Influence of Different Anions on the Surface Composition of Ionic

May 29, 2009 - be noted that the ASF given for O 1s by Lovelock et al., 0.540, was a typographical error and should have been printed as 0.580, as is ...
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Influence of Different Anions on the Surface Composition of Ionic Liquids Studied Using ARXPS C. Kolbeck,† T. Cremer,† K. R. J. Lovelock,† 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, and Erlangen Catalysis Resource Center (ECRC), UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ReceiVed: April 1, 2009

Angle-resolved X-ray photoelectron spectroscopy has been used to study the influence of different types of anions on the surface composition of ionic liquids (ILs). We have investigated nine ILs with the same cation, 1-octyl-3-methylimidazolium [C8C1Im]+, but very different anions. In all cases, an enrichment of the cation alkyl chains is found at the expense of the polar cation head groups and the anions in the first molecular layer. This enhancement effect decreases with increasing size of the anion, which means it is most pronounced for the smallest anions and least pronounced for the largest anions. A simple model is proposed to explain the experimental observations. 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 and industrial applications, such as the BASIL process.2 The nature of the interface of an IL with solids, liquids, and gases is of great interest to a large number of areas3-5 (and references therein). Particularly, an understanding of IL interfaces at a molecular level is crucial in order to explain fundamental macroscopic surface properties such as surface tension6 and electron-transfer processes in electrochemistry.7,8 For the IL-vacuum (or IL-gas) interface, the composition of, and molecular arrangement at, the surface will be different to that of the bulk due to the unbalanced forces, which are present as a result of the nonisotropic environment. On the basis of the very low vapor pressure of aprotic ILs,9-11 ultrahigh vacuum (UHV) conditions have been applied to investigate IL surfaces using X-ray photoelectron spectroscopy (XPS),5,12-23 metastable impact electron spectroscopy (MIES),13,19 high-resolution electron energy loss spectroscopy (HREELS),19 low-energy ion scattering (LEIS),21 and direct recoil spectroscopy (DRS).24,25 Other surface-sensitive methods without UHV requirements such as sum frequency generation (SFG),26-32 X-ray and neutron reflectometry,31,33,34 surface tension measurements,6,32,35 grazing incidence X-ray diffraction,36 and simulations have also been applied.37-39 Most of these surface studies have mainly concentrated on nonfunctionalized imidazoliumcontaining ILs,3 and only few surface investigations of functionalized ILs have been reported.5,18,23,30 The discussion on IL surface structure often involves two different but interrelated aspects, surface composition and molecular orientation at the surface. Surface composition involves the identification of the molecules present in the near* To whom correspondence should be addressed. E-mail: florian.maier@ chemie.uni-erlangen.de. † Lehrstuhl fu¨r Physikalische Chemie II. ‡ Lehrstuhl fu¨r Chemische Reaktionstechnik. § Erlangen Catalysis Resource Center.

surface region and whether there is enhancement of certain ions (or parts of ions) and depletion of other ions (or parts of ions) with respect to the bulk composition. This identification becomes even more important when small amounts of surface-active impurities are present in the IL.5,12,17,18,40 Molecular orientation at the surface concerns detailed ordering and geometry effects of the ions (or part of the ions) present at the surface. With regard to surface composition, a consensus has been established that both cations and anions are present in the surface region of a wide range of pure imidazolium-based ILs, particularly those ILs containing shorter alkyl chains such as [C2C1Im]+.5,19,21 For ILs containing longer aliphatic alkyl chains (i.e., C4H9 or longer) it has been shown that more alkyl carbon is present in the near-surface region than nonalkyl parts of either the charged imidazolium ring or the anion.16,27,32,37,38 Using angle-resolved X-ray photoelectron spectroscopy (ARXPS), the influence of different substituents on the surface composition was published recently by our group.5 The degree of surface enrichment of aliphatic alkyl carbon chains gradually increases in [CnC1Im][Tf2N] ILs (n ) 2-16) with increasing chain length, as was also observed by Lockett et al. for [CnC1Im][BF4] (where n ) 4-8) using also ARXPS.16 Moreover, we found that aliphatic alkyl chains seem to generally dominate the composition at the outer surface of pure ILs, independent of whether the chains are attached to the cation or the anion, as demonstrated for the case study of [C2C1Im][OcOSO3].5 Our findings were confirmed by Baldelli et al.32 When functional groups with the potential of additional intra- and intermolecular attractive interactions are introduced within the chains such as ether moieties, the surface composition is found to be very similar to the bulk one.5 In contrast to results on the surface composition of ILs, there is more uncertainty concerning preferential molecular orientation of ions in the near-surface region. This uncertainty is partly related to different experimental techniques and their variation in sensitivity to orientation effects at the outer surface.3 Particular focus has surrounded the orientation of the imidazolium ring of the cation and the longer alkyl chain of the cation. It has been shown that the alkyl chain is oriented approximately along the surface normal with a chain density lower than that of

