Molecular-Level Understanding of Solvation Structures and

Aug 4, 2015 - *E-mail: [email protected] (Z.Y.)., *E-mail: [email protected] (X.C.). ... More interestingly, the N–O stretching band exhibits a...
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Molecular-Level Understanding of Solvation Structures and Vibrational Spectra of an Ethylammonium Nitrate Ionic Liquid around Single-Walled Carbon Nanotubes Guobing Zhou, Zhen Yang,* Fangjia Fu, Yiping Huang, Xiangshu Chen,* Zhanghui Lu, and Na Hu College of Chemistry and Chemical Engineering, Jiangxi Inorganic Membrane Materials Engineering Research Center, Jiangxi Normal University, Nanchang 330022, People’s Republic of China S Supporting Information *

ABSTRACT: Molecular dynamics simulations have been performed to explore the solvation structures and vibrational spectra of an ethylammonium nitrate (EAN) ionic liquid (IL) around various single-walled carbon nanotubes (SWNTs). Our simulation results demonstrate that both cations and anions show a cylindrical double-shell solvation structure around the SWNTs regardless of the nanotube diameter. In the first solvation shell, the CH3 groups of cations are found to be closer to the SWNT surface than the NH3+ groups because of the solvophobic nature of the CH3 groups, while the NO3− anions tend to lean on the nanotube surface, with three O atoms facing the bulk EAN. On the other hand, the intensities of both C−H (the CH3 group of the cation) and N−O (anion) asymmetric stretching bands at the EAN/SWNT interface are found to be slightly higher than the corresponding bulk values owing to the accumulation and orientation of cations and anions in the first solvation shell. More interestingly, the N−O stretching band exhibits a red shift of around 10 cm−1 with respect to the bulk value, which is quite contrary to the blue shift of the O−H stretching band of water molecules at the hydrophobic interfaces. Such a red shift of the N−O stretching mode can be attributed to the enhanced hydrogen bonds (HBs) of the NO3− anions in the first solvation shell. Our simulation results provide a molecular-level understanding of the interfacial vibrational spectra of an EAN IL on the SWNT surface and their connection with the relevant solvation structures and interfacial HBs.

1. INTRODUCTION In the past decades, single-walled carbon nanotubes (SWNTs) have always attracted intensive attention because of their unique chemical, mechanical, thermal, electrical, and optical properties.1−4 However, the main limitation of their practical applications is the significant difficulty in preparing stable dispersions of SWNTs in both aqueous and organic solvents because of their hydrophobic nature and the strong van der Waals attractions among them. To solve this problem, considerable efforts have already been devoted to improving the weight fraction of individually dispersed SWNTs in various solvents, including a chemically functionalized SWNT surface, with the aid of surfactants or polymers.5−11 Compared to chemical modifications, the route of the surfactants or polymers physically adsorbed on the SWNT surface can avoid changes in the intrinsic properties of the tube. However, it would be more preferable to disperse SWNTs using only solvent without the aid of additional dispersing agents (e.g., surfactants or polymers). Recently, room-temperature ionic liquids (ILs) have been regarded as a promising alternative to these commonly used solvents for making stable SWNT dispersions.12,13 Especially compared to water and organic solvents, the physical and chemical properties of ILs can be finely tuned by varying both the cation and anion, which provides a possible route for the design and fabrication of ILs with desired solvent properties. To exploit such an approach, a molecular-level understanding of the dispersion mechanism of SWNTs in different ILs is very necessary. One of the most crucial properties is the solvation structure at the IL/SWNT interface, © 2015 American Chemical Society

which is of great importance in stabilizing the SWNT dispersions. Experimentally, vibrational spectroscopy is one of the most important approaches to probing the dispersion mechanism of SWNTs in different ILs. Meanwhile, the vibrational modes of ILs can make a sensitive response to their local environments on the SWNT interface. Recent IR spectra of Zhang and coworkers14 showed that the imidazolium-based ILs on the surface of multiwalled carbon nanotubes (MWCNTs) display a red shift of the C−H stretching vibration mode in the imidazolium ring. Meanwhile, they found that the imidazolium ring of the cation prefers to be parallel with the MWCNT surface, which is well consistent with the previous hypothesis of a π−π-stacking interaction (or a cation−π interaction) between the nanotube and imidazolium ring.12,13,15 Nevertheless, the Xray diffraction results of Su and Ding16 revealed that the d spacing in the nanocomposite of the nanotube and [Omim][BF4] can be up to 4.31 Å, which is too big to form a strong π−π-stacking interaction. As a result, they ascribed the red shift of the imidazolium’s C−H stretching vibrations in their IR spectra to the static-assisted CH−π interaction. On the contrary, the Raman spectra proposed by Li and co-workers17 showed that there is no charge transfer for SWNTs in the imidazolium-based ILs compared to the pristine SWNTs. Furthermore, they found that the IR spectra of the vibrational Received: Revised: Accepted: Published: 8166

