Influence of Different Substituents on the Surface ... - ACS Publications

2854

J. Phys. Chem. B 2009, 113, 2854–2864

Influence of Different Substituents on the Surface Composition of Ionic Liquids Studied Using ARXPS K. R. J. Lovelock,† C. Kolbeck,† T. Cremer,† N. Paape,‡ P. S. Schulz,‡ P. Wasserscheid,‡,§ F. Maier,*,† and H.-P. Steinru¨ck†,§ Lehrstuhl fu¨r Physikalische Chemie II, Lehrstuhl fu¨r Chemische Reaktionstechnik, Erlangen Catalysis Resource Center (ECRC), UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ReceiVed: December 3, 2008

Angle resolved X-ray photoelectron spectroscopy has been used to study the surface composition of various nonfunctionalized and functionalized 1,3-dialkylimidazolium ionic liquids. For [CnC1Im][Tf2N] (where n ) 2-16), an enrichment of the aliphatic carbon was observed for longer chains (n g 4). Enrichment of the aliphatic carbon also occurs for alkyl chains attached to the anion, as observed for [C2C1Im][OcOSO3]. Oligo(ethyleneglycol)ether (PEG) functionalities in the cation lead to a surface composition close to bulk stoichiometry and thus a loss in enrichment of the chains. This effect is attributed to attractive interactions between the oxygen atoms on the cation to the hydrogen atoms on the imidazolium ring for [Et(EG)2MIm][Tf2N] and [Me(EG)3MIm][Tf2N]. 1. Introduction Ionic liquids (ILs), salts with a melting point below 100 °C, have shown great promise in both experimental and theoretical environments in the past decade.1 There are at least 106 potential primary ILs of which many have very interesting physicochemical properties, leading to a burgeoning area of research.2 The main focus has been on a relatively small range of nonfunctionalized 1,3-dialkylimidazolium salts.1,3,4 However, many functionalized, or task-specific, ILs are now being developed.5,6 Industrial applications, such as the BASIL process, have also been employed.2 The nature of the IL interface with solids, liquids, and gases is of great interest to a large number of areas.7 For heterogeneous reactions with catalytically active species dissolved in the IL phase, for example, in supported ionic liquid phase (SILP) catalysis,8-11 knowledge of the liquid-gas interface is vital because transport of reactants is directly influenced by the interaction of gaseous molecules with the IL ions at the surface. An understanding of IL interfaces at a molecular level is also crucial in order to explain fundamental macroscopic surface properties such as surface tension12 and electron transfer processes in electrochemistry.4,13 It has been shown for a number of nonfunctionalized ILs that heterogeneity exists in the bulk phase, leading to polar regions (comprised of the anion and cation headgroups) and nonpolar regions (comprised of the alkyl chains).14-18 Moreover, different structural bulk phases and phase transitions, such as “liquid crystal-to-liquid”, are observed for imidazolium ILs with extended alkyl chains.19,20 For the ILvacuum (or IL-gas) interface, the composition of and molecular arrangement at the surface will be different to the bulk due to the unbalanced forces (as a result of the nonisotropic environment). Changing the structure of the cation or anion will lead to changes in the nature of the interface. * To whom correspondence should be addressed. † Lehrstuhl fu¨r Physikalische Chemie II. ‡ Lehrstuhl fu¨r Chemische Reaktionstechnik. § Erlangen Catalysis Resource Center (ECRC).

On the basis of the very low vapor pressure of ILs, ultrahigh vacuum (UHV) techniques can be applied for their investigation.21,22 This field is relatively new as most liquids cannot be studied at room temperature under standard UHV conditions due to evaporation of the sample. The techniques already used to investigate IL surfaces at UHV include X-ray photoelectron spectroscopy (XPS),23-37 ultraviolet photoelectron spectroscopy (UPS),36,38-40 near edge X-ray fine structure (NEXAFS),40-42 metastable impact electron spectroscopy (MIES),26,36 time-offlight secondary ion mass spectrometry (ToF-SIMS),27,43-45 high resolution electron energy loss spectroscopy (HREELS),36 and low energy ion scattering (LEIS).25 Due to the surface sensitive (but not surface specific) nature of many of these techniques, the near-surface region of an IL is usually probed. Studies of ILs have focused mainly on characterizing nonfunctionalized ILs and species dissolved therein. Angle-resolved XPS (ARXPS) has been applied to investigate surface composition of ILs and surface enhancement of species dissolved in ILs.31-33 A recent study focused on functionalized ILs for the first time.35 There are major benefits to studying the surfaces of ILs using XPS. First, elemental identification is possible, meaning XPS can be used to detect the presence of surface-active contaminants, such as silicone impurities, with a much higher sensitivity compared to other techniques such as NMR.27,30,33 Also, an important advantage of carrying out experiments on ILs at UHV is the greater purity of ILs at greatly reduced pressures, even without heating the IL above room temperature.30 Techniques used to investigate the surfaces of ILs, other than UHV techniques, include direct recoil spectroscopy (DRS),46,47 sum frequency generation (SFG),48-51 and X-ray and neutron reflectometry.52,53 It has been established that both cations and anions are present in the surface region of a wide range of imidazolium-based ILs.7 It is less clear whether there are preferential distribution or molecular orientation effects of ions in the near-surface region, particularly at the outer surface. Particular focus has surrounded the orientation of the imidazolium ring of the cation and the longer alkyl chain of the cation; for different studies, the imidazolium ring has been found to be either perpendicular or parallel to the surface normal.7 Lockett

