Surface Characterization of Functionalized Imidazolium-Based Ionic

Aug 2, 2008 - The surface composition of oligo(ethylene glycol) ether functionalized bis(trifluoromethylsulfonyl)imide ionic liquids has been studied ...
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Langmuir 2008, 24, 9500-9507

Surface Characterization of Functionalized Imidazolium-Based Ionic Liquids Claudia Kolbeck,† Manuela Killian,† Florian Maier,*,† Natalia Paape,‡ Peter Wasserscheid,‡ and Hans-Peter Steinru¨ck† Lehrstuhl fu¨r Physikalische Chemie II, Department Chemie and Pharmazie, and Lehrstuhl fu¨r Chemische Reaktionstechnik, Department Chemie- and Bioingenieurswesen, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ReceiVed April 22, 2008. ReVised Manuscript ReceiVed June 3, 2008 The surface composition of oligo(ethylene glycol) ether functionalized bis(trifluoromethylsulfonyl)imide ionic liquids has been studied by means of X-ray photoelectron spectroscopy (XPS). For [Me(EG)MIM][Tf2N], [Et(EG)2MIM][Tf2N], and [Me(EG)3MIM][Tf2N], which vary by the number of ethylene glycol (EG) units (from 1 to 3), we have shown that the stoichiometry of the surface near region is in excellent agreement with the bulk stoichiometry, which confirms the high purity of the ionic liquid samples investigated and rules out pronounced surface orientation effects. This has been deduced from the experimental observation that the angle-resolved XP spectra of all elements present in the IL anions and cations (C, N, O, F, S) show identical signals in the bulk and surfaces sensitive geometry, i.e., at 0° and 70° emission angle, respectively. The relative intensity ratios of all elements were found to be in nearly perfect agreement with the nominal values for the individual ILs. In contrast to these findings, we identified surfaceactive impurities in [Me(EG)MIM]I, which is the starting material for the final anion exchange step to synthesize [Me(EG)MIM][Tf2N]. Sputtering of the surface led to a depletion of this layer, which however recovered with time. The buildup of this contamination is attributed to a surface enrichment of a minor bulk contamination that shows surface activity in the iodide melt.

1. Introduction Ionic liquids (ILs)ssalts with melting points below 100 °Cshave found increasing scientific interest over the past decade.1 Attracted by their structural diversity and their unique profiles of physicochemical properties (e.g., extremely low volatility,2,3 unusual solvation and miscibility properties,4 electroconductivity5), many research groups have studied applications of these materials in catalysis,6 electrochemistry,5 analytics,7 and as ″engineering fluids″ (e.g., separation technologies8). Several industrial applications have been recently reported.9 Introducing functional groups within the IL molecules, and, thus, tuning their properties over a wide range, ionic liquids can be adapted to specific applications, a concept that is known as ″task specific ionic liquids″ (TSILs).1,10 Among the TSILs, ionic liquids with ether functionalities are of particular interest. The motivation for applying oligoether substituents (PEG-substituents) stems from * Corresponding author: Florian Maier, Lehrstuhl fuer Physikalische Chemie II, Egerlandstr. 3, D-91058 Erlangen, Germany. E-mail: florian.maier@ chemie.uni-erlangen.de. † Lehrstuhl fu¨r Physikalische Chemie II, Department Chemie and Pharmazie. ‡ Lehrstuhl fu¨r Chemische Reaktionstechnik, Department Chemie- and Bioingenieurswesen. (1) Ionic liquids in synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, 2008. (2) Earle, M. J.; Esperanca, J.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831–834. (3) Wasserscheid, P. Nature 2006, 439, 797–797. (4) Swatloski, R. P.; Rogers, R. D.; Holbrey; J. D. WO 03/029329, 2003. (5) Endres, F.; El Abedin, S. Z. Phys. Chem. Chem. Phys. 2006, 8, 2101–2116. (6) Welton, T. Coord. Chem. ReV. 2004, 248, 2459–2477. (7) Anderson, J. L.; Armstrong, D. W.; Wie, G. T. Anal. Chem. 2006, 78, 2892–2902. (8) Jork, C.; Kristen, C.; Pieraccini, D.; Stark, A.; Chiappe, C.; Beste, Y. A.; Arlt, W. J. Chem. Thermodyn. 2005, 37, 537–558. (9) Maase, M. In Multiphase homogeneous catalysis; Cornils, B., Herrmann, W. A., Horvath, I. T., Leitner, W., Mecking, S., Olivier-Bourbigou, H., Vogt, D., Eds.; Wiley-VCH: Weinheim, 2005; pp 560-566. (10) Visser, A. E.; Swatlowski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H.; Rogers, R. D. Chem. Commun. 2001, 135–136.

