in the layer bound to iron are tilted somewhere between the two orientations shown in Figure 9, then θ must lie between 0 and 50°. Figure 10a shows that the strength of the r+ mode is insensitive to the orientation of the methyl groups for tilts less than 50°. Therefore, if the average tilt of the oleate molecules in the upper layer of the bilayer also lie in the range 0-50°, then the SF signal from the r+ mode will be approximately equal and opposite37 for the two layers and will tend to cancel. Figure 10b shows that the intensity of the r- mode is more sensitive to tilt than the r+ mode in the range 0 < θ < 50°. Therefore, any difference between the tilts of the methyl groups in the two layers will result in a net SF signal from the r- mode (Figure 3). The difference in sign of the r- and r+ modes in the bilayer arises because the net signal from the r- mode arises from molecules in the upper layer, which are presumably more tilted (Figure 10b), and the net signal from the r+ mode arises from molecules in the lower layer, which, because the intensity is insensitive to tilt, are presumably more densely packed (Figure 10a). When the upper layer is removed by rinsing, the methyl groups in the monolayer are in a less centrosymmetric environment than in the (36) As with the r- mode, when the methyl groups are inverted at the surface, the line strength of the r+ mode changes sign.
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bilayer and give rise to strong peaks from both the r+ and r- modes (Figure 5). We note that the discussion in this section has not considered the possibility of gauche defects in the oleate chains. We are unable to comment confidently on the conformational order of the hydrocarbon chains of the oleate molecules because our analysis of the strength of the CH2 resonances has had to be based on the groups adjacent to the double bond. We believe it likely that the adsorbed oleate molecules will contain gauche defects: unsaturated phospholipids in cell membranes contain gauche defects that facilitate close packing of the molecules.38 A schematic diagram of the bilayer structure that we believe potassium oleate forms at iron surfaces is shown in Figure 8. Surface Structure of Sodium Octanoate. The SF spectra of iron under solutions of sodium octanoate (Figure 7) contained no resonances even at potentials at which this molecule reduces friction. The only firm conclusion we can draw from these SF spectra is that the octanoate molecules adsorbed at iron surfaces did not have a net polar orientation. Several structures can explain how octanoate adsorbs at iron to give isotropic lubricating films: disordered monolayers, perfect bilayers, multilayers, islands of octanoate, and adsorbed micellar aggregates. The adsorption of micelles can be discounted as the cmc of sodium octanoate is 350 mM,27 whereas a reduction in friction was observed at 60 mM. The formation of multilayers, as suggested by Brandon et al.,3 also seems unlikely as our ac impedance measurements indicate that octanoate forms a thinner film than oleate, which we believe forms a bilayer. We note, however, that an AFM study of the adsorption of octanoate at iron indicates the formation of a nonuniform layer consisting of islands of iron octanoate.5 The SF spectra presented here would suggest that the octanoate molecules in such islands have an isotropic (disordered) arrangement. Surface Structure and Friction. The structures of the adsorbed films of oleate and octanoate determined from the SF spectra suggest how these molecules modify the friction of iron surfaces. Although the SF spectra were not recorded in lubro, i.e., at the contact between iron and another surface, the structures determined at the ironwater interface will be present when iron first makes contact with another solid. The surface structures determined from the SF spectra can therefore give us some indication of the mechanisms of lubrication. The mild steel-iron contact has a high coefficient of friction (µc ) 0.45) under water. This value was greatly reduced in the presence of 10 mM solutions of oleate (µc ) 0.09). Corrosion and wear of the steel made it more difficult to determine µc at intermediate concentrations: we found, however, that the coefficient of friction was generally constant at concentrations >0.5 mM. SFS showed that disordered or weakly adsorbed films of oleate adsorbed to iron at low concentrations (2 mM). We conclude that some reordering of the film in the contact must occur under pressure since the coefficient of friction is constant with concentration despite changes in the film structure under static conditions. In particular, it is likely that the top layer of the bilayer is removed in the contact since SFS indicated that this layer was only physisorbed. We also conclude, however, that the formation of bilayers must play some role in the stability of the lubricating film that
We have used a combination of sum-frequency spectroscopy (SFS), friction measurements, and capacitance measurements to provide an insight at the molecular level into the mechanisms by which carboxylate-terminated hydrocarbon molecules lubricate iron surfaces in aqueous solution. The formation of bilayers of oleate molecules, which was inferred from their vibrational spectra at a concentration of 10 mM, seems to be important in both the reduction of wear and friction at the iron surface. The exact role of the bilayer will depend on the contact between the surfaces: under some conditions, the bilayer may be entrapped and act as additional spacer between the surfaces; under other conditions, the physisorbed upper layer could be squeezed out, and the bilayer simply serves to provide the most closely packed surface film. The sumfrequency vibrational spectra of sodium octanoate are consistent with AFM measurements and suggest that friction was reduced by repulsions between disordered adsorbates. The coefficients of friction indicated that the ordered films formed by oleate were more effective in lubricating iron surfaces than the disordered films formed by octanoate. These experiments have also illustrated that SFS, while being ideally suited for studying anisotropic surface films, can also be used to detect the formation of an almost centrosymmetric adsorbate, namely a bilayer, even at a surface that has a low background SF signal.
(37) Gennis, R. Biomembranes; Springer-Verlag: New York, 1989.
Acknowledgment. This work was funded by ROPA
occurs somewhere between 2 and 10 mM since it is straightforward to make friction measurements at 10 mM oleate concentration and corrosion and wear are not a problem. The fact that measurements of the coefficient of friction after washing were irreproducible confirms this hypothesis: although the lower layer is probably chemisorbed and persists after washing, the presence of an upper layer seems to be necessary for lubrication. SFS indicates the formation of bilayers of oleate at the iron-water interface: the exact role of the bilayer in lubrication is unclear, however, and the mechanism of lubrication will certainly depend on the conditions in the contact. For example, at low pressures, where the contact region is small, or at low sliding speeds, our measurements suggest that the upper layer of the bilayer would not be stable enough to be present in the contact region. On the other hand, at high pressures, where the contact region is large, or at high sliding speeds, it is possible that the bilayer could be entrapped in the contact region and act as an additional spacer to stop the two surfaces from touching. Despite uncertainty in the role of the bilayer, we note, however, that even if the upper layer of the bilayer is removed during lubrication, its formation provides the most closely packed layer directly bound to the iron surface, which means oleate reduces the friction of iron. A comparison between the SF spectra and friction measurements of iron surfaces in contact with oleate and octanoate is enlightening. Oleate, which has a long hydrocarbon chain, forms lubricating films at iron that are orientationally ordered. Octanoate, which has a shorter hydrocarbon chain, forms disordered layers at iron surfaces. The coefficients of friction of the mild steeliron contact under water, a solution of oleate, and a solution of octanoate are 0.45, 0.09, and 0.25, respectively. We infer that repulsions between layers of oriented oleate molecules are more effective in reducing friction than the interactions between disordered islands of octanoate. Conclusion
Vibrational Spectra of Lubricants Adsorbed at Fe-H2O
Grant No. RG20136 from the EPSRC. We thank John Clint (Hull University) for preparing the LB films. D.C.D. thanks Emmanuel College, University of Cambridge, for a Research Fellowship. A.F. thanks the Royal Society for a Fellowship. We are particularly grateful to Florence
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Hollway (Unilever Research, Port Sunlight Laboratory) for insightful discussions about lubrication and for performing the tribological measurements. LA980641W
