Surface Structure of Quaternary Ammonium-Based Ionic Liquids

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Surface Structure of Quaternary Ammonium-Based Ionic Liquids Studied Using Molecular Dynamics Simulation: Effect of Switching the Length of Alkyl Chains Seiji Katakura, Naoya Nishi, Kazuya Kobayashi, Ken-ichi Amano, and Tetsuo Sakka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00799 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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The Journal of Physical Chemistry

Surface Structure of Quaternary Ammonium-Based Ionic Liquids Studied Using Molecular Dynamics Simulation: Effect of Switching the Length of Alkyl Chains Seiji Katakura, Naoya Nishi,∗ Kazuya Kobayashi, Ken-ichi Amano, and Tetsuo Sakka Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615–8510, Japan E-mail: [email protected] Phone: +81–75–383–2493. Fax: +81–75–383–2490

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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

Abstract The surface structure of four quaternary ammonium-based ionic liquids (QaILs) at the QaIL|vacuum interface has been analyzed using molecular dynamics simulation to investigate the effect of switching the length of alkyl chains (𝑘) of the quaternary ammonium cations on the surface structure. These four QaILs are composed of a common anion, bis(trifluoromethane sulfonyl)amide (TFSA− ) and different cations: butyltrimethylammonium (N+ , 𝑘 = 1), di 1114 butyldimethylammonium (N+ , 𝑘 = 2), tributylmethylammonium (N+ , 𝑘 = 3), and tetra 1144 1444 butylammonium (N+ , 𝑘 = 4), where 𝑘 represents the number of butyl chains. All the QaILs 4444 show the same features as well-studied imidazolium-based ionic liquids: the formation of the interfacial ionic layers and the orientational preference that non-polar parts of ions point to the vacuum phase. The thickness of the first ionic layer decreases with increasing 𝑘. This results from two-dimensional nano-segregation between polar and non-polar parts of ions, where the state of the polar parts changes from the continuous phase for small 𝑘 to dispersed one for large 𝑘 because of the enlargement of the non-polar domain with increasing 𝑘. Orientational distributions of the butyl chains of the Qa cations indicates that the orientational preference of the butyl chains pointing to the vacuum phase is weakened with increasing 𝑘, especially significantly from 𝑘 = 1 to 2. Even for 𝑘 = 4, N+ still shows the orientational preference 4444 in spite of its symmetric structure. A linear relation is found between the interfacial potential differences and the surface densities of the Qa cations, suggesting a possibility to control surface-absorptivity of dipolar gas molecules in ILs by changing the cation size.

Introduction Ionic liquids(ILs), which are solely composed of ions and are liquid around room temperature, have several attractive properties such as negligible volatility, non-flammability, and thermal, chemical, and electrochemical stability. ILs exhibit unique behaviors and structures in the nano order. 1,2 In particular, the IL structure at the electrified interface has attracted attention 3 in a variety of research fields. Both the theoretical 4,5 and experimental 6 studies revealed that the electric double

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layer structure cannot be expressed by conventional electric double layer theories, such as GouyChapman theory. Even in very recent years, novel behaviors of ILs have been discovered such as partial disorder of Coulombic ordering of ions in nano-pores 7 and extraordinarily long potential screening. 8 The free surface of ILs is the simplest interface in the sense that substance interacting with IL ions is absent. Therefore, the surface structure of ILs provides the scientific basis for revealing the nature of the nanostructure of ILs at the various interfaces. ILs also have promising applications in the IL-gas two-phase system such as gas chromatography, 9–11 nanoparticle synthesis, 12 CO2 capture and separation, 13–16 and gas sensing. 17,18 The mass transfer and chemical reaction occurring at the surface (IL|gas interface) are the critical steps in these applications and are subject to be governed by the properties of ILs at the surface. Because ILs are composed of cations and anions, ILs can be designed by tuning their ionic structure and combinations. Although this designability has succeeded in controlling the physicochemical properties of IL bulk, it is not trivial to design IL ions for controlling the surface properties of ILs. As the spatial and orientational distributions of IL ions at the IL surface determine the surface properties, 19 it is important to clarify the surface structure to develop a strategy for choosing and tuning IL ions to control the surface properties. The surface structure of ILs has been studied by experimental and computational techniques such as x-ray reflectometry (XR), 20–25 neutron reflectometry (NR), 26 sum frequency generation (SFG), 21,27–29 ellipsometry, 30 metastable atom electron spectroscopy (MAES), 31 direct recoil spectroscopy (DRS), 32,33 angle-resolved x-ray photoelectron spectroscopy (ARXPS), 34–36 Rutherford back-scattering spectroscopy (RBS), 37–39 low energy ion scattering (LEIS), 40 inelastic and reactive atom scattering, 41,42 and molecular dynamics simulation (MD). 43–49 Most of these studies focused on imidazolium-based ILs with 1-alkyl-3-methylimidazolium (C+𝑛mim ) cations. These studies have revealed mainly two general features for the surface structure of C+𝑛mim -based ILs. One is the spontaneous formation of ionic layers at the surface, whose layer thickness corresponds to the diameter of ions. 20,50 The other feature is a preferential orientation of ions at the first ionic layer with non-polar parts of ions pointing to the gas phase. 34,51,52 This orientation seems to involve the close packing of

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polar parts of ions and results in the high-density region at the first ionic layer. The ionic layer and the orientational preference are not only to C+𝑛mim -based ILs but also to other ILs, such as quaternary ammonium-based ILs (QaILs) 22,28,53,54 and quaternary phosphonium-based ILs (QpILs). 55,56 The surface structure of C+𝑛mim -based ILs depends on the alkyl chain length of C+𝑛mim , as studied using MD simulations 45–47,50,57,58 and experiments, such as NR, 26 RBS, 38,39 SFG, 29,59 MAES, 31 neutral impact collision ion scattering spectroscopy (NICISS), 60 reactive atom scattering (RAS), 61 LEIS, 40 ARXPS, 34,62 and DRS. 33 These studies revealed that C+𝑛mim ions with longer alkyl chains show a higher orientational preference and a thicker non-polar region at the surface. Comparisons of pyrrolidinium-based ILs with C+𝑛mim -based ILs found the similarity of the two kinds of cations in the dependence of the surface structure on the alkyl chain length. 42,63,64 In the non-polar region at the surface, alkyl chains are not fully packed. Hence, other parts of IL ions, such as polar-parts of the cation and the polar/non-polar parts of the anion, are also exposed at the surface. Previous MD simulations demonstrated the exposure of these parts at the surface of ILs such as [C+2mim ][BF4 – ] 45 and [C+6mim ][TFSA− ] 65 (TFSA− = bis(trifluoromethanesulfonyl)amide). The surface coverage of alkyl chain increases with increasing the alkyl chain length of a cation. 21,31,40,45,58,65 The polar-parts of ions are still exposed at the surface of [C+8mim ][BF4 – ] 45 and [C+12mim ][TFSA− ], 58 whose cations have long alkyl chains and form distinct alkyl layer at the surface. These alkyl chains of cation at the alkyl layer have the orientational preference pointing to the vacuum phase, but it does not mean the formation of the fully packed alkyl layer at the surface of ILs. Experimental studies also proposed that there are spaces between alkyl chains for example by a study using SFG and XR at the surface of [C+3mim ][I− ], [C+4mim ][BF4 – ], and [C+4mim ][PF6 – ]. 21 It is controversial whether long alkyl chains of cation cover whole the surface or not. A DRS study for [C+12mim ][PF6 – ] 33 suggested that PF6 – anions are exposed at the surface. A LEIS study 40 revealed that TFSA− are exposed at the surface even for the case with C+𝑛mim and Qp cations with long alkyl chains. On the other hand, a MAES study changing the length of the alkyl chain of the cation indicated that the long alkyl chains fully cover the surface of ILs and hide the anions. 31 These different results between DRS, LEIS, and MAES are likely due to the different probing depth. 31

