Tracing Dynamics, Self-Diffusion, and Nanoscale Structural

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Tracing Dynamics, Self-Diffusion, and Nano-Scale Structural Heterogeneity of Pure and Binary Mixtures of Ionic Liquid 1Hexyl-2,3-dimethylimidazolium Bis(fluorosulfonyl)imide with Acetonitrile: Insights from Molecular Dynamics Simulations Mohammad Hossein Kowsari, and Leila Tohidifar J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b08396 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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The Journal of Physical Chemistry

Tracing Dynamics, Self-Diffusion, and Nano-Scale Structural Heterogeneity of Pure and Binary Mixtures of Ionic Liquid 1-Hexyl2,3-dimethylimidazolium Bis(fluorosulfonyl)imide with Acetonitrile: Insights from Molecular Dynamics Simulations

Mohammad H. Kowsari*, Leila Tohidifar Department of Chemistry and Center for Research in Climate Change and Global Warming (CRCC), Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran

*

Corresponding author. Tel.: +98 24 3315 3207. Fax: +98 24 3315 3232. E-mail address: [email protected] and [email protected]

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Abstract All-atom molecular dynamics (MD) simulations of 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide ([C6mmim][FSI]) ionic liquid (IL) and its binary mixtures with acetonitrile (ACN) are reported for the first time. The presence of ACN as a cosolvent, similar to the effect of increasing temperature, causes an enhancement to the ion translational motion and fluidity in the IL, leading to significant improvement of ionic conductivity and self-diffusion which is well explained by a microscopic structural analysis. In neat IL and concentrated IL mixture, self-diffusion of the cation is higher than that of corresponding anion; however, further adding of ACN into the diluted mixtures with the IL molar fraction (xIL) below 0.50 results in more weakened interactions of the nearest ACN-anion neighbors rather than those of ACNcation neighbors so that the number of isolated anions is more than that of isolated cations at this condition, and the anions diffuse faster than the cations as expected based of their relative sizes. The velocity autocorrelation function (VACF) analysis indicates the inverse relation between the xIL and the mean collision time of each species. Additionally, at a fixed xIL, both the mean collision time and the velocity randomization time of ACN are shorter than those of the ions. The gradual addition of ACN changes the morphology of nano-segregated domains and tends to disrupt ionic clusters (i.e., it scatters and decomposes both the polar and non-polar domains) compared to those of pure IL, whereas both the radial and spatial distribution functions show the stabilization role of ACN on the close contact ion pair association. On the other hand, increasing of ACN causes a weakening of the structural correlations of the cation-cation and anion-anion neighbors in the solutions. ACN molecules appeared as a bridge with balanced affinities between the polar and non-polar domains, and no indication was observed for aggregation of ACN


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molecules in the studied mixtures that can rationalize good miscibility with imidazolium-based ILs.



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1. Introduction In general, room-temperature ionic liquids (RTILs) are known as green chemical compounds, which have been attracted in academia and chemical industries. These neoteric chemical substances are often the best candidates for numerous applications because of their unique interesting properties such as ultra-low vapour pressure, no flammability, high thermal and chemical stability, broad liquidus range, and wide electrochemical window.1-3 However, the rest handling of ILs in large scale is partly limited because of several disadvantages such as slow glassy dynamics, high viscosity, and relative high cost. One suitable strategy to overcome these problems and likely tuning the properties of ILs is mixing them with simple non-expensive molecular cosolvents. In this way, optimum design of such mixtures is possible with proper choice of the IL, the cosolvent, and their suitable molar fractions. Thus, we need information about the details of the relation between altering the mixture composition and the target properties of such binary mixtures. For this purpose, focus on the dynamics, thermodynamics, microscopic structure, noncovalent interactions, the electrostatic and hydrogen bonding, between simple candidate cosolvent molecules and ionic species is really important. The more widely studied ILs in the literature are 1,3-dialkylimidazolium ILs, especially 1alkyl-3-methylimidazolium ones, may be due to easy to preparation, and having flexible designable ionic structure, dual nature solubility, relatively high conductivity, and low melting point.4 In addition, they are fully miscible with low-viscous aprotic molecular solvents, e.g. acetonitrile (ACN), CH3CN, opening full range of composition and consequently, the extra finetuning of many physicochemical properties. The ILs/ACN mixtures have been suggested as a novel thermally stable electrolyte with less toxicity and high efficiency for a range of applications, including Li-ion batteries, solar cells, and supercapacitors.5-7 As the excellent


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partners with fine relation, ACN considerably improves the ionic mobility and conductivity of ILs, which in turn are able to noticeably decrease the volatility, and hence, the hazard of ACN.8 On the other hand, spectroscopic measurements and quantum-chemical calculations evidence has demonstrated strong hydrogen bonds between the acidic proton attached to the C2 position of the imidazolium ring and anions. According to recent studies, replacement of the hydrogen atom at this location with a methyl group, also known as C2 methylation, eliminates the corresponding hydrogen bonds, and tends to significant changes in the properties of the ILs, such as enhancing the melting point, vaporization enthalpy, viscosity, and especial improving of electrochemical stability.9-15 The end change is more important in the electrochemical application of such ILs. Up to now, several research groups16-20 have investigated the reasons for unexpected increment of melting point and viscosity in the case of alkyl substituent at the imidazolium C2 position by means of IR and Raman spectroscopy, as well as quantum-chemical and MD calculations. Hunt16 and Ludwig et al.17,18 believe that the entropy factor and defect hypothesis can help us to rationalize this phenomenon. In addition to better electrochemical stability of C2 methylated imidazolium-based ILs relative to ordinary C2 protonated ones, the ILs with magic bis(fluorosulfonyl)imide anion, [(FSO2)2N]-, also known as [FSI]-, have been strongly attracted as promising electrolytes in novel Li-ion batteries because of relatively high ionic conductivity, low viscosity, high chemical stability, and forming robust solid-electrolyte interphase (SEI).21-29 Based on the reports of Matsumoto et al., the viscosity of less commonly known [FSI]--based ILs is significantly lower than that of the more widely used bistriflimide, [(CF3SO2)2N]-, also known as [TFSI]- or [NTf2]-, based ILs. Additionally, the increase of viscosity for [TFSI]--based ILs in the presence of Li is about 66-119%, whereas that of the corresponding [FSI]--based ILs is remarkably smaller (28-



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33%).30 Thus, the proposed target IL in this study, 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide ([C6mmim][FSI]) (see structural formula in Figure 1a and 1b for [C6mmim]+ and [FSI]-, respectively), especially when its properties are modified by adding ACN (Figure 1c) as a cosolvent, can be used as a new proper candidate electrolyte for electrochemical applications. For this purpose, fundamental understanding of the physicochemical properties and microscopic structure of this IL and its binary mixtures with ACN is required to discover best application conditions. A number of groups reported MD simulations on [FSI]--based ILs, especially with the subject of force field development.31-34 Also numerous studied evaluated the applications of this class of ILs as electrolytes in the electrochemical industries.35-41 However, the number of last simulations or experimental reports on the [FSI]--based ILs are very scarce rather than [TFSI]--based ILs. The researchers suggest that fluidity enhancement is obtained for ILs in the presence of water or organic solvents like ACN. We indicated the fluidity enhancement of 1-ethyl-3methylimidazolium bistriflimide ([emim][TFSI]) when it is mixed with benzene in the (1:1) binary mixture using MD simulation42 The mixtures of ACN with ILs have been studied by experimental and molecular simulation techniques.43-50 Since high diffusion coefficient (~ 4.3×10-9 m2·s-1) and relatively low shear viscosity (~ 0.34 cP) at room temperature are estimated for ACN,51 thus it can improve the dynamical properties of ILs.44 The hydrogen-bonding interactions in the mixture of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and ACN were investigated by Zheng et al. using attenuated total reflection infrared (ATR-IR) spectroscopy, 1H nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations. They identified that with increasing the concentration of ACN, the ion clusters break into ion pairs; however, the close contact of cation


