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Langmuir 1996, 12, 1511-1519
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Effect of Fluorine on the Bonding and Orientation of Perfluoroalkyl Ethers on the Cu(111) Surface Jerry M. Meyers and Shane C. Street Department of Chemistry, University of Illinois, Urbana, Illinois 61801
Scott Thompson and Andrew J. Gellman* Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received June 8, 1995. In Final Form: December 4, 1995X By using small fluorinated ethers as models for perfluoropolyalkyl ether lubricants, we have been able to determine the effect of fluorine on the bonding and orientation of the alkylethers adsorbed on the Cu(111) surface. The desorption energies have been determined by using temperature-programmed desorption and the orientations by Fourier transform infrared absorption reflection spectroscopy (FTIRAS).
The model compounds studied were dioxolane (CH2OCH2OCH2) and perfluorodioxolane
(CF2OCF2OCF2), diethyl ether ((CH3CH2)2O) and perfluorodiethyl ether ((CF3CF2)2O), dimethoxymethane ((CH3O)2CH2) and perfluorodimethoxymethane ((CF3O)2CF2), dimethyl ether ((CH3)2O) and perfluorodimethyl ether ((CF3)2O). All of the molecules studied adsorb molecularly and reversibly on the Cu(111) surface exhibiting first-order desorption kinetics. Upon fluorination of the alkyl ethers, the adsorbatemetal bond was weakened by 5-7 kcal/mol. The hydrogenated ethers exhibit repulsive adsorbate-adsorbate interactions, while the fluorinated ethers have attractive adsorbate-adsorbate interactions. At saturation coverages, the FT-IRAS spectra show that the diethyl ethers are oriented with their molecular axes parallel to the Cu(111) surface. 1,3-Dioxolane was oriented with its ring plane parallel to the metal surface.
1.0 Introduction Perfluoropolyalkyl ethers (PFPAEs) are an important class of lubricants in the computer and aerospace industry. Some of their physical properties that make them important are excellent thermal stability, excellent oxidative stability, wide liquid ranges, low volatility, and small temperature dependency of their viscosity.1-3 In the computer industry, they are used to lubricate the headdisk interface of hard drive systems.4 PFPAEs have also been considered for use by the Air Force as gas turbine engine oils.2 The four commercially available PFPAEs are Fomblin Z (CF3O(CF2CF2O)x(CF2O)yCF3) where the ratio of x/y is ∼2/3, Fomblin Y (CF3O(CF(CF3)CF2O)x(CF2O)yCF3) where the ratio of x/y is ∼40/1, Demnum (CF3CF2CF2O(CF2CF2CF2O)xCF2CF3), and Krytox (CF3CF2CF2O(CF(CF3)CF2O)xCF2CF3). However, one problem with these lubricants is that they decompose in the presence of metals and metal oxides at temperatures above 450 K.1,5,6 To solve this problem, extensive effort has been expended to find additives for the PFPAE base oils which stabilize them at elevated temperatures. The best candidates for such additives are specially designed PFPAEs.2 Considering the importance of the established and potential applications for this class of lubricants, it is no surprise that a number of studies have investigated the interaction of PFPAEs with metal surfaces. The studies in the literature can be divided into two groups: those * Author to whom correspondence should be sent. X Abstract published in Advance ACS Abstracts, March 1, 1996. (1) Zehe, M. J.; Faut, O. D. Trib. Trans. 1990, 33, 634. (2) Snyder, C. E., Jr.; Gschwender, L. J.; Tamborski, C. Lub. Eng. 1981, 37, 344. (3) Vurens, G. H.; Mate, C. M. App. Surf. Sci. 1992, 59, 281. (4) Bhushan, B. Tribology and Mechanics of Magnetic Storage Devices; Springer-Verlag: New York, 1990. (5) Herrera-Fierro, P.; Jones, W. R., Jr.; Pepper, S. V. J. Vac. Sci. Technol. A 1993, 11 (2), 354. (6) Carre, D. J. ASLE Trans. 1985, 29 (2), 121.