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+

N

CM +

CB1

N

CB2 CB3

CM

CB1

CB4

N1144+, k =2

N1444+, k =3

O

+

F

CB3

S C

CB2

F CB4

N4444+, k =4

CB2 CB3

CB4

N

CB1

CM

CB1

CB2 CB3

N1114+, k =1

+

N

NBT

OBT

-

SBT

O O F

F

CB4

FBT

CBT

F

TFSA−

Figure 1: Chemical structure of QaILs and definition of atom names and the number of butyl chains, 𝑘, in this paper. Hydrogen atoms are not shown but explicitly considered in the simulation. Compared with the studies on the alkyl chain length dependence, a limited number of studies focused on the number of butyl chains (𝑘). An MD study 44 by Sarangi et al. focused on cation symmetry and compared the surface structure of symmetric 1-alkyl-3-alkylimidazolium (C𝑛 C+𝑛im ) with asymmetric C+𝑛mim . The study showed that the symmetric cations have lower orientational preferences than the corresponding asymmetric ones. The authors discussed that the lower orientational preferences for C𝑛 C+𝑛im are caused by the geometric constraint of C𝑛 C+𝑛im where if one alkyl chain points to the vacuum phase then the other has to point to the bulk phase. A question is what if a cation have more than two alkyl chains. The introduction of the third alkyl chain is difficult, if not impossible, for the imidazolium ions. However, some other types of IL cation can have more than two alkyl chains, such as Qa cations and Qp cations. Hereafter in the present paper the Qa and Qp cations with four normal alkyl groups are denoted as N+𝑎𝑏𝑐𝑑 and P+𝑎𝑏𝑐𝑑 , respectively, where 𝑎, 𝑏, 𝑐, and 𝑑 represent the number of carbon atoms of the alkyl groups. The bulk structure of these ILs has been studied by small angle x-ray scattering, 66–69 wide angle x-ray scattering, 67,69,70 NMR, 71 Raman scattering spectroscopy, 72–74 broadband dielectric spectroscopy, 75,76 and MD. 67–70,77–81 In contrast to the bulk studies, a limited number of studies have focused on the

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surface structure of QaILs, 22,28,40,53–55,82–85 some of which used cations having more than two alkyl chains. 22,28,53,55,56,83–85 A comparison between N+6444 and C+4mim has been made by Baldelli et al. using SFG, 53 revealing that the butyl chain in both the cations has similar orientation preferences at the surface. The same research group also studied 28 the surface of a series of [N+11iPr𝑛 ][TFSA− ] (iPr = isopropyl, 𝑛 = 3, 4, 6, 10), which revealed that the orientation of the alkyl chain strongly depends on the length and that the longer alkyl chains have 𝑔𝑎𝑢𝑐ℎ𝑒 conformation. In our previous studies, we revealed ionic multilayer structure at the surface of QaILs and QpILs by using XR. 22,55,84 We have also reported an MD simulation 85 at the surface of [N+1444 ][TFSA− ], and showed the multilayer structure and ionic orientations in which the non-polar parts of the ions point to the vacuum phase. Recently Peñalber-Johnstone et al. reported a surface study for various types of QpILs using SFG and MD simultaneously. 56 In a comparison of several [P+888𝑛 ][Cl – ] (𝑛 = 4, 5, 6, 10, 12, 14), the SFG spectra showed an abrupt change in the CH stretching region from 𝑛 = 10 to 12, and the MD simulation revealed an abrupt increase in 𝑔𝑎𝑢𝑐ℎ𝑒 occurrence from 𝑛 = 8 to 10. They also found that any one of the four alkyl chains is nearly equally probable to occupy the outer most region of the surface. In the study, [P+8888 ][Cl – ] and [P+4444 ][Cl – ], whose cations have a symmetric structure, even showed the SFG signal in the CH stretching region, which may indicate that inversion symmetry of these symmetric cations is broken at the surface due to their preferential orientation. In spite of the findings of the previous studies described above, it has not been clarified how the surface structure of ILs is affected and can be controlled by the systematic change in the number of alkyl chains of the IL cations. In the present study, we performed MD simulations of the surface of a series of QaILs (shown in Fig. 1) to investigate the surface structure depending on the number of butyl chains from 𝑘 = 1 to 4 with a combination of methyl groups and butyl chains for the four alkyl groups of the Qa cations. The most significant difference among the four QaILs was the orientational preference of the Qa cations, which decreased with increasing 𝑘. The preference was also observed for 𝑘 = 4, which is the most symmetric cation among the four Qa cations. In contrast with the Qa cations, the influence of 𝑘 on the orientational preference of TFSA− was remarkably small. Another notable difference was that the increase in 𝑘 causes the formation of the thinner first

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ionic layer at the surface.

