(489f) Assessing the Reliability of Computing Ion Pair Lifetimes and Diffusivity to Predict Experimental Viscosity Trends of Ionic Liquids

One of the most important properties for ionic liquids is viscosity. A common method being used to predict the viscosity of ionic liquids computationally is quantitative structure-property relationship (QSPR) analysis, which is extremely quick to run computationally, but is difficult to extend beyond the training set used. Another method used to predict viscosities is classical molecular dynamics simulations, which can be applied to any ionic liquid. Unfortunately, viscosity is a computationally expensive property to calculate using molecular dynamics, and thus is not feasible for a high-throughput screening process. Instead, two other properties, ion pair lifetime and self-diffusivities, can be calculated using molecular dynamics with much less computational expense. These properties were calculated for 24 ionic liquids, covering a range of cation and anion classes. All calculations were performed at 400 K to reduce the computational expense. The values of self-diffusivity and ion pair lifetime were then compared with the experimental viscosities of the ionic liquid at 298 K. Experimental viscosities were found to have a strong correlation with both the ion pair lifetime and the inverse diffusivity. For both properties, the ionic liquids were divided into two groups based on the nature of the anion, one group being linear/bent anions and the other containing planar/spherical anions. It was found that both the ion pair lifetime and the diffusivity can be used to predict the ionic liquids with the lowest viscosity with an 80% success rate, with 1/15th the computational cost of calculating viscosity directly. This predictive capability with low computational expense would allow for rapid screening of ionic liquids to determine those with low viscosities.