Surface Science 604 (2010) 1694–1697

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Surface Science

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Surface segregation in binary mixtures of imidazolium-based ionic liquids

Ryutaro SoudaInternational Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

E-mail address: SOUDA.ryutaro@nims.go.jp.

0039-6028/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.susc.2010.06.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 February 2010Accepted 21 June 2010Available online 27 June 2010

Keywords:Secondary ion mass spectroscopySurface segregationLiquid surfaces

Surface composition of binary mixtures of room-temperature ionic liquids has been investigated using time-of-flight secondary ion mass spectrometry at room temperature over a wide composition range. Theimidazolium cations with longer aliphatic groups tend to segregate to the surface, and a bis(trifluoromethanesulfonyl)imide anion (Tf2N

−) is enriched at the surface relative to hexafluorophosphate(PF6−). The surface of an equimolar mixture of Li[Tf2N] and 1-butyl-3-methylimidazolium hexafluoropho-sphate ([bmim][PF6]) has a nominal composition of [bmim][Tf2N] because of surface segregation and ligandexchange. The surface segregation of cations and anions is likely to result from alignment of specific ligand-exchanged molecules at the topmost surface layer to exclude more hydrophobic part of the molecules.

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1. Introduction

Room temperature ionic liquids (RTILs) are a new class ofchemicals that have recently prompted a significant amount ofresearch. They are composed solely of ions, and destabilization ofthe ionic lattice by asymmetrically shaped cations and anions isthought to lower the melting point of RTILs relative to that of typicalionic compounds. The unique physicochemical properties of RTILs,including high thermal stability, high ionic conductivity, andnegligible vapor pressure, are ideal for a number of applications as asolvent for green chemical synthesis [1–3] and a conductive media forlithium ion batteries [4,5] and fuel cells [6,7]. For these practicalapplications, the interfacial properties of RTILs are quite importantbecause chemical reactions are first catalyzed by the species at theinterface. To date, the structure and composition of liquid-vacuum(air) interface of imidazolium-based RTILs have been studiedextensively using a variety of surface sensitive techniques, such assum frequency generation (SFG) [8–11], direct recoil spectrometry(DRS) [12,13], X-ray photoelectron spectroscopy (XPS) [14–17], time-of-flight secondary ion mass spectrometry (TOF-SIMS) [14,18–20],metastable impact electron spectroscopy (MIES) [21,22], highresolution electron energy loss spectroscopy (HREELS) [21], andhigh-resolution Rutherford backscattering spectroscopy (HRBS)[23,24]. A consensus has been made that both cations and anionsare sharing the surface, but there exist conflicting reports on theorientation of the imidazolium ring [8–13]. Molecular dynamics (MD)simulations showed that cation is aligned with the imidazolium ringbeing parallel to the surface and that the longer aliphatic chains of theimidazolium cations are likely to protrude outside from the surface

[25,26]. The predicted alignment is in good agreement with theexperimental results of SFG [8–11], angle-resolved XPS [16,17], andHRBS [23,24]. The surface enrichment of aliphatic carbon was alsoobserved for alkyl chains attached to the anion [17]. The surfacecomposition of a Pt salt additive ([Pt(NH3)4]Cl2) in 1-ethyl-3methy-limidazolium ethylsulfate ([emim][EtOSO3]) has been analyzed usingangle-resolved XPS (AR-XPS) [27]; it was revealed that the [Pt(NH3)4]+

(Cl−) ion is enriched (depleted) in the near surface region, suggestingthat larger and more polarizable ions tend to segregate to the surface.This phenomenon resembles that observed previously for aqueoussolutions of alkali halides and other ionic compounds [28–30]. Minorimpurities, such as silicone and hydrocarbons, also tend to segregate tothe surface of RTILs [14,31–34]. Thus, the surfaces of pure and slightlycontaminated RTILs have been studied extensively, but no systematicstudies on the surface structure and composition of RTIL mixtures existto the best of the author's knowledge. This paper is devoted to theanalyses of surface segregation of cations and anions in some binarymixturesof RTIL basedonTOF-SIMS. TOF-SIMS is oneof themost surfacesensitive techniques that probe the topmost surface layer, so that it isexpected that the surface enrichment of specific cation and anionmoieties, if any, can be investigated straightforwardly based on thesputtered ion intensities as a function of their composition in the bulk.The interactions underlying surface segregation of both cations andanions in concentrated and dilute solutions are discussed, togetherwitha ligandexchange phenomenon. To this end, the cations interactions areinvestigated specifically using mixtures of 1-octyl-3-methylimidazo-liumhexafluorophosphate ([omim][PF6])with [emim][PF6] and [omim]tetrafluoroborate ([omim][BF4]) with [emim][BF4], together with theanions interactions in a mixture of 1-butyl-3-methylimidazolium bis(trifluoromethanelsulfonil)imide ([bmim][Tf2N]) with [bmim][PF6].The surface composition of Li[Tf2N] in [bmim][PF6] is also investigatedto explore the possibility of ligand exchange.