10.1021/jp902978r CCC: $40.75  2009 American Chemical Society Published on Web 05/29/2009

Surface Composition of Ionic Liquids Studied Using ARXPS

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TABLE 1: Summary of ILs Investigated in This Study

archetypal self-assembled monolayers on a metal substrate.31,37,38 For different studies, the imidazolium ring has been found to be either perpendicular or parallel to the surface normal.3 The location of the anion relative to the cation has only been addressed by few systematic surface studies employing ILs with the same cation but different anions. Using SFG, Rivera-Rubero et al. found for nine different anions with [C4C1Im]+ that the imidazolium ring lays parallel to the surface plane and the butyl chain projects into the gas phase, independent of the anion identity.41 Jeon et al. found using SFG and X-ray reflectivity that, for [C4C1Im][X] (where [X]- ) I-, [BF4]-, and [PF6]-), the loosely packed butyl chains were projected toward the gas/ liquid interface, whereas the tightly packed charged cores (i.e., imidazolium cores and anions) were in contact with the neighboring IL molecules in the bulk.31 From their results, the authors also suggested that I- was located beneath the imidazolium core (i.e., the charged ring), whereas [BF4]- and [PF6]were located alongside. Conclusions on surface orientation have also been made based on the macroscopic property surface tension.29,42 Apart from pure ILs, only few surface studies of more complex IL systems such as of IL solutions12,15,43 and IL mixtures44 have been reported. In order to obtain a better understanding of the factors that determine the IL surface structure, the present ARXPS investigation addresses the influence of the anion on the composition of the near-surface region of nine [C8C1Im][X] ILs. We have chosen [C8C1Im]+ as the common cation for three reasons. First, there is a broad range of ILs with this cation with very different anions available that are liquid at room temperature.10 Second, these ILs can be prepared with surface contamination below XPS detection limits (see section 2.1). Third, surface enrichment of the octyl chain was already thoroughly studied for [C8C1Im]-

[Tf2N] in our work on the influence of substituents on the IL surface composition.5 The anions [X]- under investigation are very different in chemical nature and include anions such as halides, [BF4]-, [PF6]-, and [TfO]- and also more complex anions containing perfluoroalkyl groups such as [Tf2N]-, [(C2F5SO2)2N]- ([Pf2N]-), or [PF3(C2F5)3]- ([FAP]-). The anions were chosen to cover different sizes (ranging from Clto [FAP]-), shapes (from spherical to elongated anions), and coordination abilities (from strongly coordinating halides to weakly coordinating anions with perfluoroalkyl groups). Many of the anions studied here are commonly used in IL applications. We excluded certain anions (e.g., [MeOSO3]-, [OcOSO3]-, and [B(CN)4]-) from this ARXPS study since they exhibit XPS carbon signals that strongly overlap with carbon signals from the [C8C1Im]+ cation.45 We show that the different anions have a considerable effect on the degree of surface enrichment of the octyl chains. The anion location is also commented upon for all ILs. The ILs studied herein are shown in Table 1; they are listed by their molecular volume (see also Table 2). 2. Experimental Section 2.1. Synthesis of ILs and Density Measurements. NMR spectra were performed on a JEOL ECX 400 MHz spectrometer in d6-DMSO. 1-Methylimidazole, alkyl halides, diethylsulfate, and octanol were purchased from Aldrich and were distilled prior to use. Li[Tf2N] was obtained from Merck Solvent Innovation GmbH, Cologne. Li[Pf2N] was obtained from Iolitec (purity > 97%). Synthesis of [C8C1Im][X] (X ) Cl, Br). Alkyl halide (1 equiv) was added dropwise to ice-cooled 1-methylimidazole (1 equiv) under an argon atmosphere. The reaction mixture was vigorously stirred for 18 h and then heated to 60 °C for 4 h to complete

