A Highly Viscous Imidazolium Ionic Liquid inside Carbon Nanotubes

Article pubs.acs.org/JPCB

A Highly Viscous Imidazolium Ionic Liquid inside Carbon Nanotubes Tomonori Ohba† and Vitaly V. Chaban*,‡ †

Graduate School of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan MEMPHYS - Center for Biomembrane Physics, Syddansk Universitet, Odense M, 5230, Denmark

‡

S Supporting Information *

ABSTRACT: We report a combined experimental (X-ray diffraction) and theoretical (molecular dynamics, hybrid density functional theory) study of 1-ethyl-3-methylimidazolium chloride, [C2C1MIM][Cl], inside carbon nanotubes (CNTs). We show that despite its huge viscosity [C2C1MIM][Cl] readily penetrates into 1−3 nm wide CNTs at slightly elevated temperatures (323−363 K). Molecular simulations were used to assign atom− atom peaks. Experimental and simulated structures of RTIL inside CNT and in bulk phase are in good agreement. We emphasize a special role of the CNT−chloride interactions in the successful adsorption of [C2C1MIM][Cl] on the inner sidewalls of 1−3 nm carbon nanotubes.

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INTRODUCTION Nanoconfined liquids1−9 play an important role in tentative nanofluidic applications,10,11 chemical reactions on the nanoscale (reaction vessels),12,13 and controlling biochemical activity.7,14 It is therefore urgent to understand the structure and dynamics in closed environments, since those are often drastically different as compared to the bulk phase of the same liquid.1 Although nonfunctionalized carbon nanotubes (CNTs) exhibit pronounced hydrophobic behavior, they can incorporate many polar liquids, including water and acetonitrile.4,15 Spontaneous filling is possible even at ambient pressure and room temperature, provided the surface tension of the liquid is not extremely high. This work reports an investigation of the structure and dynamics of a highly viscous imidazolium-based room-temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium chloride ([C2C1IM][Cl]), confined inside carbon nanotubes. The research interest in this particular RTIL in conjunction with pristine CNTs is motivated by the following factors: (a) hydrophilicity of [C2C1IM][Cl], (b) very high shear viscosity (341 cP at 323 K),16 and (c) presence of a halogen anion in combination with an organic cation (possessing delocalized valence electron density). [C2C1IM][Cl]-phobic behavior of pristine CNTs is expected to introduce major changes in the confined liquid structure and dynamical properties of RTIL. We for the first time prove, both experimentally and in computer simulations, that [C2C1IM][Cl] readily penetrates 1−3 nm wide CNTs, despite its hydrophilicity and huge viscosity.16 Furthermore, this RTIL engenders a specific phase in the severe confinement induced by the narrow CNT. The X-ray diffraction (XRD) technique17−19 for ions and molecules confined in nanoconfinements is a powerful tool for observation of molecular structures. Structural patterns of © 2014 American Chemical Society

encapsulated liquids can be evaluated using XRD. However, the detailed structure is problematic to obtain from an XRD analysis of a scattering pattern and electron radial distribution functions (RDFs) if the system appears reasonably complicated. Atomistic-precision molecular dynamics (MD) simulations provide a significant assistance in the assignment of experimental peaks to atom−atom correlations, based on computational models. The successful joint application of XRD and hybrid reverse Monte Carlo computer methodology was demonstrated by one of us recently.5,19 XRD experiments can also be used to validate computational models by direct comparison of certain structure properties. The validated models can subsequently be used for extended study of the systems of interest (for instance, dynamic properties) and provide more trustworthy physical insights than simulation studies alone.

