Nanocrystallization of Imidazolium Ionic Liquid in ... - ACS Publications

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Nanocrystallization of Imidazolium Ionic Liquid in Carbon Nanotubes Tomonori Ohba,*,† Kenji Hata,‡ and Vitaly V. Chaban§ †

Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan § Instituto de Ciência e Tecnologia, Universidade Federal de São Paulo, 12231-280, São José dos Campos, São Paulo, SP, Brazil ‡

S Supporting Information *

ABSTRACT: Room-temperature ionic liquids (ILs) have been of considerable worldwide interest as universal solvents, reaction media, gas scavengers, and electrolytes, particularly in supercapacitors. The behavior of ILs confined in nanoscale cavities is essential for high-performance capacitors, yet it is not well understood. Here, the structural properties of the 1-ethyl-3-methylimidazolium chloride IL confined in small diameter carbon nanotubes (CNTs) are characterized by experimental structural and vibrational analyses, complemented by molecular simulations. The IL poorly fills the 1 nm CNTs and the included IL possesses a similar local structure to that seen in the bulk. For 2 and 3 nm diameter CNTs, highly ordered cationic networks associated with restricted vibrational motion are observed. Anions are relatively mobile in the ordered cationic network within the CNTs. Rigid crystalline cationic networks and mobile chloride anions distinguish the unique properties of the confined IL.

1. INTRODUCTION

Knowledge of the molecular aspects of the IL behavior is increasingly important for diverse applications. Immobilized ILs can unfortunately block the entrance to narrow nanopores. For example, water was adsorbed by ILs in 6 nm nanopores, whereas very little water adsorption was observed in 3 nm nanopores.22,23 Polymorphous IL crystal formation was observed in multiwall CNTs.24 However, bulk liquid-like structures and faster dynamics were observed in molecular dynamics simulations for heterogeneous nanopores but not homogeneous nanopores.25 Thus, relatively uniform nanopores immobilize ILs by inducing nanoscale crystallization. Capacitances are considerably affected by electrode curvatures; convex cylindrical CNT surfaces reduce the electrical field.26 Both electric double-cylinder and wire-in-cylinder capacitor behaviors were observed respectively in wide and narrow internal CNT nanopores.27,28 Although these reports help to understand structures and behaviors of ILs confined in nanopores, they are based mostly on theoretical simulations. Furthermore, they do not characterize the actual structures of ILs in the confined systems. The goal of our present work is to determine several important aspects of IL encapsulation in CNTs. First, in what particular way are molecular structures of the IL confined in CNTs? Second, does the confined IL structure depend on the nanopore size in the context of cation−anion coordination? Third, does the ionic mobility of the confined IL differ from the bulk IL?

Room-temperature ionic liquids (ILs) are solvent-free electrolytes and are commonly used in solution chemistry, catalysis, chemical analysis, electrochemistry, and enzymatic reactions for the dissolution of cellulose.1−7 They have also been used as environmentally friendly nonvolatile solvents for organic reactions and industrial chemistry1,3,5,8 and have great potential for gas separations, liquid−liquid extraction, and liquid membrane separations.9 Ionic liquids are available for CO2 separation, as ionic liquids have been designed that can capture a high capacity of CO2.6,10 Significant amounts of CO2 can be captured by ILs equipped with amine functional groups and be later released via heating at 373 K or lower.6 Various combinations of an organic cation and an inorganic anion exhibit a wide variety of physical chemical properties, as discussed by Chiappe and Pieraccini.11 The properties are determined by the effective ionic concentration as well as the ionic conductivity.12 ILs are important in chemistry13 and perform well in electron double-layer capacitors with high electrochemical windows and no additional solvents. Unfortunately, relatively high viscosities limit their prospective performance.14−17 The capacitor performance of ILs was observed to exhibit a dependence on the nanopore size. Specifically, 0.7 nm slit nanopores exhibit maximum capacitance, corresponding to two ionic sizes where all the ions are located in the vicinity of the nanopore walls.18 Ushaped changes in capacitance were also observed with nanopore sizes in the range 0.75−1.26 nm in molecular dynamics simulations.19 High capacitance performance was observed when using vertically aligned carbon nanotubes (CNTs) in an ionic liquid electrolyte system rather than traditional activated carbon.20 The complex capacitance behavior is a combined effect of pore sizes and adsorption potentials.19,21 © XXXX American Chemical Society

