Comparison of Electronic and Physicochemical Properties between

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Comparison of Electronic and Physicochemical Properties between Imidazolium-Based and Pyridinium-Based Ionic Liquids Chongchong Wu, Alex De Visscher, and Ian Donald Gates J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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The Journal of Physical Chemistry

Comparison of Electronic and Physicochemical Properties between Imidazolium-Based and Pyridinium-Based Ionic Liquids Chongchong Wua, Alex De Visschera, b, Ian D. Gatesa* a

Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Alberta, Canada b Department of Chemical and Materials Engineering, Faculty of Engineering and Computer Science, Concordia University, Montreal, Quebec, Canada ABSTRACT To compare 1-butyl-3-methylimidazolium ([BMIM]+) and 1-butyl-3-methylpyridinium ([BMPy]+)-based ionic liquids (ILs) and investigate the influence of intramolecular and intermolecular interactions on physicochemical properties, a systematic study was performed on the electronic structures and physicochemical properties of [BMIM]+ tetrafluoroborate ([BMIM][BF4]), [BMIM]+ hexafluorophosphate ([BMIM][PF6]), [BMIM]+ hydrogen sulfate ([BMIM][HSO4]),

[BMIM]+

methylsulfate

([BMIM][MSO4]),

[BMIM]+

ethylsulfate

([BMIM][ESO4]), [BMPy]+ tetrafluoroborate ([BMPy][BF4]), [BMPy]+ hexafluorophosphate ([BMPy][PF6]),

[BMPy]+

hydrogen

sulfate

([BMPy][HSO4]),

[BMPy]+

methylsulfate

([BMPy][MSO4]), and [BMPy]+ ethylsulfate ([BMPy][ESO4]) using density functional theory and molecular dynamics simulation. The results reveal that aggregation behavior exists in [HSO4]- and [ESO4]--based ILs, and the differences between their densities and self-diffusion coefficients are smaller when there is an aggregation effect in ILs. A dimer is formed by two strong hydrogen bonds between two [HSO4]- anions in [HSO4]-based ILs, and the existence of hydrogen bonds in ILs increases density and decreases self-diffusion coefficient. The intermolecular interaction strength of [BMIM]+-based ILs is stronger than that of [BMPy]+-based ILs.

*Corresponding Author:

Ian Gates Phone: +1-403-220-5752 Email: [email protected]

ACS Paragon Plus Environment

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1. INTRODUCTION

Ionic liquids (ILs), often referred to as “green solvents”, are salts in liquid state at room temperature with extremely low vapor pressure, low combustibility, and favorable solvating properties for a large range of polar and nonpolar compounds.1 They are alternative solvents for extraction of sulfur and nitrogen-containing compounds from oil.2,3 In addition, they are also reported to be efficient at extracting naphthenic acids.4-4 Furthermore, researchers have utilized ILs to separate bioactive compounds, produce catalysts and electrodes, and pretreat cellulose.5–7 It is also noteworthy that ILs have potential applications in pharmaceutics and medicine.8 ILs are obtained by combining ions with specific characteristics to tune them to specific capabilities, hence, they are called “designer solvents”.9 Bulky and asymmetric organic cations such as 1-butyl-3-methylimidazolium and 1-butyl-3-methylpyridinium, and common anions including

tetrafluoroborate,

hexafluorophosphate,

hydrogen sulfate,

methylsulfate,

and

ethylsulfate can be used for the synthesis of ILs.10 It has been demonstrated that the properties of ILs can be altered to some extent by varying the cation, anion, or substituent groups.11 For instance, the density of imidazolium-based ILs rises with increasing molecular weight of the anion, whereas the density drops with increasing length of alkyl-substituted chain bound to the cation or by introducing a third alkyl substituent on the imidazolium ring at the C2 position.12 Efficient application of ILs in multidisciplinary areas requires tuning of ILs so that they have desirable physicochemical properties.13 Considering that their physicochemical properties, such as density, self-diffusion coefficient, and vapor pressure, are tied to their intramolecular and intermolecular interactions, a fundamental issue in designing ILs is understanding the strength of intramolecular and intermolecular interactions, specifically, the intermolecular interactions in the bulk ionic fluid.14,15 There are many kinds of ILs, making it time-consuming and laborious to 2


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select ILs for specific applications through experimental investigations.16 Quantum chemistry and molecular simulations provide a relatively easy method to investigate molecular interactions and to characterize the physicochemical properties of ILs so that the relationships between molecular structures and physicochemical properties can be understood at the molecular level.17 Imidazolium-based ILs and pyridinium-based ILs are commonly used ILs and they are similar with respect to cationic ring structure and atomic composition.18,19 Nevertheless, there are few studies that compare their electronic and physiochemical properties by using density functional theory (DFT) and molecular dynamics simulation to investigate the influence of intermolecular interactions on physiochemical properties of ILs. Here, we use DFT to explore the differences between imidazolium-based ILs and pyridinium-based ILs, and in particular, the electronic structures of the imidazolium cation and pyridinium cation and the intermolecular interactions, including van der Waals (vdW) interactions, hydrogen bonds, π-stacking, and electrostatic interactions. Moreover, molecular dynamics simulations are conducted to investigate the influence of cations and anions on physicochemical properties

such

as

density

and

self-diffusion

coefficient.