April 30, 2015 July 12, 2015 August 4, 2015 August 4, 2015 DOI: 10.1021/acs.iecr.5b01624 Ind. Eng. Chem. Res. 2015, 54, 8166−8174

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Industrial & Engineering Chemistry Research

present work attempts to provide some contributions toward this end. In the present work, a series of MD simulations have been carried out to investigate the solvation structures and vibrational spectra of a protic IL on the SWNT surface. A prototype protic IL of ethylammonium nitrate (EAN) is considered here because of its simple structure and water-like properties.28 It should be noted that the external solvation behavior of a protic IL on the SWNT surface is only considered in this work because the external solvation is more important for the understanding of the dispersion of SWNT in ILs than the corresponding internal solvation. Therefore, the main purpose of this work aims at the details of external solvation structures of EAN on the SWNT surface with different diameters, including the interfacial density, orientation, and charge distribution of both EA+ cations and NO3− anions. Meanwhile, the relevant vibrational spectra of both the cations and anions in the first solvation shell and their relationship with the solvation structures and hydrogen-bond (HB) dynamics are also investigated and discussed in detail. This paper is organized as follows. In section 2, we present the details of MD simulations. Then, the simulation results are shown and discussed in section 3. Finally, we offer a few general conclusions and remarks in section 4.

modes of the imidazolium cations are independent of the SWNT content in the bucky gels. These phenomena from both Raman and IR spectra suggested that the interaction nature between the imidazolium-based ILs and the SWNTs is only the van der Waals interaction rather than the previous π−πstacking explanation.12,13,15 As pointed out by Schatz,18 experimental observations of the objects on a nanoscale are often fraught with enormous difficulties. It is well-known that the thickness of the solvation shell at the IL/SWNT interface is only several nanometers,19,20 so that different arguments may result from various experiments. Actually, it is still a tremendous challenge for current experimental methods to directly explore the solvation structures at the liquid/solid interfaces. Until very recently, Zobel et al.21 employed the X-ray pair distribution function analysis to experimentally illustrate the restructuring behavior of polar and nonpolar solvents on the nanoparticle surface for the first time. In the face of limitations in the experimental methods, several groups have turned to molecular dynamics (MD) simulations to provide the missing understanding of the solvation structures at the IL/SWNT interface.17,19,22,23 As a powerful analysis tool, MD simulations can offer a molecularlevel insight into the fundamental properties of solvation structures and dynamics at different interfaces. Several years ago, the pioneer MD results of Shim and Kim22 showed that there is a smeared-out, cylindrical shell-like solvation structure of both EMI+ cations and BF4− anions on the nanotube surface irrespective of the nanotube diameter, while the corresponding confined structures inside the nanotube are highly related to the nanotube diameter. They found that the imidazolium rings of EMI+ cations in the first internal and external solvation shells prefer to be parallel to the nanotube surface because of the π−π-stacking interaction between the nanotubes and cations. Similar orientational behavior of Bmim+ cations on the SWNT surface has also been observed for the [Bmim][BF4] IL, and the BF4− anions tend to cling to the nanotube surface with three F atoms.23 Additionally, Aparicio and Atilhan19 studied the choline-based ILs on the graphite and SWNT surface by using MD simulations, showing that aromatic rings of benzoate and salicylate cations in the first solvation shell tend to be parallel to the graphite and SWNT surface. Unlike the carbonbased interfaces, recent MD simulations of the imidazoliumbased ILs around the metal nanoparticle revealed that the polar groups of both the cations and anions can be in contact with the metal surface simultaneously while the side chains of the cations are away from the metal surface. Although the above MD simulations have provided detailed and reliable data on the solvation structures of various ILs around the SWNTs and metal nanoparticles, they have not allowed us to construct a reasonable relationship between the simulated solvation structures and the experimental results, such as the relevant vibrational spectra. Furthermore, most of previous experiments and simulations only focused on the aprotic IL/SWNT systems, especially for the imidazolium-based ILs. As an important subset of the ILs, however, the protic ILs, which are formed by an equimolar combination of a Brønsted acid and a Brønsted base, have received little attention. Compared to aprotic ILs, protic ILs have a number of unique properties, such as building a three-dimensional hydrogen-bonded network like water.25−28 Clearly, much more work is needed to understand the solvation structures and dynamics of protic ILs at various interfaces and their relationship to the relevant vibrational spectra. This