2. EXPERIMENTAL SECTION The ionic liquid 1-ethyl-3-methylimidazolium chloride (>98%) was purchased from Sigma-Aldrich Co. (St. Louis, MO). Three Received: September 28, 2015 Revised: December 3, 2015

A

DOI: 10.1021/acs.jpcc.5b09423 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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nm. The acceptance ratio in the CEMC simulations was chosen as 0.5. Molecular dynamics (MD) simulations were also performed to observe the dynamics properties. MD simulations were executed for 7, 10, and 20 ion pairs in a 2.46 nm long CNT composed of 720 carbon atoms in a unit cell of 2.46 × 100 × 100 nm3. The densities of the 7, 10, and 20 ion pairs in the CNT were 0.19, 0.27, and 0.54 g mL−1, respectively. The unit cell in the MD simulation was reduced from those in the CEMC simulations to save calculation time. The initial configurations of 7, 10, and 20 ion pairs in the 2.46 nm CNT for the MD simulation were obtained from the final configurations in the CEMC simulation of 60 ion pairs in the longer 6.15 nm CNT. The potential models were the same as those in the CEMC simulation. The MD simulation used the leapfrog Verlet integration algorithm in the NVT ensemble with coupling to a thermal bath held at approximately 300 K. The time step was 0.5 fs, and the total integration time was 1.0 ns. The corresponding simulation details have been reported previously.35,36 The self-diffusion coefficients in the Einstein relation were obtained from the slope of the mean-square displacement averaged over the trajectories of individual ions in three dimensions for the last 100 ps.37

varieties of high-pressure CO-converted, and chemical-vapordeposited single- and double-wall CNTs were provided by Unidym Inc. (Menio Park, CA), the Hata group (AIST, Japan), and NanoLab Inc., (Waltham, MA), respectively. All the CNTs were heated at 673 K for 1 h in an O2 atmosphere to remove their end-caps. N2 adsorption isotherms were measured at 77 K using a commercial volumetric apparatus (Autosorb-1, Quantachrome Instruments Co., Boynton Beach, FL). Prior to those measurements, samples were subject to evacuation below 0.1 Pa at 423 K for more than 2 h. XRD measurements were performed at SPring-8 (Hyogo, Japan) with a wavelength of 0.03294 nm at 303 K. Beforehand, the CNTs were put into glass capillary tubes and evacuated at 423 K for 24 h. Vaporphase ILs were introduced into the internal nanopores of previously evacuated CNTs at 373 K for 1 week. To perform Xray diffraction (XRD) measurements of bare, empty CNTs under the same conditions as those filled ILs, the latter were heated at 423 K for 1 day in vacuo. The electron radial distribution functions for the adsorbed IL in the CNTs were obtained from Fourier transforms of the differential XRD patterns between the CNTs with the IL and the bare CNTs. Scanning transmission electron microscope (TEM) and energy dispersive spectroscopic (EDS) mapping images were observed for the IL-filled CNTs at 120 kV (JEM-2100F, JEOL Co., Tokyo, Japan). Fourier transform IR spectra of liquid-phase and adsorbed 1-ethyl-3-methylimidazolium chloride were acquired with a JASCO FT/IR-410 (Tokyo, Japan).

4. RESULTS AND DISCUSSION CNT nanopore structures and porosities were characterized by N2 adsorption isotherms at 77 K (Figure 1). CNTs with 1, 2, and 3 nm average diameters were used. Resulting structural data are shown in Table 1. Nanopore volumes were obtained from linear fitting of the large αS regions from the N2