Specifically,

1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate

([BMIM][PF6]),

([BMIM][HSO4]),

1-butyl-3-methylimidazolium

1-butyl-3-methylimidazolium tetrafluoroborate ([BMPy][PF6]),

1-butyl-3-methylimidazolium

ethylsulfate

([BMPy][BF4]),

methylsulfate

([BMIM][ESO4]),

hydrogen

sulfate

([BMIM][MSO4]),

1-butyl-3-methylpyridinium



hydrogen

sulfate

hexafluorophosphate ([BMPy][HSO4]),

1-butyl-3-methylpyridinium methylsulfate ([BMPy][MSO4]), and 1-butyl-3-methylpyridinium ethylsulfate ([BMPy][ESO4]) were selected for investigation. The structures of cations and anions are depicted in Figure 1.

3



2. COMPUTATIONAL METHODS

The double numerical plus polarization (DNP) basis set, which adds the polarized d and pfunction to non-hydrogen atoms and hydrogen atoms, separately, has been reported to be accurate for investigating IL interactions.20 The generalized gradient approximation (GGA) functional by Perdrew and Wang (PW91)21 has been found to be suitable to study exchange-correlations.22 Therefore, here, geometry optimizations were performed by using the GGA-PW91 functional and DNP basis set in the Dmol3 module of the Material Studio 8.0 software package.23,24 Electron density, electrostatic potential, and population analysis were conducted at the same level. The electrostatic potential was mapped on the electron density isosurface. In this study, the amorphous cell module was used to construct amorphous cubic periodic boxes for bulk ILs including 100 cations and 100 anions at temperature 298.15 K and pressure 101.325 kPa. In the simulations, the COMPASS II force field,25 which was used by many researchers to conduct IL molecular dynamics study, was employed.26,27 In addition, the Ewald summation method and atom based method was adopted to study electrostatic and vdW interactions during the construction.28 The cells were relaxed in a 200 ps molecular dynamics run using the isothermal-isobaric (NPT) ensemble. The Berendsen barostat and the Andersen thermostat were used for all molecular dynamics calculations.29 This was followed by a 200 ps molecular dynamics production run with frames outputted every 100 fs into the trajectory file. Electrostatic and vdW terms were determined by the Ewald summation method with 0.0001 kcal/mol accuracy by using a 15.5 Å cut-off value and 0.5 Å buffer width. The trajectories calculated by NPT were used to analyze the density. The densities of ILs were also recalculated by both 1 ns equilibration run and 2 ns NPT molecular dynamics simulation. After the 1 ns 4


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canonical (NVT) equilibration run, 2 ns NVT molecular dynamics simulations were performed, with sampling every 100 fs, and resulting trajectories utilized for self-diffusion parameters calculations. It should be noted that the properties obtained in the current study are mainly used for comparison purposes, not for prediction. Therefore, although boxes containing a few hundreds of ion pairs and a few nanoseconds simulation are necessary to obtain reliable molecular dynamics properties for ILs,30 the boxes in the current study consist of 100 ion pairs and the simulation time is 2 ns. 3. RESULTS AND DISCUSSION 3.1. Characterization of the Cations Charges, electrostatic potential, and Mulliken bond orders31 for [BMIM]+ and [BMPy]+ cations are displayed in Figure 2. The positive charge of [BMIM]+ is “formally” carried by the quaternary nitrogen atoms (Figure 2), which is consistent with the resonance structures.32 Judging from electrostatic potential legends in Figure 2, the electrostatic potential of [BMIM]+ is more positive than [BMPy]+. As expected, the electrostatic potential for the imidazolium and pyridinium ring of [BMIM]+ and [BMPy]+ is more positive than that of the butyl chain (Figure 2). Therefore, two expected conclusions can be deduced from electrostatic potential analysis. One is that electrostatic interaction is the main interaction between cation and anion, and the other is that anions prefer to locate nearer the imidazolium and pyridinium rings than the butyl chain owing to the more positive electrostatic potential of the aromatic ring than that of the butyl chain. In addition, it is anticipated that the presence of two nitrogen atoms in the imidazolium ring against only one nitrogen on the larger pyridinium ring will induce a greater effect, i.e., a lesser aromaticity. The harmonic oscillator measure of aromaticity (HOMA) values of [BMIM]+ and [BMPy]+ are calculated to be 0.8870 and 0.9730, respectively. The closer HOMA is to unity, the stronger the aromaticity of the ring,33 confirming that pyridinium ring has stronger aromaticity. 5



Moreover, the Mulliken bond orders of the pyridinium ring are more uniformly distributed in contrast to those in the imidazolium ring (Figure 2), further verifying that the aromaticity of pyridinium ring is stronger.