that involve actual PFPAEs1-7 and those using small model compounds which are used to simulate the surface chemistry of parts of the larger PFPAE chain.8-15 Kasai and co-workers7 have investigated the degradation of these perfluoropolyethers in the presence of Al2O3 by 19F NMR, XPS, and mass spectrometry. They concluded that the degradation process occurs in two stages. In the first stage, which proceeds slowly, the acidic Al2O3 surface attacks the perfluoropolyether forming F2CdO which reacts with the Al2O3 surface to form AlF3 and CO2. The AlF3 is highly Lewis acidic and reacts extremely rapidly with the PFPAE. In the second and faster stage of the process, an acidic Al+ site on the AlF3 surface forms a bidentate linkage with the two oxygen atoms in the acetal linkage (OCF2O) of the perfluoropolyether. This induces a partial positive charge on the carbon atom of the acetal linkage, which then causes a fluorine atom from an adjacent CF2 to transfer to the acetal carbon. This fluorine transfer results in the scission of the perfluoropolyether backbone into a chain terminated by fluoromethoxy (OCF3) and a chain terminated by either a fluoroformate (OCOF) or an acylfluoride (COF). Thiel and co-workers have investigated the surface chemistry of several hydrogenated and fluorinated alkyl ethers on clean and oxidized Ni(100)8 and clean (7) Kasai, P. H.; Tang, W. T.; Wheeler, P. App. Surf. Sci. 1991, 51, 201. (8) Jenks, C. J.; Joyce, J. A.; Thiel, P. A. J. Vac. Sci. Technol. A 1994, 12 (4), 2101. (9) Walczak, M. M.; Thiel, P. A. Surf. Sci. 1989, 224, 425. (10) Napier, M. E.; Stair, P. C. J. Vac. Sci. Technol. A 1992, 10 (4), 2704. (11) DeKoven, B. M.; Meyers, G. F. J. Vac. Sci. Technol. A 1991, 9 (4), 2570. (12) Napier, M. E.; Stair, P. C. J. Vac. Sci. Technol. A 1991, 9 (3), 649. (13) Napier, M. E.; Stair, P. C. Surf. Sci. 1993, 298, 201. (14) Walczak, M. M.; Leavitt, P. K.; Thiel, P. A. J. Am. Chem. Soc. 1987, 109, 5621. (15) Walczak, M. M.; Leavitt, P. K.; Thiel, P. A. Trib. Trans. 1990, 33, 557.
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Ru(001)9,14,15 surfaces. On the Ni(100) surfaces, they used temperature-programmed desorption (TPD) and HREELS to determine that perfluorodiethoxymethane, (CF3CF2O)2CF2 (PFDEM), does not decompose on the clean or oxidized metal surface. Desorption of the PFDEM monolayer occurs at 172 K on the clean surface and at 187 K on the oxidized surface. This study is important because PFDEM contains the acetal linkage (OCF2O) at which decomposition of the PFPAEs is thought to occur.7 Thiel also studied the effect of fluorination on the bonding of alkyl ethers on the Ru(001) surface. The desorption energies of diethoxymethane ((CH3CH2O)2CH2), 1,2-diethoxyethane (CH3CH2OCH2CH2OCH2CH3), and perfluoro-1,2-diethoxyethane (CF3CF2OCF2CF2OCF2CF3) were determined by TPD measurements. The chemisorption bond strengths of diethoxymethane and 1,2-diethoxyethane were 13-16.5 and 14-15 kcal/mol, respectively. For both of these molecules, approximately 0.05 monolayers were found to decompose on the Ru(001) surface. Upon fluorination of 1,2-diethoxyethane, the chemisorption bond strength decreased to 10.5-11 kcal/mol and no decomposition was detected. Stair and Napier have investigated the surface chemistry of three model perfluorinated ethers on clean, polycrystalline iron surfaces10,12,13 using TPD and XPS. The perfluorinated ethers studied were perfluoro-1-methoxy-2-ethoxyethane, CF3OCF2CF2OCF2CF3 (mPFAE1), perfluoro-1-methoxy-2-ethoxypropane, CF3OCF2CF(CF3)OCF2CF3 (mPFAE2), and perfluoro1,3-diethoxypropane, CF3CF2OCF2CF2CF2OCF2CF3 (mPFAE3). The predominant reaction pathway for these molecules on the clean polycrystalline iron surface was molecular desorption; however, at high exposures decomposition was observed. For mPFAE1 and mPFAE2, decomposition occurred at 140 K while decomposition occurred at 150 K for mPFAE3. From the XPS spectra, it was determined that the composition mechanism involved C-F bond scission of the CF3O endgroup of mPFAE1 and mPFAE2 and of the CF3 or CF2O group of mPFAE3. Stair and Napier have also investigated the surface chemistry of mPFAE1 on an oxidized polycrystalline iron and a clean polycrystalline gold surface.12 By using XPS and secondary ion mass spectrometry (SIMS), they determined that mPFAE1 was unreactive on both of these surfaces. In our study, we have used model perfluorinated ethers to determine the effect of fluorination on the bonding and orientation of the alkyl ethers adsorbed on the Cu(111) surface. The desorption energies have been determined by using temperature-programmed desorption and the orientations by Fourier transform infrared absorption reflection spectroscopy (FT-IRAS). All of the molecules studied adsorb and desorb molecularly on the Cu(111) surface without decomposing. The most significant result of this work is the observation that upon fluorination of the alkyl ethers the adsorbate-metal bond was weakened by 5-7 kcal/mol. Equally importantly, we have found that the surface interaction parameters for all the hydrogenated ethers indicate repulsive adsorbateadsorbate interactions. In direct contrast, the fluorinated ethers were found to have attractive adsorbate-adsorbate interactions. At saturation coverages, the FT-IRAS spectra show that the linear ethers are oriented with their molecular axes parallel to the Cu(111) surface. 1,3Dioxolane was oriented with its ring plane parallel to the metal surface. 2.0 Experimental Section The work described in this paper has been performed in two stainless steel ultrahigh vacuum chambers. In the first chamber,
Meyers et al. a base pressure of 1.0 × 10-10 was achieved by an ion pump and titanium sublimation pump. The first chamber is equipped with an Extrel quadrupole mass spectrometer which was used for the thermal desorption measurements. In addition, the chamber is equipped with optics for low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It also contains an Ar+ ion sputtering gun and leak valves for the adsorption of gases onto the crystal surface. The second UHV chamber is equipped with FT-IRAS, AES, TPD, and argon ion sputtering and is fully described elsewhere.16 A base pressure of ν(CH)ax.25 Figure 9 shows the spectra of the dioxolane multilayer and monolayer on the Cu(111) surface. Many modes are labeled with their assignments, and complete assignments are given in Table 4. Assignments are made on the basis of literature references,24,26 where it must be noted that the identification of ring modes in ref 26 was based on the symmetry class assumptions for a planar ring system. The spectra indicate that dioxolane in the monolayer lies with the ring “plane” essentially parallel to the surface. The plane defined by the O-C-C-O atoms is parallel to the surface, with the methylenic group tilted away from (25) Caillod, J.; Saur, O.; Lavalley, J.-C. Spectrochim. Acta 1979, 36A, 185. (26) Barker, S. A.; Bourne, E. J.; Pinkard, R. M.; Whiffen, D. H. J. Chem. Soc. 1959, 59, 802.