3. THEORETICAL SECTION The infrared (IR) spectrum of the 1-ethyl-3-methylimidazolium chloride was calculated by density functional theory [B3LYP/631G(d)], as implemented in the Gaussian 09 package.29 To obtain the confined structure, canonical ensemble Monte Carlo (CEMC) simulations were performed at 303 K for 20, 30, 50, and 60 ion pairs adsorbed in a 2.5 nm diameter CNT. An armchair-type CNT having a length of 6.15 nm was positioned in the center of the 6.15 × 100 × 100 nm3 unit cell. Here the CNT was composed of 1800 carbon atoms. Ion pair numbers 20, 30, 50, and 60 in the CNT correspond to adsorbed densities of 0.22, 0.32, 0.54, and 0.65 g mL−1, respectively. The parameters used to represent the pairwise interaction energies were those successfully applied previously (see Supporting Information Table S1).30,31 Fixed partial charges were assumed in the simple point-charge approximation. Polarizable models should be used for an adequate physical description of air/ electrolyte interfaces,32,33 but although polarizable potential models are generally more accurate, the polarizability term can be neglected, as it has been demonstrated to be significantly reduced in confined nanospaces.34 The intramolecular structure of the 1-ethyl-3-methylimidazolium cation was obtained from density functional theory and was held fixed in the course of the CEMC simulation. The intermolecular interaction potentials between ions were calculated by means of the Lennard-Jones and Coulomb interaction models. Lorentz−Berthelot mixing rules and Ewald summation methods were adopted for the Lennard-Jones interactions between different molecules and long-range Coulombic interactions, respectively. Periodic boundary conditions were applied in all three Cartesian directions. The simulation was performed at constant temperature, 303 K. The calculation cycle consisted of 12 × 106 steps, and the last 3 × 106 steps were used for the calculation of radial distribution functions. In the displacement step, molecular ions were randomly translated and rotated by no more than 0.05

Figure 1. Gas adsorption isotherms for the investigated CNTs. (a) N2 adsorption isotherms in the CNTs at 77 K and (b) the αS plots: (●) 1 nm CNT; (■) 2 nm CNT; (▲) 3 nm CNT. B

DOI: 10.1021/acs.jpcc.5b09423 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. CNT Nanopore Structures and Adsorption Properties of the 1-Ethyl-3-methylimidazolium Chloride IL inside the CNTs total pore volume (mL g−1) nanopore volume (mL g−1) specific surface area (m2 g−1) external surface area (m2 g−1) diameter (nm) diameter from TEM (nm) adsorbed amount (g g−1) density (g mL−1)

1 nm CNT

2 nm CNT

3 nm CNT

1.23 ± 0.13 0.16 ± 0.01 967 ± 5 491 ± 8

1.52 ± 0.02 0.72 ± 0.13 1540 ± 7 200 ± 30

0.79 ± 0.01 0.71 ± 0.01 1010 ± 5 40 ± 10

1.4 ± 0.0 1.0 ± 0.2 0.002 ± 0.00 0.003 ± 0.00

2.5 ± 0.3 2.0 ± 0.6 0.18 ± 0.01 0.22 ± 0.01

2.9 ± 0.1 2.9 ± 0.8 0.13 ± 0.02 0.16 ± 0.03

adsorption isotherms. Diameters were calculated based on the assumption of a cylindrical nanopore. CNT diameters obtained from the N2 adsorption isotherms agree well with those observed from the TEM images in the preceding report.38 Details have been reported previously.38,39 Figure 2 shows

Figure 3. Experimentally determined structures of the 1-ethyl-3methylimidazolium chloride IL. (a−c) XRD patterns of CNTs (black curves), CNTs with the adsorbed IL (blue curves), and the differential patterns (red curves): (a) 1 nm CNT, (b) 2 nm CNT, and (c) 3 nm CNT; (d) bulk ionic liquid (green curve) at 303 K. (e) Electron radial distribution functions of the confined IL and the bulk IL.

