Computing Transport Properties of Molecular and Ionic Fluids Using Atomistic Simulations

We report the results of first atomistic simulation study to compute the thermal conductivity of ionic liquid as well as the effect of water content on the transport properties (such as viscosity and thermal conductivity) of these liquids. Atomistic simulations are conducted to examine the dependence of the transport properties of ionic liquids (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and 1-ethyl-3-methylimidazolium ethylsulfate) on temperature and water content. A nonequilibrium molecular dynamics procedure is utilized along with an established fixed charge force field. It is found that the simulations quantitatively capture the temperature dependence of the viscosity. They also qualitatively capture the drop in viscosity that occurs with increasing water content. Using mixture viscosity models, it is shown that the relative drop in viscosity with water content is actually less than that which would be predicted for an ideal system. This finding is at odds with the popular notion that small amounts of water cause an unusually large drop in the viscosity of ionic liquids. The simulations suggest that due to preferential association of water with anions and the formation of water clusters, the excess molar volume is negative. This means that dissolved water is actually less effective at lowering the viscosity of these mixtures when compared to a solute obeying ideal mixing behavior. The experimental results for thermal conductivity of 1-ethyl-3-methylimidazolium ethylsulfate were not yet available but the simulation results were in good agreement with the experimental results of other ionic liquids. Classical atomistic simulations are also used to compute the enthalpy of vaporization of a series of ionic liquids comprised of the 1-alkyl-3-methylimidazolium cations paired with the bis(trifluoromethylsulfonyl)imide anion. The calculations show that the enthalpy of vaporization is the lowest for neutral ion pairs. Nonneutral clusters have much higher vaporization enthalpies than their neutral counterparts, and thus are not expected to make up a significant fraction of volatile species. The enthalpy of vaporization increases slightly as the cation alkyl chain length increases and as temperature decreases.