Cation Exchange at the Interfaces of Ultrathin Films of Fluorous Ionic Liquids on Ag(111).

In the context of applications with thin ionic liquid (IL) films on solid supports, we studied the ion distribution within mixed thin IL films by angle-resolved X-ray photoelectron spectroscopy. After the deposition of 1-methyl-3-octylimidazolium hexafluorophosphate, [C8C1Im][PF6], on top of a wetting layer (WL) of 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate, [PFBMIm][PF6], on Ag(111) at room temperature (RT), we find a preferential enrichment of the [PFBMIm]+ cation at the IL/vacuum interface. In a similar deposition experiment at 82 K, this cation exchange at the IL/solid interface does not occur. Upon heating the film from 82 K to RT, we observe the replacement of [C8C1Im]+ by [PFBMIm]+ at the IL/vacuum interface between ∼160 and ∼220 K. No further changes in the surface composition were observed between 220 K and RT. Upon further heating the mixed IL film, we find the complete desorption of [PFBMIm][PF6] from the mixed film below 410 K, leaving a WL of pure [C8C1Im][PF6] on Ag(111), which desorbs until 455 K.

Introduction

Thin films of ionic liquids (ILs) have been the focus of ultrahigh vacuum (UHV) surface science in the past 10 years as they provide powerful ways for molecular level studies of liquid/solid interfaces in general.[1−10] The detailed knowledge of the structure and the formation of the IL/solid interface, in particular, enables a more complete description and control of the interface properties and the system’s overall stability and performance in applications where ILs are in contact with solid surfaces, such as catalysis,[11,12] sensors,[13] lubrication,[14,15] separation,[16,17] and electrochemistry.[18]

Physical vapor deposition (PVD) has proven to be a suitable route for the preparation of ultrathin IL films from the submonolayer range up to several tens of nanometers.[1,3,5,6,10] At temperatures of 140 K and below, scanning tunneling microscopy (STM) provides a potent way to elucidate IL island growth and surface order on solid supports for submonolayer coverages.[3,4,19,20] For higher temperatures, where no long-range ordered layers are formed, or for coverages beyond the first layer, spectroscopic methods such as angle-resolved X-ray photoelectron spectroscopy (ARXPS)[1,2,7,8] or reflection absorption infrared (IR) spectroscopy[5,6] are especially powerful to study these highly dynamic liquid systems. In particular, ARXPS proved itself as a well-established method to investigate the IL/support interactions, molecular enrichment and orientation at the interfaces, wetting behavior, and the IL film growth from a submonolayer to multilayers.[1,7,8,10,21−23]

In light of more complex IL systems, composed of more than one type of ion pairs, we recently studied thin films of IL mixtures by a sequential deposition of two ILs with the same cation and two different anions, 1-methyl-3-octylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [C8C1Im][Tf2N], and 1-methyl-3-octylimidazolium hexafluorophosphate, [C8C1Im][PF6].[8] Upon deposition of [C8C1Im][PF6] onto a wetting layer (WL) of [C8C1Im][Tf2N] on Ag(111), the replacement of [Tf2N]− by [PF6]− was observed by ARXPS. We proposed that two driving forces are cooperatively responsible for the exchange of the anions, that is, the larger adsorption energy of [C8C1Im][PF6] at the IL/Ag interface and the lower surface tension of [C8C1Im][Tf2N] at the IL/vacuum interface.[8]

As we have already studied mixed films of ILs with two different anions and the same cation, mixtures of ILs with two different cations and the same anion ([PF6]−) were the subject of the present work. One cation is the previously investigated [C8C1Im]+ (see Figure a). The other cation is 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium, [PFBMIm]+, a dialkylimidazolium cation with a perfluorinated ethyl end group at the butyl side chain (see Figure b). In the context of fluorous phases in ILs, first reports of imidazolium-based ILs with fluorinated alkyl side chains, for example, by Merrigan et al., highlighted their use as surfactants, as they promote the formation and stabilization of dispersions of perfluorocarbons in conventional (nonfluorinated) ILs.[24] Perfluoroalkylation is also reported to generally improve the tribological behavior, for example, by decreasing viscosity and increasing hydrophobicity.[14]

Figure 1

Molecular structures and van der Waals models of (a) 3-methyl-1-octylimidazolium hexafluorophosphate, [C8C1Im][PF6], and (b) 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate, [PFBMIm][PF6].