3.2. Geometric Structures Radial distribution functions (RDFs) are commonly used to describe the probability of finding another particle at a certain distance from a reference particle34. The RDFs of anion-hydrogen atoms in the ring/the methyl chain/the butyl chain were depicted in Figure S1. The peaks for anion-hydrogen atoms in the ring/the methyl chain are stronger than that for anion-hydrogen atoms in the butyl chain, therefore, it was deduced that the anions tend to be located near hydrogen atoms of the pyridinium, imidazolium rings, and methyl chains, consistent with electrostatic potential analysis. In addition, the RDFs for anion-cation were provided in Figure S2. According to Figure S2, the possibility of finding anions around [BMPy]+ is higher than that around [BMIM]+. Some typical arrangements of cations and anions in a cubic periodic box are presented in Figure 3. It also confirmed that anion-π+ stacking is more conspicuous in [BMPy]+-based ILs (Figure 3f-g) than that in [BMIM]+-based ILs (Figure 3a-e). Moreover, the T-shaped and parallel-displaced π-π stacking conformations are identified in the IL cluster. Concerning [BMIM][HSO4] and [BMPy][HSO4], the RDGs for the intermolecular interactions among [HSO4]- anions were shown in Figure. 4. It was noted that there are two distinct peaks around 1.5 Å and 2.4 Å for both [BMIM][HSO4] and [BMPy][HSO4]. The Bondi’s van der Waals radii of O and H atoms are 1.52 Å, and 1.20 Å35, respectively. Hence, the location of these two peaks are within the summary of van der Waals radii of O and H atoms (2.72 Å), indicating the formation of two strong hydrogen bonds between O atoms and H atoms. As shown in Figure 3c and 3h, [HSO4]- anions form an octatomic ring ([HSO4]- dimer) through two strong 6


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O···H hydrogen bonds. It is reported that some ILs with chains longer than (or equal to) the butyl chain could cause aggregation behavior.36,37 For cations with a butyl chain, the influence of anions on aggregation is pronounced. The intermolecular RDFs between the ethyl chain of [ESO4]- and butyl chain of [BMIM]+ and [BMPy]+ both have a strong peak, revealing the tendency of ethyl chain and butyl chain to aggregate (Figure 5). It was deduced from Figure 3 that prominent aggregation behavior does not exist in [BF4]-, [PF6]-, [MSO4]--based ILs. For [ESO4]--based ILs, the interactions between nonpolar butyl chains and ethyl chains in [ESO4]- cause aggregation (Figure 3e, j). The long alkyl chain of [ESO4]- makes it possible for them to aggregate through nonpolar interactions, which is one type of vdW interaction. With regards to [HSO4]--based ILs, there are several peaks for the intermolecular RDFs between [HSO4]- and the pyridinium or imidazodium ring (Figure 5). As shown in Figure 3c and 3h, the [HSO4]- dimer is surrounded by pyridinium or imidazolium rings, leading to aggregation behavior. According to earlier molecular simulation and experimental studies, aggregation not only exists in nonpolar domains, but also between polar parts and charged parts.38–40 Hence, aggregation behavior is formed between polar pyridinium or imidazolium ring and charged [HSO4]- dimer.

3.3. Intermolecular Interaction Energy The intermolecular interaction energy has significant impacts on physicochemical properties of ILs. The average intermolecular interactional energy was calculated by molecular dynamics, and the results are presented in Figure 6. To understand the nonpolar and polar interactions separately in this study, the total non-bonding energy is divided into vdW interaction energy and electrostatic energy. It is deduced that the total non-bonding energy and vdW interaction energy of [BMIM]+-based ILs are more negative than that of corresponding [BMPy]+-based ILs, confirming 7



that the intermolecular interaction strength of [BMIM]+-based ILs is stronger than that of [BMPy]+-based ILs, as shown in Figure 6. Furthermore, the electrostatic energy accounts for the vast majority of the total non-bonding energy. However, the electrostatic energy is more negative for [BMPy]+-based ILs than that for [BMIM]+-based ILs. According to geometry analysis, anion-π+ interactions are less common in [BMIM]+-based ILs than that in [BMPy]+-based ILs. Therefore, despite the higher electrostatic potential of [BMIM]+, the electrostatic energy is more negative for [BMPy]+-based ILs compared to [BMIM]+-based ILs. As depicted in Figure 7, the differences of vdW interaction energy between different types of ILs are obvious. Moreover, it suggests that the strength of vdW interactions in descending order are [BMIM]+ > [BMPy]+, [BF4]- > [PF6]-, and [ESO4]- > [MSO4]- > [HSO4]-. According to geometry structure analysis, the anion dimers are prone to be surrounded by the [BMPy]+ cation for [BMPy][HSO4], and anions in [BMIM][HSO4] tend to interact with cations through anion-π+ interactions. Both [BMPy][HSO4] and [BMIM][HSO4] have two strong hydrogen bonds formation and obvious aggregative behavior. Compared with other [BMIM]+ and [BMPy]+-based ILs structures, there is smaller variation in the structure arrangements for [BMPy][HSO4] and [BMIM][HSO4], which could help explain the smaller electrostatic energy and vdW interaction energy differences between these two types of ILs.

