Influence of Microstructure and Interaction on Viscosity of Ionic Liquids

Subscriber access provided by University Libraries, University of Memphis

Article

Influence of Microstructure and Interaction on Viscosity of Ionic Liquids Xiaochun Zhang, Feng Huo, Xiaomin Liu, Kun Dong, Hongyan He, Xiaoqian Yao, and Suojiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 23 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Influence of Microstructure and Interaction on Viscosity of Ionic Liquids Xiaochun Zhang, Feng Huo, Xiaomin Liu, Kun Dong, Hongyan He, Xiaoqian Yao, Suojiang Zhang*

Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China

Corresponding author: [email protected].

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 43

Abstract High viscosity of ionic liquids (ILs) is one of the bottlenecks for its application in industry. Understanding the relationship between structure and viscosity is a key issue for directional designing ILs with low viscosity. In this work, the microstructures and interactions of three representative imidazolium-based ILs were studied by quantum chemistry calculations and molecular dynamics simulations in order to investigate the origin of different viscosity. An all-atom force field for difluorophosphate ([PO2F2]) anion was developed. The sandwich structures of hydrogen bond network were observed. The relationship between the number/energy of hydrogen bond and viscosity was proposed. The sequence of interaction energy is consistent with the trend of experimental viscosity. The simulation studies suggest that the hydrogen bond and interaction energy play important roles in determination of the viscosity of ILs.

2


Page 3 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Ionic liquids (ILs) are a new kind of solvents solely composed of cations and anions, which have some special physicochemical properties, such as near-zero vapor pressure, low melting temperature, high thermal stability, wide electrochemical window, and adjustable solvation behavior. Therefore, they have been widely applied in chemistry, chemical engineering, material, biology and many other fields.1-13 Due to the unique properties, ILs have become one of the hottest areas in academia, and been regarded as potential candidates for developing clean technology. From the point of chemical technology and engineering, viscosity is the key property for evaluating performance of liquid flow, which affects not only mixing, stirring, separation and transportation, but also the mass transfer and heat transfer. Therefore, the high viscosity is considered as one of the main drawbacks both at laboratory and industrial level in general.14 However, most of the available ILs are highly viscous solvents, which are 2-3 orders of magnitudes larger than conventionally utilized organic solvents, i.e. the viscosity of tetrahexylphosphonium dithiomaleonitrile ([P6666][dtmn]) even up to 16150 cP.15 Therefore, it is crucial to understand the relationship between IL molecular structure and its macroscopic viscosity for designing ILs with low viscosity. As is well known, the size and shape of ILs have an effect on viscosity. Generally, the viscosity for anion with planar structure and small size is low, for example, the viscosity for 1-butyl-3-methylimidazolium tricyanomethane ([bmim][C(CN)3])16 and 1-ethyl-3-methylimidazolium thiocyanate ([emim][SCN])17 is 27.3 and 23.6 cP at 298

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 43

K, respectively. On the contrary, the viscosity for anion with non-planar structure, large size and fluorination of the alkyl chain is high, such as, the viscosity for 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4])18 is 180 cP. Similarly, increasing the length of alkyl side chain, carbon chain branch and fluorination on cation also will increase viscosity.19 All of the above can be ascribed to increase size and thus increase van der Waals interaction in anion or cation, except for [BF4] anion, which is due to the more rigid feature and absence of conformational degree of freedom.20 Moreover, the interaction energy and electrostatic interaction were reported to have an impact on the viscosity of ILs.19, 21 In

the

present

work,

1-ethyl-3-methylimidazolium

difluorophosphate

([emim][PO2F2]), 1-ethyl-3-methylimidazolium dicyanoamide ([emim][N(CN)2]) and [emim][SCN] were chosen to investigate the effect of anion on the viscosity of ILs. Among of them, [PO2F2] anion is stereo and contains two fluorine atoms, while [SCN] anion and [N(CN)2] anion are planar. In comparison, [SCN] anion is linear and the size is smaller than [N(CN)2] anion. Based on such structure indication, the viscosity would follows the order of [emim][PO2F2] > [emim][N(CN)2] > [emim][SCN], whereas the experimental viscosity for [emim][PO2F2],22 [emim][SCN]17 and [emim][N(CN)2]23 is 35, 23.6 and 21 cP at 298 K, respectively. In other words, the sequence of experimental viscosity is [emim][PO2F2] > [emim][SCN] > [emim][N(CN)2]. That is, contrary to the assumption, the viscosity of [emim][SCN] is higher than [emim][N(CN)2], although [SCN] anion is linear and smaller than [N(CN)2] anion. Therefore, the aim of this work is to explain the origins of the

4


Page 5 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


different viscosity in [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] by quantum chemistry calculations and molecular dynamics simulations. The article is organized as follows. First, an all-atom (AA) force field for [PO2F2] anion was developed. Second, the microstructures and interactions of [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] were analyzed by quantum chemistry calculations, including geometries and interaction energies. Then, the effects of intermolecular interaction energy and hydrogen bond on viscosity were investigated by molecular dynamics simulations, including relationship between intermolecular interaction energy and viscosity, number and energy of hydrogen bond, distribution of anions around cations, distribution of hydrogen bond angle, temperature effect on hydrogen bond. Finally, conclusion remarks were addressed. 2. Development of an all-atom force field The all-atom (AA) force field for the [PO2F2] anion was developed with the functional form of the standard AMBER model24: U =

∑

K r ( r − r0 ) 2 +

bonds

∑

K θ (θ − θ 0 ) 2 +

angels

∑

dihedrals

Kχ 2

[1 + cos( n χ − δ )] (1)

+ ∑ 4ε ij [(σ ij / rij )12 − (σ ij / rij ) 6 ] + ∑ q i q j / rij i< j

i< j

where U represents the total energy of the system. The first three terms in eq 1 is the bonded interactions: bonds, angles and torsions. The bond lengths and angles are represented by harmonic potentials with equilibrium values of r0 and θ0, and the dihedral angles are represented by conventional cosine series. The nonbonded interactions include van der Waals (VDW, in Lennard-Jones 6-12 form) and Columbic interactions. ε, σ and q are the energy well depth, collision diameter and charge, 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 43

respectively. The Lennard-Jones (LJ) parameters for interactions between unlike atoms are obtained from Lorentz-Berthelot combining rules. The nonbonded interactions separated by exactly three consecutive bonds (1-4 interactions) are reduced by a scale factor, which is optimized as 1/2 for VDW and 1/1.2 for Columbic interactions. The structures and atom type notations of [emim] caion, [PO2F2] anion, [SCN] anion and [N(CN)2] anion are shown in Figure 1. The structure of [PO2F2] anion was optimized at the hybrid density functional theory B3LYP with the 6-31++G** basis set using Gaussian 09 package. After the optimized structure was obtained, the vibration frequencies were checked to verify a true minimum energy structure. All the equilibrium bond lengths (r0) and angles (θ0) were extracted from the optimized structure. The force constants for bonds and angles were adjusted by fitting vibration frequencies obtained from molecular mechanics to quantum chemistry (QM) frequencies. The VDW parameters for P, O and F atoms were taken from AMBER99.24 The one-conformation, two-step restraint electrostatic potential (RESP) method, which developed by Bayly et al,25 was used to get atom charges by fitting the electrostatic potential generated from QM calculations at B3LYP/6-31++G** level. 4 layers and 523 points were used for fitting electrostatic potential. All atom charges were optimized at the first step, and the charges on equivalent atoms were restricted to be the same at the second step. The restraint weight factors were 0.0005 and 0.001 for the first step and second step, respectively. The more detail information for developing force field can be found in the work of Liu et al.26 The force field

