Theoretical Study on Amino Acid-Based Ionic Pairs and Their

Article pubs.acs.org/JPCC

Theoretical Study on Amino Acid-Based Ionic Pairs and Their Interaction with Carbon Nanostructures Cesar Herrera,† Rafael Alcalde,† Mert Atilhan,*,‡ and Santiago Aparicio*,† †

Department of Chemistry, University of Burgos, Plaza Misael Bañuelos s.n., Burgos 09001, Spain Department of Chemical Engineering, Qatar University, Doha, Qatar

‡

S Supporting Information *

ABSTRACT: Quantum chemistry methods were used to analyze the properties of ionic pairs formed by combination of the 1-ethyl-3-methylimidazolium cation with anions derived from alanine, glycine, serine, and phenylalanine amino acids, which appear in the corresponding ionic liquids. Anion−cation pairs were studied from structural and energetic viewpoints using density functional theory together with the use of natural bond orbital and atoms in a molecule approaches. Interactions of the mentioned ionic pairs with carbon nanostructures carried out with graphene sheets and single-walled carbon nanotubes, with ions placed on the outer surface and when confined inside the nanotube, were analyzed from first principles. Interaction energies, density of states, and charge density allow inferring the mechanism of interaction between the ion pairs and graphene or carbon nanotubes.

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INTRODUCTION Ionic liquids (ILs) have attracted great attention both in industry and in academia in recent years due to their relevant physicochemical properties,1−5 which make these fluids suitable alternatives to traditional approaches for many technological problems such as efficient lubrication in hydraulic systems and absorbers in gas scrubbing, etc.6,7 Probably the most relevant characteristic of ILs is the possibility of controlling their properties through a judicious combination of cations and anions,8 and thus, ILs could be designed for many requested applications.9−11 Academic and industrial research on ILs has shifted from first- and second-generation ILs,12,13 such as classical combinations of imidazolium cations with fluorinated anions, with well-known problems rising from the instability of halogen-containing anions, to new types of ionic liquids. The new ionic liquids studied in recent years have been designed considering criteria such as low toxicity,11,14 biodegradability,15 production at competitive costs,16 and simple synthetic procedures from common and accessible reactants.17,18 Therefore, new ILs have been designed such as those containing ammonium, guanidinium, cholinium, or piperazinium cations paired with anions such as lactate, salicylate, phosphate, alkylsulfate, or acetate.19−26 Moreover, the use of generally regarded as safe (GRAS) criteria,27 selection of ions from active pharmaceutical ingredients,28 or use of ions derived from natural materials29 has extended ILs toward new groups of compounds with better properties than first- and secondgeneration ILs. Amino acid-based ionic liquids (AILs) are among the most interesting approaches available for development of taskspecific ionic liquids with suitable environmental and toxicological properties at low economical costs.30,31 Use of © 2014 American Chemical Society

amino acids as a versatile platform for developing ILs rises from the presence of an amino group and a carboxylic acid residue in the same molecule. Likewise, the presence of several relevant side groups and a chiral atom confers these molecules as a suitable choice for adjusting ILs properties through selection of a suitable amino acid. Moreover, amino acids may lead both to anions and to cations for ILs. Other advantages of the use of amino acids as ILs source are their high biodegradability, the well-known ability to produce amino acids in large scale at low costs, and the possibility of adding additional functionality through modification of chemical groups. Therefore, considering all these properties several authors have analyzed the most relevant characteristics of amino acid-based ILs. Fukumoto et al.32 synthesized the first ILs from the 20 natural amino acids, for which most relevant physicochemical properties were analyzed later by the same authors.33 These authors showed that ion functionalization allowed one to control ILs properties, such as viscosity, melting point, polarity, or hydrophobicity, in a straightforward manner, thus pointing to AILs as a suitable platform for developing task-specific ILs from natural sources. Yang et al.34 measured the thermophysical properties of 1ethyl-3-methylimidazolium ([EMIM]) aminoacetate (Gly) ionic liquid, which were analyzed using the interstice model. Zhang et al.35 designed AILs in combination with phosphonium cations for CO2 capture purposes, which showed suitable physical properties for CO2 absorption. Gardas et al. carried out a study on the properties of AILs with ammonium- and phosphonium-based cations and developed quantitative Received: January 7, 2014 Revised: April 8, 2014 Published: April 11, 2014 9741

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Figure 1. Results of geometry optimization for ion pairs studied in this work. Optimization calculated at the B3LYP/6-311++g(d,p) theoretical level. Values inside each panel show counterpoise-corrected interaction energy, ΔE, calculated at the (black) B3LYP/6-311++g(d,p) and (bold) MP2/6311++g(d,p) theoretical levels and (red) relevant interatomic distances. Red dashed lines show possible hydrogen-bonding interactions. Atom color code: (red) oxygen, (blue) nitrogen, (gray) carbon, and (light gray) hydrogen. Ion pairs are grouped using three different interaction sites: interaction between the COO anion group and the acidic hydrogen, H2, placed in the C2 position of imidazolium ring, with one oxygen interacting with H2 and the other one closer to the ethyl group (position a) or the methyl group (position b), and interaction between the COO anion group and hydrogen atoms in the imidazolium ring opposite to H2 (position c).

chemistry methods have been applied successfully to study the interaction of molecular amino acids with graphene and carbon nanotubes.59 The AILs studied in this work were all based on [EMIM] cation paired with anions obtained from alanine, [ALA], glycine, [GLY], serine, [SER], and phenylalanine, [PALA], amino acids, Figure 1. These ionic liquids were selected to analyze the effect of the functionalization in the amino acid on the systems properties, evolving from the presence of a simple H atom in [GLY], to a −CH3 group in [ALA], to a −CH2OH group in [SER], to a −CH2−phenyl in [PALA]. We carried out extensive quantum chemistry studies for all these AILs using several approaches to analyze the properties of the involved anion−cation pairs and the behavior of these pairs with regard to graphene and single-walled carbon nanotubes (SWNTs). Use of quantum chemistry methods to study the properties of ion pairs has led to relevant structural, energetic, and topological information for characterization of different types of ionic liquids,60−63 and thus, these methods would allow one to analyze the properties of the selected AILs and their behavior on graphene surface, on SWNTs, and when confined inside SWNTs.

structure−property relationships for predicting their properties.36 Muhammad et al.37 analyzed the thermophysical properties of AILs composed of imidazolium cations, showing the moderate viscosity of these fluids, which is of pivotal importance for their industrial applications. Tong et al.38 measured surface properties and other relevant physical properties of imidazolium−AILs. Likewise, Liu et al.39 carried out a molecular dynamics study on imidazolium−AILs ionic liquids, showing the structural changes at the nanoscopic level rising from the changes in anion and cation molecular structures. AILs have attracted remarkable attention for several technical applications such as biochemical processes,40,41 CO2 capture,35,42,43 synthesis development,44 or catalysis studies.45 A very relevant field of study for ILs is their behavior with regard to carbon nanostructures (graphene and carbon nanotubes), because of its importance both for basic science and for applied purposes.46−50 The behavior of ILs on graphene/graphite surfaces,51,52 solvation of carbon nanotubes by ILs,53 and confinement of ILs54 between graphene sheets47,55 or inside carbon nanotubes56,57 have shown very relevant structural and physical properties with possible applications in several technological fields. To date, the properties of AILs with regard to graphene and/or carbon nanotubes have not been studied in the literature. Therefore, we report in this work a computational study on (i) the properties of AILs ionic liquids in which amino acid-derived anions are paired with [EMIM] cation and (ii) their behavior with regard to graphene and carbon nanotubes. We studied in previous works the behavior of relevant types of ILs, such as those based on cholinium and piperazinium cations, with regard to carbon nanostructures using a computational chemistry approach;50,58 these studies are extended to AILs in this work. Although studies of AILs on carbon nanostructures are absent in the literature, quantum

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COMPUTATIONAL DETAILS This study is divided into two sections: (i) analysis of isolated anion−cation pairs and (ii) analysis of ion pairs on graphene and SWNTs. For the study of anion−cation pairs, density functional theory (DFT) was used to analyze the properties of the involved ionic pairs for the selected AILs, Figure 1. Geometry optimizations for ion pairs were done using the DFT approach with the Becke gradient-corrected exchange functional64 and Lee−Yang−Parr correlation functional65 with three parameters (B3LYP)66 method. Single-point calculations for second-order Møller−Plesset perturbation theory (MP2) were 9742

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[BF4];78nevertheless, the nonplanar C1 conformer is favored over the Cs one, and thus, only the C1 conformer was considered in this work.78,79 Three interaction positions were analyzed in Figure 1: (i) interaction between the COO anion group and the acidic hydrogen, H2, placed in the C2 position of the imidazolium ring (the carbon atom between the two imidazolium nitrogen atoms) with one oxygen interacting with H2 and the other one closer to the ethyl group (position a) or to the methyl group (position b) and (ii) interaction between the COO anion group and hydrogen atoms in imidazolium ring opposite to H2 (position c; atoms C4 and H4, close to the methyl group, and C5 and H5 close to the ethyl group). Results reported in Figure 1 show that interaction through positions a and b is equivalent for the four studied anions, with average differences of 0.49% and 0.05% calculated at the B3LYP and MP2 levels, respectively. Anion−cation interaction through position c is weaker than through positions a/b (12.12% and 10.53% weaker on average, calculated at the B3LYP and MP2 levels, respectively) because of the acidic character of the H2 site, but nevertheless, interaction energies through position c are also remarkable. This is in agreement with the more acidic character of the H2 atom in comparison with the rear protons involved in the position c interaction (H4 and H5), which was justified by Hunt et al. through the charge distribution along the imidazolium ring.80 In spite of the controversies about the role developed by hydrogen bonding in alkylimidazolium hydrogen bonds81 and the prevailing role of Coulombic-type forces over covalent type on the properties of ionic pairs, the interionic distances reported in Figure 1 are short enough (lower than 1.80 Å for all studied anions and positions) for considering the possibility of developing relevant anion−cation hydrogen bonds mainly through a and b positions but also through the c site. These results agree with previously published results by Mou et al. for [EMIM][GLY].82 The interaction energies reported in Figure 1 follow the order [GLY] > [ALA] > [SER] > [PALA] for all studied interaction sites, but for a fixed interaction position differences between [GLY] and [PALA] containing pairs are lower than 3% at both the B3LYP and the MP2 levels, and thus, the presence of the studied functional groups in the studied anions does not involve relevant changes in either the structural or the energetic properties of the ion pairs with the [EMIM] cation. For example, hydrogen-bonding distances through positions a/ b are in the 1.66−1.72 Å range and in the 1.74−1.79 Å range for interaction through position c for the studied anions. Relevant properties of ion pairs are reported in Table 1. Anion−cation interaction leads to a remarkable charge transfer (calculated according to ChelpG method), Δq. This charge transfer is almost equivalent for positions a and b for a fixed anion, which is in agreement with the equivalent interaction energies reported in Figure 1, and lower for position c. The lower Δq values for interaction through position c are in contrast with the lower interaction energies through that site in comparison with a/b sites because a lower Δq should lead to larger Coulombic interaction, and thus, the origin could be in the weaker interaction, hydrogen bonding, through position c when compared with the interaction through the acidic H2 site. Likewise, Δq for a fixed interaction position follows the order [GLY] < [ALA] < [SER] < [PALA], which is in agreement with the decreasing interaction energies on going from [GLY] to [PALA] because of the weakening of anion−cation Coulombic interactions with increasing charge transfer. The ChelpG charges were also calculated for isolated ions to allow