the fact that the replacement of alkyl groups by oligoether groups has been shown to decrease the ionic liquid’s viscosity significantly. This effect has been demonstrated both for substituents at the anion (e.g., for PEG-sulfate ions11) and for PEG-substituents at the cation (e.g., for PEG-functionalized imidazolium dialkylphosphates12). Moreover, ether functionalities in ionic liquids possibly interact with metal ions in IL solutions yielding an increase of metal ion solubility and of metal complex stability. While ″bulk″ properties (e.g., liquid range, polarity, coordination behavior, viscosity, density, heat capacity) of common ILs have been studied to some extent, the investigation of their surface and interface properties just started a few years ago.13–16 However, surfaces and interfaces are pivotal for many applications of ILs. For example, the nature of the interface IL/gas is extremely relevant for catalytic reactions involving gaseous reactants in contact with bulk ILs or IL films supported on surfaces (a concept known as ″supported ionic liquid phase (SILP)″ catalysis17–19). One of the surface-sensitive techniques employed is X-ray photoelectron spectroscopy (XPS). XPS, also denoted as electron spectroscopy for chemical analysis (ESCA), is a well-established method to analyze the chemical composition of the near-surface region. Due to the (11) Himmler, S.; Ho¨rmann, S.; van Hal, R.; Schulz, P. S.; Wasserscheid, P. Green Chem. 2006, 8, 887–894. (12) Kuhlmann, E.; Himmler, S.; Giebelhaus, H.; Wasserscheid, P. Green Chem. 2007, 9, 233–241. (13) Gannon, T. J.; Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R. Langmuir 1999, 15, 8429–8434. (14) Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R.; Seddon, B. Phys. Chem. Chem. Phys. 2001, 3, 2879–2885. (15) Hardacre, C. Annu. ReV. Mater. Res. 2005, 35, 29–49. (16) Aliaga, C.; Santos, C. S.; Baldelli, S. Phys. Chem. Chem. Phys. 2007, 9, 3683–3700. (17) Riisager, A.; Fehrmann, R.; Haumann, M.; Gorle, B. S. K.; Wasserscheid, P. Ind. Eng. Chem. Res. 2005, 44, 9853–9859. (18) Riisager, A.; Fehrmann, R.; Flicker, S.; van Hal, R.; Haumann, M.; Wasserscheid, P. Angew. Chem., Int. Ed. 2005, 44, 815–819. (19) Riisager, A.; Jorgensen, B.; Wasserscheid, P.; Fehrmann, R. Chem. Commun. 2006, 994–996.

10.1021/la801261h CCC: $40.75  2008 American Chemical Society Published on Web 08/02/2008

Functionalized Imidazolium-Based Ionic Liquids

sensitivity of the core level binding energies to the local chemical environment, one can also determine the chemical state (e.g., oxidation state) of a particular atom. Since the inelastic mean free path of photoelectrons is only in the order of a few to 10 atomic layers (depending on kinetic energy and material), this technique is inherently surface sensitive and can be employed for measuring composition depth profiles of the surface near region.20 XPS is usually performed under ultrahigh vacuum (UHV) conditions with pressures below 10-6 mbar, which makes investigations on liquid surfaces difficult, due to the unavoidable vapor pressure of conventional liquids. Because of their low vapor pressure of typically lower than 10-9 mbar, this restriction does not hold for ionic liquids. Starting from 2005, a number of XPS studies under different experimental conditions on neat ionic liquids, ionic liquid mixtures, and ionic liquid solutions demonstrated that XPS is a very powerful tool in this field. Various ionic liquids were investigated: In most cases, they contained an imidazoliumbased cation ([BMIM], [EMIM], [OMIM], [HMIM]), while for the anions [PF6]-, [BF4]-, [Tf2N]-, [EtOSO3]-, Cl- or Br- were used.21–33 In some of the studies, XPS was combined with other surface science techniques such as low energy ion scattering (LEIS),23 UV photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES), high-resolution electron energy loss spectroscopy (HREELS),25,29 and secondary ion mass spectroscopy (SIMS).26 In a very recent, systematic AR-XPS study, evidence for orientation of alkyl chains away from surface toward the vacuum was reported.31 Furthermore, in particular also through our own work, it was demonstrated that XPS is particularly well suited to accurately quantify the composition of the IL in the surface near region and to identify surface contaminations21,26,33 or the surface enrichment of dissolved species.32 Furthermore, it has been reported that the surfaces of ionic liquids can be cleaned by in situ sputtering with inert gas ions26 as it is well-known for solid-state materials. However, we are not aware of systematic studies of IL cleaning using sputter techniques. The present study addresses the surface composition of functionalized ionic liquids by X-ray photoelectron spectroscopy with the following major topics. (1) Systematic characterization of three functionalized ionic liquids carrying ether groups in the (20) Seah, M. P.; Briggs, D. Practical Surface Analysis, 2nd ed.; John Wiley & Sons: Chichester, 1990; Vol. 1, Auger and X-ray Photoelectron Spectroscopy. (21) Smith, E. F.; Villar-Garcia, I. J.; Briggs, D.; Licence, P. Chem. Commun. 2005, 5633–5635. (22) Fortunato, R.; Afonso, C. A. M.; Benavente, J.; Rodriguez-Castellon, E.; Crespo, J. G. J. J. Membr. Sci. 2005, 256, 216–223. (23) Caporali, S.; Bardi, U.; Lavacchi, A. J. Electron Spectrosc. Relat. Phenom. 2005, 151, 4–8. (24) Seyama, M.; Iwasaki, Y.; Tate, A.; Sugimoto, I. Chem. Mater. 2006, 18, 2656–2662. (25) Ho¨fft, O.; Bahr, S.; Himmerlich, M.; Krischok, S.; Schaefer, J. A.; Kempter, V. Langmuir 2006, 22, 7120–7123. (26) Smith, E. F.; Rutten, F. J. M.; Villar-Garcia, I. J.; Briggs, D.; Licence, P. Langmuir 2006, 22, 9386–9392. (27) Kwon, J. H.; Youn, S. W.; Kang, Y.-C. Bull. Korean Chem. Soc. 2006, 27, 1852–1853. (28) Silvester, D. S.; Broder, T. L.; Aldous, L.; Hardacre, C.; Crossley, A.; Compton, R. G. Analyst 2007, 196–198. (29) Krischok, S.; Eremtchenko, M.; Himmerlich, M.; Lorenz, P.; Uhlig, J.; ¨ ttking, R.; Beenken, W. J. D.; Ho¨fft, O.; Bahr, S.; Kempter, V.; Neumann, A.; O Schaefer, J. A. J. Phys. Chem. B 2007, 111, 4801–4806. (30) Lovelock, K. R. J.; Smith, E. F.; Deyko, A.; Villar-Garcia, I. J.; Licence, P.; Jones, R. G. Chem. Commun. 2007, 4866–4868. (31) Lockett, V.; Sedev, R.; Bassell, C.; Ralston, J. Phys. Chem. Chem. Phys. 2008, 10, 1330–1335. (32) Maier, F.; Gottfried, J. M.; Rossa, J.; Gerhard, D.; Schulz, P. S.; Schwieger, W.; Wasserscheid, P.; Steinru¨ck, H.-P. Angew. Chem., Int. Ed. 2006, 45, 7778– 7780. (33) Gottfried, J. M.; Maier, F.; Rossa, J.; Gerhard, D.; Schulz, P. S.; Wasserscheid, P.; Steinru¨ck, H.-P. Z. Phys. Chem. 2006, 220, 1439–1453.