Methods –computational details– MD simulations at the surface of QaILs were performed by using DL_POLY classic. 86 The QaILs are composed of TFSA− and Qa cations, whose four alkyl groups are either of methyl and butyl. The number of butyl chains of the Qa cations (𝑘) was varied from one to four; butyltrimethylammonium (N+1114 , 𝑘 = 1), dibutyldimethylammonium (N+1144 , 𝑘 = 2), tributylmethylammonium (N+1444 , 𝑘 = 3), and tetrabutylammonium (N+4444 , 𝑘 = 4) were studied. These cations and TFSA− were modeled by an all-atom non-polarizable force field, CL&P, 87 which is widely used for the MD simulations of the bulk of ILs including QaILs 69,79 and the surface of ILs. 42,46,48,65,88 Relative permittivity was set to 𝜀r = 2 to take into account electronic polarization, 89,90 which corresponds to the reduction of the Coulombic interaction by the scaling of atomic charges by a factor of 0.7. In an IL bulk simulation, the scaling factor was recommended between 0.7 and 0.8 91 to reproduce experimental dynamic properties such as diffusion coefficients, although the structure is less sensitive to the scaling factor. In the present study, we chose 𝜀r = 2 (corresponding to the scaling factor of 0.7) by considering the fact that experimental values of refractive index of QaILs, 𝑛vis , are about 1.4 30,92 at a visible wavelength and the relationship 𝜀r = 𝑛2vis . The long-range electrostatic interactions were evaluated by using Smooth Particle Mesh Ewald (SPME) method, 93 with an accuracy of 1 × 10−5 . As in previous simulations, meaningful result could be obtained using conventional 3-dimensional periodicity without the need for the employment of a slab correction. A real space cutoff distance ̊ 87 was set to 10 Å for the nonbonded interactions, which is slightly less than a typical value, 12 A. The structure of QaILs and definitions of atom names in the present study are shown in Fig. 1. QaIL slabs were settled in orthorhombic MD cells in the surface simulations (Fig. 2). Ion pairs were put into the MD cells to simulate the QaIL slabs (Table 1). In all the cases of QaILs, the number of atoms in the slab was over 20,000 (Table 1). The size for the in-plane direction to the 7



̊ ∼ 50 A) ̊ and the thickness of the slab 𝑑slab was ∼ 100 A. ̊ The size surface was (𝑙𝑥 , 𝑙𝑦 ) = (∼ 50 A, of the MD cells was set to the same as the slab size for the in-plane direction (𝑙𝑥 , 𝑙𝑦 ) and to 300 Å for the out-of-plane direction (𝑙𝑧 ) (Fig. 2). These values identifying the geometries of the MD cells are summarised in Table 1. Table 1: Number of ion pairs and atoms, and cell size. QaIL

𝑁ion

𝑁atom

3 𝑙𝑥 × 𝑙𝑦 × 𝑙𝑧 / Å

[N+ ][TFSA− ] 1114 [N+ ][TFSA− ] 1144 [N+ ][TFSA− ] 1444 [N+ ][TFSA− ] 4444

560 460 360 340

22960 23000 21240 23120

51.5×51.5×300.0 51.9×51.9×300.0 50.0×50.0×300.0 51.0×51.0×300.0

Figure 2: Geometry of MD cell.

Initial configurations of the QaIL surface simulations were prepared from bulk simulations in the following way. First of all, initial configurations of the bulk systems were prepared by putting the ion pairs randomly inside cubic MD cells at a low density. Then, simulations for the bulk systems were performed for 1 ns. During the bulk simulations of QaILs pressure and temperature were controlled to 1 atm and 423 K by using Berendsen barostat and thermostat, 94 respectively (𝑁𝑃 𝑇 ensemble). The final configurations of the bulk simulations were used to make the cells for the QaILs|vacuum interface system by combing these primitive cells and the image cells at +𝑙𝑧 . The values of 𝑙𝑧 were reconfigured to be 300 Å for the surface simulations (Fig. 2). Since molecules crossing the periodic boundaries of the 𝑧-direction in bulk cells conflict the new periodic boundaries, these parts of molecules were translated along the 𝑧-axis. The configurations were equilibrated for 1 ns to prepare the initial configurations of the surface simulations. These initial configurations were prepared 6 times for each QaIL with the procedures described above, and then 8


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the surface simulations were performed for 2 ns with collecting trajectories for each initial configuration. During the surface simulations of QaILs, the temperature was controlled at 423 K by using the Nose-Hoover thermostat (𝑁𝑉 𝑇 ensemble). 95,96 The high temperature was chosen to increase statistical reliability. The integration time step was 2 fs for all the simulations.

–data analysis– To improve statistical reliability, we prepared 6 different initial configurations for the QaILs|vacuum interface systems and performed the surface simulations for each QaIL. As the slabs in the MD cells have two surfaces, 12 trajectories of the surface were obtained for each QaIL. Actually, we found that 6 trajectories (3 initial configurations) were reliable enough to discuss the results shown in Results and Discussion (see Fig. S1 and S2). We used 12 trajectories to improve the data quality further. We also found that 12 trajectories for 2 ns produce higher statistical reliability than 2 trajectories for 12 ns (see Fig. S3). To average information obtained from these trajectories of the surface, the Gibbs dividing surfaces were determined for all the surfaces and defined at 𝑧 = 0 with the positive direction of the 𝑧-axis from the vacuum phase to the IL phase (Fig. 2). The Gibbs dividing surfaces located at 𝑧 = 0 satisfy the following equation. 𝑧bulk

0

∫𝑧vacc

𝜌M (𝑧)d𝑧 =

∫0

) ( − 𝜌M (𝑧) − 𝜌M,bulk d𝑧,

(1)

where 𝜌M (𝑧) is the mass density distribution and 𝜌M,bulk is mass density in the bulk IL phase. The integral limits, 𝑧vacc and 𝑧bulk , are chosen sufficiently far from the interfacial transition region so that 𝜌M (𝑧vacc ) = 0 and 𝜌M (𝑧bulk ) = 𝜌M,bulk are satisfied. Fig. S4 shows a graphical example of how to determine the Gibbs dividing surface. We obtained 𝜌M,bulk by averaging 𝜌M (𝑧) for ±30 Å in the 𝑧-direction from the center of the IL slab, 𝑧0 . We also used 𝑧0 as the upper limit of the integral range, 𝑧bulk , in the right side of Eq.(1). After the Gibbs dividing surfaces were determined, number density distributions 𝜌𝑁,i (𝑧) of atom i from the simulations were averaged for each QaIL. ̊ were analyzed. The 𝑧 Orientational distributions of ions at the topmost ionic layer (𝑧 < 10 A)

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position of the ions was represented by the nitrogen atoms (N for the cations and NBT for the anion, see Fig. 1). The orientational distribution 𝑝(𝜃) is defined as

∬

𝑝(𝜃) sin 𝜃d𝜃d𝜙 = 4𝜋

(2)

where we assumed that the surface is azimuthally isotropic and therefore it can be simplified as

∫

𝑝(𝜃) sin 𝜃d𝜃 = 2

(3)

where 𝜃 is the orientational angle between 𝑧-axis and an intramolecular vector of ions (see Fig. 3 (a)). Because of this definition, 𝜃 = 0◦ (𝜃 = 180◦ ) means that the vector points to the bulk phase (the vacuum phase), and 𝜃 = 90◦ means that the vector is parallel to the interface. Note that 𝑝(𝜃) describes the ratio of the population at 𝜃 to that for isotropic distribution. In the case of isotropic distribution (𝑝(𝜃) = 1), the population distribution of 𝜃 is sin 𝜃, because solid angle depends on 𝜃. When an ion has two or more intramolecular vectors (e.g., N+4444 has four N to CB4 vectors), we regarded these vectors to be independent of each other to obtain the 𝑝(𝜃) profiles. We averaged 𝑝(𝜃) obtained from the 12 interfaces as well as the number density distributions. To compare the orientations of the four different cations, we also calculated 𝑝(𝜃) of resultant vectors of the intramolecular vectors from N to CB4 (res. N-CB4 ) as a measure of the orientation of the Qa cation. An example of the resultant vectors of N+1444 is shown in Fig. 3 (b).

