Anion Enhancement at the Liquid-Vacuum Interface of an Ionic Liquid

Subscriber access provided by UNIV OF LOUISIANA

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Anion Enhancement at the Liquid-Vacuum Interface of an Ionic Liquid Mixture Yong Zhang, Yehia Khalifa, Edward Maginn, and John T. Newberg J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07995 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Anion Enhancement at the Liquid-Vacuum Interface of an Ionic Liquid Mixture Yong Zhang†,§, Yehia Khalifa‡,§, Edward J. Maginn*,† and John T. Newberg*,‡ † Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556 ‡ Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716

ABSTRACT The liquid-vacuum interface was investigated for a ionic liquid (IL) mixture containing

1-ethyl-3-methylimidazolium

acetate,

[C2MIM][OAc],

and

1-ethyl-3-

methylimidazolium bis(trifluoromethanesulfonyl)imide, [C2MIM][TFSI]. Herein we detail a quantitative connection between molecular simulations and angle resolved X-ray photoemission for an IL-vacuum interface. Results show that for a mixture with a low concentration of [TFSI] , the anion [OAc] is slightly depleted from the interface while the [TFSI] anion is significantly enhanced relative to the bulk. Both experiments and simulations reveal that the mole fraction of [TFSI] increases significantly from the bulk value in the top 17 Å. Furthermore, simulations show that [TFSI]- has a preferred orientation at the liquid-vacuum interface.

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

Introduction Ionic liquids (ILs) are salts with a melting point below 100 ˚C. The successful use of ILs in a wide-range applications, including nanoparticle synthesis, gas storage and separation, depends heavily on the nature of the surface structure and composition of the interface.1 In recent years mixtures of ILs have been garnering attention relative to pure ILs as they provide the ability to fine-tune unique physical properties by exploiting different anions and cations for different applications.2,3 A particular emphasis on the interface1,4 has gained traction due to the distinctive character that is inherently different from the bulk ion population. For example, there has been direct evidence indicating precise controlling of interfacial properties such as differential capacitance,5,6 surface tension7,8 and electronic properties that affect catalytic activity9,10 by mixing different anions and cations. While there has been significant effort examining the interface of pure ILs,1,2,4,11-15 less attention has been given to the interfacial properties and structure of IL mixtures2,16,17 which is the focus of this study. Experimental techniques utilized to study the IL-vacuum interface of IL mixtures include high resolution Rutherford backscattering (HRBS),18-21 low-energy ion scattering (LEIS),22 reactive atom scattering (RAS),7,23 small angle X-ray scattering,23 neutron scattering,23 time of flight secondary ion mass spectrometry (ToF-SIMS)19,24 and X-ray photoelectron spectroscopy (XPS)9,10,25-28 The aforementioned scattering7,18-23 and mass spectrometry19,24 techniques, some of which are highly surface sensitive, have allowed for the examination of interfacial composition. Examination of the IL-vacuum interface with electron photoemission (XPS) has allowed for the elucidation of changes in electronic properties9,10,26-28 and angle resolved XPS (AR-XPS) to probe interfacial composition.25,29 Herein we utilize AR-XPS to examine interfacial composition.


2

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A number of these surface science studies have examined the effects of varying the cation while utilizing a common anion.7,19,23-25,30 Souda et al.24 used ToF-SIMS to study [C8MIM]0.5[C2MIM]0.5[BF4] (1-octyl-3-methyl-imidazolium tetrafluoroborate and 1-ethyl-3methyl-imidazolium tetrafluoroborate) where the [C8MIM]+ cation with the larger C8 alkyl tail was found to be enhanced at the interface compared to the smaller [C2MIM]+ cation. Similar conclusions were found using [PF6]– (hexafluorophosophate) as the common anion. Around the same time, Maier et al.25 utilized AR-XPS to examine a mixture of imidazolium cations with [TFSI] (bis(trifluoromethanesulfonyl)imide) as the common anion using a mixed composition of [C12MIM]0.1 [C2MIM]0.9 [TFSI] and observed no obvious preferential enhancement of the larger [C12MIM]+ cation compared to [C2MIM]+ at the IL-vacuum interface. The lack of enhancement of the larger [C12MIM]+ cation prompted the authors to suggest that their results do not rule out the possible formation of surface islands from lateral interactions at the IL-vacuum interface. This was followed by studies from Nakajima et al.19 who used HRBS and ToF-SIMS to examine a mixture composed of [C10MIM]𝑥 [C2MIM]1−𝑥 [TFSI] for x = 0.1 and 0.5. For both values of x their results revealed a surface enhancement of the larger [C10MIM]+ cation at the IL-vacuum interface compared to [C2MIM]+ and [TFSI]. Bruce et al.23 utilized small-angle X-ray scattering and neutron scattering to show similar conclusions regarding the enhancement of the larger [C12MIM]+ cation relative to [C2MIM]+ at the liquid-vacuum interface for all mixtures in the range of [C12MIM]𝑥 [C2MIM]1−𝑥 [TFSI] for x = 0 to 1. Smoll et al.7 studied the same system of [C12MIM]𝑥 [C2MIM]1−𝑥 [TFSI] (x = 0 to 1) using RAS with laser induced fluorescence and found an enhancement of the larger [C12MIM]+ cation at the surface for all x > 0. Heller et al.29 explored mixtures of [C8MIM]0.5[(MeO)2IM]0.5[PF6] (where [(MeO)2IM]+ is 1,3-di(methoxy)imidazolium)


3


Page 4 of 30

using AR-XPS and found enhancement of the [C8MIM]+ at the surface relative to the stoichiometric ratio in the mixture. Mixtures that utilize two different anions with a common cation have also been examined with surface sensitive techniques.7,18,20-22,24 Souda et al.24 used ToF-SIMS to measure the surface enhancement of [TFSI] relative to [PF6] in [C4MIM][TFSI]0.1 [PF6]0.9 . Nakajima et al.18,20,21 used HRBS to examine equimolar anion mixtures to show that there is a general tendency of larger anions such as [TFSI] to be enhanced at the interface compared to [BF4] ,20,21 [PF6],18,21 [TfO] (trifluoromethanesulfonate),21 and to a lesser extent [Cl] (chloride).18,21 Villar-Garcia et al.22 used LEIS to show that [TFSI] is strongly enhanced at the interface of mixtures containing [C4MIM][TFSI]𝑥 [Cl]1−𝑥 and [C4MIM][TFSI]𝑥 [I]1−𝑥 (x = 0 to 1). For xTFSI ≥ 0.2, chloride and iodide were no longer observed at the interface due to the shallow probing depth (high surface sensitivity) of LEIS. Smoll et al.7 using RAS observed the enhancement of [TFSI] in the system of [C4MIM][TFSI]𝑥 [BF4]1−𝑥 for x < 0.5, whereas for x > 0.5 [TFSI] is similar to the stoichiometric bulk ratio. While there have been a number of molecular dynamic (MD) simulations examining the ILvacuum interface of pure ILs,31-37 fewer studies have examined the interface of IL mixtures.7,38,39 Palchowdhury and Bhargava38 studied the structure at the liquid-vapor interface of three different equimolar IL mixtures composed of either a common cation ([C4MIM][Cl]0.5[PF6]0.5 and [C4MIM][Cl]0.5[TfO]0.5) or a common anion ([C8MIM]0.5[C2MIM]0.5[TfO]). The longer C8 alkyl cation and larger anions with a distributed charge were found to be enhanced at the outermost layer of the interface compared to the bulk. In addition, orientational ordering was observed at the interface for the longer alkyl chain of the cations and anions with a smaller charge density. Garcia et al.39 studied the IL-vacuum interface of [C4MIM][TFSI]x[Cl]1-x (x = 0.05 to 1). The enhancement