1695R. Souda / Surface Science 604 (2010) 1694–1697

2. Experimental

RTIL samples were purchased from Kanto Regents (Tokyo, Japan).They were used as received without further purification. The weighedsamples were diluted in methanol at a concentration of 0.2 mmol/ml.The concentrations of [omim][PF6], [bmim][Tf2N], and Li[Tf2N] werefurther reduced by serial dilutions. The RTIL mixtures (from 1:1 to 1:3000) were prepared by blending these methanol solutions. Aftercomplete mixing, aliquots of 20 μL were spin-coated (5000 rpm for30 s) onto a mirror-finished polycrystalline Ni plate. Prior to spin-coating, the Ni substrate was cleaned with methanol in an ultrasonicbath. The samples were inserted immediately into a load-lockchamber and degassed at room temperature. After degassing thesample to attain the vacuum in 10−5 Pa range, it was transferred to anultrahigh vacuum (UHV) chamber.

The TOF-SIMS measurements were performed in the UHVchamber by excitation of the samples with a He+ beam (2 keV) thatwas generated in a differentially-pumped electron-impact-type ionsource. The dc primary ion intensity at the sample was 20 nA, and theoperation spot size was ca. 5 mm in diameter. The beamwas choppedinto pulses (40 ns in width and 20 kHz in repetition rate) usingelectrostatic deflection plates. The normal working pressure duringthe TOF-SIMS measurement was 1×10−8 Pa, but the presentexperiment was performed in a 10−7 Pa range. The sample wasbiased and a grounded stainless steel mesh was placed close to thesample surface (extraction field of 125 V/mm). The secondary ionswere pulse counted using a microchannel plate after travelingthrough a field-free linear TOF tube; the TOF-SIMS spectrum wascreated using a multichannel scaler. The measurements were made atroom temperature within a beam fluence of less than 1011 ions cm−2.

3. Results and discussion

Fig. 1 shows positive TOF-SIMS spectra obtained from spin-coatedfilms of (a) [emim][PF6], (b) [omim][PF6], and (c) their equimolarmixture. The emim+ and omim+ cations (m/z=111 and 195) aresputtered together with fragment ions. The emim+ cation iscomparable to or greater than the fragment ions in intensity whereasthe omim+ intensity is considerably small relative to the fragment ion

Fig. 1. TOF-SIMS spectra of positive ions sputtered from spin-coated thin films of(a) [emim][PF6], (b) [omim][PF6], and (c) their 1:1 mixture.

intensities from the octyl chain (e.g., C2H3+, C2H5

+, C3H3+, C3H5

+, andC3H7

+). This behavior might be explained as the exposure of thealiphatic chain to the vacuum: The emission of the imidazolium cationtends to be self-blocked by its long side chain protruding to thevacuum side. The spectral features from their mixture resemble thoseof pure [omim][PF6]; only a very small amount of the emim+ cation isrecognizable. The amount of emim+ in the topmost layer is at most 2–3% of that of omim+ as estimated from the deconvolution of the TOF-SIMS spectrum. The depletion (enrichment) of emim+ (omim+) in thetopmost layer is thought to be driven by the exclusion of the longeraliphatic chain of the imidazolium cation from the bulk. The similarphenomenon is observed using equimolarmixture of [bmim][PF6] and[emim][PF6] (not shown). In this case, the surface composition ofemim+ is estimated to be about 1/3 of that of bmim+. Thus, the cationcoverage in the topmost layer of the binary mixtures is found to bedependent on the length of aliphatic chains.