2. SIMULATION DETAILS In this work, the force field of EAN was taken from the recent optimized potentials for liquid simulations (OPLS) all-atom model developed by Acevedo and Tirado-Rives.29,30 The C atoms of SWNTs were treated as uncharged particles interacting through the Lennard-Jones (L-J) potential, which has been widely used to describe the SWNT properties.31−33 All L-J parameters and partial atomic charges used in this work were summarized and listed in Table S1 of the Supporting Information. The mixed L-J parameters were derived from selfparameters using the Lorenz-Berthelot mixing rules. A cutoff of 1.0 nm was applied for the nonbonded interactions, and the long-range electrostatic interactions were calculated by using the particle-mesh Ewald method.34 In each MD simulation, Newton’s equations of motion was integrated by using the velocity-Verlet algorithm with a time step of 1.0 fs, and the periodic boundary condition was used in all three directions. Both the temperature and pressure were controlled by using the Berendsen algorithm with coupling times of 0.1 and 2.0 ps, respectively. All MD simulations are performed by using the modified Tinker 6.1 code.35 First, a bulk EAN system, comprised of 2316 pairs of EA+ cations and NO3− anions, was arranged within a cubic simulation cell of 70.82 × 70.82 × 70.82 Å3. The following NPT MD simulation was run for 2 ns to relax the bulk EAN at T = 353.0 K and P = 1.0 atm. It should be noted that the isobaric simulation was achieved by changing the cell volume in both the x and y directions, while the cell length of the z direction was fixed at 70.82 Å. Herein, four (n, n) armchair-type SWNTs with diameters of 5.386, 10.772, 16.157, and 21.543 Å were considered, corresponding to (4, 4), (8, 8), (12, 12), and (16, 16), respectively. Each nanotube length is equal to the cell length of the z direction. Then, one SWNT was accommodated in this cubic simulation cell with its axis parallel to the z direction. Besides the ion pairs inside the SWNTs, each cation (or anion) outside the nanotube was removed when its center of mass from the nearest C atom of SWNT was less than 3.0 Å. Finally, these initial simulation structures for all EAN/SWNT 8167

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From the viewpoint of energy, similar solvation structures of cations (or anions) around various SWNTs can be further supported by the calculated interaction energy Einter between cations (or anions) and SWNTs. As shown in Figure S5 of the Supporting Information, Einter is found to be proportional to the surface area S for all SWNTs. This indicates that Einter per unit area is independent of the nanotube diameter, which is well consistent with the similar solvation structures above. Accordingly, Figure 2 shows the distribution snapshots of N

systems, which contain one nanotube and 2276, 2213, 2115, and 2033 ion pairs for the cases of (4, 4), (8, 8), (12, 12), and (16, 16), respectively, were set up. For each initial structure above, an additional NPT MD of 10 ns was carried out for equilibration, and then the next NVT MD of 10 ns was performed for data analysis with the trajectories stored every 100 fs. During all MD simulations, the SWNTs were immobile and rigid with a C−C bond length of 1.415 Å and an axial length of 70.82 Å. For all EAN/SWNT systems, typical equilibrium snapshots are shown in Figures S1−S4 of the Supporting Information. Additionally, another two NVT MD simulations following the above final configuration were performed to calculate the relevant IR spectra and the continuous HB dynamics. For calculation of the IR spectra, the NVT MD simulation was run for 200 ps with a smaller time step of 0.5 fs, and the velocity trajectories were saved every 0.5 fs. Meanwhile, another NVT simulation was carried out for 500 ps, and the coordinate trajectories were updated every 5 fs, which is short enough to reasonably obtain continuous HB dynamics.

