Ionic liquids, salts that are molten near ambient conditions, have come under intense study in recent years due to their unique properties. Many ionic liquids have negligible vapor pressure, high thermal stability, excellent solvation power, and a broad liquidus range. They have found use in a number of industrial applications such as gas storage, acid scavenging, hydrosilyation, and electroplating.

Classical atomistic-level simulations have been shown to be an effective means for estimating the physical properties of ionic liquids. Simulations have been used to compute pure properties such as liquid and crystalline density, liquid structure, heat capacity, enthalpy of vaporization, compressibility and volumetric expansivity, self-diffusivity and viscosity. Several mixture properties have also been computed, though to a lesser degree. In most studies, classical potential models (“force fields”) are proposed for a particular ionic liquid and then a handful of properties are computed. Validation of the force field is done by comparing the computed properties against available experimental data, most often liquid densities. Unfortunately, it has been shown that several different parameterizations of a force field can yield essentially the same liquid density, making this property not a particularly good choice for force field validation studies.

In this work we test the ability of a force field to obtain accurate predictions for a wide range of thermodynamic and transport properties for a single ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([emim][EtSO4]). [emim][EtSO4 ] is a good ionic liquid to study because many of its properties have been measured experimentally, so an overall assessment of the accuracy of force field-based simulations for the prediction of several different ionic liquid properties can thus be obtained. It is also easy to prepare in a halide-free manner from diethyl sulfate and is therefore comparatively inexpensive. It is water miscible, air stable and has a relatively low viscosity. Preliminary studies suggest that it also has low toxicity.

To carry out this study, a new fixed charge force field is developed for both the anion and cation. Three different simulation techniques are used: equilibrium molecular dynamics (EMD), reverse nonequilibrium molecular dynamics (RNEMD) and continuous fractional component Monte Carlo (CFC MC). For the neat ionic liquid, the following properties are computed: density, volumetric expansivity, heat capacity, enthalpy of vaporization, rotational relaxation time, self-diffusivity, shear viscosity and thermal conductivity. In addition, the following properties are computed for water / [emim][EtSO4 ] mixtures: density, excess molar volume, enthalpy of mixing, partial molar enthalpy, water solubility as a function of partial pressure, rotational relaxation time, self-diffusivity, shear viscosity and thermal conductivity. We find that most properties agree with experimental values to within 10%, although some properties may require the use of a polarizable force field to achieve better accuracy. To our knowledge, no other simulation study has been carried out in which so many different properties have been computed for a single ionic liquid with a single force field.