In addition to the growth and interface composition of mixed thin films of [PFBMIm][PF6] and [C8C1Im][PF6] deposited in UHV by PVD on Ag(111), we also discuss the stability of pure and mixed IL thin films toward desorption. Furthermore, we address the roles of surface free energy versus interface adsorption energy by comparing the surface compositions in the thin film and the macroscopic mixture systems. Such studies play an important role with respect to the nanoscale applications of IL films at elevated temperatures.[8,9,25,26]

Experimental Section

[C8C1Im][PF6] was purchased from Sigma-Aldrich (purity > 95%). [PFBMIm][PF6] was prepared via a two-step reaction: in the first step, [PFBMIm]I was synthesized by the alkylation of 1-methylimidazole with 1,1,1,2,2-pentafluoro-4-iodobutane. The target compound was then obtained through ion metathesis using an aqueous potassium hexafluorophosphate solution. A detailed synthesis procedure alongside spectroscopic and thermal data is available in the Supporting Information. For reference measurements, we also prepared bulk mixtures of [PFBMIm][PF6] and [C8C1Im][PF6] with different compositions. Acetonitrile (Sigma-Aldrich, purity 99.8%) was used as a cosolvent to ensure proper mixing of the respective ILs.[8,27] These IL mixtures were spread as macroscopic films (∼0.1 mm thickness) on polycrystalline Ag foil and carefully degassed before introducing them into the vacuum chamber.

For our measurements on the ultrathin layers, we deposited well-defined amounts of the two different ILs onto the Ag(111) crystal (15 mm diameter, 2 mm thickness) via PVD using two effusion cells, developed in our group explicitly for IL deposition.[8] We checked the IL flux during PVD using a quartz crystal microbalance. To remove volatile impurities, the ILs were thoroughly degassed in UHV for more than 24 h at evaporator temperatures between 370 and 460 K. The evaporation temperatures ranged from 423 to 443 K for [C8C1Im][PF6], and from 443 to 463 K for [PFBMIm][PF6]. During deposition, the chamber background pressure was below 2 × 10–9 mbar for [C8C1Im][PF6] and below 5 × 10–9 mbar for [PFBMIm][PF6]. According to the literature, aprotic ILs evaporate as discrete neutral ion pairs.[3,9,25,26,28,29] Compared to the spectra of macroscopically thick IL films prepared ex situ, no signs of decomposition were observed in the X-ray photoelectron (XP) spectra of the films prepared by PVD (see Table S1 in the Supporting Information).

The experiments were performed using ARXPS in a UHV system, which has been described previously.[7] The ARXP spectra were acquired with a nonmonochromated Al Kα X-ray source (SPECS XR 50, 1486.6 eV, 240 W) and a hemispherical electron analyzer (VG SCIENTA R3000). We used a pass energy of 100 eV, which resulted in an overall energy resolution of ∼0.9 eV.[7] Peak fitting and background subtraction were done using CasaXPS V2.3.16Dev6. For the Ag 3d and F 1s peaks, a Shirley background,[30] for the P 2p peaks a two-point linear background, and for the C 1s peaks a three-point linear background were subtracted. For the N 1s spectra, an additional background was subtracted to account for the overlapping shake-up and plasmon satellites and inelastically scattered electrons of the Ag 3d core levels[31−34] (for further details, see ref (7)). The IL spectra were fitted with a Voigt profile (30% Lorentzian contribution). For the thin-film experiments, the binding energies were referenced to the Ag Fermi edge, yielding a value of 368.2 eV for the Ag 3d5/2 level. The XP spectra of the macroscopic film of [PFBMIm][PF6] were referenced to the binding energy of Chetero at 287.0 eV. For temperature-dependent XPS, the sample was heated continuously during the measurements at a rate of 2 K/min. The acquisition time for F 1s spectra ranged from 2 to 5 min, for C 1s spectra from 5 to 10 min, and for Ag 3d spectra from 1 to 2 min.