done for the structures optimized with the B3LYP functional for comparative purposes. The 6-311++g(d,p) basis set was used both for B3LYP optimizations and for MP2 single-point calculations. Atomic charges were calculated to fit the electrostatic potential according to the ChelpG67 scheme. Interaction energies for the ionic pairs, ΔE, were calculated as the differences among the pair and sum of corresponding cation and anion energies at the same theoretical level, with basis set superposition error (BSSE) corrected through the counterpoise procedure.68 Calculations were carried out with the Gaussian 03 package.69Atoms in a molecule (AIM),70 carried out using the AIM2000 program,71 and natural bond orbital (NBO)72 calculations, carried out with Gaussian 03,69 were done to get a deeper insight into anion/cation interactions. Calculations for ionic liquids + carbon nanostructures were done using Siesta 3.2.73 The Perdew−Burke−Ernzerhof (PBEGGA) parametrized generalized gradient approximation, which is a direct generalized-gradient parametrization of the free electron gas with no free parameters, was used for all calculations.74 Double-ζ polarized (DZP) basis sets for all involved atoms were applied together with the normconserving Troullier−Martins pseudopotentials.75 Dispersion correction using Grimme’s method was used76 to take account of van der Waals type interactions. C6 and R0 parameters were obtained from Grimme’s parametrization76 for all possible pairwise interactions. The geometric mean was used to calculate the C6(i,j) cross paremeters, the scale factor d for the scaling function in Grimme’s potential was fixed to 20, and the overall fitting factor s6 was set to 1.66, which is recommended in the Siesta manual for the DZP basis set. Calculations were carried out with an energy mesh cutoff of 300 Ry and a k-point mesh of 8 × 8 × 8 in the Monkhorst− Pack scheme.77 Model systems were built using a 20 Å × 20 Å graphene sheet in the armchair configuration, with a single ionic pair placed on top of the sheet, and periodic boundary conditions (PBC) were applied in the three space directions. PBC in the direction normal to the graphene sheet surface was large enough (30 Å) to avoid interaction between image layers. For SWNT calculations, an armchair SWNT (10,10) was used with a single ionic pair placed above and inside the SWNT, which was placed in a 15 × 30 × 30 Å3 simulation box. The large size of the simulation box avoids interaction between adjacent boxes. Calculations for isolated ion pairs, required for calculation of interaction energies, were done by placing isolated ions inside the simulation boxes used for graphene sheets. Structural relaxation, for isolated ionic pairs and graphene and SWNT-containing systems, was done by conjugate gradients, keeping fixed graphene or SWNT coordinates, with convergence criteria being forces acting on all atoms not to exceed 0.04 eV/Å.

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RESULTS AND DISCUSSION Isolated Ion Pairs. Analysis of the interaction between the studied carbon nanostructures and ion pairs requires previous knowledge of the anion−cation interaction in the absence of graphene or SWNTs, which is reported in this section. The structures of anion−cation ion pairs were optimized at B3LYP/ 6-311++(g,d), and thus, several interaction positions were studied and interaction energies were calculated, Figure 1. It should be remarked that the ethyl group bonded to the imidazolium ring in the studied [EMIM] cation leads to several conformers, for which both the planar Cs and the nonplanar C1 conformers appear in several ionic liquids such as [EMIM]9743

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anions and cations shows the energy gap HOMO(anion) − LUMO(cation) (4.08, 4.03, 3.67, and 3.67 eV for [GLY], [ALA], [SER], and [PALA], respectively) is remarkably lower than the gap HOMO(cation) − LUMO(anion) (14.94, 14.75, 14.31, and 14.01 eV, for [GLY], [ALA], [SER] and [PALA], respectively), and thus, change transfer upon ion pair formation evolves from the HOMO(anion) to the LUMO(anion), as shown in the shapes of the frontier orbitals reported in Figure 2. Moreover, the lower the HOMO(anion)−LUMO(cation) energy gap, the larger the charge transfer, as shown in Table 1 for the studied anions. The HOMO−LUMO energy gaps for ion pairs are roughly equivalent for interactions through positions a/b and do not change remarkably with the type of anion (roughly 4 eV), whereas this gap is remarkably lower for interaction through position c (roughly 3 eV). Orbitals lower lying shown in Figure 2 show orbital mixing, especially HOMO-3, which shows the development of hydrogen bonding via ion pair formation. Density of states, DOS, for the calculated ion pairs are reported in Figure 3, showing the effect of interaction position (Figure 3a) and type of anion (Figure 3b). For a fixed anion, although characteristics of interactions through positions a/b are very similar, as discussed in previous sections, DOS is remarkably different for these two interaction sites, especially in the vicinity of the HOMO, whereas the LUMO region is not affected. Interaction through position c leads to completely different behavior with regard to positions a/b, which is in agreement with the lower HOMO−LUMO energy gap. Likewise, the type of anion involved in the ion pair changes the DOS regions in the vicinity of both the HOMO and the LUMO orbitals. The effects on the HOMO region may be justified because this rises mainly from anion orbitals, but although the LUMO rises from the cation LUMO, Figure 2, it is also affected by the presence of the studied functional groups in the considered anions. Vibrational frequencies for relevant sites are reported in Table 3; interactions through the acidic H2 atom lead to strong red shiftings of the corresponding C−H stretching vibrations, ν2 in Table 3, in comparison with values for the isolated [EMIM] cation, which are consistent with development of hydrogen bonding. Likewise, ν2 red shiftings are larger for interaction through position b than through position a for pairs involving [GLY] and [ALA] and reverse for pairs involving [SER] and [PALA] anions. Development of intermolecular hydrogen bonding through positions a/b changes remarkably the length of C2−H2, which is 1.0776 Å, for the isolated [EMIM] cation. Moreover, there is a perfect linear relationship between the enlargement of the C2−H2 bond and the corresponding red shift of ν2 but also a roughly linear variation of interaction energy with ν2 red shifts, Figure 4.

Table 1. Anion−Cation Charge Transfer from ChelpG Ion Total Charges, Δq, Dipole Moment, μ, HOMO and LUMO Energies, EHOMO and ELUMO, and HOMO−LUMO Energy Gap, ΔEGa ion pair

interaction site

Δq/e

μ/D

EHOMO/ eV

ELUMO/ eV

ΔEG/ eV

a

0.087

12.05

−5.03

−1.06

−3.97

b c a

0.085 0.065 0.116

12.14 15.77 12.08

−5.17 −4.73 −5.03

−1.03 −1.69 −1.06

−4.14 −3.05 −3.97

b c a

0.110 0.089 0.157

12.24 15.93 11.76

−5.14 −4.60 −5.31

−1.03 −1.71 −1.09

−4.11 −2.88 −4.22

b c a

0.139 0.105 0.159

12.21 15.87 12.64

−5.33 −5.01 −5.09

−1.09 −1.74 −1.17

−4.24 −3.27 −3.92

b c

0.133 0.083

12.67 16.90

−5.20 −4.79

−1.12 −1.80

−4.08 −2.99

[EMIM] [GLY]

[EMIM] [ALA]

[EMIM] [SER]

[EMIM] [PALA]

a

All properties calculated for optimized structures at the B3LYP/6311++g(d,p) theoretical level. Interaction sites as in Figure 1.

comparison with ionic pairs. In the [EMIM] cation, ChelpG charges for H2, H4, and H5 atoms are 0.212, 0.200, and 0.211, which do not justify the different acidity for the H2 atom in comparison with the H4 and H5 atoms, but according to Hunt et al.80 the charge for the corresponding C−H moieties should be considered to justify the different acidity of imidazolium interaction sites, and thus, 0.134, 0.067, and 0.087 charges were calculated for C2−H2, C4−H4, and C5−H5 sites, respectively, which justifies the preferential interaction through a/b positions (involving C2−H2) than through position c (involving C4−H4 and C5−H5). Upon ion pair formation through a/b positions, charges in the H2 atom and C2−H2 imidazolium moiety change, being remarkably more positive in comparison with isolated imidazolium ion, and the anion oxygen atom interacting with the H2 atom is more negatively charged (with the exception of the [PALA] anion) than the corresponding isolated anions, Table 2. These charge transfers upon ionic pair formation lead to large dipole moments for all interaction sites (ranging from 11.8 to 16.9 D) with pairs formed through positions a/b showing lower dipole moments than through c site. The energy for frontier orbitals and the corresponding HOMO−LUMO gaps, ΔEG, are also reported in Table 1. HOMO orbitals for the ion pairs come from the corresponding anion HOMOs, whereas ion pair LUMOs come from the cation LUMO, Figure 2. Analysis of frontier orbitals for isolated