4

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of [TFSI] at the interface was observed even at low [TFSI] bulk concentrations. Detailed analysis revealed that the long alkyl chain of [C4MIM]+ prefers to point toward the vacuum and [TFSI] tends to arrange parallel to the vacuum interface with CF3 groups pointing toward the vacuum region. These orientations were found to be independent of composition. Smoll et al.7 studied the IL-vacuum interface in [C12MIM]x[C2MIM]1–x[TFSI] (x = 0 to 1) and found that the C12 alkyl chain is enhanced at the outer layer relative to the bulk. Under this is a layer dominated by head groups of imidazolium and [TFSI] anions as x increases. While there have been a number of surface science and MD simulation studies examining the structure of the IL-vacuum interface of IL mixtures, the body of IL literature combining both XPS with MD simulations is currently limited to pure ILs.40 In the current study we combine both ARXPS and MD simulations to examine the IL-vacuum interface of the IL mixture [C2MIM][TFSI]0.1[OAc]0.9 (where [OAc]– is acetate). While IL-vacuum interfaces of pure ILs for imidazolium-based [TFSI]– and [OAc]– ILs have been examined via XPS25,26,41-48 and MD simulations37, to our knowledge the surface structure of [TFSI]– mixed with [OAc]– has not been examined by XPS or MD. In this study it will be shown that both AR-XPS and MD results yield a more significant enhancement of the [TFSI]- anion at the liquid/vacuum interface relative to the bulk, despite [TFSI]- having a significantly lower concentration in the bulk compared to [OAc]and [C2mim]+. While these results are in qualitative agreement with previous studies examining [TFSI]– at the IL-vacuum interface of IL-mixtures examined by scattering techniues7,18-22 and mass spectrometry19,24, herein we also provide, for the first time, a quantitative comparison of ILvacuum interfacial concentrations between AR-XPS measurements and MD simulation for an IL mixture.


5


Page 6 of 30

Simulation Procedure MD simulations were carried out using the package LAMMPS.49 The General Amber Force Field (GAFF)50 was used to describe the intra- and intermolecular interactions. This force field has been used widely on organic and ionic liquid systems and has been shown to be reliable.51 In order to derive atomic charges used with GAFF, electronic structure calculations were carried out on an isolated ion to optimize the structure at the B3LYP/6-311++g(d,p) level using Gaussian0952 (see Figure 1). The atomic charges were then derived based on the optimized structure by fitting the electrostatic potential surface obtained from these calculations using the RESP method.53

Figure 1. Optimized structures of [C2MIM]+, [TFSI]- and [OAc]- with labels of selected atom types. Such charges have a total value of ±1 e on the cation and anion, respectively. To approximate the effect of charge transfer and polarizability in the bulk phase, the partial charges were scaled uniformly by 0.8.51 The optimized structure of each ion and the derived atomic partial charges used in the current work are provided in Tables S1 to S3 in the Supporting Information (SI). The long range electrostatic interactions were calculated using the particle-particle particle-mesh (PPPM) method54 with a real space cutoff of 12 Å. The same cutoff was used for van der Waals


6

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interactions and a tail correction was applied55 which is like that used for correcting a single homogeneous phase. Short simulations were also carried out using a 2-dimensional Ewald method and truncated van der Waals interactions (no tail correction). The results were found to be very similar to those found using the method in the current work and therefore validated the procedure used in the current study. For all simulations, a time step of 1 femtosecond (fs) was used and periodic boundary conditions were applied in all directions. The simulation box was built up by placing 900 [C2MIM][OAc] and 100 [C2MIM][TFSI] molecules randomly in a cubic box using the package Packmol.56,57 The systems were then equilibrated for 2 ns in the isothermal-isobaric (NPT) ensemble followed by a 2 ns simulation in the canonical ensemble (NVT) during which the simulation box dimensions slowly changed with the total volume staying constant. At the end of this NVT simulation, the box had a dimension of 50 Å in both x and y directions and a longer z dimension of 115 Å. This deformed box was further equilibrated for 2 ns in the NVT ensemble. Following this, the simulation box was extended by 50 Å in the z-direction with the atomic coordinates untouched, thereby forming two liquid-vacuum interfaces. Starting from this configuration, a 120 ns trajectory was generated in the NVT ensemble and the last 60 ns was used in the analysis. The Nosé-Hoover thermostat58 and the extended Lagrangian approach59 were applied to control the temperature and pressure, respectively. A time constant of 100 fs was used in both thermostat and barostat. Simulations were carried out at an elevated temperature of 350 K because of the sluggish dynamics at room temperature. We expect the liquid structure among the species observed at this temperature will also be observed at lower temperatures.60 The pressure was fixed at one atmosphere in all constant pressure simulations with isotropic volume fluctuations.


7


Page 8 of 30

Experimental Methods Both ionic liquids [C2MIM][TFSI] (Iolitech, 99%) and [C2MIM][OAc] (Iolitec, 95%) were purchased and used without further purification and stored in a desiccator. Prior to XPS analysis an IL mixture with xTFSI = 0.103 was prepared gravimetrically in lab air by weighing out a total mass of 0.110 g (0.08677 g of [EMIM][OAc] and 0.02294 [EMIM][TFSI]). This mixture was chosen for two reasons, i) to illustrate the dramatic effect a small concentration of [TFSI] – has on the surface composition of the mixture, and ii) the ability to directly and uniquely compare [TFSI]– to [OAc]– by utilizing O 1s spectra, and [TFSI]– to [C2MIM]+ utilizing N 1s spectra. While imidazolium-based [TFSI]- ILs are hydrophobic, [Im][OAc] ILs are known to be hydrophilic and therefore susceptible to water uptake from lab air. However, it has been shown previously by some of the authors of this study using gravimetric analysis that the uptake of water into bulk [C4MIM][OAc] is exceedingly slow, taking on the order of many hours to days to reach saturuation.61 Given the time scale of weighing the IL mixture was on the order of a few minutes, we expect the effect of water uptake on the measured mass in the 100 mg regime to be minimal. Moreover, for any water absorption that may have occurred after taking the mass measurement (during sample handling, etc.), this adventitious water would be removed upon placing the sample into an ultra-high vacuum chamber prior to XPS analysis. AR-XPS of the IL-vacuum interface was performed at 0˚ and 80˚ relative to the sample normal using a Thermo Scientific K-alpha X-ray photoelectron system equipped with a monochromatic aluminum K-alpha micro-focused X-ray source. Experiments were performed at room temperature using a 0.2 mm X-ray spot size. XPS survey spectra were collected at a pass energy of 100 eV with a dwell time of 10 ms and a step size of 1.0 eV (averaged over 3 scans). High-resolution spectra taken at 0˚ were collected at a 20 eV pass energy with a step energy of 0.10 eV and a dwell


