Structural Properties and Vibrational Spectra of Ethylammonium

Feb 22, 2016 - The angular bracket represents that the ensemble averaging is summing over all tagged anions at different reference initial time. For c...
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Structural Properties and Vibrational Spectra of Ethylammonium Nitrate Ionic Liquid Confined in Single-Walled Carbon Nanotubes Guobing Zhou,† Yunzhi Li,† Zhen Yang,*,† Fangjia Fu,† Yiping Huang,† Zheng Wan,† Li Li,† Xiangshu Chen,*,† Na Hu,† and Liangliang Huang*,‡ †

College of Chemistry and Chemical Engineering, Jiangxi Inorganic Membrane Materials Engineering Research Center, Jiangxi Normal University, Nanchang 330022, People’s Republic of China ‡ School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States S Supporting Information *

ABSTRACT: The structures and relevant vibrational spectra of an ethylammonium nitrate (EAN) ionic liquid (IL) confined in single-walled carbon nanotubes (SWCNTs) with various diameters have been investigated in detail by using classical molecular dynamics simulation. Our simulation results demonstrate that the EAN IL confined in larger SWCNTs can form well-defined multishell structures with an additional cation chain located at the center. However, a different singleshell hollow structure has been found for both the cations and the anions in the 1 nm SWCNT. For the cations confined in SWCNTs, the CH3 groups stay closer to the nanotube walls because of their solvophobic nature, while the NH3+ groups prefer to point toward the central axis. Accordingly, the NO3− anions tend to lean on the SWCNT surface with three O atoms facing the central axis to form hydrogen bonds (HBs) with the NH3+ groups. In addition, in the 1 nm SWCNT, the CH3 groups of cations exhibit an obvious blue shift of around 16 cm−1 for the C−H stretching mode with respect to the bulk value, and the N−H stretching mode of NH3+ groups is split into two characteristic peaks with one peak appearing at a higher frequency. Such a blue shift is attributed to the existence of more free space for the C− H bonds of confined CH3 groups, while the splitting phenomenon is due to the fact that more than 60% of the confined NH3+ groups have one dangling N−H bond. For the anions confined in the 1 nm SWCNT, the N−O stretching mode of NO3− has a maximum red shift of around 24 cm−1 with respect to the bulk value, which is attributed to enhanced HBs between anions and cations. Our simulation results reveal a molecular-level correlation between confined structural configurations and the corresponding vibrational spectra changes for the ILs confined in nanometer scale environments.

1. INTRODUCTION The structures of fluids confined in nanometer scale environments have attracted extensive attention during the past years because of their fundamental importance in biological, chemical, physical, and material processes.1−5 In particular, the nanoconfined fluids can exhibit various unique properties by comparison with their bulk counterparts. Compared to the fluids in the bulk phase or on the surfaces, experimental observations of nanoconfined fluids are still of great challenge due to enormous difficulties of accessing the interior confinement region in well-defined model experiments.6 As one of the promising approaches, the combination of spectroscopic measurements and theoretical simulations can provide a molecular-level interpretation for the unique properties of nanoconfined fluids. Therefore, understanding of the relationship between the confined structures and the corresponding vibrational spectra is critical for experimental scientists to probe the structures of nanoconfined fluids. Molecular dynamics (MD) simulations offer a direct and fundamental insight into both the structures and the vibrational spectra of fluids under nanometer scale confinement. A systematic study via MD simulations can construct the © XXXX American Chemical Society

structure−spectrum relationship under confinement, which in turn provides design principles to related processes. As an ideal nanometer scale confinement, single-walled carbon nanotubes (SWCNTs) have uniform hydrophobicity and tunable pore sizes. They have been widely used in studies where the size effect plays an important role. For example, depending on the diameter of SWCNTs, confined water can form layered cylindrical structures, which is significantly different from the bulk water.2,7−12 In the armchair SWCNTs (n = 7−10), the confined water layers are twisted to form a spiral-like chain of water molecules along the nanotube axis.7−9 Alternatively, two different structures with the hexagonal and the octagonal rings have been observed for the water molecules in the (9,9) SWNT.10,11 When a slightly larger (10,10) SWCNT was used, an additional linear water chain was observed, located in the center of the octagonal water configuration.12 Such confinement-induced structural evolution depends strongly on the ratio of molecular size and host material dimension. In Received: January 11, 2016 Revised: February 21, 2016