10.1021/jp810637d CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

Ionic Liquids Studied Using ARXPS

J. Phys. Chem. B, Vol. 113, No. 9, 2009 2855

TABLE 1: Summary of ILs Investigated in This Study

et al. used ARXPS to show, for nonfunctionalized [CnC1Im][BF4] (where n ) 4, 6, 8), that the longer, hydrophobic alkyl chain is oriented toward the vacuum side, although no mention is made of the average angle of the chains to the surface normal.31 On the basis of their SFG results on [CnC1Im][BF4] (where n ) 4-11), Iimori et al. suggest that the outer surface consists of regions where straight alkyl chains protrude in bundles and ionic regions that are dominated by the ionic headgroups.49 Jiang et al. used multiscale coarse graining (MSCG) simulations for [CnC1Im][NO3] (where n ) 2-12) to show that the longer the alkyl chain, the more the outer surface is composed of alkyl chains rather than polar groups.54,55 These simulations also show that the angle of the alkyl chains to the surface normal, for n ) 10 and 12, was ∼30° (similar to the angle for alkyl chains of gold-thiolate self-assembled monolayer systems56). Recent SFG studies by Santos and Baldelli, performed on [CnC1Im][Cn′OSO3] (n and n′ ) 1-4), reveal that the alkyl chains of both the cation and the anion project toward the gas phase.57 However, little is known about the effect of functionalization of ILs on the nature of the interface. Our group recently made an initial ARXPS study on ILs with ether functionalized chains at the imidazolium cation.35 On the basis of these findings the present ARXPS investigation addresses the variation in the composition of the nearsurface region of eight nonfunctionalized and two functionalized ILs, all of which contain an imidazolium-based cation. In contrast to former studies of our group, the surface sensitivity has now been further increased by employing higher electron emission angles. The structures, names, and chemical formulas are given in Table 1. These investigations are split into three sections. The first section focuses on the nonfunctionalized [CnC1Im][Tf2N] (where n ) 2-16). It will be shown that as the alkyl chain length increases, the amount of hydrophobic alkyl

carbon groups at the outer surface increases. To the authors’ knowledge, this study is the first XPS investigation of ILs with alkyl chains longer than octyl (C8H17). The second section concerns the IL [C2C1Im][OcOSO3]. It will be shown that the outer surface of the IL is composed mainly of hydrophobic alkyl carbon groups from the anion. A comparison will be made to the more commonly studied IL [C2C1Im][EtOSO3], which does not contain a long hydrophobic alkyl chain. The final section deals with the ether-functionalized ILs [Et(EG)2MIm][Tf2N] and [Me(EG)3MIm][Tf2N]. It will be demonstrated that the outer surface is enriched by fluorine groups from the anion and not by the more hydrophilic ether-functionalized chains, that is, no enhancement of the ether-containing alkyl chains occurs. These oligoether substituted ILs (PEG-ILs) are of particular interest due to their very low viscosity.58 2. Experimental Section 2.1. Synthesis of ILs. All ILs investigated have been prepared and characterized in our group. NMR spectra were recorded on a JEOL ECX 400 MHz spectrometer in d6-DMSO. In the following, the successive steps of the synthesis are described. Nonfunctionalized ILs. 1-Methylimidazole, alkyl halides, diethylsulfate, and octanol were purchased from Aldrich and were distilled prior to use. The lithium bis[(trifluoromethyl)sulfonyl]imide was obtained from Merck - Solvent Innovation GmbH, Cologne. Step 1 - Synthesis of [CnC1Im][X] (n ) 2-16, X ) Cl, Br). Alkyl halide (1 equiv) was added dropwise to ice-cooled 1-methylimidazole (1 equiv) under argon atmosphere. The reaction mixture was vigorously stirred for 18 h and then heated to 60 °C for 4 h to complete reaction. 1-Alkyl-3-methylimidazolium halides were obtained as white solids (>99%).

2856 J. Phys. Chem. B, Vol. 113, No. 9, 2009 Step 2 - Synthesis of [CnC1Im][Tf2N] (n ) 2-16). At room temperature, a 30% solution of the [CnC1Im][X] (1.0 equiv) in water was added to a 30% solution of Li[Tf2N] in water (1.0 equiv). The resulting aqueous solutions were extracted five times with the same volume of dichloromethane. After combining the organic phases and removal of the volatile solvent, the bis[(trifluoromethyl)sulfonyl]imide ILs were obtained. Synthesis of [C2C1Im][OcOSO3]: Alkylation. Diethylsulfate (1 equiv) was slowly added to 1-methylimidazole (1 equiv) that was precooled to 4 °C by an external cooling bath. After 2 h of stirring under argon atmosphere, the mixture was heated up to 60 °C and was stirred under argon for another 6 h. The product, 1-ethyl-3-methylimidazolium ethylsulfate ([C2C1Im][EtOSO3]), obtained was a slightly yellowish liquid. Transesterification. Octanol (2 equiv) were mixed with 1-ethyl-3-methylimidazolium ethylsulfate ([C2C1Im][EtOSO3]). Some drops of the catalyst, methanesulfonic acid, were added to the mixture. The reaction mixture was heated to 70 °C with constant stirring. Ethanol formed during the reaction was removed under vacuum. After 7 h, more octanol (0.5 equiv) was added, and after stirring for another 30 min the remaining ethanol was removed under reduced pressure (final pressure: 10-2 mbar). After neutralizing the obtained IL with sodium hydroxide, a 2-fold (volumetric) excess of diethylether was added to extract the remaining alcohol from the product. This process was repeated five times. The rest of the diethylether was removed under reduced pressure (10-2 mbar). The product obtained was a colorless liquid. Functionalized PEG-ILs. Step 1 - Synthesis of PEGImidazoles. A solution of 2.5 equiv sodium hydroxide, dissolved in the same amount of ice, 120 mg of hexadecyltrimethylammonium hydrogensulfate, 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. Benzenesulfonyl chloride (1 equiv) 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 twice 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 PEGbenzenesulfonate. A solution of 3.0 equiv sodium hydroxide, dissolved in the same amount of ice, 120 mg of hexadecyltrimethylammonium hydrogensulfate, and 1.0 equiv of 1Himidazole (1-H-Im) was prepared. PEG-benzenesulfonate (1.1 equiv) was added dropwise at 70 °C. The reaction mixture was stirred for 12 h 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 (10-2 mbar) to yield the final product. The yields of PEG imidazoles are Et(EG)2Im ) 76%, Me(EG)3Im ) 88%. Step 2 - Synthesis of PEG-Functionalized Iodide ILs. 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 the reaction. The product was obtained in quantitative yield in all cases in the form of a yellow liquid. Step 3 - Synthesis of PEG-Functionalized [Tf2N]- ILs. At room temperature, a 30% solution of the PEG-functionalized iodide IL (1.0 equiv) in water was added to a 30% solution of Li[Tf2N] in water (1.0 equiv). The resulting aqueous solutions were extracted five times with the same volume of dichlo-