8

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


time of 50 ms averaged over 10 scans. For the 80˚ measurements, the spectra were averaged over 30 scans. A flood gun was utilized with a current of 125 µA and extraction voltage of 40 V. Previous XPS studies of ILs typically investigate a single sample for a number of IL compositions.1,16,17,62 However, repeating XPS measurements on independent IL samples of the same composition is strongly lacking in the XPS literature of ILs. To this end, our focus herein was to carefully examine one IL mixture (xTFSI = 0.103) to provide a confident, repeatable measurement and acquire an average value of surface composition for comparison with MD simulations. More specifically, the IL mixture was deposited into four shallow wells (1.2 mm diameter) bored into an aluminum substrate (see SI, Figure S1 for more details) for XPS analysis. The wells were made sufficiently small to restrict movement of the viscous IL mixture during angle resolved XPS measurements. The IL laid flat to the naked eye inside of the wells. Furthermore, the wells were aligned along the tilting axis of an angular resolved module such that distance between the IL-vacuum interface and the acceptance aperture (i.e., sample height) was not altered significantly upon rotating the Al sample holder between 0˚ and 80˚. A final fine-tuning of the sample height was optimized automatically by the K-alpha XPS system by optimizing the XPS signal prior to the start of analysis. For each of the four samples the module was rotated between 0˚ and 80˚, thus a total of seven rotations were performed with the sample module for these experiments. The total collection time for each sample was under 3 hours, which is below an observed 4 hour threshold where X-ray induced damage becomes detectible for imidazolium based ILs in the N 1s region.63 No foreign elements or Al peaks were observed (see survey spectra, Figure S2), indicating i) there was no measurable IL contamination at the IL-vacuum interface, ii) there was no dissolution of the Al substrate into the IL mixtures, and iii) the X-ray spot size (0.2 mm) and focal point of spectrometer were maintained within the IL sample diameter (1.2 mm).


9


Page 10 of 30

Spectra taken for O 1s, N 1s and C 1s at both 0˚ and 80˚ are overlaid in Figure S3 illustrating the repeatability of the measurements for all four IL samples. These results confirm that the IL mixture did not move macroscopically throughout the experiment, especially for the 80 tilted samples where gravity can pull down on a droplet potentially changing the angle of acquisition and also exposing the substrate through sample dewetting. The experiments herein show that the development of small 1.2 mm dia. sample wells that can be manipulated along the rotational axis of the sample holder get around the potential problem of ILs being pulled down and spread from gravitational effects. This was accomplished through the use of a small (0.2 mm) X-ray spot size, which in turn allowed for the use of small liquid samples.

Results and Discussion Ion concentration profiles as a function of liquid depth (z) from the IL-vacuum interface for a [C2MIM][TFSI]0.1[OAc]0.9 mixture (Figure S4a) were calculated using canonical ensemble MD simulations and the ion center of mass. Figure 2a shows concentration profiles averaged across the two IL-vacuum interfaces and are normalized to the average bulk value for each species obtained from the center 50 Å of the z = 115 Å liquid region. Due to the low mole fraction and small number of [TFSI], a relatively large fluctuation is seen in its concentration at the middle of the liquid phase. The distributions of all the species show a well-organized structure at the liquidvacuum interface, which extends at least 15 Å into the liquid. The oscillation of the [C2MIM]+ cations and [OAc]– anions enforced by opposite charges extends even deeper as shown in Figure 2a. The [TFSI] anion center of mass (COM) was found to be located at the vacuum side of the


10

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. MD simulations of concentration profiles as a function of liquid depth (z) for (a) [C2MIM]+, [TFSI] and [OAc] ions and (b) oxygen atoms within [TFSI] and [OAc]. Concentrations are normalized to the average bulk value within the middle 50 Å of the liquid phase. The results displayed are an average between the two IL-vacuum interfaces in the simulation cell. interface relative to those of [C2MIM]+ and [OAc] although, considering the sizes of the ions, all species are mixed together and no well-defined layered structure can be identified. The concentration of [TFSI] at the interface is significantly enhanced relative to bulk. This enhancement is the highest of the three species. A slight enhancement is observed for [C2MIM]+ at the interface relative to the bulk. Different from [TFSI] and [C2MIM]+, the [OAc] distribution includes a depletion at shallow depth (z = ~8 Å). The peak height is lower than the average value observed in the liquid bulk, corresponding to the [TFSI] peak and a slight enhancement at a deeper depth (z = ~14 Å). Following the same procedure, the concentration profiles as a function of liquid


11


Page 12 of 30

depth were also calculated for oxygen and nitrogen atoms to compare with XPS data. The concentration profile of oxygen atoms from [TFSI] and [OAc] (OTFSI and OOAc, respectively) are shown in Figure 2b. It is evident from the profiles that the concentration of OTFSI at the interface is around 4 times of that in the bulk, whereas OOAc is depleted at shallower depth and slightly enhanced at deeper depth in the liquid, consistent with the overall ion concentration results. We will now compare concentration profiles from MD results with experimentally obtained XPS data. This method has been explored previously with aqueous solutions64,65 and recently by Shimizu et al.40 in probing the surface composition of different pure ILs composed of imidazolium cations with [TFSI] as the counterion. The extent of enhancement of [TFSI] at the interface can be investigated in relation to the [OAc] anion utilizing O 1s spectra given oxygen is present in both ions. To compare the enhancement observed in the MD simulations with the AR-XPS results, we introduce the variable S(z) (Eqn. 1) which is a conversion of the MD concentration profile into an exponentially decreasing concentration profile from the interface going into the bulk, the integral of which is equivalent to AR-XPS intensity66 (Eqn. 2): 𝑧

S(z) = 𝑒 − 𝜆 cos 𝜃 𝜌(𝑧) 𝑑

I = ∫0 𝑆(𝑧) d𝑧

(1) (2)

where 𝜌(𝑧) is the ion concentration obtained from MD simulation, d is the commonly defined probing depth of 3λ cos 𝜃 for XPS and 𝜆 is the inelastic mean free path of photoelectrons at a given kinetic energy emitted at angle 𝜃 relative to the sample normal. While Eqn (2) theoretically should be integrated from 0 to , an MD simulation cell has a finite liquid depth (z). Eqn (2) integrates to a value of d which accounts for 95% of the signal66 and is smaller than the liquid depth simulated by MD. Values of 𝜆 were calculated to be 32 Å for O 1s (35 Å for N 1s) using the Gries predictive