Figure 3: (a) Schematic illustration of an intramolecular vector and the orientational angle 𝜃. (b) Illustration of the resultant vector of three N to CB4 vectors in N+1444 cations. 10


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∑

Potential profiles Φ(𝑧) were calculated by integrating charge density distributions 𝜌chg (𝑧) = i 𝑞i 𝜌𝑁,i (𝑧)

by using the Poisson equation, where 𝑞i is the partial charge of atom i. The one-

dimensional form of the Poisson equation is 𝜌chg (𝑧) d2 Φ(𝑧) = − , 𝜀0 𝜀r d𝑧2

(4)

where 𝜀0 is the permittivity of vacuum. The integral constant of the first integration was set to match the potentials at the two 𝑧-sides of the periodic boundary cell (tinfoil boundary condition) because SPME was used in this simulation. The integral constant of the second integration was set to satisfy that the potential in the bulk QaIL phase is 0, which was calculated by averaging the ̊ potential for 𝑧 = 𝑧0 ± 20 A.

Results and discussion –top-view snapshots and surface coverage– As described in Introduction, previous studies have revealed that the non-polar parts of cations and anions prefer to point to the gas (or vacuum) phase at the surface of ILs. 31,33,38,48 Depending on the size of the non-polar parts of cations (alkyl chain length), the surface coverage of the alkyl chain changes; 42 with increasing the length, more alkyl chains cover the surface. 31,33,60 Here we investigate the surface coverage (in other words surface composition) of each part of the ions depending on 𝑘. Top-view snapshots at the surface of [N+1114 ][TFSA− ], [N+1144 ][TFSA− ], [N+1444 ][TFSA− ], and [N+4444 ][TFSA− ] are shown in Fig. 4. The red/green parts represent the polar/non-polar parts of the Qa cations and the blue/yellow parts represent that of TFSA− , respectively. The radii of the spheres are van der Waals radii of the atoms. At the surface of [N+1114 ][TFSA− ] (Fig. 4 (a)) both of the polar (blue) and non-polar (yellow) parts of TFSA− were exposed at the surface. Such exposed anions have been reported in a previous study by XR and SFG 21 for C+4mim that has one butyl chain like N+1114 . In a previous LEIS study, 40 11



Figure 4: Top-view snapshots of the surface of QaILs: (a) [N+1114 ][TFSA− ], (b) [N+1144 ][TFSA− ], (c) [N+1444 ][TFSA− ], and (d) [N+4444 ][TFSA− ]. (e) Surface coverage of polar/non-polar parts for the four QaILs, estimated from Fig. S6, with following coloring rule: (red) N, CM , CB1 , HM , and HB1 ; (green) CB2−B4 and HB2−B4 ; (blue) NBT , SBT and OBT ; and (yellow) CBT and FBT . The snapshots are generated by using VMD. 97 fluorine peak was observed for [N+1114 ][TFSA− ], indicating that TFSA− is exposed at the surface. In Fig. 4 (a), the polar parts of cation (red) were also exposed to the vacuum phase due to the low number densities of the non-polar parts of cation (green) at the surface of [N+1114 ][TFSA− ]. From these results, we can expect that both polar and non-polar part of ions can be experimentally detected. However, in the LEIS study, 40 no nitrogen peak was observed for [N+1114 ][TFSA− ] as well as for other TFSA− -based ILs, such as [C+2mim ][TFSA− ]. The authors concluded that the amount of nitrogen of N+1114 and TFSA− at the outer atomic surface is lower than the detection limit, which is 2%. In the present study, at the surface of [N+1114 ][TFSA− ], we also estimated the surface amount of the nitrogens of TFSA− and N+1114 to be ∼ 2% and ∼ 0.1%, respectively (Fig. S5), by evaluating blue and red colored areas in Fig. S5. These values correspond to the low value discussed in the LEIS study. 40 Fig. 4 (a) demonstrates that the non-polar parts of [N+1114 ][TFSA− ] are not large enough to cover the whole area of the surface. This low surface coverage of non-polar parts is important to have an image of the surface structure. In the later section, the highest orientational preference of N+1114 among the four Qa cations will be shown, but it does not mean the formation of a fully packed 12


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alkyl layer. Here we compare the top-view snapshots of the four QaILs in Fig. 4 (a), (b), (c), and (d). With increasing the number of butyl chains, 𝑘, from one for [N+1114 ][TFSA− ] up to four for [N+4444 ] [TFSA− ] the surface coverage of the butyl chain (green) increased and those of the other parts decreased. We estimated the surface coverage of the polar (red) and non-polar (green) parts of the Qa cations and the polar (blue) and non-polar (yellow) parts of TFSA− exposed at the surface. We converted Fig. 4 into four-valued images and analyzed the coverages of these four parts (Fig. 4 (e)). The converted figures are shown in Fig. S6. As shown in Fig. 4 (e), the surface coverage of the butyl chain (green) monotonically increased with increasing 𝑘, from the lowest for [N+1114 ][TFSA− ] (23 %) to the highest for [N+4444 ][TFSA− ] (55 %). The rate of change of this coverage decreases with increasing 𝑘, indicating that some of the additional butyl chains, exposed to the surface, are tilting towards bulk. On the other hand, the coverages of the other parts decreased for larger 𝑘, especially for the polar parts of the cations (red), indicating that the additional butyl chain mainly covers the polar-part of the cation. This coverage of the polar-part of the cation (red) was lowest for [N+4444 ] [TFSA− ] (11 %). The non-zero value indicates that the butyl chains of the Qa cations are not large enough to cover the whole surface even with 𝑘 = 4.