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spectra and the molecular level structural configuration is still missing. To this end, we design a series of classical MD simulations to explore the properties of a confined IL, including the structures, vibrational spectra, and hydrogen bonding network. SWCNTs with different diameters are applied as the host materials. The IL, namely, ethylammonium nitrate (EAN), is composed of C2H5NH3+ cations and NO3− anions.31 We aim at constructing the fundamental relationship between the confined structural configuration and the corresponding vibrational spectra of EAN IL. Detailed analyses, including the diameter effect on the confined IL structures, the cylindrical radial distribution, the orientation of IL ions, the structural and dynamic properties of the hydrogen bonding network, and the vibrational spectra for cations and anions, have been provided and discussed. The paper is organized as follows: in section 2, we present the details of MD simulations. The simulation results are shown and discussed in section 3. Finally, we offer a few general conclusions and remarks in section 4.

subnanometer small SWCNTs, only a one-dimensional singlefile water chain is possible.1−3,13,14 In a milestone work of Hummer and co-workers,1 a single-file water chain was observed when confined in the subnanometer (6, 6) SWCNT. Such a single-file water chain demonstrated a unique pulselike diffusion pattern, due to a novel hydrogen bond (HB) network. In the MD simulation of Wang et al.,13 a similar single-file water chain was observed in a smaller (5, 5) SWCNT (diameter, 0.68 nm). Such a single-file water chain was confirmed later by Raman spectra experiments of Cambre et al.14 It is accepted now that vibrational spectroscopy is one of the most efficient and effective experimental approaches to probing the structures of nanoconfined fluids. Vibrational signals are sensitive to change of local structure environments. Together with theoretical simulations, scientists have made some progress in the relationship between the confined structures and the corresponding vibrational spectra of water molecules in SWCNTs. For example, Weinwurm and Dellago15 performed MD simulations to study the spectroscopic properties of a single-file water chain in a (6, 6) SWCNT and found that the calculated O−H stretching mode had two characteristic peaks related to the hydrogen-bonded and the dangling OH bonds. In larger SWCNTs, however, the stacked-ring structures of confined water molecules correspond to one splitting-like peak. Such spectroscopic characteristics were observed by Byl et al.16 in their MD simulations, and they attributed the highand low-frequency peaks to OH bonds with weak inter-ring hydrogen bonds and the OH bonds with bulk-like intraring hydrogen bonds, respectively. As one of the most promising green solvents, ILs confined in nanometer scale environments have been intensively studied by experimental and theoretical scientists in recent years due to their potential applications in solar cells, supercapacitors, fuels, and catalytic processes.6,17−23 Up to now, most of the previous MD studies mainly focused on the confined structures and melting transition of various ILs in carbon nanotubes with different diameters.17,24−28 Shim and Kim24 have reported that 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]) IL can form a single-file ion chain in the (7,7) SWCNT with the diameter of 0.95 nm. As the diameter increases, the cations and the anions could form zigzag or chiral single files in the (8,8) and the (10,10) nanotubes, respectively. Different multishell solvation structures were also observed in larger SWCNTs. Similarly, single- and multishell structures of 1-butyl3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) IL confined in carbon nanotubes were also observed by Singh et al.25 and Dong et al.26 in their MD simulations. Recent MD simulation by Dou and co-workers28 has demonstrated that a shell−chain structure of the [Bmim][PF6] IL confined in SWCNTs possesses a long-range crystalline order at low temperatures. Such crystalline structures display a dramatic structure transformation (i.e., the melting transition) at about 500 K, much higher than the melting point of bulk [Bmim][PF6] IL. Their results indicate a confinement-induced melting temperature shift of the studied ILs, which agrees well with the experimental observations.29 A similar temperatureinduced structural transformation was also reported for the 1hexyl-3-methylimidazolium bromine ([C6mim]Br) IL confined in carbon nanotubes.30 Despite the progress, little has been reported about the structure−spectra relationship of confined ILs. A direct correlation between the observed experimental