In the present experiment, some of the spin-coated films are notuniform in thickness as evidenced by that Ni+ (m/e=58 and 60) issputtered from the substrate to some extent (see Fig. 1(a)). The Ni+

ion is recognizable for the RTIL films except for [omim][PF6] and[omim][BF4]. The same occurs when pure RTILs are deposited insteadof theirmethanol solutions. In general, themorphology of a thin liquidfilm changes inevitably with time because of the surface tension andinteraction with the substrate. The morphological change of the spin-coated films can be identified from the disappearance of lightinterference patterns after the TOF-SIMS measurement. [omim][PF6]and [omim][BF4] can avoid the morphological change because ofhigher viscosity. The secondary ion intensities are likely to undergothe film morphology effect, but the TOF-SIMS spectra from theequimolar mixtures of [omim][PF6]/[emim][PF6] and [bmim][PF6]/[emim][PF6] can be deconvoluted using those of the pure RTILs. In thisstudy, therefore, the ion intensities from RTIL mixtures are compareddirectly to each other because they are expected to provideinformation about how the surface coverage of cations and anionschanges as a function of bulk composition.

In Fig. 2 are shownnegative TOF-SIMS spectra from the spin-coatedfilms of (a) [bmim][Tf2N], (b) [bmim][PF6], and (c) their 1:10mixture.The spectra are dominated by fragment ions, such as C−, CH−, O−, F−,C2−, C2H− and C2H2

−; intact PF6− (m/e=145) and Tf2N− (280)

Fig. 2. TOF-SIMS spectra of negative ions sputtered from spin-coated thin films of(a) [bmim][Tf2N], (b) [bmim][PF6], and (c) their 1:10 mixture.

Fig. 4. TOF-SIMS intensities of PF6−andTF2N− ions sputtered frommixtures of [bmim][ PF6]and [bmim][ Tf2N] as a function of relative concentration of the latter.

1696 R. Souda / Surface Science 604 (2010) 1694–1697

anions are also sputtered considerably. The intensity of the Tf2N−

anion is enriched relative to that of the PF6− anion for the mixture,indicating that the larger anion is concentrated at the topmost surfacelayer. This tendency is consistent with the surface segregation ofcations.

The surface coverage of cations in the mixtures of [emim][BF4] and[omim][BF4] is investigated as a function of their composition in thebulk; the TOF-SIMS intensities are displayed in Fig. 3. The enrichmentof the omim+ cation at the surface is clearly recognizable. The surfacesegregation of the Tf2N− anion is also observed for the [bmim][Tf2N]/[bmim][PF6] mixtures as shown in Fig. 4. Although the data points inboth measurements are scattered because of the film morphologyeffect etc., the surface coverage of the cations and anions can beroughly estimated from their intensities relative to those of the purefilms. The topmost surface layer of the diluted solution containing 1–2% of [omim][BF4] ([bmim][Tf2N]) consists of approximately equalamount of the omim+ and emim+ cations (Tf2N− and PF6− anions).For a more diluted solution (0.1%), the omim+ cation and Tf2N− anionare enriched at the surface with coverage of 5–7%.

Fig. 5 shows TOF-SIMS intensities of (a) cations and (b) anionssputtered from the mixture of [bmim][PF6] and Li[Tf2N]. The bmim+

intensity is almost constant over a wide concentration range of Li[Tf2N]. This result shows strong preference of the bmim+ cation in thetopmost layer. The Li+ ion is thought to be expelled from the surface,but its intensity is considerably high because lighter ions aresputtered more efficiently. The small size of Li+ also enables ejectionfrom deeper layers (the escape depth of Li+ is more than 10monolayers when water adsorbs on a LiI film [35]). It is expectedthat the concentration in the subsurface sites is fundamentallyidentical to that in the bulk, so that a linear decay of the Li+ intensityreflects its bulk concentration. On the other hand, the TOF-SIMS anionintensity clearly shows that Tf2N− is enriched at the surface. Theresults are similar to those of the [bmim][Tf2N]/[bmim][PF6] mixturesshown in Fig. 4, but the intensity of Tf2N− at higher Li[Tf2N]concentration rather decreases. This result is explained as the countercation effect: Tf2N− gains smaller kinetic energy during collisionswith the lighter Li+ ion, suggesting that the Li+ ion is present in thesecond layer forming a Li+–Tf2N− bond. In any case, the steepdecrease of PF6− relative to Tf2N− in intensity with increasing Li[Tf2N]concentration is attributable to the enrichment of the Tf2N− anion on

Fig. 3. TOF-SIMS intensities of emim+ and omim+ ions sputtered from mixtures of[emim][BF4] and [omim][BF4] as a function of relative concentration (in mole unit) ofthe latter.

the topmost surface. Consequently, the surface of the equimolarmixture tends to have a nominal composition of [bmim][Tf2N]. Theapparent difference in evolutions between the Li+ and Tf2N− (orbmim+ and PF6−) ions strongly suggests that ligand exchange occursin the mixtures.