6


Page 7 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


parameters developed for [PO2F2] anion are listed in Supporting Information Table S1. Force fields for [emim] cation, [SCN] anion and [N(CN)2] anion were derived from Liu et al,26 Chaumont et al,27 and Schröder et al,28 respectively.

3. Simulation methods 3.1 Quantum chemistry calculations The structures of [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] ion pairs were optimized at B3LYP/6-31++G** level by using Gaussian 09 software. The initial configurations of the ion pairs were mainly based on the charge distribution, electron densities and electrostatic potential of the isolated cation and anion.29, 30 The relative ion clusters of the three ILs with n=2, 3 were also optimized. Each optimized structure was recognized as a true minimum without any imaginary frequency. For comparison, the interaction energies were calculated with B3LYP/6-31++G** and B2PLYPD/6-31++G** level, respectively. B2PLYPD is a new empirical hybrid function, which considered the correction from perturbation theory and dispersion correlation, and it has been successfully used to calculate the thermodynamic data.31-33 For interaction energies, basis set superposition errors (BSSEs) were considered using the counterpoise method.34

3.2 Molecular dynamics simulations Molecular dynamics (MD) simulations were carried out for [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] contained 500 ion pairs at 0.1 MPa using MDynaMix package.35 Simulations were performed at 288, 298 and 308 K for [emim][PO2F2], while [emim][SCN] and [emim][N(CN)2] were at 298 K. The

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 43

standard periodical boundary conditions with 2/0.2 fs multiple time-step algorithms were used.36 Intramolecular forces and long-range forces including LJ and Columbic interactions were cut off at 15 Å. The LJ tail correction was added to the energy and pressure, and the Columbic interactions were dealt with the Ewald summation.37 The Nose-Hoover NpT ensemble36 was adopted with coupling constants of 700 and 30 fs. All systems were first started at 700 K for 1000 ps, and then gradually lowered to the target temperature. The equilibrated time lasted for more than 5.0 ns, and each production phase lasted for 5.0 ns. Trajectories were dumped for further analysis.

4. Results and discussion 4.1 Quantum chemistry calculations 4.1.1 Geometry and interaction energy Ion pairs as the basic structural units are very important because ILs are composed of ions completely. ILs may have several conformations when the anion is located at different positions around cation, but it was ignored and only the lowest energy conformers were analyzed here. The optimized structures of cation-anion ion pairs

for

[emim][PO2F2],

[emim][SCN]

and

[emim][N(CN)2]

obtained

at

B3LYP/6-31++G** level are shown in Figure 2. Generally, a hydrogen bond can be identified if the distance between the proton on the donor group and the acceptor atom is shorter than van der Waals distance and the angle is greater than 90°. The van der Waals distances for H…O, H…F, H…N and H…S are 2.72, 2.7, 2.75 and 3.0 Å, respectively. From Figure 2, it can be seen that four H…O hydrogen bonds exist between [emim] cation and [PO2F2] anion. Two hydrogen bonds are between the two

8


Page 9 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O atoms of anion and H5 atom of cation with the distances of 2.415 and 1.897 Å, respectively. The other two occur between the two O atoms of anion and H1 atom of CH3 group and HT atom of C2H5 group. The distances are 2.102 and 2.449 Å, respectively. Similarly, four hydrogen bonds are form in [emim][SCN]. Among of them, CR-H5…S, CR-H5…N, CT-H1…S and CT-HT…N bond distances are 2.96, 2.065, 2.799 and 2.43 Å, respectively. However, for [emim][N(CN)2], only three hydrogen bonds can form with the CR-H5 … N1, CT-H1 … N1 and CT-HT … N1 distances of 2.346, 2.185 and 2.165 Å, respectively. The hydrogen bond in [emim][N(CN)2] is the least among the studied three ILs. X-ray and IR spectra also have shown that hydrogen bonds exist between cation and anion in typical imidazolium-based ILs.38-40 The interaction energy of ion pair reflects the strength of cation-anion interaction, and there may have relationship between the strength of cation-anion interaction and viscosity. Therefore, the interaction energies of ion pairs of [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] were calculated. The interaction energy of ion pair can be calculated as follows, ∆E (kJ / mol ) = 2625.5[ Eionpair (au ) − ( Ecation ( au ) + Eanion ( au ))]

(2)

where Eionpair is the energy of the ion pair, and Ecation and Eanion are the energies of cation and anion, respectively. The calculated interaction energies both at B3LYP/6-31++G** and B2PLYPD/6-31++G** levels are presented in Supporting Information Table S2. As shown in Table S2, after the BSSE correction, there is little difference between the interaction energies calculated at B3LYP/6-31++G** and

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 43

B2PLYPD/6-31++G** levels. The sequence of interaction energy is as follow: [emim][PO2F2] > [emim][SCN] > [emim][N(CN)2]. The experimental viscosity follows the same trend. Therefore, it can conclude that the interaction between cation and anion is stronger, viscosity is higher.

4.1.2 Ion cluster and local hydrogen bond network The existence of the cluster in some ILs was found by experiment and simulation,41-47 which plays an important part in ILs-involving reactions and processes. Figure 3 shows the representative ion clusters for ([emim][PO2F2])n, ([emim][SCN])n and ([emim][N(CN)2])n with n=2, 3 obtained at B3LYP/6-31++G** level. It is found that these ions are connected by the hydrogen bonds to form hydrogen

bond

network

in

([emim][PO2F2])n,

([emim][SCN])n

and

([emim][N(CN)2])n with n=2, 3. For example, for the two ion pairs of [emim][PO2F2], the hydrogen bond networks are composed by nine hydrogen bonds of H…O and H… F, and H atoms including H atoms of CH3 group, H5 atom of ring, H4 atoms of ring and H atoms of C2H5 group of [emim] cation. Moreover, in the hydrogen bond networks of ([emim][PO2F2])n, ([emim][SCN])n and ([emim][N(CN)2])n, the anions are located between two cations like sandwich, which is obviously in ion clusters with n=2. The hydrogen bonds in ([emim][PO2F2])n are more than those in ([emim][SCN])n and ([emim][N(CN)2])n. The hydrogen bond networks were also found in [emim][BF4], [bmim][PF6] and [emim][Tf2N] in our previous work demonstrated by simulation and experimental IR spectra.48, 49

4.2 Molecular dynamics simulations

10


Page 11 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.2.1 Liquid density Force field can be validated by several properties, such as density,26, spectroscopic data,61,

62

heat of vaporization.63,

64

50-60

However, the density is most

frequently used and can be directly gained from isothermal-isobaric molecular dynamics simulations. The density of [emim][PO2F2] was calculated as 1.326 g/cm3 at 298 K and 0.1 MPa. The experimental value22 is 1.314 g/cm3. The error is 0.91%. Therefore, the proposed force field for [emim][PO2F2] is reliable.

