Elsevier

Chemical Physics

Volume 542, 1 February 2021, 111051
Chemical Physics

Molecular-level insights into composition-dependent structure, dynamics, and hydrogen bonds of binary ionic liquid mixture from molecular dynamics simulations

https://doi.org/10.1016/j.chemphys.2020.111051Get rights and content

Abstract

In this work, we have performed a series of molecular dynamics (MD) simulations to explore structure, dynamics, and hydrogen bonds (HBs) of the imidazolium-based ionic liquid (IL) mixture containing [Emim][BF4]x[NTF2](1−x), where the molar fraction x is 0.0, 0.25, 0.50, 0.75, and 1.0, respectively. Our simulation results demonstrate that the association extent between cations and both anions become weaker and weaker as the concentration of [BF4] anion increases due to the weakened interactions between them. Meanwhile, all ions in the IL mixture are found to diffuse faster at the higher concentration of [BF4] anion while the order of diffusion rate is always [Emim]+ > [BF4] > [NTF2] owing to different molecular weights regardless of the composition. Furthermore, the weakened HBs between cations and anions are found to be at the higher concentration of [BF4] anion, leading to a faster rotation for all ions in the IL mixture. Compared to the diffusion rates among different ions, however, there is unexpectedly a much larger difference in their rotation rates with the fixed order of [BF4] > [Emim]+ > [NTF2], where the rotational relaxation times of [Emim]+ and [NTF2] are much more than that of [BF4] by at least one order of magnitude. This can be attributed to that the rotational motions of spherical [BF4] anions only require the transient HB breakage with cations so that their rotations should be affected by the continuous HB strength, which is significantly different from those of both [Emim]+ and [NTF2] dominated by the intermittent HB strength. Our simulation results provide a molecular-level understanding composition-dependent structure, dynamics, and HBs in IL mixtures.

Introduction

Room temperature ionic liquids (ILs) have attracted considerable attention in numerous fields during the past years, including heterogeneous catalysis, nanoparticle synthesis, extraction, electrochemistry, and gas capture [1], [2], [3], [4], [5], [6], [7], [8], [9]. This is mainly due to their unique physiochemical properties such as high viscosity, high thermal stability, negligible vapor pressure, and large electrochemical window, to name just a few [10]. Furthermore, the physiochemical properties of ILs can be tuned by combining appropriate cations and anions, and therefore ILs are also called “designer solvents”. Despite the extraordinary properties of ILs, the low conductivity and high viscosity of ILs largely restrict their potential applications in electrochemistry [11]. It is reported that the mixing of ILs with known properties is an economical way to improve their performance in electrochemistry. For example, some works pointed out that the mixture of [Emim][BF4] and [Emim][NTF2] ([Emim]+= 1-ethyl-3-methylimidazolium, [BF4]=tetrafluoroborate, and [NTF2]= bis((trifluoromethane)-sulfonyl)imide) with well-tailored properties can act as an optimal electrolyte in supercapacitors, owing to their outstanding capacity in extending the electrochemical window and improving charge storage density [12], [13], [14], [15]. More importantly, IL mixtures relative to pure ILs have more ability to design desirable physical and chemical properties for specific applications by tuning types and proportions of ILs [16], [17], [18]. However, there would generate complex structure in IL mixtures when combining distinct pure ILs. In this context, it is urgently necessary to relate the complex structure of distinct IL mixtures to their properties.