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TABLE 2: Quantitative Analysis of the XP Spectra of [C8C1Im][X] for the Carbon Atoms of the Cationa ratios I(Calkyl)/I(Chetero) at IL

0°

70°

80°

[C8C1Im]Cl [C8C1Im]Br [C8C1Im]I [C8C1Im][BF4] [C8C1Im][PF6] [C8C1Im][TfO] [C8C1Im][Tf2N] [C8C1Im][Pf2N] [C8C1Im][FAP]

1.52 1.47 1.47 1.40 1.39 1.37 1.36 1.40 1.36

2.20 1.96 2.01 1.71 1.68 1.69 1.63 1.60 1.66

3.11 3.06 2.74 2.38 2.35 2.40 2.25 2.11 2.01

liquid IL molecular density/g cm-3 volume/nm3 1.01 1.17 1.31 1.08 1.24 1.14 1.27 1.37 1.48

0.38 0.39 0.41 0.43 0.46 0.50 0.62 0.70 0.72

a The experimentally determined ratios of Calkyl relative to Chetero are given; the experimental values are derived from XP spectra taken at 0, 70, and 80°. The nominal ratio Calkyl/Chetero is 7:5 ) 1.4:1. In addition, the IL measured liquid density values and molecular volumes (determined using liquid density) are shown.

the reaction. 1-Alkyl-3-methylimidazolium halides were obtained as white solids (purity > 99%). Synthesis of [C8C1Im][Tf2N] or [C8C1Im][Pf2N]. At room temperature, a 30% solution of the [C8C1Im]Cl (1.0 equiv) in water was added to a 30% solution of Li[Tf2N] or Li[Pf2N] 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 product was obtained (purity > 99%). Synthesis of [C8C1Im]I. [C8C1Im]I was synthesized in the group of Dr. Pete License at the University of Nottingham (purity > 99%). [C8C1Im][FAP]. [C8C1Im][FAP] was kindly given by Merck and used as supplied (level of purity unknown). [C8C1Im][TfO], [C8C1Im][BF4], and [C8C1Im][PF6]. [C8C1Im][TfO], [C8C1Im][BF4], and [C8C1Im][PF6] with purities above 95% were purchased from Sigma-Aldrich and used as supplied. Mass Density Measurements. After extensive degassing over at least 4 h at a reduced pressure of 10-2 mbar at 40 °C, the mass densities of all samples were measured at room temperature (20 °C) by weighing a defined volume of the IL (at least 0.100 mL) on a chemical balance (for [C8C1Im][BF4], [C8C1Im][TfO], [C8C1Im][Tf2N], [C8C1Im][Pf2N], and [C8C1Im][FAP]). With this method, a relative error in mass density dF/F values below 1.5% was achieved. For the relatively viscous ILs ([C8C1Im]Cl, [C8C1Im]Br, and [C8C1Im]I) and also for [C8C1Im][PF6], density measurements were performed at a 0.02% accuracy level using the vibrating tube method.46 2.2. XPS Measurements. As thoroughly described elsewhere,5 the thin IL films were prepared by deposition of the corresponding IL onto a planar Au foil in air introduced in our UHV systems, and ARXP spectra were recorded under polar angles of 0 (normal emission), 70, and 80° (grazing emission), corresponding to an information depth of 7-9, 2-3, and 1.0-1.5 nm, respectively. A preferential increase in the 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 usually employed as a reference for the reported binding energies in our previous work. ILs, however, have been shown to charge, even for lowviscosity ILs.5 Under the experimental conditions of the XPS setup in use, peak positions were reproduced with variations of about (0.15 eV.5 To facilitate visual comparison of the IL spectra, we used an internal standard for the binding energy