12

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) Exponentially decaying oxygen atom concentration profiles, applying Eqn (1) to MD data in Figure 2b. (b) AR-XPS O 1s data of the IL mixture collected at 0˚ (solid black) and 80˚ (dashed black). The O 1s data is normalized to the OTFSI component at 532.6 eV. formula from NIST Electron Inelastic-Mean-Free-Path Database (v. 1.2)67 based on a weighted average of the density of the material. The exponentially decaying concentration profiles of oxygen atoms normalized to the bulk are displayed in Figure 3a. The solid red and blue lines represent [TFSI] and [OAc], respectively, and correspond to 𝜃 = 0˚. The dashed red and blue lines are of the same elements at a more surface sensitive grazing angle of 80˚. The curves in Figure 3a show an OTFSI peak concentration at shallow liquid depth which drops off at deeper depth due to the exponentially decaying factor. Below the top layer, for the dashed lines, a sharper drop off in signal is observed due to the enhanced surface sensitivity. By restricting the signal from deeper layers at 80˚, it can be seen that


13


Page 14 of 30

[TFSI] further dominates the interface due to the probing depth being reduced to ~17 Å (3𝜆 cos 80˚). The integration of Eqn. (1) allows for a direct comparison to XPS experiments. The experimental acquisition of high resolution AR-XPS O 1s spectra (Figure 3b) provides evidence of a measured enhancement of the [TFSI] anion between 0˚ (shown in solid black) and 80˚ (dashed black). The four and two iso-electronic oxygen atoms belonging to [TFSI] (red) and [OAc] (blue) are observed at 532.6 eV and 530.4 eV, respectively. The spectra at 80˚ show a clear decrease in [OAc] intensity as we probe less of the bulk, indicating the enhancement of the [TFSI] anion at the interface relative to [OAc]. For N 1s spectra (Figure 4a) there are two peaks at 401.8 eV and 399.5 eV, corresponding to the nitrogen atoms on the [C2MIM]+ cation and the imide nitrogen on the [TFSI] anion. The decrease in intensity of [C2MIM]+ nitrogen signal at 80˚ (red spectrum) signifies the enhancement of [TFSI] at the interface relative to the bulk. For C 1s spectra (Figure 4b) there is a main peak centered at 286.4 eV that contains carbons on the [C2MIM]+ cation and [OAc] anion that are similar to the bulk stoichiometry (see Figure S5 for component analysis and assignment). The peak near 292.8 eV is indicative of the fluorocarbons on the [TFSI] anion. The decrease in carbon

Figure 4. AR-XPS (a) N1s and (b) C 1s spectra showing IL mixture collected at 0˚ (black) and 80˚ (red). Spectra are normalized to the TFSI components.


14

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intensity at 80˚ (red spectrum) in [C2MIM]+ and [OAc]– ions relative to [TFSI] is another indicator of the enhancement of [TFSI] at the IL-vacuum interface. Given the integrated XPS component areas from O 1s and N 1s spectra are proportional to the mole of material (n), we are able to make a direct quantitative comparison between AR-XPS results and MD simulations (see SI, Eqns. S1 and S2). More specifically, the oxygen atoms of the O 1s spectra were used to quantify the relative population of anions [TFSI]- and [OAc]-, while the nitrogen atoms of the N 1s spectra were used to quantify the relative population of [TFSI] - and [C2MIM]+ Together, the use of O 1s and N 1s spectra allow for independent analyses of [TFSI]  relative to [OAc] and [C2MIM]+, respectively. Note, using the same core shell orbital to quantify two different ion species does not require the use of XPS sensitivity factors. Given that there are 4 moles of oxygen and 1 mole of nitrogen per mole of [TFSI], 2 moles of oxygen per [OAc], and 2 moles of nitrogen per mole of [C2MIM]+, the mole fraction of [TFSI] to [OAc] (for O 1s spectra) and [TFSI] to [C2MIM]+ (for N 1s spectra) are given by Eqns. (3) and (4) respectively. 𝑂 1𝑠 𝑥𝑇𝐹𝑆𝐼 =

𝑂 1𝑠 𝑛𝑇𝐹𝑆𝐼 𝑂 1𝑠 𝑂 1𝑠 𝑛𝑇𝐹𝑆𝐼 + 𝑛𝑂𝐴𝑐

(Eqn. 3)

𝑁 1𝑠 𝑥𝑇𝐹𝑆𝐼 =

𝑁 1𝑠 𝑛𝑇𝐹𝑆𝐼 𝑁 1𝑠 𝑁 1𝑠 𝑛𝑇𝐹𝑆𝐼 + 𝑛𝐶 2𝑀𝐼𝑀

(Eqn. 4)

The average xTFSI (± 2 std. dev.) are reported for both O 1s and N 1s XPS data at 0˚ and 80˚ in Table 1. xTFSI results from corresponding MD simulations utilizing Eqn. (1) for oxygen and nitrogen concentration profiles is also included for comparison. The measured XPS peak areas for the four individual spots is provided in Tables S4 and S5 for O 1s and N 1s, respectively, at 0˚ and 80˚ along with observed mole ratios and mole fractions in Tables S6 and S7 respectively. Let us first examine the comparison between MD and AR-XPS results based on O 1s spectra. O 1s A nominal 𝑥TFSI bulk value of 0.100 for MD is determined from the simulation cell and oxygen


15


Page 16 of 30

O 1s atom mole ratio of 1:9 for [TFSI] to [OAc] ions. For AR-XPS a nominal 𝑥TFSI bulk value of

0.103 is expected from the gravimetric preparation of our mixture (mole ratio of 1.03:8.97 for O 1s [TFSI] to [OAc]). As seen from Table 1, O 1s MD results for a 0˚ probing angle give 𝑥TFSI = O 1s 0.118, which exceeds the nominal value of 0.100. For AR-XPS measurements taken at 0˚, 𝑥TFSI =

0.152, which also exceeds the nominal value of 0.103. While the XPS analysis of pure ILs at 0˚ can be similar to the bulk,17,63,68 deviation from stoichiometric values for IL mixtures probed at 0˚ has been observed by Heller et al.29 who examined [C8MIM]0.5 [(MeO)2IM]0.5 [PF6] and an equimolar mixture of [C8MIM][PF6] and [(MeO)2IM][TFSI]. Our findings from both MD and ARXPS indicate that the ratio of [TFSI] to [OAc] via O 1s analysis is consistent with Heller et al.29, showing a deviation from the stoichiometric bulk value while probing at 0˚. In going from 0˚ to O 1s the more surface sensitive 80˚ probing angle, the calculated value of 𝑥TFSI from MD increases from O 1s 0.118 to 0.252. Similarly, AR-XPS experimental values for 𝑥TFSI increase from 0.152 to 0.201.

Thus, the quantitative assessment of oxygen atoms present in [TFSI] and [OAc] via MD and ARXPS both reveal that [TFSI] is significantly more enhanced at the IL-vacuum interface relative to bulk than the [OAc] anion. Table 1. Comparison between MD and AR-XPS values of xTFSI.

