Gregorio Garcíaa,
Mert Atilhanb and
Santiago Aparicio*a
aDepartment of Chemistry, University of Burgos, 09001 Burgos, Spain. E-mail: sapar@ubu.es
bDepartment of Chemical Engineering, Qatar University, P.O. Box 2713, Doha, Qatar
First published on 5th September 2014
The effect of the confinement of ionic liquid (choline benzoate) cluster inside C540 fullerene has been studied through both molecular dynamic and density functional theory simulations. The solvation of the fullerene by the ionic liquid is also analysed and compared with pristine fullerene cages. The mobility of the studied endohedral fullerenes under external electric fields is simulated as a function of the intensity of applied fields. Density functional calculations allowed inferring both the changes in the electronic structure of C540 because of the ionic liquid and the interactions between the molecules forming the ionic liquid inside C540. This is the first available study on endohedral fullerenes with confined ionic liquids published in open literature. More importantly, theoretical simulations carried out in this work could be used both in the prediction of the properties and as a guide to experimentalist for the rational designing of new materials based on ionic liquids confined in fullerenes for specific applications.
Studies on the encapsulation of molecules in fullerenes are lesser in comparison with metallofullerenes. Dodziuk et al.14 reported a broad molecular mechanics study on EHFs formed by the encapsulation of small molecules (H2, water), hydrocarbons, ketones, amines @C60, C70, C76, C80 and C82 fullerenes, reporting the stabilization energy upon encapsulation and analysing the host–guest sizes in relationship with stabilization upon confinement. Despite the simplicity of the computational approach used, Dodziuk et al.14 reported the development of weak host–guest interactions for most of the studied compounds. The confinement of H2 clusters in C60 were studied theoretically by Ren et al.15 and Barajas-Barraza et al.,16 by analysing the molecular arrangement as a function of the dihydrogen cluster sizing. The encapsulation of water molecules led to several studies considering the incarceration of a single water molecule17–19 and water clusters.20,21 Charkin et al.22 analysed the properties of several guest molecules (e.g. benzene) in fullerenes using methods based on the density functional theory (DFT). DFT methods were also used by Ganji et al.23 for the study of NH3 confined in C60, considering not only single ammonia molecules but also the confinement of ammonia clusters. Despite the attraction of using fullerene cages to isolate single molecules or molecular clusters of fixed size, studies on the confinement of more complex molecules have not been reported.
Ionic liquids (ILs) have attracted considerable attention in the scientific community because of their remarkable physical and chemical properties,24 the possibility of tuning these properties through a suitable combination of involved ions,25,26 and their applications in very different technologies.27,28 Therefore their behaviour with respect to carbon nanostructures,29,30 graphene,31–36 carbon flakes,37 graphene nanoribbons,38,39 or nanopores arrays29,40 has been the subject of several studies. The scarce literature on the behavior of fullerenes with ILs systems is mainly devoted to empty fullerene cages solvated in ILs.39,41–43 Our research group used molecular dynamics simulations for the study of C60 solvated in the N-methylpiperazinium IL.39 Nevertheless, according to our knowledge regarding the studies of ILs, or more complex molecules in general, confined inside fullerene cages (ionic liquid – EHFs) are absent in the literature.
To focus on the rational design of new materials based on ILs confined in endohedral fullerenes for determinate tasks, a deeper knowledge on the influence of encapsulated molecules (guest) on the fullerene (host) features and vice versa is required. Unfortunately, it is difficult to define a priori which molecules lead to concrete features. Up to now, a trial and error approach has been widely used in experiments. This is a typical context wherein theoretical simulations can provide new insights to experimentalists regarding the suitability of new systems. Thus, here, we propose a computational study through both classical molecular dynamics (DM) and density functional theory (DFT) simulations on the characteristics of IL molecules confined inside endohedral fullerenes. For this purpose, choline benzoate IL (CH_BE, Fig. 1) confined inside C540 fullerene was selected as a first case study. Discarding classical ILs, choline-based cation ((2-hydroxyethyl)trimethylammonium) is among the most suitable new options for developing ILs because of the environmental, toxicological and economical aspects44–46 and the aromatic character of the selected anion. Moreover, our group reported a recent study on the behaviour of CH_BE with regard to graphene and single-walled carbon nanotubes.36 To study the confinement of a single ion or a single IL molecule inside a fullerene cage, we decided to study the confinement of several IL molecules. Therefore, the selected system for this study was an IL cluster composed of 6 ion pairs confined inside a C540 cage (6_CH_BE@C540) with the objective of checking the suitability of isolating small IL clusters inside fullerene cages. The fullerene cage was selected to allow the proper fit of the IL cluster.