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and anion in the ion pair form is not broken. Also, by adding ACN, the strength of hydrogen bonds between the nitrogen site of ACN and imidazolium ring C-Hs of [bmim]+ is improved.43 Chaban et al.44 used MD simulation and found that high dilution of five imidazolium-based ILs including [BF4]- and 1-alkyl-3-methylimidazolium [amim]+ cations (alkyl = ethyl, butyl, hexyl, octyl, and decyl) with ACN allows for ionic conductivity increase by more than 50 times for long alkyl side chain ILs and more than 10 times for short alkyl side chain ILs. They also demonstrated that the motion of ions is a function of the motion of ACN molecules at low IL concentrations; altogether the ionic conductivity depends on the mixture content and microscopic structure.44 At the same time, Stoppa et al. studied roughly similar 1-alkyl-3-methylimidazolium tetrafluoroborate + ACN mixtures (alkyl = ethyl, butyl, and hexyl) using broadband dielectric spectroscopy. They defined two broad areas for IL + ACN mixtures over the entire composition range, distinct by a transition state around the IL molar fraction, xIL, of 0.2. At lower IL concentrations area, xIL ≤ 0.2, the IL behaves as a rather weakly associated conventional electrolyte in ACN, while at higher IL concentrations area, xIL ≥ 0.2, it takes on its IL nature, lubricated by ACN.45 With regard to the last computational studies on the microscopic structural heterogeneity in ILs, a number of MD simulations revealed nano-domains formation in ILs including cations with relatively long alkyl chains.52-56 Wang and Voth observed that the terminal methyl group of relatively intermediate length alkyl chains of the 1-alkyl-3-methylimidazolium cations in [Cnmim][NO3] ILs (where, n = 4, 6, and 8) aggregated and constructed an uncharged domain, which existed separately from another domain that formed due to strong electrostatic interactions between the anions and positively part of cations (imidazolium ring).55 This nano-scale property can be applied to explain some of the interesting properties of ILs at a molecular level.56-63



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Shimizu et al. using MD simulations showed that replacement of the alkyl side chain of 1-alkyl3-methylimidazolium cations in [Cnmim][NTf2] ILs (n = 3, 6, 9) by ether-substituted alkyl side groups, resulting [(C1OC1)(n/3)mim][NTf2], leads to disappearance of nanostructured nature because of the low affinity of oxygen-substituted alkyl chains to associate side by side; thus, these more polar ether chains cause to thin layers of non-polar domains.61 MD simulations also evaluated the nano-segregation in the case of ILs mixtures with various solvents.60,63 The results indicated that solute-solvent interactions play an effective role in the morphology of segregated domains. For instance, non-polar n-hexane guest molecules are found in close to the non-polar parts,60 whereas the polar solutes such as water are localized in the polar domains because of electrostatic correlations with the charged groups.60,63 The other groups of solvents, namely dipolar solvents, like ACN, showed an interesting behavior so that they act as a bridge between the polar and non-polar domains as a consequence of interacting in the interfacial region with both of the mentioned domains.60 Nano-segregation phenomenon also causes to determine the solute-solvent interactions in ILs that are quite complex. In this paper we attempt to fully understand the microscopic molecular details of the dynamics, self-diffusion, and structure of the ionic species of [C6mmim][FSI] IL in the presence of ACN as a non-aqueous apolar solvent. For this purpose we calculated a wide variety of quantities by using the atomistic MD simulation as a proper microscopic tool to discuss how adding different molar fractions of ACN cosolvent molecules influence on fine-tuning the different properties of [C6mmim][FSI] IL, and how the ionic states and the nano-segregation phenomenon of the solution are affected by gradual presence of this cosolvent. For the first time in this study, with the combination of refined flexible all-atom force fields for ACN (see below, section 2), we successfully improved the calculated numerical value of the self-diffusion


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coefficient of this solvent in good agreement with the experimental data compared to other the recent computational reports. To our knowledge, this is the first study on pure [C6mmim][FSI] and also on the simulation of its binary mixtures with ACN. In comparison to previous similar work of Chaban et al.,44 we have chosen a new target IL and the refined force field parameters for ACN with the aim of improving the quality of the dynamic predictions. We also use a much broader range of microscopic structural quantities, in particular the visualization of the molecular liquid structure and the nano-segregation phenomena, to deeper discuss how the gradual adding of ACN cosolvent molecules influences the ion translational motion and fluidity of ILs.

2. Computational methods Molecular dynamics simulations of [C6mmim][FSI] IL and ACN in the pure states and their binary mixtures were performed using the DL_POLY 2.18 package64 in the isothermal-isobaric (NpT) ensemble. In the simulations, periodic boundary conditions were employed, and temperature and pressure were controlled at T = (600, 500, 450, 400, 350, and 300) K and p = 1 atm by means of Nośe-Hoover thermostat/ barostat65,66 with applied relaxation times of 0.2 and 1.0 ps, respectively. Since the calculation of pair potentials assumes a spherical cutoff (Rcutoff), the standard explicit long-range corrections to the system potential energy and virial (pressure) were applied. Electrostatic energies were estimated by the Ewald summation method.67 It is to be emphasized that since the boiling point of ACN is about 355.15 K, pure ACN system was only simulated at T = (350 and 300) K. The number of total ion pairs, molecules, atoms, and the shortrange van der Waals cutoff of all simulated systems with final simulation box sizes are listed in Table 1.



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The initial simulation box structures of [C6mmim][FSI] IL and ACN in the pure states and their binary mixtures were prepared by replicas of randomly placed anion-cation pairs and ACN molecules in the boxes, see Table 1. Manually constructed structures, with xIL > 0, were firstly equilibrated using annealing with three consecutive runs at (600, 500, and 450) K, each with a total time length run of 1 ns. Subsequently, each system was equilibrated and annealed with three runs at (400, 350, and 300) K, each with a total time length run of 3 ns, except the run at 350 K that consisted of a total time length run of 1.5 ns. The final runs at (400 and 300) K were performed for data analysis each including a total trajectory time length of 10 ns with the data interval collection of 1.0 ps. Afterwards, only for calculating the velocity autocorrelation functions (VACFs), five consecutive short runs (each with a time length run of 10 ps and data interval collection of 0.005 ps) were carried out, and a trajectory averaging on five runs was performed to improve reproducibility and statistical accuracy of the reported VACF results. In the case of pure ACN, the system was firstly equilibrated at 400 K for 1 ns, and the following annealing simulation process was carried out at 350 and 300 K, each for 1 ns. Finally, the properties of pure ACN were obtained from the runs involving 2 ns trajectories. In addition, for testing the effect of time length run on the calculated transport properties, we performed pure IL simulation runs at 300 and 400 K with 20 ns time length trajectories. Comparison of the results showed no significant difference between the MSD results of runs with different time lengths. Thus it can be said the applied production run time for calculating the properties is enough (see Figure S1 and Table S1 of the Supporting Information for details). The force field parameters for [C6mmim]+ and [FSI]- ions were derived from the nonpolarizable all-atom force field developed by Canongia Lopes et al.31,68-70 The new refined


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combination of six-site flexible force fields reported by Grabuleda et al.71 and Nikitin and Lyubartsev72 was used to simulate ACN in this work with the aim of improving the realism of the interaction potential. In 2012, Chaban et al.44 demonstrated that the model of Nikitin applied in their ILs/ACN mixture simulations correctly reproduced thermodynamic properties of bulk ACN; however, it underestimated self-diffusion compared to the experimental values. We combined the AMBER force filed parameters adopted for ACN by Grabuleda et al.71 with the reported partial atomic charges obtained from the quantum-mechanical calculations, MP2/6-311++G(3df,3p), by Nikitin and Lyubartsev.72 Our simulation results showed that current refine force field parameters provide a good unprecedented improvement in calculating the self-diffusion of ACN in agreement with the experimental data.73

3. Results and discussion 3.1. Dynamical and transport properties We calculated the MSDs, normalized VACFs, self-diffusion coefficients for the center of mass of ions and ACN molecules, and the relative diffusivities as a function of xIL to achieve more insight into the dynamic behavior and the microscopic motion of the species using extensive molecular dynamics simulations at 300 and 400 K. The ionic conductivity of solutions was also estimated from the ionic self-diffusion coefficients using Nernst-Einstein equation. Table 2 indicates the self-diffusion coefficients of the ions of [C6mmim][FSI] IL and ACN molecules in the case of pure and the various molar fractions of their binary mixtures, which were calculated at 300 and 400 K from the MD trajectories using the Einstein relation: D=