Lovelock et al. romethane. After combining the organic phases and removal of the volatile solvent, the bis[(trifluoromethyl)sulfonyl]imide ILs were obtained. 2.2. XPS Measurements. The thin IL films were prepared by deposition of the corresponding IL onto a planar Au foil (20 mm × 15 mm × 0.1 mm). These samples were then introduced in the UHV system via a loadlock, with exposure to air minimized to approximately one minute. After at least 6 h of pumping, a pressure of ∼5 × 10-10 mbar was achieved, just above the base pressure in the vacuum system, confirming the absence of volatile impurities. The XPS measurements were performed at room temperature (in case of [C16C1Im][Tf2N], spectra were taken after heating to ∼40 °C in order to melt the IL and obtain a smooth surface) with an ESCALAB 200 VG system using Al KR radiation (hν ) 1486.6 eV). Individual core level spectra were recorded with a pass energy of 20 eV, yielding an overall energy resolution of 0.9 eV. To vary the surface sensitivity of the measurements, spectra were collected under ϑ ) 0° (normal emission) and for ϑ ) 70° and ϑ ) 80° (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 compounds59 at the kinetic energies used (∼800-1300 eV), measurements at 0° probe the near-surface region (information depth, (ID) 7-9 nm, depending on the kinetic energy); measurements at 70° (ID, 2-3 nm), and 80° (ID, 1-1.5 nm) probe the topmost layers. A preferential 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 measurement” at 0°. The Au 4f7/2 signal (EB ) 83.55 eV) was used as a reference for the reported binding energies. ILs have been shown to charge, even for low viscosity ILs.60 Under the experimental conditions of the XPS setup in use, peak positions were reproduced with variations of (0.15 eV. We refrain from referencing the binding energy scale to the position of the alkyl carbon, which has been used in the literature,31 since the binding energy of the C1 s peak assigned to the alkyl chain relative to the imidazolium carbon was found to vary with chain length (see Section 3.1). For C 1s spectra of ILs containing the [Tf2N]- anion, a three point linear background subtraction was used. For all other spectra, a two point linear background subtraction was used. All peaks were fitted using Gaussian lineshapes. When constraints were imposed on the fitting they are explained for the particular IL. Using the areas under the fitted peaks and taking into account the sensitivity factors for the different elements, quantitative information was obtained on the stoichiometry of the near-surface region analyzed. The atomic sensitivity factors (ASFs) used are those reported by Kolbeck et al. for our specific experimental setup and are given in Table 2.35 In Figure 2a,b and Figure 4, spectra taken under different angles are depicted in one plot for direct visual comparison. In our XPS setup, measurements at 70 and 80° electron emission angle gave a reduction in overall XPS intensity compared to 0° emission angle. These changes in absolute intensity with emission angle were due to an instrumental artifact. To correct for this reduction, all spectra recorded at 70 and 80° were multiplied by an empirical constant determined by measurements of an IL comprised of small ions. As in our previous study, [C2C1Im][Tf2N] was selected as the reference substance.35 For cross-checking, a clean gold foil was also used.35 It should be emphasized that the relative surface compositions given and



TABLE 2: Quantitative Analysis of the XP Spectra of [CnC1Im][Tf2N]a C 1s (hetero) C1s (alkyl) N 1s (cation) C 1s (anion) N 1s (anion) O 1s (anion) S 2p (anion) F 1s (anion) position peak maxima (eV) ASF [C2C1Im][Tf2N] nominal 0° [C4C1Im][Tf2N] nominal 0° [C6C1Im][Tf2N] nominal 0° [C8C1Im][Tf2N] nominal 0° [C10C1Im][Tf2N] nominal 0° [C12C1Im][Tf2N] nominal 0° [C16C1Im][Tf2N] nominal 0°

286.6 0.205 5.0 5.1 5.0 5.1 5.0 5.2 5.0 5.1 5.0 5.4 5.0 5.5 5.0 5.7

284.8 0.205 1.0 0.8 3.0 3.1 5.0 5.2 7.0 7.0 9.0 9.2 11.0 11.8 15.0 15.5

401.9 0.350 2.0 2.1 2.0 2.1 2.0 2.0 2.0 2.1 2.0 2.0 2.0 2.0 2.0 2.0

292.7 0.205 2.0 2.1 2.0 2.1 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.9 2.0 1.8

399.3 0.350 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

523.5 0.540 4.0 4.0 4.0 3.9 4.0 3.8 4.0 3.9 4.0 3.8 4.0 3.9 4.0 3.7

168.8 0.400 2.0 2.0 2.0 2.1 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.9 2.0 1.9

