Force Fields for Studying the Structure and Dynamics of Ionic Liquids: A Critical Review of Recent Developments

Abstract

Classical molecular dynamics simulations are a valuable tool to study the mechanisms that dominate the properties of ionic liquids (ILs) on the atomistic and molecular level. However, the basis for any molecular dynamics simulation is an accurate force field describing the effective interactions between all atoms in the IL. Normally this is done by empirical potentials which can be partially derived from quantum mechanical calculations on simple subunits or have been fitted to experimental data. Unfortunately, the number of accurate classical non-polarizable models for ILs that allow a reasonable description of both dynamical and statical properties is still low. However, the strongly increasing computational power allows one to apply computationally more expensive methods, and even polarizable-force-field-based models on time and length scales long enough to ensure a proper sampling of the phase space. This review attempts to summarize recent achievements and methods in the development of classical force fields for ionic liquids. As this class of salts covers a large number of compounds, we focus our review on imidazolium-based ionic liquids, but show that the main conclusions are valid for non-imidazolium salts, too. Insight obtained from recent electronic density functional results into the parametrization of partial charges and on the influence of polarization effects in bulk ILs is highlighted. An overview is given of different available force fields, ranging from the atomistic to the coarse-grained level, covering implicit as well as explicit modeling of polarization. We show that the recently popular usage of the ion charge as fit parameter can looked upon as treating polarization effects in a mean-field matter.

Novel 2-alkyl-1-ethylpyridinium ionic liquids: synthesis, dissociation energies and volatility

“…At the reference temperature, T = 460 K, the 2-alkyl-1-ethylpyridinium based ionic liquids presents significant lower volatility than the imidazolium ionic liquids series. 50 Analogous to imidazolium based ionic liquids, 49 decreases along the alkyl side chain length of the cation, similarly as it was observed for the imidazolium series. 50 The non-electrostatic potential is expected to increase nearly linearly along the alkyl chain length.…”

“…Additionally, they are required for validation and parameterization of the models (force fields) used to describe ionic liquids in different simulation techniques. 3,6,19,49 Despite the importance of the vapor pressures of ionic liquids and the related thermodynamic properties of vaporization,…”

Thermophysical properties of imidazolium tricyanomethanide ionic liquids: experiments and molecular simulation

“…by theoretical/molecular simulation methods [12][13][14][15][16][17][18][19][20] and several predictive models have been proposed. 11,[21][22][23][24] Currently it is well accepted that ILs are highly structured fluids in which the ion pairs arrange themselves into polar and non-polar domains.…”

Probing spatial locality in ionic liquids with the grand canonical adaptive resolution molecular dynamics technique

“…The full atomistic force field parameters for modeling 1-ethyl 3-methyl imidazolium chloride were taken from a study by Dommert et al 39 , in particular such model considers reduced/scaled charges of +0.8e and -0.8e instead of the full formal charges +1.0 e and -1.0e.…”