At present, a large number of experimental studies have been performed to explore properties, such as viscosity, self-diffusion coefficient, excess molar volume, and HB dynamics, in IL mixtures with one common cation but different anions [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. For example, in the binary IL mixture of 1-buthyl-3-methylimidazolium bis(tri-fluoromethanesulfonyl)imide ([Bmim][NTF2]) and 1-buthyl-3-methylimidazolium dimethyl phosphate ([Bmim][Me2PO4]), Matthews et al. [19] showed that, compared to the [NTF2] anions, it is favorable for [Me2PO4] anions to form HB with the H2 atom on the imidazolium cations and thereby there is an apparent difference in their HB lifetimes. On the contrary, there isn’t any preference of forming HBs between the imidazolium cations and the triflate ([OTF]) and [NTF2] anions [20]. On the other hand, Cha and Kim investigated the HBs in the binary mixtures of imidazolium-based ILs via IR and NMR spectroscopy, and their results demonstrated that the number of HBs between the cation and different anions is proportional to the anion concentration in the mixture during the IR timescale [21]. Such unique properties of IL mixtures are supposed to result from the balance of several intermolecular forces, including coulombic, van der Waals, and HBs interactions [19]. Prior study pointed out that the structure of IL mixtures with a range of compositions are dominated by the Coulombic interactions [22]. Meanwhile, the nature of HB is also crucial in determining the microscopic properties, especially for the mixture containing various anions. However, few experiments can provide direct and adequate information about structural features and interaction mechanisms in IL mixtures.

Molecular dynamic (MD) simulation, as a powerful analysis tool, has been widely employed to provide in-depth understanding of interaction mechanism in IL mixtures containing one common cation [31], [32], [33], [34], [35], [36], [37]. For example, Rayal and Balasubramanian [31] conducted MD simulations to investigate two different binary ILs (i.e., [Bmim][Cl]x[PF6](1−x) and [Bmim][BF4]x[PF6](1−x)) with various proportions of the anions. Their simulation results demonstrated that there is a big difference in the organization manner of anions around the cations, which mainly attributes to the proportion and radii size of anions. They further revealed that the preference of both [Cl] and [PF6] anions forming strong HBs with the most acidic proton of the imidazolium ring increases as the concentration of [PF6] anions increases in the [Bmim][Cl]x[PF6](1−x) system. Nevertheless, it is worth noting that the preference distinction in the [Bmim][BF4]x[PF6](1−x) system is negligible because of their smaller differences for two anions. In addition, Lepre et al. [32] explored the properties of 1-buthyl-3-methylimidazolium tricyanomethanide ([Bmim][C(CN)3]) and 1-buthyl- 3-methylimidazolium acetate (Bmim] [OAC]) IL mixture and they observed negative enthalpy and positive molar volume in the mixture system, which can be attributed to a rearrangement of the HB network of the mixture that favors the interaction of the acetate anion with the imidazolium cation at position C2. Kapoor and Shah [33] performed MD simulations to explore the translational motions of two binary IL mixtures ([Bmim][Cl] + [C4mim][MeSO4] ([MeSO4] = methylsulfate) and [Bmim][Cl] + [Bmim][NTF2]). They found that the diffusion coefficients of all ionic species decreased upon increasing the concentration of [Cl] anions, indicating their dynamical properties are closely related to the composition in the mixtures. Despite the progress in understanding the behaviors of IL mixtures from experiments and simulations, a complete understanding of the effect of various molar composition of anion in IL mixtures on the structure, dynamics, as well as the HBs properties has not been well established up to now.

To this end, here a series of MD simulations were performed with the objective of getting insight into the effect of composition of anions on the structure, dynamics, and HBs of the [Emim][BF4]x[NTF2](1−x) IL mixture, wherein x covers five different molar fractions (0, 0.25, 0.50, 0.75, and 1.0). Besides, we want to mention that the reason of choosing these particular systems is that: (i) both of them are widely ILs due to their superior performances; (ii) the two anions were selected based on the larger differences of their sizes and their abilities to form HBs with cation; (iii) they have optimal performances as electrolytes in supercapacitors. The primary goal of this work is to demonstrate the influence of various composition for anions on the structure, dynamics, and HBs properties at a molecular-level. This paper is organized as follows: In Section 2, we present the details of MD simulations. Then, the simulation results are shown and discussed in Section 3. Finally, we offer a few general conclusions in Section 4.