688.8 1.000 6.0 5.9 6.0 5.8 6.0 5.8 6.0 5.9 6.0 5.7 6.0 5.5 6.0 5.5

a The nominal and the experimentally determined composition in number of atoms and mean binding energy values are given for the various elements constituting the ILs; the experimental values are derived from XP spectra taken at 0°. The atomic sensitivity factors (ASF) taking the transmission function of our electron analyzer into account are taken from ref 35.

conclusions drawn in this publication are not affected by these correction constants. 3. Results and Discussion Previous XP studies of ILs from various groups have shown the presence of silicon impurities in the near-surface region of ILs that could not be detected using NMR or other bulk sensitive techniques.23,27,30,33 In this study, a silicon impurity was observed for [C16C1Im][Tf2N] only and was removed by Ar+ ion bombardment prior to ARXPS, a procedure that has proven to be successful for ILs previously (see Figure S1, Supporting Information).30,31,35 Also, no signs of halide contamination or additional hydrocarbon impurities were observed for all ILs studied herein. 3.1. Nonfunctionalized [CnC1Im][Tf2N] ILs. Figure 1a shows a survey scan for [C8C1Im][Tf2N]; this IL will be used as a case study to describe the principal XPS features. XPS signals were observed for all expected elements. The C 1s and N 1s spectra are shown in Figure 1b,c, respectively, for normal emission (0°). In the C 1s region, three peaks are observed; the C 1s peak at the highest binding energy, that is, at 292.7 eV, is labeled as 3 in Figure 1b. It is assigned as arising from the two CF3 groups of the [Tf2N]- anion, as has been concluded previously.25-27 These carbon atoms are the most electropositive due to bonding to electron withdrawing fluorine atoms and were fitted with one peak. The carbon signals at lower binding energies are assigned to the imidazolium cation. There are carbon atoms with different chemical environments. These environments can broadly be classified as Chetero, corresponding to carbon atoms bonded to the heteroatoms nitrogen (or oxygen, see Sections 3.2 and 3.3), and Calkyl, corresponding to atoms only bonded to other carbons and hydrogen. The peak at 284.8 eV, labeled as 1 in Figure 1b, can be identified as arising from Calkyl, as has previously been determined.23,25,27 It was fitted with one component. The peak at 286.6 eV, labeled as 2, can be unequivocally identified as arising from Chetero. It appears at higher binding energy than the peak due to the alkyl carbons, because of the presence of at least one bond to a more electronegative atom. It has been shown in high resolution spectra that peak 2, due to Chetero, is composed of more than one component.23,25,27,28,31 However, when using a nonmonochromated Al KR source, as

was done in this study, the lower resolution does not justify fitting the Chetero signals with more than one component: various attempts yielded an unacceptably large number of constraints and equivocal results. Therefore, the Chetero peak (2) was fitted with one component only for all C 1s spectra in this work. This procedure provided very reliable results, employing only one empirically derived constraint for the full-width-at-halfmaximum (fwhm) values of peaks 1 and 2, namely fwhm(Chetero) ) fwhm(Calkyl) × 1.11. This constraint was found to give good fits for all [CnC1Im][Tf2N] ILs. For each individual [CnC1Im][Tf2N] IL, C 1s spectra at 0, 70, and 80° were fitted, and an average binding energy (BE) peak separation of BE(Chetero) - BE(Calkyl) was calculated; subsequently this value was used to refit all spectra at 0, 70, and 80°. Hence, it is not possible, as mentioned in Section 2.2, to reference the binding energy scale of imidazolium ILs with different chain lengths to the BE of Calkyl. The peak separation for [CnC1Im][Tf2N], where n ) 2-16, is lowest for [C2C1Im][Tf2N], then increases with n and reaches a plateau for [C8C1Im][Tf2N] onward, as shown in Figure 1e. The reason for this observation is that, while the alkyl carbons are treated as equivalent in the fitting procedure, there are actually subtle differences; the alkyl carbons closer to the ring (e.g., between the second and the eighth alkyl carbon atom) are clearly affected by the charge on the imidazolium ring, whereas those further away are not affected. Hence, a plateau is reached when these contributions dominate the spectrum. Fitting the other regions is a far simpler task than for the C 1s region. All [CnC1Im][Tf2N] ILs contain three nitrogen atoms, as shown by the structure at the bottom of Figure 1. The N 1s peak at the higher binding energy in Figure 1c, that is, at 401.9 eV, is due to the nitrogen atoms from the cation, as the imidazolium ring is positively charged. The two nitrogen atoms of the imidazolium ring are indistinguishable by XPS and hence were fitted with one peak. The peak at lower binding energy, 399.3 eV, is due to the nitrogen atom of the anion, which carries a relatively large amount of the anion’s negative charge.61 This nitrogen atom was also fitted with one peak. No constraints were used for the fwhm of both peaks. For the peak separation an empirical value of 2.62 eV was determined and used throughout the fitting procedure. The fluorine and the oxygen atoms are chemically equivalent and hence only one single peak

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Lovelock et al.

Figure 1. XP spectra of [C8C1Im][Tf2N], recorded under 0° (black dots) electron emission angle with respect to the surface normal: (a) survey scan, (b) C 1s region, and (c) N 1s region along with corresponding fits (green, blue, individual components; red, summation). (d) C 1s XP spectra of [CnC1Im][Tf2N] (where n ) 2-16), recorded under 0° electron emission angle. (e) Peak separation of the C 1s peaks due to Chetero and Calkyl for [CnC1Im][Tf2N] as a function of chain length, n; for details see text.