These were used along with the nanopore volumes to calculate the adsorption densities. The bulk density of 1-ethyl-3methylimidazolium chloride at 353 K is 1.112 g mL−1. An extremely small adsorption density in the 1 nm CNTs indicates that adsorption of the IL in the smallest nanopores is fairly poor due to severely restricted nanopores. Even though they offer large enough nanopores, only 20% IL was adsorbed in the 2 and 3 nm CNTs because of its high viscosity and the winding nanoporous structures of CNTs. Another reason for the low adsorbed densities is that nanopores narrower than the IL molecular size would not allow IL adsorption, but N 2 adsorption nevertheless already accounts for these smaller nanopores. The simulated adsorbed density of an IL in the 3 nm CNT was also significantly smaller than bulk IL.10 The weak XRD peak for the 1 nm CNT at 15 nm−1 indicates that the interionic distance was elongated from 0.37 to 0.42 nm in the extremely dilute IL. In the 2 and 3 nm CNTs, the XRD patterns have three strong peaks at 18, 30, and 52 nm−1. The interionic distances are shortened by 0.02 nm, and the intramolecular distances corresponding to the 1-ethyl-3methylimidazolium cation are slightly elongated. The electron radial distribution functions of the bulk IL and the adsorbed IL are shown in Figure 3e. The peaks before 0.2 nm correspond to 1-ethyl-3-methylimidazolium cation intramolecular scattering (i.e., covalent bonds). The peaks after 0.2 nm are mainly intermolecular. The bulk IL exhibits nonzero densities before 0.16 nm, within 0.34−0.56 nm, and within 0.68−0.92 nm. These peaks are attributed to the intramolecular distances in the 1-ethyl-3-methylimidazolium cation (0.16 nm), first-nearest-neighbor intermolecular distances (0.34−0.56 nm), and second-nearest-neighbor intermolecular distances (0.68− 0.92 nm). The shoulder at 0.22 nm originates from the expectedly strong coordination of the imidazole ring and the chloride anion.40 The strong imidazole ring−chloride anion correlation was also observed elsewhere.41 The distributions in the CNTs are sharper than those of the bulk IL, indicating that their structures are more ordered. The sharp distribution between 0.12 and 0.14 nm corresponds to the covalent bonds

Figure 2. Images of 1-ethyl-3-methylimidazolium chloride and CNTs. Scanning TEM image (a) and EDS mapping images (b−d) of the IL adsorbed in the 3 nm CNTs. Red, green, and purple dots represent carbon, nitrogen, and chlorine atoms, respectively.

scanning TEM images and EDS mapping of 2 nm CNTs containing the IL. The EDS image of the carbon atoms is mainly due to the CNTs. In turn, the coincident nitrogen and chlorine atom signals indicate that IL cations (containing nitrogen) and anions (containing chlorine) were adsorbed in the CNTs. Figure 3a−d shows XRD patterns of the bare CNTs, CNTs with the adsorbed IL, differential XRD patterns between the first two, and the XRD pattern for the bulk IL. The scattering factor s is defined as 4π sin θ/λ. The CNT XRD peaks at 15−16, 30, and 50 nm−1 correspond to inter-CNT distances of 0.39−0.42 nm and intra-CNT distances of 0.21 and 0.13 nm (i.e., (10) and (11) peaks), respectively. The XRD pattern of the bulk IL has a main peak at 17 nm−1, a shoulder at 9.7 nm−1, and small peaks at 35 and 54 nm−1. The main peak was previously attributed to scattering between the carbon atom in the 1-ethyl-3-methylimidazolium cation and the chloride anion.40 The peaks with large scattering factors have contributions from intramolecular correlations in the 1-ethyl-3methylimidazolium cation. These peak positions inside the CNTs were slightly shifted, and enhanced peaks were observed for the 2 and 3 nm CNTs. Furthermore, a weak peak was observed for the 1 nm CNT. Quantities of the adsorbed IL in the CNTs were evaluated from the scattering intensities in the wide 93−105 nm−1 scattering factors, as shown in Table 1. C

DOI: 10.1021/acs.jpcc.5b09423 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Structures of the 1-ethyl-3-methylimidazolium chloride IL determined by Monte Carlo simulations. (a) Snapshot of the ionic liquid adsorbed inside the 2.5 nm CNT. Gray, blue, and red spheres represent carbon, nitrogen, and hydrogen atoms of the 1-ethyl-3-methylimidazolium cation, respectively. The chloride anions are yellow spheres, and carbon atoms of the nanotube are black spheres. (b) Radial distributions of cations (solid curves) and anions (dashed curves). The zero coordinate coincides with the CNT center. (c) Radial distribution functions for the cation− cation (blue), anion−anion (red), and cation−anion pairs (green).