scale; the C 1s signal of the octyl chain of the common [C8C1Im]+ cation was set to 285.0 eV.45 For C 1s spectra of ILs with perfluoroalkyl-group-containing anions ([C8C1Im][TfO], [C8C1Im][Tf2N], [C8C1Im][Pf2N], and [C8C1Im][FAP]), a three point linear background subtraction was applied; for all other spectra, a two point linear background subtraction was used. All peaks were fitted using Gaussian line shapes. It has been shown previously that when using a nonmonochromated Al KR source as in this study, it is best to fit C 1s peaks arising from the cation with two components only, Calkyl and Chetero.5 These peaks are labeled as 1 and 2, respectively, in the chemical structure given in Figure 2. This procedure provided very reliable results, employing only one empirically derived constraint for the full-width at half-maxima (fwhm) values of peaks 1 and 2, namely, fwhm(Chetero) ) fwhm(Calkyl) × 1.11. Applying this constraint gave good fits for all [C8C1Im][X] ILs and has previously given good fits for a range of nonfunctionalized and functionalized ILs.5 From the areas under the fitted peaks and by taking into account the sensitivity factors for the different elements, quantitative information can be obtained on the stoichiometry of the near-surface region. The atomic sensitivity factors (ASFs) used for C 1s, N 1s, O 1s, F 1s, and S 2p spectra are those reported previously;5 additional ASFs for Cl 2p, Br 3d, I 3d5/2, B 1s, and P 2p were determined as described by Kolbeck et al.18 All ASF values for the corresponding elements are given in Table 3 and in Supporting Information, Table S.1. It must be noted that the ASF given for O 1s by Lovelock et al., 0.540, was a typographical error and should have been printed as 0.580, as is given herein.5 However, the correct value of 0.580 has been used in all calculations for both papers. 2.3. Sample Purity. There are major benefits to studying the surfaces of ILs using XPS. First, elemental identification is possible, meaning that XPS can be used to detect the presence of surface contaminants, such as silicone impurities, that cannot be detected with other techniques such as NMR.5,12,14,17,18,40,46 An additional important advantage of carrying out experiments in UHV is the increased purity of the ILs at greatly reduced pressures, even without heating to above room temperature.40 Also, no signs of halide contamination or additional hydrocarbon impurities were observed using ARXPS for all ILs studied herein. Thus, in situ cleaning methods such as argon sputtering were unnecessary.5,18 It should be noted that extended exposure to X-rays during XPS can lead to spectral changes. Damage affecting the N 1s region of [C8C1Im][BF4] under X-ray irradiation has previously been observed.40 Similar damage over time under X-ray irradiation was observed for all ILs investigated herein, most commonly observed in the N 1s region at binding energies lower than that of the imidazolium nitrogen, suggesting the presence of a noncharged decomposition product (see also Supporting Information, Figure S.2).40 The most significant damage occurred for [C8C1Im][BF4] and [C8C1Im][FAP], both of which contain significant amounts of fluorine; samples could be studied for a minimum of 4 h with our standard nonmonochromatized X-ray source (power 150 W) before significant and observable (which means 5% of the total N 1s intensity) damage occurred. Spectra reported here were collected at much lower X-ray exposure times than 4 h. It should be noted that no significant damage was observed over 12 h of irradiation time for [C8C1Im][PF6] in XPS, suggesting no obvious trend for sample decomposition.

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Figure 1. XP survey scan spectra of (a) [C8C1Im]Cl, (b) [C8C1Im][BF4], and (c) [C8C1Im][Pf2N] recorded under a 0° electron emission angle with respect to the surface normal.

3. Results and Discussion Figure 1a, b, and c shows survey scans for [C8C1Im]Cl, [C8C1Im][BF4], and [C8C1Im][Pf2N], respectively. These three ILs were chosen as case studies to explain the results. [C8C1Im][BF4] was studied using ARXPS by Lockett et al., thus providing a relevant opportunity for comparison.16 As noted in section 2.3, XPS signals were observed for all expected elements, with no indication of impurities (above the limits of XPS detection, ∼1%). For all elements, a table containing quantitative analysis of all spectra (taken at 0, 70, and 80°) is given in the Supporting Information, Table S.1. Comparisons between the BE positions for both the Chetero and the anion elements are given elsewhere.45 The C 1s region for all 9 [C8C1Im][X] ILs studied contains 12 carbon atoms from the imidazolium cation. These carbon atoms can be decomposed into two types, Calkyl and Chetero, labeled as 1 and 2 on the structure in Figure 2, respectively.5 Chetero is located at a higher binding energy (∼286.4-287.0 eV) than Calkyl (285.0 eV) due to the C-N bond(s) to the more electronegative nitrogen atoms of the imidazolium ring.45 The procedure for determining the relative amounts of Calkyl and Chetero has been explained previously for [C8C1Im][Tf2N].5 The binding energy peak separation for Calkyl and Chetero is dependent on the nature of the anion, which will be explained in detail elsewhere.45 In Figure 2a-f, the C 1s region is shown for [C8C1Im]Cl, [C8C1Im][BF4], and [C8C1Im][Pf2N] at 0 and 80°, along with fitted components. At 0°, the measured intensity ratios Calkyl/Chetero for all three ILs approximately match the nominal ratio, 7:5 ) 1.4, as shown in Figure 2g, suggesting that within the probing depth of 7-9 nm, there is a homogeneous distribution of alkyl chains and imidazolium rings for these three ILs. At 80° and, thus, a probing depth of 1.0-1.5 nm, the intensity of the Chetero component decreases relative to

