Article pubs.acs.org/JPCC
Effect of Tethering Strategies on the Surface Structure of AmineFunctionalized Ionic Liquids: Inspiration on the CO2 Capture Huabin Xing, Yan Yan, Qiwei Yang, Zongbi Bao, Baogen Su, Yiwen Yang, and Qilong Ren* Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Zhejiang University, Hangzhou 310027, China S Supporting Information *
ABSTRACT: Amine-functionalized ionic liquids (ILs) show great potential in CO2 capture due to their high performance. It is of primary importance to study the structure and orientation of cations and anions at the liquid−gas interface, which can significantly affect the interfacial transportation of CO2. We illustrated the significance of the tethering strategy of functional group on the interfacial structure of ILs for the first time by investigating the interfacial composition and orientation of cation-/anion-tethered amine-functionalized ILs through molecular dynamics simulation. The results showed that when the amine group is tethered to the cation, strong interaction between the amine group and anions results in a compact interfacial structure without the cation/anion enrichment. However, for amino-acid-based ILs with an anion-tethered strategy, the amine group tends to have a higher preference in the outer layer of interface. These results are highly instructive for the design of amine-functionalized and even other functionalized ILs. replacement for the volatile organic solvent.9,22−24,26−31 In particular, the amine-functionalized ILs have received a great deal of attention from researchers due to their high performance for effective and selective CO2 capture.4,9,22−24,32−36 Bates et al.9,35 first studied the CO2 fixation with amine-functionalized ILs by incorporating a primary amine moiety into the alkyl chain of the imidazolium cation, showing a significantly higher uptake of CO2 than traditional ILs. Besides the ILs with amine group on the cation, a number of amino acid ILs bearing amine functionality in the anion have also been reported and utilized as absorbent to capture CO2.4,22−24,32−34 As we know, the physicochemical properties of ILs are sensitive to their water content,37,38 which may be a disadvantage if ILs were used to remove CO2 from coal-fired power plants flue gas that inevitably contains water. Recently, Brennecke et al. found that water can be used to reduce the viscosity of the CO2complexed ILs while only slightly decreasing the CO 2 capacity.24 Because the amine-functionalized ILs are very promising candidates for CO2 capture, knowledge of the interfacial structure, especially the distribution and orientation of the amine group at the IL−gas interface, is quite important. However, little study has been reported for the interfacial structure of amine-functionalized ILs. In particular, the effect of the location of amine group (in anion or cation), or rather the tethering strategy, on the interfacial structure of aminefunctionalized ILs remains unclear. As a consequence, the
I. INTRODUCTION Functionalized ionic liquids (ILs) have shown great potential in a variety of applications including gas absorption, liquid−liquid extraction, and biphasic catalysis because their performance is significantly improved by incorporating the functional group into the cation/anion.1−9 For applications involving interface and heterogeneous systems, it is of primary importance to study the structure and orientation of these ions at the interface, which presents a great impact on surface properties and transportation of chemical species across such interfaces.10−19 However, despite a great quantity of cation-functionalized ILs3,6,9,20,21 and anion-functionalized ILs22−24 being reported, the choice of the tethering location of the functional group is often somewhat arbitrary and lacks enough consideration. Hence, in this paper, we compared the structure and orientation of cation- and anion-tethered functionalized ILs at the interface to illustrate the great importance of the tethering strategy of the functional group. During the past two decades, CO2 capture and sequestration have become an extensively investigated topic resulting from the substantial increase in CO2 emission. One of the most wellestablished industrial processes for CO2 recovery is chemical absorption of CO2 by monoethanolamine, diethanolamine, and methyldiethanolamine aqueous solutions.25,26 However, the use of a substantial amount of water will inevitably increase energy consumption and costs and may lead to corrosion problems.25 Quite distinct from the common solvents, ILs are one class of solvents that bear many unique properties, such as negligible vapor pressure, high thermal and chemical stability, and the tunability of structure and function. Thus, ILs have been recognized as promising solvents for CO2 capture, as a © XXXX American Chemical Society
Received: March 12, 2013 Revised: July 15, 2013
A
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Figure 1. Chemical structures and atom labels of ILs studied in the present work.
precision by many researchers,39−42 and thus the AMBER force field was selected in this work. An all-atom force field of the nonpolarizable ILs model was expressed in the functional form in AMBER framework, which is given in eq 1.43 The total potential energy includes four kinds of potential energy: nonbonded interactions, stretching of covalent bonds, bending of valence angles, and molecular dihedrals.