Therefore, the main goal of this work is to the study the energetic, structural and dynamic properties of 6_CH_BE@C540 in comparison with the pristine C540. Properties of CH_BE@C540 and C540 were studied in vacuum and CH_BE was also used as the surrounding solvent. In addition, considering the recent results by Xu et al.19 on the electrically driven transport of endohedral fullerenes encapsulating water molecules, the behaviour of 6_CH_BE@C540 (in vacuum and when surrounded by CH_BE solvent) was also analyzed as a function of EEF intensity under external electric fields (EEFs). To our knowledge, this is the first available study on endohedral fullerenes with confined ionic liquids published in open literature.
Label | N | Medium | IL pairs @C540 | IL pairs outside C540 |
---|---|---|---|---|
1 × PRIST_C540_VAC | 1 | Vacuum | 0 | 0 |
1 × PRIST_C540_IL | 1 | CH_BE | 0 | 185 |
1 × 6_CH_BE@C540_VAC | 1 | Vacuum | 6 | 0 |
1 × 6_CH_BE@C540_IL | 1 | CH_BE | 6 | 185 |
Molecular dynamics simulations for all the studied systems were performed at 303 K in the NVT ensemble using the box size reported in the previous paragraph, and with the temperature controlled using the Nosé–Hoover thermostat. Coulombic interactions were handled with the Ewald summation method48 with a cut-off radius of 15 Å. Tuckerman–Berne double time step algorithm,49 with long and short time steps of 1 and 0.1 fs, was considered for solving the equations of motion. Lorentz–Berthelot mixing rules were used for Lennard-Jones terms. Simulations extending up to 10 ns were performed.
Simulations under EEFs were performed for static fields applied in the z-direction with 0.02, 0.04, 0.06, 0.08 and 0.10 V × Å−1 intensities. These non-equilibrium simulations were also performed using a Nosé–Hoover kinetic thermostat. Using kinetic thermostats for non-equilibrium simulations may lead to problems because of the EEF-induced drift motion contributing to the total kinetic energy, which is not separated by the thermostat from purely thermal effects, thus leading to an overestimation of kinetic energy. Nevertheless, the contribution of the drift motion is strongly dependent on the intensity of the applied field; results previously reported by our group showed that in the case of CH_BE this contribution is almost negligible for fields lower than 0.10 V × Å−1.50 Therefore, using kinetic thermostats for the studied systems should lead to reliable results, which is in agreement with the simulations of ILs under moderate EEFs available in the literature.51–54 Similarly, kinetic thermostats were also used for the simulation on EHFs under EEFs.19
Second, the Gaussian 09 (Revision D.01) package61 was used to compute the binding energy and study the interaction network of the confined CH_BE cluster. These calculations were carried out at the PBE/6-31G* theoretical level. Interactions between different molecules forming the confined IL cluster were analyzed by atoms in molecules (AIM)62 and natural bond orbital (NBO)63 theories. Interacting molecules were localized by analyzing NBO charge transfers between different units. Previously, the effect of a C540 cage on intermolecular interactions between ions was tested. First, this analysis was carried out taking into account the C540 cage. Then, NBO calculations were performed for the 6_CH_BE cluster, with cluster geometry obtained upon confinement in C540, but the C540 surrounding cage was not considered. After comparing both analyses, our calculations showed that the surrounding cage leads to negligible effects on the calculation of the charge transfer interactions between confined molecules. Therefore, for the sake of simplicity, the C540 cage was not considered in NBO and AIM studies. Then, we carried out a topological analysis of electronic density over different interacting ion pairs using the AIM2000 program.64
System | dC–C/Å | RC540/Å | ϕ/deg | ELJ(C540)/eV per atom |
---|---|---|---|---|
1 × PRIST_C540_VAC | 1.4530 ± 0.0184 | 10.7399 ± 0.3804 | 11.04 ± 3.23 | 50.92 ± 0.00 |
1 × PRIST_C540_IL | 1.4529 ± 0.0174 | 10.7311 ± 0.3984 | 8.89 ± 3.37 | 51.88 ± 0.01 |
1 × 6_CH_BE@C540_VAC | 1.4522 ± 0.0173 | 10.7340 ± 0.3889 | 9.42 ± 2.08 | 51.40 ± 0.01 |
1 × 6_CH_BE@C540_IL | 1.4509 ± 0.0180 | 10.7224 ± 0.3964 | 8.08 ± 4.25 | 52.31 ± 0.01 |