4.2.2 Intermolecular interaction energy In the above section, the correlation between gas phase interaction energy and viscosity was obtained. Here, the relationship between the intermolecular interaction energy calculated from liquid phase by MD simulations and experimental viscosity was further studied. The van der Waals energy and electrostatic energy contribute to the total intermolecular interaction energy. The intermolecular interaction energies for [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] are -519.28, -508.25 and -499.25 kJ/mol, respectively. The sequence of intermolecular interaction energy is in agreement with the ordering of experimental viscosity. It is revealed that the intermolecular interaction energy is higher, the viscosity is higher. Analogous conclusions were inferred by Tsuzuki et al.58 and Atihan et al.14 Therefore, viscosity of the studied ILs would be determined not only by the sizes, but also by the strength of the cation-anion interaction. In addition, the electrostatic energies for [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] are -447.34, -443.44 and -426.03 kJ/mol, which is 86%, 87% and 85% of the total intermolecular interaction energy, respectively. It

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 43

reflects that the electrostatic energy contributes the main part of the total intermolecular interaction energy.

4.2.3 Hydrogen bond The existence of hydrogen bond is the major intermolecular structural feature in ILs,

30, 48, 65, 66

which is closely related to some important properties and applications.

The hydrogen bond interactions in [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] were analyzed by site-site radial distribution functions (RDFs). Figure 4 (a) shows the RDFs between various kinds of H atoms (H5, H41, H1, HT, HC, see Figure 1 (a)) in [emim] cation and O atoms in [PO2F2] anion. The first peak locations are 2.43, 2.48, 2.58, 2.58 and 2.83 Å for H5…O, H41…O, H1…O, HT…O and HC…O, respectively. Meanwhile, the first peak values decrease with the order of H5 > H41 > H1 > HT > HC. The RDFs results indicate that the hydrogen bonds can form between H5, H41, H1, HT and O atom, and the order of activity is H5 > H41 > H1 > HT, while hydrogen bond can not form between HC and O. The RDFs for the different kinds of H atoms in cation with F atoms in [PO2F2] anion are plotted in Figure 4(b). The first maximum locations are 2.58, 2.63, 2.73, 2.68 and 2.83 Å for H5…F, H41…F, H1…F, HT…F and HC…F, respectively, meaning that the hydrogen bonds can form between H5, H41, HT and F. The similar interaction of H-F was found in [bmim][PF6].26 Compared with the RDFs in Figure 4 (a) and (b), it can be found that the O and F atoms prefer to distribute around H5 atoms than other H atoms, however, O atoms is closer to H atoms (H5, H41, H1, HT) than F atoms, and the interactions of H…O are stronger than those of H…F. For example, the first peaks for H5…O and H5…F are at 2.43

12


Page 13 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and 2.58 Å with the value of 3.75 and 1.58, respectively. Figure 5 (a) exhibits the RDFs of all kinds of H atoms in [emim] cation and S atoms in [SCN] anion. The first peak locations of H…S are 2.73, 2.78, 2.93, 2.93 and 3.18 Å for H5, H41, H1, HT and HC, respectively, which reveal that the hydrogen bonds can form between H5, H41, H1, HT and S atom. The first peak values for H5… S, H41…S, H1…S and HT…S are 2.75, 2.32, 2.0 and 1.76, respectively. Among the four hydrogen bonds, the first peak location for H5…S is the nearest, and the relative value is the highest. Thus, the interaction of H5…S is the strongest. The interactions of H…N are displayed in Figure 5 (b), the first peak locations are 2.53, 2.53, 2.78, 2.63 and 2.98 Å for H5, H41, H1, HT and HC, respectively. Only H5, H41 and HT atoms can form hydrogen bond with N atoms in [emim][SCN]. Whereas, the hydrogen bond of H5…S is weaker than that of H5…N by comparing the first peak location and values. The RDFs of different kinds of H atoms in [emim] cation and N atoms in [N(CN)2] anion are shown in Figure 6 (a) and (b). It is found that the hydrogen bonds exist between H5, H41, HT atom and N1 atom with the distance of 2.58, 2.68 and 2.73 Å, respectively. The hydrogen bond of H5…N1 is the strongest among the three hydrogen bonds. Note that all H atoms in [emim] cation, including H5, H41, H1, HT and HC, are not form hydrogen bond with N2 atom in [N(CN)2] anion.

4.2.4 Number and energy of hydrogen bond In order to more clearly compare the hydrogen bond in [emim][PO2F2], [emim][SCN] and [emim][N(CN)2], the number and energy of hydrogen bonds in

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 43

three ILs were investigated. As discussed above, in all interactions of H atoms in [emim] cation with X atoms (X= O, F, S, N, N1) in three anions, H5…X and H4…X are stronger. Thus, only the number and energy of H5…X and H4…X were calculated. The number and energy of hydrogen bonds in [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] are listed in Table 1, 2 and 3, respectively. As seen in Table 1, both the average number and interaction energy of hydrogen bond are follow the order of CR-H5…O > CW-H41/H42…O > CR-H5…F > CW-H41/H42…F. Moreover, the interaction energy of hydrogen bond for CR-H5/CW-H4…O are nearly six times larger than those of CR-H5/CW-H4…F. It is evident that the hydrogen bonds of CR-H5/CW-H4…O are stronger than those of CR-H5/CW-H4…F, and CR-H5…O is the strongest, which agree with results of RDFs in Figure 5 (a) and (b). For [emim][SCN], the intensity of the hydrogen bond interaction for CR-H5…S is weaker than that of CR-H5…N. For example, the interaction energies of hydrogen bond for CR-H5…S and CR-H5…N are -30.63 and -31.34 kJ/mol, respectively, as presented in Table 2. Whereas, the hydrogen bond interactions of CW-H41/H42…S are stronger than those of CW-H41/H42…N. For instance, the interaction energy of hydrogen bond for CW-H41…S is -13.79 kJ/mol, while that for CW-H41…N is -12.74 kJ/mol. In addition, it is obviously that the hydrogen bonds of CR-H5…S/N are stronger than those of CW-H41/H42…S/N by comparing the average number and interaction energy of hydrogen bond. Likewise, the hydrogen bond of CR-H5…N1 is also stronger than that of CW-H41/H42…N1 in [emim][N(CN)2], as seen in Table 3. From Table 1, 2 and 3, it can be found that both the total average number and

14


Page 15 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interaction energy of hydrogen bond follow the order of [emim][PO2F2] > [emim][SCN] > [emim][N(CN)2], which is consistent with the sequence of experimental viscosity. It is revealed that the viscosity is in direct proportion to the hydrogen bond ability in the studied ILs. Thus, we can explain why the viscosity of [emim][SCN] is higher than [emim][N(CN)2], it is because the hydrogen bond and interaction energy in [emim][SCN] are stronger than [emim][N(CN)2], although [SCN] anion is linear and smaller size.