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METHODS Experimental Section. [C2C1IM][Cl] (>98%, SigmaAldrich Co.) was used in this study. Supergrowth CNTs were prepared by Hata, which have a significantly large aspect ratio of length to diameter without metal impurities.20 The average CNT diameter was approximately 2 nm.19,21 The TEM image and diameter distribution evaluated from 200 CNTs in the TEM image are provided in Figure S1 (Supporting Information). The CNTs were put into a glass capillary tube and subsequently evacuated at approximately 1 Pa and 423 K for 2 h. [C2C1IM][Cl] vapor was adsorbed on the CNTs at 373 K for a week to reach equilibrium and avoid [C2C1IM][Cl] Received: March 20, 2014 Revised: May 20, 2014 Published: May 21, 2014 6234

dx.doi.org/10.1021/jp502798e | J. Phys. Chem. B 2014, 118, 6234−6240

The Journal of Physical Chemistry B

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were propagated using the GROMACS simulation suite.27−29 Analysis of structure and dynamics was done using supplementary utilities distributed with the GROMACS package27−29 and in-home tools. All manipulations with nanotubes, such as construction of ideal geometry and division of ions between inner and outer regions, were performed using the set of tools (MDCNT) developed by V.V.C. We started with the force field model for [C2C1IM][Cl] proposed by Lopes and Padua.30 Despite its generality, this model provides a poor reproduction of the available experimental properties (specific density, shear viscosity). For instance, the calculated viscosity for bulk [C2C1IM][Cl] at 363 K is equal to 154 cP, whereas the experimental viscosity at this temperature is 39.1 cP.16 The source of this significant difference comes from the neglected electron transfer from anion to cation. We refined the model using the method proposed in ref 22 (Figure 1). As a result, the electrostatic

condensation in the glass capillary tube. The glass capillary tube was sealed prior to the XRD measurements. XRD measurements for [C 2 C 1 IM][Cl]-adsorbed CNTs, CNTs, and [C2C1IM][Cl] were performed at SPring-8 at a wavelength of 0.03294 nm and 303 K. The electron radial distribution functions for the [C2C1IM][Cl] in CNTs and bulk were obtained from Fourier transforms of the XRD patterns calculated by subtracting the patterns for the [C2C1IM][Cl]adsorbed CNTs from the pattern for CNTs in vacuo. Computations. The structure and dynamics data have been obtained using molecular dynamics simulations with pairwise potential functions. Polarizability of ions was included implicitly via the approach developed recently.22,23 The refined force field models (see below) have been used in order to capture specific chloride anion−1-ethyl-3-methylimidazolium cation and chloride anion−carbon nanotube interactions. Twenty-four nm long (10,10)(15,15), (17,17)(22,22), and (22,22)(27,27) doublewalled CNTs were placed in parallelepipedic periodic simulation boxes and surrounded by 5000 [C2C1IM][Cl] ion pairs. Double-walled CNTs were chosen in order to minimize interactions between inner and outer ions. Molecular dynamics (MD) simulations in the constant pressure constant temperature (NPT) ensemble were performed at 363 K and 1 bar to obtain RTIL-filled CNTs. Each system was simulated for 20 ns. The equilibration (filling) was controlled on the basis of the time evolution of potential energy components, total dipole moment of the box, and structure correlation functions, such as radial distribution functions. The configurations derived by the described procedure, therefore, exhibit natural ionic density and structure patterns inside the nanotubes. It is important to obtain RTIL-filled CNTs in the introduced way, since generation of configurations using various packing algorithms may result in spurious atom arrangements, which cannot be removed during subsequent MD. This artifact may happen due to specific spatial constraints in the CNT interior and eventually lead to unphysical simulation results. The RTIL-filled configurations (Figure S2, Supporting Information) were subject to an additional 100 ns MD run with a time-step of 0.001 ps. Only the trajectories for ions in the inner cavities of CNTs were recorded. All reported properties correspond to these equilibrium trajectories. We also report selected physical chemical properties for bulk [C2C1IM][Cl], simulated in the cubic box using 1000 ion pairs. The simulations were done separately at 303, 323, and 363 K to investigate the impact of ionic thermal motion. The electrostatic interactions were simulated using direct Coulomb law up to 1.3 nm of separation between interaction sites. The interactions beyond 1.3 nm were accounted for by the computationally efficient particle-mesh Ewald (PME) method.24 It is important to use the PME method in the case of ionic systems, since electrostatic energy beyond the cutoff usually contributes 40−60% of the total electrostatic energy in the system. The Lennard-Jones 12−6 interactions were smoothly brought down to zero from 1.1 to 1.2 nm using the classical shifted force technique. The constant temperature (303, 323, and 363 K) was maintained by a Bussi−Donadio− Parrinello velocity rescaling thermostat25 (with a time constant of 1.0 ps), which provides a correct velocity distribution for a given ensemble. The constant pressure was maintained by a Parrinello−Rahman barostat26 with a time constant of 4.0 ps and a compressibility constant of 4.5 × 10−5 bar−1. The Cartesian coordinates of interaction sites were saved every 50 ps for a subsequent analysis. All molecular dynamics trajectories

