Monte Carlo Simulations of the Ionic Liquid 1-butyl-3-methylimidazolium hexafluorophosphate

We report the first molecular simulation study of 1-n-butyl-3-methylimidazolium hexafluorophosphate, a widely studied ionic liquid. Monte Carlo simulations are carried out in the isothermal-isobaric ensemble to calculate the molar volume, cohesive energy density and liquid structure as a function of temperature and pressure. A united atom forcefield is developed using a combination of ab initio calculations and literature parameter values. This forcefield treats the anion hexafluorophosphate as a spherically symmetric interaction site and is later modified to account for the atomistic details of the anion. The results obtained from different forcefields are compared against each other to determine the influence of the molecular representation of the anion on the thermophysical properties of the ionic liquid. The accuracy of the forcefields in predicting the volumetric properties is assessed by a direct comparison of the results with experimental observations. Calculated molar volumes (or densities) are within 5 % of experimental values, and a reasonable agreement is obtained between computed and experimental values of the isothermal compressibility and volume expansivity. Local structure, presented in the form of radial distribution functions, shows that the anions are found to preferentially cluster in two favorable regions near the cation. We also assess the applicability of the molecular simulations to calculate the Henry's constant of gases with a wide range of solubilities. The results of the Widom test particle insertion method and expanded ensemble simulations are reported. A comparison between the simulation results and experiments shows good agreement. The study reveals inherent difficulty associated with the Widom test particle insertion method in determining the excess chemical potential, while the expanded ensemble method appears to be somewhat better. Local organization of solvent molecules about the solute molecules is used to identify interactions governing the observed solubility behavior.