Figure 2. ARXP spectra of the C 1s region: [C8C1Im]Cl at (a) 0 and (d) 80°, [C8C1Im][BF4] at (b) 0 and (e) 80°, and [C8C1Im][Pf2N] at (c) 0 and (f) 80°. (g) Ratio of the Calkyl/Chetero intensities for [C8C1Im]Cl, [C8C1Im][BF4], and [C8C1Im][Pf2N] as a function of emission angle (angles are measured relative to the surface normal).

the intensity of the Calkyl component, indicating that there are more alkyl carbons present in the near-surface region than ring carbon atoms. The intensity ratio Calkyl/Chetero at 80° follows the trend 3.1 for [C8C1Im]Cl > 2.4 for [C8C1Im][BF4] > 2.1 for [C8C1Im][Pf2N], as shown in Figure 2g. This trend scales with the size of the anions; most alkyl chains are present for the smallest anion Cl-, whereas for the much larger [Pf2N]-, the surface enrichment of alkyl carbon relative to ring carbon is less pronounced. For all nine ILs studied, the ratio Calkyl/Chetero measured at 80° follows this trend: Cl- ∼ Br- > I- > [PF6]- ∼ [BF4]- ∼ [TfO]- > [Tf2N]- > [Pf2N]- > [FAP]- (see Table 2). In order to quantify this observation, the molar volume M/F or the molecular volume M/F/NA is used, where M, F, and NA are the molar mass, the mass density at room temperature, and the Avogadro constant, respectively. It has previously been shown that many key physicochemical properties of ILs can be directly related to molecular volumes;47 molecular volumes are not only available from measurements but can also be predicted from calculations.48 The values for molecular volumes of the ILs studied herein, given in Table 2, are determined from our liquid density measurements. Figure 3 shows the intensity ratio Calkyl/Chetero for all nine ILs at 0 and 80° (70° results are omitted here), plotted against the molecular volume of the ILs. The error bars for the intensity ratios are (7%, representing a conservative estimation for the mean error for all data sets and fitting procedures; the error in the molecular volumes is given by the density measurements ((1.5%). At 0° (i.d. 7-9 nm), the surface

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Kolbeck et al. TABLE 3: Elements Chosen for the Ratio Aanion/Ncation for [C8C1Im][X] (where X ) anion and A ) element selected from anion)a anion, X -

Cl BrI[BF4][PF6][TfO][Tf2N][Pf2N][FAP]a

Figure 3. (a) Ratio of the intensities for Calky/Chetero for [C8C1Im][X], recorded under 0 (green circles) and 80° (red triangles) electron emission angles with respect to the surface normal, against the ionic liquid molecular volume (calculated using liquid densities). A line for the nominal ratio (blue dashed line) is also added. (b) Ratios of the intensities Aanion/Ncation for [C8C1Im][X] (where X ) anion and A ) element selected from anion) at different electron emission angles.