3. RESULTS AND DISCUSSION 3.1. Solvation Structures. To illustrate the solvation structures of an EAN IL on the SWNT surface with various diameters, we have analyzed the two-dimensional density distribution of N atoms of both the EA+ cations and NO3− anions projected on the plane perpendicular to the nanotube axis, as shown in Figure 1. We can see clearly from Figure 1a−d

Figure 2. Snapshots (top views) of N atoms of cations (upper panel) and anions (lower panel) around the SWNTs of (4, 4), (8, 8), (12, 12), and (16, 16).

atoms (both EA+ cations and NO3− anions) around the SWNTs, where each snapshot is taken from the superposition of 10 representative configurations separated by 500 ps to better clarify each solvation structure. We can see clearly from this figure that the solvation structures shown in Figure 1 can be further confirmed through the corresponding equilibrium snapshots. It should be noted that the long-ranged electrostatic interaction between the cations and anions led to more pronounced multishell solvation structures for an EAN IL around the SWNTs than water.36 To better analyze the unique properties of cations and anions around the SWNTs in the following, the first solvation shell corresponds to the radial distances of the cation’s and anion’s N atoms with respect to SWNT surfaces of less than 7.2 and 5.5 Å, respectively. Figures 3 and 4 present the spatial distribution functions (SDFs) of the N atoms of cations and anions in the

Figure 1. Two-dimensional density distributions (top views) of N atoms of cations (upper panel) and anions (lower panel) around the SWNTs of (4, 4), (8, 8), (12, 12), and (16, 16).

that all density profiles of EA+ cations near the nanotube surface display a well-defined double-shell structure regardless of the nanotube diameter, where the maximum density peaks occur in the first solvation shell with a radical distance of only 5.0 Å from the SWNT surface and are much higher than the corresponding second density peaks. Furthermore, the accumulation of cations on the SWNT surface becomes more and more pronounced as the nanotube diameter increases because the larger surface area of bigger SWNTs can provide more spaces to accommodate the big cations. However, parts e−h in Figure 1 show an opposite double-shell solvation structure for NO3− anions, where the maximum density peaks appear in the second solvation shell instead of the first shell. Additionally, a detailed comparison between the cations and anions reveals that their density profiles in the first solvation shell are not consecutive and complementary with each other, indicating that the first solvation shell on the SWNT surface is composed of abundant EA+ cations and a few NO3− anions.

Figure 3. SDFs (top views) of N atoms of cations (red) and anions (green) around the SWNTs of (a) (4, 4), (b) (8, 8), (c) (12, 12), and (d) (16, 16) with an isosurface value of 0.002 Å−3 in the first solvation shell. 8168

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Figure 5. Schematic illustrations for (a−c) the definitions of vectors and angles and (d) HBs between NH3+ and NO3−.

group) to the C atom (CH2 group). Then the orientation of the cations on the SWNT surface can be characterized via the angle α1 between u1 and u3 and the angle α2 between u1 and u4, as well as the angle β1 between u2 and u3 and the angle β2 between u2 and u4. As shown in Figure 6b, the β1 distribution is found to have a high peak around 70°, suggesting that the CH3 group is closer to the SWNT surface than the NH3+ group. Such an orientational behavior of cations can be confirmed directly from the SDFs of the N atoms and C atoms (CH3 group) of the cations, as shown in Figure S7 of the Supporting Information. Similarly, Zeng and co-workers37 have also found that the CH3 group of a methanol molecule tends to gather around the SWNTs, and they ascribe such orientational behavior to the van der Waals interaction between the CH3 group and the SWNT. However, recent X-ray reflectivity and vibrational sum frequency spectroscopy at the EAN/air interface have revealed that the EA+ cations are oriented with the alkyl chains toward the gas phase.38 In other words, the ethyl groups of the EA+ cations at the interface still prefer to point toward the interface even in the absence of SWNTs. Therefore, the above orientational behavior of EA+ cations on the SWNT surface is mainly due to the solvophobic nature of the CH3 groups, which is also responsible for the predominant accumulation of EA+ cations near the SWNTs shown in Figures 1−4. Additionally, the previous experimental and theoretical results have shown that a relatively strong CH−π interaction can be formed between the CH bond of alkane and the benzene molecule.39,40 More recently, the experimental results further verified that such a CH−π interaction can also be found between the bigger π bonds of the nanotubes and the CH bonds of the ILs.16 Although the CH−π interaction is not the fundamental cause of the cation’s accumulation on the SWNT surface, this interaction can further promote such an accumulation. We can see from Figure 6a,c that all distribution patterns of α1 almost display a broad plateau in the range from 60° to 130°, while the main peaks of angle α2 are found to be 90° regardless of the nanotube diameter, indicating that the ethyl groups of the cations prefer to be perpendicular to the SWNT surface and each NH3+ group can swing along the C−C bond of the ethyl