2. SIMULATION DETAILS In this work, four different (n,n) armchair-type SWCNTs were considered: n = 8, 15, 22, and 30, with diameters of 1.08, 2.02, 2.96, and 4.04 nm, respectively. For convenience, they are denoted as 1, 2, 3, and 4 nm SWCNTs in the following sections. During all MD simulations, the SWCNTs were immobile and rigid with a C−C bond length of 1.415 Å32 so that the repeating unit length in the axial direction is 2.448 Å. Meanwhile, the periodic boundary condition was only used in the axial direction (i.e., z direction). Therefore, the length of each SWCNT must be an integral multiple of 2.448 Å to meet the periodic boundary condition in the MD simulation. Furthermore, it is necessary to prolong the length of the smaller SWCNTs to accommodate enough ion pairs to obtain reliable statistical results. Accordingly, the length of the four SWCNTs was chosen to be 39.14, 23.48, 7.83, and 7.83 nm, respectively. We randomly placed 110, 336, 275, and 543 EAN IL ion pairs in the four SWCNTs so that all the four systems have a comparable IL density with the bulk value, as shown in previous studies.25,27,33−35 The confined densities for the EAN IL in these SWCNTs were calculated with an effective volume V = π(D − σc)2L/4, where σc represents the Lennard-Jones atomic size of carbon atoms of the SWCNT and L is the length of the SWCNT. In each simulation, the EAN IL was treated with the optimized potentials for liquid simulations of all-atom force field (OPLS) as developed recently by Acevedo and Tirado-Rives.36,37 The carbon atoms of SWCNTs were fixed and regarded as uncharged sites during the simulation. The interactions between EAN IL and SWCNTs were calculated through the site−site interaction method via the Lennard-Jones (L-J) potential. All L-J parameters and partial atomic charges used in this work were summarized and listed in Table S1 of the Supporting Information, and the mixed L-J parameters were from the Lorenz−Berthelot mixing rules. Using the aforementioned initial configurations, each MD simulation was carried out in the canonical (NVT) ensemble: temperature was set to be 353.0 K and controlled by the Berendsen algorithm with a coupling coefficient of 0.1 ps. The Newton’s equation of motion was integrated by the velocity− Verlet algorithm with a time step of 1.0 fs. A cutoff of 1.5 nm was applied for the nonbonded interactions, while the longrange electrostatic interactions were treated by the particlemesh Ewald method.38 We ran each calculation for a total of B

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defined as the ratio of local density ρ(r) within a cylindrical shell at position r in the radial direction to the confined average density ρ of the system, i.e., g(r) = ρ(r)/ρ. The twodimensional density distribution in the xy plane is also shown in the inset of Figure 1. For the 1 nm SWCNT, there are two pronounced peaks at the ends of the cylindrical radial distribution. In addition, the two-dimensional density distribution shows that both the cations and the anions form only a single-shell hollow structure in the vicinity of the 1 nm SWCNT wall. It is also obvious that the peak, representing CH3 groups of the cations stays closer to the inner wall, compared with that of NH3+ groups. This suggests that the CH3 groups of cations prefer to point toward the 1 nm SWCNT surface rather than the NH3+ groups. Recent results of X-ray reflectivity and vibrational sum frequency spectroscopy at the EAN/air interface have revealed a similar result that the EA+ cations are oriented such that the CH3 groups point toward the gas phase.39 Therefore, one can conclude that above the preferential location of the cations at the interface of SWCNTs is mainly due to the solvophobic nature of CH3 groups. The comparison between the NO3− anions and the CH3 groups of cations illustrates that the former peak is higher and stays closer to the nanotube surface, implying that the anions prefer to cling to the 1 nm SWCNT surface compared to the cations. On the other hand, in larger SWCNTs, i.e., the 2, 3, and 4 nm SWCNTs, we observe from Figure 2 that the number of peaks for both cations and anions increases as the nanotube diameter increases. There is a peak of the cations at the nanotube axis. This means that the EAN IL confined in larger SWCNTs can form well-defined multishell structures with an additional cation chain at the nanotube center. Such observation is verified later by the corresponding two-dimensional density distributions (see Figure S5 of the Supporting Information). Similar “shell− chain” structures have been reported for water molecules and the imidazolium-based ILs confined in the carbon nanotubes in previous studies.7,28 Like the CH3 groups in the 1 nm SWCNT, the CH3 groups in larger SWCNTs also tend to have preferential distributions near the nanotube walls closer than that of NH3+ groups. By comparing the results of Figures 1 and 2, one shall notice that the peaks for cations and anions confined in the 1 nm SWCNT are significantly higher than the corresponding peaks in other larger SWCNTs. For example,