Section snippets

Simulation details

In this work, the MD simulations were carried out to explore the properties of a series of binary IL mixture consisting of [Emim][BF4] and [Emim][NTF2] with various molar fraction. The IL mixture can be expressed as [Emim][BF4]x[BF4](1−x), with the molar fraction × varying from 0.0 (corresponding to the pure [Emim][NTF2]) to 1.0 (corresponding to the pure [Emim][BF4]) with increment every 0.25. For each molar fraction system, the IL mixture containing 512 pairs of cations and anions was

Results and discussion

We first present the radial distribution functions (RDFs) to explore the effect of two anions with different proportions on the structural properties in bulk [Emim][BF4]x[NTF2](1−x) IL mixture. The RDFs is defined as the ratio of local density ρ(r) within a spherical shell at position r in the radial directions to the corresponding partial bulk density ρbulk, i.e., gr=ρ(r)/ρbulk. The geometric center of cation imidazolium ring, B and F atoms of [BF4] anion, and N and O atoms of [NTF2] anion

Conclusion

In this work, we have employed the MD simulation to explore the structure, dynamics, and HBs properties of [Emim][BF4]x[NTF2](1−x) mixture, where molar fractions × cover 0, 0.25, 0.50, 0.75, 1.0. Our simulation results show that the proportion of two different anions in IL mixture have a great influence on the structural properties, where the tendency of aggregating between two anions and [Emim]+ cation both become more and more weak as the proportion of [BF4] anions increase in IL mixture. On

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21463011 and 21863005), Natural Science Foundation of Jiangxi Province (No. 20171BAB203012), Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No.U1501501, the Sponsored Program for Cultivating Youths of Outstanding Ability in Jiangxi Normal University.

References (71)

  • T. Sato et al.

    Electrochim. Acta

    (2004)
  • Y.C. Pei et al.

    Sep. Purif. Technol.

    (2009)
  • N.J. Brooks et al.

    Chem. Sci.

    (2017)
  • X.P. Wang et al.

    Chem. Phys.

    (2019)
  • K. Gholizadeh et al.

    J. Mol. Liq.

    (2017)
  • C.E. Resende Prado et al.

    J. Mol Struct. THEOCHEM

    (2007)
  • X.Q. Sun et al.

    Chem. Rev.

    (2012)
  • G.B. Damas et al.

    J. Phys. Chem. B

    (2014)
  • G.G. Eshetu et al.

    Angew. Chem. Int. Ed.

    (2014)
  • D.R. MacFarlane et al.

    Energy Environ. Sci.

    (2014)
  • M. Smiglak et al.

    Chem. Commun.

    (2014)
  • P. Zhang et al.

    Adv. Mater.

    (2014)
  • S.J. Zhang et al.

    Chem. Soc. Rev.

    (2014)
  • T. Welton

    Biophys. Rev.

    (2018)
  • M.A. Taige et al.

    Chem.

    (2012)
  • C. Lian et al.

    ACS Energy Lett.

    (2016)
  • N.C. Osti et al.

    J. Phys. Chem. C

    (2018)
  • C. Lian et al.

    J. Phys. Chem. C

    (2018)
  • A. Fang et al.

    J. Phys. Chem. C

    (2019)
  • A.S.L. Gouveia et al.

    Phys. Chem. Chem. Phys.

    (2019)
  • H. Niedermeyer et al.

    Chem. Soc. Rev.

    (2012)
  • G. Chatel et al.

    Green Chem.

    (2014)
  • R.P. Matthews et al.

    Phys. Chem. Chem. Phys.

    (2016)
  • S. Cha et al.

    Phys. Chem. Chem. Phys.

    (2015)
  • M. Chakraborty et al.

    J. Phys. Chem. B

    (2018)
  • J.N.C. Lopes et al.

    J. Phys. Chem. B

    (2005)
  • F. Castiglione et al.

    Phys. Chem. Chem. Phys.

    (2010)
  • J.-M. Andanson et al.

    J. Phys. Chem. Lett.

    (2011)
  • I.J. Villar-Garcia et al.

    Chem. Sci.

    (2014)
  • M.T. Clough et al.

    Chem. Sci.

    (2015)
  • H.F.D. Almeida et al.

    J. Chem. Eng. Data

    (2016)
  • A. Finotello et al.

    J. Phys. Chem. B

    (2008)
  • M. Brussel et al.

    Phys. Chem. Chem. Phys.

    (2012)
  • R.S. Payal et al.

    Phys. Chem. Chem. Phys.

    (2013)
  • L.F. Lepre et al.

    Phys. Chem. Chem. Phys.

    (2016)
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