Table 2. Partial Charges, q, Calculated According to the ChelpG Method for Ion Pairs in Relevant Sitesa ion pair

q(H2)/e

Δq(H2)/e

q(C2−H2)/e

Δq(C2−H2)/e

q(O)/e

Δq(O)/e

[EMIM][GLY] [EMIM][ALA] [EMIM][SER] [EMIM][PALA]

0.388 0.344 0.282 0.282

+0.176 +0.132 +0.070 +0.070

0.212 0.215 0.215 0.226

+0.078 +0.081 +0.081 +0.092

−0.907 −0.839 −0.902 −0.756

−0.040 −0.005 −0.079 +0.051

a

All values calculated for optimized structures of ion pairs interacting through position b and calculated at the B3LYP/6-311++g(d,p) theoretical level. Interaction sites as in Figure 1. H2 stands for the acidic hydrogen in the imidazolium ring bonded to the C2 atom placed between the two imidazolium nitrogens; C2−H2 stands for the moiety formed by the C2 and H2 atoms; O stands for the oxygen atom in the corresponding anion closer to H2 atom (Figure 1); Δq stands for the difference between the corresponding charge in the ion pair and in the corresponding isolated ion. 9744

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Figure 2. Frontier orbitals for the [EMIM][ALA] ion pair interacting through position b, calculated at the B3LYP/6-311++g(d,p) theoretical level. Interaction site as in Figure 1. Isosurfaces calculated at 0.02.

Figure 3. Density of states, DOS, as a function of orbital energy, E, for the reported ionic pairs calculated at the B3LYP/6-311++g(d,p) level. DOS were shifted for the sake of visibility. The names of the interaction sites (a, b, and c) as in Figure 1

Table 3. Vibrational Frequencies, ν, for the Reported Ions and Ionic Pairs Calculated at the B3LYP/6-311++g(d,p) Leveld ion/ion pair

ν1/cm−1

ν2/ cm−1

[EMIM] [EMIM][GLY]

3291.0 3290.1a,b,c 3289.1a,b,c 2722.2a,b,c 3290.0a,b,c 3288.9a,b,c 2774.3a,b,c 3291.4a,b,c 3289.6a,b,c 2677.0a,b,c 3292.2a,b,c 3289.7a,b,c 2796.9a,b,c

3277.1 2642.6a,b,c 2508.9a,b,c 3276.5a,b,c 2634.0a,b,c 2521.8a,b,c 3275.5a,b,c 2526.8a,b,c 2607.3a,b,c 3276.2a,b,c 2520.9a,b,c 2598.9a,b,c 3275.3a,b,c

[EMIM][ALA]

[EMIM][SER]

[EMIM][PALA]

a,b,c

Interaction sites as in Figure 1. dReported vibrational frequencies correspond to (ν1) symmetric stretching for ring C5−H5 in the imidazolium cation and (ν2) stretching for ring C2−H2 in the imidazolium cation. H2 stands for the acidic hydrogen atom placed in the imidazolium ring between the nitrogen atoms; H5 stands for hydrogen atoms placed in the imidazolium ring opposite to H2.

Figure 4. Enlargement of the C2−H2 bond in the [EMIM] cation upon ion pair formation in comparison with the value for isolated [EMIM], Δr(C2−H2), in relationship to red shifting of the ν2 vibrational frequency, Δν2, with ν2 corresponding to the stretching vibration of the C2−H2 bond in [EMIM], and with counterpoisecorrected interaction energy, ΔE. All values calculated the (black) B3LYP/6-311++g(d,p) theoretical level.

AIM analysis applied to the studied ion pairs would allow one to infer details about the topology of anion−cation interactions. Anion−cation interactions through the three studied positions are characterized by the appearance of two binary critical points, BPC1 and BCP2, corresponding to the stronger and weaker interactions, respectively, together with a ring critical point, RCP1, Figure 5. Likewise, the appearance of ring paths leading cyclic structure in the region of anion−cation interaction confirms the development of strong hydrogen bonding, especially through positions a/b. The characteristics of the relevant critical points are reported in Table 4. Positive values of the Laplacian of the electron density shows the closedshell character of interionic interactions. Likewise, the electron density for interactions through positions a/b is larger than through position c and also their Laplacians, in agreement with interaction energies reported in Figure 1. BCP1 points

correspond to the stronger interactions for each pair, e.g., developed with C2−H2 site for positions a/b, whereas BCP2 corresponds to a weaker interaction, e.g., developed with ethyl or methyl hydrogens for positions a/b, and thus, electron densities and their Laplacians for BCP2 are lower than for BCP1. For interaction site through position b, electron density and their Laplacians corresponding to BCP1 (corresponding to the stronger hydrogen bonds) roughly follow the order [GLY] > [ALA] > [SER] > [PALA], in agreement with interaction energies reported in Figure 1, and although this trend is also followed for interaction through position c, it is not so well defined for position a, Table 4. NBO results are reported in Table 5. Anion−cation interactions are analyzed considering three main properties within the second-order perturbation theory: (i) second-order 9745

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Figure 5. AIM analysis of [EMIM][SER] ionic pairs calculated for optimized structures at the B3LYP/6-311++g(d,p) theoretical level. Relevant intermolecular binary critical points, BCPs, and ring critical points, RCPs, are reported. Color code: (small red dots) BCPs, (yellow red dots) RCPs, (black spheres) carbon atoms, (gray spheres) hydrogen atoms, (blue spheres) nitrogen atoms, and (red spheres) oxygen atoms. Pink lines show bond paths and yellow lines ring paths.

perturbation energy, E(2), (ii) the energy difference among the donor and the acceptor, ΔEij, and (iii) the Fock matrix element between the donor and acceptor, Fij. Strong intermolecular interactions, hydrogen bonding in all studied systems and positions, are characterized by the orbital interactions from the oxygen lone pairs in anion COO oxygen atoms, LP(O), with the antibonding orbitals of the corresponding C−H acceptor sites, LP(O) → σ*(C−H). Effective interactions would rise from large E(2) values (effective donor−acceptor charge transfer) and low ΔEij and large Fij (suitable donor−acceptor symmetry). For each interaction site, two types of interactions are obtained (short and long in Table 5), rising from development of strong and weaker interactions, as mentioned in previous sections. Short interactions are characterized by very large E(2) values, whereas long interactions, e.g., interactions with side alkyl chains, led to lower E(2) values. Interactions through LP(2) are stronger than through LP(1) for all pairs interacting through the short positions because ΔEij are roughly double for LP(1) than for LP(2). For interactions through position b, the total E(2) rising from interactions with LP(1) and LP(2) pairs follows the order [GLY] > [ALA] > [SER] ≈ [PALA], which is in agreement with interaction energies reported in Figure 1. This trend is also roughly followed by interactions through position c, but the opposite trend is obtained for interaction through position a, in agreement with the behavior reported in Figure 4. Mou et al.82 reported a linear trend between the C−H acceptor elongation and the population of the σ*(C−H) acceptor orbital in the [EMIM] cation upon ion pair formation with the [GLY] anion. Results reported in Figure 6 are in agreement with this linear trend for the three studied interaction positions. The results show that increasing population of the σ*(C−H) acceptor orbital leads to larger C−H elongations, interactions through positions a/b have remarkably larger σ*(C−H) populations than interactions through position c, which would justify the larger elongations of C2−H2 bonds in comparison with C5−H5, the larger red shifting of the corresponding C−H vibrations, and the larger interaction energies (Figure 1) for positions a/b than for position c interactions. For a fixed interaction position, there is a rough trend between σ*(C−H) population with [EMIM] C−H elongation, C−H red shifting, and interaction energy upon ion pair formation: the larger the σ*(C−H) population the larger the remaining properties (Figures 1, 4, and 6). It should be remarked that although the [EMIM][PALA] pair for interaction through position a shows the largest σ*(C−H) population, [EMIM] C−H elongation, and [EMIM] C−H red

shifting, considering all pairs interacting through that position (Figures 4 and 6) it has the lowest interaction energy for that interacting position (Figure 1). For the remaining positions [EMIM][GLY] has the largest interaction energy and thus the largest σ*(C−H) population, [EMIM] C−H elongation, and [EMIM] C−H red shifting; thus, the properties evolve with the trend [GLY] > [ALA] > [SER] ≈ [PALA]. Therefore, charge transfer from oxygen lone pairs in the corresponding anion toward the [EMIM] σ*(C−H) acceptor orbital develops a key role in the anion−cation interaction: the more effective the transfer, the stronger the interaction. Analysis of NBO results allow one to analyze the effect of the presence of the studied functional groups in the anions on the effectiveness of this charge transfer. All studied anions have the common structure R−CHNH2−COO; thus, we can analyze the effect of having R = a single H atom ([GLY]), a −CH3 group ([ALA]), a −OH group ([SER]), or a −phe group ([PHE]) to the charge transfer between the anion and the [EMIM] cation. Considering a fixed interaction position, e.g., position b, the studied functional groups decrease the occupancy of LP(O) in anion COO groups. These LP(O) occupancies are calculated for isolated anions and related with the second-order perturbation energies for the corresponding interactions, Figure 7, showing that the presence of the studied functional groups decreases total occupancy of LP(O) when compared with [GLY], thus resulting in a reduction of total E(2) corresponding to the LP(O) → σ*(C−H) interactions, leading to lower interaction energies, Figure 1. Ion Pairs on Graphene Sheets. The results reported in the previous section for the mechanism of anion−cation interactions showed the preferential interaction through positions a/b in comparison with position c; likewise, similar characteristics were inferred for interactions through positions a and b. Therefore, studies on interactions between ion pairs and carbon nanostructures (graphene and SWNTs) were carried out only for ion pairs interacting through position b. First, considering that studies for ion pairs/carbon nanostructures were carried out using a different computational approach to that reported in the previous section for isolated ion pairs, isolated ion pairs (in the absence of carbon nanostructures) were also calculated using the same approach as the one used for the study of ion pairs/carbon nanostructures. Therefore, isolated anions, cations, and ion pairs were placed in 30 × 30 × 30 Å3 simulation boxes and PBC was applied; box dimensions were large enough to avoid interactions between ions belonging to neighbor simulations boxes. Interaction energy was calculated for the four studied ion pairs interacting through 9746