composition of all nine primary ILs investigated was approximately the same as the nominal composition (see also Supporting Information, Table S.1), suggesting that the average of the first 7-9 nm corresponds to a homogeneous distribution of cations and anions. The largest enhancement for alkyl carbon is observed for the smallest anion, Cl-, and the smallest enhancement is observed for the largest anion, [FAP]-. The amount of enhancement observed is thus clearly correlated with the size of the anion. However, the change in surface enhancement of Calkyl from chloride to iodide (i.e., the halides) is relatively large for a relatively small change in molecular volume. Conversely, the change in surface enhancement of Calkyl from [BF4]- to [FAP]- is relatively small for a relatively large change in molecular volume. In order to draw a full picture of the surface composition, anion-related signals were also investigated. Figure 3b shows the intensity ratio for one central atom from the anion, labeled A, and the two nitrogen atoms from the imidazolium ring (representing the center of the polar head group of the cation); the nominal ratio is 0.5 for all ILs studied. Selections for the central atom A of the corresponding anion are given in Table 3; for example, phosphorus was chosen for [PF6]- and [FAP]-. The signal-to-noise level for the measured ratios is quite low because the anion-related XP spectra of element A are quite small. Additionally, for 70 and 80° measurements, damping of the A and N 1s signals by surface alkyl carbon atoms occurs. Overall, we estimate the total error bars to (10%. For seven of the nine ILs the A/N intensity ratio shows a weak increase with increasing surface sensitivity. This observation leads to the tentative conclusion that for all the ILs shown, the mean center of the anion in the near-surface region is located nearly at the same (or even slightly shorter) distance from the outer surface

element chosen, A

ASF (core level)

Cl Br I B P S N N P

0.550 (Cl 2p) 0.550 (Br 3d) 5.61 (I 3d5/2) 0.115 (B 1s) 0.300 (P 2p) 0.400 (S 2p) 0.350 (N 1s) 0.350 (N 1s) 0.300 (P 2p)

The ASFs (atomic sensitivity factors) are also given.

as compared to the imidazolium ring irrespective of the nature of the anion. Jeon et al. have proposed that, using SFG and X-ray reflectivity for [C4C1Im]I, the anion is located beneath the imidazolium ring.31 There is no evidence from our data to suggest that the iodide anion, or any other anion, is preferentially located beneath the imidazolium ring; such a scenario would lead to significant damping of the anion signal relative to the cation signal at grazing emission. Jeon et al. also proposed, for [C4C1Im][BF4] and [C4C1Im][PF6], that the anion is located at the same level as the imidazolium ring; this conclusion fits well with our data presented here.31 A comparison can be directly made between the results obtained by us and those of Lockett et al. for [C8C1Im][BF4].16 Both experiments show an increase in the amount of Calkyl at increasing surface sensitivity. A smaller ratio, and consequently lower enhancement, was observed by Lockett et al. at an 80° electron emission angle (about 1.9 instead of 2.3 in our case).16 This difference is likely due to different fitting procedures employed (fitting either three or one peak for Chetero) and/or different acceptance angles of the corresponding electron analyzer. As noted elsewhere, ARXPS is not very sensitive to detailed geometry effects such as molecular orientation but is instructive for determining the depth distribution of certain elements, that is, the surface composition.5 Since the systems investigated are liquids and therefore the surface is mobile and fluid, the data recorded represent an average of the true situation at any time. In the most surface sensitive geometry at 80°, with an information depth of 1-1.5 nm, 65% of the XPS intensity arises from the first 0.3-0.5 nm, which is below the size of most of the IL ions studied herein. This high surface sensitivity allows one to derive information on the surface composition and to speculate on the arrangement of the molecules in the topmost layer. For all of the ILs in this study, the outer surface is dominated by the presence of alkyl carbon atoms compared to the ILs’ nominal composition. The strong implication therefore is that the topmost IL layer consists of cations oriented to a certain degree with the octyl chains preferentially protruding into the vacuum. The corresponding imidazolium rings (i.e., the charged cation head groups) are located, on average, below this aliphatic carbon overlayer and at the same or a slightly larger distance from the outer surface than the center of the anions (which is expected to coincide roughly with the center of negative charge). For the halide ILs and, in particular, [C8C1Im]Cl, this layered model, as schematically shown in Figure 4, fits our data particularly well, assuming the head groups of the cations and the anions form a confined layer, labeled as the “ionic sublayer” in Figure 4. The degree of packing and, therefore, the density of the polar groups depend on the size of the anions and, most

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Figure 4. Proposed model of the highly ordered IL-vacuum interface for [C8C1Im]Cl. The first molecular layer consists of octyl chains protruding mainly to the vacuum (called the “aliphatic carbon overlayer”), and underneath are the ionic parts of the cation and the chloride anions (called the “ionic sublayer”). For the larger anions, the first molecular layer is considerably less ordered (for details, see text). The indicated extension of 1.2 nm of the first molecular layer of [C8C1Im]Cl corresponds roughly to the information depth in our ARXPS setup for an electron emission angle of 80° relative to the surface normal.