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imidazolium-based cation, namely, [R-(EG)xMIM][Tf2N], by varying the number x of ethylene glycol (EG) groups from 1 to 3, where R represents either a methyl or an ethyl group. For each of these three ILs (see Table 1), bulk and surface composition are identical with only minor surface contaminations in some cases. (2) Investigation of [Me(EG)MIM]I as one of the educts in the synthesis of [Me(EG)MIM][Tf2N]; the corresponding data showed major contaminations. (3) Measurements to identify the bulk or surface nature of these contaminations. In addition, we also present data for the neat IL [EMIM][Tf2N], which served as a reference system for the quantitative analysis of the functionalized ILs.

2. Experimental Section All ILs investigated have been prepared and characterized in our group. NMR spectra were performed on a JEOL ECX 400 MHz spectrometer, in d6-DMSO. Mass spectrometric analysis was performed on a MALDI CFRplus (AXIMA, Kratos Analytical, U.K.), equipped with a nitrogen laser (337 nm, 3 ns pulse, maximum pulse rate 10 Hz) at 1.9 µJ. In the following, the successive steps of the synthesis are described. 2.1. Synthesis of PEG-Functionalized Bis(trifluoromethylsulfonyl)imide Ionic Liquids. 1-Methylimidazole (>99%) and iodomethane (>99%) were purchased from Fluka and were distilled prior to use. Li[Tf2N] was obtained from Merck-Solvent Innovation GmbH, Cologne. Step 1: Synthesis of PEG-Imidazoles: A solution of 2.5 equiv of sodium hydroxide, dissolved in the same amount of ice, 120 mg of hexadecyltrimethylammonium hydrogen sulfate, and 1.0 equiv of an oligo(ethylene glycol) monoalkylether of the general formula R-EGx-OH (x ) 1-3, EG ) -O-CH2-CH2-) was prepared. A 1.1 equiv portion of benzenesulfonyl chloride was added dropwise at 70 °C. The reaction mixture was refluxed for 3 h at 70 °C. The precipitate was removed by filtration, and the aqueous solution was extracted two times with dichloromethane. The combined organic phases were washed once with distilled water, concentrated to a small volume, and dried under reduced pressure to yield the PEG-benzenesulfonate. A solution of 3.0 equiv of sodium hydroxide, dissolved in the same amount of ice, 120 mg of hexadecyltrimethylammonium hydrogen sulfate, and 1.0 equiv of 1H-imidazole (1-H-IM) was prepared. A 1.1 equiv portion of PEG-benzenesulfonate was added dropwise at 70 °C. The reaction mixture was stirred overnight at room temperature and then for 2 h at 70 °C. Water was added to the reaction mixture until the precipitate was dissolved. The aqueous solution was extracted five times with dichloromethane, and the combined organic phases were concentrated to a small volume and distilled under reduced pressure to yield the final product. The yields of PEG imidazoles are MeEGIM ) 87%, Et(EG)2IM ) 76%, and Me(EG)3IM ) 88%. Step 2: Synthesis of PEG-Functionalized Iodide Ionic Liquids: Iodomethane (1.0 equiv) was slowly added to 1.0 equiv of PEGimidazole that was precooled to 0 °C by an external cooling bath. After 2 h of stirring under argon atmosphere, the mixture was heated up to 50 °C and was stirred under argon for another 3 h to complete reaction. The product was obtained in quantitative yield in all cases in the form of a yellow liquid. IL4 [Me(EG)MIM]I. δH (400 MHz, d6-DMSO, Me4Si) 9.20 (s, 1H, NCHN); 7.76 (s, 1H, NCHCHN); 7.72 (s, 1H, NCHCHN); 4.34 (t, 2H, 3JH-H ) 4.94 Hz, NCH2); 3.85 (s, 3H, NCH3); 3.65 (t, 2H, 3J H-H ) 4.94 Hz, CH2O); 3.21 (s, 3H, OCH3) ppm. δC (100 MHz, d6-DMSO, Me4Si) 137.3 (NCHN); 123.9 (NCHCHN); 123.1 (NCHCHN); 70.09 (CH2O); 58.66 (OCH3); 49.17 (NCH2CH2O); 36.47 (NCH3) ppm. M (cation), 141; M (anion), 127. Step 3: Synthesis of PEG-Functionalized Bis(trifluoromethylsulfonyl)imide Ionic Liquids: At room temperature, a 30% solution of the PEG-imidazolium iodide (1.0 equiv) in water was added to a 30% solution of Li[Tf2N] in water (1.0 equiv). The resulting aqueous

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Table 1. Molecular Structures and Physical Properties of the Ionic Liquids Investigated

* Molecular weight. ** Mass density Fmass at room temperature taken from ref 35. *** Mass density Fmass of IL1-IL3 calculated from mass measurements at room temperature of 10-20 µL of IL. # Molar density Fmol calculated from M and Fmass. ## Molar density Fmol,XPS calculated from XPS intensities of corresponding N 1s, F 1s, and S 2p signals of IL1-IL3 relative to those of IL0 and the value of the molar density of IL0. ### Molar density Fmol,XPS calculated from XPS intensity of the N 1s 0° spectrum after extended sputtering (spectrum V of Figure 4a).