option of tethering strategy of amine group in designing aminefunctionalized ILs is still arbitrary in the view of interfacial chemistry, which hinders the study of CO2 capture by ILs. In this work, we investigated the structure and orientation of four different kinds of amine-functionalized ILs at the ILs/ vacuum interfaces through molecular dynamics (MD) simulation as a function of tethering sites and amine functionalization, including1-aminopropyl-3-methylimidazolium tetrafluoroborate ([apmim][BF4]), 1-ethyl-3-methylimidazolium glysine ([emim][Gly]), 1-ethyl-3-methylimidaz-olium alanine ([emim][Ala]), and 1-ethyl-3-methylimidazolium lysine ([emim][Lys]). Because 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) is well-studied with no aminefunctionalization, it is taken here as a basic IL for comparison against [apmim][BF4]. The structure of the amine-functionalized ILs is illustrated in Figure 1. Our work shows that the amine addition into the cation results in the absence of preferential ordering of amine group in the interface, whereas an amine group into the anion has a high possibility of extending toward the vacuum, especially that of [emim][Lys] due to its long aliphatic chain. The study indicates a sharp contrast of interfacial structure of cation- and anion-tethered functionalized ILs, and is highly instructive for carbon dioxide (CO2) capture by functionalized ILs. The paper is organized as follows. Section II presents information about the models and simulation details. Section III reports the simulation results, and the discussion is given. Section IV presents concluding remarks.
E=
∑
K r(r − r0)2 +
bonds
+
angles
Kφ
∑ dihedrals N
+
∑ i=1
∑
2
K (θ − θ0)2 θ
[1 + cos(ηφ − γ )]
⎧ ⎡⎛ ⎞12 ⎛ ⎞6 ⎤ q q ⎫ σ σ ⎪ ⎪ ∑ ⎨4 ∈ij ⎢⎢⎜⎜ ij ⎟⎟ − ⎜⎜ ij ⎟⎟ ⎥⎥ + i j ⎬ r r r ⎝ ij ⎠ ⎦ ij ⎪ j=i+1 ⎪ ⎣⎝ ij ⎠ ⎩ ⎭ N
(1)
where the symbols have their conventional meaning. The combination rule for Lennard-Jones terms is demonstrated in eq 2. εij = (εiεj)1/2 , σij = (σi + σj)/2
(2)
The force-field parameters were taken directly from the literatures,39−42 which are based on the AMBER with several additional parameters obtained from ab initio calculations. The atomic charges were calculated by using the one-conformation two-step standard restraint electrostatic potential (RESP) fitting method,44 and the final RESP atom charges are listed in Table S1 in the Supporting Information. MD simulations were performed with GROMACS 4.5.445−47 with each system composed of 512 ion pairs. The leapfrog algorithm was adopted with a time step of integration of 1 fs. Periodic boundary conditions were applied in three dimensions.
II. FORCE-FIELD AND SIMULATION DETAILS AMBER force field was one of the most widely used force fields for IL simulation, and it was found to be capable of simulating imidazolium-based ILs and amine-functionalized ILs with good B
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ILs are in good agreement with the experimental data and consistent with the simulations from other authors, with the calculated density of [apmim][BF4] 4.6% lower than the experimental value, showing the largest deviation from the experimental value. Because no force-field parameters were adjusted to fit the experimental values, this level of agreement is reasonable. A schematic of molecular structure of all the ILs is shown in Figure 1 to aid the discussion.
