MD Simulation studies on the structure dynamics and interfacial properties of RTILs

Abstract

The thesis entitled Molecular Dynamics Simulation Studies on the Structure, Dynamics, and Interfacial Properties of Room Temperature Ionic Liquids” presents the results of investigations on room temperature ionic liquids. Classical molecular dynamics simulations were employed to characterize their microscopic structure, dynamics and interfacial properties. Chapter 1 presents a general introduction to room temperature ionic liquids (RTILs). The methods of synthesis, some important physico-chemical properties, and few applications of the RTILs are discussed. Results of various experimental and theoretical studies on these liquids are reviewed. The chapter also contains a brief introduction to the classical molecular dynamics (MD) method employed in this thesis. Chapter 2 presents a study on the complex dynamics exhibited by the IL, 1-nbutyl, 3-methylimidazolium hexafluorophosphate ([bmim][PF6]) in the temperature range, 250K-450K. The activation energy for the self diffusion of the anion was found to be marginally higher than that for the cation. The calculated electrical conductivity was found to agree well with experimental data. Structural relaxation has been studied through the decay of coherent and incoherent intermediate scattering functions over a range of temperatures and wave vectors relevant to the system. The relaxation data has been used to identify and characterize two processes – β and α. The dynamical heterogeneity of the ions determined through the non-Gaussian vii parameter indicated the motion of cation to be more heterogeneous than that of the anion. The faster ones among the cations were found to be coordinated to faster anions, while slower cations were surrounded predominantly by slower anions. Thus the dynamical heterogeneity in this ionic liquid was shown to have structural signatures. In Chapter 3, the far-infrared region of the vibrational spectrum of four RTILs has been examined using two computational methods: normal mode analysis (NMA) and the power spectra obtained from velocity auto correlation functions. A distinct band has been observed around 30 to 70 cm−1 in all the liquids. The peak position shifted to higher frequencies with an increase in the strength of the cation-anion interaction. These bands were shown to arise primarily from inter-ionic interactions, but cannot be solely ascribed to localized motions of the hydrogen bond between the cation and the anion. They could be nearly reproduced by local interactions. These findings were confirmed from density functional theory (DFT) based gas phase calculations on ion pairs as well as through periodic calculations of crystalline [bmim][PF6]. Chapter 4 discusses the influence of the symmetry of the imidazolium cation on the structure and properties of the ionic liquid-vapour interface. The anion chosen was bis(trifluoromethylsulfonyl)imide (NTf2). At the interface, both cations and anions were present, and the alkyl chains of the former preferred to orient out into the vapour phase. A large fraction of cations were oriented with their ring normal parallel to the surface and alkyl chains perpendicular to it. These orientational preferences were reduced in ionic liquids with symmetric cations. The electrostatic potential difference between the liquid and the vapour phases was positive and decreased with increasing length of the alkyl group. The calculated surface tension of the liquids too decreased with increasing alkyl chain length and its value for a liquid with a symmetric cation was marginally higher than that for one with an asymmetric, isomeric cation. Chapter 5 presents studies on free standing nanoscopic clusters of [bmim][PF6] with diameters in the range of 2-8 nm. The butyl tail groups of the [bmim]+ ion protrude outwards from the surface of the cluster while the ring centres lie beneath. The number densities of cation ring centres showed a non-uniform distribution near the surface in comparison to that of anions. An electrostatic potential drop of - 0.17 V has been calculated across the cluster-vapour interface for the largest cluster studied. The effective interaction potential between the clusters was found to exhibit a short-ranged, strong attractive well. A linear dependence of this well depth on the cluster size was observed, consistent with the predictions of the interpenetration model for inter-micellar interactions