solution was extracted five times with the same volume of dichloromethane. After combining the organic phases and removal of the volatile solvent, the PEG-functionalized bis(trifluoromethylsulfonyl)imide ionic liquids were obtained as almost colorless liquids. The total yields of [PEG-MIM][Tf2N] (with respect to starting material 1-H-IM) are [Me(EG)MIM][Tf2N] 78%, [Et(EG)2MIM][Tf2N] 72%, and [Me(EG)3MIM][Tf2N] 74%. IL1 [Me(EG)MIM][Tf2N]. δH (400 MHz, d6-DMSO, Me4Si) 9.05 (s, 1H, NCHN); 7.67 (s, 1H, CH3NCHCHN); 7.62 (s, 1H, CH3NCHCHN); 4.32 (t, 2H, 3JH-H ) 4.94 Hz, NCH2CH2O); 3.84 (s, 3H, NCH3); 3.65 (t, 2H, 3JH-H ) 4.94 Hz, NCH2CH2O); 3,24 (s, 3H, OCH3) ppm. δC (100 MHz, d6-DMSO, Me4Si) 137.3 (NCHN); 123.9 (CH3NCHCHN); 123.1 (CH3NCHCHN); 120.0 (CF3); 70.05 (NCH2CH2O); 58.43 (OCH3); 49.20 (NCH2CH2O); 36.13 (NCH3) ppm. M (cation), 141; M (anion), 280. IL2 [Et(EG)2MIM][Tf2N]. δH (400 MHz, d6-DMSO, Me4Si) 9.02 (s, 1H, NCHN); 7.67 (s, 1H, CH3NCHCHN); 7.62 (s, 1H, CH3NCHCHN); 4.32 (t, 2H, 3JH-H ) 4.94 Hz, NCH2CH2O); 3.84 (s, 3H, NCH3); 3.75 (t, 2H, 3JH-H ) 4.94 Hz, NCH2CH2O); 3.52 (t, 2H, 3J 3 H-H ) 7.00 Hz, OCH2CH2OCH2CH3); 3.45 (t, 2H, JH-H ) 7.00 3 Hz, OCH2CH2OCH2CH3); 3.37 (q, 2H, JH-H ) 7.00 Hz, OCH2CH3); 1.06 (t, 3H, 3JH-H ) 7.00 Hz, OCH2CH3). δC (100 MHz, d6-DMSO, Me4Si) 137.3 (NCHN); 124.1 (s, CH3NCHCHN); 123.1 (CH3NCHCHN); 120.0 (CF3); 66.03-70.19 (CH2O); 49.40 (NCH2CH2O); 36.19 (NCH3); 15,41 (CH2CH3) ppm. M (cation), 199; M (anion), 280. IL3 [Me(EG)3MIM][Tf2N]. δH (400 MHz, d6-DMSO, Me4Si) 9.06 (s, 1H, NCHN); 7.72 (s, 1H, NCHCHN); 7.69 (s, 1H, NCHCHN); 4.32 (t, 2H, 3JH-H ) 4.94 Hz, NCH2); 3.84 (s, 3H, NCH3); 3.74 (t,

2H, 3JH-H ) 4.94 Hz, CH2O); 3.35-3.55 (m, 8H, OCH2CH2); 3.19 (s, 3H, OCH3) ppm. δC (100 MHz, d6-DMSO, Me4Si) 137.3 (NCHN); 123.7 (NCHCHN); 123.3 (NCHCHN); 68.63-71.78 (CH2O); 58.64 (OCH3); 49.27 (NCH2CH2O); 36.36 (NCH3) ppm. M (cation), 229; M (anion), 280. 2.2. XPS Measurements. The thin IL films were prepared by deposition of the corresponding IL onto a planar Au foil (10 mm × 10 mm × 0.1 mm), which was cleaned by Ar+ ion bombardment and subsequently annealed under UHV conditions. These samples were then introduced in the UHV system via a loadlock, with the exposure to air minimized to approximately 1 min. After at least 6 and up to 12 h of pumping, a pressure of ∼5 × 10-10 mbar was achieved, which hardly exceeds the base pressure in the vacuum system, confirming the very low vapor pressure of the ionic liquid and absence of volatile impurities. The XPS measurements were performed with a VG ESCALAB 200 system using Al KR radiation (hν ) 1486.6 eV); the experimental data shown below were recorded with a pass energy of 20 eV, yielding an overall energy resolution of 0.9 eV. The Au 4f7/2 signal (EB ) 83.55 eV) was used as a reference for the reported binding energies. To vary the surface sensitivity of the measurements, spectra were collected under ϑ ) 0° (normal emission) and for ϑ ) 70° (grazing emission). Due to the small acceptance angle of (4° of the electron analyzer, the probe depth varies mainly with cos(ϑ). Considering the inelastic mean free path of ∼3 nm of photoelectrons in organic compounds34 (34) Roberts, R. F.; Allara, D. L.; Pryde, C. A.; Buchanan, D. N. E.; Hobbins, N. D. Surf. Interface Anal. 1980, 2, 5–10. (35) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103–6110.

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at the kinetic energies used (∼800-1300 eV), measurements at 0° probe the surface near region (information depth, ID: 7-9 nm, depending on the kinetic energy) whereas measurements at 70° only probe the topmost layers (ID: 2-3 nm). To correct for reduced overall transmission at 70° detection angle, the corresponding ionic liquid spectra are multiplied by an empirical factor determined by measurements of clean gold foils under identical conditions. Thus, for a homogeneous distribution of the various elements in the investigated sample, identical signals are expected in both geometries. On the other hand, an increase in core level intensity with increasing detection angle and, thus, with increasing surface sensitivity, indicates a higher concentration of this element in the topmost layers as compared to the ″bulk″. This could be due to the surface enrichment of a contamination33 or of one component in a mixture or solution of ILs32 or due to a pronounced preferential orientation of cations or anions in the IL.31