Figure 4. SDFs (side views) of N atoms of cations (red) and anions (green) around the SWNTs of (a) (4, 4), (b) (8, 8), (c) (12, 12), and (d) (16, 16) with an isosurface value of 0.002 Å−3 in the first solvation shell.

first solvation shell. We can see clearly from Figure 3 that the radial distance of the cation’s N atoms (around 7.2 Å) from the SWNT surface is slightly larger than that of the anion’s N atoms (around 5.5 Å). Meanwhile, the SDFs of the cation’s C atoms (CH3 group; around 5.6 Å) and the anion’s N atoms (around 5.5 Å) show that their radial distances from the SWNT surface are almost identical with each other, as shown in Figure S6 of the Supporting Information. These phenomena suggest that the solvation of SWNTs in EAN is a combinative contribution from both the CH3 groups of the cations and the NO3− anions. Further, we can observe explicitly from Figure 4 that the cations and anions lie on the SWNT surface with a short stripelike distribution pattern. Meanwhile, the proportion of cations close to the SWNT surface is much higher than that of anions for each SWNT/EAN system and increases slightly with the nanotube diameter, which is well consistent with the results from Figure 1. To further provide insight into the details of the first solvation structure around the SWNTs, we present the orientational distributions of the cations and anions with respect to the SWNTs. As shown in Figure 5a,b, we define two vectors u1 and u2 (across the C atom of the CH3 group) of the SWNT: the former is the nanotube axis, and the latter is perpendicular to the nanotube axis. Meanwhile, one vector u3 for the cations is from the C atom (CH3 group) to the N atom (NH3+ group), and the other u4 is from the C atom (CH3 8169

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Figure 6. Angular distributions of (a) α1, (b) β1, (c) α2, (d) β2, (e) α3, and (f) β3 in the first solvation shell around the SWNTs with various nanotube diameters.

group. Meanwhile, Figure 6d shows that the β2 distribution appears as two big peaks located at about 40° and 90°, respectively. It can be inferred that the main orientational patterns of the cations in the first solvation shell correspond to the “CH2−up” and “CH2−down” forms because of rotation of the NH3+ group along the C−C bond of the ethyl group. The “CH2−up” form is the CH2 group locating above the vector u3, corresponding to the β2 peak of 40°, while the “CH2−down” form is the CH2 group locating below the vector u3, corresponding to the β2 peak of 90°. In the “CH2−up” form, the NH3+ group is supposed to point toward the SWNT surface to form HBs with the NO3− anions in the first solvation shell. In the “CH2−down” form, the ethyl group is nearly tangential to the plane of the SWNT and the NH3+ group is supposed to point toward the bulk EAN to form HBs with the NO3− anions in the second solvation shell. On the other hand, Figure 5c presents that the orientation of the anion is represented by its dipole vector u4, forming the angles α3 and β3 with u1 and u2 (across the N atom of the anion). As shown in Figure 6e,f, the α3 distribution displays a big peak at around 90° and the β3 distribution shows a high and narrow peak at around 30°, suggesting that the anions prefer to lean on the nanotube surface, with three O atoms facing the bulk EAN rather than the SWNT surface. Such an orientational distribution is favorable for HB formation between the cations and anions in the first solvation shell, matching the orientation of the “CH2−up” cations in the first solvation shell. Herein, it should be emphasized that the NO3− anions in the bulk EAN are in the tetrahedral geometry rather than the planar geometry in the gas phase, as shown in Figures S8 and S9 of the Supporting Information. To evaluate the influence of the interfacial solvation structures on the charge distribution, we have calculated the two-dimensional electrostatic charge distributions of EAN IL around various SWNTs, as shown in Figure 7. We can find from this figure that a well-defined multishell charge distribution exists in the vicinity of SWNTs regardless of the nanotube diameter. Actually, various multishell charge structures have also been observed for ILs around various materials, including graphene, graphite, sapphire, and metal

Figure 7. Two-dimensional atomic charge distributions (top views) of the cations and anions around the SWNTs of (a) (4, 4), (b) (8, 8), (c) (12, 12), and (d) (16, 16).