The coverage and growth of IL films were characterized by measuring the attenuation of the Ag 3d signal of the crystal at the emission angles of ϑ = 0° and 80° relative to the surface normal. When using Al Kα radiation, the information depth (ID) at normal emission ϑ = 0° is 7–9 nm (depending on the kinetic energy). At ϑ = 80°, ID is only 1–1.5 nm, which means that only the topmost surface layers are probed. Thereby, ID is defined as 3 times the inelastic mean free path, λ, of the electrons.[8] For Ag 3d electrons with a kinetic energy of about 1.1 keV, λ is 2.5 nm in IL films.[7,8] For a homogeneous two-dimensional growth, the Ag 3d signal at an angle ϑ decreases from I0 for the clean surface to I for a film of thickness d, according to I/I0 = exp(−d/(λ·cos ϑ)).[2,7,8]

In accordance with previous publications, coverages are given in ML; thereby, we define 1 ML as a closed double layer of ions, irrespective of their relative arrangement.[1,7−9] The monolayer height is calculated as the cubic root of the molecular volume Vm based on mass density values from the literature. Using the literature value for Vm, hML of [C8C1Im][PF6] was calculated to be 0.77 nm.[8] The hML value of [PFBMIm][PF6] was estimated to be 0.71 nm, by comparison of the value of 0.70 nm for [C4C1Im][PF6] (calculated from its known mass density) and the increase of Vm upon the exchange of hydrogen by fluorine.[35]

Results and Discussion

Ion Exchange in Mixed Films at RT

We start with analyzing the ARXP spectra collected at 0° and 80° emission angles after the successive deposition of [PFBMIm][PF6] and [C8C1Im][PF6] on Ag(111) at room temperature (RT). The corresponding C 1s and F 1s spectra are depicted in Figure .

Figure 2

F 1s and C 1s spectra measured at 0° (left) and 80° (right) for clean Ag(111) (I), after the deposition of 0.5 ML of [PFBMIm][PF6] (II) on Ag(111), and after the subsequent deposition of 1.0 ML of [C8C1Im][PF6] on top of the existing WL of [PFBMIm][PF6] (III) at RT.

The very different chemical environments of the fluorine atoms in the [PFBMIm]+ cation and the [PF6]− anion (of both ILs) yield a distinct chemical shift of the respective F 1s binding energy, leading to two clearly distinguishable peaks in the F 1s region. For [PFBMIm][PF6], the FCF peak at 689.1 eV corresponds to the five fluorine atoms in the fluorinated alkyl chain of the [PFBMIm]+ cation, and the FPF peak at 686.9 eV corresponds to the six fluorine atoms of the [PF6]− anion. For [C8C1Im][PF6], only the FPF peak at 686.9 eV is observed. Similarly, the C 1s spectra show distinct binding energy differences for the atoms with different chemical environments. For [C8C1Im][PF6], the Chetero peak at 286.9 eV corresponds to the five carbon atoms next to the nitrogen atoms in the cation, and the Calkyl peak at 285.3 eV corresponds to the remaining seven atoms of the alkyl chain.[8] For [PFBMIm][PF6], three peaks are observed: the CCF peak at 293.7 eV and the CCF peak at 291.3 eV because of the carbon atoms in the fluorinated alkyl chain and the Chetero peak at 286.9 eV containing the signals of the remaining six carbon atoms, including that of the carbon atom next to the perfluorinated ethyl end group of the chain. The differences in binding energy allow for a quantitative analysis of the IL stoichiometry and for a direct comparison of the relative occurrence of the respective ions in the bulk and at the interfaces of the ultrathin films of ILs. Changes in the signal intensity upon variation of the surface sensitivity in ARXPS (i.e., comparing the spectra at 0° and 80°) directly reflect the enrichment or depletion of the corresponding ions (or specific substituents at the molecular ions) at the IL/vacuum interface.