→ 1 d → lim 〈[ ri (t) - ri (0)]2 〉 6 t →∞ d t

(1) 11 ACS Paragon Plus Environment


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→

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→

where, the quantity 〈[ ri (t) - ri (0)]2 〉 is the mean-square displacement (MSD) of species i, and can be seen in Figure (2) for the ions of studied systems at 400 K. We used the relatively long 10 ns trajectories to calculate the MSD of ions and accurately define the diffusive regime of MSD plots through computing the well-known β exponent very nearby unit. The linear fitting was performed to determine the slope of MSD in the time range of 3-7 ns. For pure ACN, only a 2 ns trajectory was sufficient to calculate reliable diffusive regime. However, as mentioned above, we also tested the longer (20 ns) trajectories for calculating the MSD and self-diffusion of pure [C6mmim][FSI] at 300 and 400 K (see Figure S1 and Table S1 in the Supporting Information for details). Table 2 and Figure 2 represent the composition dependence of self-diffusion coefficients and the computed MSDs for cations, anions, and ACN molecules over the entire composition range so that the dilution of IL with ACN causes a significant increase in the MSDs, and self-diffusion coefficient values of the ions and ACN, which can be attributed to changes in the cation-cation, anion-anion, ACN-ACN, cation-anion, cation-ACN, and anion-ACN neighboring structural correlations. It further enhances dissociation of nano-cluster aggregated forms of ion pairs, and leads to greater ion mobility (see structural analysis, section 3.2). For instance, in pure IL system (xIL = 1.0) at 300 K, computed D+ and D- are 0.69 and 0.57 (×10-12 m2·s-1), respectively. As xIL is decreased to 0.75 and 0.50 with the addition of ACN, the value of D+ is 2.92 and 5.68 (×10-12 m2·s-1). Also at the same compositions, the value of D- is 2.58 and 5.56 (×10-12 m2·s-1), respectively so that at 300 K, for xIL = 0.75 and 0.50, D+ increases to ~ 4 and 8 times and D- rises up to ~ 4 and 10 times, respectively. As the molecular content more increases (xIL = 0.25 and 0.10), a very significant increment is observed in the discussed diffusion values, whereas at xIL = 0.10, D+ and D- are ~ 525 and 735 times of magnitude higher than the pure IL simulated system 12 ACS Paragon Plus Environment

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unexpectedly. This remarkable trend can be rationalized in this way that in pure IL, [C6mmim]+ and [FSI]- are bound together due to van der Waals and Coulombic interactions, and form large ionic clusters involving the polar networks and non-polar aggregated domains that cause to hindered transport. Addition of ACN concentration leads to the insert of a great number of ACN cosolvent molecules, which are moving significantly faster than the ions, to the interfacial region of polar networks and non-polar domains; hence, the dissociation of ionic clusters result in an abrupt growth of ionic mobility. For simulations at 400 K, the totally same trend is followed. However, the computed enhancement of the D+ and D- of the most dilute mixture with xIL = 0.10 is ~ 39 and 50 times higher than the corresponding diffusion values in pure IL at 400 K. It is worth noting that these ratios are more reliable than those ratios from simulations calculated at 300 K (525 and 735) because of very slow glassy dynamics (β < 1) of pure ILs, especially the ones with a long alkyl side chain and relatively higher melting point. In addition, it is difficult to obtain high accurate dynamical quantities of ILs from equilibrium MD simulations with non-polarizable force fields at low temperature ~ 300 K.74-77 As shown in Table 2 for the pure ACN system (xIL = 0.00) at 300 K, the computed DACN (4499.83 ×10-12 m2·s-1) is in very good agreement with the available experimental values measured by NMR which are 4310, 4340, 4370, 4850, and 5100 (×10-12 m2·s-1) at 298 K.73 As xIL is increased to 0.10 and 0.25, the value of DACN decreases to 1302.30 and 311.15 (×10-12 m2·s-1), respectively, so that DACN decreases to ~ 3.5 and 14.5 times, respectively. As the IL content further increases (xIL = 0.50 and 0.75), a very significant decay is observed in the discussed diffusion coefficient values, 47.53 and 8.93 (×10-12 m2·s-1), and then DACN is ~ 95 and 504 times of magnitude lower than that of the pure ACN simulated system, respectively. For mixture



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simulations at 400 K, the same general trend is followed in the composition dependence of DACN. However, the changes in the relative computed DACN with the composition of mixtures at 400 K are lower than those of at 300 K. According to the data shown in Table 2, temperature dependence of the self-diffusion coefficients is also observed for ions and ACN molecules. The diffusion coefficients of species will be considerably enhanced at higher temperatures (400 K). The translational motion of ACN molecules is several times faster than the ionic species (see Table 2 and Figure 2); however, [C6mmim]+ cations and [FSI]- anions exhibit comparable MSD and ionic self-diffusion relative to each other over the whole studied composition range due to interionic association. Diffusion coefficients of all species alter over 1.5-2 orders of magnitude in the whole studied composition. The D+, D-, and DACN values motivate us to estimate the diffusivity ratios of DACN/D+, DACN/D-, and D-/D+ as a function of the IL molar fraction at 400 K (Figure 3). Similar ratios of self-diffusion coefficients have been recently reported by Marekha et al.73 for four [bmim]+based ILs using experimental 1H NMR measurements. The important observation in Figure 3 is that self-diffusion of [FSI]- rather than that of [C6mmim]+ follows inversion trend at the concentrated and diluted solutions, i.e. at xIL < 0.50, D- constitutes ~ 112-120 % of the D+. In contrast, at xIL ≥ 0.50, D+ is larger. In other words, anions diffuse faster than cations in the diluted IL solutions by ACN, whereas at the concentrated IL solutions, the opposite trend is observed. This phenomenon can be rationalized based on the hypothesis of “hyper anion preference” (HAP) parameter that was first proposed by Chen et al.78. They claimed that in pure ILs and concentrated IL solutions, in addition to the neutral ion associations, the negatively charge aggregates (hyper anions) form, which are enriched with anions and are bigger in size than those with positively charge. Therefore, the lower value of diffusion is reported for anions.


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While at high concentrations of cosolvent, these negatively charge aggregates get smaller and are dissociate to individual anions, resulting in the D-/D+ ratio inverts. We can present here a complementary explanation for the diffusion mechanism of anions with the help of our structural analysis reported below in section 3.2. In the presence of ACN molecules, distinct changes occur in the interactions between the species that impress transport properties. In the pure and concentrated [C6mmim][FSI] IL, the cations and anions are bound together. Addition of ACN leads to simultaneous dissociation of ionic aggregations and there appear correlations between the ions and ACN molecules. It seems that in the ACN-rich region (diluted IL solution with

xIL < 0.50), in addition to the separation of ionic aggregations,

interactions of the nearest ACN-anion neighbors are much weakened compared to those of ACNcation neighbors so that the number of isolated anions is more than that of isolated cations at this condition. Therefore, a significant enhancement is observed in the D-/D+ ratio and in the relative dynamics of [FSI]- anions in diluted [C6mmim][FSI] IL (Figure 3). The results of structural analysis (see section 3.2, Figures 8 and 10) complementary confirm our dynamical observations and the above discussion. The computed diffusivity ratios of DACN/D+ and DACN/D- as a function of the IL molar fraction at 400 K are also showed in Figure 3. For instance, at xIL = 0.10, DACN/D+ and DACN/Dare reported 3.3 and 2.8, respectively. As xIL is increased to 0.25, the ACN molecules diffuse 5.2 and 4.6 times faster than the cations and the anions, respectively. As the IL content further increases, for xIL equal to 0.50 and 0.75, the value of DACN/D+ are ~ 5.3 and 4.8, and the value of DACN/D- are ~ 5.6 and 5.7, respectively. For more understanding the dynamical details of ionic species and ACN molecules at short times,

we

also

calculated

the

normalized

velocity


autocorrelation

function,


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VACF(t ) = 〈 vi (t ).vi (0)〉 / 〈 vi (0).vi (0)〉 , for the centers of mass of the species in the pure IL and binary mixtures at 400 K, and only for pure ACN at 350 K. In definition of the VACF, vi (0) is the initial velocity of the center of mass of the particle i, and vi (t) is its velocity at time t; the brackets 〈 〉 represent an ensemble average over all time origins. The computed VACFs of [C6mmim]+, [FSI]-, and ACN for the binary mixture with xIL = 0.50 at 400 K are compared in Figure 4. As shown, not only mean collision time (first zero), but also velocity randomization time (second zero) are shorter for ACN than those of the ions. The first zero (the x-intercept) in the VACF of ACN, cation, and anion was estimated to be around 0.15, 0.20, and 0.26 ps, respectively. These observations are in agreement with the MSD trend as a result of lower molar mass and charge of ACN in comparison with the ions. After the negative region of VACFs corresponding to cage effects and oscillating of VACFs around zeros, the second zeros, representing the velocity randomization times, are identified around 1 and 1.6 ps for ACN and the ions, respectively. The point here is that the greater number of collisions involving lighter ACN molecules randomizes their velocities sooner than those of the heavier [C6mmim]+ cations and [FSI]- anions. The same trend is also found in the relative second zero times of species at the other studied molar fractions as shown in Figure 5. The effect of the IL molar fraction on the VACFs of [C6mmim]+, [FSI]- and ACN was also investigated. Figure 5 depicts IL composition dependence of VACFs where the mean collision times for cations, anions, and ACN molecules follow a clear trend with xIL: 1.00 ≤ 0.75 < 0.50 < 0.25 < 0.10. But for each species, the velocity randomization time is independent of the xIL. The observed distinction in the mixture composition dependence of the mean collision times may be attributed to this fact that at the IL concentrated mixtures, the number of cations and anions increase, which cause the ions to get closer to each other and to the ACN molecules, result in