15.0 ns, where the first 5.0 ns was for equilibration and the latter 10.0 ns, collected every 100 fs, was for trajectory analyses. The snapshots of the four studied systems at equilibrium are shown in Figures S1−S4 of the Supporting Information. After the 15.0 ns calculation, we performed two successive NVT calculations: one for the vibrational spectra analysis and the other for HB dynamics. For the vibrational spectra calculations, we ran each calculation for 200 ps at a smaller time step of 0.5 fs. The velocity trajectory was saved every 0.5 fs. The other NVT calculations for HB analysis were carried out for 500 ps, and the coordinates were collected every 5 fs for that time period.

3. RESULTS AND DISCUSSION 3.1. Structural Properties. As shown in Figure 1, in order to illustrate the distribution of EAN IL confined in the 1 nm

Figure 1. Cylindrical radial distributions of the CH3 groups and the NH3+ groups in cations as well as the NO3− anions confined in the 1 nm SWCNT. The insets correspond to the two-dimensional density distributions.

SWCNT, we define the following characteristic atoms for cylindrical radial distribution analysis: C atom of the CH3 group and N atom of the NH3+ group in the cations and N atom in the NO3− anions. The cylindrical radial distribution function7 is

Figure 2. Cylindrical radial distributions of the CH3 groups and the NH3+ groups in cations as well as the NO3− anions confined in 2 nm (left panel), 3 nm (middle panel), and 4 nm (right panel) SWCNTs. C

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the α1 and α2 peaks at 80° and 60°, respectively. The “CH2-left” form is when the CH2 group is located on the left side of the vector u3, corresponding to the α1 and α2 peaks at 100° and 120°, respectively. For both the “CH2-right” and the “CH2-left” orientations, the CH3 groups of cations prefer to point toward the nanotube walls while keeping the polar NH3+ groups directed to the nanotube axis. For the orientation of the NO3− anions, as shown in Figure 4(e) and (f), the dipole vector u5 is illustrated by the α3 and β3 angles, formed with respect to the vectors u1 and u2 (connecting the nanotube axis and the N atom of the anions and pointing from the former to the latter). The results suggest that the α3 curves for the four studied SWCNTs have a peak at around 90°, while the β3 curves have a peak at around 170°. This indicates that the NO3− anions prefer to lean on the nanotube surface with three O atoms facing the central axis. Such a configuration favors the formation of NH3+−NO3− hydrogen bonds between the cations and the anions under confinement. It is worth noting that the angular distributions of both cations and anions demonstrate relatively sharp peaks in the 1 nm SWCNT. Specifically, it can be seen clearly from Figure 4 that the maximum P(α3) and P(β3) values for anions in 1 nm SWCNTs are up to about 0.1 and 0.14, respectively, while the corresponding values in larger SWCNTs are almost less than 0.050 and 0.050, respectively. On the other hand, the maximum P(α1), P(α2), P(β1), and P(β2) values for cations in 1 nm SWCNTs are about 0.054, 0.045, 0.13, and 0.11, respectively, whereas the corresponding values in larger SWCNTs are almost less than 0.030, 0.030, 0.065, and 0.050, respectively. We attribute such results to the fact that in the small 1 nm SWCNT both cations and anions of the EAN ILs are induced to form more ordered structures under severe confinement effects. 3.2. Vibrational Spectra. Vibrational spectroscopy is one of the most sensitive tools to probe the structural properties of confined fluids. Here, we have explored the vibrational spectra for both the EA+ cations and the NO3− anions confined in the SWCNTs, expecting to construct a correlation with the aforementioned structural and orientation properties, as well as the HB structures and dynamics. In this work, the vibrational spectra are obtained from MD simulations through the Fourier transformation of the velocity autocorrelation function (VACF)

