Molecular dynamics simulation of the ionic liquid 1-n-butyl 3- methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim]
[Tf
2N]
Wei Zhao 1, Hossein Eslami 1,2, Welchy Leite Cavalcanti 1,3, and Florian Müller- Plathe 1,*
1 Physikalische Chemie, Technische Universität Darmstadt, Petersenstrasse 20, D-64287 Darmstadt, Germany
2 Department of Chemistry, College of Sciences, Persian Gulf University, Boushehr 75168, Iran
3 Bremen Center for Computational Materials Science, Universität Bremen, Germany Email: [email protected]
Introduction
Recently, numerous studies have been performed to obtain fundamental understanding of the unique charachteristics of ionic liquids composed of different cations and anions. In spite of this, microscopic understanding about the ionic state and ion dynamics, which directly influences the physicochemical properties of ionic liquids, are still scarce. In the case of [bmim][Tf2N], studied in this work, to the best of our knowledge, only one molecular dynamics study by Sieffert and Wipff [1] was found in the literature which was based on the [bmim] model developed by Andrade et al. [2] and a [Tf2N] model developed by Lopes et al. [3]. In this work we tried to do molecular dynamics simulations by using the [bmim] and [Tf2N] models developed by Lopes et al. [3]. However, a severe problem with these models is in the description of dynamic behavior.
In the present work, we propose a refined model based on the model of [bmim][Tf2N]
developed by Lopes et al [3]. This model includes three types of modifications based on the original model: (i) an effective dielectric constant εeff is introduced to reduce the electrostatic interactions; (ii) Smaller radii of hydrogen atoms on the imidazolium ring of [bmim] are chosen in order to strengthen the ability of these hydrogen atoms to form hydrogen bonds with anions in general; (iii) Lennard-Jones parameters of the outer atoms, fluorine and oxygen atoms of [Tf2N], are optimized to reproduce various experimental properties.
Key words: Ionic liquids, Molecular modelling, Force field, Molecular dynamics
2
2 Model description
In the present fully flexible all-atom model, the total potential takes the form:
=
bonds k r ,ij
(
−)
2 + angles kφ ,ijk(
−)
2 + dihedrals kτ ,ijkl[
− τ − τ ]
U
total ∑
rij ij
r0 ,i
j ∑
ijk 2 φ ijk 0 ,ij
k ∑
ijkl
1 cos p ( )
2 0
LJ σ
12 σ 6 EL q q 1 ε1 − r 2 (1)
+ ∑ 4ε
ij − ij +
∑
i j + RF ij
ij r r ij r ε ε r 2ε + 1 r 3
ij ij ij 0 eff ij RF cutoff
Harmonic potentials were used to represent the bond stretching and bending. The parameters for the bonded interactions are taken unchanged from the previous model developed by Lopes et al. [3] The nonbonded interactions are composed of van der Waals interactions in forms of Lennard-Jones potentials and electrostatic interactions which are handled by using the reaction field method with a universal finite cutoff (of 1.2 nm).
For the [Tf2N] anions, optimization of the LJ potential parameters of fluorine and oxygen atoms were carried out to reproduce the density and self-diffusion coefficicents of cations and cations of [bmim][ Tf2N] system. We found that it is possible to reproduce these properties by tuning only the size of the fluorine and oxygen atoms. In our model, we introduced an effective dielectric constant εeff =1.8 into the calculation of electrostatic interactions.
Molecular simulations
Molecular dynamics imulations of a system consisting of 256 [bmim][Tf2N] ion- pairs were performed both for parameter optimization and model-validation. First, simulations of the present system based on optimized force field were performed at 278, 298, 318, 338, and 358 K (at constant pressure of 1 bar) to check the temperature- dependence of various properties. A parallel simulation based on the model by Lopes et al [15, 16] was performed as a reference system. To calculate the density and the isothermal compressibility, the same molecular system was simulated at 298K at different pressures, 100, 200, 300, 400, and 500 bar. All the simulations were performed for 10 ns. The first 2 ns was used for system equilibration and the trajectories of the later 8 ns were used for data analysis.
In all systems, a cutoff of 1.2 nm was applied for all the non-bonded interactions. The LJ parameters between different types of atoms were derived from the Lorenz- Berthelot mixing rule. All simulations were performed in the NPT ensemble with a time step of 2 fs.
φ
In all simulations, the temperature was kept constant using the Berendsen thermostat [4]
with a coupling time constant of 0.2 ps. A pressure coupling based on
4
Berendsen barostat [4] is applied with a coupling constant of 2ps. All simulations were performed using our molecular simualtion package YASP [5].
Conclusion
In this present work, we report a fully flexible all-atom model for a ionic liquid [bmim]
[Tf2N]. During the parameter optimization process, we hold the principle that the modifications to the original model should be kept at a minimum in order to get good transferability of most of parameters. Although the parameter optimizations is carried out only for a few key parameters, this model predicts well a wide range of thermodynamic as well as dynamic properties in comparison with experiments.
References:
1. N. Sieffert and G. Wipff, J. Phys. Chem. B 110 (2006) 13076.
2. J. de Andrade, E. S. Boes, and H. Stassen, J. Phys. Chem. B 106 (2002) 13344.
3. J. N. C. Lopes, and A. A. H. Padua, J. Phys. Chem. B 108 (2004) 16893.
4. H. J. C. Berendsen, J. P. M. Postma, W. F. Van Gunsteren, A. Dinola, J. R.
Haak, J. Chem. Phys. 81 (1984) 3684.
5. F. Müller-Plathe, Comput. Phys. Commun. 78 (1993), 77; K. Tarmyshov and F.
Müller-Plathe, J. Chem. Inf. Mod. 45 (2005) 1943.