As a first step, we deposited 0.5 ML of [PFBMIm][PF6] onto Ag(111) at RT (Figure II). At this coverage, we obtain a closed WL with a so-called checkerboard arrangement of alternating anions and cations, as schematically depicted in Figure II. This arrangement in a homogeneous WL was derived from detailed coverage-dependent studies of the growth behavior of [PFBMIm][PF6] on Ag(111) from submonolayer to multilayer coverages (see Figure S1 in the Supporting Information). A similar WL with checkerboard arrangement was observed for thin films of [C8C1Im][PF6] on Ag(111)[8] and for other ILs in low-temperature STM.[4] In this homogeneous WL, both the cation and the anion are directly in contact with the metal surface. For the F 1s signals, the ratio FCF/FPF of 0.95 (with an estimated error of ±15%) at 0° is close to the nominal ratio of the five FCF atoms in the cation to the six FPF atoms in the anion (5/6 = 0.83). The ratio of the signals at 80° (which are 6 times more surface-sensitive than that at 0°) contains information on the orientation of the ions within the WL. The FCF/FPF ratio of 1.58 reveals a strong enhancement of the cationic FCF signal compared to the anionic FPF signal, which indicates a pronounced preferential orientation of the pentafluorobutyl (PFB) side chains toward the vacuum side, that is, away from the metal surface. In the C 1s spectrum at 80° (Figure II), the CCF/Chetero ratio of 0.68 is much larger than the nominal ratio of 0.33 for the two fluorinated CCF (=CCF + CCF) atoms of the PFB chain and the six Chetero atoms of the cation, providing further support for the proposed preferential orientation of the PFB side chains toward the vacuum side. Notably, we also observe a pronounced surface enrichment of the PFB side chain at the IL/vacuum interface of macroscopic films (see Table S1 in the Supporting Information) with the FCF/FPF ratios of 0.95 at 0° and 1.44 at 80°.

Figure 3

Schematic view of the film structure after deposition of [C8C1Im][PF6] onto a WL of [PFBMIm][PF6] on Ag(111) at RT. The Roman numerals correspond to the spectra in Figure .

Upon subsequent deposition of 1.0 ML of [C8C1Im][PF6] on top of the WL of [PFBMIm][PF6] at RT, we obtain the spectra depicted in Figure III. At 0° emission, we observe with FCF/FPF = 0.35 a value close to the nominal ratio of 0.28, as calculated from the molar fractions in the mixed film. At 80° emission, however, we observe a much higher ratio of FCF/FPF = 0.82, which again indicates a strong enrichment of the PFB chains at the IL/vacuum interface. This observation is quite surprising at first sight, as prior to the deposition of [C8C1Im][PF6], the [PFBMIm]+ cations were in direct contact with the Ag(111) surface. Our data thus indicate an immediate (on the timescale of our experiment) ion exchange at the IL/solid interface during the deposition of [C8C1Im][PF6]; for a schematic sketch, see Figure III. A similar behavior was recently discovered for the deposition of [C8C1Im][PF6] onto a WL of [C8C1Im][Tf2N] on Ag(111).[8] Here, the [Tf2N]− anions were initially also directly in contact with the IL/Ag interface, but after the subsequent deposition of [C8C1Im][PF6], the [Tf2N]− anions enriched at the IL/vacuum interface of the mixed film. For this system, the larger adsorption energy of [C8C1Im][PF6] at the IL/Ag interface and the lower surface tension of [C8C1Im][Tf2N] at the IL/vacuum interface were made responsible as cooperatively acting driving forces.[8] For the system studied here, we propose the same driving forces to be the deciding factors for the preferential enrichment of PFB at the vacuum interface. The surface tensions of fluorinated ILs are generally considerably lower than that for nonfluorinated ILs.[24,36,37] Although we are not aware of the quantitative determination of the surface tension of [PFBMIm][PF6], we found a pronounced preferential surface enrichment of the PFB side chains also for the macroscopic films of the same mixtures (Figure S4). This is taken as strong evidence for the lower surface tension of [PFBMIm][PF6] as compared to that of [C8C1Im][PF6].

When analyzing the C 1s spectra, we find a CCF/Calkyl ratio of 0.23 at 80°, which is larger than the nominal ratio of 0.14, thus indicating a slight preferential surface enrichment of the PFB groups over the longer octyl chains in this mixed thin film. When comparing the two chain signals to the Chetero signal at 80°, we find that both the CCF/Chetero ratio of 0.23 (nominally 0.13) and the Calkyl/Chetero ratio of 1.01 (nominally 0.88) are larger than the nominal values. This observation indicates that the PFB and the octyl chains are both enriched at the IL/vacuum interface compared to the cationic imidazolium head group.