3. Results and Discussion We investigated three functionalized ILs (IL1-3), with different lengths of the functional group, a functionalized IL (IL4) representing the starting material for IL1, and a nonfunctionalized IL (IL0), which served as a reference for the quantitative analysis of the XP spectra. An overview of the physical properties is given in Table 1, which contains a schematic drawing of the molecular composition, the molecular weight (M), the mass density (Fmass) as measured in this study (for IL0, we refer to ref 35), and the molar density (Fmol) as calculated from M and Fmass for each IL. For IL1-IL4, the latter value has to be compared to the molar density calculated from the XP spectra (Fmol,XPS), which is also denoted in Table 1. In the following, we present the XPS results for the different ILs in detail. 3.1. [Me(EG)MIM][Tf2N] (IL1). As a first step, we have investigated the XP spectra of [Me(EG)MIM][Tf2N] (C9H13F6N3O5S2, M ) 421.34 g/mol, denoted as IL1). The corresponding XP spectra for the C 1s, N 1s, O 1s, S 2p, and F 1s regions are shown in panels a-e of Figure 1, respectively. Prior to plotting the spectra, a linear background was subtracted in each case. For all levels the spectra for normal emission (ϑ ) 0°, black) and in grazing emission (ϑ ) 70°, red) show identical intensities, within the margin of error. This indicates a homogeneous distribution of the IL molecules at the surface and in the bulk, within the escape depth of the photoelectrons. For the C 1s and the N 1s spectra, two peaks are observed, which are attributed to nonequivalent C and N atoms, respectively (see below). The asymmetric line shape of the S 2p spectra is due to the spin-orbit splitting of the S 2p1/2 and S 2p3/2 levels, with an intensity ratio of 1:2. In addition to the spectra shown in Figure 1, survey spectra were measured to detect possible contaminations. However, there was no sign of other elements, ruling out surface contaminations such as Si, which were observed for other ILs.33 By analysis of the peak areas and by considering the sensitivity factors for the different elements, quantitative information on the overall stoichiometry of the investigated sample can be derived from the spectra. The simplest approach to account for the atomic sensitivity factors (ASF) is to use the literature values, relative to the F 1s signal, as given in Table 2.36 However, this analysis does not account for the particular transmission function of a specific analyzer at a given pass energy and, in our case, yields deviations of about -10% at low and +15% at high kinetic energies. Therefore, we have chosen a different approach, namely, calibrating the sensitivity factors for the IL constituents using two ultraclean and well-characterized ILs, namely, [EMIM][Tf2N] (denoted as IL0 below) and [EMIM][EtOSO3], as (36) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. H.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211.

Figure 1. XP spectra of IL1 [Me(EG)MIM][Tf2N], recorded under 0° (black) and under 70° (red) electron emission angle, with respect to the surface normal: (a) C 1s, (b) N 1s, (c) O 1s, (d) S 2p, and (e) F 1s region.

reference, which belong to the ILs most studied with XPS in literature.26,29,32,33 Due to their small size compared to the inelastic electron mean free path, possible molecular orientation effects are expected to be of minor importance. The corresponding averaged atomic sensitivity factors (ASF-corr) for the elements of interest are included in Table 2. The results of the quantitative analysis are collected in Table 2, where the numbers of atoms as derived from the various XP peaks are denoted. The values are normalized such that the sum of C, N, O, S, and F atoms yields the sum over the nominal values, e.g., 25 for [Me(EG)MIM][Tf2N], i.e., C9H13F6N3O5S2. Since the spectra for ϑ ) 0° and 70° are identical within the margin of error ((3%), we will only use the average of both geometries (mean value) in the following. Note that the small C 1s component at 284.4 eV for IL1 (see below), which is seen at 70° only, leads to the larger value for carbon in this geometry (7.3 vs 6.9 at 0°), which lies within the error bars. Overall, the agreement between the measured and the nominal stoichiometry is very good. In the following, we will analyze the observed results for the C 1s, N 1s, and O 1s regions in more detail. The C 1s spectra in Figure 2a show two peaks at binding energies of 286.6 and 292.7 eV. They are assigned to the carbon atoms in the cation and the anion, respectively. The assignment of the peak at 292.8 eV to the anion is straightforward, due to the known high electron affinity of the F atoms.26,29 The ratio for Ccation:Canion of 7.1:2.0 is in very good agreement with the expected values of 7.0:2.0. The nonequivalent carbon atoms in the cation, namely, the three aromatic ones in the imidazolium ring and the four carbons of

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Table 2. Quantitative Analysis of the XP Spectra of IL1-IL3a C 1s (cation) N 1s (cation) O 1s (c. + a.) C 1s (anion) N 1s (anion) S 2p (anion) F 1s (anion) position peak max (eV) ASFb) ASF-corrc) IL1 (total, 25) nominal 0° 70° mean

286.6 0.25 0.205 C 7 6.9 7.3 7.1

401.9 0.42 0.350 N 2 2.0 1.9 1.9

532.6 0.66 0.580 O 5 5.1 5.0 5.1

292.7 0.25 0.205 C 2 2.0 1.9 2.0

399.3 0.42 0.380 N 1 1.0 0.9 0.9

168.7 0.54 0.400 S 2 2.0 1.9 1.9

688.8 1.00 1.000 F 6 5.9 6.0 6.0

IL2 (total, 29) nominal 0° 70° mean

10 10.0 10.6 10.3

2 2.0 1.8 1.9

6 6.1 5.9 6.0

2 2.0 2.0 2.0

1 1.0 1.0 1.0

2 2.0 1.9 2.0

6 6.0 5.7 5.8

IL3 (total, 31) nominal 0° 70° mean

11 11.0 11.3 11.2

2 2.1 1.9 2.0

7 7.1 7.2 7.2

2 1.8 1.8 1.8

1 1.0 0.9 1.0

2 2.0 2.1 2.0

6 6.0 5.8 5.9

a In the upper part, the binding energies and the atomic sensitivity factors of the various core levels are given (for details, see text). In the lower part the nominal and the experimentally determined composition in number of atoms is given for the various elements constituting the ILs. The numbers derived from the experimental spectra at 0° and 70° agree within the margin of error. The ″mean″ value presents their average. b Atomic sensitivity factors (ASF) taken from ref 36. c Corrected ASF values (taking the transmission function of electron analyzer into account).

