Ionic liquids studied across different scales: A computational perspective

Abstract

For theoreticians, ionic liquids represent a major challenge. This is due to the fact that intermolecular interactions are particularly strong because of ionic liquids' ionicity. This, in turn, causes a subtle interplay between different scales which is encoded in the measured macro- and mesoscopic properties and also in the molecular electrostatic characteristics. Therefore, force fields have to describe the microscopic processes correctly in order to reproduce macroscopic properties accurately over a large range of state variables. Herein, imidazolium-based ionic liquids were studied at different scales, going from the detailed quantum electronic scale to the classical atomistic scale. It is indicated how the information gained at each level could be used for the other scales. In particular, the issue of deriving suitable partial charges for use in classical force fields is addressed. The Blöchl method was employed to generate partial charges reproducing the multipole distribution accurately for bulk systems. This led naturally to absolute ionic charges of less than /l e/, i.e., charge scaling. So, the monopole structure of the herein introduced force field mimics the quantum chemical behaviour observed in the liquid phase. This led to a substantial improvement in the description of dynamical properties of immediate experimental interest, such as electric conductivity. For further insight, the electric dipole moment of the ions was taken as physical indicator of their electronic structure. The electric dipole moment was found to fluctuate strongly and to depend on polarisation. Hence, our scale-combined study offers a gateway to rational design of models, based on the relevant underlying physics rather than on mere numerical parameterisation, and thereby to (possibly) more direct physical interpretation of experimental results.

Establishing Consistent van der Waals Volumes of Polyatomic Ions from Crystal Structures

“…Several studies showed that ions constituting ILs have non-integer charges. [52][53][54][55] Furthermore, bonding within the ion pair in ILs is not purely electrostatic but exhibits considerable amounts of dispersive interactions. [56,57] Thus, the treated species are not ions in the classical sense but may be regarded as "quasi-ions" or "effective ions".…”

Multiscale modeling of the trihexyltetradecylphosphonium chloride ionic liquid

“…[35][36][37][38][39][40][41][42][43] Various simulation methods are available at different length and time scales depending on the specific targets requested from the studied IL systems. [35][36][37][38][39][40][41][42][43] Various simulation methods are available at different length and time scales depending on the specific targets requested from the studied IL systems.…”

Direct Correlation between Ionic Liquid Transport Properties and Ion Pair Lifetimes: A Molecular Dynamics Study

“…We find that for a given cation, ILs having a [CNPyr] anion have the fastest dynamics, followed by those with a [124triz] anion. Those with the small spherical [PF 6 ] anion actually had the slowest dynamics.…”