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Fig. 2 (a) Radial distribution function, g(r), and (b) the corresponding running integrals of g(r), N, between the center of mass of C540 and the C540 carbon atoms for the reported systems with system labelling as in Table 1. In panels (c) and (d), carbon atoms around a pentagonal site in C540 are reported (top and side views), with atoms color code used to define atoms corresponding to the maxima in g(r) shown in panel (a). In panels (c) and (d), only the pentagonal site is shown for the sake of visibility, and in panel (d) the remaining atoms of C540 are also omitted. Dashed gray vertical lines in panel (b) show the position of maxima in g(r) reported in panel (a). All the values were obtained from molecular dynamics simulations at 303 K. |
The energetics of C540 fullerene upon CH_BE confinement and solvation is reported in Table 2, in which intramolecular Lennard-Jones energy for C540, ELJ(C540), is reported for the studied systems. Reported results show that encaging CH_BE molecules leads to an increment in ELJ(C540) of roughly 0.5 eV (∼0.5%) for both C540 in vacuum and surrounded by IL, whereas the solvation of C540 with CH_BE molecules increases ELJ(C540) by roughly 0.93 eV (∼1%) for both pristine and endohedral C540. These results are in agreement with increasing planarity around the pentagonal sites in C540 upon both CH_BE encaging and solvation.
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Fig. 3 (a) Radial distribution functions, g(r), and (b) the corresponding running integrals, N, between the center of mass of C540 and the center of mass of CH cation and BE anion for ions confined inside C540 with system labelling as in Table 1. r = 0 Å corresponds to the C540 center of mass, and the vertical gray dashed line shows the position of C540 carbon atoms, defined as the average C540 radius reported in Table 2. All the values obtained from molecular dynamics simulations at 303 K. In the case of 1 × 6_CH_BE@C540_IL system only g(r) inside the C540 is reported. |
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Fig. 4 Radial distribution functions, g(r), between the carbon atoms in C540 (C) fullerene and selected atoms in CH and BE ions, for 1 endohedral C540 fullerene encaging 6 CH_BE ion pairs surrounded by CH_BE ionic liquid (1 × 6_CH_BE@C540_IL in Table 1). Atoms' code as in Fig. 1. All the values obtained from molecular dynamics simulations at 303 K. Only ions inside the C540 fullerene are considered in the calculation of g(r). |
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Fig. 5 (a) Representative snapshot and (b) spatial distribution function for CH and BE ions confined inside C540 for 1 endohedral C540 fullerene encaging 6 CH_BE ion pairs surrounded by CH_BE ionic liquid (1 × 6_CH_BE@C540_IL in Table 1). All the values obtained from molecular dynamics simulations at 303 K. Color code: (blue) CH cation and (green) BE anion. In panel (b), dashed red line show the position of C540 surface, defined with the average C540 radius reported in Table 2. |
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Fig. 6 Orientation of ions confined inside C540 fullerene. All the values calculated for 1 endohedral C540 fullerene encaging 6 CH_BE ion pairs surrounded by CH_BE ionic liquid (1 × 6_CH_BE@C540_IL in Table 1). All the values obtained from molecular dynamics simulations at 303 K. |
The energetics of endohedral systems was also analysed from molecular dynamics simulations. Intermolecular Lennard-Jones energy, Einter(LJ), between the ions and C540 atoms was calculated for 1 × 6_CH_BE@C540_VAC and 1 × 6_CH_BE@C540_IL to infer the effects of external solvation shell ions on the confined ones. Einter(LJ) between confined ions and C540 atoms changes for BE from −3.61 ± 0.01 eV to −4.92 ± 0.01 eV, and for CH from −2.80 ± 0.01 eV to −3.28 ± 0.01 eV from 1 × 6_CH_BE@C540_VAC to 1 × 6_CH_BE@C540_IL. Therefore, it may be concluded that BE–C540 interaction is remarkably stronger than the CH–C540 interaction when the endohedral fullerene is in vacuum or is surrounded by IL. Similarly, the external solvation of the endohedral fullerene with IL leads to an improvement in the interaction between the confined ions and the internal fullerene surface, most remarkably for BE ions (increasing Lennard-Jones interaction 1.31 eV) but also for CH cations (increasing 0.48 eV), which is in agreement with the structural changes reported in the previous sections.