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0.029 45 (0.18422)

0.01558 (0.06948)

0.00818 (0.03812)

position b; calculated values were −390.77, −377.26, −375.33, and −364.71 kJ mol−1 for [EMIM][GLY], [EMIM][ALA], [EMIM][SER], and [EMIM][PALA], respectively. These results are in good agreement with values reported in Figure 1, both in the trends of changes of interaction energy with the type of anion and in the values of these interaction energies, leading to −3.8% average deviation with values calculated at the B3LYP/6-311++g(d,p) level. The structures of ion pairs on top of graphene were optimized, and the most favored configurations are reported in Figure 8. The reported configurations showed a planar arrangement of the ion pairs on the graphene surface, the moiety formed by the imidazolium group of the cation, and the −N−C−COO group of the anion adopts a planar shape laying parallel to the graphene sheets. The average interplanar distances between the ion moiety and the graphene sheet are reported in Table 6 and were found to be in the range from 3.02 to 3.06 Å range for the four studied anions, with almost negligible effect of the type of anion. Rajesh et al.59 studied the properties of several amino acids on top of graphene sheets; their results for histidine, which contains the imidazolium moiety included in the [EMIM] cation, showed a planar arrangement on top of the sheet at 3.21 Å, which is in good agreement with the results reported in this work for the studied ion pairs. The ion pairs containing the [EMIM] cation lie closer to the graphene sheets (5.3% on average) than molecular histidine, because of the presence of ionic systems. Literature available studies on the use of first-principles for the study of ion pairs interactions with graphene are very scarce, and thus, comparison with results reported in this work is difficult. Ghatee and Moosavi83 analyzed the properties of [MMIM] and [BMIM] cations paired with [Cl] and [PF6] anions on top of coronene and circumcoronene using the B3LYP/6-311g level, although the shape of the studied ions led to a configuration not so perfectly planar as the ones studied in this work; their results could be used to do a rough comparison with systems studied in this work. The results of Gatee et al.83 for [BMIM][PF6] showed preferential interaction of the anion with the graphene sheet, which led to a nonparallel configuration of the imidazolium ring on top of the graphene surface. Results reported in this work show that the shape of the studied amino acid-based anions allows development of a planar moiety when interacting with the cation on top of the graphene sheet and thus improving the interaction with the sheet because both the anion and the cation may interact very effectively with the sheet at the same time, Figure 8. This behavior is in contrast with [BMIM][PF6],83 for which, because of the anion spherical shape, interaction with the sheet is developed mainly through the anion. Interaction energies between the studied ion pairs and graphene sheets were also calculated, Table 1, showing very effective interaction between the ions and the graphene. Ghatee et al.83 reported adsorption energies of −0.11 and −0.92 eV for ion pairs containing alkylimidazolium and [Cl] or [PF6] anions, respectively, which are lower than the ones reported in this work for amino acidbased pairs. The reason for the large adsorption energies for [EMIM]-amino acid-based anions pairs rise from the structure of adsorption reported in Figure 8, which allows simultaneous anion−cation interaction with the graphene sheets. To obtain deeper insight into the role of the type of anion for a fixed cation into the mechanism of interaction with the graphene sheet, calculations for [EMIM][PF6] were carried out using the same approach as for amino acid-based compounds, which

a

Critical points numbering and interaction sites from Figure 2. ρ and ∇2ρ reported in atomic units.

0.00614 (0.02780) 0.01830 (0.09036) 0.03381 (0.23496) 0.02005 (0.10188) 0.03254 (0.21667)

0.00602 (0.02708)

0.00899 (0.04526) 0.01185 (0.05027) 0.03079 (0.20080) 0.00615 (0.02803) 0.01824 (0.08984) 0.03369 (0.23592) 0.01938 (0.09684) 0.03476 (0.25142)

0.00537 (0.02426)

0.00913 (0.04542) 0.01221 (0.05232) 0.03046 (0.19924) 0.00597 (0.02700) 0.01832 (0.09072) 0.03534 (0.25476) 0.01800 (0.08752) 0.03326 (0.22959)

0.00641 (0.02844)

RCP1

0.00885 (0.04396) 0.01141 (0.04814)

BCP2 BCP1

0.03181 (0.21196) 0.00590 (0.02678)

RCP1 BCP2

0.01836 (0.09119) 0.03568 (0.25798)

BCP1 RCP1

[EMIM] [GLY] [EMIM] [ALA] [EMIM] [SER] [EMIM] [PALA]

BCP2

0.01820 (0.08849)

BCP1

0.03302 (0.22739)

ion pair

0.00644 (0.02862)

c b a

Table 4. Electron Density, ρ, and (parenthesized) Laplacian of Electron Density, ∇2ρ, at Intermolecular Binary Critical Points, BCPs, and Ring Critical Points, RCPs, Obtained from AIM Analysis of Ionic Pairs Calculated for Optimized Structures at the B3LYP/6-311++g(d,p) theoretical levela

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Table 5. NBO Analysis of the Reported Ionic Pairs Calculated at the B3LYP/6-311++g(d,p) Level; Second-Order Perturbation Energy, E(2), Energy Difference among the Donor, i, and the Acceptor, j, ΔEij, and Fock Matrix Element between the Donor and the Acceptor, Fija ionic pair

interaction site

interaction type

donor NBO (i)

acceptor NBO (j)

E(2)/kJ mol−1

ΔEij/au

Fij/au

[EMIM][GLY]

a

short short long long short short long long short short long long short short long long short short long long short short long long short short long long short short long long short short long long short short long long short short long long short short long long

LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2) LP(1) LP(2)

BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD* BD*

21.00 104.39 15.44 14.90 25.52 127.07 14.77 18.70 21.55 92.63 2.22 0.71 21.63 105.35 15.15 14.73 24.52 124.93 15.10 17.91 20.25 83.18 2.72 0.79 27.82 123.80 15.61 20.13 22.68 110.42 15.27 16.23 20.33 85.56 2.43 0.71 21.67 81.25 15.31 23.22 22.89 110.79 15.10 16.82 21.05 75.19 5.44 2.47

1.02 0.65 1.07 0.64 1.00 0.65 1.07 0.64 1.05 0.67 1.05 0.61 1.02 0.65 1.07 0.64 1.00 0.65 1.07 0.64 1.06 0.67 1.05 0.61 1.00 0.65 1.08 0.64 1.01 0.65 1.07 0.64 1.06 0.67 1.05 0.61 1.01 0.63 1.09 0.65 1.01 0.65 1.07 0.64 1.05 0.65 1.06 0.62

0.07 0.12 0.06 0.04 0.07 0.13 0.06 0.05 0.07 0.11 0.02 0.01 0.07 0.12 0.06 0.04 0.07 0.13 0.06 0.05 0.07 0.11 0.02 0.01 0.07 0.13 0.06 0.05 0.07 0.12 0.06 0.05 0.07 0.11 0.01 0.01 0.07 0.10 0.06 0.06 0.07 0.12 0.06 0.05 0.07 0.10 0.03 0.02

b

c

[EMIM][ALA]

a

b

c

[EMIM][SER]

a

b

c

[EMIM][PALA]

a

b

c

a

a, b, and c interaction sites as in Figure 1; type of interaction: short, stands for short interactions in Figure 1 (roughly 1.7 Å donor−acceptor separation) and long for long interactions in Figure 1 (roughly 2.1−2.4 Å donor−acceptor separation). Data for NBOs involved in intermolecular interactions are reported. For all ionic pairs, the donor NBO from oxygen lone pairs (LP) in the anion COO group and acceptor NBO from antibonding orbitals (BD*) in cation CH groups.

could also be compared with results from Ghatee et al.83 The calculated structure of isolated ion pairs is reported in Figure S1 (Supporting Information); two possible main interaction positions were studied, but the interaction in which the [PF6] anion is placed above the imidazolium ring leads to a larger

interaction energy, Figure S1b (Supporting Information), in agreement with literature information.84 These two possible interacting pairs were placed on top of graphene and optimized, Figure S2 (Supporting Information). Two main conclusions may be inferred when comparing results for amino acid anions 9748

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Figure 6. Relationship between enlargement of the C−H acceptor bond in the [EMIM] cation upon ion pair formation in comparison with the value for isolated [EMIM], Δr*, and increment in population of [EMIM] σ*(C−H) orbital upon ion pair formation in comparison with the value for isolated [EMIM], Δσ*. C−H stands for the C2−H2 bond for interactions through positions a/b and C5−H5 for interaction through position c. All values calculated at the B3LYP/6311++g(d,p) theoretical level.