Figure 1. Shear viscosity of [C1C2IM][Cl] versus electrostatic charge scaling factor at 303 K (red solid line), 323 K (green dashed line), and 363 K (blue dash-dotted line). Horizontal dashed lines of an appropriate color correspond to experimental viscosities. At all temperatures, the scaling factor with excellent performance of the model is 0.89−0.90.

charges on all atoms of [C2C1IM][Cl] were uniformly multiplied by 0.89. The applied methodology is not physically rigorous (as atomic polarizabilities are somewhat different, even for the elements of the same period). However, as previous works suggest,22,23,31,32 the resulting FF models perform better than the original models, especially in the case of ionic transport. Hybrid density functional theory was used to investigate adsorption of [C1C2IM][Cl] on the inner CNT sidewall. We describe an electron density using a high-quality hybrid exchange-correlation functional, omega B97X-D.33 Within this functional, the exchange energy is combined with the exact energy from Hartree−Fock theory. Furthermore, empirical atom−atom dispersion correction is included, being of vital importance in the case of nanotube containing systems. According to Chai and Head-Gordon, the omega B97X-D functional simultaneously yields satisfactory accuracy for thermochemistry, electron kinetics, and noncovalent interactions.33 The 6-31G* basis set, containing vacant d-orbitals, was used for all atoms, including hydrogen atoms, since this basis set is continuously applied to describe a great variety of carbonaceous compounds. The ab initio calculations (geometry optimization, electronic structure, electronic localization, partial atomic charges, etc.) were performed using the Gaussian 09 program. 6235



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RESULTS AND DISCUSSION In an attempt to get insights into possible specific (non-van der Waals) interactions between [C1C2IM][Cl] and carbon nanotubes, we have performed hybrid DFT calculations of a single ion pair, [C1C2IM]+ and [Cl]−, inside single-walled armchair CNT(13,13). The nanotube consists of 26 carbon atoms in each ring and 9 rings along the z-axis (Figure 2). The

Figure 3. Distances between the chloride anion and the carbon atoms of the CNT for a geometrically relaxed structure. The numbers along the x-axis only refer to different atoms of the nanotube and do not reflect the positions of these atoms in the genuine structure of the CNT.

Figure 2. Highest occupied molecular orbital localized on the chloride anion: (a) top view; (b) side view. (c) Localization of the entire valence electron density in the CNT(13,13) + [C1C2IM][Cl] system.

initial geometry of CNT was ideal: r(C−C) = 0.142 nm, α(C− C−C) = 120°. The rim atoms were saturated using hydrogen atoms. Note that without terminating hydrogen atoms selfconsistent field equations cannot converge and, therefore, the wave function cannot be approximated for future analysis of charge density distribution and other related properties. The hydrogen atoms introduce minimum perturbation of the electronic structure of the CNT, compared to any other rim atom. Since we are interested particularly in the electrostatic potential in this system, the use of periodic DFT is not desirable. The ion pair was initially located at the geometrical center of the CNT. Together with saturating atoms and encapsulated ions, the size of this system is 280 atoms containing 1508 electrons. The general system wave function was expressed through 14 422 basis functions. Geometry optimization using the conjugate gradient method was performed until the requested four convergence criteria were achieved. The geometry convergence criteria were defined as follows. The maximum force on any atom should not exceed 0.30 kJ mol−1 nm−1; the maximum displacement should not exceed 5.3 × 10−4 nm; the root-mean-square of all forces should not exceed 0.23 kJ mol−1 nm−1; and the root-mean-square of all displacements should not exceed 3.5 × 10−4 nm. The recorded energy alteration between the two last geometry optimization iterations was 0.026 kJ mol−1. Figure 2a,b depicts localization of the highest occupied molecular orbital (HOMO) of the chlorine atom. Figure 2c depicts localization of all valence electrons in the system for comparison. The most vibrant and most unexpected observation is communalization of charge density between the chlorine atom and a group of neighboring carbon atoms of the CNT. The observed delocalization of the anion’s electron density is rather unexpected, as this feature, essentially, puts in question the hydrophobic character of the CNT. At the same time, it was suggested before that the CNT forms stable clusters with lithium ions, whose binding energy to the CNT sidewall definitely exceeds weak dispersion forces.34 Delocalization between cation and anion can also be observed (Figure 2). Charge transfer in RTILs is a known feature of cation−anion electronic structure in the case of most RTILs.22 Figure 3 summarizes interatomic distances between the center of the chloride anion and a few adjacent carbon atoms of the nanotube. The smallest distances are 0.33, 0.34, and 0.35