–number density distribution profiles– To clarify the structure of the ionic layers at the surface of the four QaILs and its dependence on 𝑘, we investigated number density distributions of atoms i 𝜌N,i (𝑧). Fig. 5 (a1), (b1), (c1), (d1) represent 𝜌N,i (𝑧) of atoms constituting the cations and Fig. 5 (a2), (b2), (c2), (d2) represent that of TFSA− for the four QaILs. Note that the 𝜌N,i (𝑧) profiles are scaled by the number of atom i in an ion. For example, 𝜌N,CB4 (𝑧) profiles of N+1114 , N+1144 , N+1444 , and N+4444 was scaled by 1, 2, 3, and 4, respectively. As described in the prior section showing the top-view images, we classified polar / non-polar parts of ions (see Fig. 4). First of all, we will focus on [N+1114 ][TFSA− ] (Fig. 5 (a1) and (a2)) to see common features among the four QaILs. In Fig. 5 (a1) N and CM , which are the polar parts of N+1114 , showed distinct 13



̊ roughly corresponding to the diameter of the ions. On high-density layer with a thickness of 10 A, the other hand, the non-polar parts (CB2 , CB3 , and CB4 ), especially CB4 , showed a low-density dip in the same region. The non-polar parts of N+1114 are distributed in the topmost region of the first ionic ̊ with a systematic trend; with increasing the distance of the C atoms of the butyl layer (𝑧 < 0 A) chain from the quaternary N atom in the ionic structure (from CB1 to CB4 ), the C atoms are more distributed to the vacuum phase from the surface. On the other hand, CM is distributed in the inner region compared with the butyl chain. In Fig. 5 (a2) the anion TFSA− had the same two features as N+1114 in which polar parts formed first ionic layer and non-polar parts preferred the outer region of this layer. These distribution difference between the polar and non-polar parts means that they are segregated at the QaIL surface. Since the polar and non-polar parts are covalently bonded, the surface segregation indicates that both the cation and anion at the first layer have an orientational preference in which the non-polar parts, such as alkyl and CF3 groups, point to the vacuum phase. Previous studies showed this orientation of the alkyl chain of C+𝑛mim by MD 44,45,57,65,98 and experiments 31,99,100 and that of CF3 of TFSA− by MD 48 and experiments. 31,38 Alkyl chains and CF3 of several QaILs, [N+11iPr𝑛 ][TFSA− ] (𝑛 = 3, 4, 6, 10), 28 [N+1113 ][TFSA− ], 82 and [N+1444 ][TFSA− ], 85 also have these orientational preferences. As shown in Fig. 4 (a), the non-polar parts of the anion (yellow) were exposed to the vacuum phase, agreeing with these previous studies. A careful comparison of the profiles at Fig. 5 (a1) and (a2) at −10 < 𝑧 < 0 Å indicates that the non-polar parts of TFSA− were less protruded from the surface compared with the non-polar parts of N+1114 . However, for the charge density distribution at the outermost surface, the non-polar parts of TFSA− contribute much more than that of the cation. Therefore the outermost surfaces of the four QaILs are negatively charged, which are indicated in Fig. S7 as the charge density distribution and will ̊ polar parts be further discussed in the later section on the potential profiles. In 0 < 𝑧 < 10 A, of N+1114 showed higher density than bulk density and non-polar parts showed lower density than that of polar parts. The orientation of the N+1114 in the first layer, in combination with the mutual exclusion of ions, enables such high ionic density at the first ionic layer. Quantitative analysis of the orientation will be shown later.

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Figure 5: Number density distributions as a function of 𝑧 at the surface of (a) [N+1114 ][TFSA− ], (b) [N+1144 ][TFSA− ], (c) [N+1444 ][TFSA− ], and (d) [N+4444 ][TFSA− ]: (1) Atoms of the cations; N (red), CM (blue-green), CB1 (grey, dotted), CB2 (grey, dashed), CB3 (grey, solid), and CB4 (black); and (2) atoms in the anions; NBT (blue), SBT (grey, dashed), OBT (grey, solid), CBT (black), and FBT (grey, dotted). Black dotted lines represent the bulk number density. All the number densities are normalized by the atomic compositions of ions.

15



To compare these number density distributions for the four QaILs, the bulk normalized number densities (𝜌N,i (𝑧)∕𝜌bulk ) are shown in Fig. 6. In Fig. 6 (a), the polar part of all the cations showed ̊ The thickness of the first ionic layer decreased with distinct high-density layer around 𝑧 ∼ 5 A. increasing 𝑘, indicating that the large cation formed the thin first ionic layer. In Fig. 6 (b), the polar part of TFSA− also showed the less pronounced first ionic layer for larger 𝑘. Therefore, the polar parts of both the cation and anion should play a role to determine the thickness of the first ionic layer. In other words, it is likely that the polar parts of cation and anion are two-dimensionally aggregated at the surface and form the polar domain that is smaller for larger 𝑘. This difference in the thickness of the polar domain corresponds to the difference in the segregation state among the four QaILs. Here we compare [N+1114 ][TFSA− ] (Fig. S8 (a)) and [N+4444 ] ̊ where the polar-parts formed [TFSA− ] (Fig. S8 (b)) as the two extreme cases. In 0 < 𝑧 < 10 A, the layer, the polar parts for [N+1114 ][TFSA− ] (Fig. S8 (a2)) formed a continuous domain, whereas those for [N+4444 ][TFSA− ] (Fig. S8 (b2)) formed a dispersed domain. Oppositely, the non-polar domain was dispersed for [N+1114 ][TFSA− ] but continuous for [N+4444 ][TFSA− ] (Fig. S8 (a3) and (b3)). Such two-dimensional segregation of polar and non-polar domains at the surface is in contrast to the nanostructural organization in IL bulk phase, 79,101 in which polar and non-polar parts form domains and are segregated with each other. In bulk, when the alkyl chain of the cation is short, the polar domain forms continuous microphases whereas an increase in the alkyl chain length leads to an enlargement of the non-polar domain and the continuous polar domain eventually become dispersed. 101 The change from continuous to dispersed phase also seems to occur at the surface of these four QaILs. Formation of the distinct non-polar domain along the surface has been reported at the surface of [C+8mim ][BF4 – ], 45 and [C+8mim ] octylsulfate. 88 In particular, the formation of two-dimensional structure, lamellar-like structure, is proposed at the surface of [C+8mim ] octylsulfate, 88 in which both cation and anion have octyl chains. This lamellar-like structure seems to be an extreme case of the twodimensional segregation of the polar and non-polar parts in ILs at the surface. Because [N+1114 ] [TFSA− ] did not form the lamellar-like structure in the present study, the lamellar-like structure

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Figure 6: Bulk normalized number density distributions as a function of 𝑧 at the surface of; [N+1114 ] [TFSA− ] (green), [N+1144 ][TFSA− ] (blue), [N+1444 ][TFSA− ] (gray), and [N+4444 ][TFSA− ] (red): (a) N of cations, (b) NBT of TFSA− anions, and (c) CB4 of cations. The inset in (a) shows the first peaks of N of cations.