Adsorption at 82 K and Subsequent Evolution with Temperature

To investigate the dynamics of the ion-exchange process in the mixed thin films in detail, we deposited IL films at a lower temperature, that is, at 82 K on Ag(111). The spectra are depicted in Figure . Figure shows a scheme of the IL film on the surface for the different steps of the experiment. As a first step, we deposited a WL of [PFBMIm][PF6] (Figure II). In the F 1s spectra at 0°, we observe a ratio of FCF/FPF = 0.70 (±15%), which, within the margin of error, agrees with the nominal ratio of 0.83. At 80°, the ratio FCF/FPF = 1.10 indicates a surface enrichment of the PFB side chain in the WL. However, the fact that the ratio for 82 K is smaller than the value of 1.58 found at RT (see above) indicates a less pronounced surface enrichment at the lower adsorption temperature. Accordingly, in the C 1s spectrum at 80°, the ratio CCF/Chetero of 0.58 (nominal 2/6 = 0.33) was also lower than the value of 0.68 for the WL at RT. The less pronounced preferential orientation is likely due to a “hit-and-stick” adsorption at this low temperature, that is, after arrival, the ion pairs neither move on the surface nor do they assume the optimum adsorption geometry.

Figure 4

F 1s and C 1s spectra at 0° (left) and 80° (right) emission for clean Ag(111) (I), after the deposition of 0.5 ML of [PFBMIm][PF6] at 82 K (II), after the deposition of 1.0 ML of [C8C1Im][PF6] on top of the existing film of [PFBMIm][PF6] at 82 K (III), and after heating the composite IL film to RT (IV). The arrows highlight changes in peak intensities compared to the respective spectrum below.

Figure 5

Schematic view of the film structure in the heating experiment after the deposition of [C8C1Im][PF6] onto a WL of [PFBMIm][PF6] on Ag(111) at 82 K. The Roman numerals correspond to the spectra in Figure .

In the second step, we deposited 1.0 ML of [C8C1Im][PF6] on top of the WL of [PFBMIm][PF6] (Figure III). In the F 1s spectra at 0°, the FCF/FPF ratio of 0.28 is in accordance with the nominal ratio of 0.28. At 80°, the strong attenuation of the CCF and FCF signals of [PFBMIm]+, as compared to the first deposition step (II), shows that the [C8C1Im][PF6] layer covers the WL of [PFBMIm][PF6] homogeneously (Figure III). This behavior demonstrates that no ion exchange at the IL/Ag interface occurs because of the lack of mobility at this low temperature.

Next, we investigated the development of the composite layer as a function of temperature by heating this film at a rate of 2 K/min from 82 K to RT, while measuring the XP spectra in situ. The final spectra at RT are depicted in Figure IV; a selected set of spectra acquired at 80° emission during heating is shown in Figure S5 in the Supporting Information. The graphs in Figure are the result of the quantitative analysis of the individual C 1s (only Calkyl and Chetero signals; CCF is not shown because of the low signal-to-noise ratio) and F 1s (FCF and FPF) signals in 80° emission during heating.

Figure 6

Thermal evolution of the respective F 1s and C 1s signals at 80° upon continuous heating of the layered IL film to RT after the deposition of 1.0 ML of [C8C1Im][PF6] on top of a previously deposited WL of [PFBMIm][PF6] on Ag(111) at 82 K. Heating rate, 2 K/min.

Upon heating, we observe a decrease of the FPF signal (blue). Simultaneously, we find an increase of the Calkyl signal (yellow). We attribute these changes until 150 K to the enrichment of the alkyl chains at the IL/vacuum interface, which leads to the attenuation of the [PF6]− ions below. In this temperature range, we find no change in the intensity of the FCF peak (red) of the [PFBMIm]+ cations at the IL/Ag interface, which further indicates that the alkyl enrichment is limited to the IL/vacuum interface because of the reorientation effects in the outermost layer. Starting at ∼160 K, we observe an increase of the FCF signal, which indicates the onset of the ion exchange at the IL/Ag interface and subsequent diffusion of the [PFBMIm]+ cations to the IL/vacuum interface. At this temperature, the Calkyl signal has passed its maximum and decreases upon heating to higher temperatures. This behavior reflects the gradual enrichment of the [PFBMIm]+ cations at the IL/vacuum interface at the expense of the [C8C1Im]+ cations, which is completed at ∼220 K, where the FCF signal has reached its saturation value. In a reference heating experiment (Figures S6 and S7 in the Supporting Information), after the deposition of a pure 1.5 ML [C8C1Im][PF6] film onto Ag(111) at 82 K, we find the Calkyl peak to increase up to 220 K during heating. This indicates that the decrease of the Calkyl peak in the mixed film above 180 K is due to the competing enrichment of the PFB side chains at the vacuum interface, following the cation-exchange process at the IL/Ag interface. The FPF peak in Figure continues to decrease up to 220 K because of the attenuation of the [PF6]− anions by the surface-enriched alkyl and PFB chains. The intensity of the Chetero peak (black) shows overall only a very small decrease. Above 220 K, the RT surface composition of the mixed IL film is reached (see Figure IV), and no further changes in the signal intensities are observed. As expected, after heating to RT, the spectra look almost identical to those of the film deposited at RT (compare Figure IV with Figure III). The ratio FCF/FPF = 0.28 of the F 1s signals at 0° still shows the nominal ratio of 0.28, whereas at 80°, the ratios in the F 1s and C 1s spectra (FCF/FPF = 0.75, CCF/Chetero = 0.20, CCF/Calkyl = 0.17, and Calkyl/Chetero = 1.14) match those from the deposition at RT.