3.4. Averaged Noncovalent Interaction Analysis In 2010, Yang et al.41 constructed a noncovalent interaction index based on the study of reduced density gradient (RDG) as a function of electron density ρ(r). In 2013, their group developed an averaged noncovalent interaction index (avgNCI) along with a fluctuation index to characterize the magnitude of interactions and fluctuations.42 The averaged reduced density gradient (avgRDG) was defined by using the RDG of each single structure obtained from a 8


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dynamics trajectory. By plotting avgRDG with respect to sign(λ2)ρ(r) (effective density), noncovalent interaction regions could be identified through analyzing spikes in the figure. The peaks in the sign(λ2)ρ0 region, repulsive interaction is indicated. The strength of interaction can be reflected by the absolute value of effective density, and larger absolute value means stronger interactions. In addition, to visually observe the strength and position of those noncovalent interactions, the low-gradient (RDG=0.25) isosufaces were colored according to the corresponding values of effective density. The surfaces were colored on a blue-green-red scale with blue indicating strong attractive interactions (such as dipole-dipole or hydrogen bonding), green demonstrating weak interactions (such as vdW interactions), and red representing steric interactions. The plots of RDG versus sign(λ2)ρ and gradient isosurfaces are displayed in Figures 8 and 9, respectively. The existences of spikes from -0.02 a.u. to 0.02 a.u. in Figure 8a-e indicate that vdW interaction and electrostatic interaction are present in all those ten types of ILs. It is noted that there are two prominent red spikes for [BMIM][HSO4] and two black spikes for [BMPy][HSO4] between 0.07 a.u. and 0.10 a.u. in Figure 8c, affirming the presence of two types of strong hydrogen bonds in both [BMIM][HSO4] and [BMPy][HSO4], consistent with geometry analysis. In addition, as depicted in Figure 9e and f, there are two blue circular regions in the [HSO4]dimers of both [BMIM][HSO4] and [BMPy][HSO4] RDG isosurfaces, further confirming the presence of two types of strong hydrogen bonds in the [HSO4]- dimers. The location of the two red spikes does not overlap (Figure 8c), which suggests that the strength of the two types of hydrogen bonds in the [HSO4]- dimers of [BMIM][HSO4] are different. Similarly, the strength of the two types of hydrogen bonds in the [HSO4]- dimers of [BMPy][HSO4] are also dissimilar. However, the distance between the two black spikes is shorter than that of the two red ones, 9



meaning that the difference of the strength of hydrogen bonds in the [HSO4]- dimers of [BMPy][HSO4] is smaller than that of [BMIM][HSO4].

3.5. Densities To investigate the influences of intermolecular and intramolecular interactions on the densities of ILs, the densities of these ten types of ILs at room temperature calculated by the COMPASSII force field are shown in Figure 10. Table 1 lists densities for [BMIM][BF4], [BMIM][PF6], [BMIM][HSO4], [BMIM][MSO4], [BMPy][BF4], and [BMPy][MSO4] from experiment43-45 and the values determined from the analysis conducted here. The maximum deviation between experimental density and calculated value is 3%, indicating the reliability of the results of the analysis conducted here. The densities of ILs with same anions and different cations are slightly different; whereas the densities of ILs with same cations and different anions have much larger variation (Figure 10). Therefore, it is deduced that the influence of the anions on the density of the ILs is greater than that of the cations. In general, the densities follow the order of [BMIM]+> [BMPy]+, [PF6]-> [BF4]-, and [HSO4]-> [MSO4]-> [ESO4]- (Figure 10). It is noted that the experimental densities of ILs follow the order of [BMIM][PF6]> [BMIM][BF4], [BMIM][HSO4]>[BMIM][MSO4], [BMIM][PF6]> [BMPy][PF6], and [BMIM][MSO4]> [BMPy][MSO4], which has the same tendency with the simulation data and further confirms the accuracy of the predicted densities. There are several factors accounting for the density trend. One factor that rationalizes the higher density of [BMIM]+-based ILs than [BMPy]+-based ILs is the different attractive non-bonded interactions. [BMIM]+-based ILs have higher interaction energy than [BMPy]+-based ILs. Therefore, the interactions between [BMIM]+ and anions are stronger than that between [BMPy]+ and the same anions, leading to the more compressed distribution of [BMIM]+ and 10


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anions. Consequently, the densities of [BMIM]+-based ILs are larger than that of [BMPy]+-based ILs. The other factor is the steric hindrance; the steric hindrance of [ESO4]- is larger than that of [MSO4]- and [HSO4]-. Hence the densities of ILs with [ESO4]- anions are smaller than [MSO4]and [HSO4]--based ILs. Furthermore, based on geometry and avgNCI analysis, strong hydrogen bonds are formed in [HSO4]--based ILs, which can explain the higher density of [HSO4]--based ILs than [MSO4]- and [ESO4]--based ILs. Additionally, the higher density of [PF6]--based ILs in comparison with [BF4]--based ILs is ascribed to the relatively larger molecular mass of [PF6]- over that of [BF4]-. Interestingly, the density differences between [BMIM][HSO4]/[BMPy][HSO4] and [BMIM][ESO4]/[BMPy][ESO4] are smaller than that between other [BMIM]+-based ILs and [BMPy]+-based ILs. Geometry analysis demonstrates the existence of aggregation in [BMIM][HSO4], [BMIM][ESO4], [BMPy][HSO4], and [BMPy][ESO4], implying that aggregation effects influence the densities of ILs.