Lennard-Jones (LJ) interactions and electrostatic forces were cut off at a radius of 1.5 nm. Electrostatic interactions were treated with a particle-mesh Ewald (PME) summation.48,49 Each simulation system consists of 512 ion pairs. Nowadays, one of the main CO2 emission sources is the flue gas from the coal-fired power plants, whose temperature is generally very high (373−473 K) and the pressure is ∼1 atm.25,26 The absorption performances of amine-functionalized ILs at different temperature have been investigated by researchers, and this kind of IL showed satisfactory absorption capacities at 373 K and 1 atm.24 In addition, the oscillations decrease and the sampling of the simulation improves with increasing temperature, and thus the simulations were conducted at a high temperature (450 K) and 1 atm. Other researchers also carry out the simulation of vacuum−IL interface at similar temperature and pressure.11,12 The system was first simulated in the NPT ensemble (P = 1.0 atm) for the bulk simulation to get an appropriate density. After equilibration, the direction normal to the surface of ILs was elongated so that the liquid slab occupied a third of the simulation box in the middle with two equivalent interfaces. The system was then equilibrated for another 2 ns in the NVT ensemble, and a subsequent trajectory of duration 1.5 ns was obtained for analysis, with configurations from the MD trajectory stored every 0.5 ps for analysis. The simulations were carried out at 298 and 450 K using the Berendsen thermostat. Simulations conducted at 450 K were used for analyzing the surface structure, while results at 298 K were used only to validate the simulation. Because the system shows symmetry along the axis normal to interface (Z axis), we bisect the simulation box along the Z axis and show properties of the positive side of the slab in this paper. Table 1 presents the
III. RESULTS AND DISCUSSION III.1. Mass Density. To investigate the structures at the interface, we have calculated the mass densities profiles of cations and anions along the Z axis respectively, as shown in Figure 2. The calculation was based on the average of the coordination and weight of each atom of the molecule. These distributions were calculated with respect to their average bulk densities. As can be seen in Figure 2, the vacuum side is on the left of the Z axis, where the mass densities of cations and anions are zero, and the bulk liquid is on the left with the interfacial region between it and the vacuum. In Figure 2, noticeable oscillations are observed for both cations and anions of all of the systems, even in the bulk side, which suggests nanostructural organization in ILs as reported from previous reports.55,56 Although the density profiles (DPFs) of [apmim][BF4] in Figure 2 show evident oscillations for both cation and anion, the relative mass density for cation and anion in the interfacial region shows a similar value as that in the bulk side, which suggests that neither cation enrichment nor anion enrichment exists in the interfacial region, giving rise to no difference between the bulk liquid and the interfacial region. However, the DPFs of [bmim][BF4] in Figure 2 demonstrate enhanced densities for both [bmim]+ and [BF4]− in the interfacial region, with the peak values of DPFs of anion at 23.7 Å and of cation at 23.1 Å both larger than 1.0. Thus, by comparing the DPFs of [apmim][BF4] and [bmim][BF4], the sharp contrast between two systems indicates that the amine addition into the butyl group of the cation has a great influence on the interfacial density oscillations. [emim][Gly], [emim][Ala], and [emim][Lys] share the similar DPFs, as illustrated in Figure 2. One distinct phenomenon observed here is that it is the anion of these three amino acid-based ILs rather than the cation that lies on the outermost part of the interface, which is in sharp contrast with the interfacial structure of [bmim][BF4] with the cation occupying the outmost part of the interface. In addition, the maximum value of the relative mass density for the [emim]+ in [emim][Gly], [emim][Ala], and [emim][Lys] is 1.08, 1.13, and 1.26 respectively, giving rise to the increasing extent of enhancement of cation densities in the inner part of
Table 1. Liquid Densities of ILs at 298 K from This Work, Literature, and Experiments density (g/cm3) simulation ILs
this work
literature
experiments
[bmim][BF4] [apmim][BF4] [emim][Gly] [emim][Ala] [emim][Lys]
1.170 1.292 1.151 1.100 1.027
1.174,401.19441 n/a 1.219,521.17953 1.17152 1.11152
1.20150 1.35551 1.158953 1.120954 n/a
calculated densities for ILs at 298 K under 1.0 atm as well as experimental data and the simulated densities from the literature. As shown in Table 1, the predicted densities for all
Figure 2. Mass density profiles of cations and anions of ILs at 450 K along the Z axis. C