Notably, the temperature range of the cation exchange corresponds well to the glass transition temperature of 190 K for bulk [C8C1Im][PF6].[38,39] It was found previously that the bulk glass transition is in good agreement with the temperature-dependent phase changes in thin IL films on Ag(111).[4,8] For bulk [PFBMIm][PF6], a melting temperature of 339 K was measured. We thus conclude that the [PFBMIm]+ cations (either by themselves or accompanied by [PF6]− anions) within the mixed thin film become mobile at a much lower temperature.

Temperature Stability of Mixed Films

To assess the stability of the mixed IL films on Ag(111) toward temperature, we heated the deposited mixed films from RT to above 500 K with a constant heating rate of 2 K/min. As a reference, we also studied the thin films of the pure ILs. The graphs in Figure show the quantitative analysis of the respective F 1s signals; the corresponding sets of F 1s spectra acquired during heating can be found in Figures S8–S10 in the Supporting Information.

Figure 7

Intensity of F 1s (blue diamonds: F of [PF6]−, red diamonds: F of [PFBMIm]+) and Ag 3d (gray circles) peaks at 0° upon continuous heating of thin IL films on Ag(111) above RT after the deposition of (a) 1.4 ML of [C8C1Im][PF6],[8] (b) 1.4 ML of [PFBMIm][PF6], and (c) 1.0 ML of [C8C1Im][PF6] on top of 0.5 ML of [PFBMIm][PF6]. The stability range of the multilayers is shaded in dark gray and that of the WL in light gray. The desorption temperatures of the multilayers and WLs, that is, the rate maxima (which correspond to the inflection points of the decreasing signals) are indicated by vertical arrows. Heating rate, 2 K/min.

The desorption of 1.4 ML of pure [C8C1Im][PF6] from Ag(111) (Figure a) occurs in two well-separated steps.[8] The film is stable up to about 385 K, followed by the desorption of the multilayers until 415 K. Thereafter, desorption of the WL starts at 435 K, and at 455 K, the film is completely desorbed.

The desorption of 1.4 ML of pure [PFBMIm][PF6] also occurs in two, but not as well-resolved, steps (see Figure b). The two F 1s signals related to the anion and cation remain constant up to about 370 K, and thereafter show a first decrease because of the desorption of multilayers. At ∼420 K, the coverage of the WL is reached. Within this temperature range, the intensity of the Ag 3d signal increases accordingly. The desorption of the WL follows until, at ∼455 K, the IL film is completely desorbed. Up to ∼415 K, the anion and cation signals decrease simultaneously with a constant FCF/FPF ratio of ∼0.9 (see also the spectra in Figure S9 in the Supporting Information), which is in line with the general observation of IL desorption as ion pairs.[9,25,28] At a higher temperature, that is, in the desorption range of the WL, the FCF peak becomes larger than the FPF peak (see the spectrum for 434 K in Figure S9), yielding a FCF/FPF ratio of ∼1.3. This behavior indicates the partial decomposition of [PFBMIm][PF6] in direct contact with the Ag(111) surface and the desorption of the products in this temperature range. This could also explain the fact that no clear plateau for the WL is observed. Because of the decomposition, no conclusions on the adsorption energy can be deduced.

Interestingly, the multilayer desorption of [PFBMIm][PF6] starts at a significantly lower temperature than that for [C8C1Im][PF6]. Also, the temperature range for the multilayer desorption of [PFBMIm][PF6] is broader than that for [C8C1Im][PF6], and the separation between the desorption of multilayer and WL steps is less defined. According to Hessey and Jones,[9] the desorption of IL multilayers occurs by a direct emission of ion pairs into the gas phase via a high-mobility transition state. The broader multilayer desorption range might, in this regard, be related to a lower mobility of [PFBMIm][PF6] compared to [C8C1Im][PF6].