4.2.5 Distribution of anions around cations The distribution of anions around cations in ILs can be more clearly depicted in a more intuitive fashion as spatial distribution functions (SDFs). The three-dimension probability distribution for O atoms in [PO2F2] anion, S atoms in [SCN] anion and N1 atoms in [N(CN)2] anion around [emim] cation are shown in Figure 7, 8 and 9. There are mainly four high probability regions, the largest density is around H5, and the other three regions are around H41, H42 and between H41-H42. The results agree well with that the interactions between H5/H4 in cation and X atoms in three anions are stronger, and H5-X is the strongest, as displayed in Figure 4, 5 and 6. Compared with Figure 7, 8 and 9, it can be seen that the distributions of [PO2F2] anion and [N(CN)2] anion around [emim] cation are similar, but the distribution region for [PO2F2] anion is larger than [N(CN)2] anion, while the distribution region for [SCN] anion around [emim] cation is the smallest. The results are in accord with that the average number and interaction energy of hydrogen bonds for H5/H4…O in [emim][PO2F2] are the largest, and H5/H4 … S in [emim][SCN] are the smallest

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 43

among the three ILs. In addition, unlike the distribution of Cl and [PF6] anion around [dmim] cation,26 the three anions, [PO2F2] anion, [SCN] anion and [N(CN)2] anion, are not distribute in front of H5 and H41/H42 atoms in [emim] cation. Take [emim][PO2F2] as an example, there is a cavity for O atoms in [PO2F2] anion in front of H5 atoms in [emim] cation, and most regions are found above and below H5, as shown in Figure 7 (a).There are no distributions for O atoms in [PO2F2] anion in front of H41 and H42 in [emim] cation, see Figure7 (b). In order to know the specific distribution angle, the hydrogen bond angles for the different kinds of H atoms in [emim] cation and X atoms (X= O, F, S, N, N1) in three anions were analyzed. Figure 10 (a) shows the distribution of hydrogen bond angles for the different kinds of H atoms in [emim] cation and O atom in [PO2F2] anion. It can be found that the angle for CR-H5…O is 118°, CW-H41…O is 134°, CT-H1… O is about 120°, CT-HT…O is 128°and CT-HC…O is about 123°, respectively. The results indicate that CR-H5…O, CT-H1…O and CT-HC…O are mainly distribute about 120°, while CW-H41…O and CT-HT…O are mainly distribute about 130°. Similar, as shown in Figure 10 (b), the CR-H5…F, CT-H1…F and CT-HC…F are mainly distribute about 120 ° , while CW-H41 … F and CT-HT … F are mainly distribute about 130°. The same phenomenon can be found for H atoms in [emim] cation and S/N atom in [SCN] anion, and N1 atom in [N(CN)2] anion, as plotted in Supporting Information Figure S1 and S2. The hydrogen bond angles in [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] are bent, not linear. The results are in line with the three anions are not distribute in front of H5 and H41/H42 atoms

16


Page 17 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in [emim] cation, as shown in Figure 7, 8 and 9. Bent hydrogen bond were found to exist in protic ILs.67

4.2.6 Temperature effect on hydrogen bond It is well known that the viscosity of ILs is sensitive to temperature. The viscosity is decreased with temperature elevation.19, 68 Since there is the relationship between hydrogen bond and viscosity, whether the temperature will effect on hydrogen bond? The hydrogen bond of CR-H5…O in [emim][PO2F2] is the strongest in the studied three ILs, as seen the interaction energy of hydrogen bond in Table 1, 2 and 3. Thus, the effect of temperature on the interaction of CR-H5…O in [emim][PO2F2] was investigated as representative. The temperatures are 288, 298 and 308 K. As shown in Figure 11, as temperature rises from 288 to 298 and 308 K, the first peak value for CR-H5…O slightly decreases. The total energies for CR-H5…O at 288, 298 and 308 K were calculated, which are -63.01, -60.92 and -60.16 kJ/mol, respectively. The energy also decreases with increasing temperature. The results show that the interaction for the hydrogen bond of CR-H5…O will slightly decrease with the increase of temperature, although the temperature only increases by 10 K.

5. Conclusions The microstructures and interactions for [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] were studied for investigating the effect factors on viscosity by quantum chemistry calculations and molecular dynamics simulations. An all-atom force filed was developed for the [PO2F2] anion. The density of [emim][PO2F2] was calculated and compared to experimental density and found to be

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 43

in reasonable agreement. Hydrogen bonds were found to exist in an ion pair of [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] according to quantum chemistry calculations. For the ion clusters of ([emim][PO2F2])n, ([emim][SCN])n and ([emim][N(CN)2])n, the cations and anions connect with each other by hydrogen bond to form network in which the anions are located between cations like sandwich. The hydrogen bond interactions were further analyzed by site-site radial distribution functions. It was found that H5, H41, H1, HT atom in [emim] cation could form hydrogen bonds with O atom in [PO2F2] anion and S atom in [SCN] anion, and the order of activity is H5 > H41 > H1 > HT. While the hydrogen bonds exist between H5, H41, HT atom in [emim] cation and F atom in [PO2F2] anion, and N atom in [SCN] anion and N1 atom in [N(CN)2] anion. The average number and interaction energy of hydrogen bond in three ILs were calculated. For [emim][PO2F2], the average number and interaction energy of hydrogen bond for CR-H5/CW-H4…O are higher than those of CR-H5/CW-H4…F. The interaction of CR-H5…O will slightly decrease with increasing temperature from 288 to 298 and 308 K. For [emim][SCN], the average number and interaction energy of hydrogen bond for CR-H5…S are lower than those of CR-H5…N, while those of CW-H4…S are higher than CW-H4…N. In the three ILs, the average number and interaction energy of hydrogen bond for CR-H5…X (X represents the O, F, S, N, N1 in three anion) are higher than CW-H4…X. In addition, both the total average number and interaction energy of hydrogen bond are follow the sequence of [emim][PO2F2] >

18


Page 19 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[emim][SCN] > [emim][N(CN)2], which are consistent with the order of experimental viscosity, indicating that the hydrogen bond of ILs is stronger, the viscosity is higher. The relationship between the interaction energy calculated by quantum chemistry calculations and molecular dynamics simulations and viscosity were analyzed. It was found that the interaction energy is higher, the viscosity is higher. Thus, it can conclude that it is because the hydrogen bond and interaction energy in [emim][SCN] are stronger than those in [emim][N(CN)2] that the viscosity of [emim][SCN] is higher than [emim][N(CN)2], although [SCN] anion is linear and smaller size. The results indicate that hydrogen bond and interaction energy between cation and anion play important roles in determining the viscosity of ILs except size and shape. It is expected that this work can provide useful information for understanding the viscosity of ILs and rational designing ILs with low viscosity for the development of clean technology.

