Room temperature ionic liquids: classical and ab initio molecular dynamics simulation studies

Abstract

The thesis presents results of investigations on imidazolium cation based room temperature ionic liquids (RTILs), using ab initio molecular dynamics (AIMD) and classical molecular dynamics (MD) simulations. Chapter 1 introduces RTILs and provides comprehensive survey of earlier studies and a brief overview of the simulation methods and the analyses employed. Chapter 2 presents the results of AIMD andMD simulations of 1,3-dimethylimidazolium chloride [mmim][Cl] at 425 K. The structure of the melt is characterized through radial (RDF) and spatial distribution functions (SDF). Transport properties such as diffusion coefficients of ions, shear viscosity and electrical conductivity of the liquid have been calculated. The dynamics of dipole and ion solvation in the liquid have been studied using solvation time correlation functions. An atomistic MD simulation study of a planar vapor-liquid interface of 1,n-butyl-3- methylimidazolium hexafluorophosphate ([bmim][PF6]) is presented in Chapter 3. Layering of the ions near the interface is observed as oscillations in the corresponding number density profiles. The amplitude of the oscillations in the electron density profile were found to be relatively diminished due to out-of-phase oscillations in the contributions from the anion and the cation. The butyl chains were found to be preferentially oriented along the interface normal, imparting a hydrophobic character to the surface. Chapter 4 presents results of AIMD studies carried out on liquid [bmim][PF6] and its mixture with CO2. The simulations predict the formation of a weak cation-anion hydrogen bond. The anions were strongly polarized in the condensed phase. In the mixture, CO2 molecules were found to interact better with the anion than with the cation. Such molecules present in the vicinity of the ions exhibit larger deviations from linearity in their instantaneous configurations. The backbone of the CO2 molecules were aligned tangential to the PF6 spheres and they were preferentially located in the octahedral voids of the anion. Chapter 5 presents a refined atomistic potential for [bmim][PF6] to overcome the drawbacks of the model used earlier. The refined model predicts the density of the liquid vii at different temperatures between 300K and 500K within 1.4% of the experimental value. The calculated diffusion co-efficients of ions and the surface tension of the liquid agree well with experiment. The development of a coarse grained model for the family of 1-n-alkylimidazolium hexafluorophosphate ILs is presented in Chapter 6. Good agreement with experiments is obtained for physical properties such as density and surface tension. The calculated neutron and X-ray weighted structure factors agree well with results from scattering experiments. Intermediate range ordering is observed for ILs containing cations with a longer alkyl chain, and its origin is discussed. In an effort to identify the relationship between CO2 solubility in ILs and the microscopic interactions between CO2 and the anions present in a IL, gas phase DFT calculations were carried out. Chapter 7 presents structures, energies and vibrational features of several energy optimized anion-CO2 complexes. The dominance of Lewis acid-base interactions was identified. In the optimized configurations, the CO2 molecule was found to adopt a non-linear geometry. The calculated binding energy of the anion-CO2 complexes were found to be inversely correlated with the experimentally measured solubility of CO2 in ILs containing such anions. Chapter 8 provides results from classical MD studies on [bmim][PF6] - CO2 mixtures studied at varying concentration of CO2. The volume expansion of the mixture as computed from the simulations agree well with the experiments. The diffusion co-efficients of both the ions and that of CO2 increases with increase in the CO2 concentration. The AIMD simulation of a RTIL electrolyte, [emim][F].2.3HF is presented in Chapter 9. The presence of a hydrogen bond between the acidic ring hydrogen of the cation and the fluoride anion is observed. The fluoride ions associate with HF molecules to form polyfluoride anions. The intermolecular structure of the solution is determined using RDF and SDFs. The distribution, geometry and vibrational characteristics of the polyfluoride anions have been characterized.