3.6. Self-diffusion Coefficients Analysis The self-diffusion coefficient is important to understand mass transfer rates as well as transport properties such as viscosity and ionic conductivity.46-48 In this study, the mean square displacement (MSD) was used to calculate the self-diffusion coefficient. After 1 ns NVT dynamics, MSD versus time data set can be obtained through forcite analysis. A typical curve of MSD as a function of time was provided in Figure S3. The exponent βt was calculated according to Equation (1) to determine whether the system is in the diffusion region:

βt =

∆

(1)

where t is time, ∆ is the MSD at t. If the value of βt is less or bigger than 1, it demonstrates that the system is subdiffusive or superdiffusive, respectively. On the other hand, 11



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βt=1 denotes normal diffusion.49 The βt value of [BMIM][BF4] was shown in Figure S4. The system is subdiffusive initially, and gets closes to normal diffusion after around 1 ns. The MSD data set, which was in the diffusion region, was used for linear regression where the self-diffusion coefficient is equal to the slope of the regressed fit divided by 6. The effect of structures on self-diffusion coefficients is shown in Figure 11. For ILs with the same anions and

different cations,

the

variation

of

self-diffusion

coefficients for

[BMIM][BF4]/[BMPy][BF4] and [BMIM][PF6]/[BMPy][PF6] are more obvious, exhibiting that [BF4]- and [PF6]- anions impact the self-diffusion coefficient significantly. The differences of self-diffusion

coefficients

[BMIM][HSO4]-/[BMPy][HSO4]-,

for

[BMIM][MSO4]-/[BMPy][MSO4]-, and [BMIM][ESO4]-/[BMPy][ESO4]- are smaller, especially for [BMIM][HSO4]-/[BMPy][HSO4]- and [BMIM][ESO4]-/[BMPy][ESO4]-. Based on geometry analysis, [HSO4]- and [ESO4]--based ILs have conspicuous aggregations, which can account for the smaller self-diffusion coefficients differences for [BMIM][HSO4]-/[BMPy][HSO4]- and [BMIM][ESO4]-/[BMPy][ESO4]-. On other other hand, with the same cations, the self-diffusion coefficients for [HSO4]--based ILs are smaller than other anions-based ILs. According to geometry

and

avgRDG

analyses,

two

prominent

hydrogen

bonds

exist

in

both

[BMIM][HSO4]-/[BMPy][HSO4]-. The presence of hydrogen bonds induces IL clustering, thereby, the lowest self-diffusion coefficients of [HSO4]--based ILs among these ILs. 4. CONCLUSIONS Density functional theory and molecular dynamics simulation were used to explore the interactions, densities, and self-diffusion coefficients of [BMIM]+-based ILs and [BMPy]+-based ILs. The electrostatic interaction energy accounts for the vast majority of total non-bonding interaction energy. From geometry analysis, it is concluded that vdW interactions, hydrogen bonds，electrostatic interactions, anion-π+， and π+-π+ stacking widely occur in these types of ILs. 12


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Two strong hydrogen bonds are found in the [HSO4]- dimer through geometry and avgNCI analyses. Anions have greater influence on the density than cations. Aggregation behavior and hydrogen bond formation can alter the density and self-diffusion coefficients of ILs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. RDFs for anion-hydrogen atoms in the ring/the methyl chain/the butyl chain, RDFs of cation-anion, time dependences of MSD, and βt values of [BMIM][BF4]. AUTHOR INFORMATION Corresponding Author *Ian Gates. Email: [email protected]. Phone: +1-403-220-5752 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Financial support provided by the Department of Chemical and Petroleum Engineering, University of Calgary, is acknowledged as well as support from the University of Calgary Beijing Research Site. The authors are also grateful for the computational support from School of Chemical Engineering of the China University of Petroleum, Qingdao Campus, China.

13



References (1)

(2)

(3)

(4) (5)

(6)

(7)

(8) (9) (10) (11)

(12)

(13)

(14)

(15)

(16) (17)