The effects of host–guest interactions upon ions confinement inside C540 have been also studied thoroughly by analysing the changes in the electronic structure of the 1 × 6_CH_BE@C540_VAC system using density functional theory methods for complementing the information inferred from the molecular dynamics simulations in previous sections. In a first step, the lattice constant parameter for C540 has been optimized; the minimal energy as a function of lattice constant was found for a value of 35.20 Å (Fig. 7). There are no comparison data obtained experimentally for lattice constant.
Band diagrams for C540 and 1 × 6_CH_BE@C540_VAC along the symmetry directions of the fcc cubic Brillouin zone are plotted in Fig. 8. At the Γ point, a change in the energy gap is inferred, which is increased roughly 0.3 eV due to the confined ionic liquid. Although the lowest unoccupied level (LUMO) practically remains constant, the highest occupied orbital (HOMO) is stabilized by 0.28 eV, whereas the Fermi level (EF) suffers destabilization of 0.37 eV. Such increase in the EF level leads to the lowest unoccupied levels to cross with Fermi level. Unoccupied levels whose energy is lower than EF are evident along all the directions of the Brillouin zone, suggesting long-range interactions between the guest and the C540 cage.60 Six unoccupied states whose energies are lower than EF were found. One of each band would be related with those levels from the cations, which have lost one electron. Changes in the BS diagram have been analyzed with partial density of states (PDOS). Because of the π-electronic structure of C540, the guest PDOS were obtained considering only the contribution of p-orbitals of the atoms (see Fig. 9). As expected, in the case of the C540 cage, total DOS and p-orbital PDOS show a very similar behaviour. For the host–guest system, PDOS due to the atoms of the confined liquid have also been calculated. Total DOS and p-orbital PDOS show very similar profiles, indicating that the band structure is mainly due to the C540 cage. IL confined cluster atoms show slight contributions to the total DOS. In contrast, both HOMO and LUMO levels show an important contribution from the oxygen atoms of the anions. The energy stabilization of the HOMO states could be related with the allocation of the anion negative charge.
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Fig. 8 Electronic band structure for (a) pristine C540 and (b) 6_CH_BE@C540 systems. Dotted lines represent Fermi level. |
A binding energy value of −14.01 eV has been estimated for the confined IL cluster and C540 cage at PBE-D/6-31G* level. To our knowledge, binding energies for ILs confined in fullerenes are not reported yet. However, this large binding energy value is indicative of an energetically favoured confinement process for CH_BE IL. The interaction network of ions confined in C540 has been studied through NBO and AIM theories. In the context of AIM theory, the localization of the critical points (CP) is a very suitable tool for the characterization of intermolecular interactions. Although there are four types of critical points, only bond critical points (BCP) were considered to analyze the intermolecular interactions of confined ions. BCPs are characterized through the charge density (ρ), its Laplacian (∇2ρ), and the ellipticity (ε), defined as |(λ1/λ2) − 1|, wherein λi is the largest/smallest (i = 1/i = 2) curvature of the charge density in a direction perpendicular to the bond path. Ellipticity could be used to measure the extent to which charge is preferentially accumulated, and provides a criterion for structural stability. Thus, large ellipticity values are related with weak bonds. NBO and AIM parameters for those interactions having AIM parameters fulfilling the criteria to be considered intermolecular interactions are reported in Table S1 (ESI†), being plotted in Fig. 10. Almost all the inferred intermolecular interactions arise from the lone pair of both the oxygen atoms in the anion acting as donors and the hydrogen atoms in the cation acting as acceptors. Only NBO charge transfers whose values are larger than 0.05 eV satisfy AIM criteria. As noted in Fig. 10, the confined IL cluster adopts a spherical configuration. As expected, the largest NBO charge transfer correspond with large ρ and small ε, and also with the shortest donor–acceptor distances. Moreover, the strongest interactions are those wherein the phenyl motif of the benzoate anion tends to be parallel to C540 aromatic rings with a typical π–π stacking distance, which is indicative of a π-stacking interaction.