prefers coordination at the imidazole due to Coulombic attraction. Simulated radial distribution peaks (Figure 4c) between the cation−cation, anion−anion, and cation−anion pairs are located at 0.64 nm, 0.52 and 0.62 nm, and 0.32 nm, respectively. The shorter anion−anion distribution peak increases with increasing density through condensation of the IL. The 0.26, ca. 0.4, and ca. 0.5 nm peaks in the electron radial distributions are therefore attributed to the cation−anion, anion−anion, and cation−cation and/or anion−anion distributions, respectively. The simulated distances are slightly longer than the experimental values because the cation center-of-mass was used for the distance calculations for simplicity. Therefore, significant peaks at 0.26 and ca. 0.5 nm occur because of stronger correlations between cation−anion and cation−cation pairs in the CNTs as compared with those in the liquid phase. The intramolecular vibrations of the 1-ethyl-3-methylimidazolium cation adsorbed in CNTs were characterized with infrared (IR) spectroscopy (Figure 5a). The experimental IR spectra of the bulk IL were assigned by comparison with density functional theory results as follows: a sharp peak at 1173 cm−1 for asymmetric stretching of the imidazole ring; small peaks around 1400 cm−1 for bending of the imidazole ring and the alkyl chains; a sharp peak at 1572 cm−1 for symmetric stretching of the imidazole ring; a small peak at 1655 cm−1 for asymmetric stretching of the imidazole ring and broad peaks at 3065 and 3400 cm−1 for the asymmetric stretching of the alkyl chains and the stretching of CH bonds in the imidazole ring, respectively. The experimental IR peaks in the bulk IL were compared with those of the IL adsorbed in the CNTs. In the 1 and 2 nm CNTs, only the asymmetric stretching vibration and the CH stretching vibration of the imidazole rings were observed. The sharp peaks between 1300−1800 cm−1 and 3500−4000 cm−1 are due to water vapor in the atmosphere that could not be removed. Thus, molecular vibrations in the 1 and 2 nm CNTs are severely restricted for the imidazole ring, although the restriction is somewhat relieved in the 2 nm CNT because small peaks at 1572 and 1655 cm−1 associated with vibrations of the imidazole ring are observed. In contrast, the IL in the 3 nm CNT exhibits IR spectra similar to those in the bulk. The normalized peaks at 1655 and 3400 cm−1 are strong relative to those in the bulk.

constituting the 1-ethyl-3-methylimidazolium cation. These distributions indicate that the intramolecular flexibility of the cation is restricted in the CNTs. Peaks at 0.26 nm, which are attributed to the strong coordination between an imidazole ring and the chloride anion, are significant for the 2 and 3 nm CNTs. The same peak for the 1 nm CNT is at 0.28 nm. Thus, the distance between an imidazole ring and the chloride anion is elongated in the 1 nm CNTs. This may happen because of adsorption of both ions on the inner CNT sidewall; that is, new competitive interactions emerge in the system. The first- and second-nearest-neighbor intermolecular distributions in the 1 nm CNT are similar to those in the bulk IL. Peaks in the distribution with distances longer than 0.8 nm are attenuated, similar to the trends in the bulk IL. The apparent distribution peaks in the 2 and 3 nm CNTs are at 0.34−0.53, 0.61−0.75, 0.86, 1.1, and 1.3 nm. This long-range ordering indicates that the IL in the 2 and 3 nm CNTs gives rise to nanocrystals, which are also observed in multiwall CNTs via electron diffraction.24 Inspection of snapshots from the CEMC simulation (Figure 4a) reveals the 1-ethyl-3-methylimidazolium cations are both tilted and parallel to the CNT. The snapshot suggests that the cations are mainly adsorbed on the sidewalls and that most of the anions are located in the vicinity of the imidazole ring. Shi and Sorescu also proposed that cations preferentially adsorb on the CNT wall and then, with increasing loading, adsorb on the second-layer sites.6 Simulated distributions (Figure 4b) of the IL along the longitudinal center axis of the CNT indicate that the cations are positioned at 0.73 and 0.86 nm from the center for the tilted and parallel configurations, respectively. Routa and Cummings proposed the alignment of the imidazolium ring and alkyl chain in a cation molecule along the carbon slit pore walls at low density and the formation of higher order layers at high density.42 On the contrary, the alignment of the imidazolium ring with the CNT wall in this system is somewhat restricted by the curvature effect, although the parallel configuration of cations becomes preferable at higher densities. In turn, the anions are relatively closer to the pore center at a distance of 0.66 nm. This indicates that the cations strongly interact with the CNT sidewall, whereas the anions interact more weakly. Indeed, adsorption of the bulky cation decreases its surface energy and is thermodynamically favorable. In turn, the anion D

