Dynamics in an Idealized Ionic Liquid Model

Abstract

An idealized four-site ionic liquid model having characteristics approximating those of 1-butyl-3-methylimidazolium hexafluorophosphate ([Im(41)][PF(6)]) is introduced as a low-cost alternative to existing all-atom models for purposes of simulating solute-based dynamics over nanosecond and longer time scales. The structural and energetic properties of the model are in reasonable agreement with those of [Im(41)][PF(6)] and similar ionic liquids, but the dynamics are unrealistically slow. A temperature shift of approximately 100 K is required to produce agreement between the viscosity and diffusion coefficients of the model and experimental values. Several aspects of the ion dynamics such as subdiffusive translational motions, non-Gaussian van Hove distributions, and jumplike displacements in both positions and orientations, are similar to behavior observed in supercooled liquids. Translational diffusion coefficients and rotational correlation times show roughly the proportionalities to viscosity expected from hydrodynamic models, and slip hydrodynamic calculations provide reasonable accuracy in some cases. But anomalously high rotational diffusion coefficients which decouple from viscosity at low temperature are also observed. These anomalies are explained in terms of the prevalence of 180 degrees rotational jumps coupled to the presence of marked heterogeneity in rotational motions, especially about one molecular axis. Comparisons between the dynamics observed in the ionic liquid (IL) model and a neutral mixture (NM) counterpart help to explain the origins of the distinctive dynamics in ionic liquids compared to conventional solvents. The requirement for balancing electrostatic interactions in the IL leads to uniform and interleaved distributions of cations and anions resembling a distorted ionic lattice, similar to the structure of molten NaCl. The resistance to reorganizing this structure is what leads to the slow dynamics of ionic liquids. The coupling among large collections of ions is presumably responsible for the similarity of ionic liquids to supercooled conventional liquids.

Simulations of room temperature ionic liquids: from polarizable to coarse-grained force fields

“…As can be seen on the panel a) of figure 5, the electrostatic potential (extracted from a DFT calculation) of these molecules is rather smooth, so that it is possible to pass from the all-atom representation of panel b) to the coarse-grained one of panel c). Such a coarsegraining approach was proposed for BMIM-PF 6 by Roy et al 105 In their representation, the BMIM + cation is represented by three interaction sites (one for the imidazolium ring, one for the methyl group and one for the butyl group) while the PF − 6 anion is replaced by a single site. The cation is kept rigid during the simulation, its geometry is therefore chosen according to its average conformation in the liquid.…”

Understanding the different (dis)charging steps of supercapacitors: influence of potential and solvation

“…For the pure ionic liquid, three different CDC electrode materials are considered (see Table 1). Following our previous works, we use for the ionic liquid the coarsegrained model of Roy and Maroncelli, which yields the correct structural, thermodynamic and dynamic properties of [BMI] [PF 6 ] [30,31]. Three and one sites describe the cation and the anion, respectively [18,19].…”

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.…”

“…[39][40][41]43,59,60 The united-atom (UA) model is traditionally one of the most popular coarse-graining schemes used to model IL systems [61][62][63][64] as well as hydrocarbon systems in general. One critical problem in computational studies is that the bulky and asymmetric tetraalkylphosphonium cations generally consisting of a large number of atoms, and the strong correlations between ionic groups due to long-range electrostatic interactions, lead to their viscous behavior, and thus molecular simulations at large length and long time scales are needed to obtain reliable results.…”