NBO charge distribution for the confined IL molecules has been also analysed (Table 3). All the equivalent atoms show similar charge values, independent of their position in the confined cluster. According to the molecular structure of the cation, a positive charge should be allocated to N atoms, which yield a charge of −0.29 a.u. The calculated total charge of the cation is 1.16 a.u. On the other hand, a negative charge in the anion should be allocated to both the O atoms, which show a charge of −0.73 a.u. with the total anion charge being −0.85 a.u. This effect is related to a charge transfer of around 0.3 eV between the cation and the anion. This charge transfer energy is very similar to the change in the HOMO state due to confined IL. Then, a relationship between the stabilization of the HOMO state and charge transfer between IL ions could be established.
HOMO and LUMO states for the 1 × 6_CH_BE@C540_VAC system also show an important contribution from the confined IL cluster (Fig. 11). As expected from PDOS results, HOMO and LUMO orbitals are mainly located on anions. LUMO orbitals are mainly delocalized over all the anion, whereas HOMO is mainly located over COO− motifs. Although Fig. 11 only plots the HOMO and LUMO orbital, an analogous behaviour was inferred for six orbitals below and above.
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Fig. 11 Isovalues of (a) HOMO and (b) LUMO molecular orbitals for 6_CH_BE cluster confined in C540. Values were calculated at PBE-D/6-31G* level. |
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Fig. 12 Radial distribution functions, g(r), between the center of mass of C540 and the center of mass of CH cation and BE anion for (a) 1 empty C540 fullerene surrounded by CH_BE ionic liquid (1 × PRIST_C540_IL in Table 1), and (b) 1 endohedral C540 fullerene encaging 6 CH_BE ion pairs surrounded by CH_BE ionic liquid (1 × 6_CH_BE@C540_IL in Table 1). All the values obtained from molecular dynamics simulations at 303 K. r = 0 Å correspond to C540 center of mass, and the vertical gray dashed line shows the position of C540 carbon atoms, defined as the average C540 radius reported in Table 2. |
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Fig. 13 Radial distribution functions, g(r), between the carbon atoms in C540 (C) fullerene and selected atoms in CH and BE ions, for (a) 1 empty C540 fullerene surrounded by CH_BE ionic liquid (1 × PRIST_C540_IL in Table 1), and (b) 1 endohedral C540 fullerene encaging 6 CH_BE ion pairs surrounded by CH_BE ionic liquid (1 × 6_CH_BE@C540_IL in Table 1). Atoms code as in Fig. 1. All the values obtained from molecular dynamics simulations at 303 K. In panel b, only ions outside the C540 fullerene are considered in the calculation of g(r). r = 0 Å corresponds to the position of C540 carbon atoms. |
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Fig. 14 Running integrals, N, differences obtained from the corresponding radial distribution functions reported in Fig. 13 between values for 1 empty C540 fullerene surrounded by CH_BE ionic liquid (N1×PRIST_C540_IL, defined as in Table 1), and 1 endohedral C540 fullerene encaging 6 CH_BE ion pairs surrounded by CH_BE ionic liquid (N1×6_CH_BE@C540_IL, defined as in Table 1). Atoms' code as in Fig. 1. For N1×6_CH_BE@C540_IL only ions outside C540 were considered, as in Fig. 13. |
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Fig. 15 (a) Representative snapshot and (b) spatial distribution function for CH and BE ions in the first external solvation shell of C540 for 1 endohedral C540 fullerene encaging 6 CH_BE ion pairs surrounded by CH_BE ionic liquid (1 × 6_CH_BE@C540_IL in Table 1). All the values obtained from molecular dynamics simulations at 303 K. Color code: (blue) CH cation and (green) BE anion. In panel (b), dashed red line show the position of C540 surface, defined with the average C540 radius reported in Table 2. |