Figure 2. XP spectra of IL0 (green), IL1 (black), IL2 (red), and IL3 (blue) taken under 0° (solid lines) and under 70° (dashed lines) electron emission angle, with respect to the surface normal: (a) C 1s, (b) N 1s, (c) O 1s, (d) S 2p, and (e) F 1s region.

the substituents (see Figure 1a), cannot be resolved with the resolution of our spectrometer. This fact is not too surprising, as the C 1s peaks of comparable ether units, e.g., in poly(ethylene glycol) polymers, are observed at 286.4 eV,37 which also represents the typical binding energy of C atoms in imidazolium cations (see spectrum for IL0). The small additional C 1s peak at 284.4 eV, which is observed only at 70°, is assigned to a minor

contamination at the surface, which is so small that it is seen in the surface sensitive geometry only. The N 1s spectra in Figure 2b show two symmetric peaks at 399.3 and at 401.9 eV. They are assigned to the N atoms in the anion and cation, respectively. The absolute intensity ratio of 0.9:1.9 is in very good agreement with the expected ratio of 1.0:2.0, which also supports the peak assignment. In the O 1s region only one peak is observed. This is surprising, since both, anion and cation, contain oxygen in chemically different environments. The fact that only one peak is resolved indicates that the binding energies of both must be very similar (see below). 3.2. [Et(EG)2MIM][Tf2N] (IL2). To investigate the influence of the length of the functional group, we measured XP spectra of [Et(EG)2MIM][Tf2N] (C12H19F6N3O6S2, M ) 479.72 g/mol, denoted as IL2). This IL contains two ether units (as compared to one for IL1) and an ethyl end group (as compared to the methyl end group for IL1 and also IL3, see below). The C 1s, N 1s, O 1s, S 2p, and F 1s regions of IL2 are shown in Figure 2 (red curves), along with the corresponding spectra for IL1 (black curves) from Figure 1 and also the spectra for [EMIM][Tf2N] (C8H11F6N3O4S2, M ) 391.31 g/mol, denoted as IL0, green curves), which served as a reference for our analysis. Note that all spectra of an individual region are plotted on the same intensity scale, which allow a direct comparison of the observed intensities. In all cases, spectra are shown for normal emission (ϑ ) 0°, solid) and in grazing emission (ϑ ) 70°, dashed). The spectra for IL2 in both geometries are identical within the margin of error, indicating a homogeneous distribution of the molecules at the surface and in the bulk and the absence of surface contaminations. The numbers of atoms as derived from the various peaks are denoted in Table 2. Again, very good agreement with the nominal values is found. Inspection of the C 1s spectra shows two peaks, at 286.6 and 292.7 eV, due to the C atoms in the cation and anion, respectively. The intensity ratio of 10.3:2.0 is very close to the nominal value of 10.0:2.0. The two peaks in the N 1s spectra are again observed at 399.3 and 401.9 eV, with an intensity ratio of 1.0:1.9, which is in excellent agreement with the nominal values for the nitrogen atoms in the cation and anion, respectively. 3.3. [Me(EG)3MIM][Tf2N] (IL3). As third IL, we investigated [Me(EG)3MIM][Tf2N] (C13H21F6N3O7S2, M ) 509.44 g/mol, denoted as IL3). The XP spectra are also included in Figure 2 (blue curves). As for the two other ILs discussed so far,

Functionalized Imidazolium-Based Ionic Liquids

the spectra for normal and grazing emission are identical within the margin of error. The numbers of atoms derived from the peak intensities are included in Table 2, and again, good agreement with the nominal values is found. For the O 1s signal, only one peak is observed, even though the number of oxygen atoms in the cation and in the anion is now similar. As compared to the spectrum for IL1, a small increase in peak width is found that will be discussed in the following. 3.4. Comparison [Me(EG)MIM][Tf2N], [Et(EG)2MIM][Tf2N], and [Me(EG)3MIM][Tf2N] The spectra in Figure 2 for IL1 to IL3 together with the reference IL0 are plotted on the same intensity scale, which allows a quantitative comparison of the data. The analysis of the binding energies of the peaks in the C 1s, N 1s, S 2p, and F 1s spectra show that they are identical with (0.1 eV for all four ILs studied. Only the O 1s peak of IL0 has a somewhat smaller full width at half-maximum than those found for IL1-IL3 and is shifted to a lower binding energy by ∼0.2 eV. We attribute this to the fact that IL0 has oxygen atoms only in the anion, while IL1-IL3 additionally have (an increasing number of) oxygen atoms in the cation, which leads to the observed broadening and shift in binding energy. By fitting our data with two peaks of the expected intensity ratio (not shown), we obtain reasonable fits when using a peak separation of 0.55 ( 0.15 eV, yielding mean values of 532.4 and 533.0 eV for the oxygen atoms in the anion and cation, respectively. For a more detailed analysis, measurements with better resolution are required. The contributions of initial and final state effects to the small binding energy difference observed in XPS will be subject of future studies. Since IL1, IL2, and IL3 consist of the same anion [Tf2N]-, all differences in peak intensities should be found in the cation signals. Indeed, the relative C 1s intensities of the cation-derived peak at 286.6 eV increases relative to the anion-derived peaks at 292.7 eV, when going from [Me(EG)MIM][Tf2N] to [Me(EG)3MIM][Tf2N], i.e., from 1 to 3 EG units. The corresponding intensity ratios are 7.1:2.0, 10.3:2.0 and 11.2:1.8, respectively. The O 1s signal has more or less the same intensity, since the number of O atoms increased from five to seven per IL ion pair, when going from 1 to 3 EG units. Overall, the intensities of all anion-derived and also imidazolium-derived signals (N 1s, S 2p, F 1s) decrease with increasing size of the cation, since the density of IL pairs is reduced due to the larger size of the cation, when going from 1 to 3 EG units. Comparing absolute N 1s, F 1s, and S 2p intensitiessas deduced by numerical peak integration after linear background subtractionsto the corresponding signals of [EMIM][Tf2N], which again serves as a reference (Fmol ) 3877 mmol/cm3),35 comprising the same number of N, F, and S atoms per ion pair, the molar densities of IL1-IL3 can be derived from XPS. The mean values are included in Table 1 (Fmol,XPS); the indicated errors are derived by the statistical scatter of the molar densities, obtained independently from the individual N 1s, F 1s, and S 2p analysis of IL0-IL3. The comparison with the molar densities (Fmol) deduced from the experimentally determined mass density (FM) yields excellent agreement, with deviations of below (2% for all three ILs investigated: e.g., for IL1, we find values of Fmol,XPS of 3.92 ( 0.03 vs 3.85 ( 0.06 × 10-3 mol/cm.3 This very good agreement further corroborates the ultraclean nature of the IL surfaces and the potential of XPS for quantitative analysis of such systems. 3.5. [Me(EG)MIM]I (IL4). To obtain additional information on the ILs studied above and as an example that IL surfaces are not necessarily clean, we present our results for [Me(EG)MIM]I (C7H13N2OI, M ) 268.10 g/mol, denoted as IL4) in the following.