nm. The van der Waals radii of chlorine and carbon are 0.175 and 0.170 nm, respectively. Therefore, these atoms must be separated by at least 0.345 nm under an assumption of a weak nonbonded interaction. The computed minimum distance does not account for thermal expansion of the system. However, two carbon atoms are located closer, supporting our hypothesis of a certain amount of electron transfer. Importantly, there are no specific interactions between C2C1IM+ and CNT. The cation is bound to the anion but is located at a significant distance from the nanotube sidewall. Although both the CNT and the imidazole cationic ring demonstrate features of aromatic compounds, no π−π stacking is found. Obviously, the imidazolium cation and CNT can interact stronger in the absence of chloride anion, i.e., upon encapsulation of another RTIL into the CNT. Apart from electronic structure numerical simulations, charge transfer between the CNT and RTIL can be confirmed using Raman spectroscopic measurements. Since the specific interaction between [C1C2IM][Cl] and carbon nanotube is proven, it can be quantified by a calculation of the pairwise binding energy. The basis set superposition error corrected value amounts to 167 kJ mol−1. This quantity, indeed, cannot be explained by exclusively dispersion attraction. As a comparison, the OPLS/AA force field provides 112 kJ mol−1 for the same interaction, whereby the cation−CNT interaction brings 78 kJ mol−1 and the anion−CNT interaction brings only 34 kJ mol−1. In addition to underestimation of system enthalpy by 33%, the applied empirical FF does not reproduce the structure of encapsulated ions. According to OPLS/AA, the cation exhibits a greater affinity to CNT, as compared to CNT−anion binding. In order to approach the potential energy surface, suggested by the empirical nonpolarizable FF, to the potential energy surface, suggested by the hybrid DFT method, we iteratively adjusted Lennard-Jones (12,6) parameters for CNT carbon− anion cross-interaction, until the energy and equilibrium distance from the FF coincided with the energy from hybrid DFT. The final values for carbon−chlorine sigma amount to 0.29 nm (sigma parameter) and 3.0 kJ mol−1 (epsilon parameter). No other interaction parameter in the system has been altered. It is noteworthy that it was possible to obtain excellent coincidence of both anion−CNT distance and ion pair−CNT binding energy simultaneously. These results reflect the fact that adjustment of the Lennard-Jones part (and not the 6236