DOI: 10.1021/acs.jpcc.5b09423 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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increasing density, as anticipated by the decreasing free space within the CNT. The preceding study suggested that faster selfdiffusivity of ionic liquids in CNTs was 10 × 10−9 m2 s−1, which was rather faster than in the bulk ionic liquid.10 On the contrary, ionic liquids in carbon and silica mesopores displayed slower diffusion coefficients than in the bulk.43 Energetically favorable sites on heterogeneous silica surfaces provided strong attraction potentials with the ionic liquids, resulting in the observation of temperature independent properties. From the comparison with the self-diffusion coefficients of cations and anions, the cations weakly adhered to the CNT wall, whereas anions moved relatively freely in the inner spaces of the CNTs. These simulated results supported the experimental structural aspects, as mentioned above.

5. CONCLUSION In summary, unique structures of 1-ethyl-3-methylimidazolium chloride in CNTs were experimentally characterized. Their long-range ordering in the 2 and 3 nm CNTs, as well as the strong correlations between the chloride anion and the imidazole ring of the 1-ethyl-3-methylimidazolium cation, was fingerprinted in the electron radial distribution functions of XRD. The 1-ethyl-3-methylimidazolium cation was found to be immobile according to the vibrational analysis. However, the imidazole ring possesses a freedom of movement for certain vibrations. These vibrations are strongly correlated with the motion of the chloride anions. Thus, chloride anions can migrate across the imidazole rings within the relatively ordered nanoscale framework maintained by the 1-ethyl-3-methylimidazolium cations. Further studies will be necessary to evaluate the mobilities of other ionic liquids in confined nanoscale spaces. Similar trends are expected in ILs in which bulky cations exhibit a strong electrostatic coupling with small anions, such as halides, dicyanoamide, and possibly tetrafluoroborate.

Figure 5. (a) Vibrational spectra of the IL in CNTs. IR spectra of 1ethyl-3-methylimidazolium chloride in CNTs and in bulk. The black, red, blue, and green curves correspond to the IL in the 1, 2, and 3 nm CNTs and in bulk, respectively. (b) Infrared peak area ratios normalized with respect to the ratio at 1173 cm−1. Dashed lines represent the peak area ratios of the IL in bulk.

The broad peak at 2100 cm−1 could not be assigned by comparison to the density functional theory calculation. The peak area ratios in Figure 5b also suggest that the peaks at 1655 and 3400 cm−1 for the ionic liquid adsorbed in the CNTs are significantly larger than those in the bulk ionic liquid. They are attributed to asymmetric stretching and CH stretching vibrations in the imidazole ring. Thus, the alkyl chains have more freedom to vibrate relative to the imidazole ring. Overall diffusion of the 1-ethyl-3-methylimidazolium cation appears severely restricted, however. The mobility of the IL in the 2.5 nm CNT was evaluated from the self-diffusivity behavior seen in the MD simulations, as shown in Figure 6. The snapshots in the MD simulations in

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09423. Table S1 provides parameters for the pairwise interaction potentials used in the numerical simulations; Figure S1 shows the snapshots in the MD simulations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.O.). Notes

The authors declare no competing financial interest.

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Figure 6. Self-diffusion coefficients of 1-ethyl-3-methylimidazolium cation (blue) and chloride anion (red).

ACKNOWLEDGMENTS The authors thank Drs. J. Kim, N. Tsuji, and S. Kohara for their help in recording the XRD data at SPring-8. TEM and EDS observations were performed at the Chemical Analysis Center, Chiba University. This research was supported by the Japan Society for the Promotion of Science KAKENHI Grants 26706001 and 15K12261 and research fellowships from the Kurita Water and Environment Foundation. V.V.C. thanks CAPES.

Figure S1 suggested that 1-ethyl-3-methylimidazolium cations were connected on the CNT walls and chloride anions were preferentially positioned in the inner nanopores, agreeing with the structures implied from the Monte Carlo simulations and experiments. The self-diffusion coefficients of 1-ethyl-3-methylimidazolium cations are approximately 10 × 10−9 m2 s−1 and do not change with density. On the contrary, those of chloride anions decrease from 100 × 10−9 to 40 × 10−9 m2 s−1 with E

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DOI: 10.1021/acs.jpcc.5b09423 J. Phys. Chem. C XXXX, XXX, XXX−XXX