O 1s 𝑥TFSI

N 1s 𝑥TFSI

Nominala

0˚

80˚

MD

0.100

0.118

0.252

AR-XPS

0.103

0.152 (0.005)b

0.201 (0.005)

MD

0.091

0.103

0.210

AR-XPS

0.093

0.147 (0.003)

0.200 (0.023)

Nominal MD values based on a [TFSI] to [OAc] mole ratio of 1:9 for O 1s and a [TFSI]  to [C2MIM]+ mole ratio of 1:10 for N 1s. Nominal AR-XPS values also take into consideration the actual delivered mass in preparing the sample gravimetrically. b AR-XPS results are the average of four measurements. Values in parentheses are 2 std. dev.

a


16

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N 1s When examining nitrogen atoms, the nominal value of 𝑥TFSI for MD is 0.091 given by the mole N 1s ratio of 1:10 for [TFSI] to [C2MIM]+. MD simulations for 0˚ probing angle given 𝑥TFSI = 0.103, N 1s which is greater than the nominal value. The expected nominal bulk value of 𝑥TFSI for AR-XPS

measurements is 0.093 (mole ratio of 1.03:10) from gravimetric preparation. AR-XPS results for N 1s measurements at 0˚ give 𝑥TFSI = 0.147, which also exceeds the nominal value. Thus, both MD and

AR-XPS indicate, from a nitrogen atom perspective, that the ratio of for [TFSI] to [C2MIM]+ at the IL-vacuum interface exceeds the nominal value when probing at 0˚. For a grazing angle of 80˚, xTFSI increases for both MD (0.210) and AR-XPS (0.200) relative to 0˚ measurements. These results are consistent with a molecular level picture of the [TFSI] anion being significantly more enhanced at the interface compared to the bulk than the [C2MIM]+ cation. Our findings are qualitatively consistent with previous experimental surface science studies that find [TFSI] to be enhanced at the IL-vacuum interface relative to other anions using a common cation18,20-22,24. In most cases, our measured AR-XPS values of xTFSI are consistently greater than the calculated values from MD (the exception being N 1s at 80˚). Interestingly, Nakajima et al.20 observed a greater surface mole fraction of xTFSI from experimental HRBS results when compared to MD for the equimolar mixture of [C2MIM][TFSI]0.5[BF4]0.5 (xTFSI = 0.72 from HRBS relative to xTFSI = 0.59 from MD). Nakajima et al.20 attributed this deviation to a deterioration of the accuracy of the MD force fields (developed for neat ILs) utilized for mixtures.


17


Page 18 of 30

Both AR-XPS and MD show a more significant enhancement of [TFSI] at the IL-vacuum interface relative to the bulk than both [OAc] and [C2MIM]+. To further understand the interfacial liquid structure, next we discuss results from MD simulations examining individual atom concentration profiles and bond vector orientations. Figure 5 shows the concentration profiles for selected atoms. On average, the F and C atoms in [TFSI] are most probable to be found closer to the vacuum followed by the terminal carbon (CT) in the ethyl group of [C2MIM]+ (see Figure 1). The CH in [OAc] has almost the same peak position as that of CT atoms. The peaks of the N, S and O atoms of [TFSI] lie slightly deeper into the liquid. The peak positions of N atoms of [C2MIM]+ (in the imidazolium ring) and the carboxylate group of [OAc] are also located slightly deeper in the liquid phase. These observations on the relative ordering of the atoms within

Figure 5. Concentration profiles of selected atoms as a function of liquid depth (z). Concentrations are normalized to the average bulk value within the middle 50 Å of the liquid phase. The results displayed are an average between the two IL-vacuum interfaces in the simulation cell. Atom names are defined in Figure 1. [C2MIM]+ and [TFSI]– are similar to what was reported previously in simulations of a pure imidazolium [TFSI]– IL.33 It is interesting to note that the atoms (C and F in [TFSI], CT in


18

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[EMIM]+, and CH in [OAc]) that face the vacuum carry relatively small charges whereas those that are located on the liquid side of the interface carry larger partial charges. To further elucidate the interfacial versus bulk atomic arrangements we also performed ion orientation analyses as a function of z. The S-S vector (pointing from first atom to the second) in [TFSI], CH-CO vector in [OAc], and N1-CT and N2-N1 vectors in [C2MIM]+ were chosen to represent the orientation of each ion, respectively. The distribution of the cosine values of the angles as a function of liquid depth is summarized in Figure 6, where the colors refer to the relative probability of a configuration. For these plots, a uniform distribution means all values of cos(α) are equally probable, a cosine value of 1.0 or -1.0 indicates that the vector is parallel or antiparallel to the interface normal (perpendicular to the interface), and a value of 0 means the vector is perpendicular to interface normal (parallel to the interface). As shown in Figure 6, all of the studied vectors have a preferred orientation at the IL-vacuum interface, which becomes isotropic with increasing liquid depth. The S-S vector in [TFSI] was found to have a preferred orientation at 90˚ (cosine value of 0) at the interface (note that this distribution is much broader than those of other vectors examined in the current work), which allows both CF3 groups to locate close to the vacuum and occupy the very outer layer of the interface while NTFSI resides deeper in the liquid, consistent with the results shown in Figure 2. The same [TFSI] orientation was also observed at the IL-vacuum interface for pure [C2MIM][TFSI]19 and [C4MIM][TFSI]x[Cl]1-x (x = 0.05 to 1.00) mixtures.39 For both N1-CT and N2-N1 vectors in [C2MIM]+, a single preferred distribution appears at a liquid depth of ~9 Å. The angle for the preferred distribution is around 0 for both vectors. These results suggest that the ethyl group of [C2MIM]+ prefers to stay close to the vacuum, with a direction almost perpendicular to the interface surface. These observations agree with previous studies on both pure ILs and IL mixtures.25,31,69 For the CH-CO vector in [OAc], two


19


Page 20 of 30

Figure 6. Vector orientation distribution (relative to the interface normal) as a function of liquid depth calculated for the [C2MIM][OAc] and [C2MIM][TFSI] mixture: S-S vector (pointing from first atom to the second) in [TFSI], N1-CT and N2-N1 vectors in [C2MIM]+and CH-CO vector in [OAc]. Atom names are defined in Figure 1. Note the different scale in each plot. preferred distributions are seen at the interface. A more localized one at ~180 and a widely distributed one slightly deeper into the liquid phase with an angle between 30-120. The more localized one corresponds to the outermost depletion peak in the concentration profile shown in Figure 2a, which is almost perpendicular to the interface with the methyl group located close to the vacuum. The CO and CH atoms of these [OAc] anions form the first peaks in Figure 5 at a depth between 5 and 10 Å. The other preferred orientation of the CH-CO vector corresponds to the second peak at a depth of about 14 Å in the concentration profile (Figure 2a). These [OAc] anions have a widely distributed orientation and the methyl group slightly prefers to point toward


20

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the inside of the liquid phase. The CO and CH atoms of these ions form the second peak in Figure 5 around z = 15 Å. These results suggest that there are two layers of [OAc] at the interface with opposite orientations. Souda et al.24 and Nakajima et al.21 have argued that anion size is the driving force for surface enhancement of anions relative to other anions. Recent studies conducted by Smoll et al. 7 and Heller et al.29 propose this preferential enhancement of certain ions effectively lowers the surface tension. Nakajima et al.18 have also argued that the enhancement of [TFSI] to the surface reduces the overall surface energy due to better screening of the electric field. On the other hand, VillarGarcia et al.22 conclude from their study that anion size has little impact on the composition of the interface but rather the driving force of surface enhancement lies in the strength of cation-anion interaction where the weakest cation-anion interactions are readily broken and thereby rejected to the surface. The observed concentration profiles in our work point to a lower entropy at the IL interface relative to the bulk. Therefore, for this interfacial structure to be stable in terms of free energy, the interactions between ions must be enthalpically favored. More specifically, our results suggest the interactions between atoms with large partial charges and interactions between small partial charges are the driving force for the interfacial enhancement of [TFSI].