Acknowledgment This work was supported by the National High Technology Research and Development Program of China (2013AA06540201), National Natural Scientific Foundation of China (21106146, 91434111, 21276255) and Beijing Natural Science Foundation (2142029).

Supporting Information Available: Force field parameters for [PO2F2] anion developed in this work; Interaction energies (∆E) of ion pair of [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] at B3LYP/6-31++G** and B2PLYPD/6-31++G** level; Distribution of hydrogen bond angles for CX-HX…S (a) and CX-HX…N (b) in

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 43

[emim][SCN]; Distribution of hydrogen bond angle for CX-HX … N1 in [emim][N(CN)2]. This material is available free of charge via the Internet at http://pubs.acs.org.

References 1.

Zhang, S. J.; Sun, J.; Zhang, X. C.; Xin, J. Y.; Miao, Q. Q.; Wang, J. J. Ionic liquid-based green

processes for energy production. Chem. Soc. Rev. 2014, 43, (22), 7838-7869. 2.

Zhang, X. P.; Zhang, X. C.; Dong, H. F.; Zhao, Z. J.; Zhang, S. J.; Huang, Y. Carbon capture with

ionic liquids: overview and progress. Energy Environ. Sci. 2012, 5, (5), 6668-6681. 3.

Bara, J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin, D. L.; Noble, R. D.

Guide to CO2 Separations in Imidazolium-Based Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2009, 48, (6), 2739-2751. 4.

van Rantwijk, F.; Sheldon, R. A. Biocatalysis in ionic liquids. Chem. Rev. 2007, 107, (6),

2757-2785. 5.

Ranke, J.; Stolte, S.; Stormann, R.; Arning, J.; Jastorff, B. Design of sustainable chemical

products -The example of ionic liquids. Chem. Rev. 2007, 107, (6), 2183-2206. 6.

Hapiot, P.; Lagrost, C. Electrochemical reactivity in room-temperature ionic liquids. Chem. Rev.

2008, 108, (7), 2238-2264. 7.

Plaquevent, J. C.; Levillain, J.; Guillen, F.; Malhiac, C.; Gaumont, A. C. Ionic Liquids: New

Targets and Media for alpha-Amino Acid and Peptide Chemistry. Chem. Rev. 2008, 108, (12), 5035-5060. 8.

Han, X.; Armstrong, D. W. Ionic liquids in separations. Acc. Chem. Res. 2007, 40, (11),

1079-1086. 9.

Plechkova, N. V.; Seddon, K. R. Applications of ionic liquids in the chemical industry. Chem. Soc. 20


Page 21 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Rev. 2008, 37, (1), 123-150. 10. Earle, M. J.; Seddon, K. R. Ionic liquids. Green solvents for the future. Pure Appl. Chem. 2000, 72, (7), 1391-1398. 11. Petkovic, M.; Seddon, K. R.; Rebelo, L. P. N.; Pereira, C. S. Ionic liquids: a pathway to environmental acceptability. Chem. Soc. Rev. 2011, 40, (3), 1383-1403. 12. Rogers, R. D.; Seddon, K. R. Ionic liquids-Solvents of the future? Science 2003, 302, (5646), 792-793. 13. Macfarlane, D. R.; Forsyth, M.; Howlett, P. C.; Pringle, J. M.; Sun, J.; Annat, G.; Neil, W.; Izgorodina, E. I. Ionic liquids in electrochemical devices and processes: managing interfacial Electrochemistry. Acc. Chem. Res. 2007, 40, (11), 1165-1173. 14. Atilhan, M.; Jacquemin, J.; Rooney, D.; Khraisheh, M.; Aparicio, S. Viscous Behavior of Imidazolium-Based Ionic Liquids. Ind. Eng. Chem. Res. 2013, 52, (47), 16774-16785. 15. Del Sesto, R. E.; Corley, C.; Robertson, A.; Wilkes, J. S. Tetraalkylphosphonium-based ionic liquids. J. Organomet. Chem. 2005, 690, (10), 2536-2542. 16. Carvalho, P. J.; Regueira, T.; Santos, L. M. N. B. F.; Fernandez, J.; Coutinho, J. A. P. Effect of Water on the Viscosities and Densities of 1-Butyl-3-methylimidazolium Dicyanamide and 1-Butyl-3-methylimidazolium Tricyanomethane at Atmospheric Pressure. J. Chem. Eng. Data. 2010, 55, (2), 645-652. 17. McHale, G.; Hardacre, C.; Ge, R.; Doy, N.; Allen, R. W. K.; Maclnnes, J. M.; Bown, M. R.; Newton, M. I. Density-viscosity product of small-volume ionic liquid samples using quartz crystal impedance analysis. Anal. Chem. 2008, 80, (15), 5806-5811. 18. Zhao, H.; Liang, Z. C.; Li, F. An improved model for the conductivity of room-temperature ionic

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 43

liquids based on hole theory. J. Mol. Liq. 2009, 149, (3), 55-59. 19. Yu, G. R.; Zhao, D. C.; Wen, L.; Yang, S. D.; Chen, X. C. Viscosity of ionic liquids: Database, observation, and quantitative structure-property relationship analysis. AIChE. J. 2012, 58, (9), 2885-2899. 20. Zhou, Z. B.; Matsumoto, H.; Tatsumi, K. Low-Melting, Low-iscous, Hydrophobic Ionic Liquids: Aliphatic Quaternary Ammonium Salts with Perfluoroalkyltrifluoroborates. Chem. Eur. J. 2005, 11, (2), 752-766. 21. Bandres, I.; Alcalde, R.; Lafuente, C.; Atilhan, M.; Aparicio, S. On the Viscosity of Pyridinium Based Ionic Liquids: An Experimental and Computational Study. J. Phys. Chem. B 2011, 115, (43), 12499-12513. 22. Matsumoto, K.; Hagiwara, R. A New Series of Ionic Liquids Based on the Difluorophosphate Anion. Inorg. Chem. 2009, 48, (15), 7350-7358. 23. Fletcher, S. I.; Sillars, F. B.; Hudson, N. E.; Hall, P. J. Physical Properties of Selected Ionic Liquids for Use as Electrolytes and Other Industrial Applications. J. Chem. Eng. Data. 2010, 55, (2), 778-782. 24. Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. A Second Generation Force Field for the Simulation of Proteins, Nucleic Acids, and Organic Molecules. J. Am. Chem. Soc. 1995, 117, (19), 5179-5197. 25. Bayly, C. I.; Cieplak, P.; Cornell, W. D.; Kollman, P. A. A Well-Behaved Electrostatic Potential Based Method Using Charge Restraints for Deriving Atomic Charges-the Resp Model. J. Phys. Chem. 1993, 97, (40), 10269-10280. 26. Liu, Z. P.; Huang, S. P.; Wang, W. C. A Refined Force Field for Molecular Simulation of