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Figure 3. XP spectra of IL4 ([Me(EG) MIM]I), measured under 0° (black) and under 70° (red) electron emission angle, with respect to the surface normal: (a) C 1s, (b) N 1s, (c) O 1s, (d) I 3d, and (e) I 4d region.

This IL is particularly interesting as it is the starting material for the final anion metathesis in the synthesis of IL1, i.e., [Me(EG)MIM][Tf2N]. The corresponding XP spectra for C 1s, N 1s, O 1s, I 3d, and I 4d regions are shown in panels a-e of Figure 3, respectively, and partly in Figure 4a (I). It is quite evident that, in contrast to the ILs studied above, the intensities for normal and grazing emission (0 vs 70°) are significantly different. The N and I signal intensities are much lower at grazing emission, which is indicative of a depletion of these elements (and, thus, the IL) in the surface near region. The O 1s signal is damped only to a lower degree. The quantitative analysis of the peak intensities, which is depicted in Figure 4b (column I), shows the number of atoms as derived from the spectra in the two geometries (0°, black circles; 70°, red squares). It reveals pronounced deviations from the nominal values (dashed horizontal lines), being more pronounced for 70°. These observations indicate that the chemical composition found in the surface near region does not correspond to the clean pure ionic liquid. The C 1s signal in Figure 3 contains two components at 284.9 and 286.6 eV. The latter was also observed for IL1 and is assigned to the C atoms of the cation; it shows the same behavior with emission angle as the C, N, and I signals. The peak at 284.9 eV displays much higher intensity at grazing emission, indicating that it originates from a species enriched at the surface of IL4. From these observations and the quantitative analysis in Figure 4b (I), we conclude that IL4 is covered by a thin contamination layer, which predominantly consists of carbon. It also contains some oxygen, since the deviation of the O signal from the nominal

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Figure 4. (a) C 1s, N 1s, O 1s, and I 3d XP spectra of IL4 ([Me(EG)MIM]I) taken under 0° (black) and under 70° (red) electron emission angle, with respect to the surface normal. The spectra have been collected (I) after introduction in the vacuum system (data from Figure 3), (II) after sputtering with Ar ions (500 eV, 5 µA, 10 min) at 270 K, (III) 12 h at 300 K after step II, (IV) after sputtering with Ar ions (1000 eV, 5 µA, 5 min) at 300 K, and (V) 36 h at 300 K after step IV. (b) Atomic C, N, O, and I (corrected ASF for I3d5/2, 5.65) composition at stages I-V, as derived from XP spectra in Figure 4a taken under 0° (black circles) and 70° (red squares). The nominal composition of IL4 is indicated by dashed lines; for details see text.

value is less pronounced than those for N and I. Survey spectra have been collected in order to identify other constituents of this contamination layer; however, not even traces of other elements such as Si are found. To study the origin and the nature of the contamination, the IL surface was sputtered with Ar ions (500 eV, 5 µA, 10 min). To slow down possible annealing and diffusion processes, it was cooled to 270 K for the first sputtering step. The XP spectra of the C 1s, N 1s, and I 3d5/2 region are shown in Figure 4a (II) along with the corresponding spectra from Figure 3 before sputtering (I). The N 1s and I 3d spectra and also the C 1s component at 286.6 eV clearly show an increase in intensity in both geometries after sputtering (see also Figure 4b (II)). Furthermore, a relative increase of these signals at 70° as compared to that at 0° is found. This indicates that the contamination layer has been partly removed from the surface. To investigate a possible recovery of the contamination layer, we measured XP spectra after leaving the sample at 300 K for 12 h. The corresponding data in Figure 4a and b (III) demonstrate that the contamination layer indeed has partly recovered. This is concluded from the decrease of the N 1s and I 3d5/2 peaks and also of the C 1s peak at 286.6 eV, in particular for grazing emission. As a next step, we again sputtered the surface with Ar ions (1000 eV, 5 µA, 5 min)ssee XP spectra in Figure 4a (IV). The cleaning effect is more pronounced than that seen after the first sputtering step, with the N 1s, I 3d5/2 peaks and also of the C 1s peak at 286.6 eV significantly gaining intensity, in particular

Kolbeck et al.