Conclusion The structure of the liquid-vacuum interface of a mixture of [C2MIM][OAc] and [C2MIM][TFSI] with a 9:1 molar ratio was studied using both AR-XPS experiments and MD simulations. Quantitative agreement of both techniques show a more significant enhancement of [TFSI] at the interface relative to the bulk, when compared to [OAc] and [C2MIM]+. Simulations reveal that all ions have well-organized structures at the liquid-vacuum interface. [TFSI] as well


21


Page 22 of 30

as the ethyl side chain of [C2MIM]+ and the methyl group of [OAc] were found to occupy the outer surface of the interface facing towards the vacuum. The other moieties of [C2MIM]+ and [OAc] tend to stay on the liquid side of the interface. The observed structuring at the interface is believed to be the result of the competition between unfavorable entropy and favorable enthalpy, resulting from the interactions between atoms with large partial charges and interactions between atoms with small partial charges.

ASSOCIATED CONTENT Supporting Information Optimized structure of each ion and derived partial atomic charges, snapshot of the experimental substrate containing the IL mixture and angle resolved module, survey spectra at both 0˚ and 80˚, high resolution C 1s of the IL mixture, overlay of four different high resolution spectra of the components of IL mixture, summary tables of individual spots and average mole ratios and fractions. Notes §These authors contributed equally. The authors declare no competing financial interests. ACKNOWLEDGMENT YZ and EM acknowledge the support by the U.S. Department of Energy, Basic Energy Science, Joint Center for Energy Storage Research under contract no. DE-AC0206CH11357. Computational resources were provided by the Center for Research Computing (CRC) at the University of Notre Dame. JN acknowledges support from an ACS PRF doctoral new investigator award 56249-DNI5.


22

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Corresponding Author * [email protected] * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §These authors contributed equally. Funding Sources YZ and EM acknowledge the support by the U.S. Department of Energy, Basic Energy Science, Joint Center for Energy Storage Research under contract no. DE-AC0206CH11357. Computational resources were provided by the Center for Research Computing (CRC) at the University of Notre Dame. JN acknowledges support from an ACS PRF doctoral new investigator award 56249-DNI5.

References (1) Lovelock, K. R. J. Influence of the Ionic Liquid/Gas Surface on Ionic Liquid Chemistry. Phys. Chem. Chem. Phys. 2012, 14, 5071-5089. (2) Niedermeyer, H.; Hallett, J. P.; Villar-Garcia, I. J.; Hunt, P. A.; Welton, T. Mixtures of Ionic Liquids. Chem. Soc. Rev. 2012, 41, 7780-7802. (3) Chatel, G.; Pereira, J. F.; Debbeti, V.; Wang, H.; Rogers, R. D. Mixing Ionic Liquids “Simple Mixtures” Or “Double Salts”? Green Chem. 2014, 16, 2051-2083. (4) Hayes, R.; Warr, G. G.; Atkin, R. At the Interface: Solvation and Designing Ionic Liquids. Phys. Chem. Chem. Phys. 2010, 12, 1709-1723. (5) Costa, R.; Pereira, C. M.; Silva, A. F. Structural Ordering Transitions in Ionic Liquids Mixtures. Electrochem. Commun. 2015, 57, 10-13.


23


Page 24 of 30

(6) Costa, R.; Voroshylova, I. V.; Cordeiro, M. N. D.; Pereira, C. M.; Silva, A. F. Enhancement of Differential Double Layer Capacitance and Charge Accumulation by Tuning the Composition of Ionic Liquids Mixtures. Electrochim. Acta 2018, 261, 214-220. (7) Smoll, E.; Tesa-Serrate, M.; Purcell, S.; D’Andrea, L.; Bruce, D.; Slattery, J.; Costen, M. L.; Minton, T. K.; McKendrick, K. G. Determining the Composition of the Vacuum–liquid Interface in Ionic-Liquid Mixtures. Faraday Discuss. 2017, 206, 497-522. (8) Palchowdhury, S.; Bhargava, B. L. Surface Structure and Dynamics of Ions at the LiquidVapor Interface of Binary Ionic Liquid Mixtures: Molecular Dynamics Studies. J. Phys. Chem. C 2016, 120, 5430-5441. (9) Villar-Garcia, I. J.; Lovelock, K. R. J.; Men, S.; Licence, P. Tuning the Electronic Environment of Cations and Anions using Ionic Liquid Mixtures. Chem. Sci. 2014, 5, 25732579. (10) Men, S.; Licence, P. Tuning the Electronic Environment of the Anion by using Binary Ionic Liquid Mixtures. Chem. Phys. Lett. 2017, 681, 40-43. (11) Steinrueck, H. Recent Developments in the Study of Ionic Liquid Interfaces using X-Ray Photoelectron Spectroscopy and Potential Future Directions. Phys. Chem. Chem. Phys. 2012, 14, 5010-5029. (12) Aliaga, C.; Santos, C. S.; Baldelli, S. Surface Chemistry of Room-Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2007, 9, 3683-3700. (13) Lynden-Bell, R. M.; Del Popolo, M. G.; Youngs, T. G. A.; Kohanoff, J.; Hanke, C. G.; Harper, J. B.; Pinilla, C. C. Simulations of Ionic Liquids, Solutions, and Surfaces. Acc. Chem. Res. 2007, 40, 1138-1145. (14) Maginn, E. J. Molecular Simulation of Ionic Liquids: Current Status and Future Opportunities. J. Phys. Condes. Matter 2009, 21, 373101. (15) Zahn, S.; Brehm, M.; Bruessel, M.; Holloczki, O.; Kohagen, M.; Lehmann, S.; Malberg, F.; Pensado, A. S.; Schoeppke, M.; Weber, H., et al., Understanding Ionic Liquids from Theoretical Methods. J. Mol. Liq. 2014, 192, 71-76. (16) Lovelock, K. R. J.; Villar-Garcia, I. J.; Maier, F.; Steinrück, H. P.; Licence, P. Photoelectron Spectroscopy of Ionic Liquid-Based Interfaces. Chem. Rev. 2010, 110, 5158-5190. (17) Steinrück, H. P. Surface Science Goes Liquid! Surf. Sci. 2010, 604, 481-484. (18) Nakajima, K.; Oshima, S.; Suzuki, M.; Kimura, K. Surface Structures of Equimolar Mixtures of Imidazolium-Based Ionic Liquids using High-Resolution Rutherford Backscattering Spectroscopy. Surf. Sci. 2012, 606, 1693-1699.