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decreasing the mean collision times of species. As a result, for each of the three species, the first zero point occurs in the shorter time at higher molar fractions of [C6mmim][FSI] IL. For instance, the mean collision times for the cations at xIL (equal to 1.00 and 0.75) are observed around 0.17 ps, whereas, the value of this quantity for xIL equal to 0.50, 0.25 and 0.10 is around 0.19, 0.24, and 0.48 ps, respectively. As shown in Figure 5, the first zero times and negative region on the VACF plots cannot be distinguished clearly for the diluted systems with the relatively low molar fraction of IL (xIL = 0.10). On the other hand, the density of the studied system is an important factor that affects the detailed behaviors of VACFs. Figure 5 also shows that there is a direct relation between the depth well of the first minimum of VACF and the density of the studied system. By increasing xIL, and consequently, the density of the system, the mean collision times of all species shift to the shorter times; the first depth well of VACF is enhanced, and the negative region of the VACF plot is clearly identified. In the VACF graph of ACN molecules at the lower panel of Figure 5, it is difficult to detect clearly the mean collision time for pure ACN case, which is a highly diluted solution, and collision of molecules with each other is very rare event at this dilute system. Thus, the negative region and the minimum point in the VACF of pure ACN are approximately eliminated. By increasing the xIL, the density of the mixture increases, which results in shorter first zero times. Ionic conductivity, σ, is another key parameter that we investigated to found the influence of adding ACN as a cosolvent on the transport properties of [C6mmim][FSI] IL, especially for electrochemical applications. The obtained σ values in the case of pure IL and its binary mixtures with ACN at 300 and 400 K are listed in Table 3. The ionic conductivity was estimated by Nernst-Einstein equation:

σ =

∑

ρ q D

(2)



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where, e is the electric charge unit (1.602 ×10-19 C), q is the ion charge, and Di and ρ are the self-diffusion coefficient and density of species i, respectively. The simulation results indicate that the addition of ACN to [C6mmim][FSI] IL makes an increase in the magnitude of ionic conductivity of IL so that its value for the pure IL at 400 K is only 6.43 ×10-3 S·cm-1. On the other hand, ionic conductivity of IL is about 13.53 and 22.07 (×10-3 S·cm-1) for xIL = 0.75 and xIL = 0.50, respectively, meaning that 2 and 3.5 times increase is observed respectively. While the amount of ACN reaches to xIL equal to 0.25 and 0.10, the corresponding values of σ are 46.89 and 98.67 (×10-3 S·cm-1), respectively, and enhancement is remarkable. There is a totally similar trend at 300 K. Whereas at 300 K, σ increases by ~ 4.3 and 8 times upon dilution to xIL equal to 0.75 and 0.50, respectively (Table 3). As mentioned above (in the report of self-diffusion data), the ratios between the dynamical quantities of simulated systems, especially for xIL = 1.00, at 400 K are more reliable than those from the simulations at 300 K. Based on simulation evidences, we can deduce that heating and addition of ACN to [C6mmim][FSI] IL are suitable means to reach good dynamical conditions for electrochemical applications.

3.2. Structural analysis The details of structural arrangements of species and nano-scale structural heterogeneity in [C6mmim][FSI] IL and its binary mixtures with ACN at different xIL were studied by calculating the partial site-site radial distribution functions (RDFs), the snapshots visualization of equilibrated simulation boxes with a color coding scheme, and the spatial distribution functions (SDFs) analysis from MD simulations at 300 K, as shown in Figures 6-12. According to the previous experiments and simulations, the ILs based on 1-alkyl-3methylimidazolium have segregated polar and non-polar structural domains in nano-scale. This


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nano-segregated phenomenon is attributed to the interaction of the alkyl side chains of cations because of the attractive van der Waals interactions and construction of non-polar domains that are separated from the polar network (imidazolium ring and anion) consisting of a high-charge density.52-56 Historically, this idea was extracted from the RDFs of the self (CT–CT) atoms (terminal carbons of the alkyl side chains) that were concluded by MD simulations so that an intense peak was found for (CT–CT) RDFs, showing strong correlations compared to that of the other interactions between carbons in the side chain (Cn–Cn, n = 1,2,3,…). Therefore, the nonpolar domains are constructed due to arranging the terminal groups of alkyl side chains as side to side.54,60 In the present work, similar behavior was observed by calculating (Cn–Cn) RDFs for [C6mmim][FSI] IL as well (see Figure 6a). For this IL, the polar parts are composed of [FSI]- and imidazolium ring of [C6mmim]+ that contains the highest density of cation charge due to electrostatic interactions, whereas aggregation of the alkyl side chains form the non-polar islands. To show this phenomenon clearly, a color coding scheme is utilized for visualization of polar and non-polar parts.53 We used this procedure to visualize the segregation in [C6mmim][FSI] IL at 300 K (Figure 6b). In this color scheme, the polar parts are colored red. They exist as separate structures embedded in the aliphatic regions. The second methylene group of hexyl side is considered as initiator of the non-polar alkyl side chain of the cation that is painted green. The point here is that since dimensions of the simulation box are about 4.1×4.0×4.0 nm, the relative size of the non-polar parts of [C6mmim][FSI] IL is estimated to be around ~ 1-2 nm. According to structural studies of the 1-alkyl-3-methylimidazolium ILs based on [TFSI]-, the strongest structural correlations between the [TFSI]- and imidazolium cations are related to the interactions of the oxygen atoms of anions and the hydrogen atom on the CR atom of the



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imidazolium ring.61,79 In this work, a similar correlation was investigated between the cation and anion in the novel C2 methylated [C6mmim][FSI] IL based on the partial site-site RDF analysis (Figure 7a). A more detailed structural analysis of [C6mmim][FSI] IL/ACN mixtures was also obtained by calculating the RDFs between the selected atomic sites of IL and ACN. The RDFs between the hydrogen site on CW (H4a) in the imidazolium ring of the cations and the oxygen, nitrogen, and flourine atoms in the anions are shown in Figure 7a at 300 K. As can be seen, for the (H4a–O) RDF, a relatively intense peak is identified at 2.5 Å, whereas both of the (H4a–N) and (H4a–F) RDFs show the first peak at around 4.8 Å with a lower intensity. This is an evidence of strong hydrogen bonding between the headgroup of the cation (imidazolium ring) and [FSI]-, which seems to be one of the causes of forming large ionic clusters and increase of viscosity in the IL environment. We also investigated the structural correlations between [C6mmim][FSI] IL and ACN in details. Current MD simulations illustrate the strong structural correlation (i.e., hydrogen bonding) between ACN and the imidazolium ring of the cation, as shown in Figure 7b, by representing RDFs between the nitrogen atom (NC) in the ACN and selected hydrogen atoms of the cation (H4a, H6, HM, HC) for the mixture with xIL = 0.50. These RDFs for other [C6mmim][FSI]/ACN mixtures are roughly similar. Based on these RDFs, the first peak intensities of (NC–X) RDFs were observed as the following trend: X = H4a > H6 > HM > HC. Accordingly, we can remark that a strong structural correlation (and hydrogen bonding) occurs between the ACN molecules and the headgroup of the cation. By comparison of (O–H4a) and (NC–H4a) RDFs in Figure 7, it seems that in the mixtures of [C6mmim][FSI] IL and ACN, the nitrogen atom in ACN and the oxygen atom of [FSI]- are very similar with each other and interact with the headgroup of the cation.


