On the computation and contribution of conductivity in molecular ionic liquids

Abstract

In this study we present the results of the molecular dynamics simulation of the ionic liquids: 1-butyl-3-methyl-imidazolium tetrafluoroborate and trifluoromethylacetate as well as 1-ethyl-3-methyl-imidazolium dicyanamide. Ionic liquids are characterized by both a molecular dipole moment and a net charge. Thus, in contrast to a solution of simple ions in a (non-) polar solvent, rotational and translational effects influence the very same molecule. This study works out the theoretical framework necessary to compute the conductivity spectrum and its low frequency limit of ionic liquids. Merging these computed conductivity spectra with previous simulation results on the dielectric spectra of ionic liquids yields the spectrum of the generalized dielectric constant, which may be compared to experiments. This spectrum was calculated for the three ionic liquids over six orders of magnitude in frequency ranging from 10 MHz to 50 THz. The role of rotation and translation and their coupling term on the generalized dielectric constant is discussed in detail with a special emphasis on the zero-frequency limit. Thereby, the frequency dependence of the cross correlation between the collective rotational dipole moment and the current is discussed.

Spotlight on ionic liquids

“…These effects have been addressed in a number of articles describing MD simulations of ILs. 31,33,[68][69][70][71][72][73][74][75][76][77] [69][70][71][72][73][74] reported several studies of both equilibrium and nonequilibrium solvation, orientational dynamics, vibrational relaxation, and electron-transfer reactions in ͓emim͔ + / ͓PF 6 ͔ − . Song 77 has presented a method to extend dielectric continuum models for solvation to include precise molecular shapes and an extension beyond the DebyeHückel model for high concentrations of charges, up to neat ILs.…”

The influence of polarizability on the dielectric spectrum of the ionic liquid 1-ethyl-3-methylimidazolium triflate

“…21 The important point to stress here is that it is the generalized dielectric constant, S*(o), which is measured experimentally by dielectric relaxation spectroscopy 15,44,45 and which can be computed by molecular dynamics simulation. 21,41,[46][47][48] Whilst the nuclear degrees of freedom describing translation and rotation are usually explicitly considered in computer simulations, the electronic degrees are either implicitly contained in the applied force field or may be explicitly modelled by introducing (electronic) polarizability. Since in the former case electrostatic forces are restricted to the interactions between permanent charge distributions, these cannot respond to an external electric field, E 0 (o), of very high frequency, o.…”

“…Altogether, the GDC S*(o) reads sinðotÞ=o ¼ t). 41 Therefore, the term 4pis(0)/o is subtracted from the GDC both in computational analysis…”

Soft Ionization of Thermally Evaporated Hypergolic Ionic Liquid Aerosols

“…[2][3][4][5][6][7][8][9][10][11] The unique properties of ILs are a direct result of the interactions between the ions 12,13 and this led to an explosion of interest to utilize ILs for broad applications such as catalysts, 14 batteries, 8,9,15 and hypergolic fuels. [16][17][18][19] Some energetic ionic liquids, the dicyanamide, 16 nitrocyanamide, 19 and azide systems, 17 have shown promising potential for propellant applications, 18 as substitutes for conventional energetic compounds, monomethyl hydrazine/nitrogen tetroxide (MMH/NTO), with several advantages including thermal stability, environmental friendliness, and low volatility.…”