24

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Nakajima, K.; Miyashita, M.; Suzuki, M.; Kimura, K. Surface Structures of Binary Mixtures of Imidazolium-Based Ionic Liquids using High-Resolution Rutherford Backscattering Spectroscopy and Time of Flight Secondary Ion Mass Spectroscopy. J. Chem. Phys. 2013, 139, 224701. (20) Nakajima, K.; Nakanishi, S.; Chval, Z.; Lísal, M.; Kimura, K. Surface Segregation in a Binary Mixture of Ionic Liquids: Comparison between High-Resolution RBS Measurements and Molecular Dynamics Simulations. J. Chem. Phys. 2016, 145, 184704. (21) Nakajima, K.; Nakanishi, S.; Lísal, M.; Kimura, K. Surface Structures of Binary Mixture of Ionic Liquids. J. Mol. Liq. 2017, 230, 542-549. (22) Villar-Garcia, I. J.; Fearn, S.; Ismail, N. L.; McIntosh, A. J. S.; Lovelock, K. R. J. Fine Tuning the Ionic Liquid-Vacuum Outer Atomic Surface using Ion Mixtures. Chem. Commun. 2015, 51, 5367-5370. (23) Bruce, D. W.; Cabry, C. P.; Lopes, J. N. C.; Costen, M. L.; D’Andrea, L.; Grillo, I.; Marshall, B. C.; McKendrick, K. G.; Minton, T. K.; Purcell, S. M. Nanosegregation and Structuring in the Bulk and at the Surface of Ionic-Liquid Mixtures. J. Phys. Chem. B 2017, 121, 6002-6020. (24) Souda, R. Surface Segregation in Binary Mixtures of Imidazolium-Based Ionic Liquids. Surf. Sci. 2010, 604, 1694-1697. (25) Maier, F.; Cremer, T.; Kolbeck, C.; Lovelock, K. R. J.; Paape, N.; Schulz, P. S.; Wasserscheid, P.; Steinrück, H. P. Insights into the Surface Composition and Enrichment Effects of Ionic Liquids and Ionic Liquid Mixtures. Phys. Chem. Chem. Phys. 2010, 12, 1905-1915. (26) Men, S.; Lovelock, K. R. J.; Licence, P. X-Ray Photoelectron Spectroscopy of Pyrrolidinium-Based Ionic Liquids: Cation-Anion Interactions and a Comparison to Imidazolium-Based Analogues. Phys. Chem. Chem. Phys. 2011, 13, 15244-15255. (27) Men, S.; Mitchell, D. S.; Lovelock, K. R. J.; Licence, P. X-Ray Photoelectron Spectroscopy of Pyridinium-Based Ionic Liquids: Comparison to Imidazolium- and Pyrrolidinium-Based Analogues. Chem. Phys. Chem. 2015, 16, 2211-2218. (28) Men, S.; Licence, P. Probing the Electronic Environment of Binary and Ternary Ionic Liquid Mixtures by X-Ray Photoelectron Spectroscopy. Chem. Phys. Lett. 2017, 686, 74-77. (29) Heller, B. S.; Kolbeck, C.; Niedermaier, I.; Dommer, S.; Schatz, J.; Hunt, P.; Maier, F.; Steinrück, H. Surface Enrichment in Equimolar Mixtures of Non‐ functionalized and Functionalized Imidazolium‐ based Ionic Liquids. Chem. Phys. Chem. 2018, 19, 17331745.


25


Page 26 of 30

(30) Heller, B. S.; Kolbeck, C.; Niedermaier, I.; Dommer, S.; Schatz, J.; Hunt, P.; Maier, F.; Steinrück, H. Surface Enrichment in Equimolar Mixtures of Non‐ functionalized and Functionalized Imidazolium‐ based Ionic Liquids. Chem. Phys. Chem. 2018, 19, 17331745. (31) Lynden-Bell, R. Gas-Liquid Interfaces of Room Temperature Ionic Liquids. Mol. Phys. 2003, 101, 2625-2633. (32) Herrera, C.; García, G.; Alcalde, R.; Atilhan, M.; Aparicio, S. Interfacial Properties of 1Ethyl-3-Methylimidazolium Glycinate Ionic Liquid regarding CO2, SO2 and Water from Molecular Dynamics. J. Mol. Liq. 2016, 220, 910-917. (33) Pensado, A. S.; Malfreyt, P.; Padua, A. A. H. Molecular Dynamics Simulations of the Liquid Surface of the Ionic Liquid 1-Hexyl-3-Methylimidazolium Bis(Trifluoromethanesulfonyl)Amide: Structure and Surface Tension. J. Phys. Chem. B 2009, 113, 14708-14718. (34) Tesa-Serrate, M. A.; Marshall, B. C.; Smoll, E. J., Jr.; Purcell, S. M.; Costen, M. L.; Slattery, J. M.; Minton, T. K.; McKendrick, K. G. Ionic Liquid-Vacuum Interfaces Probed by Reactive Atom Scattering: Influence of Alkyl Chain Length and Anion Volume. J. Phys. Chem. C 2015, 119, 5491-5505. (35) Shimizu, K.; Tariq, M.; Freitas, A. A.; Padua, A. A. H.; Lopes, J. N. C. Self-Organization in Ionic Liquids: From Bulk to Interfaces and Films. J. Braz. Chem. Soc. 2016, 27, 349-362. (36) Yan, T.; Li, S.; Jiang, W.; Gao, X.; Xiang, B.; Voth, G. A. Structure of the Liquid− Vacuum Interface of Room-Temperature Ionic Liquids: A Molecular Dynamics Study. J. Phys. Chem. B 2006, 110, 1800-1806. (37) Konieczny, J. K.; Szefczyk, B. Structure of Alkylimidazolium-Based Ionic Liquids at the Interface with Vacuum and Water - a Molecular Dynamics Study. J. Phys. Chem. B 2015, 119, 3795-3807. (38) Palchowdhury, S.; Bhargava, B. Segregation of Ions at the Interface: Molecular Dynamics Studies of the Bulk and Liquid–vapor Interface Structure of Equimolar Binary Mixtures of Ionic Liquids. Phys. Chem. Chem. Phys. 2015, 17, 19919-19928. (39) García, G.; Atilhan, M.; Aparicio, S. Interfacial Properties of Double Salt Ionic Liquids: A Molecular Dynamics Study. J Phys. Chem. C 2015, 119, 28405-28416. (40) Shimizu, K.; Heller, B. S.; Maier, F.; Steinrück, H.; Canongia Lopes, J. N. Probing the Surface Tension of Ionic Liquids using the Langmuir Principle. Langmuir 2018, 34, 44084416. (41) Ikari, T.; Keppler, A.; Reinmöller, M.; Beenken, W. J.; Krischok, S.; Marschewski, M.; Maus-Friedrichs, W.; Höfft, O.; Endres, F. Surface Electronic Structure of Imidazolium-