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formation seems to require the long linear alkyl chains for both the cation and anion. In Fig. 6 (b), the polar parts of TFSA− showed a less distinct first ionic layer compared to the cation. This trend agrees with previous MD studies for the surface of TFSA− -based ILs. 44,48,57 In TFSA− , not only the center N atom but also four O atoms have large negative charges. These relatively delocalized charges in TFSA− probably produced a variety of coordination of TFSA− around the cations and caused the delocalized distribution of the polar parts of TFSA− . In Fig. 6 (c), the number densities of non-polar parts were lower than that of bulk density in ̊ while polar parts of cations showed high-density. The low-density region of non0 < 𝑧 < 10 A, polar parts of cation was significant for the case of N+1114 , whereas there was a small difference among N+1144 , N+1444 , and N+4444 . This trend reflects that the orientational preference is the highest for N+1114 among the QaILs, as shown in the next section quantitatively. When a Qa cation has two or more butyl chains, the orientations of the butyl chains influence each other, because all of them are covalently bonded to the center nitrogen atom. As N+1114 does not have this restriction, the number densities of non-polar parts of N+1114 markedly decreased in the polar part of the first layer. ̊ except The number density distributions of non-polar parts of cations showed a peak at 𝑧 > 5 A, for N+1444 . These peak positions shifted to negative direction with increasing 𝑘, also indicating that larger Qa cation forms a thinner layer. In the above discussion, we have focused on the first ionic layer. Next, we will focus on the ionic multilayers. In Fig. 6 (a), only [N+4444 ][TFSA− ] among the four QaILs showed weak multilayer formation, implying that ILs with a large cation tend to form distinct ionic multilayers. A study using coarse-grained MD 50 suggested the monolayer formation for C+𝑛mim with the alkyl chain shorter than butyl chain and multilayer formation for longer ones. The multilayer formation for large cations is also proposed by all-atom MD studies. 45,46,58 Experimentally, some XR studies 22–25,55,84 using large cations, such as N+1888 , 22,55,84 C+18mim , 23 and C+22mim , 24,25 confirmed the existence of the ionic multilayers. On the other hand, XR 20 and NR 26 studies using relatively small cations, such as C+4mim and C+8mim , did not obtain clear evidence of the multilayer formation. MD also has difficulty to determine whether small ions form the ionic multilayers or not, because

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the density distributions for such ions obtained from MD tend to show an ambiguous first ionic layer compared with large ions. The amplitude of the oscillation in the density distribution due to the multilayer formation decreased by increasing the temperature. 43,65,102 Moreover, the amplitude seems to decrease by employing long (∼ 20 ns) simulation time 57,58 and by averaging the density distributions from independent initial configurations. 44 Since the averaging method was employed in the present study, the oscillation amplitude in Fig. 6 is small compared with that before averaging (see Fig. S1). Individual density distributions before averaging (Fig. S1) showed the oscillation similar to that of some of the previous MD studies. 43,45,48,65,102 As the oscillation disappeared for 𝑘 = 1, 2, 3 by averaging the density distributions, these three QaILs will not form the ionic multilayers at the surface.

–orientation of ions– The orientations of butyl chains of the Qa cations were analyzed by using the intramolecular vector from N to CB4 . The results are shown in Fig. 7 (a). The 𝑝(𝜃) profile of butyl chain of N+1114 was ∼ 0.7 in 𝜃 < 80◦ and monotonically increased with increasing 𝜃 in 𝜃 > 80◦ . The maximum of 𝑝(𝜃) at 𝜃 = 180◦ means that butyl chain of N+1114 preferentially points to the vacuum phase at the surface, as predicted from the number density distribution (Fig. 5 (a)). The monotonic increase indicates that the most stable orientation of N+1114 is that its butyl chain perpendicularly points to the vacuum phase. The butyl chain of C+4mim at the surface of [C+4mim ][TFSA− ] has the same orientational preference in a previous MD study. 57 For the other three Qa cations (𝑘 ≥ 2) having more than one butyl chain, 𝑝(𝜃) also indicates that butyl chains prefer to points to the vacuum phase but much less prominently than N+1114 . Note that the orientation of butyl chains of the three Qa cations is restricted because they are covalently bonded to the same quaternary nitrogen atom. For the Qa cations with 𝑘 ≥ 2, 𝑝(𝜃) had minima at 𝜃 ∼ 90◦ and had local maxima at 𝜃 ∼ 0◦ . The corresponding minima were observed in a previous MD study at the surface of [C3 C+3im ][TFSA− ] and [C5 C+5im ][TFSA− ], 44 where the cations have two alkyl chains. In the same study, their monoalkyl analogues, [C3 C+1im ] [TFSA− ] and [C5 C+1im ][TFSA− ], did not show such minima, and showed orientational distribution 19



similar to that of [N+1114 ][TFSA− ]. At 𝜃 ∼ 90◦ , 𝑝(𝜃) is less than unity for the four Qa cations, indicating in-plane butyl chains at the surface are unstable. This corresponds to the lower density of butyl chains and higher density of the polar parts at the first ionic layer than that in bulk (Fig. 6 (a) and (c)). The local maxima of 𝑝(𝜃) at 𝜃 ∼ 0◦ for 𝑘 ≥ 2 indicates that some of the butyl chains also point to the bulk phase, which is the opposite direction to the preferred orientation. According to a previous MD study, 44 when imidazolium cations have two alkyl chains (C𝑛 C+𝑛im ), such an orientation of alkyl chains to point to the bulk phase become pronounced because of the geometric constraint of the two alkyl chains in C𝑛 C+𝑛im . In analogy with C𝑛 C+𝑛im , if one butyl chain for the Qa cations with 𝑘 ≥ 2 points to vacuum phase, orientations of the other butyl chains are not independent of the first one anymore. As butyl chains do not prefer the in-plane orientation, the orientation pointing to the bulk phase increases for 𝑘 ≥ 2. As a result, the introduction of the second and further butyl chain causes the decrease in 𝑝(𝜃) at 𝜃 = 180◦ and oppositely the increase in 𝑝(𝜃) at 𝜃 = 0◦ . ̊ were also analyzed. The orientations of butyl chains in the subsurface region (10 Å < 𝑧 < 20 A) The 𝑝(𝜃) profiles are shown in Fig. S9. These 𝑝(𝜃) profiles are about 1 for all the 𝜃, indicating the isotropic orientation in the subsurface region. We discuss the orientations of the cations by regarding the resultant vectors of intramolecular vectors from N to CB4 (res. N-CB4 ) as a measure of the orientation (Fig. 7 (b)). The orientation angle of this resultant vector indicates the mean orientation of butyl chains in a cation (see Fig. 3). In the case of N+1114 , the vector is the same as the vector from N to CB4 (Fig. 7 (a)), because N+1114 have only one butyl chain. The increase in 𝑝(𝜃) at 𝜃 ∼ 0 for N+1144 ,N+1444 , and N+4444 in Fig. 7 (a), was not observed in Fig. 7 (b), indicating that the orientations of butyl chains in these three Qa cations influence one another. The 𝑝(𝜃) profiles of N+1144 and N+1444 had a small difference. The orientation of N+1114 is the most strongly restricted; 𝑝(𝜃) of N+1114 showed the highest maximum at 𝜃 = 180◦ among the four QaILs. N+4444 , which is the most symmetric among the four Qa cations, also preferred the vacuum direction. In a recent SFG study, 56 similar symmetric cations P+4444 and P+8888 showed CH3 stretching vibration peaks. Since anisotropic orientation leads to the break of inversion symmetry

