Structural and dynamical properties of ionic liquids: The influence of ion size disparity

Abstract

The influence of ion size disparity on structural and dynamical properties of ionic liquids is systematically investigated employing molecular dynamics simulations. Ion size ratios are varied over a realistic range (from 1:1 to 5:1) while holding other important molecular and system parameters fixed. In this way we isolate and identify effects that stem from size disparity alone. In strongly size disparate systems the larger species (cations in our model) tend to dominate the structure; the anion-anion distribution is largely determined by anion-cation correlations. The diffusion coefficients of both species increase, and the shear viscosity decreases with increasing size disparity. The influence of size disparity is strongest up to a size ratio of 3:1, then decreases, and by 5:1 both the diffusion coefficients and viscosity appear to be approaching limiting values. The conventional Stokes-Einstein expression for diffusion coefficients holds reasonably well for the cations but fails for the smaller anions as size disparity increases likely due to the neglect of strong anion-cation correlations. The electrical conductivity is not a simple monotonic function of size disparity; it first increases up to size ratios of 2:1, remains nearly constant until 3:1, then decreases such that the conductivities of the 1:1 and 5:1 systems are similar. This behavior is traced to the competing influences of ion diffusion (enhancing) and ion densities (reducing) on conductivities at constant packing fraction. The temperature dependence of the transport properties is examined for the 1:1 and 3:1 systems. In accord with experiment, the temperature dependence of all transport properties is well represented by the Vogel-Fulcher-Tammann equation. The dependence of the diffusion coefficients on the temperature/viscosity ratio is well described by the fractional Stokes-Einstein relation D proportional to (T/eta)(beta) with beta approximately = 0.8, consistent with the exponent observed for many molten inorganic salts.

Thermo-molecular orientation effects in fluids of dipolar dumbbells

“…In particular, we are unaware of any other computer simulation studies of size-asymmetric dipolar dumbbells (but see Ref. 18), although extensive work has been done on the size-asymmetric primitive model consisting of charged hard spheres [19][20][21][22][23].…”

Ion pairing and phase behaviour of an asymmetric restricted primitive model of ionic liquids

“…[1][2][3] However, in our dense canonical ensemble, room-temperature system the effect is rather dramatic, due to the system undergoing a phase transition (from a solid to liquid), as b increases from zero. In Figure 5,we see how the mean-square displacement, MSD, changes from insignificant values (as in a solid) to those typical of a fluid as b increases from 1.5 Å to 1.75 Å.…”

“…We shall denote our model as the Asymmetric Restricted Primitive Model (ARPM) and it is depicted in Figure 1. It should be noted that the ARPM is similar in spirit to a range of simple asymmetric models that were investigated by Spohr and Patey, [1][2][3] and by Lindenberg and Patey, 4,5 although they included dispersion interactions, which we do not. We will generally consider fluids at room temperature (298 K), while Patey and co-workers primarily studied models at elevated temperatures.…”

“…Several modified RPM models, have been proposed which allow other mechanisms to be explored. For instance, Spohr and Patey performed studies on charged Lennard-Jones particles with size-asymmetry, 1 charge-asymmetry, competing influences of size and charge asymmetry, 2 and solvent polarization. 3 Lindenberg and Spohr also simulated melt-ing temperatures for systems containing charge-asymmetric Lennard-Jones ions.…”

Charge Ordering Induces a Smectic Phase in Oblate Ionic Liquid Crystals

“…The existing computer simulation literature on ionic liquid crystals [3] is mainly focused on so-called roomtemperature ionic liquids [4,5] and high-temperature melts of simple ionic crystals [6]. The overwhelming majority of these studies considers real molecules, ranging from fully atomistic detail to coarse-grained descriptions.…”