Figure 7. Relationship between total second-order perturbation theory, E(2)total, which is the sum of values corresponding to short and long interactions for LP(1) and LP(2) donors in Table 5, with total occupancy of LPs in anion oxygen atoms of the COO group calculated for isolated anions, LP(O)total. E(2)total calculated for interactions through position b at the B3LYP/6-311++g(d,p) theoretical level.

with those for [PF6]: (i) the interaction energy with graphene is remarkably larger for pairs containing amino acid cations than for those with [PF6]; (ii) in the case of [PF6] pairs, the [EMIM] cation is tilted on the graphene surface, whereas for amino acid anions the [EMIM] cation lays parallel to the graphene sheet, Figure 8. Therefore, changing amino acid anions by [PF6] hinders development of planar anion−cation pairs and thus the relevance of the anion−cation mechanism of interaction on the form the ion pairs are adsorbed on top of graphene sheets. The planar structure of alkylimidazolium− amino acid-based pairs allows a very effective interaction with the graphene sheet because both anion and cation are able to develop effective interactions with the sheet, which is in contrast with [EMIM][PF6]. The effect of the type of amino acid anion on the adsorption energy is not very strong; although values evolve with the trend [GLY] > [ALA] > [SER] > [PALA], the energy decreases just 10.2% on going from [GLY] to [PALA]. In the case of [EMIM][PALA], two adsorption possibilities were inferred: (i)

Figure 8. Geometry of ionic pairs (lowest energy) on top of the graphene surface calculated at the GGA/Grimme/DZP level. Top and side views of (a) [EMIM][GLY], (b) [EMIM][ALA], (c) [EMIM][SER], and (d) [EMIM][PALA].

cation imidazolium ring parallel to the sheet and (ii) anion phenyl ring parallel to the sheet, Figure 8. For the case of the phenyl ring parallel to the sheet, AB stacking is inferred with the ring placed further of the sheet than when the imidazolium ring is placed parallel to the sheet, Table 6. Rajesh et al.59 analyzed the adsorption of phenylalanine amino acid on graphene, showing parallel AB stacking at 3.33 Å, which is in good agreement with results in this work, although the presence of ionic species decreases the distance with the graphene sheet in comparison with molecular amino acids. Moreover, adsorption energies calculated for [EMIM][PALA] showed that the position with the imidazolium ring parallel to 9749

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Table 6. Graphene or SWNT(10,10)−Ionic Pair Interaction Energy, ΔE, and Interplanar Distance, deq, between Graphene Sheets or SWNT (10,10) and the Cation Imidazolium Ring (for all the ionic pairs) or Anion Phenyl Ring (for [EMIM][PALA])a graphene [EMIM] [GLY] [EMIM] [ALA] [EMIM][SER] [EMIM] [PALA]

SWNT(10,10), topd

SWNT(10,10), insidee

ΔE/eV

deq/Å

ΔE/eV

deq/Å

ΔE/eV

deq/Å

−2.85

3.06

−2.65

3.07

−3.56

3.06f

−2.88

3.02

−2.13

3.05

−3.84

2.95f

−2.60 −2.56b −1.51c

3.04 3.04b 3.16c

−2.35 −1.79b −1.09c

3.08 3.09b 3.26c

−3.80 −5.03

2.96f 3.08f

a Values calculated at the GGA/Grimme/DZP level. bImidazolium ring on surface. cphenyl ring on surface. dion pairs on top of SWNT. eion pairs confined inside of SWNT. fthe shortest C or N (from ions) to C (from SWNT) distance is reported.

the sheet is more favorable than the position with the phenyl ring (1.05 eV larger adsorption energy) because when the phenyl ring is placed parallel to the sheet cation interaction with the sheet almost vanishes and the anion interaction with the sheet is weaker (anion COO group lying perpendicular to the graphene surface). The effect of interplanar distance was calculated and reported in Figure 9; these plots were calculated from the corresponding optimized structures reported in Figure 8 translating the ion pair toward and away from the graphene sheet with the structure of the ion pair fixed. Results reported in Figure 9 show well-defined potential wells; all studied ion pair/graphene interactions are relevant at distances up to 4.5 Å of the sheet (roughly 0.5 eV interaction energy at that distance), thus confirming the very effective mechanism of adsorption. On the contrary, moving ion pairs closer to the sheet from the equilibrium positions leads to strong repulsive forces; roughly at 2.5 Å all adsorption energies are positive. A detailed analysis of the various contributions to the interaction energy is reported in Figure 10. The kinetic energy contribution shows well-defined minima at roughly 0.5 Å further of the equilibrium position, Figure 10a. Likewise, exchange-correlation and electrostatic contributions show maxima at almost the same distances as the kinetic contribution. Therefore, the balance between kinetic, exchange-correlation, and electrostatic terms (K+E+E) in the vicinity of the equilibrium position takes account of 34% of the interaction energies reported in Table 6. Nevertheless, the largest contribution to the interaction energy rises from the van der Waals contributions (MM, 66% on average for the four ion pairs with the imidazolium ring on top of the graphene plane, Figure 10d), which are quantified in this work according to Grimme’s approach.76 This is particularly important when comparing [EMIM][PALA] with the imidazolium or phenyl ring on top of graphene, for which most of the difference in the interaction energy (roughly 1 eV) rises from the weaker van der Waals contribution for the ion pair with phenyl ring on top of graphene sheet, Figure 10d. Charge difference density, Δρ, for the [EMIM][GLY] pair on top of the graphene sheet is reported in Figure 11 (analogous results were obtained for the remaining ion pairs). Δρ was calculated according to eq 1 Δρ = ρgraphene + IL − (ρgraphene + ρIL )

Figure 9. Graphene−ionic pairs interaction energy, ΔE, as a function of the distance between graphene sheets and the cation imidazolium ring (for all the ionic pairs) or anion phenyl ring (for [EMIM][PALA]), d, in comparison with the lowest energy position, deq, calculated at the GGA/Grimme/DZP level. Values were obtained from optimized structures reported in Figure 8, which were shifted toward and away from the direction perpendicular to the graphene sheets.

where ρgraphene+IL, ρgraphene, and ρIL stands for the electron density for IL adsorbed on graphene, isolated graphene, and isolated ionic pair, respectively. Three slices are reported in Figure 11: (i) on the graphene surface, (ii) on the imidazolium ring, and (iii) perpendicular to the graphene surface passing through the imidazolium C2−H2 site. Results in Figure 11a show charge density difference on graphene upon ion pair adsorption. The graphene region below the [GLY] anion shows electron enrichment (red regions), especially below the anion COO group. The graphene region below the imidazolium ring shows a complex behavior, but an enrichment below C2−H2 is inferred. Likewise, the graphene surface below the anion− cation hydrogen-bonding region (C2−H2---OCO) shows very remarkable electron enrichment. The behavior in the direction perpendicular to the graphene surface is reported in Figure 11c, the graphene surface shows electron enrichment, and the region between the sheet and the imidazolium ring has remarkable electron excess, especially in the region below the C2−H2 group, from which strong interaction between the cation and the graphene sheet through the acidic C2−H2 site is inferred. On the contrary, the behavior in the region between the anion and the sheet shows a zone of charge depletion. Moreover, results in Figure 11b show the changes on the ions charge density upon adsorption, concluding the increment in

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electron density in the imidazolium ring but also in the anion COO group. Ghatee et al.83 reported that the anion−cation interaction energy increases upon adsorption on graphene, i.e., interionic interaction is reinforced on graphene sheets. The anion−cation interaction energy for [BMIM][PF6] increases 17% upon graphene adsorption.83 Results obtained in this work also show increments on anion−cation interaction energy upon adsorption, although these changes are more moderate than for [BMIM][PF6]: 2.0%, 4.1%, 2.6%, 2.4%, and 3.7% for [EMIM][GLY], [EMIM][ALA], [EMIM][SER], [EMIM][PALA] (interacting through imidazolium), and [EMIM][PALA] (interacting through phenyl), respectively. These improvements in interionic interactions may be justified considering the charge density differences reported in Figure 11b. The electronic density of states for the systems ion pair + graphene in the optimized structures reported in Figure 8 is shown in Figure 12. DOS for pristine graphene is in agreement with previous literature results,85 showing the zero-gap semiconductor behavior with the Fermi energy exactly at the Dirac point. There are relevant changes in the DOS of graphene after adsorption of the corresponding ion pairs with increasing intensity in the whole region below the Fermi level, EF. The contributions of the electronic levels belonging to the adsorbed ions to the total DOS appear in the whole region below EF, whereas changes in the intensity of conduction bands are less remarkable. The peak appearing just above EF does not change its intensity upon ions adsorption but is blue shifted 0.14, 0.25, 0.27, 0.26, and 0.26 eV for [EMIM][GLY], [EMIM][ALA], [EMIM][SER], [EMIM][PALA] (interacting through imidazolium), and [EMIM][PALA] (interacting through phenyl), respectively. Increasing DOS in the vicinity of the EF shows the ions−graphene charge transfer reported in previous sections. Ion Pairs on SWNTs. After the study of the interaction of the studied ion pairs with the planar graphene surface, similar analysis will be carried out for interactions on the curved SWNTs surface. Figure 13 shows the optimized structures, whereas relevant geometrical parameters and interaction energies are in Table 6. First, a planar structure of the ion pairs on SWNT is obtained as previously reported for graphene surfaces. Second, the interaction between ion pairs and the sheets becomes weaker on going from planar graphene to

Figure 10. Contributions to the total interaction energy as a function of the distance between graphene sheets and the cation imidazolium ring (for all the ionic pairs) or anion phenyl ring (for [EMIM][PALA]), d, in comparison with the lowest energy position, deq, calculated at the GGA/Grimme/DZP level. Values were obtained from optimized structures reported in Figure 4, which were shifted toward and away from the direction perpendicular to the graphene sheets. Contributions: (a) kinetic (Pauli repulsion), (b) exchange correlation, (c) electrostatic (sum of Hartree contribution plus ion−ion and ion− electron terms), and (d) molecular mechanics (Grimme’s contribution).76

Figure 11. Contour plots of the charge difference density for [EMIM][GLY] on top of graphene calculated at the GGA/Grimme/DZP level. Slices on (a) the graphene layer, (b) the imidazolium ring, and (c) perpendicular to the graphene surface through the C−H bond (for carbon placed between N atoms). 9751

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Figure 12. Density of states, DOS, as a function of orbital energy, E, calculated for the systems formed by ionic pairs + graphene sheets. DOS for graphene sheets in the absence of ionic pairs is reported inside each panel for comparative purposes. All values obtained at the GGA/Grimme/DZP level.