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Figure 7: Distribution of orientational angle 𝜃 of (a) vector N to CB4 , (b) resultant vector of vectors N to CB4 , (c) vector SBT to CBT , and (d) vector SBT to SBT at the first ionic layer where the N or NBT ̊ Coloring rule: [N+ ][TFSA− ] (green), [N+ ][TFSA− ] (blue), atoms of the ion are in 𝑧 < 10 A. 1114 1144 [N+1444 ][TFSA− ] (gray), and [N+4444 ][TFSA− ] (red). 𝜃 ∼ 180◦ corresponds to the direction from the IL phase to the vacuum phase. 𝜃 ∼ 0◦ corresponds to the opposite direction. Horizontal black lines at 𝑝(𝜃) = 1 represent the isotropic orientation.

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of the cations, which is the selection principle of SFG, such anisotropic orientation revealed in the present study would contribute to the SFG signals. The conformation of the butyl chains of N+4444 predominantly contributes to the anisotropic orientation. The effect of the bond length and angle in butyl chains is small. When all the four butyl chains of N+4444 are in the 𝑡𝑟𝑎𝑛𝑠 conformation the resultant vector of N-CB4 would become 0 because of its symmetry. When some chains are in the 𝑔𝑎𝑢𝑐ℎ𝑒 conformation and the four butyl chains relatively point to the vacuum phase, the resultant vector also points there. The CH2 stretching vibration peaks of several Qp cations in SFG spectra in the previous study 56 demonstrates the existence of such 𝑔𝑎𝑢𝑐ℎ𝑒 conformation. Actually, we observed the 𝑔𝑎𝑢𝑐ℎ𝑒 conformation of the butyl chains at the surface in snapshots. The orientation distributions of CF3 of TFSA− are shown in Fig. 7 (c). Intramolecular vector from SBT to CBT was calculated to evaluate the orientation of CF3 of TFSA− . The 𝑝(𝜃) showed a maximum at 𝜃 = 180◦ for all the QaILs. The maximum indicates that CF3 prefers pointing to the vacuum phase at the surface. In previous experimental studies of QaILs by MAES 31 and RBS 37 and MD studies, 48,57 CF3 of TFSA− showed similar orientation. These 𝑝(𝜃) profiles of all the QaILs were surprisingly similar, suggesting that Qa cations at the surface existing next to TFSA− do not affect the orientation of TFSA− . The orientation distributions of S-S are shown in Fig. 7 (d). Intramolecular vector from SBT to SBT in TFSA− was calculated to evaluate the orientation of TFSA− . The 𝑝(𝜃) profiles of TFSA− were convex in all the QaILs. It means that TFSA− prefers the orientation perpendicular to the surface. As is the case with the Qa cations, TFSA− at the perpendicular orientation results in the higher density of the first ionic layer at the surface. The dependence of 𝑝(𝜃) on the Qa cations is small as is the case of CF3 (Fig. 7 (c)), but the convex curvature of 𝑝(𝜃) of [N+4444 ][TFSA− ] was largest, which means TFSA− showed the higher surface-normal preference for [N+4444 ][TFSA− ].

–potential normal to the interface– The charge density distribution 𝜌chg (𝑧) of the QaILs is shown in Fig. 8 (a). The profiles of 𝜌chg (𝑧) ̊ at the first showed oscillation with a period corresponding to the ionic layer thickness (∼ 10 A) 22


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̊ The outer region in the first layer was negatively charged and the inner region layer (z < 10 A). was positively charged. In other words, there is an electric dipole in the first layer. The dipole arose from a slight difference of |𝜌chg (𝑧)| of cation and anion (Fig. S7). By comparing 𝜌chg (𝑧) with 𝜌N,i (Fig. 5) in detail, the negative charge at the outer region was attributed to CF3 group of the anion. The butyl chains of the Qa cations are more extruded to the vacuum phase than the CF3 group. However, the contribution to 𝜌chg (𝑧) of the butyl chain (CB2 , CB3 , CB4 ) was negligible because its net charge is zero. The imbalance of the charges, that is, magnitude of the electric dipole, was smaller for the QaIL with larger 𝑘. In general, a QaIL with larger 𝑘 is more hydrophobic. As the vacuum phase is low dielectric media, the position of Qa cation relative to TFSA− shifts to the vacuum phase with increasing 𝑘, leading to the smaller electric dipole in the first layer. The potential profiles Φ(𝑧) at the IL surface have been studied by MD. 44,45,48,65,98,103 The Φ(𝑧) profiles for the QaILs in the present study are shown in Fig. 8 (b). The interfacial potential difference between IL phase and vacuum phase ΔΦ = Φvac − ΦIL was dominated by potential change around z = 0. The ΔΦ values were negative for all the QaILs, reflecting the direction of the electric dipole in the first layer. The negative ΔΦ values agree with previous MD studies at the surface of other ̊ to the IL bulk TFSA− -based ILs. 44,48,65 The convergence of Φ(𝑧) within the first layer (z < 10 A) potential indicates that the first layer maintains electroneutrality. Therefore, ΔΦ is determined not by monopole but by dipole induced by different charge density distribution of cation and anion in the first layer. Here we assume that ΔΦ reflects the average structure of the ion pair at the surface. In this aspect, ΔΦ can be described as the product of the magnitude of the dipole per ion pair (𝜇IP ) and the surface density of ion pairs (𝜎surf ), as follows

ΔΦ =

−1 𝜇 𝜎 . 𝜀r 𝜀0 IP surf

(5) 2∕3

We estimated the surface density of ion pairs simply from 𝜎surf = 𝜌bulk , where 𝜌bulk is the volume density of ion pair in bulk, and estimated from the averaged value in z > 20 Å (the values are listed

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Figure 8: (a) Charge density distributions and (b) potential profile of [N+1114 ][TFSA− ] (green), [N+1144 ][TFSA− ] (blue), [N+1444 ][TFSA− ] (gray), and [N+4444 ][TFSA− ] (red). In (a), the offsets are −3 0, 1.5 × 10−4 , 3 × 10−4 , and 4.5 × 10−4 𝑒 Å .