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chlorine in a perfect agreement with hybrid DFT results. This additionally suggests a strong interaction between imidazole hydrogen atoms (especially hydrogen connected to carbon between two nitrogen atoms) and the anion inside the carbon nanotube. This interaction is also pronounced in the bulk phase. As suggested by ab initio calculations at zero temperature, correlation of cation with the CNT at 303 K is poorer than it could have been expected on the basis of the aromaticity-guided considerations. The diameter of CNT does not influence this observation. Note that the second peak inside CNT(10,10) is more sharp than that inside CNT(22,22), since in a smaller tube the second peak (ca. 0.7 nm) appears quite close to the opposite sidewall of the tube. Figure S3 (Supporting Information) presents a few snapshots of ionic organization inside CNT(10,10) and CNT(22,22). Figure 5a shows the XRD patterns of [C1C2IM][Cl]adsorbed CNTs and CNTs in vacuo as well as [C1C2IM][Cl] in the bulk. The XRD patterns of CNTs have relatively broad peaks around s = 16, 30, and 51 nm−1. On the other hand, strong peaks at s = 18, 30, and 52 nm−1 and weak peaks around s = 60, 79, and 89 nm−1 appeared in the XRD pattern of [C 1C2 IM][Cl]-adsorbed CNTs. As the XRD peaks of [C1C2IM][Cl] in the bulk were broad around s = 10, 18, 36, and 60 nm−1, adsorbed [C1C2IM][Cl] had a more ordered structure inside CNTs than bulk [C1C2IM][Cl]. However, a less packed structure of [C1C2IM][Cl] in CNTs was suggested by high small-angle X-ray scattering. The electron radial distribution functions of [C1C2IM][Cl] in CNTs and the bulk were obtained from the differential XRD patterns between [C1C2IM][Cl]-adsorbed CNTs and CNTs and the XRD pattern of bulk [C1C2IM][Cl], respectively, as shown in Figure 5b. The electron radial distributions were apparently different from each other. The mean nearest- and second-nearestneighbor distances were 0.4 and 0.7 nm in bulk [C1C2IM][Cl], respectively, corresponding to the intermolecular distance between [C1C2IM] and [Cl]. In the case of adsorbed [C1C2IM][Cl], several sharp peaks appeared at 0.25, 0.40, 0.48, 0.63,0.72, 0.86, 1.07, 1.28, 1.50, and 1.73 nm, which suggested an ordered conformation of [C1C2IM][Cl]. Figure 6 presents computed RDFs, which are analogous by definition to electron RDFs. A few features should be emphasized. First, RTIL in both tubes engenders much more ordered systems than the bulk RTIL. It should be noted that most of these well-defined peaks come from interactions with

electrostatic part) of the system potential energy was a correct methodological choice. The following part of the paper is devoted to classical molecular dynamics and X-ray diffraction investigation of [C1C2IM][Cl] encapsulated in CNT(10,10) and CNT(22,22), whereas the corresponding results of bulk [C1C2IM][Cl] are used for comparison. Figure 4 depicts RDFs derived between

Figure 4. Radial distribution functions calculated between the inner surface of the CNT and each symmetrically nonequivalent interaction center constituting cation and anion. The y-scale is omitted, since normalization of the surface-atom RDF is ambiguous and comparison of heights is valid only for the same surface. The simulations were conducted at 303 K and 1 bar.

the inner surface of the CNT and the interaction centers of the RTIL. Both inside CNT(10,10) and CNT(22,22), the only atom which can approach the CNT sidewall up to 0.22 nm is imidazole hydrogen. Another atom exhibiting even stronger position correlations but at larger distance (at 0.34 nm) is

Figure 5. XRD patterns of [C1C2IM][Cl]-adsorbed CNTs and CNTs (a). The XRD pattern of [C1C2IM][Cl] in bulk is for comparison. The electron radial distribution functions obtained from the XRD patterns (b). 6237



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Figure 7. Radial distribution functions between the chlorine atom and the hydrogen atom of the imidazole ring (red solid line), the chlorine atom and the carbon atom of hydrocarbon chains (dash-dotted green line), and the chlorine atom and the carbon atom of the imidazole ring (blue dashed line).

Figure 6. Integral radial distribution functions obtained for all atoms, including CNT atoms and hydrogen atoms, in the simulated systems at 303 K.