Mahmood, H.; Moniruzzaman, M.; Yusup, S.; Welton, T. Ionic Liquids Assisted Processing of Renewable Resources for the Fabrication of Biodegradable Composite Materials. Green Chem. 2017, 19 (9), 2051–2075. Lin, J.; Lü, R.; Wu, C.; Xiao, Y.; Liang, F.; Famakinwa, T. A Density Functional Theory Study on the Interactions between Dibenzothiophene and Tetrafluoroborate-Based Ionic Liquids. J. Mol. Model. 2017, 23 (4), 145. Lü, R.; Wu, C.; Lin, J.; Xiao, Y.; Wang, F.; Lu, Y. The Study on Interactions between 1 ‐ Ethyl ‐ 3 ‐ Methylimidazolium Chloride and Benzene / Pyridine / Pyrrole / Thiophene. J. Mol. Liq. 2017, 237, 289–294. Wu, C.; De Visscher, A.; Gates, I. D. Interactions of Biodegradable Ionic Liquids with a Model Naphthenic Acid. Sci. Rep. 2018, 8 (1), 176. Cao, Y.; Zhang, R.; Cheng, T.; Guo, J.; Xian, M.; Liu, H. Imidazolium-Based Ionic Liquids for Cellulose Pretreatment: Recent Progresses and Future Perspectives. Appl. Microbiol. Biotechnol. 2017, 101 (2), 521–532. Watanabe, M.; Thomas, M. L.; Zhang, S.; Ueno, K.; Yasuda, T.; Dokko, K. Application of Ionic Liquids to Energy Storage and Conversion Materials and Devices. Chem. Rev. 2017, 117 (10), 7190–7239. Ventura, S. P. M.; e Silva, F. A.; Quental, M. V.; Mondal, D.; Freire, M. G.; Coutinho, J. A. P. Ionic-Liquid-Mediated Extraction and Separation Processes for Bioactive Compounds: Past, Present, and Future Trends. Chem. Rev. 2017, 117 (10), 6984–7052. Egorova, K. S.; Gordeev, E. G.; Ananikov, V. P. Biological Activity of Ionic Liquids and Their Application in Pharmaceutics and Medicine. Chem. Rev. 2017, 117 (10), 7132–7189. Niedermeyer, H.; Ashworth, C.; Brandt, A.; Welton, T.; Hunt, P. A. A Step towards the a Priori Design of Ionic Liquids. Phys. Chem. Chem. Phys. 2013, 15 (27), 11566–11578. Gardas, R. L.; Coutinho, J. A. P. A Group Contribution Method for Heat Capacity Estimation of Ionic Liquids. Ind. Eng. Chem. Res. 2008, 47, 5751–5757. Shah, J. K.; Brennecke, J. F.; Maginn, E. J. Thermodynamic Properties of the Ionic Liquid 1-N-Butyl-3-Methylimidazolium Hexafluorophosphate from Monte Carlo Simulations. Green Chem. 2002, 4 (2), 112–118. Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.; Brennecke, J. F. Thermophysical Properties of Imidazolium-Based Ionic Liquids. J. Chem. Eng. Data 2004, 49 (4), 954–964. Marium, M.; Auni, A.; Rahman, M. M.; Mollah, M. Y. A.; Susan, M. A. B. H. Molecular Level Interactions between 1-Ethyl-3-Methylimidazolium Methanesulphonate and Water: Study of Physicochemical Properties with Variation of Temperature. J. Mol. Liq. 2017, 225, 621–630. Fumino, K.; Reimann, S.; Ludwig, R. Probing Molecular Interaction in Ionic Liquids by Low Frequency Spectroscopy: Coulomb Energy, Hydrogen Bonding and Dispersion Forces. Phys. Chem. Chem. Phys. 2014, 16 (40), 21903–21929. Kurnia, K. A.; Neves, C. M. S. S.; Freire, M. G.; Santos, L. M. N. B. F.; Coutinho, J. A. P. Comprehensive Study on the Impact of the Cation Alkyl Side Chain Length on the Solubility of Water in Ionic Liquids. J. Mol. Liq. 2015, 210, 264–271. Chen, L.; Bryantsev, V. S. A Density Functional Theory Based Approach for Predicting Melting Points of Ionic Liquids. Phys. Chem. Chem. Phys. 2017, 19 (5), 4114–4124. Červinka, C.; Pádua, A. A. H.; Fulem, M. Thermodynamic Properties of Selected Homologous Series of Ionic Liquids Calculated Using Molecular Dynamics. J. Phys. Chem. B 2016, 120, 2362–2371. 14


Page 14 of 30

Page 15 of 30 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


(18) (19)

(20)

(21) (22)

(23) (24) (25)

(26)

(27)

(28) (29) (30) (31)

(32)

(33)

(34)

(35) (36)