is observed in the F 1s and in the O 1s spectra, respectively. The two sulfur atoms are chemically equivalent; the observed double peak structure of the S 2p level (see Supporting Information) is due to spin-orbit splitting into the S 2p1/2 and S 2p3/2 levels (in a ratio of 1:2). The C 1s XP spectra for [CnC1Im][Tf2N] with increasing length of the alkyl chain from n ) 2 to 16 are shown in Figure 1d, for ϑ ) 0°. As the chain length increases, the intensity of the corresponding C 1s peak (1) increases, as expected from the increased number of alkyl carbons, Calkyl, present. The intensity of the peaks due to Canion (3) and Chetero (2) both decrease with increasing n, due to the decrease in overall molar density with increasing size of the cation. Table 2 gives a summary of the results of the quantitative analysis of the spectra of the different constituents as determined in the bulk-sensitive geometry (i.e., at an electron emission angle

of 0°, which corresponds to an escape depth of ∼7-9 nm, depending on kinetic energy) compared to the nominal composition. The very good agreement of the experimentally determined and the nominal values (with deviations of typically much smaller than 10%) indicates that at the depth probed, all ILs have an approximately homogeneous mixture of cations and anions, within the experimental error. As the electron emission angle is increased, the depth probed by XPS decreases; hence the surface sensitivity is greater at larger electron emission angles. At ϑ ) 80° (depth probed ∼ 1-1.5 nm), the spectra indicate a nonstoichiometric composition, especially for ILs with longer alkyl chains (n g 4). [C8C1Im][Tf2N] will again be used as a case study to explain this phenomenon. For a more detailed analysis, an extra set of experiments was taken at 40°, in addition to sets at 0, 70, and



Figure 2. XP spectra of [C8C1Im][Tf2N] recorded under 0 (black), 40 (blue), 70 (red), and 80° (green) electron emission angle with respect to the surface normal: (a) C 1s region, (b) N 1s region. Ratios of signal intensities for [C8C1Im][Tf2N] as a function of electron emission angle: (c) Calkyl/Chetero, (d) Ncation/Nanion. Ratios of signal intensities for [CnC1Im][Tf2N] recorded at different electron emission angles as a function of chain length, n: (e) Calkyl/Chetero, (f) Ncation/Nanion.

80°. This IL was chosen as it is a part of a related study involving a range of ILs with [C8C1Im]+ as the common cation.62,63 The changes in intensity with electron emission angle are shown in Figure 2a,b for the C 1s and the N 1s regions. From 0 to 40°, very little differences are observed in the C 1s and also the N 1s spectra. This observation is as expected as the depth probed at 40° is approximately three-quarters of that probed at 0°. At 70°, the intensity I(Calkyl) of peak 1 in the C1s spectra increases and the intensities I(Ncation) and I(Nanion) of the two N 1s peaks decreases. At 80°, an even more pronounced increase can be observed for I(Calkyl), accompanied by a simultaneous decrease for I(Canion), I(Chetero), I(Ncation), and I(Nanion), indicating that Calkyl is enriched at the surface at the

expense of the other atoms. These findings become even more evident in Figure 2c, where the intensity ratio I(Calkyl)/I(Chetero), that is, the ratio of aliphatic carbons (1) versus the hetero carbons (2), is plotted versus emission angle (for the labeling of the atoms see bottom of Figure 2). For an isotropic distribution of ions, a constant ration of 7:5 ) 1.4 is expected for [C8C1Im][Tf2N] for all angles. With increasing emission angle, the measured relative amount of hydrophobic alkyl carbon increases, which indicates an inhomogeneous distribution of the different carbon species in the near-surface region with a larger amount of Calkyl than Chetero at the surface. Our results conform with the investigation of Lockett et al. on [CnC1Im][BF4] (where n ) 4-8) in terms of surface enrichment of the alkyl groups.31


Lovelock et al.

Figure 3. XP spectra of [C2C1Im][OcOSO3] recorded under 0° (black dots) electron emission angle with respect to the surface normal: (a) C 1s region and (b) O 1s region, along with corresponding fits (blue, green, individual components; red, summation). Ratio of signal intensities for [C2C1Im][OcOSO3] as a function of emission angle: (c) Calkyl/Chetero, (d) OS-O-C/OSO3.

Moreover, our results also greatly extend the range of ILs investigated up to n ) 16. The ratio of I(Ncation)/I(Nanion) can be used as the best indication of the relative amounts of positively and negatively charged polar headgroups (i.e., the imidazolium ring and the anion, respectively). It shows little change with increasing angle, as shown in Figure 2d; only for 80°, a minor decrease from its nominal value of two for an isotropic distribution is observed, indicating that the anion is located slightly above the imidazolium ring. Hence, the imidazolium ring and the anion are relatively homogeneously distributed and are very likely at approximately the same distance from the outer surface. The error bars for the intensity ratio I(Calkyl)/I(Chetero) are all (10%; this value was calculated based on fitting using different constraints for the fwhm (for example, using 1.0, 1.1, 1.2 eV and observing the difference in the intensity ratio I(Calkyl)/

I(Chetero)). However, the error bars for the intensity ratio I(Ncation)/ I(Nanion) are considerably smaller (all (5%) because no constraints are required. The major contribution to the error is from the linear background subtraction. These observations found for [C8C1Im][Tf2N] (in addition to C 1s and N 1s spectra, F 1s, O 1s, and S 2p ARXP spectra are provided as Supporting Information) are representative for all [CnC1Im][Tf2N] ILs investigated here. At 0°, that is, in the bulk sensitive geometry, the Calkyl/Chetero intensity ratio is as predicted by stoichiometry, as can be seen in Table 2 and Figure 2e (the black squares and the blue dashed line overlap well). However, at 70 and 80°, that is, in the surface sensitive geometry, the amount of alkyl carbon is greater than the nominal amount. This effect is most clearly observed for the ILs with the longest alkyl chains, [C12C1Im][Tf2N] and [C16C1Im][Tf2N] at 80°, where the ratio of I(Calkyl)/I(Chetero) is nearly twice as large as expected