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of ions at the surface of ILs, which results in a charge separation and dipole at the surface layer. The electrostatic potential in [bmim][BF4] is more negative than that in [apmim][BF4], which indicates that amino addition into the cation increases the electrostatic potential. For all of the ILs, [emim][Ala] has the smallest potential drop while [emim][Gly] has the largest potential drop across the interface. III.3. Number Density. To further explore the interfacial structure and orientation of the alkyl chains in cations and anions, we calculated the number density profiles for the most representative atoms of cations and of anions (Figures 5 and 6) with the distribution of the amine group represented by the DPFs of the N atoms in the amine group. In Figure 5, the distribution of the [BF4]− is represented by the DPFs of the B atoms, and the bulk liquid is on the left and the vacuum is on the right. As can be seen in Figure 5a, the B atoms of [bmim][BF4] tend to stay in the liquid side and are in the innermost part of the interfacial region, whereas the butyl group of the cation tends to be on the vacuum side. Moreover, the C10 atoms of the terminal methyl group of the butyl group are present in the outmost part of the surface, followed in sequence by C9 atoms, C8 atoms, C7 atoms, and N1 atoms, which indicates that the butyl group of [bmim]+ in [bmim][BF4] tends to be perpendicular to the surface, which is consistent with the experimental observations by Rivera-Rubero15and simulation results conducted by Lynden-Bell.58 After substituting the terminal methyl group of the butyl chain by amine group, the structure of the alkyl chain at the interface changes significantly, as illustrated in Figure 5b. Although the anion [BF4]− stays in the innermost side of the interfacial region as [BF4]− does in [bmim][BF4], the alkyl chain of the cation shows a much different behavior from the butyl chain of [bmim]+. The N10 atoms, C9 atoms, and C8 atoms locate at almost the same position at the interface, while the C7 atoms and N1 atoms stay in the inner part of the interface. Thus, the alkyl chain of [apmim]+ in [apmim][BF4] exhibits a more compact interfacial structure. In addition, the N10 atoms, C9 atoms, and C8 atoms show no enhancement at the interface. Therefore, by comparing the DPFs of [apmim][BF4] with the DPFs of [bmim][BF4] in Figure 5, the alkyl chain of [apmim]+ shows no distinct preferential ordering at the interface. Figure 6shows the number density of atoms of anions in [emim][Gly], [emim][Ala], and [emim][Lys], respectively. As can be observed in Figure 6a, the amine group of the anion of [emim][Gly] stays in the outmost side of the surface, followed closely by the C12 atoms attached to the amine group and C11 atoms to which the oxygen atom of COO− is attached, which indicates that the anion [Gly]− tends to be parallel to the surface normal with the amine group protruding toward the vacuum side. However, in the case of [Ala]−, which carries a methyl group at C12 atoms, the C14 atoms attached to the C12 atoms are present in the outermost part of the surface rather than the amine group, and the amine group and C12 atoms share almost the same behavior as DPFs with the C11 atoms locating furthest from the vacuum side, as shown in Figure 6b. Therefore, the added methyl group of C12 position leads to a much smaller chance of amine group protruding toward the vacuum. Unlike [emim][Gly] and [emim][Ala], [emim][Lys] has two amine groups, one of which is at the end of the aliphatic alkyl chain of [Lys]−. In this work, the amine group refers to the terminal amine group (N18) of the alkyl chain of anion. As can be seen in Figure 6c, the terminal amine group (N18) of the alkyl chain locates closest to the vacuum side, and
the interfacial region in the sequence of [emim][Gly] < [emim][Ala] < [emim][Lys]. III.2. Charge Density and Electrostatic Potential. Figure 3 shows the charge density profiles of [apmim][BF4] along the
Figure 3. Charge density profiles of [apmim][BF4] at 450 K along the Z axis.
Z axis, and the others are in the Supporting Information. The charge density is calculated from the partial charge on each site averaged over the planes perpendicular to the charged interface. As can be seen in Figure 3, the charge-density profile shows a region of positive charge on the extreme outer edge of ILs, followed by a region of negative charge, which is also observed in 1-1-octanol-3-methylimidazolium tetrafluoroborate [C8OHmim][BF4].57 Strong oscillations are seen in the charge-density profiles along the Z axis, indicating nanostructural ordering in ILs. By evaluating the double integral of the charge density using eq 3, the electrostatic potential (φ) across the interface can be calculated. φ(z 0) − φ( −∞) = −
1 ϵ0
Z
Z′
∫−∞ dz′ ∫−∞ ρq (z″) dz″
(3)
where ρq(z″) is the charge density at the position z″. Figure 4 shows the variation of the potential through the interface of all the ILs. The electrostatic potential in all of the considered ILs is negative relative to the vacuum, as can be seen in Figure 4. This stems from the preferential ordering or nonisotropic ordering
Figure 4. Variation of the electrostatic potential along the Z axis of all the ILs. D
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Figure 5. Number density profiles of individual atoms of [apmim][BF4] and [bmim][BF4] at 450 K.
