Structural and dynamical properties of ionic liquids: Competing influences of molecular properties

Abstract

Room temperature ionic liquids differ from molten salts in many ways, our work concentrates on two distinguishing features. These are large cation-anion size disparities and at least one ionic species where the center of mass and the center of charge do not coincide. In earlier work, we examined the influences of these features in isolation on simple spherical models. This paper extends this work to ionic liquid models where both features are present, and where the characteristic distance sigma(+-) (') determining the strength of the Coulombic attractions is unconstrained. We consider the interplay among these molecular features and elucidate their relative importance to the behavior of ionic liquids. Particular attention is focused on the transport properties. We find that size disparity, charge location, and sigma(+-) (') can all have large (often competing) effects. In our models, size disparity and small charge displacements lead to weakly bound, directional ion pairs, and the resulting asymmetric ion-counterion distribution gives rise to increased diffusion coefficients, consequently lower viscosity, and increased conductivity. These observations are analogous to effects reported in the literature, and we see similarities between the directional ion pairs in our models and directional cation-anion pairing through weak hydrogen bonding in room temperature ionic liquids. In our models, large charge displacements lead to strongly bound, long-lived, directional ion pairs, and in this regime the trends noted above are reversed, increased viscosities, and decreased conductivities are observed. Recently, creating more strongly hydrogen bonded, directional ion pairs has been put forward as possible means of achieving larger viscosity reductions. The trend reversal that we observe suggests that this might not work in practice.

Ionizable side chains at catalytic active sites of enzymes

“…The environment does not resemble the infinitely dilute ideal fluid for which the law of mass action is appropriate (Eisenberg 2011c). The catalytic active site seems more like an ionic liquid (Kornyshev 2007; Siegler et al 2010; Spohr and Patey 2010) than an ideal gas. The ionic liquid of the catalytic active site differs from classical ionic liquids because some of its components are side chains of proteins, ‘tethered’ to a polypeptide backbone, not free to move into the bulk solution.…”

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.…”

Intrinsic Mobility of Gaseous Cationic and Anionic Aggregates of Ionic Liquids

“…[20] Close intrinsic mobilities for the IL aggregates are also indicated by their cross-sections as calculated (Table 1) using the TM model and the MobCal software [31][32][33] from structures fully optimised at the B3LYP 6-311G(d,p) level using Gaussian 03.…”