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We further investigated the influence of adding ACN (altering the IL molar fraction) on the interionic, intermolecular, and ion–molecular structural correlations as provided in Figure 8 and Figure S2 of the Supporting Information. As can be seen in Figure 8a, the (NC–N) RDFs representing the structural correlation of ACN–anion neighbors show only one peak around 5.7 Å with relatively higher intensity for the concentrated IL system (xIL = 0.75) compared with that of the other composition mixtures. The decrease of IL content from the IL molar fraction of 0.75 to 0.50 leads to an obvious reduction in the height of (NC–N) peak, and such trend occurs slightly for more dilutions. In Figure 8b, the (NC–CRM) RDFs representing the structural correlation of ACN–cation neighbors show that the reduction of the intensity of peaks with adding ACN is lower than that of (NC–N) RDF case. From the graphs (a) and (b) illustrated in Figure 8 and Figure S2(a), we can conclude that the molecular-ion interactions are affected by the mixture composition, which are gradually reduced by adding ACN. The (NC–CRM) RDFs have three close peaks at around 3.5, 4.6, and 5.4 Å and a broader weak peak with maximum intensity around 9-10 Å. A roughly similar pattern can be observed in the (N–CRM) RDFs (Figure 8f). These (NC–CRM) and (N–CRM) RDF peaks suggest the distinct locations in which an ACN molecule or [FSI]- can interact with the imidazolium ring of the cation. We also detected these regions by the SDF analysis, see below. Figure 8c shows the (NC–NC) RDFs representing the structural correlation between the neighboring ACN molecules in the solutions. Unlike the RDFs for xIL equal to 0.75 and 0.50 in which the intensity of the first (NC–NC) peaks is identical, the RDFs suggest lower peak intensities and weaker ACN–ACN correlations at higher contents of ACN molecules, which can be attributed to decrease of system density; this (NC–NC) RDF pattern indicates that no effective aggregation has occurred between the ACN molecules in all the studied systems. This result is



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consistent with the observations of Bardak et al.80. The point here is that unlike the height of (NC–NC) peaks, the position of RDF peaks is insensitive to the content of ACN. As shown in Figure 8d/e, by representing the (CRM–CRM) and (N–N) RDFs, the structural correlations decrease for the nearest neighbors of cation–cation and anion–anion with adding ACN to [C6mmim][FSI] IL. The first peaks of both of these RDF types are broadened and slightly shifted to the larger distances by increasing the ACN cosolvent, and the second peaks are removed for the most diluted mixture with xIL = 0.10. Especially the (CRM–CRM) RDF shows no cation–cation structural correlation at xIL = 0.10. In contrast, Figure 8f and Figure S2(b) depict increase of the structural correlations between the nearest cation–anion neighbors (resulting in the stabilization of the ion pairs) by representing the change in the (CRM–N) and (H4a–O) RDFs with adding ACN, respectively. The mixture with the high level of ACN content, xIL = 0.10, has the strongest first and second peaks of the (CRM–N) RDF at 3.5 and 5.4 Å, but there is not the third (CRM–N) peak at this mixture as seen in other more concentrated IL mixtures. In contrast of the effect of the mixture composition on the height of the first and second peaks of the (CRM–N) RDF, the height of the third broader weak peak at larger distances around 9-10 Å is gradually reduced by adding ACN. In the three right panels of Figure 8, the variation of blue RDF sets belonging of the system with xIL = 0.10 at distances larger than 10 Å is a good evidence of the strong dissociation of large ionic clusters in diluted IL. This is also shown in the corresponding snapshot of the simulation box in Figure 10. In addition, the locations of the coordination layers of cation–anion pairs representing by the (CRM–N) RDFs are not changed with adding ACN molecules. This is in agreement with the conclusion of Zheng et al.43 and Chaban et al.44 on similar mixtures.


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Based on the (CRM–N) RDFs (Figure 8f), as the concentration of IL decreases to xIL = 0.10, the nearest cations and anions tend to better cooperate with themselves and form neutral ion pairs, which are stabilized by ACN molecules. On the other hand, increasing of ACN causes to the weakening of cation–cation and anion–anion associations so that the species are switched to the isolated form (see Figure 8d/e). According to section (3.1), which illustrates the improvement of dynamical properties at diluted IL mixtures, it can be said that by adding ACN, the number of free ion pairs should be more than that of large ionic clusters, consisting of many associated ion pairs, which are constructed by strong association of polar and non-polar domains. These results are consistent with the findings of Chaban et al.44. In Figure 9, the (NC–X) and (CC–X) RDFs (CC corresponds to the carbon atom of methyl group in ACN) are plotted for the mixture with xIL = 0.50; where, X represents the different atomic sites on the alkyl side chain or imidazolium ring of [C6mmim]+. In Figure 9a, it is observed that the nitrogen atom (NC) of ACN has great affinity to the polar network of cation in the entire range of composition such that the probability of finding the nitrile (C≡N) group of ACN near the cationic part of the polar domains (imidazolium ring) is higher than that of the non-polar domains. This fact is deduced from the high intensity of the first peak of (NC–H4a) RDF in shorter distance compared to that of the other three RDF types. As can be seen, the maximum intensity of the first peak of (NC–H4a) RDF is located around 2.7 Å. But that of the (NC–CT), (NC–CRM), and (NC–C2) RDFs is estimated around 3.9, 3.5, and 3.7 Å, respectively. The (CC–CT) RDF (Figure 9b) exhibits strong first peak, and its intensity is comparable with those of the (CC–C2), (CC–H4a), and (CC–CRM) RDFs. This behavior can be attributed to this fact that the alkyl side chain of cation has a low-charge density (non-polar part of IL). Therefore, the methyl group of ACN inclines to the non-polar regions of the cation. As revealed in Figure 9, the



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ACN molecules appear between the polar and non-polar domains that can rationalize miscibility with many compounds such as imidazolium-based ILs. These observations are consistent with the reported results of Pádua et al.,59 Shimizu et al.,60 and Bardak et al.80. The (NC–X) and (CC– X) RDFs at various molar fractions of IL are also showed in Figure S3 of the Supporting Information. The location of each type of RDF peak is not changed with changing the molar fraction of IL. In order to do more structural analysis consistent with dynamical finding of the effect of adding ACN to IL, we investigated the morphology of the segregated polar networks and nonpolar domains of [C6mmim][FSI] IL in the studied systems. The color coding of the snapshots in the simulation boxes of equilibrated systems with the xIL equal to 1.00, 0.75, 0.50, 0.25, and 0.10 at 300 K is shown, without ACN molecules to simplify viewing, in Figure 10 and along with the ACN molecules in Figure S4 of the Supporting Information. According to these simulation boxes, the most important extracted point is that there is a strong structural correlation between the ions in the pure [C6mmim][FSI] IL, which construct large ionic aggregations (clusters). As described in section (3.1), addition of ACN causes to the simultaneous dissociation of ionic aggregations, and based on the RDFs (Figure 8d and 8e), the correlations between the cationcation and anion-anion neighbors are further weakened. So it seems that when the molar fraction of the studied IL gradually decreases from 1.00 to 0.10 (Figure 10), the dissociation of ionic clusters occurs while, at xIL = 0.10, the size of ionic clusters effectively reduces, and they convert into thinner scattered parts between the ACN molecules. To explain further, we analyzed the tail-tail hexyl side chain (CTHT3) distribution by calculating (CT–CT) RDF for the systems with different molar fractions of IL at 300 K (Figure 11a). Interestingly, at xIL = 1.00, (CT–CT) RDF shows an intense peak with the height of 2.6 that