Figure 6. Number density profiles of individual atoms of [emim][Gly], [emim][Ala], and [emim][Lys] at 450 K.
the C11 atoms stay furthest from the vacuum side, with the other atoms in the middle of them. Compared with [Gly]− in [emim][Gly] and [Ala]− in [emim][Ala], the terminal amine group in [Lys]− has a higher possibility of extending to the vacuum side due to its long aliphatic chain. Therefore, the structure of the anion has a great impact on the surface structure of the anion-tethered ILs. Niedermaier et al.59,60 measured the surface structure of one IL containing an aminoethylsulfonate anion by angle-resolved X-ray photoelectron spectroscopy and found the surface enrichment of the amine groups. This can be considered as one experimental support for our simulation results, although the molecular structures of ILs in both works are not the same and the studied ILs in this work cover a much wider structural diversity. Comparing Figure 5b with Figure 6, the DPFs of cationtethered ILs and anion-tethered ILs are much different, and the effect of the position of an amine group is evident in the interfacial structure of cation and anion-tethered functionalized ILs. Amine addition into the cation results in absence of preferential ordering of amine group in the interface, whereas an amine group in anion has a higher possibility to extend toward the vacuum, especially [emim][Lys] due to its long aliphatic chain. The methyl group of [emim][Ala] reduces the tendency of amine group to protrude toward the vacuum. It is obvious that the more amine group existing in the outermost region of the interface, the more favorable for the uptake of CO2, which indicates anion-tethered amine functionalized ILs a better candidate for cation-tethered ILs. To better observe the interfacial structure of the aminefunctionalized ILs, we show the simulation snapshots from the MD trajectory data in Figure 7. We have used the same color
Figure 7. Snapshots of the simulation boxes of amine-functionalized ILs.
code as Pensado et al.57 and Canongia Lopes and Pádua56 to represent the polar groups in blue and the nonpolar groups in yellow. To identify the amino group in ILs, we represented the amine group in red. A more compact structure is observed in [apmim][BF4] compared with amino-acid-based ILs. In [emim][Ala], because the alkyl chain of [Ala]− is too short E
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Figure 8. Orientational ordering parameters of ILs along the Z axis.
Figure 9. RDFs presentation of [bmim][BF4] and [apmim][BF4]: (a) RDFs between B atom on [BF4]− and heavy atoms on cations and (b) RDFs between F/N10 atoms and H atoms on cations.
interface representing the orientation of the butyl chain is in the range of 0.4 to 0.5, which indicates a strong preference to point away from the bulk liquid. Moreover, the positive value of the N1−N3 vector in the interfacial region representing the orientation of the imidazolium ring suggests that the ring tends to align parallel to the surface normal, which explains the surface enrichment of [bmim]+ cation of [bmim][BF4] in the interfacial region, as demonstrated in Figure 2. However, the orientational ordering parameters of [apmim][BF4] show a sharp contrast with those of [bmim][BF4] (as can be seen in Figure 8b), as the preferential ordering of imidazolium ring vanishes and the value of N1−N10 vector at the interface is negative, which indicates that the alkyl chain of [apmim]+ tends to lie flat on the surface rather than extend toward the vacuum. Because the value of N1−N3 vector for [emim]+ is positive in the interfacial region, the ethyl chain of amino-acid-based ILs is likely to be in alignment with the surface normal, as observed in
to cover the surface layer, it is easy to observe the amine group from the vacuum side. The amine group of [emim][Lys] shows a more pronounced trend toward the vacuum side. III.4. Orientational Ordering. Figure 8 shows the orientational ordering parameters defined as the average of the second Legendre polynomial ⟨P2(θ )⟩ =
1 (3 cos2 θ − 1) 2
(4)
In eq 4, θ is the angle between a specific direction vector in the molecule-fixed frame and the Z axis perpendicular to the liquid slab. P2(θ) varies between 1 and −0.5, with a value of 1 indicating two considered vectors parallel and a value of −0.5 implying them perpendicular. By calculating the orientational ordering parameters P2(θ), the orientation of the cation/anion relative to the surface can be decided. As can be seen in Figure 8a, the peak value of N1−C10 vector in [bmim][BF4] at the F
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Figure 10. RDFs presentation of [emim][Gly]: (a) RDFs between O2 atom on [Gly]− and heavy atoms and (b) RDFs between O2 atoms on [Gly]− and H atoms.