role in determination of the confined RTIL structure. Rather, structure is determined by specific nonbonded interactions between the cation and the chloride anion. This feature is due to strong electrostatic interactions which maintain the genuine structure of [C1C2IM][Cl] after it enters the nanotube. However, some additional effects may be expected for very large CNTs (dCNT ≫ 3 nm) in the vicinity of the hydrophobic carbon sidewall. The second carbon sidewall (in the doublewalled CNTs) exhibits a negligible impact on the confined structure. However, it is important for correct prediction of the transport properties of confined RTIL ions. In the remaining part of the paper, the dynamics of [C1C2IM][Cl] will be briefly discussed in terms of linear velocity autocorrelation functions (VACFs, Figure 8) and diffusion constants (Table 2). On the basis of normalized VACFs, motion of cation and anion occurs in different ways, with the motion of anion being more self-correlated. The impact of temperature is essentially negligible, as well as the impact of CNT (Table 3). The main difference between (10,10) and (22,22) is in the minimum of the VACF position in the case of cation, while no difference is observed for anion. Nevertheless, self-diffusion constants obtained from these VACFs exhibit a significant difference. Whereas diffusion constants of both cation and anion in the bulk phase inside CNT(22,22) suggest a highly viscous liquid, the diffusion of RTIL inside CNT(10,10) is nearly 1.5 orders of magnitude larger. This evidently happens due to the destruction of the electrostatically driven network in bulk [C1C2IM][Cl] after the

the carbon sidewalls (not only with the inner tube). Indeed, if CNT atoms are omitted from consideration (Figure S4, Supporting Information), the sharp peaks beyond 0.5 nm are substituted by a uniformly decaying smooth line. Second, peaks located between 0.2 and 0.3 nm correspond to distance correlations involving hydrogen atoms (Figure 6). In the RDFs obtained from XRD, this set of correlations is reflected by a single maximum (Figure 5b). The most intensive peak is located between 0.3 and 0.5 nm and includes carbon−carbon, carbon−nitrogen, carbon−chlorine, and nitrogen−chlorine impacts. Table 1 details the assignment of particular density maxima to particular atom−atom pairwise correlations. Interestingly, no long-range order is observed in the bulk phase at 303 K, even though [C1C2IM][Cl] is expected to transition into a glassy phase under these conditions. The electron RDF contains a poorly defined peak at 0.70− 0.84 nm, whereas the computed RDF is zero at this region of center−center separations. In order to get insights into this difference, we decomposed the integral RDF into components, as shown in Figure 7. These RDFs suggest an existence of non1 values up to 0.84 nm, some of which are somewhat above 1 and some of which are somewhat below 1. The integral curve (Figure 6) is, in turn, very close to unity. Radial distributions obtained for [C1C2IM][Cl] inside CNTs (10,10), (17,17), and (22,22) are very similar. In the meantime, all of them are qualitatively different from the RDF in bulk [C1C2IM][Cl]. This interesting observation brings us to the conclusion that the curvature of the CNT does not play a major

Table 1. Assignment of Peaks on Electron RDFs to Non-Bonded Atom Pairs distance (nm)

atom pair

0.25 0.40 0.48 0.63 0.72; 0.86; 1.07; 1.28

H(a)−Cl; H(a)−C(CNT) N(a)−Cl N(a)−Cl Cl−Cl with C(CNT)

0.22 0.38 0.48 0.70−0.84

H(a)−Cl C(t)−Cl C(r)−Cl Cl−C(t); Cl−C(r); Cl−H(a)

comments inside CNT hydrogen atom is able to closely approach the CNT carbon sidewall confinement breaks symmetry of cation−anion interaction, providing two similar peaks for the N−Cl distance cation-separated anion−anion correlation is manifested by a fuzzy peak correlations of all RTIL atoms with CNT sidewalls in bulk RTIL strong imidazole−chloride coordination rather weak correlation between anion and carbon atoms of the imidazole ring fuzzy wide peak on electron RDF which is clearly decomposed into two peaks (see Figure 7) poorly pronounced peaks corresponding to the second coordination shell 6238



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liquid enters the carbon nanotube. Compare the structural patterns in bulk and confined RTIL obtained from experiment (Figure 5). As temperature increases, the difference between CNT(10,10) and other systems decreases but still clearly persists at 363 K. An ability of narrow CNTs to drastically modify ionic motion and, therefore, to alter the phase behavior is an exciting possibility deserving an additional reconnaissance.

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CONCLUSION With the help of atomistic simulations, we assigned peaks in the experimental electronic RDFs to particular atom−atom pairs. Hybrid density functional theory with explicit correction for dispersion (weak attractive) forces was employed to observe the adsorption of ions on the inner surface of the carbon nanotube. A specific interaction of CNT and chloride anion was found and incorporated into the developed force field for empirical simulations. Several principal differences between the RTIL structure inside CNTs and in the bulk phase were found and discussed. The ability of a narrow CNT to drastically accelerate confined ions was observed on the basis of atomistic molecular dynamics simulations. Overall, the introduced work emphasizes how efficient the bundle of X-ray diffraction, molecular dynamics simulations, and density functional theory can be for the investigation of nanoconfined liquid properties.