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suggests relatively more aggregation of the terminal methyl group of hexyl chains compared to that of the other molar fractions of IL. Whereas dilution of IL by ACN molecules to xIL equal to 0.50, 0.25, and 0.10 leads to weakening of (CT–CT) correlations so that the lowest affinity of these tail groups to each other is observed for xIL = 0.10. We can claim that at the high level of ACN content, the non-accumulated hexyl side chains find similar pattern in short length alkanelike domains. For more clarification, the simplified pattern of the obtained snapshots of the microscopic structure of corresponding simulation boxes are shown in Figure 11b. Only the terminal methyl groups of the hexyl side chains of imidazolium cations are showed for better clarification. The SDFs analyses were used for gaining more insight into the average arrangements of the center of masses of the nearest neighboring anions and ACN molecules in the surroundings of a reference [C6mmim]+ in the case of xIL = 0.50 at 400 K. The different view sides of SDFs are shown in Figure 12 where the blue and gray density clouds belong to the spatial distribution of the nearest ACN and [FSI]- anions around a reference cation, respectively. These SDFs are calculated from analyzing long 10 ns trajectories by the Travis code81 and visualizing the output using VMD82. As can be seen in the first row of Figure 12, the main regions with high blue probability density of the center of mass of ACN molecules around the [C6mmim]+ reference at the first shell are strongly localized around the ring of cation in out-of-plane (both above and below the plane of the imidazolium ring), which is connected with two other parts of the U-shaped blue density clouds around H4a and the region between the two methyl side chains of [C6mmim]+. Additionally, the region between the two CW sites of ring, and finally, the region between the 2methyl side chain (CCR) and the first –CH2- group of hexyl side chain are the other regions of the



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reference cation surrounded by blue density cloud of the nearest neighbor ACN molecules. This observation confirms the tendency of the nitrogen atom of ACN molecules (with partially negative charge) to associate with the positive regions of [C6mmim]+ cations. Similar trends are generally observed from the (NC–X) and (NC–CRM) RDF's analysis as shown in Figures 7b and 8b, respectively. The second row of Figure 12 shows different views of the SDF of the center of masses of the anions around the reference cation, represented by the separated capping gray probability density clouds. A similar SDF pattern of the anions around the reference cation was reported by Zhang and Maginn for [C4mmim][PF6] IL.20 The SDF of [FSI]- anions around the reference cation has roughly similar high probability regions in comparison with the SDF of ACN molecules around the cation, and is compatible with the (CRM–N) RDF peaks as shown in Figure 8f. The different view sides of both density clouds of the anions and ACN molecules around the reference cation can be seen in the last row of Figure 12. The localized high capping gray density regions of the anions around the imidazolium ring of the reference cation are separate from each other, whereas at the same distances the corresponding U-shaped blue density clouds of ACN molecules around the reference cation are connected to each other and also to the distinct localized gray density regions of the anions around the ring of the reference cation (see the last row of Figure 12). This is another evidence for the stabilization role of ACN molecules on the close contact ion pair association. Besides, a part of the blue density cloud of ACN is observed in the region between the two CW sites of the ring; however, the corresponding part of the gray density cloud of anions slightly shifts from the inter-region of the two CW sites of the ring to the broader region below the H5a site and near the hexyl side chain of the reference cation.


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4. Conclusion In the current study, extensive MD simulations of [C6mmim][FSI] IL and its binary mixtures with ACN were performed for the first time in order to find the influence of gradual adding ACN on the fine-tuning of dynamical behavior, self-diffusion, ionic conductivity, and structural properties of the IL at the molecular level. Analysis of the obtained quantitative and qualitative dynamical properties, especially the MSDs, demonstrated that upon addition of ACN, an enhancement of the ion translational motion in IL occurs as a consequence of weakened structural correlations between the cation-cation and anion-anion neighbors in the solutions, which leads to the break of large ionic clusters to smaller domains and isolated ion pairs. Whereas both the radial and spatial distribution functions showed the stabilizing role of ACN on the close contact ion pair association. The net effect is a significant improvement of the self-diffusion and ionic conductivity behavior of [C6mmim][FSI] IL upon mixing with ACN; consequently, this IL/ACN mixture can be introduced as a good tunable solvent medium and efficient electrolyte for organic synthesis and electrochemical applications. In neat IL and concentrated IL mixtures, the self-diffusion of cation is higher than the corresponding anion value, but an inverse expected trend was identified for diluted IL with xIL < 0.50. It seems that in the ACN–rich region, interactions of the nearest ACN–anion neighbors are more weakened than those of ACN–cation neighbors so that the number of isolated anions is more than that of isolated cations at this condition. The VACF analysis indicated the inverse relation between the xIL and the mean collision time of each species. By increasing xIL, the first depth well of VACF is enhanced, and the negative region of the VACF is clearly identified.



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Besides, at a fixed xIL, both the mean collision time and the velocity randomization time of ACN are shorter than those of the ions. For the first time in this study, with the combination of refined flexible all-atom force fields for ACN, we successfully improved the calculated numerical value of the self-diffusion coefficient of this solvent in good agreement with the experimental data. We further investigated the structural properties of [C6mmim][FSI] IL and its binary mixtures with ACN by calculating the RDFs, SDFs, and the snapshots visualization of equilibrated simulation boxes with a color coding scheme. Structural analysis predicted the nano-segregation for all systems; however obvious differences were found in the morphology of polar and nonpolar domains as a function of ACN concentration so that when the xIL decreases from 1.00 to 0.10, the polar domains scattered and the non-polar domains decomposed and reduced due to affinity reduction of the side by side orientation of hexyl side chains. The lowest affinity was observed between the tail–tail hexyl side chains of the cations at xIL = 0.10. Based on the RDF results, we concluded that the nitrile group of ACN has a great affinity to the imidazolium ring of the cation belonging to the polar network, and the methyl group of ACN inclines to the hexyl side chain of the cation belonging to the non-polar domains. Thus in the binary mixtures, the ACN molecules reside as a bridge between the polar and non-polar domains in agreement with previous MD simulations. In future work, we will study the thermodynamic properties of the IL/ACN mixtures as a function of temperature and the IL molar fraction.

ASSOCIATED CONTENT


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Supporting Information. A figure showing the effect of simulation length run on the calculated MSD of the ions at 300 and 400 K, the calculated (NC–H4a), (O–H4a), (NC–X), and (CC–X) RDFs at various IL molar fractions at 300 K, the snapshots of equilibrated simulation boxes for xIL = 1.00, 0.75, 0.50, 0.25, and 0.10 at 300 K, and a table displaying the effect of length run on the calculated ionic self-diffusion coefficient for pure [C6mmim][FSI] IL from the simulations with 10 ns and extended 20 ns time length runs at 300 and 400 K. “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] and [email protected]. Tel.: +98 24 3315 3207. Fax: +98 24 3315 3232.

ACKNOWLEDGMENTS The support for this work by the Department of Chemistry of the Institute for Advanced Studies in Basic Sciences (IASBS) are gratefully acknowledged. M. H. K. also acknowledges support from the Center for Research in Climate Change and Global Warming (CRCC) in IASBS.



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REFERENCES [1] Keskin, S.; Kayrak-Talay, D.; Akman, U.; Hortaçsu, Ö. A Review of Ionic Liquids Towards Supercritical Fluid Applications. J. Supercrit. Fluids 2007, 43, 150-180. [2] Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-Liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621-629. [3] Feng, R.; Zhao, D.; Guo, Y. Revisiting Characteristics of Ionic Liquids: A Review for Future Application Development. J. Environmental Protection 2010, 1, 95-104. [4] Phung Le, M. L.; Cointeaux, L.; Strobel, P.; Leprêtre, J-C.; Judeinstein, P.; Alloin, F. Influence of Solvent Addition on the Properties of Ionic Liquids. J. Phys. Chem. C 2012, 116, 7712-7718. [5] Diaw, A.; Chagnes, A.; Carre, B.; Willmann, P.; Lemordant, D. Mixed Ionic Liquid as Electrolyte for Lithium Batteries. J. Power Sources 2005, 146, 682-684. [6] Hogan Jr, C. J.; de la Mora, J. F. Tandem Ion Mobility-Mass Spectrometry (IMS-MS) Study of Ion Evaporation from Ionic Liquid-Acetonitrile Nanodrops. Phys. Chem. Chem. Phys. 2009, 11, 8079-8090. [7] Kobrak, M. N. Characterization of the Solvation Dynamics of an Ionic Liquid via Molecular Dynamics Simulation. J. Chem. Phys. 2006, 125, No. 064502. [8] Chaban,V. V.; Prezhdo, O. V. How Toxic Are Ionic Liquid/Acetonitrile Mixtures?. J. Phys. Chem. Lett. 2011, 2, 2499-2503. [9] Andriko, Y. O.; Reischl, W.; Naure, G. E. Trialkyl-Substitude Imidazolium-Based Ionic Liquids for Electrochemical Applications: Basic Physicochemical Properties. J. Chem. Eng. Data 2009, 54, 855-860.