curved SWNT surfaces, and thus, the interplanar distances increase on going from graphene to SWNT surfaces. Although the increment in interplanar distances between the imidazolium ring and the sheets on going from graphene to SWNT is lower than 2%, this leads to a strong decrease in interaction energies: 7%, 26%, 10%, and 30% for [GLY], [ALA], [SER], and [PALA], respectively. This is also true for the case of [PALA] in which the phenyl ring is placed on top of SWNT. Therefore, curved surfaces decrease remarkably the interaction between the corresponding ions and the carbon sheets. The interaction energy between ion pairs and SWNTs was split into the different contributions and compared with values for pairs on top of graphene, Figure 14. Results reported in Figure 14 show that whereas for pairs on top of graphene the percentage contributions are roughly 34% and 66% for K+E+E and MM contributions, respectively (with the exception of [EMIM][PALA]phe for which the percentages are 47% and 53%), ongoing to SWNT these percentages are almost 50/50%. The K+E+E contributions do not change remarkably on going from graphene to SWNT but the MM term decreases remarkably, and thus, the changes on MM contributions to the total interaction energy are the origin of the changes reported in the table between graphene and SWNT sheets. This may be produced by the weakening of ions stacking on the sheets on going to SWNTs in spite of the small increase in the interplanar distances reported in Table 6. Figure 15 shows the calculated DOS for the pristine SWNT (10,10) and for systems including adsorbed ion pairs. The metallic character of pristine SWNT is maintained upon ion pair adsorption. Paek et al.86 calculated DOS for pristine SWNT(10,10); the shape of this DOS is slightly different than

Figure 13. Geometry of ionic pairs (lowest energy) on top of the SWNT (10,10) surface calculated at the GGA/Grimme/DZP level. Top and side views of (a) [EMIM][GLY], (b) [EMIM][ALA], (c) [EMIM][SER], and (d) [EMIM][PALA].

that reported in Figure 15 in which peaks are not sharply defined. Results from Paek et al.86 for pristine SWNT showed that the energy difference with the first van Hove singularity (v1) is roughly 1.6 eV, whereas 2.78 eV is obtained in this work. These differences should be explained considering the different computational methods.87,88 The v1 values suffer very minor changes upon ion pairs adsorption for any of the studied ion pairs: 2.67, 2.76, 2.69, 2.97, and 2.85 eV for v1 with [EMIM][GLY], [EMIM][ALA], [EMIM][SER], [EMIM][PALA] (interacting through imidazolium), and [EMIM][PALA] (interacting through phenyl), respectively. Likewise, a remarkable feature appears in the vicinity of the Fermi level for all studied systems at −0.28, 0.00, −0.10, 0.00, and −0.03 for 9752

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Figure 14. Contributions to the total interaction energy for ion pairs on top of graphene and SWNT (10,10). Values were obtained from optimized structures reported in Figures 4 and 13. Contributions: (K +E+E) kinetic (Pauli repulsion) + exchange-correlation + electrostatic (sum of Hartree contribution plus ion−ion and ion−electron terms) and (MM) molecular mechanics (Grimme’s contribution).76

Figure 15. Density of states, DOS, as a function of orbital energy, E, for the reported ionic liquids on top of SWNT (10,10) calculated at the GGA/Grimme/DZP level. (b) Extended plot in the vicinity of the Fermi levels.

[EMIM][GLY], [EMIM][ALA], [EMIM][SER], [EMIM][PALA] (interacting through imidazolium), and [EMIM][PALA] (interacting through phenyl), respectively. The remaining DOS peaks increase their intensity upon ions adsorption in comparison with pristine SWNT but maintaining their positions and shapes. Figure 16 shows Δρ for the [EMIM][GLY] pair on top of SWNT showing that the ion pair−SWNT interaction leads to regions below the [GLY] anion (Figure 16a−d) with electron loss, whereas for regions below the imidazolium cation (Figures 16e−j) electron enrichment is inferred, which is similar to the mechanism of interaction reported in Figure 11c on top of graphene sheets. Likewise, the region below the anionic −NH2 group (Figure 16a) shows a remarkable spot of electronic enrichment, which points to anion−SWNT strong interactions through this group. Ion Pairs Confined Inside SWNTs. The optimized structures of ion pairs are reported in Figure 17 and the interaction energies in Table 6. Interaction energies increase remarkably upon ion confinement in comparison with results on top of graphene or SWNT sheets, especially for [EMIM][PALA], Table 6. In the case of this last ion pair, the strong structural change of the [PALA] anion upon confinement should be remarked to allow a suitable adjustment to the SWNT internal cavity, Figure 18. Likewise, the [EMIM] cation is moved out of the anion COO plane upon confinement in such a way that although anion−cation interaction is developed through the C2−H2 acidic site the second interaction through the cation methyl group is sterically hindered, Figure 18. The calculated DOS for the pristine SWNT (10,10) and for the systems including the [EMIM][GLY] ion pair, confined

Figure 16. Contour plots of the charge difference density for the [EMIM][GLY] ion pair on top of SWNT (10,10) calculated at the GGA/Grimme/DZP level. Slices perpendicular to SWNT.

and in comparison with results on top of SWNT, is reported in Figure 19. The energy difference for the first van Hove singularity is 2.74 eV for the system with the [EMIM][GLY] ion pair confined, which is slightly larger than for the ion pair on top of the SWNT. Moreover, the peak in the vicinity of the Fermi level appears close to 0 eV for the system comprising the confined ion pair which is very different from the value for the ion pair on top of the SWNT, Figure 19b. Charge density differences for the confined [EMIM][GLY] are reported in Figure 20. Isosurfaces for planes cutting the SWNT are reported at different sites separated by 0.5 Å. The reported results show a strong interaction between the confined ion pairs at the surrounding nanotube, leading to changes in the charge distribution, both for the nanotube and for the ion pairs. The nanotube shows alternating areas of charge enrichment (red spots) and charge depletion (blue spots). The confined ions show charge enrichment at all studied positions, as the centered red spots show for all panels of Figure 20.

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CONCLUSIONS The properties of the ion pairs formed by the 1-ethyl-3methylimidazolium cation and amino acid-based anions and their behavior when adsorbed on top of graphene and SWNTs and when confined inside SWNTs are studied using a combination of DFT approach and an empirical Grimme’s9753

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Figure 17. Geometry of ionic pairs (lowest energy) confined inside SWNT (10,10) calculated at the GGA/Grimme/DZP level. [EMIM] cation is plotted in cyan color, and the corresponding anions are plotted in green for the sake of visibility. The shortest ion−SWNT distances (lower than 3.2 Å), in Angstroms, are reported within each panel.

Figure 19. Density of states, DOS, as a function of orbital energy, E, for the reported ionic liquids confined inside SWNT (10,10) calculated at the GGA/Grimme/DZP level.

Figure 18. Comparison between the optimized structures of the [EMIM][PALA] ion pair confined and on top of SWNT (10,10). Structures calculated at the GGA/Grimme/DZP level. [EMIM] cation is plotted in cyan color and [PALA] anion in green for the sake of visibility: (sticks) ions confined, (lines) ions on top of SWNT (10,10).

occupancy of LP(O) in the anion COO group for the studied anions in comparison with [GLY]. The properties of the ion pairs adsorbed on graphene sheets are characterized by large interaction energies (in the −2.56 to −2.85 eV range) with ion pairs placed at 3.02−3.06 Å of the graphene sheet with the imidazolium ring parallel to the graphene surface. In the case of the [EMIM][PALA] ion pair, adsorption with the imidazolium ring parallel to the graphene surface is roughly 1 eV more favorable than with the phenyl ring parallel to the sheet. The large interaction energies for all studied pairs rise from both anion and cation interacting with the graphene sheet. Analysis of the contributions to the total interaction energy shows that van der Waals-type contribution takes account of roughly two-thirds of the total energy for the studied pairs. The density of states shows very small changes

type contribution to describe van der Waals-type interactions. Anion−cation interactions are characterized by development of hydrogen bonding, which is strongly through the H2 acidic position in the imidazolium ring but also with hydrogen atoms in the imidazolium alkyl chains, and also, although weaker than through H2, with imidazolium hydrogen atoms opposite to the H2 site. A large charge transfer characterizes the anion−cation interaction, with important charge concentration in the C−H2 site, which justifies the prevailing interaction through this site. The anion−cation interaction energies follow the order [GLY] > [ALA] > [SER] > [PALA], which is justified through AIM and NBO approaches considering the decreasing total 9754

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Figure 20. Contour plots of the charge difference density for the [EMIM][GLY] ion pair confined inside SWNT (10,10) calculated at the GGA/ Grimme/DZP level. Slices perpendicular to SWNT at the positions (0.5 Å steps) reported in the left panel.

Notes

upon ion pairs adsorption with increasing intensity of the peaks in the vicinity of the Fermi level because of the charge transfer, which leads to electron enrichment both in the region below the imidazolium ring and in the graphene surface below the ions but also close to the anion COO group. Adsorption on top of SWNT (10,10) leads to interaction energies lower than on graphene sheets. Nevertheless, ions are placed with imidazolium rings parallel to the SWNT surface with structures very close to those on top of graphene. The curvature of the surface weakens ions/sheets interactions, especially the van der Waals contributions, which are roughly one-half of the total interaction energy on top of SWNT in contrast with the larger contributions on top of graphene. Nevertheless, analysis of charge transfer shows large regions of electron enrichment in the space between the ions and the SWNT carbon atoms, leading to very effective interaction also in curved interfaces. The density of states upon ions adsorption shows small changes in the energy difference for the first van Hove singularity (in the range of increasing 6.8% for [EMIM][PALA] or decreasing 4.0% for [EMIM][GLY]), but features in the vicinity of the Fermi level appear for adsorbed ion pairs. Ion pairs confined inside of SWNT (10,10) show large structural changes in comparison with ions adsorbed on top of graphene or SWNTs, which is more remarkable for the case of the large [PALA] anion. Nevertheless, ion confinement is characterized by very large interaction energies, which are 34%, 80%, 62%, and 181% larger in comparison with ions placed on top of SWNT for [EMIM][GLY], [EMIM][ALA], [EMIM][SER], and [EMIM][PALA], respectively. Confinement is also characterized by electron enrichment both in the ions and in SWNT atoms, which leads to the large interaction energies. Ions confined in SWNT led to the nanotube structure showing alternating regions of charge enrichment and charge depletion. On the other hand, confined ions showed charge enrichment at all studied positions

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The authors declare no competing financial interest.