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Fig. 16 Orientation of ions outside C540 fullerene. All values calculated for 1 endohedral C540 fullerene encaging 6 CH_BE ion pairs surrounded by CH_BE ionic liquid (1 × 6_CH_BE@C540_IL in Table 1). All values obtained from molecular dynamics simulations at 303 K. Vectors defined as in Fig. 6. Vertical gray dashed lines stand for the position of the first external solvation shell defined according to g(r) in Fig. 12. |
The energetics for the interaction between the externally solvating ions and C540 is reported in Table 4. The reported results show stronger BE–C540 than CH–C540 interactions (roughly 2 eV stronger), which may be justified considering that BE anions are closer to the C540 external surface than CH cations, as reported in previous sections. Moreover, putting C540 inside the IL weakens CH–BE interactions, both for Coulombic and Lennard-Jones contributions, in comparison with those in pure CH_IL (with no C540).67 Nevertheless, this decrease in anion–cation interaction strength upon C540 solvation evolves in parallel with a decrease in anion–anion and cation–cation Coulombic repulsive energies. Therefore, the total balance of interionic energy, defined as the difference between the anion–cation interaction energy minus the sum of anion–anion and cation–cation contributions, is −5.49 eV for pure CH_BE whereas it is −5.79 and −5.56 eV, for 1 × PRIST_C540_IL and 1 × 6_CH_BE@C540_IL, respectively. Similarly, the total ion–C540 interactions are −19.19 and −27.39 eV for 1 × PRIST_C540_IL and 1 × 6_CH_BE@C540_IL, respectively. Therefore, placing C540 fullerene into CH_BE is strongly energetically favoured.
CH_BEa | 1 × PRIST_C540_IL | 1 × 6_CH_BE@C540_IL | ||||
---|---|---|---|---|---|---|
Einter(Coul) | Einter(LJ) | Einter(Coul) | Einter(LJ) | Einter(Coul) | Einter(LJ) | |
a Values for pure CH_BE obtained at 298 K (ref. 67). | ||||||
BE–C540 | — | — | — | −10.61 | — | −15.53 |
CH–C540 | — | — | — | −8.59 | — | −11.87 |
BE–CH | −27.68 | −0.38 | −27.15 | −0.35 | −26.77 | −0.34 |
BE–BE | 11.34 | −0.16 | 10.87 | −0.16 | 10.78 | −0.15 |
CH–CH | 11.55 | −0.15 | 11.15 | −0.16 | 11.07 | −0.15 |
The dynamic properties of the studied endohedral fullerene are of importance to characterize the studied systems. Self-diffusion coefficients, D, were obtained using molecular dynamics simulation trajectories from mean square displacements and Einstein's equation. Diffusion properties for the 1 × 6_CH_BE@C540_IL system were calculated for ions confined inside the fullerene, in the first external solvation shell (defined according to the first minima in g(r) reported in Fig. 12), and the remaining fluid layers (Table 5). Ions confined inside C540 fullerene show very low D values, two orders of magnitude lower than those for bulk CH_BE in the absence of fullerene, and show a solid-like behaviour of confined ions. This slow dynamics of ionic liquids upon confinement has also been reported in carbon nanotubes39,68 and may be explained considering the formation of a crystal-like phase of ions when confined inside carbon nanostructures.69 Nevertheless, this effect is strongly dependent on the size of ions and the width of the carbon nanotube. Ohba and Chaban70 have recently reported that for 1-ethyl-3-methylimidazolium chloride confinement in (10,10) carbon nanotubes leads to an increase of 1.5 orders of magnitude for the diffusion rates in comparison with the bulk fluid, whereas in the case of (22,22) carbon nanotube, diffusion rates are similar to or slightly lower than that in bulk fluid, depending on temperature. This behaviour is justified considering the disruption of the ionic liquid network upon confinement, and thus nanotubes with large diameter can maintain the most relevant features of the bulk fluid when ions are confined. In the case of confinement in large fullerenes, when the cage is fully filled with ions, similar to this work, the mobility of the ions is very low and anion–cation intermolecular forces are very strong, which would justify the poor diffusion rates obtained from molecular dynamics simulations.