likely, on the strength of interaction between the anion and imidazolium ring; highly coordinating anions such as Cl- are expected to bond more strongly to the imidazolium ring than anions such as [Tf2N]- and [FAP]-.49 The ionic region is therefore more ordered and compact for the smaller anions, imposing a dense packing of the aliphatic chains by attractive vdWs interactions between the aliphatic chains in the ILs, similar to the situation for aliphatic self-assembled monolayers adsorbed on surfaces.50,51 For the larger anions, the interaction between the charged groups (i.e., the imidazolium ring and the anion) is weaker. Within the first molecular layer, the ionic parts of the molecules are consequently not as well confined as those for the halides; thus, this degree of disorder in the ionic sublayer leads to aliphatic chains being located further apart, resulting in a reduction in vdWs interactions and in a loss of order for the aliphatic chains. We refrain from presenting a detailed description of the surface of an IL containing larger anions, as is given for [C8C1Im]Cl in Figure 4, because many configurations could lead to similar ARXPS findings. It must also be noted that the specific chemical moieties of the anions may orient themselves; for example, the perfluoroalkyl groups have been shown to orient themselves nearer to the surface than other parts of anions.5,52 Such orientation effects could also lead to a loss in surface enrichment of alkyl chains. Moreover, other factors expressed, for example, by Kamlet-Taft solvent parameters,53 may also describe the effect of the anions on the surface structure more accurately than simply the size of the anions. However, no sufficient data are available for the ILs investigated in this study for such comparisons to be made. Considering the complexity of the system, the proposed model is probably not the only one which is consistent with the experimental findings. As an alternative, one could envisage, for example, inhomogeneous lateral arrangements, with large islands of highly oriented alkyl chains separated by nonordered regions, which depend on the size of the anion. While we cannot unequivocally rule out such arrangements, our model seems to be the most simple and plausible. In the future, additional information could be obtained from other experimental methods and also from theoretical investigations. Additionally, the investigation of the surface composition of IL mixtures and IL solutions in combination with surface tension measurements will aid the understanding of the principles of surface formation in ionic liquid systems.54 4. Summary As an extension of our previous work on the influence of functionalized and nonfunctionalized chain substituents on the

surface composition of imidazolium-based [Tf2N]- ionic liquids, we systematically investigated the influence of the anion. Using ARXPS, nine different ILs with the same cation [C8C1Im]+ but very different anions were studied; to our knowledge, no study of surface-related properties with such a broad range of ILs is available. [C8C1Im]+ was chosen mainly for two reasons. First, a number of ILs with melting points near room temperature are available with the required surface cleanness (some of them were synthesized by us under very clean conditions); second, the C 1s signals of the octyl chain and of the ring carbon atoms are well separated in the ARXP spectra, which allows determination of the degree of chain enrichment at the surface. The investigated anions were chosen to cover different sizes (ranging from Cl- to [FAP]-), shapes (from spherical to elongated anions), coordination abilities (from strongly coordinating halides to weakly coordinating anions with perfluoroalkyl chains), and by the experimental fact that all of their related ARXPS signals do not interfere with carbon signals from the cation. All ILs were free from surface contaminations, and no significant beam damage occurred over the time scale of the ARXPS experiments reported here. At an emission angle of 0°, that is, for an information depth of 7-9 nm, the measured compositions of all ILs correspond to the nominal stoichiometry, suggesting that, on average, anions and cations are homogeneously distributed within this region. At 80°, that is, an information depth of 1.0-1.5 nm, the concomitant rise in C 1s intensity of aliphatic carbon at the expense of all other IL signals unambiguously proves surface enrichment of the octyl chains in the first molecular layer. This enrichment decreases with increasing anion size; it is most pronounced for Cl- and Brand is considerably less distinctive for the larger anions [Pf2N]and [FAP]-. In contrast, no significant surface segregation effects of the anions relative to the imidazolium rings (i.e., the positively charged head group of the cation) occur in the first molecular layer. In other words, this observation means that the ionic cation head groups and the anions are located at about the same distance from the outer IL surface, forming a more or less confined polar layer; the IL surface is terminated by the octyl chains to a different degree, depending on anion size. To explain the influence of the anion on the degree of octyl chain surface enrichment (which is related to the mean orientation of the cation at the surface), we propose that mainly the small size of the anion and the strength of the interaction between the polar groups lead to the formation of a densely packed, well-oriented surface layer, for example, in the case of [C8C1Im]Cl. For the larger anions, the packing density decreases, and the ionic region