the peak value for anions in 1 nm SWCNT is about 9.0, while the corresponding maximum value in 2, 3, and 4 nm SWCNTs is about 5.0, 4.5, and 4.5, respectively. This is due to the small volume of the 1 nm SWCNT, which in turn determines a more ordered arrangement of the confined species. To further shed light on the structures of EAN IL confined in SWCNTs, we explored the orientation distribution of cations and anions inside the nanotubes. As illustrated in Figure 3, we

Figure 3. Schematic illustrations for the definitions of vectors and angles for both the cations and the anions confined in SWCNTs with various diameters.

define two vectors u1 and u2 (connecting the nanotube axis and the C atom of the CH3 group and pointing from the former to the latter) for the SWCNT: u1 represents the nanotube axis, while u2 is perpendicular to the nanotube axis. The vectors u3 and u4 for the EA+ cations are from the C atom (CH3 group) to the N atom (NH3+ group) and C atom (CH2 group), respectively. The orientation of the cations and anions inside the SWCNT can thus be characterized via corresponding angles: α1 between u1 and u3; α2 between u1 and u4; β1 between u2 and u3; β2 between u2 and u4. As shown in Figure 4, in the 1 nm SWCNT, the distributions of α1 and α2 have two shape peaks at 80°/100° and 60°/120°, respectively. However, in the larger 2, 3, and 4 nm SWCNTs, both α1 and α2 display a broad plateau ranging from 60° to 120°, with a broad peak approximately around 90°. Similarly, for β1 and β2 angle distributions, we observed sharp peaks in the 1 nm SWCNT: ∼140° for β1 and ∼110° for β2. When it comes to larger SWCNTs, the β1 curves have much lower but broader peaks around 120°, and the β2 curves display the same trend; only the peak positions are different. With the orientation distribution information, we conclude that cations in the 1 nm SWCNT have two distinct orientation patterns, namely, “CH2-right” and “CH2-left”. The “CH2-right” orientation is when the CH2 group is located on the right side of the vector u3, corresponding to

Figure 4. Angular distributions of (a) α1, (b) β1, (c) α2, (d) β2, (e) α3, and (f) β3 for both the cations and the anions confined in SWCNTs with various diameters. D

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The Journal of Physical Chemistry C of the EAN IL confined in the SWCNTs. The normal VACF is defined as40−42 Cv(t ) =

vi⃗(0)vi⃗(t ) vi⃗(0)vi⃗(0)

(1)

where vi⃗ (t) is the velocity of atom i of a cation or an anion at time t. The angular brackets denote the ensemble average, summing over all the atoms of a cation or an anion at different initial reference time. The vibrational density of states (VDOS) can then be calculated by the Fourier cosine transformation of VACF40−42 S(ω) =

∫0

∝

Cv(t )cos ωt dt

(2)

If the normal VACF is calculated only from the atoms of cations (or anions), the obtained VDOS only has the characteristic peaks of cations (or anions). Figure 5 displays

Figure 6. Cylindrical density distributions of the H atoms and the C atoms of CH3 groups confined in the 1 nm SWCNT. The right inset represents the SDFs (top views) of H atoms (red) and C atoms (green) of CH3 groups with an isosurface value of 0.0065 Å−3. The left inset corresponds to the density distributions of H atoms and C atoms of CH3 groups around the centers of hexagonal carbon rings (CHCRs) and the carbon atoms of SWCNTs, respectively.