22


Page 23 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2004, 108, (34), 12978-12989. 27. Chaumont, A.; Wipff, G. Solvation of Ln(III) Lanthanide Cations in the [BMI][SCN], [MeBu3N][SCN], and [BMI]5[Ln(NCS)8] Ionic Liquids: A Molecular Dynamics Study. Inorg. Chem. 2009, 48, (10), 4277-4289. 28. Schroder, C.; Steinhauser, O. The influence of electrostatic forces on the structure and dynamics of molecular ionic liquids. J. Chem. Phys. 2008, 128, (22), 224503-224510. 29. Zhang, X. C.; Liu, X. M.; Yao, X. Q.; Zhang, S. J. Microscopic Structure, Interaction, and Properties of a Guanidinium-Based Ionic Liquid and Its Mixture with CO2. Ind. Eng. Chem. Res. 2011, 50, (13), 8323-8332. 30. Dong, K.; Zhang, S. J.; Wang, D. X.; Yao, X. Q. Hydrogen bonds in imidazolium ionic liquids. J. Phys. Chem. A 2006, 110, (31), 9775-9782. 31. Sancho-Garcia, J. C.; Adamo, C. Double-hybrid density functionals: merging wavefunction and density approaches to get the best of both worlds. Phys. Chem. Chem. Phys. 2013, 15, (35), 14581-14594. 32. Goerigk, L.; Moellmann, J.; Grimme, S. Computation of accurate excitation energies for large organic molecules with double-hybrid density functionals. Phys. Chem. Chem. Phys. 2009, 11, (22), 4611-4620. 33. Schwabe, T.; Grimme, S. Double-hybrid density functionals with long-range dispersion corrections: higher accuracy and extended applicability. Phys. Chem. Chem. Phys. 2007, 9, (26), 3397-3406. 34. Simon, S.; Duran, M.; Dannenberg, J. J. How does basis set superposition error change the potential surfaces for hydrogen bonded dimers? J. Chem. Phys. 1996, 105, (24), 11024-11031.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 43

35. Lyubartsev, A. P.; Laaksonen, A. M.DynaMix-a scalable portable parallel MD simulation package for arbitrary molecular mixtures. Comput. Phys. Commun. 2000, 128, (3), 565-589. 36. Glenn, J. M.; Adam, H.; Mark, E. T. Molecular dynamics algorithms for path integrals at constant pressure. J. Chem. Phys 1999, 110, (7), 3275-3290. 37. Allen, M. P.; Tildesley, D. J. Computer Simulations of Liquids; Clarendon Press: Oxford, 1987. 38. Matsumoto, K.; Hagiwara, R. Structural characteristics of alkylimidazolium-based salts containing fluoroanions. J. Fluorine. Chem. 2007, 128, (4), 317-331. 39. Choudhury, A. R.; Winterton, N.; Steiner, A.; Cooper, A. I.; Johnson, K. A. In situ crystallization of low-melting ionic liquids. J. Am. Chem. Soc. 2005, 127, (48), 16792-16793. 40. Holbrey, J. D.; Reichert, W. M.; Rogers, R. D. Crystal structures of imidazolium bis(trifluoromethanesulfonyl)imide 'ionic liquid' salts: the first organic salt with a cis-TFSI anion conformation. Dalton T. 2004, (15), 2267-2271. 41. Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 2. Variation of Alkyl Chain Length in Imidazolium Cation. J. Phys. Chem. B 2005, 109, (13), 6103-6110. 42. Chen, S.; Kobayashi, K.; Kitaura, R.; Miyata, Y.; Shinohara, H. Direct HRTEM Observation of Ultrathin Freestanding Ionic Liquid Film on Carbon Nanotube Grid. ACS Nano. 2011, 5, (6), 4902-4908. 43. Hogan Jr, C. J.; Fernandez de la Mora, J. Ion-Pair Evaporation from Ionic Liquid Clusters. J. Am. Soc. Mass. Spectrom. 2010, 21, (8), 1382-1386. 44. Shigeto, S.; Hamaguchi, H.O. Evidence for mesoscopic local structures in ionic liquids: CARS signal spatial distribution of Cnmim[PF6] (n=4, 6, 8). Chem. Phys. Lett. 2006, 427, (4–6), 329-332.

24


Page 25 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45. Wang, Y.; Voth, G. A. Unique Spatial Heterogeneity in Ionic Liquids. J. Am. Chem. Soc. 2005, 127, (35), 12192-12193. 46. Canongia Lopes, J. N. A.; Pádua, A. A. H. Nanostructural Organization in Ionic Liquids. J. Phys.Chem. B 2006, 110, (7), 3330-3335. 47. Chen, S. M.; Zhang, S. J.; Liu, X. M.; Wang, J. Q.; Wang, J. J.; Dong, K.; Sun, J.; Xu, B. H. Ionic liquid clusters: structure, formation mechanism, and effect on the behavior of ionic liquids. Phys. Chem. Chem. Phys. 2014, 16, (13), 5893-5906. 48. Dong, K.; Song, Y. T.; Liu, X. M.; Cheng, W. G.; Yao, X. Q.; Zhang, S. J. Understanding Structures and Hydrogen Bonds of Ionic Liquids at the Electronic Level. J. Phys. Chem. B 2012, 116, (3), 1007-1017. 49. Dong, K.; Zhao, L. D.; Wang, Q.; Song, Y. T.; Zhang, S. J. Are ionic liquids pairwise in gas phase? A cluster approach and in situ IR study. Phys. Chem. Chem. Phys. 2013, 15, (16), 6034-6040. 50. Liu, Z. P.; Wu, X. P.; Wang, W. C. A novel united-atom force field for imidazolium-based ionic liquids. Phys. Chem. Chem. Phys. 2006, 8, 1096-1104. 51. Zhang, X. C.; Huo, F.; Liu, Z. P.; Wang, W. C.; Shi, W.; Maginn, E. J. Absorption of CO2 in the Ionic Liquid 1-n-Hexyl-3-methylimidazolium Tris(pentafluoroethyl)trifluorophosphate ([hmim][FEP]): A Molecular View by Computer Simulations. J. Phys. Chem. B 2009, 113, (21), 7591-7598. 52. Liu, X. M.; Zhang, X. C.; Zhou, G. H.; Yao, X. Q.; Zhang, S. J. All-atom and united-atom simulations of guanidinium-based ionic liquids. Sci. China. Chem. 2012, 55, (8), 1573-1579. 53. de Andrade, J.; Böes, E. S.; Stassen, H. Computational Study of Room Temperature Molten Salts Composed by 1-Alkyl-3-methylimidazolium Cations Force-Field Proposal and Validation. J. Phys. Chem. B 2002, 106, (51), 13344-13351.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 43