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in Table S5). Note that this 𝜎surf is approximate because it does not reflect enrichment of ions at the first ionic layer at the surface. The plots of the interfacial potential differences vs. the surface densities of ion pairs for the QaILs are shown in Fig. 9 (solid red diamond). Interestingly, we found that the plots have a linear relation of ΔΦ with 𝜎surf with a negative slope and a positive intercept. The linear relation can be described as ΔΦ = 𝐴 + 𝐵𝜎surf ,

(6)

where 𝐴 is the intercept and 𝐵 is the slope. The intercept 𝐴 was 0.27 V and the slope 𝐵 was -29 V 2 Å .

The positive 𝐴 and negative 𝐵 values found in Fig. (9) imply that 𝜇IP in Eq.(5) monotonically increases with increasing 𝜎surf . The 𝜇IP tendency can be explained by a relative position of cation and anion at the surface. For the case of larger 𝜎surf , which corresponds to a smaller cation, the smaller cation prefers to be located at the inner region of the surface rather than the outer region because of the low dielectric permittivity of the vacuum. Since electroneutrality is satisfied in the first ionic layer, the inner and outer region of the surface is more positively and negatively charged, respectively, leading to larger 𝜇IP . Intramolecular dipole moments of the cation and anion could also influence 𝜇IP . However, this influence can be regarded as constant, as far as the intramolecular dipole moments hardly depend on 𝜎surf . For the anion, we have already revealed the negligible 𝑘 dependence of the orientation of TFSA− in QaILs (see Fig. 7 (c) and (d)), which indicates the negligible 𝜎surf dependence of the intramolecular dipole moments of TFSA− . The orientation of Qa cations depends on 𝑘, but their intramolecular dipole moments are small. Therefore, the relative position of cation and anion is the main factor to describe the 𝜇IP tendency. To further investigate the relation between ΔΦ and 𝜎surf , we evaluated ΔΦ and 𝜎surf for other TFSA− -based ILs from the number density and potential profiles reported in several previous MD, 44,48,65 which are also shown in Fig. 9. The ΔΦ values were negative for all of the TFSA− -based ILs. The plot for [C4 C1 pyrr + ][TFSA− ] 48 (green open diamond) well below the line for the QaILs. However, if we use 𝜀r = 1 in the Poisson equation (Eq. (4)) like the authors of the references 44,48,65 did, instead 25



Figure 9: Plots of the interfacial potential differences ΔΦ as a function of the surface density of ion pairs 𝜎surf for TFSA− -based ILs. CL&P force field 87 was used in the present study, 2009Pensado, 65 and 2014Paredes 48 at 423 K. A force field 104 developed by Ludwig et al. was used in 2011Sarangi 44 at 300 K. Only the present study set relative permittivity to two. of 𝜀r = 2 to take into account electronic polarization 89,90 in the present study, the ΔΦ values become doubled (red open diamond) and the [C4 C1 pyrr + ][TFSA− ] plot is almost on this new line for the present QaILs. The plot for [C+6mim ][TFSA− ] 65 (purple open square) was not on the new line, in spite of the same temperature as that of [C4 C1 pyrr + ][TFSA− ] 48 and ours. This deviation indicates that imidazolium-based ILs have a different ΔΦ-𝜎surf relation from QaILs and pyrrolidinium-based ILs. The plots for several [C𝑛 C+𝑚im ][TFSA− ] 44 (blue open triangle) also showed a linear relation 2 between ΔΦ and 𝜎surf . However, interestingly, this line had a slope (-55 V Å ) similar to ours with 2 𝜀r = 1 (-57 V Å ). The plot for [C+6mim ][TFSA− ] was not on the line for [C𝑛 C+𝑚im ][TFSA− ], presum-

ably because of the conditions, such as temperature and force field. Note that ΔΦ also depends on the anion. For example, [C+𝑛mim ][BF4 – ] (𝑛 = 2 and 8) 45 showed positive ΔΦ, in contrast to negative ΔΦ for TFSA− -based ILs shown in Fig. 9. The linear relation is likely to be fulfilled only when a series of ILs are composed of a common anion and the same types of cations in the same temperature and force field. In this context, presumably, we can regard pyrrolidinium and Qa cations as the same types of cations. As 𝜎surf can be estimated from the bulk property, this relation is helpful to predict ΔΦ. ΔΦ will affect the stability of dipolar gas molecules at the surface. Therefore, the linear relation suggests a possibility to control absorptivity of such gas molecules at the surface.

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Conclusions The MD simulations for the QaIL|vacuum interface were performed to investigate the effect of the number of butyl chains (𝑘) of the Qa cations on the surface structure. The thickness of the first ionic layer decreased with increasing 𝑘. The butyl chains of the Qa cations have the orientational preference to point to the vacuum phase, which was the highest for 𝑘 = 1 and significantly decreased from 𝑘 = 1 to 2. Even N+4444 showed the orientational preference in spite of its symmetric structure. In contrast, the effect of 𝑘 on the orientational preference of anion was small compared with that of the cations. These findings help us to understand and control the physicochemical phenomena at the surface of ILs of multi-alkyl cations. As well as the vacuum interface of QaILs, the solid interfaces, especially for the electrified ones, 105–107 are interesting subjects to study. At the electrochemical interfaces, the electrostatic potential 𝐸 is controllable and an essential factor in determining the interfacial properties and the structure. The relationship of the interfacial structure with 𝐸 and 𝑘 has not been clarified yet for the electrochemical interface of QaILs. MD and experimental studies for the subject are in progress in our laboratory.

Supporting Information Available Force field parameters; Number density profiles before and after averaging; Number density profiles from multiple and single initial configuration; Gibbs dividing surface; Top-view snapshot with highlighting the N and NBT atoms; Top-view snapshots converted into four-valued images; Charge density profiles; Side-view snapshots; Orientational distributions in sublayer; Interfacial potential differences and surface number density of ion pair at the surface.

Acknowledgement This work was partly supported by JSPS KAKENHI (grant numbers: JP26410149, JP16H04216, JP18K05171). 27



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Surface Structure of Quaternary Ammonium-Based Ionic Liquids

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