Hou, X.; Liu, Q.; Smith, T. J.; Li, N.; Zong, M. Evaluation of Toxicity and Biodegradability of Cholinium Amino Acids Ionic Liquids. PLoS One 2013, 8 (3), e59145. Yeganegi, S.; Sokhanvaran, V.; Soltanabadi, A. Study of Thermodynamic Properties of Imidazolium-Based Ionic Liquids and Investigation of the Alkyl Chain Length Effect by Molecular Dynamics Simulation. Mol. Simul. 2013, 39(13), 1070–1078. Martínez-Magadán, J. M.; Oviedo-Roa, R.; García, P.; Martínez-Palou, R. DFT Study of the Interaction between Ethanethiol and Fe-Containing Ionic Liquids for Desulfuration of Natural Gasoline. Fuel Process. Technol. 2012, 97, 24–29. Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Physical Review B. 1992, 45(23), 13244–13249. Yan-Hong, C.; Yan-Fei, C.; Dong-Shun, D.; Ning, A.; Yong, Z. Difference for the Absorption of SO2 and CO2 on [Pnnnm][Tetz] (n=1, m=2, and 4) Ionic Liquids: A Density Functional Theory Investigation. J. Mol. Liq. 2014, 199, 7–14. Delley, B. From Molecules to Solids with the DMol[sup 3] Approach. J. Chem. Phys. 2000, 113 (18), 7756–7764. Delley, B. An All‐electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92 (1), 508–517. Sun, H.; Jin, Z.; Yang, C.; Akkermans, R. L. C.; Robertson, S. H.; Spenley, N. A.; Miller, S.; Todd, S. M. COMPASS II: Extended Coverage for Polymer and Drug-like Molecule Databases. J. Mol. Model. 2016, 22 (2), 1–10. Sasikumar, Y.; Adekunle, A. S.; Olasunkanmi, L. O.; Bahadur, I.; Baskar, R.; Kabanda, M. M. Experimental , Quantum Chemical and Monte Carlo Simulation Studies on the Corrosion Inhibition of Some Alkyl Imidazolium Ionic Liquids Containing Tetra Fl Uoroborate Anion on Mild Steel in Acidic Medium. J. Mol. Liq. 2015, 211, 105–118. Shin, B. S.; Kim, E. S.; Kwak, S. K.; Lim, J. S.; Kim, K. S.; Kang, J. W. Thermodynamic Inhibition Effects of Ionic Liquids on the Formation of Condensed Carbon Dioxide Hydrate. Fluid Phase Equilib. 2014, 382, 270–278. Darden, T.; York, D.; Pedersen, L. Particle Mesh Ewald: An N ⋅log( N ) Method for Ewald Sums in Large Systems. J. Chem. Phys. 1993, 98 (12), 10089–10092. Allen, M. P.; Wilson, M. R. Computer Simulation of Liquid Crystals. J. Comput. Aided. Mol. Des. 1989, 3 (4), 335–353. Gabl, S.; Schröder, C.; Steinhauser, O. Computational Studies of Ionic Liquids: Size Does Matter and Time Too. J. Chem. Phys. 2012, 137 (9), 94501. Mulliken, R. S. Electronic Population Analysis on LCAO–MO Molecular Wave Functions. II. Overlap Populations, Bond Orders, and Covalent Bond Energies. J. Chem. Phys. 1955, 23 (10), 1841–1846. Hunt, P. A.; Kirchner, B.; Welton, T. Characterising the Electronic Structure of Ionic Liquids: An Examination of the 1-Butyl-3-Methylimidazolium Chloride Ion Pair. Chem. A Eur. J. 2006, 12 (26), 6762–6775. Andrzejak, M.; Kubisiak, P.; Zborowski, K. K. Avoiding Pitfalls of a Theoretical Approach: The Harmonic Oscillator Measure of Aromaticity Index from Quantum Chemistry Calculations. Struct. Chem. 2013, 24 (4), 1171–1184. Khan, I.; Batista, M. L. S.; Carvalho, P. J.; Santos, L. M. N. B. F.; Gomes, J. R. B.; Coutinho, J. A. P. Vapor-Liquid Equilibria of Imidazolium Ionic Liquids with Cyano Containing Anions with Water and Ethanol. J. Phys. Chem. B 2015, 119 (32), 10287– 10303. Bondi, A. Van Der Waals Volumes and Radii. J. Phys. Chem. 1964, 68 (3), 441–451. Wang, J.; Wang, H.; Zhang, S.; Zhang, H.; Zhao, Y. Conductivities, Volumes, Fluorescence, and Aggregation Behavior of Ionic Liquids [C4mim][BF4] and [Cnmim]Br (N = 4, 6, 8, 10, 15



(37) (38)

(39)

(40)

(41) (42) (43) (44)

(45)

(46)

(47)

(48)

(49)

12) in Aqueous Solutions. J. Phys. Chem. B 2007, 111 (22), 6181–6188. Singh, T.; Kumar, A. Aggregation Behavior of Ionic Liquids in Aqueous Solutions: Effect of Alkyl Chain Length, Cations, and Anions. J. Phys. Chem. B 2007, 111 (27), 7843–7851. Bernardes, C. E. S.; Shimizu, K.; Lobo Ferreira, A. I. M. C.; Santos, L. M. N. B. F.; Canongia Lopes, J. N. Structure and Aggregation in the 1,3-Dialkyl-Imidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Family: 2. from Single to Double Long Alkyl Side Chains. J. Phys. Chem. B 2014, 118 (24), 6885–6895. Saielli, G.; Bagno, A.; Wang, Y. Insights on the Isotropic-to-Smectic a Transition in Ionic Liquid Crystals from Coarse-Grained Molecular Dynamics Simulations: The Role of Microphase Segregation. J. Phys. Chem. B 2015, 119 (9), 3829–3836. Shimizu, K.; Bernardes, C. E. S.; Canongia Lopes, J. N. Structure and Aggregation in the 1-Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Homologous Series. J. Phys. Chem. B 2014, 118 (2), 567–576. Johnson, E. R.; Keinan, S.; Mori Sánchez, P.; Contreras García, J.; Cohen, A. J.; Yang, W. Revealing Non-Covalent Interactions. J. Am. Chem. Soc. 2010, 132 (18), 6498–6506. Wu, P.; Chaudret, R.; Hu, X.; Yang, W. Noncovalent Interaction Analysis in Fluctuating Environments. J. Chem. Theory Comput. 2013, 9 (5), 2226–2234. Haynes, W. M., CRC Handbook of Chemistry and Physics. 97th Edition, CRC press, 2016. Aguirre, C.L., Toro, N., Carvajal, N., Watling, H., Aguirre, C., Leaching of Chalcopyrite (CuFeS2) with an Imidazolium-Based Ionic Liquid in the Presence of Chloride. Miner. Eng. 2016, 99, 60-66. Ortega, J., Vreekamp, R., Marrero, E., Penco, E., Thermodynamic Properties of 1-Butyl-3-Methylpyridinium Tetrafluoroborate and Its Mixtures with Water and Alkanols. J. Chem. Eng. Data. 2007, 52(6), 2269-2276. Harris, K. R., Kanakubo M., Self-diffusion, Velocity Cross-Correlation, Distinct Diffusion and Resistance Coefficients of the Ionic Liquid [BMIM][Tf2N] at High Pressure. Phys. Chem. Chem. Phys. 2015, 17 (37), 23977-23993. Harris, K. R., Makino, T., Kanakubo, M., Viscosity Scaling of the Self-diffusion and Velocity Cross-Correlation Coefficients of Two Functionalised Ionic Liquids and of Their Non-functionalized Analogues. Phys. Chem. Chem. Phys. 2014, 16(19), 9161-9170. Tsuzuki, S., Matsumoto, H., Shinoda, W., Mikami, M. Effects of Conformational Flexibility of Alkyl Chains of Cations on Diffusion of Ions in Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13(13), 5987-5993. Mondal, A.; Balasubramanian, S. Quantitative Prediction of Physical Properties of Imidazolium Based Room Temperature Ionic Liquids through Determination of Condensed Phase Site Charges: A Refined Force Field. J. Phys. Chem. B 2014, 118 (12), 3409–3422.