From these experimental results, it is clarified that surfacesegregation of both cations and anions of RTILs is controlledfundamentally by their size. The mechanism underlying this phe-nomenon seems identical to that proposed for aqueous and othersolutions of ionic compounds [28–30]: The cation and anion moietiesthat have a large volume or polarizability are expelled preferentially

Fig. 5. TOF-SIMS intensities of (a) cations and (b) anions sputtered from mixtures of[bmim][ Tf2N] and Li[TF2N] as a function of relative concentration of the latter.

1697R. Souda / Surface Science 604 (2010) 1694–1697

from the bulk to the interface. A similar phenomenon has also beenobserved for the dilute [Pt(NH3)4]Cl2 solution in [emim][EtOSO3] [27].In contrast to these ionic compounds, however, the interactionbetween the structured cations and anions in RTILs is moreanisotropic because of the presence of aliphatic and CF3 groups.They are expected to play a role in the intermolecular interaction viavan der Waals force and hydrogen bond. In fact, the thermodynamicproperties of RTILs resemble those of polar molecules like waterrather than simple ionic compounds [36,37], suggesting the impor-tance of non-ionic intermolecular interactions. As demonstrated inthe present study, the length of the aliphatic chain is a key ingredientfor determining the cation coverage at the topmost layer of binarymixtures. In this respect, molecular ordering is known to occur at thesurfaces of pure RTILs by protrusion of the aliphatic chain into thevacuum side [16,17,23–26]; charged parts of the structured cation andanion are likely to form an “ionic sublayer” [38]. The decrease of theintact cation intensities relative to the fragment ion intensities withincreasing aliphatic chain length (Fig. 1) might also be an indication ofthe cation alignment in the topmost layer because ejection of theimidazolium cations might be disturbed by their long aliphatic chain.On the other hand, the strong affinity of the Tf2N− anion for thesurface relative to the PF6− anion is expected to be caused by thepresence of the CF3 group. Indeed, the CF3+ and CF+ ions are sputteredfrom the pure [emim][Tf2N] film [18], and CF3− ion (m/e=69) isobserved intensively for both pure [bmim][Tf2N] and its mixture with[bmim][PF6] (see Fig. 2), suggesting that the CF3 group is facingtowards the vacuum side. This result is consistent with the recentHRBS measurement [23] which revealed that the Tf2N− anion intrimethylpropylammonium [Tf2N] has a cisoid conformation withtheir CF3 end groups pointing toward the vacuum side in theoutermost layer. Thus, the present and other studies strongly suggestthat RTIL is distinct from a simple “molten salt” state of free cationsand anions; nonionic interactions between the end groups of the polarmolecules is thought to play an important role in surface segregation.The surface tends to be more hydrophobic by exposing the aliphaticand CF3 groups to the vacuum side and concealing the charged part ofthe molecules, as well as the Li+ ion, in the subsurface layer. Themolecular species can be converted dynamically in the bulk as a resultof ligand exchange. Consequently, surface segregation of the specificcations and anions results from enrichment and alignment of thespecific ligand-exchanged molecules at the topmost surface layer,which is driven by exclusion of larger and more hydrophobic endgroups from the bulk. The minor impurities of hydrocarbons can besegregated to the surface by the same mechanism [34]. For surfacesegregation of silicone impurities [14], their chemical compositionshould be clarified because the end groups rather than silicon mayplay a role.

4. Summary

The surface segregation of cations and anions in binary mixtures ofsome RTILs has been investigated using TOF-SIMS. The composition ofomim+ at the surface is enriched by approximately 50 times relativeto that in the bulk for the dilute solution (0.1%) of [omim][BF4] in[emim][BF4]. Similar enrichment of Tf2N− is observed for the 0.1%

solution of [bmim][Tf2N] in [bmim][PF6]. It is thus found that largerand polarizable ions tend to occupy the topmost surface layer. Theresults resemble those of aqueous and other solutions of simple ioniccompounds. However, the enrichment of larger cations and anions atthe surface is thought to be driven by ordering of specific ligand-exchanged molecules to exclude their hydrophobic part (e.g., longeraliphatic chains and CF3 group) from the bulk.

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