at grazing emission. At normal emission the contaminationinduced C 1s peak at 284.9 eV has nearly vanished and the number of C atoms in Figure 4b (IV) almost approached the value of 7, as expected for the clean IL. The fact that the signals at grazing and normal emission are still not identical and that the contamination-induced C 1s peak did not fully vanish at grazing emission indicates that the contamination layer was not completely removed. As the last step in our series, we collected XP spectra 36 h after the second sputtering cycle. Clearly, after the first sputtering cycle, a recovery of the contamination layer occurred, as is concluded from the decrease of the N 1s and I 3d5/2 peaks and the increase of the C 1s peak at 284.9 eV (see Figure 4a and b (V)). The recovery effects indicate that segregation of the contaminations to the surface is energetically favorable. The fact that it is less pronounced for the second cycle suggests a certain depletion of the bulk contamination, at least, in the surface near region. (Note that the time interval after the second sputtering step, where a higher ion energy of 1 keV was used, was three times longer than after the first one, in order to ensure equilibrium conditions.) All in all, the measurements in this section clearly show that the investigated IL4 displays a contamination that predominantly consists of carbon and oxygen. This contamination is strongly enriched at the surface, as is deduced not only from the XP spectra but also from the fact that it can be removed by sputtering and recovers with time. It should be mentioned that for all PEG iodide ILs of this study angle-resolved XPS revealed surface contaminations as found for IL4, i.e., the pronounced shoulder in the C 1s spectra at 284.9 eV along with damping of IL related signals; most probably, a surface contamination layer for PEG iodide ILs is formed in general. Since we can only derive conclusions for the surface near region from our XPS measurements, no information on the level of bulk contamination can be derived by this method. However, bulk measurements of IL4 by NMR spectroscopy showed no impurity in the bulk IL of more than 2 mol %. This indicates that the surface contamination detected in XPS is the result of a strong surface enrichment effect of this impurity. The amount of contaminants enriched at the surface of the pristine IL4 sample can be estimated from the XPS analysis in Figure 4. At 0° (i.e., at an information depth of 7-9 nm), signals from the cations and anions of IL 4 are reduced by about the factor of 2 as compared to the nominal composition of [Me(EG)MIM]I: from the N 1s and I 3d5/2 core levels, 1.0 instead of 2.0 N atoms and 0.4 instead of 1.0 I atoms are calculated, respectively (see Figure 4b I). In the more surface sensitive geometry of 70° (i.e., at an information depth of 2-3 nm), the IL-related signals are decreased by at least a factor of 5: 0.4 instead of 2.0 N atoms and 0.1 instead of 1.0 I atoms are observed. Consequently, the contamination within the first 2-3 nm of sample IL4 corresponds to 80-90 mol %. If one compares that to the maximum bulk contamination of 2 mol %, as probed by NMR, one obtains a surface enrichment factor of 40-45. Similar surface enrichment effects are well-known in classical surface science on solid surfaces in the form of the segregation of bulk contaminations (in the ppm regime) to the surface, yielding surface concentrations of more than 25 atom %. Well-known examples are, e.g., S on nickel surfaces,38,39 with the driving force being the lower surface free energy of the adsorbate covered surface. (37) Shimada, S.; Hiori, T.; Ida, T.; Mizuno, M.; Endo, K.; Kurmaev, E. Z.; Mowes, A. J. Polym. Sci., B: Polym. Phys. 2007, 45, 162–172. (38) Sickafus, E. N. Surf. Sci. 1979, 19, 181–197. (39) Batchelor, D. R.; Dalton, M.; Tatlock, G. J. Surf. Interface Anal. 1996, 24, 875–880.

Functionalized Imidazolium-Based Ionic Liquids

As many technological processes such as multiphase catalysis strongly depend on the liquid/gas or liquid/vacuum interface, the phenomenon of surface enrichment of contaminations with low bulk concentration is of great importance. Finally, we briefly want to address the question, whether sputtering can be used to clean the surfaces. As this was not the primary topic of the present study, we have performed only selected measurements that show that surface contamination can be removed. From our measurements, however, we cannot make a detailed statement on beam damage of the IL due to the high energy ions. From the intensities of the various core levels after the second sputtering cycle (IV), we conclude that the IL is still intact. This may indicate the fact that there is only little beam damage or that the species formed during the bombardement diffuse into the bulk or desorb into the vacuum.

Summary By systematic studies of the functionalized ILs [Me(EG)MIM][Tf2N], [Et(EG)2MIM][Tf2N], and [Me(EG)3MIM][Tf2N], which vary by the number of EG units (from 1 to 3), we have shown that for these systems surface and bulk composition are identical. Moreover, it was demonstrated that these samples contained only minor amounts of surface contaminations. This is deduced from the experimental observation that the XP spectra of all elements present in the IL anions and cations (C, N, O, F, S) show identical signals in the bulk and surface-sensitive geometries, i.e., at 0 and 70° emission angles, respectively. Also, the relative intensity ratios of the various elements, including chemically shifted species, are in nearly perfect agreement with the nominal values for the individual ILs. The corresponding investigations of [Me(EG)MIM]I, which is the starting material

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in the synthesis of [Me(EG)MIM][Tf2N], showed very different results. Here, a strong damping of all IL derived signals is observed, which is significantly more pronounced in the surfacesensitive geometry. Moreover, an additional signal is found in the C 1s spectra. These observations are attributed to the existence of a contamination layer on top of the IL. Sputtering leads to a depletion of this layer, which recovers with time. While Sicontaining contaminations have been detected by XPS before,21,33 in this study, the contamination contained only the same elements as the IL (namely C and O). However, we could demonstrate that we are also able to identify such contaminations by comparing XP spectra in normal and grazing emission. The fact that no contamination of such composition could be detected in the bulk NMR-analyses of the IL4 sample in more than 2 mol % provides a clear hint for a strong surface enrichment of this impurity. Therefore, we want to re-emphasize the importance of surface enrichment of contaminations with low concentration in the bulk, since many technological processes such as multiphase catalysis strongly depend on the liquid/gas or liquid/vacuum interface. Contaminations will strongly modify the surface properties and in particular also exchange processes between the liquid phase and the gas phase. Acknowledgment. This work was supported by the German Research Council (DFG) through SPP 1191 (STE-620/7-1 and WA-1615/8-1). The authors also gratefully acknowledge funding of the DFG, which, within the framework of its ″Excellence Initiative″, supports the Cluster of Excellence ″Engineering of Advanced Materials″ (www.eam.uni-erlangen.de) at the University of Erlangen-Nuremberg. LA801261H