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Table 1: Comparison of densities from experiments and model. ILs Experimental Density obtained in 42-44 -3 Density , g cm this study, g cm-3 [BMIM][BF4] 1.205 1.226

Deviation, % 1.74

[BMIM][PF6]

1.367

1.328

2.85

[BMIM][HSO4]

1.277

1.263

1.10

[BMIM][MSO4]

1.212

1.234

1.82

[BMPy][BF4]

1.182

1.205

1.95

[BMPy][MSO4]

1.190

1.211

1.76

N N

N

(a)

(b) F

F F

B

O

O

P

F

O

O S

S

S F

F

O

O

F

F

O

O

O

OH

F

O

O

F

(c)

(d)

(e)

(f)

(g)

Figure 1. The structures of (a) 1-butyl-3-methylimidazolium ([BMIM]+), (b) 1-butyl-3-methylpyridinium ([BMPy]+), (c) tetrafluoroborate ([BF4]-), (d) hexafluorophosphate ([PF6]-), (e) hydrogen sulfate ([HSO4]-), (f) methylsulfate ([MSO4]-), and (g) ethylsulfate ([ESO4]-).

17



(a) [BMIM]+

(b) [BMPy]+ Figure 2. Charges (black), Mulliken bond orders (red), and electrostatic potential of [BMIM]+ and [BMPy]+.

18


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(a)

(b)

(c)

(d)

(e)

(f)



(g)

Page 20 of 30

(h)

(i)

(j)

Figure 3. Typical arrangement of cations and anions in (a) [BMIM][BF4], (b) [BMIM][PF6], (c) [BMIM][HSO4], (d) [BMIM][MSO4], (e) [BMIM][ESO4], (f) [BMPy][BF4], (g) [BMPy][PF6], (h) [BMPy][HSO4], (i) [BMPy][MSO4], and (j) [BMPy][ESO4].

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(a) [BMIM][HSO4]

(b) [BMPy][HSO4]

Figure 4. RDGs for intermolecular interactions between [HSO4]- anions.

21



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(a) [BMIM][ESO4]

(b) [BMPy][ESO4]

(c) [BMIM][HSO4]

(d) [BMPy][HSO4]

Figure 5. RDGs for intermolecular interactions between ethyl chains of [ESO4]- and butyl chains of (a) [BMIM][ESO4]/(b) [BMPy][ESO4]; for intermolecular interactions between [HSO4]- and imidazolium ring of [BMIM][HSO4]/pyridinium ring of [BMPy][HSO4].

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Figure 6. Intermolecular interaction energy of ILs.

23



Figure 7. vdW interaction energy of ILs.

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(a)

(b)

(c)

(d)

(e) Figure 8. Plots of avgRDG versus sign(λ2)ρ for (a) [BMIM][BF4] and [BMPy][BF4], (b) [BMIM][PF6] and [BMPy][BF6], (c) [BMIM][HSO4] and [BMPy][HSO4], (d) [BMIM][MSO4] and [BMPy][MSO4], and (e) [BMIM][ESO4] and [BMPy][ESO4]. Note: unit for sign(λ2)ρ is a.u..

25



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(a)

(b)

(c)

(d)

(e)

(f)

26


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(g)

(h)

(i)

(j)

Figure 9. RDG isosurfaces for (a) [BMIM][BF4], (b) [BMPy][BF4], (c) [BMIM][PF6], (d) [BMPy][BF6], (e) [BMIM][HSO4], (f) [BMPy][HSO4], (g) [BMIM][MSO4], (h) [BMPy][MSO4], (i) [BMIM][ESO4], and (j) [BMPy][ESO4]. Blue indicates strong attractive interactions and red indicates strong non-bonded overlap.

27



Figure 10. Calculated densities of ILs.

28


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Figure 11. Self-diffusion coefficients of ILs.

29



TOC Graphic

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Comparison of Electronic and Physicochemical Properties between

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