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ASSOCIATED CONTENT

S Supporting Information *

Calculated structures of the [EMIM][PF6] ionic pair and of this ion pair on top of graphene sheet. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Rooney, D.; Jacquemin, J.; Gardas, R. Thermophysical Properties of Ionic Liquids. Top. Curr. Chem. 2010, 290, 185−212. (2) Aparicio, S.; Atilhan, M.; Karadas, F. Thermophysical Properties of Pure Ionic Liquids: Review of Present Situation. Ind. Eng. Chem. Res. 2010, 49, 9580−9595. (3) Feng, R.; Zhao, D.; Guo, Y. Revisiting Characteristics of Ionic Liquids: A Review for Further Application Development. J. Environ. Prot. 2010, 1, 95−104. (4) Catner, E. W.; Margulis, C. J.; Maroncelli, M.; Wishart, J. F. Ionic Liquids: Structure and Photochemical Reactions. Annu. Rev. Phys. Chem. 2011, 62, 85−105. (5) Dong, K.; Zhang, S. Hydrogen Bonds: a Structural Insight into Ionic Liquids. Chem.Eur. J. 2012, 18, 2748−2761. (6) Plechkova, N. V.; Seddon, K. R. Applications of Ionic Liquids in the Chemical Industry. Chem. Soc. Rev. 2008, 37, 123−150. (7) Angell, C. A.; Ansari, Y.; Zhao, Z. Ionic Liquids: Past, Present and Future. Faraday Discuss. 2012, 154, 9−27. (8) Weis, D. C.; McFarlane, D. R. Computer-Aided Molecular Design of Ionic Liquids: An Overview. Aust. J. Chem. 2012, 65, 1478− 1586. (9) Miller, A. L.; Carlisle, T. K.; LaFrate, A. L.; Voss, B. A.; Bara, J. E.; Hudiono, Y. C.; Wisenauer, B. R.; Gin, D. L.; Noble, R. D. Design of Functionalized Room-Temperature Ionic Liquid-Based Materials for CO2 Separations and Selective Blocking of Hazardous Chemical Vapors. Sep. Sci. Technol. 2012, 47, 169−177. (10) Ferro, V. R.; Ruiz, E.; de Riva, J.; Palomar, J. Introducing Process Simulation in Ionic Liquids Design/Selection for Separation Processes Based on Operational and Economic Criteria Through the Example of Their Regeneration. Sep. Pur. Technol. 2012, 97, 195−204. (11) Ventura, S. P. M.; Goncalves, A.; Sintra, T.; Pereira, J. L.; Goncalves, F.; Coutinho, J. A. P. Designing Ionic Liquids: the Chemical Structure Role in the Toxicity. Ecotoxicology 2013, 22, 1−13. (12) Freemantle, M. An Introduction to Ionic Liquids; RSC Publishing: Cambridge, UK, 2010; pp 8−10. (13) Beadham, I.; Gurbisz, M.; Gathergood, N. Design of Safer Chemicals−Ionic Liquids. Handbook of Green Chemistry; Wiley: New York, 2012; Vol. 9, pp 137−158. (14) Alvarez-Guerra, M.; Irabien, A. Design of Ionic Liquids: an Ecotoxicity (Vibrio fischeri) Discrimination Approach. Green Chem. 2011, 13, 1507−1516. (15) Pham, T. P. T.; Cho, C. W.; Yun, Y. S. Environmental Fate and Toxicity of Ionic Liquids: a Review. Water Res. 2010, 44, 352−372. (16) Rakita, P. E. Challenges to Commercial Production of Ionic Liquids. ACS Symp. Ser. 2003, 856, 32−40. (17) Petkovic, M.; Ferguson, J. L.; Gunaratne, H. Q. N.; Ferreira, R.; Leitao, M. C.; Seddon, K. R.; Rebelo, L. P. N.; Pereira, C. S. Novel

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The Journal of Physical Chemistry C

Article

Biocompatible Cholinium-Based Ionic Liquids - Toxicity and Biodegradability. Green Chem. 2010, 12, 643−649. (18) Andreani, L.; Rocha, J. D. Use of Ionic Liquids in Biodiesel Production: a Review. Braz. J. Chem. Eng. 2012, 29, 1−13. (19) Gao, Y.; Arrit, S. W.; Twamley, B.; Shreeve, J. M. GuanidiniumBased Ionic Liquids. Inorg. Chem. 2005, 44, 1704−1712. (20) Fukaya, Y.; Iizuka, Y.; Sekikawa, K.; Ohmo, H. Bio Ionic Liquids: Room Temperature Ionic Liquids Composed Wholly of Biomaterials. Green Chem. 2007, 9, 1155−1157. (21) Yuan, X. L.; Zhang, S. J.; Lu, X. M. Hydroxyl Ammonium Ionic Liquids: Synthesis, Properties, and Solubility of SO2. J. Chem. Eng. Data 2007, 52, 596−599. (22) Yu, Y.; Lu, X.; Zhou, Q.; Dong, K.; Yao, H.; Zhang, S. Biodegradable Naphthenic Acid Ionic Liquids: Synthesis, Characterization, and Quantitative Structure-Biodegradation Relationship. Chem.Eur. J. 2008, 14, 11174−11182. (23) Egorov, V. M.; Djigailo, D. I.; Momotenko, D. S.; Chernyshov, D. V.; Trocheshnikova, I. I.; Smirnova, S. V.; Pletnev, I. V. TaskSpecific Ionic Liquid Trioctylmethylammonium Salicylate as Extraction Solvent for Transition Metal Ions. Talanta 2010, 80, 1177−1182. (24) Costa, A. J. L.; Soromento, M. R. C.; Shimizu, K.; Marrucho, I. M.; Esperanca, J. M. S. S.; Canongia, J. N.; Rebelo, L. P. N. Density, Thermal Expansion and Viscosity of Cholinium-Derived Ionic Liquids. ChemPhysChem. 2012, 13, 1902−1909. (25) Fang, W.; Shao, D.; Lu, X.; Guo, Y.; Xu, L. Extraction of Aromatics from Hydrocarbon Fuels Using N-Alkyl PiperaziniumBased Ionic Liquids. Energy Fuels 2012, 26, 2154−2160. (26) Aparicio, S.; Atilhan, M. A Computational Study on Choline Benzoate and Choline Salicylate Ionic Liquids in the Pure State and After CO2 Adsorption. J. Phys. Chem. B 2012, 116, 9171−9185. (27) Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Ionic Liquids Are Not Always Green: Hydrolysis of 1-Butyl-3-methylimidazolium Hexafluorophosphate. Green Chem. 2003, 5, 361−363. (28) Stoimenovski, J.; Dean, P. M.; Izgorodina, E. I.; McFarlane, D. R. Protic Pharmaceutical Ionic Liquids and Solids: Aspects of Protonics. Faraday Discuss. 2012, 154, 335−352. (29) Kagimoto, J.; Ohno, H. Ionic Liquids Derived from Natural Sources. In Ionic Liquids Uncoiled. Critical Expert Overviews; Plechkova, N. V., Seddon, K. R., Eds.; John Wiley & sons: Hoboken, NJ, 2013; pp 231−250. (30) Tao, G.; He, L.; Liu, W.; Xu, L.; Xiong, W.; Wang, T.; Kou, Y. Preparation, Characterization and Application of Amino Acid-Based Green Ionic Liquids. Green Chem. 2006, 8, 639−646. (31) Chen, X.; Li, X.; Hu, A.; Wang, F. Advances in Chiral Ionic Liquids Derived from Natural Aminoacids. Tetrahedron:Asymmetry 2008, 19, 1−14. (32) Fukumoto, K.; Yoshizawa, M.; Ohno, H. Room Temperature Ionic Liquids from 20 Natural Amino acids. J. Am. Chem. Soc. 2005, 127, 2398−2399. (33) Ohno, H.; Fukumoto, K. Aminoacid Ionic Liquids. Acc. Chem. Res. 2007, 40, 1122−1129. (34) Yang, J. Z.; Zhang, Q. G.; Wang, B.; Tong, J. Study on the Properties of Aminoacid Ionic Liquid EMIGly. J. Phys. Chem. B 2006, 110, 22521−22524. (35) Zhang, Y.; Zhang, S.; Lu, X.; Zhou, Q.; Fan, W.; Zhang, X. P. Dual Amino-Functionalised Phosphonium Ionic Liquids for CO2 Capture. Chem.Eur. J. 2009, 15, 3003−3011. (36) Gardas, R. L.; Ge, R.; Goodrich, P.; Hardrace, C.; Hussain, A.; Rooney, D. W. Thermophysical Properties of Amino Acid-Based Ionic Liquids. J. Chem. Eng. Data 2010, 55, 1505−1515. (37) Muhammad, N.; Man, Z. B.; Bustam, M. A.; Mutalib, M. I. A.; Wilfred, C. D.; Rafiq, S. Synthesis and Thermophysical Properties of Low Viscosity Amino Acid-Based Ionic Liquids. J. Chem. Eng. Data 2011, 56, 3157−3162. (38) Tong, J.; Hong, M.; Chen, Y.; Wang, H.; Guan, W.; Yang, J. Z. The Surface Tension, Density and Refractive Index of Aminoacid Ionic Liquids: [C3mim][Gly] and [C4mim][Gly]. J. Chem. Thermodyn. 2012, 54, 352−357.