nanoparticles.24,41−44 In our multishell charge structures, specifically, a small negative charge shell is first the closest to the SWNT surface, after which one large positive shell and one large negative shell appear in turn. Then one small positive shell exists at the outermost EAN/SWNT interface. The first negative charge shell mainly results from the accumulation of the NO3− anions in the first solvation shell. Although the nonpolar CH3 group of the cation is also close to the SWNT surface, there is no contribution to the first negative charge shell because its charge is zero. The second positive charge shell is composed of the NH3+ groups of cations in the first solvation shell, and the third negative charge shell consists of the NO3− anions in the second solvation shell. Finally, the outermost positive charge shell corresponds to the NH3+ groups of cations in the second solvation shell. Therefore, it can be concluded that there is a very close connection between the charge distribution and solvation structures of ILs at the interface. 3.2. Vibrational Spectra. Experimentally, vibrational spectra are always one of most important analysis tools to explore the structure and dynamics properties of ILs at different 8170

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Industrial & Engineering Chemistry Research interfaces.14,16,17,45−48 Therefore, it is very interesting to evaluate the relationship between the interfacial properties and the relevant vibrational spectra by using the simulation methods. More importantly, the sum of the vibrational spectra of ILs at the interfaces can be divided easily into the individual vibrational spectra for cations and anions in the simulation methods, which can avoid the overlap problems of characteristic peaks between the cations and anions. In this work, interfacial IR spectroscopy is obtained from MD simulations through Fourier transformation (FT) of the velocity autocorrelation function (VACF) of EAN in the first solvation shell. First, the normal VACF can be given as49−51 Cv(t ) =

vi⃗(0) vi⃗(t ) vi⃗(0) vi⃗(0)

discussed in the preceding section, the NH3+ groups of cations in the first solvation shell prefer to face the bulk EAN so that their local environments remain unchanged essentially. On the other hand, the other three characteristic peaks at 2923, 2957, and 3010 cm−1 should be assigned, in turn, to the symmetric stretching vibration mode of C−H and the asymmetric stretching vibration mode of C−H in the CH2 and CH3 groups, respectively. By comparison, we can find from Figure 8 that the intensities of the CH3 asymmetric stretching band on the SWNT surface are slightly larger than those of the bulk cations, and this difference increases with the nanotube diameter. Such a difference can be attributed to the increasing proportion of cations in the solvation shell with the diameter, as shown in Figures 1 and 4, where their CH3 groups tend to point toward the SWNT surfaces. Next, Figure 9 shows all IR spectra of the NO3− anions in the first solvation shell, and the corresponding IR spectra of bulk

(1)

where vi⃗ (t) is the velocity of atom i of a cation or an anion at time t. The angular brackets denote that averaging is carried out over all atoms of a cation or an anion at different reference initial times. Then the IR vibrational density of states (VDOS) S(ω) can be calculated routinely by Fourier cosine transformation of VACF:49−51 S(ω) =

∫0

∝

Cv(t ) cos ωt dt

(2)

If the normal VACF is obtained only from the atoms of cations (or anions), the obtained VDOS only includes the characteristic peaks of cations (or anions). Figure 8 shows all IR spectra

Figure 9. FTIR spectra in the range of 1200−1800 cm−1 for the anions [Sa(ω)] around the SWNTs of (a) (4, 4), (b) (8, 8), (c) (12, 12), and (d) (16, 16) in the first solvation shell. For comparison, the results for the bulk anions are also shown.

anions are also given for comparison. In the domain of 1200− 1800 cm−1, the sole characteristic peak at about 1550 cm−1 should be assigned to the N−O asymmetric stretching vibration mode of NO3− anions. Then, we can see from this figure that all intensities of the N−O asymmetric stretching band are a bit stronger than that of the bulk anions because of the accumulation of anions in the first solvation shell. Similar IR phenomena have been observed experimentally from the recent IR spectra of the [Bmim][PF6]/SWNT bucky gels, where the intensity of the asymmetric stretching band of PF6− anions becomes sharper and stronger with an increase of the SWNT composition.17 Expectedly, an increase of the SWNT content can lead to an increase of interfacial ILs including cations and anions in the bulk gels. Besides an intensity increase, however, it is very interesting that the N−O asymmetric stretching vibration mode also exhibits a red shift of around 10 cm−1 with respect to the bulk value. A similar red shift in IR spectra has also been observed experimentally for the C−H stretching mode of the imidazolium ring for the nanotube/IL composites in the previous studies.14,16,52 Although there are some controversies on the nature of the interaction between nanotubes and imidazolium-based ILs from these different experimental observations (as discussed in section 1), one