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becomes more diffuse, leading to a loss in order and also a reduced enrichment of the octyl chains at the surface. 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. We thank Dr. Pete Licence and Alasdair Taylor for supplying the [C8C1Im]I sample and the Merck company for the [C8C1Im][FAP] sample. Moreover, we would like to thank Prof. Andreas Fro¨ba and Julia Lehmann for complementary density measurements and fruitful discussion of the results. Supporting Information Available: Quantitative analysis of the XP spectra of [C8C1Im][X]. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wasserscheid, T.; Welton, T. Ionic liquids in synthesis; WileyVCH: New York, 2008. (2) Plechkova, N. V.; Seddon, K. R. Chem. Soc. ReV. 2008, 37, 123. (3) Aliaga, C.; Santos, C. S.; Baldelli, S. Phys. Chem. Chem. Phys. 2007, 9, 3683. (4) Mezger, M.; Schroder, H.; Reichert, H.; Schramm, S.; Okasinski, J. S.; Schoder, S.; Honkimaki, V.; Deutsch, M.; Ocko, B. M.; Ralston, J.; Rohwerder, M.; Stratmann, M.; Dosch, H. Science 2008, 322, 424. (5) Lovelock, K. R. J.; Kolbeck, C.; Cremer, T.; Paape, N.; Schulz, P. S.; Wasserscheid, P.; Maier, F.; Steinruck, H. P. J. Phys. Chem. B 2009, 113, 2854. (6) Carvalho, P. J.; Freire, M. G.; Marrucho, I. M.; Queimada, A. J.; Coutinho, J. A. P. J. Chem. Eng. Data 2008, 53, 1346. (7) Endres, F.; El Abedin, S. Z. Phys. Chem. Chem. Phys. 2006, 8, 2101. (8) Baldelli, S. Acc. Chem. Res. 2008, 41, 421. (9) Zaitsau, D. H.; Kabo, G. J.; Strechan, A. A.; Paulechka, Y. U.; Tschersich, A.; Verevkin, S. P.; Heintz, A. J. Phys. Chem. A 2006, 110, 7303. (10) Armstrong, J. P.; Hurst, C.; Jones, R. G.; Licence, P.; Lovelock, K. R. J.; Satterley, C. J.; Villar-Garcia, I. J. Phys. Chem. Chem. Phys. 2007, 9, 982. (11) Leal, J. P.; Esperanca, J.; da Piedade, M. E. M.; Lopes, J. N. C.; Rebelo, L. P. N.; Seddon, K. R. J. Phys. Chem. A 2007, 111, 6176. (12) Smith, E. F.; Villar Garcia, I. J.; Briggs, D.; Licence, P. Chem. Commun. 2005, 5633. (13) Hofft, O.; Bahr, S.; Himmerlich, M.; Krischok, S.; Schaefer, J. A.; Kempter, V. Langmuir 2006, 22, 7120. (14) Smith, E. F.; Rutten, F. J. M.; Villar-Garcia, I. J.; Briggs, D.; Licence, P. Langmuir 2006, 22, 9386. (15) Silvester, D. S.; Broder, T. L.; Aldous, L.; Hardacre, C.; Crossley, A.; Compton, R. G. Analyst 2007, 132, 196. (16) Lockett, V.; Sedev, R.; Bassell, C.; Ralston, J. Phys. Chem. Chem. Phys. 2008, 10, 1330. (17) Gottfried, J. M.; Maier, F.; Rossa, J.; Gerhard, D.; Schulz, P. S.; Wasserscheid, P.; Steinruck, H. P. Z. Phys. Chem. 2006, 220, 1439. (18) Kolbeck, C.; Killian, M.; Maier, F.; Paape, N.; Wasserscheid, P.; Steinruck, H. P. Langmuir 2008, 24, 9500. (19) Krischok, S.; Eremtchenko, M.; Himmerlich, M.; Lorenz, P.; Uhlig, J.; Neumann, A.; Ottking, R.; Beenken, W. J. D.; Hofft, O.; Bahr, S.; Kempter, V.; Schaefer, J. A. J. Phys. Chem. B 2007, 111, 4801.

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