this figure that each density peak of the C atoms is surrounded by two density peaks of the H atoms at both ends. Accordingly in spatial distribution functions (SDFs) of H atoms and C atoms of CH3 (i.e., the right inset of Figure 6), we can find that some H atoms in the CH3 groups locate at the gas/EAN interface due to the unique single-shell hollow structure in the 1 nm SWCNT, which can make the C−H bonds stretch more freely and then result in the stretching modes of C−H bonds displaying a blue shift. Similar blue shifts were observed for the free O−H stretching band on the surface of neutral and protonated water clusters in the gas phase.44−46 On the other hand, other H atoms in the CH3 groups locate at the EAN/ SWCNT interface. Then, we calculate the density distributions of H atoms and C atoms of CH3 groups around the centers of hexagonal carbon rings (CHCRs) and the carbon atoms of the 1 nm SWCNT, respectively, as shown in the left inset of Figure 6. It can be seen from this figure that both maximum densities of H atoms and C atoms around the CHCR are larger than those around the carbon atoms of SWCNTs, demonstrating that the H atoms and C atoms of CH3 groups reside on the top of the CHCR due to the packing constraints. In other words, the C−H bonds at the EAN/SWCNT interface prefer toward the CHCR. Such distributions mean that the C−H bonds at the EAN/SWCNT interface are as free as those at the gas/EAN interface, also leading to a blue shift. Similar distributions were also observed for the O−H bonds of water molecules confined within graphene sheets.47 As for the N−H stretching mode in the 1 nm SWCNT, Figure 5(a) shows that the vibrational peak for the asymmetric N−H stretching mode is split into two characteristic peaks: one peak at a higher frequency, which is attributed to the dangling N−H bonds, not forming HBs with NO3− anions, thus making those N−H bonds vibrate at a higher frequency.34,47 To verify our hypothesis, we calculated the average number of HBs per NH3+ group confined in the four SWCNTs. The HB formation is defined by a geometric criteria; i.e., the following distance and angular criteria are satisfied42,48

Figure 5. Fourier transformation vibrational spectra in the range of 2800−3400 cm−1 for the cations confined in (a) 1 nm, (b) 2 nm, (c) 3 nm, and (d) 4 nm SWCNTs. For comparison, the results for the bulk cations are also shown.

the vibrational spectra of the cations confined in the four studied SWCNTs. We also include the vibrational spectrum of bulk EAN cations for comparison. In the range of 2800−3400 cm−1, the peaks at 2923, 2957, and 3010 cm−1 are characteristic peaks of the symmetric stretching vibration mode of C−H and the asymmetric stretching vibration mode of C−H in the CH2 and CH3 groups, respectively, while the other two characteristic peaks at about 3250 and 3344 cm−1 are due to the symmetric and asymmetric stretching vibration modes of N−H (NH3+ group), respectively. The results in Figure 5 show that, in 2, 3, and 4 nm SWCNTs, the stretching vibration modes of C−H and N−H are almost identical to that of the bulk EAM ILs. This indicates there is almost no change for the local environment of CH3 and NH3+ groups whether they are in the bulk phase or confined in larger SWCNTs. However, in the smallest 1 nm SWCNT, we can find from Figure 5(a) that the stretching vibration modes of C−H have an obvious blue shift of around 16 cm−1 with respect to the bulk value, which is supposed to stem from the change of the local environment of CH3 with respect to the bulk EAN.43 First, we calculate the cylindrical density distributions of the H atoms and the C atoms in the CH3 groups confined in the 1 nm SWCNT, as shown in Figure 6. Then, we can see clearly from

RON < R cON and θ ONH < θcONH E

(3) DOI: 10.1021/acs.jpcc.6b00307 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C where O is the O atom of the NO3− acceptor and N is the nonhydrogen N atom of the NH3+ donor. RON is the distance between O (NO3−) and N (NH3+) atoms, while θONH is the ONH O···N−H angle. RON are upper limits for the distance c and θc and the angle of HB formation, respectively. As shown in our previous work,42 the value for θONH and θON is set to 30° and c c 3.7 Å, respectively. As shown in Figure 7, the results reveal that

Figure 9. Fourier transformation vibrational spectra in the range of 1200−1800 cm−1 for the anions confined in (a) 1 nm, (b) 2 nm, (c) 3 nm, and (d) 4 nm SWCNTs. For comparison, the results for the bulk anions are also shown.