Finally, Figure c shows the behavior of the mixed thin IL film upon heating from RT to >500 K after the deposition of 1.0 ML of [C8C1Im][PF6] on 0.5 ML of [PFBMIm][PF6] onto Ag(111). As expected from the experiment on the pure films, no desorption was observed up to 370 K. A first decrease of the IL-related signals occurs between 370 and ∼410 K. In this temperature range, [PFBMIm][PF6] desorbs as intact ion pairs, as concluded from the simultaneous decrease in FCF (red) and FPF (blue) signal intensities. Compared to the pure [PFBMIm][PF6] film, complete desorption of [PFBMIm][PF6] occurred already within the multilayer desorption regime, that is, until ∼410 K, without any indication for partial decomposition. As the signal from the pure [PFBMIm][PF6] WL on Ag(111) is observed up to 455 K (Figure b), our findings clearly show that its desorption temperature is lowered considerably by the postdeposition of [C8C1Im][PF6]. A similar effect has been reported previously for mixed thin IL films of [C8C1Im][PF6] and [C8C1Im][Tf2N],[8] where the postdeposition of [C8C1Im][PF6] destabilized the WL of [C8C1Im][Tf2N]. The multilayer desorption of [C8C1Im][PF6] partly overlaps with the desorption of [PFBMIm][PF6] from the mixed film and extends up to ∼420 K. The remaining pure [C8C1Im][PF6] WL desorbs like a pure [C8C1Im][PF6] film until 455 K.

Conclusions

We studied the ion distribution within mixed thin IL films after the deposition of [C8C1Im][PF6] on top of a WL of [PFBMIm][PF6] on Ag(111). The two ILs have the same anion, but different cations. At RT, we observe the preferential enrichment of the [PFBMIm]+ cations at the IL/vacuum interface because of an immediate ion exchange at the IL/Ag interface after the deposition of [C8C1Im][PF6]. In line with a previous study of two ILs with identical cation but different anions,[8] we suggest that two main driving forces are cooperatively responsible for the exchange process: a lower surface tension of [PFBMIm][PF6] because of its fluorinated chain, favoring its enrichment at the IL/vacuum interface, and a larger adsorption energy of [C8C1Im][PF6] on Ag(111) compared to [PFBMIm][PF6]. The partial decomposition of the WL upon heating the pure [PFBMIm][PF6] film does not allow for a direct comparison of the respective desorption temperatures of the two ILs. However, the enrichment of the [PFBMIm]+ cations at the IL/vacuum interface is stronger for the nanoscale-mixed films than for the macroscopic mixtures, where the influence of the IL/solid interface can be neglected. We thus assume that the adsorption energy of [C8C1Im][PF6] on Ag(111) is higher compared to that of [PFBMIm][PF6].

In deposition experiments at 82 K, the cation exchange at the IL/solid interface does not occur, which we attribute to the limited mobility within the IL film at this low temperature. Upon heating the film to RT, we first observe a reorientation of the cations at the IL/vacuum interface, such that the alkyl chains at the [C8C1Im]+ cations point toward the vacuum. Between ∼160 and ∼220 K, a surface enrichment of [PFBMIm]+ cations and the corresponding depletion of [C8C1Im]+ cations is found. No further changes in the surface composition take place between 220 K and RT.

Upon heating the mixed IL film above RT, we observe the complete desorption of [PFBMIm][PF6] from the mixed film until 410 K, leaving a WL of pure [C8C1Im][PF6] on Ag(111), which then desorbs until 455 K. This contrasts the situation for pure [PFBMIm][PF6], where the WL desorbs with concomitant partial decomposition between 420 and 455 K. The observed behavior thus indicates a destabilization of the WL of [PFBMIm][PF6] by the postdeposition of [C8C1Im][PF6]. These findings open a route for selectively removing specific ions or undesired components from the IL/support interfaces.[8] In the present case, postdeposition of a second IL even enables the complete removal of the intact [PFBMIm][PF6] WL from the mixed layer on the surface, whereas for the pure [PFBMIm][PF6] WL, decomposition was observed.