TABLE 3: Quantitative Analysis of the XP Spectra of [C2C1Im][OcOSO3]a C 1s (hetero) position peak maxima (eV) ASF nominal 0° 70° 80°

286.3 0.205 6.0 5.9 5.4 4.5

C 1s (alkyl) 284.8 0.205 8.0 7.8 9.2 11.4

N 1s (cation) 401.6 0.350 2.0 1.8 1.6 1.2

O 1s (S-O-C) 532.7 0.540 1.0 1.1 1.1 0.9

O 1s (SO3) 531.3 0.540 3.0 3.2 2.7 2.2

S 2p (anion) 168.1 0.400 1.0 1.1 1.0 0.9

a The nominal and the experimentally determined composition in number of atoms are given for the various elements constituting the ILs; the experimental values are derived from XP spectra at 0, 70, and 80°. The atomic sensitivity factors (ASF) taking the transmission function of our electron analyzer into account are taken from ref 35.



TABLE 4: Comparing the Ratio of Calkyl/Chetero between [C2C1Im][OcOSO3] and [C8C1Im][Tf2N] nominal 0° 70° 80°

[C2C1Im][OcOSO3]

[C8C1Im][Tf2N]

8:6 ) 1.33 1.35 1.77 2.65

7:5 ) 1.40 1.37 1.63 2.25

from the respective stoichiometry. For n ) 2-16 also the ratio of I(Ncation)/I(Nanion) is approximately as expected for a homogeneous distribution of ions with a tendency of the anion nitrogen to be located slightly above the imidazolium ring in the near-surface region. For all other elements, a table containing quantitative analysis of all spectra (taken at 0, 70, and 80°) is given in the Supporting Information. Summarizing these results, a picture can be sketched of the surface. The outer surface of the ILs is composed of more hydrophobic alkyl carbon chains than expected based on stoichiometry, and the polar headgroups are in relatively close proximity with the [Tf2N]- anion slightly above the imidazolium ring. The alkyl chains could be oriented in a number of different ways; the two extremes are approximately perpendicular to the surface, or lying relatively flat on top of the surface, ap-

proximately parallel to the surface. Simulations by Jiang et al. suggest that alkyl chains are oriented at ∼30° to the surface normal.55 The orientation of alkyl chains cannot be determined directly using ARXPS without additional assumptions concerning the ion distribution. 3.2. Imidazolium Alkylsulfate ILs - [C2C1Im][OcOSO3]. [C2C1Im][OcOSO3] is a homologue of the much studied [C2C1Im][EtOSO3]. The only difference is that the alkyl chain on the anion is C8H17 rather than C2H5, as shown by the structure in Figure 3. For [C2C1Im][EtOSO3], it has been shown using XPS that the ratio of I(Calkyl)/I(Chetero) at 0° is ∼6:2, agreeing with the nominal values.23,33 [C2C1Im][OcOSO3] contains carbon in two different environments, Calkyl and Chetero. There are eight Calkyl carbons; seven on the anion and one on the cation (labeled 1 in Figure 3). In addition, there are six Chetero carbons, one on the anion bound to oxygen and five on the cation bound to nitrogen (labeled 2 in Figure 3). The C 1s region for [C2C1Im][OcOSO3] shows two peaks, as shown in Figure 3a. These can be positively identified as arising from Calkyl and Chetero, the peak positions of which are 284.8 and 286.3 eV, respectively. The only constraint applied initially was to the fwhm (the fwhm ratio of Calkyl/Chetero used was 1:1.11, as for [CnC1Im][Tf2N]). The peak

Figure 4. XP spectra of [Et(EG)2MIm][Tf2N] recorded under 0 (black), 70 (red), and 80° (green) electron emission angles with respect to the surface normal: (a) C 1s region, (b) F 1s region. XP spectra of [Me(EG)3MIm][Tf2N] recorded under 0 (black), 70 (red), and 80° (green) electron emission angle with respect to the surface normal: (c) C 1s region, (d) F 1s region.


Lovelock et al.

TABLE 5: Quantitative Analysis of the XP spectra of [Et(EG)2MIm][Tf2N]a position peak maxima (eV) nominal 0° 70° 80°

N1s (cation)

N1s (anion)

C1s (hetero)

C1s (alkyl)

O1s

F1s

S2p

C1s (anion)

401.9

399.2

286.6

284.9

532.7

688.9

168.7

292.8

2.0 2.0 1.9 1.7

1.0 1.0 1.0 0.9

9.0 9.1 9.0 9.0

1.0 0.9 0.9 1.1

6.0 6.0 6.0 5.6

6.0 5.8 6.2 6.7

2.0 2.0 1.9 1.9

2.0 2.1 2.1 2.0


TABLE 6: Quantitative Analysis of the XP spectra of [Me(EG)3MIm][Tf2N]a position peak maxima (eV) nominal 0° 70° 80°

N1s (ring)

N1s (anion)

C1s (hetero)

O1s

F1s

S2p

C1s (anion)