54. Canongia Lopes, J. N.; Deschamps, J.; Pádua, A. A. H. Modeling Ionic Liquids Using a Systematic All-Atom Force Field. J. Phys. Chem. B 2004, 108, (6), 2038-2047. 55. Canongia Lopes, J. N.; Pádua, A. A. H. Molecular Force Field for Ionic Liquids Composed of Triflate or Bistriflylimide Anions. J. Phys. Chem. B 2004, 108, (43), 16893-16898. 56. Canongia Lopes, J. N.; Pádua, A. A. H. Molecular Force Field for Ionic Liquids III: Imidazolium, Pyridinium, and Phosphonium Cations; Chloride, Bromide, and Dicyanamide Anions. J. Phys. Chem. B 2006, 110, (39), 19586-19592. 57. Canongia Lopes, J. N.; Pádua, A. A. H.; Shimizu, K. Molecular Force Field for Ionic Liquids IV: Trialkylimidazolium and Alkoxycarbonyl-Imidazolium Cations; Alkylsulfonate and Alkylsulfate Anions. J. Phys. Chem. B 2008, 112, (16), 5039-5046. 58. Tsuzuki, S.; Shinoda, W.; Saito, H.; Mikami, M.; Tokuda, H.; Watanabe, M. Molecular Dynamics Simulations of Ionic Liquids: Cation and Anion Dependence of Self-Diffusion Coefficients of Ions. J. Phys. Chem. B 2009, 113, (31), 10641-10649. 59. Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. Why is CO2 so soluble in imidazolium-based ionic liquids? J. Am. Chem. Soc. 2004, 126, (16), 5300-5308. 60. Zhang,

X.

C.;

Liu,

Z.

P.;

Liu,

X.

M.

Understanding

the

interactions

between

tris(pentafluoroethyl)-trifluorophosphate-based ionic liquid and small molecules from molecular dynamics simulation. Sci. China. Chem. 2012, 55, (8), 1557-1565. 61. Morrow,

T.

I.;

Maginn,

E.

J.

Molecular

Dynamics

Study

of

the

Ionic

Liquid

1-n-Butyl-3-methylimidazolium Hexafluorophosphate. J. Phys. Chem. B 2002, 106, (49), 12807-12813. 62. Canongia Lopes, J. N. A.; Pádua, A. A. H. Using Spectroscopic Data on Imidazolium Cation

26


Page 27 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Conformations To Test a Molecular Force Field for Ionic Liquids. J. Phys. Chem. B 2006, 110, (14), 7485-7489. 63. Borodin, O. Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids. J. Phys. Chem. B 2009, 113, (33), 11463-11478. 64. Liu,

Z.;

Chen,

T.;

Bell,

A.;

Smit,

B.

Improved

United-Atom

Force

Field

for

1-Alkyl-3-methylimidazolium Chloride. J. Phys. Chem. B 2010, 114, (13), 4572-4582. 65. Kempter, V.; Kirchner, B. The role of hydrogen atoms in interactions involving imidazolium-based ionic liquids. J. Mol. Struct. 2010, 972, (1-3), 22-34. 66. Bowron, D. T.; D'Agostino, C.; Gladden, L. F.; Hardacre, C.; Holbrey, J. D.; Lagunas, M. C.; McGregor, J.; Mantle, M. D.; Mullan, C. L.; Youngs, T. G. A. Structure and Dynamics of 1-Ethyl-3-methylimidazolium Acetate via Molecular Dynamics and Neutron Diffraction. J. Phys. Chem. B 2010, 114, (23), 7760-7768. 67. Hayes, R.; Imberti, S.; Warr, G. G.; Atkin, R. The Nature of Hydrogen Bonding in Protic Ionic Liquids. Angew. Chem. Int. Edit. 2013, 52, (17), 4623-4627. 68. Liu, H. J.; Maginn, E.; Visser, A. E.; Bridges, N. J.; Fox, E. B. Thermal and Transport Properties of Six Ionic Liquids: An Experimental and Molecular Dynamics Study. Ind. Eng. Chem. Res. 2012, 51, (21), 7242-7254.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 43

Captions of Tables and Figures

Table 1. Number and energy of hydrogen bonds in [emim][PO2F2].

Table 2. Number and energy of hydrogen bonds in [emim][SCN].

Table 3. Number and energy of hydrogen bonds in [emim][N(CN)2].

Figure 1. The structures and atom type notations of (a) [emim] cation, (b) [PO2F2] anion, (c) [SCN] anion, (d) [N(CN)2] anion.

Figure 2. Optimized structures of [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] ion pair obtained at B3LYP/6-31++G** level. Hydrogen bonds are indicated by dotted lines, and distances are in angstroms.

Figure 3. Optimized ion clusters for ([emim][PO2F2])n, ([emim][SCN])n and ([emim][N(CN)2])n with n=2, 3 obtained at B3LYP/6-31++G** level. Hydrogen bonds are indicated by dotted lines.

Figure 4. Radial distribution functions between various kinds of H atoms in cations and O/F atoms in anions in [emim][PO2F2].

Figure 5. Radial distribution functions between various kinds of H atoms in cations and S/N atoms in anions in [emim][SCN].

Figure 6. Radial distribution functions between various kinds of H atoms in cations and N1/N2 atoms in anions in [emim][N(CN)2].

28


Page 29 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Three-dimension probability distribution for O atoms in [PO2F2] anion around [emim] cation. The yellow and red bounded contour surfaces are drawn at 14 and 7 times the average density. (b) The same as (a) except from another perspective.

Figure 8. Three-dimension probability distribution for S atoms in [SCN] anion around [emim] cation. The yellow and red bounded contour surfaces are drawn at 14 and 7 times the average density.

Figure 9. Three-dimension probability distribution for N1 atoms in [N(CN)2] anion around [emim] cation. The yellow and red bounded contour surfaces are drawn at 14 and 7 times the average density.

Figure 10. Distribution of hydrogen bond angles for CX-HX…O (a) and CX-HX…F (b) in [emim][PO2F2].