26

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Based Ionic Liquids Studied by Electron Spectroscopy. e-J. Surf. Sci. Nanotechnol. 2010, 8, 241-245. (42) Hofft, O.; Bahr, S.; Himmerlich, M.; Krischok, S.; Schaefer, J. A.; Kempter, V. Electronic Structure of the Surface of the Ionic Liquid [EMIM][Tf2N] Studied by Metastable Impact Electron Spectroscopy (MIES), UPS, and XPS. Langmuir 2006, 22, 7120-7123. (43) Keppler, A.; Himmerlich, M.; Ikari, T.; Marschewski, M.; Pachomow, E.; Hoefft, O.; MausFriedrichs, W.; Endres, F.; Krischok, S. Changes of the Near-Surface Chemical Composition of the 1-Ethyl-3-Methylimidazolium Bis(Trifluoromethylsulfonyl)Imide Room Temperature Ionic Liquid Under the Influence of Irradiation. Phys. Chem. Chem. Phys. 2011, 13, 1174-1181. (44) Kolbeck, C.; Killian, M.; Maier, F.; Paape, N.; Wasserscheid, P.; Steinrück, H. Surface Characterization of Functionalized Imidazolium-Based Ionic Liquids. Langmuir 2008, 24, 9500-9507. (45) Broderick, A.; Khalifa, Y.; Shiflett, M. B.; Newberg, J. T. Water at the Ionic Liquid-Gas Interface Examined by Ambient Pressure X-Ray Photoelectron Spectroscopy. J. Phys. Chem. C 2017, 121, 7337-7343. (46) Khalifa, Y.; Broderick, A.; Newberg, J. T. Water Vapor Electron Scattering Cross-Section Measurements using a Hydrophobic Ionic Liquid. J. Electron Spectrosc. Relat. Phenom. 2017, 222, 162-166. (47) Kolbeck, C.; Niedermaier, I.; Deyko, A.; Lovelock, K. R. J.; Taccardi, N.; Wei, W.; Wasserscheid, P.; Maier, F.; Steinrück, H. Influence of Substituents and Functional Groups on the Surface Composition of Ionic Liquids. Chem. Eur. J. 2014, 20, 3954-3965. (48) Hurisso, B. B.; Lovelock, K. R. J.; Licence, P. Amino Acid-Based Ionic Liquids: Using XPS to Probe the Electronic Environment Via Binding Energies. Phys. Chem. Chem. Phys. 2011, 13, 17737-17748. (49) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1-19. (50) Wang, J.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. Development and Testing of a General Amber Force Field. J. Comput. Chem. 2004, 25, 1157-1174. (51) Zhang, Y.; Maginn, E. J. A Simple AIMD Approach to Derive Atomic Charges for Condensed Phase Simulation of Ionic Liquids. J. Phys. Chem. B 2012, 116, 10036-10048. (52) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al Gaussian 09, Revision B.01. Gaussian, Inc., Wallingford CT 2009, .


27


Page 28 of 30

(53) Bayly, C. I.; Cieplak, P.; Cornell, W.; Kollman, P. A. A Well-Behaved Electrostatic Potential Based Method using Charge Restraints for Deriving Atomic Charges: The RESP Model. J. Phys. Chem. 1993, 97, 10269-10280. (54) Hockney, R. W.; Eastwood, J. W., Computer Simulation using Particles; CRC Press: 1988, . (55) Sun, H. COMPASS: An Ab Initio Force-Field Optimized for Condensed-Phase Applications Overview with Details on Alkane and Benzene Compounds. J. Phys. Chem. B 1998, 102, 7338-7364. (56) Martínez, J. M.; Martínez, L. Packing Optimization for Automated Generation of Complex System's Initial Configurations for Molecular Dynamics and Docking. J. Comput. Chem. 2003, 24, 819-825. (57) Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. PACKMOL: A Package for Building Initial Configurations for Molecular Dynamics Simulations. J. Comput. Chem. 2009, 30, 2157-2164. (58) Hoover, W. G. Canonical Dynamics: Equilibrium Phase-Space Distributions. Phys. Rev. A 1985, 31, 1695-1697. (59) Shinoda, W.; Shiga, M.; Mikami, M. Rapid Estimation of Elastic Constants by Molecular Dynamics Simulation Under Constant Stress. Phys. Rev. B 2004, 69, 134103. (60) Sharma, A.; Zhang, Y.; Gohndrone, T.; Oh, S.; Brennecke, J. F.; McCready, M. J.; Maginn, E. J. How Mixing Tetraglyme with the Ionic Liquid 1-N-Hexyl-3-Methylimidazolium Bis (Trifluoromethylsulfonyl) Imide Changes Volumetric and Transport Properties: An Experimental and Computational Study. Chem. Eng. Sci. 2017, 159, 43-57. (61) Broderick, A.; Khalifa, Y.; Shiflett, M. B.; Newberg, J. T. Water at the Ionic Liquid–Gas Interface Examined by Ambient Pressure X-Ray Photoelectron Spectroscopy. J. Phys. Chem. C 2017, 121, 7337-7343. (62) Steinrück, H. P.; Libuda, J.; Wasserscheid, P.; Cremer, T.; Kolbeck, C.; Laurin, M.; Maier, F.; Sobota, M.; Schulz, P. S.; Stark, M. Surface Science and Model Catalysis with Ionic Liquid-Modified Materials. Adv. Mater. 2011, 23, 2571-2587. (63) Kolbeck, C.; Cremer, T.; Lovelock, K. R. J.; Paape, N.; Schulz, P. S.; Wasserscheid, P.; Maier, F.; Steinrück, H. P. Influence of Different Anions on the Surface Composition of Ionic Liquids Studied using ARXPS. J. Phys. Chem. B 2009, 113, 8682-8688. (64) Olivieri, G.; Parry, K. M.; Powell, C. J.; Tobias, D. J.; Brown, M. A. Quantitative Interpretation of Molecular Dynamics Simulations for X-Ray Photoelectron Spectroscopy of Aqueous Solutions. J. Chem. Phys. 2016, 144, 154704-154712.


28

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(65) Baer, M. D.; Kuo, I. W.; Bluhm, H.; Ghosal, S. Interfacial Behavior of Perchlorate Versus Chloride Ions in Aqueous Solutions. J. Phys. Chem. B 2009, 113, 15843-15850. (66) Paynter, R. W. An ARXPS Primer. J. Electron. Spectrosc. Relat. Phenom. 2009, 169, 1-9. (67) Powell, C.; Jablonski, A.; Salvat, F. NIST Databases with Electron Elastic-Scattering Cross Sections, Inelastic Mean Free Paths, and Effective Attenuation Lengths. Surf. Interface Anal. 2005, 37, 1068-1071. (68) Lovelock, K. R. J.; Kolbeck, C.; Cremer, T.; Paape, N.; Schulz, P. S.; Wasserscheid, P.; Maier, F.; Steinrück, H. P. Influence of Different Substituents on the Surface Composition of Ionic Liquids Studied using ARXPS. J. Phys. Chem. B 2009, 113, 2854-2864. (69) Baldelli, S. Interfacial Structure of Room-Temperature Ionic Liquids at the Solid-Liquid Interface as Probed by Sum Frequency Generation Spectroscopy. J. Phys. Chem. Lett. 2013, 4, 244-252.


29


Page 30 of 30

TOC Graphic


30

Anion Enhancement at the Liquid-Vacuum Interface of an Ionic Liquid

Recommend Documents