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Page 40 of 75

Tables:

Table 1.The simulation box details (xIL, total [C6mmim][FSI] ion pairs, ACN molecules, atoms, and sizes) and the van der Waals cutoff distances, RCutoff, of the studied systems at 400 K. The RCutoff and the box size for pure ACN is only reported at 300 K. xIL

Nion pairs

NACN

Total atoms

box sizes (X,Y, Z) / Å

Rcutoff / Å

0.00 (pure ACN)

-

150

900

23.91 × 23.91 × 23.91

11.5

0.10

48

432

4656

39.14 × 48.99 × 35.88

17.0

0.25

48

144

2928

33.43 × 39.00 × 29.25

14.0

0.50

150

150

7350

44.17 × 44.48 × 44.48

18.5

0.75

150

50

6750

42.60 × 42.89 × 42.89

18.5

1.00

150

-

6450

42.19 × 41.55 × 41.55

18.5


Page 41 of 75

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Table 2. Simulated self-diffusion coefficients, D in 10 m2·s-1, for ions and ACN molecules from the slope of MSD plots in various IL molar fractions of the studied ([C6mmim][FSI]/ACN) systems at 300 and 400 K. The values of β are presented in parentheses.

300 K

400 K

system

D+ (β)

D- (β)

DACN (β)

D+ (β)

D- (β)

DACN (β)

xIL= 0.00

-

-

4499.83(1.03)

-

-

-

xIL= 0.10

362.56(0.99)

418.93(1.00)

1302.30(1.00)

1384.50(1.01)

1659.18(1.03)

4675.50(1.00)

xIL= 0.25

62.16 (1.00)

63.45 (0.98)

311.15(1.00)

375.36 (0.94)

422.50 (1.00)

1958(1.01)

xIL= 0.50

5.68 (0.89)

5.56 (0.99)

47.53(1.02)

142.69 (0.97)

136.26 (0.99)

760.33(0.99)

xIL= 0.75

2.92 (0.99)

2.58 (0.97)

8.93(0.74)

82.08 (1.00)

69.76 (0.98)

395.25(1.02)

xIL= 1.00

0.69 (0.90)

0.57 (0.87)

-

34.89 (0.98)

32.69 (1.02)

-



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Table 3. Estimated Nernst-Einstein ionic conductivity, , in various IL molar fractions of the studied ([C6mmim][FSI] / ACN) systems at 300 and 400 K.

IL molar fraction (xIL)

(in 10 S·cm-1) 300 K

400 K

0.10

37.86

98.67

0.25

10.69

46.89

0.50

1.27

22.07

0.75

0.69

13.53

1.00

0.16

6.43


Page 42 of 75

Page 43 of 75

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Figure Captions Figure 1. Schematic representation of (a) 1-hexyl-2,3-dimethylimidazolium cation, [C6mmim]+, (b) bis(fluorosulfonyl)imide anion, [FSI]-, and (c) acetonitrile molecule, ACN with the atomic labels based on the force field used in this study. Figure 2. Variations of the center of mass MSD of [C6mmim]+, [FSI]-, and ACN molecule in the binary mixtures with different molar fractions of IL at 400 K. Presence of ACN molecules improves mobility of the ions. Figure 3. Ratio of self-diffusion coefficients for [C6mmim]+, [FSI]-, and ACN molecule in [C6mmim][FSI]/ACN mixtures as a function of the IL molar fraction. At xIL < 0.50, the anions diffuse faster than the cations in the solution. The inset shows the self-diffusion coefficients of individual species. Figure 4. The calculated VACF for the center of mass of [C6mmim]+, [FSI]-, and ACN molecule at 400 K and xIL=0.50. Each VACF is calculated by averaging of five trajectories. Figure 5. The effect of IL molar fraction on the center of mass VACF of [C6mmim]+, [FSI]-, and ACN molecule at 400 K, except the VACF for pure ACN, which was calculated at 350 K. Figure 6. (a) The calculated self (Cn–Cn) RDFs of the selected carbon atoms of the hexyl side chains of pure [C6mmim][FSI] IL at 300 K. (b) The snapshot of equilibrated simulation box including the [C6mmim][FSI] IL. Polar networks (containing imidazolium ring with methyl side chains of cations and the anions) and non-polar domains (including hexyl side chains of the cations) are colored red and green, respectively. Figure 7. (a) Comparison of the RDFs between the hydrogen site on CW (H4a) in the imidazolium ring of the cations, and oxygen, nitrogen, flourine atoms in the anions. This is evidence of strong hydrogen bonding between the imidazolium ring of the cations and the



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oxygen sites of [FSI]- anions. (b) Calculated RDFs between the nitrogen atom (NC) of ACN molecules and selected hydrogen atoms of the cations (H4a, H6, HM, HC) for xIL = 0.5 from MD simulations at 300 K. It seems that the strong hydrogen bonding occurs between the ACN molecules as well as the anions and the headgroup (imidazolium ring) of the cations. Figure 8. RDFs between the (a) ACN–anion, (b) ACN–cation, (c) ACN–ACN, (d) cation– cation, (e) anion–anion, and (f) cation–anion neighbors to investigate the influence of adding ACN on the various structural correlations between the species in the concentration range of 0 ≤ xIL ≤ 1 at 300 K. Figure 9. The RDFs of (a) NC–X, and (b) CC–X (NC and CC corresponding to the nitrogen atom of nitrile group and the carbon atom of methyl group in ACN) at xIL = 0.50. X represents the different atom sites on the hexyl side chain and the imidazolium ring of the cation. The nitrogen atom of ACN is found near the polar H4a site of the imidazolium ring, and the methyl group of ACN has strong structural correlation with the terminal non-polar CT sites of hexyl side chains so that the ACN molecules appear as a bridge between the polar networks and non-polar domains. Figure 10. The snapshots of equilibrated simulation boxes including xIL = 1.00, 0.75, 0.50, 0.25, and 0.10 at 300 K. Polar networks (containing imidazolium rings with methyl side chains of cations and the anions) and non-polar domains (including hexyl side chains of the cations) are colored red and green, respectively. To simplify viewing, the ACN molecules are not shown. By adding ACN, the polar networks become scattered and the non-polar domains decomposed and reduced in size. Figure 11. (a) The RDF, g CT–CT (r), between the terminal carbon atoms of the hexyl side chains of cations as a function of the IL molar fraction at 300 K. (b) Comparison of the simplified representative structures of [C6mmim][FSI] and its binary mixtures with ACN in the case of xIL =


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1.00, 0.50, 0.25, and 0.10 at 300 K. Only the terminal CTH3 groups of the hexyl side chains of cations are shown. Increasing the number of ACN molecules results in the decrease of affinity of the terminal methyl groups to each other. Figure 12. Calculated SDFs of the center of masses of ACN (blue density clouds) and [FSI](gray density clouds) around [C6mmim]+ as the reference cation in the case of xIL = 0.50 at 400 K. Two first panels represent different views of the ACN and [FSI]- anion's probability around the reference cation separately, whereas the last row shows both the ACN and [FSI]- density clouds around the reference cation.



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Figure 1. Schematic representation of (a) 1-hexyl-2,3-dimethylimidazolium cation, [C6mmim]+, (b) bis(fluorosulfonyl)imide anion, [FSI]-, and (c) acetonitrile molecule, ACN with the atomic labels based on the force field used in this study.


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Figure 2. Variations of the center of mass MSD of [C6mmim]+, [FSI]-, and ACN molecule in the binary mixtures with different molar fractions of IL at 400 K. Presence of ACN molecules improves mobility of the ions.



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Figure 3. Ratio of self-diffusion coefficients for [C6mmim]+, [FSI]-, and ACN molecule in [C6mmim][FSI]/ACN mixtures as a function of the IL molar fraction. At xIL < 0.50, the anions diffuse faster than the cations in the solution. The inset shows the self-diffusion coefficients of individual species.


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Figure 4. The calculated VACF for the center of mass of [C6mmim]+, [FSI]-, and ACN molecule at 400 K and xIL=0.50. Each VACF is calculated by averaging of five trajectories.



