Molecular Modeling of the Thermophysical and Transport Properties of Ionic Liquids

Ionic liquids (ILs) are an exciting class of compounds with unique properties that makes them attractive for industrial applications. Among their valuable features, an immesurably low vapor pressure and a liquid state at or near ambient conditions are found at the top of the list. In this research dissertation, thermophysical and transport properties of ionic liquids are theoretically investigated by means of molecular simulation techniques. Quantum calculations are used as a supportive tool to force field development work. This research task is not done in the darkness but rather is guided and supported by colateral experimental studies. Cations studied include imidazolium-, pyridinium- and triazolium-based structures with different inorganic anions such as hexafluorophosphate, bis(trifluoromethanesulfonyl)imide, nitrate and perchlorate. Static properties computed include gravimetric densities, volumetric expansivities, isothermal compressibilities, heat capacities, cohesive energy densities as well as the liquid structure. Analysis of the dynamic properties of ionic liquid systems is also carried out yielding information on the rotational dynamics and transport properties such as self-diffusivity. This work forms part of a national quest for an insight into the structure-property relationship of energetic ionic liquids (EILs) headed by the US Air Force. EILs with a high nitrogen content offer several advantages over current technologies such as hydrazine. Applications may be found in armed conflict as well as in many other energetical needs provided that EILs are found to be hypergolic. Due to the fact that hypergolic fuels carry their own oxidizer, they are ideal for space applications. Triazolium-based ionic liquids are the starting class of energetic ionic liquids and are investigated as part of this dissertation. Not much experimental data is available for this class of compounds. This is partially due to the inherent danger of the experimental measurements. Therefore, a safe computer simulation can provide a great deal of insight into the property-structure relationship and the liquid structure of the system. Validation experience obtained with imidazolium- and pyridinium-based ionic liquids gives confidence in the static and dynamic property prediction of triazolium-based ionic liquids. In addition, a molecular modeling study of ethane-based hydrofluorocabons for vapor-liquid equilibria is presented. There exists industrial interest in applications of mixtures of these refrigerants with ionic liquids and thus, molecular modeling can help elucidating design problems. A force field for 1,1,1,2-tetrafluoroethane (R-134a) is proposed and validated against experimental liquid and gas densities, vapor pressures and heat of vaporizations using Gibbs ensemble Monte Carlo simulations.