Molecular Modeling of Imidazolium-Based [Tf₂N⁻] Ionic Liquids: Microscopic Structure, Thermodynamic and Dynamic Properties, and Segmental Dynamics

Abstract

The microscopic structure, thermodynamic properties, local segmental dynamics, and self-diffusion coefficients of three ionic liquids (ILs) with a common anion, namely, the bis(trifluoromethylsulfonyl) imide ([Tf2N-]), and imidazolium-based cations that differ in the alkyl tail length, namely, the 1-butyl-3-methylimidazolium ([C4mim+]), the 1-hexyl-3-methylimidazolium ([C6mim+]), and the 1-octyl-3-methylimidazolium ([C8mim+]), are calculated over the temperature range of 298.15-333.15 K and pressure range of 0.1-60 MPa. Quantum calculations based on density functional theory are performed on isolated ion pairs, and minimum energy conformers are identified. Electronic density results are used to estimate the electrostatic potential of a molecular force field that is used subsequently for long molecular dynamics (MD) simulations of bulk ILs. Thermodynamic properties calculated from MD are shown to be in excellent agreement for the bulk density and good agreement for derivative properties when compared to experimental data. The new force field is an improvement over earlier ones for the same ILs. The microscopic structure as expressed through the radial distribution function is thoroughly calculated, and it is shown that the bulk structure characteristics are very similar to those obtained from the quantum calculations on isolated ion pairs. The segmental dynamics expressed in terms of bond and torsion angle decorrelation is shown to assume a broad range of characteristic times. Molecular segments in the alkyl tail of the cations are significantly faster than segments in the vicinity of the imidazolium ring. Finally, the new force field predicts accurately the self-diffusion coefficients of the cations and the anions over the entire temperature range examined, thus confirming its validity for a broad range of physical properties.

Classical Density Functional Study on Interfacial Structure and Differential Capacitance of Ionic Liquids near Charged Surfaces

“…This charge dispersion is aimed to be consistent with partial atom charge profiles in MD simulations. 24,25 Furthermore, all charged beads, either positive or negative, are carrying an equal amount of 0.2 elementary charge so that the total charge is one positive unit on cation and one negative unit for anion. The equal amount charge on each bead benefits handling of strong electrostatic correlation in RTILs as discussed later.…”

Improved United-Atom Force Field for 1-Alkyl-3-methylimidazolium Chloride

“…Fitting to the isolated ion pairs 18,28,54 has at least two drawbacks. The first is that charges fitted from different conformers are significantly different.…”

“…40,41,[50][51][52][53][54][55] Ideally, one would like to use accurate quantum calculations to obtain these force field parameters directly. However, the quantum calculations can be carried out only for relatively small systems of a few molecules, which can be quite a different state as compared to a dense liquid.…”

“…Logothetit et al have proposed another force field for the same series of ILs. 54 Different conformers of isolated ion pairs were used to determine the partial charges via the CHELPG method; other parameters were extracted from CHARMM without further optimization. The total ionic charges on the cation and anion were about (0.88e, instead of (1e, due to the polarization between the cation and anion.…”

Theoretical Description of Ionic Liquids

“…The ionic interactions in these complexes are often strong enough to preserve the gas-phase structure also in the liquid phase (Logotheti et al, 2009) or at least make it one of the dominant conformers;…”