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Figure 5. The effect of IL molar fraction on the center of mass VACF of [C6mmim]+, [FSI]-, and ACN molecule at 400 K, except the VACF for pure ACN, which was calculated at 350 K. 50 ACS Paragon Plus Environment

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Figure 6. (a) The calculated self (Cn–Cn) RDFs of the selected carbon atoms of the hexyl side chains of pure [C6mmim][FSI] IL at 300 K. (b) The snapshot of equilibrated simulation box including the [C6mmim][FSI] IL. Polar networks (containing imidazolium ring with methyl side chains of cations and the anions) and non-polar domains (including hexyl side chains of the cations) are colored red and green, respectively. 51 ACS Paragon Plus Environment


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Figure 7. (a) Comparison of the RDFs between the hydrogen site on CW (H4a) in the imidazolium ring of the cations, and oxygen, nitrogen, flourine atoms in the anions. This is evidence of strong hydrogen bonding between the imidazolium ring of the cations and the oxygen sites of [FSI]- anions. (b) Calculated RDFs between the nitrogen atom (NC) of ACN molecules and selected hydrogen atoms of the cations (H4a, H6, HM, HC) for xIL = 0.5 from MD simulations at 300 K. It seems that the strong hydrogen bonding occurs between the ACN molecules as well as the anions and the headgroup (imidazolium ring) of the cations.


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Figure 8. RDFs between the (a) ACN–anion, (b) ACN–cation, (c) ACN–ACN, (d) cation– cation, (e) anion–anion, and (f) cation–anion neighbors to investigate the influence of adding ACN on the various structural correlations between the species in the concentration range of 0 ≤ xIL ≤ 1 at 300 K.



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Figure 9. The RDFs of (a) NC–X, and (b) CC–X (NC and CC corresponding to the nitrogen atom of nitrile group and the carbon atom of methyl group in ACN) at xIL = 0.50. X represents the different atom sites on the hexyl side chain and the imidazolium ring of the cation. The nitrogen atom of ACN is found near the polar H4a site of the imidazolium ring, and the methyl group of ACN has strong structural correlation with the terminal non-polar CT sites of hexyl side chains so that the ACN molecules appear as a bridge between the polar networks and non-polar domains.


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Figure 10. The snapshots of equilibrated simulation boxes including xIL = 1.00, 0.75, 0.50, 0.25, and 0.10 at 300 K. Polar networks (containing imidazolium rings with methyl side chains of cations and the anions) and non-polar domains (including hexyl side chains of the cations) are colored red and green, respectively. To simplify viewing, the ACN molecules are not shown. By adding ACN, the polar networks become scattered and the non-polar domains decomposed and reduced in size.



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Figure 11. (a) The RDF, g CT–CT (r), between the terminal carbon atoms of the hexyl side chains of cations as a function of the IL molar fraction at 300 K. (b) Comparison of the simplified representative structures of [C6mmim][FSI] and its binary mixtures with ACN in the case of xIL = 1.00, 0.50, 0.25, and 0.10 at 300 K. Only the terminal CTH3 groups of the hexyl side chains of cations are shown. Increasing the number of ACN molecules results in the decrease of affinity of the terminal methyl groups to each other.


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Figure 12. Calculated SDFs of the center of masses of ACN (blue density clouds) and [FSI](gray density clouds) around [C6mmim]+ as the reference cation in the case of xIL = 0.50 at 400 K. Two first panels represent different views of the ACN and [FSI]- anion's probability around the reference cation separately, whereas the last row shows both the ACN and [FSI]- density clouds around the reference cation.



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Table of Contents (TOC) Image


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Figure 1. Schematic representation of (a) 1-hexyl-2,3-dimethylimidazolium cation, [C6mmim]+, (b) bis(fluorosulfonyl)imide anion, [FSI]-, and (c) acetonitrile molecule, ACN with the atomic labels based on the force field used in this study. Figure 1 263x236mm (96 x 96 DPI)

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Figure 4. The calculated VACF for the center of mass of [C6mmim]+, [FSI]-, and ACN molecule at 400 K and xIL=0.50. Each VACF is calculated by averaging of five trajectories. Figure 4 143x121mm (300 x 300 DPI)


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Figure 5. The effect of IL molar fraction on the center of mass VACF of [C6mmim]+, [FSI]-, and ACN molecule at 400 K, except the VACF for pure ACN, which was calculated at 350 K. Figure 5 132x206mm (300 x 300 DPI)



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Figure 6. (a) The calculated self (Cn–Cn) RDFs of the selected carbon atoms of the hexyl side chains of pure [C6mmim][FSI] IL at 300 K. (b) The snapshot of equilibrated simulation box including the [C6mmim][FSI] IL. Polar networks (containing imidazolium ring with methyl side chains of cations and the anions) and nonpolar domains (including hexyl side chains of the cations) are colored red and green, respectively. Figure 6 229x264mm (96 x 96 DPI)


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Figure 7. (a) Comparison of the RDFs between the hydrogen site on CW (H4a) in the imidazolium ring of the cations, and oxygen, nitrogen, flourine atoms in the anions. This is evidence of strong hydrogen bonding between the imidazolium ring of the cations and the oxygen sites of [FSI]- anions. (b) Calculated RDFs between the nitrogen atom (NC) of ACN molecules and selected hydrogen atoms of the cations (H4a, H6, HM, HC) for xIL = 0.5 from MD simulations at 300 K. It seems that the strong hydrogen bonding occurs between the ACN molecules as well as the anions and the headgroup (imidazolium ring) of the cations. Figure 7 1029x359mm (96 x 96 DPI)



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Figure 8. RDFs between the (a) ACN–anion, (b) ACN–cation, (c) ACN–ACN, (d) cation–cation, (e) anion– anion, and (f) cation–anion neighbors to investigate the influence of adding ACN on the various structural correlations between the species in the concentration range of 0 ≤ xIL ≤ 1 at 300 K. Figure 8 260x215mm (300 x 300 DPI)


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Figure 10. The snapshots of equilibrated simulation boxes including xIL = 1.00, 0.75, 0.50, 0.25, and 0.10 at 300 K. Polar networks (containing imidazolium rings with methyl side chains of cations and the anions) and non-polar domains (including hexyl side chains of the cations) are colored red and green, respectively. To simplify viewing, the ACN molecules are not shown. By adding ACN, the polar networks become scattered and the non-polar domains decomposed and reduced in size. Figure 10 379x269mm (96 x 96 DPI)


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Figure 11. (a) The RDF, g CT–CT (r), between the terminal carbon atoms of the hexyl side chains of cations as a function of the IL molar fraction at 300 K. (b) Comparison of the simplified representative structures of [C6mmim][FSI] and its binary mixtures with ACN in the case of xIL = 1.00, 0.50, 0.25, and 0.10 at 300 K. Only the terminal CTH3 groups of the hexyl side chains of cations are shown. Increasing the number of ACN molecules results in the decrease of affinity of the terminal methyl groups to each other. Figure 11 696x414mm (96 x 96 DPI)



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Table of Contents (TOC) Image Table of Contents (TOC) Image 84x43mm (96 x 96 DPI)



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Figure S1. Investigation of the effect of simulation length run on the calculated center of mass MSD of [C6mmim]+ and [FSI]- at 300 and 400 K. Figure S1 252x172mm (300 x 300 DPI)


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Figure S3. The RDFs of NC–X (a-d), and CC–X (e-h) (NC and CC corresponding to the nitrogen atom of nitrile group and the carbon atom of methyl group in ACN) at various molar fractions of [C6mmim][FSI] IL. X represents the different atom sites on the hexyl side chain and the imidazolium ring of the cation. The nitrogen atom of ACN is found near the polar H4a site of the imidazolium ring, and the methyl group of ACN has strong structural correlation with the terminal non-polar CT sites of hexyl side chains so that the ACN molecules appear as a bridge between the polar networks and non-polar domains. Figure S3 279x215mm (300 x 300 DPI)


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Figure S4. The snapshots of equilibrated simulation boxes for xIL = 1.00, 0.75, 0.50, 0.25, and 0.10 at 300 K. Polar networks of [C6mmim][FSI] IL (containing imidazolium ring with methyl side chains of cations and the anions) and non-polar domains (including hexyl side chains of the cations), and the methyl and the nitrile groups of ACN are colored red, green, yellow, and blue, respectively. The presence of ACN molecules leads to structural variations in the [C6mmim][FSI] IL; the polar networks are scattered and the non-polar domains decomposed and reduced in size. Figure S4 382x272mm (96 x 96 DPI)


Tracing Dynamics, Self-Diffusion, and Nanoscale Structural

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