Figure 11. Radial distribution functions of H5-O in [emim]PO2F2] at 288, 298 and 308 K.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 43

Table 1. Number and energy of hydrogen bonds in [emim][PO2F2]a Ionic liquid

[emim][PO2F2]

a

Hydrogen bond

CR-H5…O CW-H41…O CW-H42…O CR-H5…F CW-H41…F CW-H42…F Total

Average Number

0.89 0.64 0.64 0.38 0.31 0.32 3.18

Hydrogen bond energy (kJ/mol) -68.45 -40.71 -37.53 -29.37 -15.65 -14.18

Interaction energy of hydrogen bond (kJ/mol) -60.92 -26.05 -24.02 -11.16 -4.85 -4.54 -131.54

Interaction energy of hydrogen bond is equal to Average number * Hydrogen bond

energy.

30


Page 31 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Number and energy of hydrogen bonds in [emim][SCN] Ionic liquid

[emim][SCN]

Hydrogen bond

Average Number

CR-H5…S CW-H41…S CW-H42…S CR-H5…N CW-H41…N CW-H42…N Total

0.50 0.39 0.39 0.51 0.36 0.38 2.53

Hydrogen energy (kJ/mol) -61.25 -35.35 -31.92 -61.46 -35.38 -32.26

bond Interaction energy of hydrogen bond (kJ/mol) -30.63 -13.79 -12.45 -31.34 -12.74 -12.26 -113.21

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 43

Table 3. Number and energy of hydrogen bonds in [emim][N(CN)2] Ionic liquid

[emim][N(CN)2]

Hydrogen bond

CR-H5…N1 CW-H41…N1 CW-H42…N1 Total

Average Number

0.69 0.49 0.51 1.69

Hydrogen bond energy (kJ/mol) -59.24 -33.22 -30.29

Interaction energy of hydrogen bond (kJ/mol) -40.88 -16.28 -15.45 -72.61

32


Page 33 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HC

H5 H1

HC

HT

CT H1

CR

CT

CT

HC

NA

NA H1

HT

CW

CW H42

H41

(a)

F P

S

C

O (b)

N2

N

N1

N1

(c)

(d)

Figure 1. The structures and atom type notations of (a) [emim] cation, (b) [PO2F2] anion, (c) [SCN] anion, (d) [N(CN)2] anion.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

F

S

C

P

2.43

2.449

2.102

H5

H1

H5

H1

HT

HT

CR

CR NA

N

2.799

O

Page 34 of 43

NA

NA

[emim][PO2F2]

NA

[emim][SCN] N2 N1

N1 2.346

2.165

2.185 H5

HT

H1

CR NA

NA

[emim][N(CN)2]

Figure 2. Optimized structures of [emim][PO2F2], [emim][SCN] and [emim][N(CN)2] ion pair obtained at B3LYP/6-31++G** level. Hydrogen bonds are indicated by dotted lines, and distances are in angstroms.

34


Page 35 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


n=2

n=3 [emim][PO2F2]

n=2

n=3 [emim][SCN]

n=2

n=3 [emim][N(CN)2]

Figure 3. Optimized ion clusters for ([emim][PO2F2])n, ([emim][SCN])n and ([emim][N(CN)2])n with n=2, 3 obtained at B3LYP/6-31++G** level. Hydrogen bonds are indicated by dotted lines.

35



4.0 3.5

H5-O H41-O H1-O HT-O HC-O

3.0

g(r)

2.5 2.0 1.5 1.0 0.5 0.0 0

2

4

6

8

10

12

14

12

14

r /0.1nm

(a) 1.8

H5-F H41-F H1-F HT-F HC-F

1.6 1.4 1.2

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 43

1.0 0.8 0.6 0.4 0.2 0.0 2

4

6

8

10

r /0.1nm

(b)

Figure 4. Radial distribution functions between various kinds of H atoms in cations and O/F atoms in anions in [emim][PO2F2].

36


Page 37 of 43

3.0

H5-S H41-S H1-S HT-S HC-S

2.5

g(r)

2.0 1.5 1.0 0.5 0.0 2

4

6

8

10

12

14

12

14

r /0.1nm

(a)

4.5 4.0

H5-N H41-N H1-N HT-N HC-N

3.5 3.0

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5 2.0 1.5 1.0 0.5 0.0 2

4

6

8

10

r /0.1nm

(b)

Figure 5. Radial distribution functions between various kinds of H atoms in cations and S/N atoms in anions in [emim][SCN].

37



3.5

H5-N1 H41-N1 H1-N1 HT-N1 HC-N1

3.0

g(r)

2.5 2.0 1.5 1.0 0.5 0.0 2

4

6

8

10

12

14

r /0.1nm

(a)

H5-N2 H41-N2 H1-N2 HT-N2 HC-N2

1.6 1.4 1.2 1.0

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 43

0.8 0.6 0.4 0.2 0.0 0

2

4

6

8

10

12

14

r /0.1nm

(b)

Figure 6. Radial distribution functions between various kinds of H atoms in cations and N1/N2 atoms in anions in [emim][N(CN)2].

38


Page 39 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


F P O

H42

H41

(a)

H41

H42

(b)

Figure 7. Three-dimension probability distribution for O atoms in [PO2F2] anion around [emim] cation. The yellow and red bounded contour surfaces are drawn at 14 and 7 times the average density. (b) The same as (a) except from another perspective.

39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

S

C

Page 40 of 43

N

H5

H42

H41

Figure 8. Three-dimension probability distribution for S atoms in [SCN] anion around [emim] cation. The yellow and red bounded contour surfaces are drawn at 14 and 7 times the average density.

40


Page 41 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N2 N1

N1

H5

H42

H41

Figure 9. Three-dimension probability distribution for N1 atoms in [N(CN)2] anion around [emim] cation. The yellow and red bounded contour surfaces are drawn at 14 and 7 times the average density.

41



Normalized Probability

0.025

CR-H5...O CW-H41...O CT-H1...O CT-HT...O CT-HC...O

0.020

0.015

0.010

0.005

0.000 40

60

80

100

120

140

160

180

160

180

Hydrogen bond angle, θ (° )

(a)

0.020

Normalized Probability

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 43

CR-H5...F CW-H41...F CT-H1...F CT-HT...F CT-HC...F

0.015

0.010

0.005

0.000 40

60

80

100

120

140

Hydrogen bond angle, θ (° )

(b)

Figure 10. Distribution of hydrogen bond angles for CX-HX…O (a) and CX-HX…F (b) in [emim][PO2F2].

42


Page 43 of 43

4.0

H5-O

3.6

4.0

288 K 298 K 308 K

3.2

288 K 298 K 308 K g(r)

2.8 2.4

g(r)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.5

2.0 1.6 2.0

2.5

1.2

3.0

r

0.8 0.4 0.0 2

4

6

8

10

12

14

r /0.1nm

Figure 11. Radial distribution functions of H5-O in [emim]PO2F2] at 288, 298 and 308 K.

43


Influence of Microstructure and Interaction on Viscosity of Ionic Liquids

Recommend Documents