Figure 7. Dependence of the average HB number per NH3+ group on the SWCNT diameter. The dashed line represents the average HB number in bulk EAN IL.

peak for the N−O stretching vibration exhibits a slight red shift with respect to the bulk value. In contrast, the corresponding characteristic peak in the 1 nm SWCNT displays a much larger red shift, about 24 cm−1 compared with the bulk case. A similar red shift for the amide I mode (i.e., the CO bond stretching) of N-methylacetamide (NMA) in protic solvents such as water and methanol has been reported by Eaton et al.50 They attributed such a red shift to the enhanced HB interaction between NMA and the surrounding solvent molecules. Here the relevant HB dynamics have been studied in terms of the continuous time correlation functions (TCFs) SHB(t), defined as42,48

in the 2, 3, and 4 nm SWCNTs the average number of HBs, 2.8, is almost identical to that of bulk EAN ILs. However, in the 1 nm SWCNT, the average number of HBs is 2.33, decreased by ∼16.8%. This is due to the fact that the NH3+ and NO3− groups confined in the 1 nm SWCNT are highly restricted compared to those in larger SWCNTs. To gain more details of the HB, Figure 8 shows the proportion distribution of HB

SHB(t ) =

⟨h(0)h(t )⟩ ⟨h(0)h(0)⟩

(4)

where the population variable of h(t) is unity when a particular HB pair is reserved for the whole time period from 0 to t, otherwise zero. The calculated SHB(t) curves for the NH3+− NO3− HBs in the four studied systems are shown in Figure 10. Figure 8. Proportion distribution of the NH3+ groups with one, two, and three HBs confined in (a) 1 nm, (b) 2 nm, (c) 3 nm, and (d) 4 nm SWCNTs. For comparison, the corresponding results in the bulk phase are also shown in (e).

number per NH3+ group in the four SWCNTs.49 It is obvious that the proportion distribution of one, two, and three HBs is almost identical in larger SWCNTs, equivalent to that of bulk EAN ILs. However, in the 1 nm SWCNT, the proportion distribution of two HBs increases by about 60%, but the proportion of three HBs is greatly reduced. The analysis illustrates that in the 1 nm SWCNT more than 60% NH3+ groups have two HBs and one dangling N−H bond, where a higher frequency is expected from those dangling N−H bonds. Lastly, we also investigate the vibrational spectra of the anions confined in SWCNTs. As shown in Figure 9, there is only one sole signal at about 1550 cm−1 in the domain of 1200−1800 cm−1, which is assigned to be the characteristic peak of the N−O asymmetric stretching vibration mode of anions. For the anions in larger SWCNTs, the characteristic

Figure 10. Continuous TCFs SHB(t) for the NH3+−NO3− HBs confined in (a) 1 nm, (b) 2 nm, (c) 3 nm, and (d) 4 nm SWCNTs. For comparison, the results for the bulk EAN IL are also shown. F

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The Journal of Physical Chemistry C The bulk SHB(t) curve is also provided for comparison. We can see from this figure that the SHB(t) curves in larger SWCNTs decay slightly slower than that of the bulk curve, which means that the HB strength inside these nanotubes is slightly enhanced, leading to a minor red shift of the N−O stretching vibration as shown in Figure 9. By comparison, the SHB(t) curve in the 1 nm SWCNT decays much slower than that of the bulk curve, indicating that there is a significant enhancement with respect to the HB strength. Such enhanced HB strength in the 1 nm SWCNT should be responsible for the much larger red shift of the N−O stretching vibration in the NO3− anions. As reported in previous studies,51 the decrease in the HB lifetime can be explained by the fast rotational motions of anions in ILs. Vice versa, the slow rotational motions of anions may lead to a prolonged HB lifetime. Hence, the rotational dynamics of anions confined in SWCNTs is studied here to provide an explanation for the above-mentioned enhanced HB strength. To this end, the dipole vector u5 is used to represent the orientation of anions. The rotational dynamics of anions can be calculated according to the TCFs definition52,53 Cr(t ) =