Shell | r/Å | 1011 × D/m2 s−1 | |
---|---|---|---|
BE | CH | ||
a Values calculated for pure CH_BE in the absence of C540. | |||
CH_BEa | — | 0.1107 ± 0.0051 | 0.0983 ± 0.0049 |
Ions inside C540 | r < RC540 | 0.0024 ± 0.0006 | 0.0035 ± 0.0004 |
Ions in the first external solvation shell of C540 | RC540 < r < 17 | 0.0015 ± 0.0008 | 0.0017 ± 0.0008 |
Ions in outer external solvation shell of C540 | r > 17 | 0.0963 ± 0.0039 | 0.0900 ± 0.0040 |
Similarly, ions in the first external solvation shell also show very low D values, even lower than those of confined, which may be justified considering the strong interaction with the C540 surface as inferred from the energetics reported in Table 4. D values for confined BE anions and in the first external solvation shell are lower than those for CH cations, which is in contrast with the behaviour of more external ions or in the bulk CH_BE. This may be justified considering the stronger BE–C540 interaction in comparison with CH–C540 because of the π–π stacking of BE ions. Banerjee71 reported the solvation of fullerenes by water, toluene and acetone, in which it was shown that molecules in the first external solvation shell had reduced mobility in comparison with bulk molecules. For example, results from Banerjee71 in water–C540 system showed D values of 2.1 × 10−9 and 2.4 × 10−9 m2 s−1 for water molecules in the first solvation shell and in the bulk phase, respectively. Viscosity of CH_BE IL and the self-diffusion coefficients are three orders of magnitude larger than that of water, but in addition to this effect, water molecules decrease their mobility roughly up to 12% on going from bulk water to C540 solvation layer, whereas the mobility of BE ions is 60 times lower in the solvation layer than in the bulk IL. Banerjee71 also reported studies on toluene–C60 showing that toluene mobility decreases up to 25% on going to fullerene solvation shells, which is almost double the effect than that for water molecules, showing the importance of π–π interaction with the fullerene. Nevertheless, changes in ionic mobility on ILs going to fullerene solvation shells are remarkably larger than those for organic solvents. The diffusion of CH_BE ions placed further into the first C540 external solvation shell approaches that of pure CH_BE, although D values are lower for 1 × 6_CH_BE@C540_IL than for pure CH_BE, showing that the disrupting effect of C540 on IL structure extends to large distances of the fullerene, which is in agreement with the behaviour of g(r) showed in Fig. 12. Similarly, the self-diffusion coefficient of C540 in CH_BE was also calculated to be (0.0004 ± 0.0002) × 10−11 m2 s−1; the very low D value should be considered with caution because of the length of the simulations (obviously fully diffusive regime was not reached for C540 in the applied simulation time), but it shows, at least qualitatively, the low mobility of the studied giant fullerene in CH_BE.
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Fig. 17 Total dipole moment along the z-direction for BE anions, μZ, for 1 endohedral C540 fullerene encaging 6 CH_BE ion pairs surrounded by CH_BE ionic liquid (1 × 6_CH_BE@C540_IL in Table 1) under EEFs. BE anions (a) confined inside C540, (b) in the first external C540 solvation shell, and (c) in outer shells. |
For ions solvating C540 in the first layer (Fig. 17b) the reported results also show an almost negligible effect of the applied fields, which is in agreement with the strength of the ions–C540 interactions reported in previous sections. Ions in regions further than the first C540 shell are only affected by the field for the stronger cases studied. Analogous results were inferred for CH cations and for the behaviour of empty fullerenes under EEFs.
In general, theoretical simulations carried out in this work could be used in both the prediction of the properties and the rational design of new materials based on IL cluster confined in fullerenes for technological applications.
Footnote |
† Electronic supplementary information (ESI) available: NBO and AIM results (Table S1). See DOI: 10.1039/c4ra07239a |
This journal is © The Royal Society of Chemistry 2014 |