Figure 8. FTIR spectra in the range of 2800−3400 cm−1 for the cations [Sc(ω)] around the SWNTs of (a) (4, 4), (b) (8, 8), (c) (12, 12), and (d) (16, 16) in the first solvation shell. For comparison, the results for the bulk cations are also shown.

of the EA+ cations in the first solvation shell, and the corresponding IR of cations in the bulk phase is also given here for comparison. In the range of 2800−3400 cm−1, the characteristic peaks at about 3250 and 3344 cm−1 should be assigned to the symmetric and asymmetric stretching vibration modes of N−H (NH3+ group), respectively. We can find from this figure that there is almost no change of the intensity and the position of the stretching vibration modes of N−H in the first solvation shell compared to the bulk phase, indicating that the local environments of the NH3+ groups on the SWNT surface should be identical with those in bulk EAN. As 8171

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Industrial & Engineering Chemistry Research common explanation can be extracted that the strong interaction between nanotubes and ILs should be responsible for such a red shift. However, the above previous explanation seems to be inappropriate for the red shift in this work because neither π−π stacking nor CH−π can be formed between the NO3− anions and the SWNTs. On the other hand, a blue shift has been found for the IR spectra of water molecules at the hydrophobic interfaces.53,54 With respect to the bulk spectrum, classic MD simulations proposed by Cicero et al.53 have revealed that a blue shift of the OD stretching mode appears for the D2O molecules on the graphene surface and is confined in the SWNT owing to the interfacial OD group not involved in HBs, so that these free OD bonds can vibrate at a higher frequency. More recently, ab initio MD simulations of Rana and Chandra54 have also shown a blue shift of the OH stretching band on the graphene surface, which results from the presence of a free OH bond without HBs and also a weakening of H2O−H2O HBs at the hydrophobic interface. In addition, similar blue shifts of the OH stretching band in the IR spectra were also found at the liquid/vapor interfaces of neutral and protonated water clusters, as well as the methanol cluster.55−57 The above previous studies have confirmed that the absence or weakening of HBs can result in a blue shift of the OH stretching band in the IR spectra. Vice versa, the presence or strengthening of HBs can lead to a red shift. In an early experiment, Eaton et al.58 found a considerable red shift of the amide I mode (i.e., the CO bond stretching) of N-methylacetamide (NMA) in protic solvents such as water and methanol because of an enhanced HB interaction between NMA and surrounding solvent molecules. To better understand the red shift of the N−O asymmetric stretching mode of anions on the SWNT surface, therefore, relevant HBs in the first solvation shell are investigated in the following. As shown in Figure 5d, the presence of HBs are defined in terms of the following distance and angular criteria59,60 RON < R cON and θ ONH < θcONH

Figure 10. Continuous TCFs SHB(t) for the NH3+−NO3− HBs around the SWNTs of (a) (4, 4), (b) (8, 8), (c) (12, 12), and (d) (16, 16) in the first solvation shell. For comparison, the results for the bulk EAN IL are also shown.

also given for comparison. We can see clearly from this figure that all interfacial curves decay slower than the bulk curve. This slow decay behavior means that the HB strength of NO3− anions near the SWNT surface is enhanced, which leads to a red shift of the N−O asymmetric stretching mode shown in Figure 9. In other words, our simulation results reveal that the enhanced interactions between the cations and anions at the nanotube/IL interfaces should be responsible for such a red shift in IR spectra rather than the previous explanation of a strong interaction between nanotubes and ILs.14,16,52