Figure 11. RDFs presentation of [emim][Ala]: (a) RDFs between O2 atom on [Ala]− and heavy atoms and (b) RDFs between O2 atoms on [Ala]− and H atoms.
Figure 12. RDFs presentation of [emim][Lys]: (a) RDFs between O2 atom on [Lys]− and heavy atoms and (b) RDFs between O2 atoms on [Lys]− and H atoms.
methyl protruding toward the vacuum and the latter due to the flexibility of the long aliphatic chain of lysine. In the bulk region, the second-order Legendre polynomial for all ILs vanishes due to isotropic orientation, as expected. III.5. Microscopic Structures. Site−site radical distribution functions (RDFs) are calculated to understand the microscopic structure of ILs and partially clarify the underlying mechanism
Figure 8c−e, and this is in agreement with the enrichment of [emim]+ at the interface, as illustrated in Figure 2. The amine group in [Gly]− tends to lie vertical to the interface, deduced from the positive value of C11−N13 vector at the interface in Figure 8c. Unlike the amine group in [Gly]−, the amine group in [emim][Ala] and [emim][Lys] prefers to adopt an orientation parallel to the interface, the former because of the G
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between different anions is much weaker, contributing to the higher preference of the amine group in the outer layer of interface and the facility of the amine group to extend toward the vacuum or gas phase. Therefore, from the viewpoint of interfacial adsorption and transfer, the amine-functionalized ILs with an anion-tethering strategy is considered of great benefit for the CO2 capture, especially when the amine group is on the terminal side of a relatively long alkyl chain.
that the tethering location of the functional group imposes a great influence on the surface structure of ILs. Because the RDFs for B−C4 and B−C5 of [bmim][BF4]and [apmim][BF4]are almost identical, only RDFs for B−C4 are presented in this work. As observed in Figure 9a, the sharp contrast is clearly seen despite the similarity of RDFs for B−C2 and B−C4 in two ILs, where the peak value of RDFs between the B−N10 atoms in [apmim][BF4] is 3.45 while the peak value of RDFs between the B−C10 atoms in [bmim][BF4] is only 2.40. This indicates that after the amine addition into the alkyl chain of the imidazolium cation a new strong interaction site is present between the amine group of cation and the anion. It can be also observed in Figure 9b that strong hydrogen-bonding interaction exists between H10 of [apmim]+ and F atoms of anion, whereas a much weaker interaction is formed between H10 of [bmim]+ and F atoms of anion. Maginn et al. also observe the strong hydrogen-bonding interaction between the amine group of cation and the anion in the similar cation-tethered aminefunctionalized ILs.36 In addition, the peak value of RDFs between N10−H10 atoms is 1.2, indicating that there is no strong interaction between two amine groups. As B atoms of anion locate in the innermost region of the interfacial region, strong interaction between the amine group and anions dramatically reduces the preference of the amine group in the outer layer of interface and the possibility of the amine group extending toward the vacuum. Moreover, stronger interaction between cations and anions with the addition of a strong interaction site in cation may also contribute to the more compact liquid structure of [apmim][BF4] and be the reason why the interface of [apmim][BF4] shows no preferential ordering with respect to the bulk liquid. Figure 10shows the RDFs of [emim][Gly], and the RDFs of [emim][Ala] and [emim][Lys] are presented in Figures 11 and 12, respectively. As can be seen in Figure 10a, the peak value of RDFs between the oxygen atoms (O2) in anion and the C2 atoms in cations is 4.31, whereas the peak value of RDFs between the amine group in anion and the C2 atoms is 1.91. RDFs between O2 atoms and H atoms in Figure 10b show similar behavior, in which the peak value of RDFs between O2−H2 and N13−H2 is 6.57 and 1.92 respectively. Thus, we can conclude that strong hydrogen bonding interaction exists between the oxygen atoms of anion and the H2 atoms of cation of [emim][Gly]; nevertheless, the interaction between the amine group of anion and cations is relatively weak, distinct from the case of cation-tethered [apmim][BF4]. In addition to the interaction between cations and anions, we also investigate the interaction between different anions represented by RDFs for O2−N13/N18 and O2−H13/N18in anions. As shown in Figures 10−12,the first peak value of RDFs for O2−N13 is no more than 0.5, and the peak value of RDFs for O2−H13 except O2−H13 in [emim][Ala] is