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Figure 8. Normalized linear velocity autocorrelation functions (VACFs) of the cation and anion at 303 K (red solid line), 333 K (green dot-dashed line), and 363 K (blue dashed line) inside CNT(10,10) and CNT(22,22).

* Supporting Information

Figures showing a TEM image and diameter distribution evaluated from 200 CNTs in the TEM image, the simulated double-walled carbon nanotubes CNT(22,22) and CNT(10,10) filled with 1-ethyl-3-methylimidazolium chloride, arrangement of ions inside CNT(22,22) and CNT(10,10), and radial distribution function (RDF) computed for all atoms of RTIL for [C2C1MIM][Cl] inside CNT(10,10). This material is available free of charge via the Internet at http://pubs.acs.org.

Table 2. Diffusion Constants of C2C1IM+ and Cl− inside CNT(10,10), CNT(22,22), and in the Bulk Phase, Computed from the Integral of Velocity Autocorrelation Functions at 303, 333, and 363 Ka T = 303 K CNT(10,10) CNT(22,22) bulk a

T = 333 K

T = 363 K

cation

anion

cation

anion

cation

anion

0.281 0.020 0.010

0.431 0.020 0.010

0.301 0.030 0.040

0.461 0.030 0.030

0.311 0.040 0.101

0.491 0.040 0.101

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*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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Table 3. Parameters of Velocity Autocorrelation Functions in Various Investigated Systems at 303, 333, and 363 K: v(0) × v(0) Value, Position of Function Minimum, and Maximum Depth Value

T (K)

cation

anion

303 333 363

0.070 0.077 0.084

0.219 0.241 0.264

303 333 363

0.071 0.078 0.085

0.221 0.243 0.266

303 333 363

0.067 0.074 0.080

0.210 0.231 0.252

minimum position (fs) cation CNT(10,10) 220 200 240 CNT(22,22) 320 300 320 Bulk 300 300 320

cation

anion

140 140 140

−1.11 −1.21 −1.24

−6.60 −7.05 −7.41

140 140 160

−1.13 −1.20 −1.24

−6.35 −6.80 −7.21

140 160 160

ACKNOWLEDGMENTS We thank Dr. K. Hata of the AIST for supplying CNTs and also Dr. J. Kim, Dr. N. Tsuji, and Dr. S. Kohara for their help in recording the XRD data at Spring-8. MEMPHYS is the Danish National Center of Excellence for Biomembrane Physics. The Center is supported by the Danish National Research Foundation. This research was supported by Research Fellowships from the Futaba Electronics Memorial Foundation, Promotion of Ion Engineering, and Murata Science Foundation. The Ukrainian-American Laboratory of Computational Chemistry (UALCC) is acknowledged for providing a computational platform. TEM observation was performed at the Center for Analytical Instrumentation, Chiba University.

maximum depth (10−2 nm2 ps−2)

anion

−1.10 −1.14 −1.14

AUTHOR INFORMATION

Corresponding Author

The subscripts indicate standard errors of the computed values.

v(0) × v(0) (nm2/ps2)

ASSOCIATED CONTENT

S

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REFERENCES

(1) Chaban, V. V.; Prezhdo, V. V.; Prezhdo, O. V. Confinement by Carbon Nanotubes Drastically Alters the Boiling and Critical Behavior of Water Droplets. ACS Nano 2012, 6, 2766−2773. (2) Chen, J.; Li, X. Z.; Zhang, Q. F.; Michaelides, A.; Wang, E. G. Nature of Proton Transport in a Water-Filled Carbon Nanotube and in Liquid Water. Phys. Chem. Chem. Phys. 2013, 15, 6344−6349.

−6.64 −6.94 −7.16

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A Highly Viscous Imidazolium Ionic Liquid inside Carbon Nanotubes

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