The influence of water on the structural and transport properties of model ionic liquids

Abstract

Molecular dynamics simulations are used to investigate the influence of water on model ionic liquids. Several models, where the ions vary in size, and in the location of the charge with respect to the center of mass, are considered. Particular attention is focused on the variation in transport properties (diffusion coefficients, shear viscosity, and electrical conductivity) with water concentration. An effort is made to identify the underlying physical reasons for water's influence. The results for our model ionic liquids fall loosely into two categories, depending on the molecular characteristics of the constituent ions. If the ion size disparity is not too large (cation:anion diameter ratio < or approximately 2:1), and if the ion charge location is such that directional ion pair bonds are relatively weak, then we find that the ionic diffusion coefficients and the electrical conductivity increase, and the viscosity decreases with increasing water concentration. This agrees with what is commonly observed experimentally for room temperature ionic liquids (RTILs). For these systems, we do not find changes in the equilibrium structure that can account for the strong influence of water on the transport properties. Rather, by varying the molecular mass of water in our simulations, we demonstrate that the dominant effect of water can be dynamical in origin. In RTIL-water mixtures, the molecular mass of water is generally much less than that of the ions it replaces. These lighter water molecules tend to displace much heavier counterions from the ion coordination shells. This reduces caging and increases the diffusivity, which leads to higher conductivities and lower viscosities. For models with a larger ion size disparity (3:1), or in charge-off-center systems, where strong directional ion pairs are important in the pure ionic liquid, the behavior can be quite different. In these systems, the diffusion coefficients and electrical conductivity can still display conventional behavior and increase when water is added even though the reasons for this can be more complex than in the simpler cases noted above. However, in these systems the viscosity can increase, sometimes quite steeply, with increasing water concentration. We trace this unusual behavior to the formation of associated structures, extended anion-water chains that can weave among the cations in the size disparate case, and strongly bound cation-water-anion clusters in the charge-off-center systems.

Insights into Ionic Liquid/Aromatic Systems from NMR Spectroscopy: How Water Affects Solubility and Intermolecular Interactions

“…When the water concentration reaches the molar ratio 10 : 1 (water/IL), further additions do not affect the relative deviation of toluene solubility, which suggests that water molecules completely solvate and separate cations and anions from their interaction with toluene. At this concentration, a continuous water network is formed, surrounding single ions and increasing the strength of the water‐anion hydrogen bonds …”

“…At this concentration, a continuous water network is formed, surrounding single ions and increasing the strength of the water-anion hydrogen bonds. [38,39] Interesting results are obtained after analyzing these {toluene + water + IL} mixtures by 1 H NMR spectroscopy. The compositional analysis of these mixtures was calculated through direct integration of the signals, [40] and the ternary diagram is shown in Figure 5 for the six ILs.…”

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

“…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.…”

“…[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.…”

“…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. 4 In a subsequent study, they investigated effects of size-asymmetry.…”