(39) Liu, X.; Zhou, G.; Zhang, S.; Wu, G. Molecular Simulation of Imidazolium Aminoacid-Based Ionic Liquids. Mol. Simul. 2010, 36, 1123−1130. (40) Zhao, H.; Jackson, L.; Song, Z.; Olubajo, O. Enhancing Protease Enantioselectivity by Ionic Liquids Based on Chiral- or ω-Aminoacids. Tetrahedron:Asymmetry 2006, 17, 1549−1553. (41) Ohira, K.; Abe, Y.; Kawatsura, M.; Suzuki, K.; Mizuno, M.; Amano, Y.; Itoh, T. Design of Cellulose Dissolving Ionic Liquids Inspired by Nature. ChemSusChem. 2012, 5, 388−391. (42) Kasahara, S.; Kamio, E.; Ishigami, T.; Matsuyama, H. Aminoacid Ionic Liquid-Based Facilitated Transport Membranes for CO2 Separation. Chem. Commun. 2010, 48, 6903−6905. (43) Zhou, Z.; Jing, G.; Zhou, L. Characterization and Absorption of Carbon Dioxide into Aqueous Solution of Aminoacid Ionic Liquid [N1111][Gly] and 2-Amino-2-methyl-1-propanol. Chem. Eng. J. 2012, 204−206, 235−243. (44) Gong, Q.; Luo, H.; Cao, J.; Shang, Y.; Zhang, H.; Wang, W.; Zhou, X. Synthesis of Cyclic Carbonate From Carbon Dioxide and Epoxide Using Amino Acid Ionic Liquid Under 1 atm Pressure. Aust. J. Chem. 2012, 65, 381−386. (45) Fang, W.; Dou, X. Y.; He, L. N.; Miao, C. X. Natural Amino Acid-Based Ionic Liquids as Efficient Catalysts for the Synthesis of Cyclic Carbonates from CO2 and Epoxides under Solvent- Free Conditions. Lett. Org. Chem. 2010, 7, 73−78. (46) Maolin, S.; Fuchun, Z.; Guizhong, W.; Haiping, F.; Chunlei, W.; Shimou, C.; Yi, Z.; Jun, H. Ordering Layers of [bmim][PF6] Ionic Liquid on Graphite Surfaces: Molecular Dynamics Simulation. J. Chem. Phys. 2008, 128, 134504. (47) Sha, M.; Wu, G.; Liu, Y.; Tang, Z.; Gang, H. Drastic Phase Transition in Ionic Liquid [Dmim][Cl] Confined Between Graphite Walls: New Phase Formation. J. Phys. Chem. C 2009, 113, 4618−4622. (48) Fukushima, T.; Aida, T. Ionic Liquids for Soft Functional Materials with Carbon Nanotubes. Chem.Eur. J. 2007, 13, 5048− 5058. (49) Shim, Y.; Kim, H. Solvation of Carbon Nanotubes in a RoomTemperature Ionic Liquid. ACS Nano 2009, 7, 1693−1702. (50) Aparicio, S.; Atilhan, M. Choline-Based Ionic Liquids on Graphite Surfaces and Carbon Nanotubes Solvation: A Molecular Dynamics Study. J. Phys. Chem. C 2012, 116, 12055−12065. (51) Dou, Q.; Sha, M. L.; Fu, H. Y.; Wu, G. Z. Molecular Dynamics Simulation of the Interfacial Structure of [C(n)mim][PF6] Adsorbed on a Graphite Surface: Effects of Temperature and Alkyl Chain Length. J. Phys.: Condens. Matter 2011, 23, 175001. (52) Zhou, H.; Rouha, M.; Feng, G.; Lee, S. S.; Docherty, H.; Fenter, P.; Cummings, P. T.; Fulvio, P. F.; Dai, S.; McDonough, J.; et al. Nanoscale Perturbations of Room Temperature Ionic Liquid Structure at Charged and Uncharged Interfaces. ACS Nano 2012, 6, 9818−9827. (53) Wang, J.; Chu, H.; Li, Y. Why Single-Walled Carbon Nanotubes Can Be Dispersed in Imidazolium-Based Ionic Liquids. ACS Nano 2008, 2, 2540−2546. (54) Perkin, S. Ionic Liquids in Confined Geometries. Phys. Chem. Chem. Phys. 2012, 14, 5052−5062. (55) Atkin, R.; Warr, G. G. Structure in Confined RoomTemperature Ionic Liquids. J. Phys. Chem. C 2007, 111, 5162−5168. (56) Chen, S.; Wu, G.; Sha, M.; Huang, S. Transition of Ionic Liquid [bmim][PF6] from Liquid to High-Melting-Point Crystal When Confined in Multiwalled Carbon Nanotubes. J. Am. Chem. Soc. 2007, 129, 2416−2417. (57) Singh, R.; Monk, J.; Hung, F. R. A Computational Study of the Behavior of the Ionic Liquid [BMIM+][PF6−] Confined Inside Multiwalled Carbon Nanotubes. J. Phys. Chem. C 2010, 114, 15478− 15485. (58) Aparicio, S.; Atilhan, M. Molecular Dynamics Study of Carbon Nanostructures in N-Methylpiperazinium Lactate Ionic Liquid. J. Phys. Chem. C 2013, 117, 22046−22059. (59) Rajesh, C.; Majunder, C.; Mizuseki, H.; Kawazoe, Y. A Theoretical Study on the Interaction of Aromatic Aminoacids with Graphene and Single Walled Carbon Nanotube. J. Chem. Phys. 2009, 130, 124911. 9756

dx.doi.org/10.1021/jp500165t | J. Phys. Chem. C 2014, 118, 9741−9757

The Journal of Physical Chemistry C

Article

3-methylimidazolium Cation and the Glycine Anion. J. Phys. Chem. B 2008, 112, 5088−5097. (83) Ghatee, M.; Moosavi, F. Physisorption of Hydrophobic and Hydrophilic 1-Alkyl-3-methylimidazolium Ionic Liquids on the Graphenes. J. Phys. Chem. C 2011, 115, 5626−5636. (84) Dong, K.; Zhang, S.; Wang, D.; Yao, X. Hydrogen Bonds in Imidazolium Ionic Liquids. J. Phys. Chem. A 2006, 110, 9775−9782. (85) Chi, M.; Zhao, Y. P. First Principle Study of the Interaction and Charge Transfer Between Graphene and Organic Molecules. Comput. Mater. Sci. 2012, 56, 79−84. (86) Paek, E.; Pak, A. J.; Hwang, G. S. Curvature Effects on the Interfacial Capacitance of Carbon Nanotubes in an Ionic Liquid. J. Phys. Chem. C 2013, 117, 23539−23546. (87) Avramov, P. V.; Kudin, K. N.; Scuseria, G. E. Single Wall Carbon Nanotubes Density of States: Comparison of Experiument and Theory. Chem. Phys. Lett. 2003, 370, 597−601. (88) Rajesh, C.; Majumder, C.; Mizuseki, H.; Kawazoe, Y. A Theoretical Study of Aromatic Aminoacids with Graphene and Single Walled Nanotubes. J. Chem. Phys. 2009, 130, 124911.

(60) Olmo, L.; López, R.; García, J. M. Effect of the Molecular Structure in the Prediction of Thermodynamic Properties for 1-Butyl3-methylimidazolium Chloride Ionic Liquid. Int. J. Quantum Chem. 2013, 113, 852−858. (61) Fumino, K.; Fossog, V.; Wittler, K.; Hempelmann, R.; Ludwig, R. Dissecting the Interaction Energy Between Anions and Cations in Protic Ionic Liquids. Angew. Chem., Int. Ed. 2013, 52, 2368−2352. (62) Grimme, S.; Hujo, W.; Kirchner, B. Performance of DispersionCorrected Density Functional Theory for the Interactions in Ionic Liquids. Phys. Chem. Chem. Phys. 2012, 14, 4875−4883. (63) Wendler, K.; Dommert, F.; Zhao, Y. Y.; Berger, R.; Holm, C.; Sitte, L. Ionic Liquids Studied Across Different Scales: A computational perspective. Faraday Discuss. 2012, 154, 111−132. (64) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098. (65) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785. (66) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648. (67) Breneman, C. M.; Wiberg, K. B. Determining Atom-Centered Monopoles from Molecular Electrostatic Potentials. The Need for High Sampling Density in Formamide Conformational Analysis. J. Comput. Chem. 1990, 11, 361−373. (68) Simon, S.; Duran, M.; Dannenberg, J. How Does Basis Set Superposition Error Change the Potential Surfaces for HydrogenBonded Dimers? J. Chem. Phys. 1996, 105, 11024. (69) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C. et al. Gaussian 03 (Revision C.02); Gaussian, Inc.: Wallingford, CT, 2004. (70) Bader, R. F. W. Atoms in Molecules: a Quantum Theory; Oxford University Press: Oxford, 1990. (71) Biegler-König, F.; Schönbohm, J.; Bayles, D. AIM2000. J. Comput. Chem. 2001, 22, 545−559. (72) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular Interactions from a Natural Bond Orbital, Donor-Acceptor Viewpoint. Chem. Rev. 1988, 88, 899. (73) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejon, P.; Portal, D. S. The SIESTA Method for Ab Initio Order-N Materials Simulation. J. Phys.: Condens. Matter 2002, 14, 2745−2779. (74) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (75) Troullier, N.; Martins, J. L. Efficient Pseudopotentials for PlaneWave Calculations. Phys. Rev. B 1991, 43, 1993−2006. (76) Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (77) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (78) Umebayashi, Y.; Fujimori, T.; Sukizaki, M.; Asada, K.; Fujii, R.; Kanzaki, S.; Ichi, I. Evidence of Conformational Equilibrium of 1Ethyl-3-methylimidazolium in its Ionic Liquid Salts: Raman Spectroscopic Study and Quantum Chemical Calculations. J. Phys. Chem. A 2005, 109, 8976−8982. (79) Turner, E. A.; Pye, C. C.; Singer, R. D. Use of ab Initio Calculations toward the Rational Design of Room Temperature Ionic Liquids. J. Phys. Chem. A 2003, 107, 2277−2288. (80) Hunt, P. A.; Kirchner, B.; Welton, T. Characterising the Electronic Structure of Ionic Liquids: An Examination of the 1-Butyl3-Methylimidazolium Chloride Ion Pair. Chem.Eur. J. 2006, 12, 6762−6775. (81) Kempter, V.; Kirchner, B. The Role of Hydrogen Atoms in Interactions Involving Imidazolium-Based Ionic Liquids. J. Mol. Struct. 2010, 972, 22−34. (82) Mou, Z.; Li, P.; Bu, Y.; Wang, W.; Shi, J.; Song, R. Investigations of Coupling Characters in Ionic Liquids Formed between the 1-Ethyl9757

dx.doi.org/10.1021/jp500165t | J. Phys. Chem. C 2014, 118, 9741−9757