4. CONCLUSIONS In this work, we have investigated the solvation structures and vibrational spectra of an EAN IL around various SWNTs by using classical MD simulations. Four different armchair-type SWNTs [(4, 4), (8, 8), (12, 12), and (16, 16)] are considered here. Our simulation results show that both EA+ cations and NO3− anions can form a cylindrical double-shell solvation structure around the SWNTs regardless of the nanotube diameter. However, the maximum density peak of cations appears in the first solvation shell while the maximum peak of anions occurs in the second shell, indicating that the EA+ cations prefer to accumulate on the SWNT surface prior to the NO3− anions. Furthermore, the proportion of cations in the first solvation shell is found to increase slightly with the SWNT diameter, and the CH3 groups of cations are closer to the SWNT surface than the NH3+ groups in the first solvation shell because of their solvophobic nature. On the SWNT surface, there are two main orientational patterns of cations: one is the “CH2−up” form in which the NH3+ group points toward the SWNT surface to form HBs with the anions in the first solvation shell, and the other is the “CH2−down” form in which the NH3+ group points to the bulk EAN to form HBs with the anions in the second solvation shell. Additionally, the NO3− anions tend to lean on the nanotube surface, with three O atoms facing the bulk EAN, which is favorable to form HBs with the cations of the “CH2−up” form in the first solvation shell. Such solvation structures of cations and anions on the SWNT surface result in a well-defined multishell charge distribution in the vicinity of SWNTs.

(3)

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 of the O (NO3−) and N (NH3+) atoms, while θONH is the O···N−H angle. Accordingly, RON and θONH are the upper limit distance c c and angle of HB formation, respectively. In this work, the θONH c value is fixed at 30° and the RON c values is set to 3.7 Å, which is

obtained from the first minimum of the corresponding radial distribution functions (RDFs) of the bulk EAN at 353 K, as supplemented in Figure S10 of the Supporting Information. First of all, our detailed HB analysis has revealed that the average HB number per anion in the first solvation shell is equal to the bulk value of 2.8, indicating that no free N−O bond exists on the SWNT surface. Next, the HB strength of the anions in the first solvation shell is analyzed by relevant continuous time correlation functions (TCFs) SHB(t), which can be defined as59 SHB(t ) =

⟨h(0) h(t )⟩ ⟨h(0) h(0)⟩

(4)

where the variable h(t) is unity when the tagged HB pair at a certain region is continuously kept from time 0 to time t and zero otherwise. Figure 10 presents all interfacial SHB(t) curves for the anions in the first solvation shell, and the bulk curve is 8172

DOI: 10.1021/acs.iecr.5b01624 Ind. Eng. Chem. Res. 2015, 54, 8166−8174

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On the other hand, the calculated IR vibrational spectra of an EAN IL on the SWNT surface are divided into two individual contributions from cations and anions. Compared to the bulk phase, we find that there is almost no change for the intensity and position of the N−H stretching vibration modes of EA+ cations in the first solvation shell, which is well consistent with the interfacial NH3+ groups toward the bulk EAN. However, the intensities of the interfacial CH3 asymmetric stretching band are found to be slightly larger than the bulk value, and this difference increases with the nanotube diameter, corresponding to the CH3 groups facing the SWNT surfaces and a higher proportion of cations in the first solvation shell of the larger nanotube. As for the NO3− anions, all intensities of the N−O asymmetric stretching band are also found to be a bit stronger than the bulk value because of the accumulation of anions in the first solvation shell. However, it is unexpected that the N− O asymmetric stretching vibration mode exhibits a red shift of around 10 cm−1 with respect to the bulk value, which is quite contrary to the blue shift of the OH stretching band of water molecules at the hydrophobic interfaces. Through a detailed analysis of continuous HB dynamics, enhanced HBs can be found for the NO3− anions in the first solvation shell, which results in such a red shift of the N−O stretching mode. In this work, we have shown a detailed picture of the solvation structures and vibrational spectra of protic ILs around the SWNTs. More importantly, our simulation results provide a molecular-level understanding of interfacial vibrational spectra and their relationship with the relevant solvation structures and HB dynamics, which are of great benefit for the experimental scientists to understand the unique behavior of various ILs at the interfaces of carbon-based nanomaterials.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01624. L-J parameters and partial atomic charges used in this work, typical equilibrium snapshots for four SWNT/ EAN systems, interaction energies of SWNTs and an EAN IL, SDFs of cations and anions around the SWNTs, RDFs between the N (NH3+) and O (NO3−) atoms, and continuous TCFs SHB(t) with the distance criterion of 3.5 Å for the NH3+−NO3− HBs (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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REFERENCES

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S Supporting Information *

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Article

ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of China (Grants 21306070 and 21463011), Natural Science Foundation of Jiangxi Province (Grant 20151BAB203014), National High Technology Research and Development Program of China (Grant 2012AA03A609), Key Technology R&D Program of Jiangxi Province (Grant 20114ACB01200), and Science and Technology Project of Universities in Jiangxi Province (Grant KJLD12005). 8173

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