401.9

399.3

286.6

532.8

689.0

168.8

292.8

2.0 2.1 1.9 1.7

1.0 1.1 1.0 1.0

11.0 11.2 11.0 10.8

7.0 6.9 6.9 6.7

6.0 5.7 6.2 6.9

2.0 2.0 2.0 1.9

2.0 2.1 2.1 2.2


separation for the two carbon peaks was constrained to an average value found for all angles (1.55 eV). Decomposition of the O 1s signal is far easier to achieve. There are four oxygen atoms in the IL, all on the anion. The negative charge is assumed to be delocalized over all three terminal oxygen atoms; therefore the corresponding peak is observed at a lower binding energy (531.3 eV) than that of the bridging oxygen atom (532.7 eV). The N 1s region shows a single component at 401.6 eV, as expected. The S 2p signals (not shown) exhibit the two expected spin-orbit split components with the 2p3/2 peak at 168.1 eV. At 0°, the nominal and experimentally determined values for all elements match within the error limit, as shown in Table 3. At 70° and even more pronounced for 80°, the amounts of Calkyl are larger than the nominal values and the amounts of Chetero are smaller than the nominal values, as seen in Table 3. This observation is evident from the ratio I(Calkyl)/I(Chetero), plotted Figure 3c, which increases from the nominal value of 1.33 at 0° to nearly 2.6 at 80°. As the cation contains only a single alkyl carbon, this enhancement has to be due to a surface enrichment of the hydrophobic alkyl chain on the anion. The intensity ratio of OS-O-C/OSO3 also increases as the electron emission angle is increased from 0 to 80°, although this effect is very small. This result suggests that the anion is oriented with the bridging oxygen atom (OS-O-C) above the three terminal oxygen atoms (OSO3), which fits to the outward orientation of the alkyl chain. Within the error limit, the ratio of the terminal oxygen atoms to the imidazolium nitrogen (not shown) exhibits virtually no change with increasing electron emission angle, suggesting that the charged headgroup of the anion is at approximately the same distance from the outer surface as the imidazolium ring. From these results, the following picture can be envisaged of the top layer of this IL surface: the alkyl carbon of the anion is at the outer surface with the bridging S-O-C oxygen oriented above the terminal SO3 oxygens, and the SO3 group and the cation at approximately the same level. Santos et al., using SFG, investigated [CnC1Im][MeOSO3] (where n ) 1, 2, 4) and found results indicating that both ions are present at the first layer of the gas-liquid interface and the alkyl chains of both the cation and

anion are extended toward the gas phase (i.e., away from the liquid phase).48 More recent studies by Santos et al. of a wider range of [CnC1Im][Cn′OSO3] ILs (n and n′ ) 1-4) confirm these findings.57 Our ARXPS results are in agreements with these results derived from SFG. A comparison can be made for the Calkyl/Chetero intensity ratio for [C2C1Im][OcOSO3] and [C8C1Im][Tf2N] as the longer alkyl chain for both ILs is composed of a C8H17 group. The nominal ratios of the two different carbon species are 1.33:1 and 1.40: 1, respectively, allowing a legitimate comparison (note that the situation is very similar but not completely identical due to the slightly different carbon content and hence ratios). Under grazing emission, the ratio for [C2C1Im][OcOSO3] is considerably larger than for [C8C1Im][Tf2N]. This difference is most clearly observed at 80°, as shown in Table 4, which implies that surface enrichment of hydrophobic aliphatic carbon is more pronounced for [C2C1Im][OcOSO3] than for [C8C1Im][Tf2N]. This difference could be related to the fact that the anionic groups are different in size and shape (the [Tf2N]- anion has an ionic group with the negative charge distributed over the larger -SO2-N-SO2core than the [OcOSO3]- anion with the negative charge located at the terminal -SO3 group), and/or the fact that the alkyl chain is bonded to the anion rather than the cation. Ongoing investigations in our group indicate that the degree of alkyl enhancement is influenced mainly by the size of the anions.62 3.3. PEG-Functionalized ILs. Two ILs containing ethylene glycol functionalities in the cation alkyl chain, [Et(EG)2MIm][Tf2N] and [Me(EG)3MIm][Tf2N], were studied using ARXPS. The structures of both ILs are given in Figure 4. This part of our study is an extension of previous investigations by Kolbeck et al. of the same ILs, where full assignment of the spectra was given.35 One important conclusion was that decomposition of XP signals of C 1s and O 1s spectra was not possible. For the C 1s spectra, it is impossible to distinguish between the imidazolium ring carbons and the chain carbons as all are Chetero carbons (apart from one Calkyl for [Et(EG)2MIm][Tf2N]), as shown in Figure 4a,c). For the O 1s spectra, the ether oxygen atoms from the cation and the sulfoxy oxygen atoms from the anion coincidentally have approximately the same binding energy and hence are indistinguishable by XPS.