the surface in its puckered conformation. In the C-H stretch region this is made apparent by the presence in the monolayer spectrum of the ν(CH)ax mode at 2865 cm-1. That the ethylenic (CH2CH2) C-C bond is roughly parallel to the surface is shown in the monolayer spectrum by the presence of the coincident νa(CH2)-C2 modes at 2950 cm-1, which have transition dipole moments vectors perpendicular to the plane of the molecule. The methylenic ν(CH)eq mode should also be in this region and can not be explicitly identified in our spectra. The ethylenic νs(CH2)C2 modes are absent in the monolayer spectrum (they are present in the multilayer spectrum at 2900 cm-1). In the lower frequency region are the ring stretching modes and the symmetric ethylenic rock mode at 921 cm-1. The ring modes are assigned in ref 26, assuming a planar geometry of the ring system so that the infrared activities of the modes are given lying parallel to certain molecular axes; the z-axis is the axis of two-fold symmetry bisecting the O-C-O bond angle, the x-axis is out of the plane of
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the flat ring, and the y-axis is in the plane but orthogonal to the z-axis. These molecular axes are useful in analyzing
the ring stretch modes even where the puckering of the ring has removed the perfect symmetry of the ring plane. The absorptions in the monolayer spectrum have infrared activities (i.e. transition dipole moment vectors) out of the plane either along the x-axis as is the case for the symmetric ethylenic CH2 rock mode or along the z-axis which has been altered by the pseudorotation of the methylenic group. Thus the ring mode at ∼1090 cm-1, with its moment along the z-axis, is strong in both the multilayer and the monolayer, but the orthogonal ring mode (along the y-axis) at 1028 cm-1 in the multilayer is not present in the monolayer. Another way to describe the character of these two ring modes is as νs(“OCO”) (zaxis) which has a transition dipole moment with some projection along the surface normal and as νa(“OCO”) (yaxis) whose associated transition dipole moment vector lies parallel to the surface. There does not appear to be any strong spectroscopic evidence for a truly disordered adsorption geometry of dioxolane at saturation coverage as has been postulated for the cyclic ether trioxane on the same surface.27 Perfluoro-1,3-dioxolane. Ab initio calculations for the molecular configuration of perfluoro-1,3-dioxolane found the unexpected result that the ring is planar.28 Infrared spectroscopy and tentative assignments allow us to identify the absorptions shown in Figure 10. The figure presents spectra of perfluorodioxolane at multilayer, saturated monolayer, and low submonolayer coverages on the Cu(111) surface. Two ring stretching modes are identified in the multilayer spectrum at 962 and 1047 cm-1. The strong absorptions at 1235 and 1293 cm-1 in (27) Hofmann, M.; Wagner, H.; Glenz, A.; Woll, C.; Grunze, M. J. Vac. Sci. Technol. A 1994, 12 (4), 2063. (28) Liang, J. University of Dayton Research Institute, private communication.
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the multilayer are associated with the νs(CF2) and νa(CF2) modes, respectively. These identifications are not specific enough to determine the orientation of adsorbed perfluorodioxolane, which appears to be complex but not random. The absorption associated with νa(CF2) shows a preferential loss of intensity to its high-frequency shoulder upon formation of the monolayer and at lower submonolayer coverages. The monolayer spectrum has the ring stretching mode at 1047 cm-1 but not the lower frequency ring mode found in the multilayer at 962 cm-1. The exact nature of the infrared activities of these ring modes is not clear, except that the associated moments lie in the plane of the ring. The presence of one ring mode in the spectrum of the monolayer argues for an orientation with the ring plane tilted with respect to the surface. The monolayer and submonolayer spectra are similar in their spectral features. There is a relative decrease in the intensity of the νa(CF2)-associated mode as the coverage decreases, but without an accurate characterization of this mode it is not possible to determine the orientation change, if any, indicated by the relative loss. 5.0 Conclusions By using small fluorinated ethers as models for perfluoropolyalkyl ether (PFPAE) lubricants, we have been able to determine the effect of fluorine on the bonding and orientation of the alkyl ethers adsorbed on the Cu(111) surface. All of the ethers studied adsorb molecularly and reversibly on the Cu(111) surface, exhibiting first-order desorption kinetics. Upon fluorination of the alkyl ethers, the adsorbate-metal bond was weakened by 5-7 kcal/ mol. Of particular interest is the effect of fluorination on the interactions between the adsorbed ethers. The interaction parameters (R) for the hydrogenated ethers were found to have positive values, indicating repulsive adsorbate-adsorbate interactions, while the fluorinated ethers were found to have negative values of R, indicating attractive adsorbate-adsorbate interactions. At saturation coverages, the FT-IRAS spectra show that the diethyl ethers are oriented with their molecular axes parallel to the Cu(111) surface. 1,3-Dioxolane was oriented with its ring plane parallel to the metal surface. Acknowledgment. This work was supported by the AFOSR under Grant No. AFOSR-F49620-93-1-02-41-URI. LA950451A