1 Ni

various diameters. Our simulation results show that the EAN IL confined in larger SWCNTs, i.e., 2, 3, and 4 nm SWCNTs, can form well-defined multishell structures with an additional cation chain located at the center. However, both the cations and the anions can only form single-shell hollow structures in the 1 nm SWCNT, which is significantly different from those in larger SWCNTs. Compared with NH3+ groups, CH3 groups of cations are found to be closer to the nanotube walls because of their solvophobic nature. Furthermore, the cylindrical radial distribution illustrates clearly that the peaks of cations and anions confined in the 1 nm SWCNT are significantly higher than those in larger SWCNTs, indicating that the ion arrangements in the 1 nm SWCNT are much more restricted and ordered than those in other nanotubes. Also, in the 1 nm SWCNT, the cations display two main orientation patterns, namely, “CH2-right”and “CH2-left”, where the polar NH3+ groups prefer to point toward the central axis of the nanotube. We also observe that the anions prefer to lean on the nanotube surface with three O atoms facing to the central axis, which is beneficial for the HB formation between cations and anions. Besides the structural properties, we have also analyzed the relevant vibrational spectra of cations and anions confined in SWCNTs and found that anomalous vibrational signals in the 1 nm SWCNT can be explained by the confined structures and the HBs. Compared with the bulk cations, the asymmetric C− H stretching vibration modes display a significant blue shift of around 16 cm−1 in the 1 nm SWCNT. Such a blue shift should result from the change of local environment of CH3 groups, where some C−H bonds prefer to point to the center axis (corresponding to the gas-phase region of the single-shell hollow structure), while others point toward the CHCR. Both orientations could provide much more free space for the C−H bonds so that they could stretch more easily and then display a blue shift. In addition, in the 1 nm SWCNT, the vibrational peak for the asymmetric N−H stretching mode is split into two characteristic peaks with one at a higher frequency. On the basis of the distribution of the HB number per cation, more than 60% NH3+ groups are found to have two HBs and one dangling N−H bond in the 1 nm SWCNT, and this dangling N−H stretching mode should be related to the characteristic peak at the higher frequency. For anions confined in SWCNTs, the asymmetric N−O stretching mode has a large red shift of about 24 cm−1 in the 1 nm SWCNT with respect to the bulk value. This is attributed to significantly enhanced HBs in the small 1 nm SWCNT. Through a detailed analysis of rotational dynamics, we conclude that the 1 nm SWCNT has a considerably enhanced confinement effect on the rotation motions of anions, which is favorable to the HB strength between cations and anions. The results discussed in this work have provided a molecular-level understanding of structures and vibrational spectra of the EAN IL confined in SWCNTs and the relationship between the vibrational spectra and confined structures as well as HBs, which is of great importance to experimental scientists for a deep understanding of the unique behaviors of confined ILs.

Ni

∑ uj(t )uj(0) j=1

(5)

where Ni is the total number of anions inside the SWCNT and the uj(t) is the unit vector of the jth anion at time t. The angular bracket represents that the ensemble averaging is summing over all tagged anions at different reference initial time. For comparison, the rotational TCF curve of anions in bulk EAN IL is also provided in Figure 11. Unlike observed in larger

Figure 11. Rotational TCFs of the anions confined in (a) 1 nm, (b) 2 nm, (c) 3 nm, and (d) 4 nm SWCNTs. For comparison, the results for the bulk anions are also shown.

SWCNTs, the rotational TCF curve of anions in the 1 nm SWCNT decays much slower than the bulk TCF curve, which agrees well with the results observed from the TCF curves of HB dynamics (see Figure 10). Such analysis confirms that the 1 nm SWCNT has an enhanced confinement effect on the rotational motions of anions, which results in the much enhanced HB strength between confined cations and anions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00307.

4. CONCLUSIONS In this work, we have systematically explored the structures and the vibrational spectra of EAN IL confined in SWCNTs with

1. Lennard-Jones parameters and partial atomic charges used in this work; 2. Typical equilibrium snapshots for G

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four SWCNT/EAN systems; 3. Two-dimensional density distributions of characteristic atoms in larger SWCNTs (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.Y.). *E-mail: [email protected] (X.S.C.). *E-mail: [email protected] (L.L.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21306070, 21463011, and 21476099), Natural Science Foundation of Jiangxi Province (No. 20151BAB203014), Joint Funds of the National Natural Science Foundation of China and Guangdong Province--Phase II (Special Program for Scientific and Applied Research of Tianhe-2 Supercomputer), the Sponsored Program for Cultivating Youths of Outstanding Ability in Jiangxi Normal University.

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