Ionic Liquids Studied Using ARXPS Both F 1s and S 2p spectra have atoms in one chemical environment only, and the N 1s spectra contained distinguishable peaks from the cation and anion, as for [CnC1Im][Tf2N] ILs. Kolbeck et al. found that at 0 and 70° no enhancement of any elements was observed within error limits.35 As shown in Tables 5 and 6, these results were confirmed with our slightly modified experimental setup. However, at 80° for both ether-containing ILs there is an enhancement of the F 1s signal by about 10%, as shown in Tables 5 and 6, indicating a greater amount of fluorine in the near-surface region. There was also a decrease in the amount of nitrogen from the cation. It has also been shown that, using high resolution Rutherford backscattering for [N(Me)3Pr][Tf2N], fluorine atoms are at the outer surface.64 Also, for [C4C1Im][TfO], Iwahashi et al. concluded by SFG that the outer surface is composed of CF3 groups.65 The conclusion of our studies is that also for the PEG ILs the outer surfaces consist of a greater than nominal amount of fluorine from the anion. Consequently, the ether-functionalized hydrophilic alkyl chain is not preferentially located at the outer surface in contrast to the findings for [CnC1Im][Tf2N] and [C2C1Im][OcOSO3] for which the topmost surface layers are dominated by the alkyl chains. In the following, we will speculate on the origin of this difference. It has been shown that the melting points for a number of ether-functionalized imidazolium ILs are higher than for the nonfunctionalized homologues (note that only glass transitions can be measured for [Et(EG)2MIm][Tf2N] and [Me(EG)3MIm][Tf2N] due to difficulties of crystallization).66 This finding suggests that the lattice energy for ether-functionalized ILs is greater, caused by additional attractive interactions between the oxygen on the cation and either the cation or anion. For ether-functionalized halide ILs, Fei et al. demonstrated using IR spectroscopy and density functional theory that an interaction exists between the oxygen atoms on the ether-functionalized chain and the hydrogen atoms on the imidazolium ring, especially the electropositive C2-H hydrogen (bonded to N-C-N carbon of ring).67 Recent molecular dynamics simulations have also shown interactions between the oxygen on the cation and the imidazolium ring, both intra- and intermolecularly.68 Attractive interactions between the oxygen atoms on the cation to the hydrogen atoms on the imidazolium ring for [Et(EG)2MIm][Tf2N] and [Me(EG)3MIm][Tf2N] would make the surface IL termination (enrichment) with the ether-functionalized chain energetically unfavorable, leading to an outer surface that is preferentially composed of the noncoordinating CF3 groups from the anion rather than any groups from the cation. 4. Summary and Conclusions We have performed a detailed investigation of the chemical composition of the near-surface region for three different classes of 1,3-dialkylimidazolium ionic liquids by use of angle resolved X-ray photoelectron spectroscopy: (1) ILs with alkyl chains of different length attached to the cation ([CnC1Im][Tf2N] with n ) 2-16), (2) an IL with a C8H17 alkyl chain attached to the anion ([C2C1Im][OcOSO3]), and (3) two ILs containing ethylene glycol functionalities in the cation alkyl chain ([Et(EG)2MIm][Tf2N] and [Me(EG)3MIm][Tf2N]). In the XP spectra, the signals of the various chemically nonequivalent atoms, that is, carbon atoms bonded to (a) F, (b) N or O, or (c) C and H atoms, can be unequivocally identified, and information on the location relative to the outer surface (i.e., the interface to vacuum) can be derived from the intensity and the angular dependence of the XP signals.

J. Phys. Chem. B, Vol. 113, No. 9, 2009 2863 The detailed analysis of the [CnC1Im][Tf2N] ILs unambiguously shows an enrichment of the alkyl chains at the outer surface at the expense of the imidazolium head groups and the anions; this effect monotonically increases with increasing chain length up to n ) 16. From the relative ARXPS intensities of the N atoms in the anion and in the cation, we conclude that both anions and cationic head groups are located approximately at the same distance from the outer surface with the anion slightly above the imidazolium ring. For [C2C1Im][OcOSO3], again a surface enrichment of the alkyl chain is found. A comparison with [C8C1Im][Tf2N] (the corresponding IL with the C8H17 alkyl chain of the same length on the cation) implies that surface enrichment of hydrophobic aliphatic carbon is more pronounced for [C2C1Im][OcOSO3] than for [C8C1Im][Tf2N]. A possible origin of this behavior is the difference in size of the anionic headgroup; the [Tf2N]- anion has a larger ionic headgroup than the [OcOSO3]-. Interestingly, for the ether-functionalized ILs there is no enrichment of the functionalized chains at the surface. Our studies indicate that the outer surfaces are similar to the bulk composition with a slight excess of fluorine from the anion, that is, CF3 groups. In agreement with previous studies, we postulate attractive interactions between the ether oxygen atoms on the cation to the hydrogen atoms on the imidazolium ring for [Et(EG)2MIm][Tf2N] and [Me(EG)3MIm][Tf2N], leading to an increase in the lattice energy in the bulk of the IL. Thus, a termination/enrichment of the IL surface with the etherfunctionalized chain is energetically unfavorable, leading to an outer surface that is preferentially composed of the noncoordinating CF3 groups from the anion rather than any groups from the cation. In order to understand the relationship between surface composition and structure of ILs, further studies of IL systems comprising different anions, functional groups, IL mixtures, and IL solutions with ARXPS are required. These studies should be combined with other surface sensitive techniques and with theoretical calculations. Acknowledgment. This work has been supported by the DFG through SPP 1191, Grants STE 620/7-2 and WA 1615/8-2, and by the Excellence Cluster “Engineering of Advanced Materials” granted to the University of Erlangen Nuremberg. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ionic liquids in synthesis. Welton, T., Ed.; Wiley-VCH: New York, 2008. (2) Plechkova, N. V.; Seddon, K. R. Chem. Soc. ReV. 2008, 37, 123. (3) Weingaertner, H. Angew. Chem., Int. Ed. 2008, 47, 654. (4) Endres, F.; El Abedin, S. Z. Phys. Chem. Chem. Phys. 2006, 8, 2101. (5) Lee, S. G. Chem. Commun. 2006, 1049. (6) Davis, J. H. Chem. Lett. 2004, 33, 1072. (7) Aliaga, C.; Santos, C. S.; Baldelli, S. Phys. Chem. Chem. Phys. 2007, 9, 3683. (8) Riisager, A.; Wasserscheid, P.; van Hal, R.; Fehrmann, R. J. Catal. 2003, 219, 452. (9) Riisager, A.; Fehrmann, R.; Haumann, M.; Wasserscheid, P. Eur. J. Inorg. Chem., 2006, 695. (10) Riisager, A.; Fehrmann, R.; Haumann, M.; Wasserscheid, P. Top. Catal. 2006, 40, 91. (11) Riisager, A.; Fehrmann, R.; Flicker, S.; van Hal, R.; Haumann, M.; Wasserscheid, P. Angew. Chem. Int. Ed. 2005, 44, 815. (12) Carvalho, P. J.; Freire, M. G.; Marrucho, I. M.; Queimada, A. J.; Coutinho, J. A. P. J. Chem. Eng. Data 2008, 53, 1346. (13) Baldelli, S. Acc. Chem. Res. 2008, 41, 421. (14) Chiappe, C. Monatsh. Chem. 2007, 138, 1035.

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