Computer Simulation Studies of Intermolecular Structure, Microheterogeneity and Dynamics in Room Temperature Ionic Liquids

Abstract

The thesis presents results of investigations on imidazolium cation based room temperature ionic liquids (RTILs) using classical molecular dynamics simulations. Chapter 1 presents a general introduction to RTILs describing their physical properties and applications. The current status of experimental and theoretical research in this area is reviewed. The chapter also contains a brief discussion on the classical molecular dynamics simulation method. In Chapter 2, the behavior of a model room temperature ionic liquid under shear is explored using non-equilibrium molecular dynamics simulations. A coarsegrained model of liquid [C10mim][PF6], subjected to planar Couette shear flow is studied. The external field reduces intermolecular structure in the liquid. However, orientational ordering of the molecules in the form of a nematic phase is observed under shear. This chapter is reproduced with permission from Institute of Physics (IOP). S. G. Raju and S. Balasubramanian, “Intermolecular correlations in an ionic liquid under shear,” J. Phys.: Condens. Matter, 21, 035105, (2009). In Chapter 3, a dilute aqueous solution of the salt, 1-n-butyl,3-methylimidazolium hexafluorophosphate ([bmim][PF6]) has been studied using atomistic molecular dynamics simulations to investigate the effect of ions on water and vice versa. In the solution, the anion is found to diffuse faster than the cation, in contrast to observations in the pure ionic liquid. Distributions of pair energies have been employed to identify ion association, and around 13% of the ions were found to exist as pairs. vii The mean potential energy of water molecules present in the coordination shell of an anion is lesser than that of water molecules coordinated to a cation. The former kind also exhibit two distinct orientational preferences with respect to the anion. The larger diffusion coefficient of the anion is related to the faster dynamics of water molecules in its hydration layer, as evidenced from the relaxation of their residence time correlation function. This chapter is reproduced with permission from S. G. Raju and S. Balasubramanian, “Aqueous solution of [bmim][PF6]: Ion and Solvent Effects on Structure and Dynamics,” J. Phys. Chem. B, 113, 4799 (2009) Copyright 2009 Americal Chemical Society. In Chapter 4, the morphology of a room temperature ionic liquid, 1,3-didecylimidazolium hexafluorophosphate has been predicted from a coarse grain molecular dynamics simulation. The liquid is seen to spontaneously self-assembly into a lamellar phase. The non-polar alkyl tails attached to the imidazolium ring of the cation, form brushes separating the parallel sheets containing the charged species. A high degree of parallel stacking of the imidazolium rings is observed. The structure factor of the liquid exhibits a sharp feature at 3.3 nm−1 and a weaker one at 1.8 nm−1. This chapter is reproduced by permission of Royal Chemical Society. S. G. Raju and S. Balasubramanian, “Emergence of nanoscale order in room temperature ionic liquids: simulation of symmetric 1,3-didecylimidazolium hexafluorophosphate,” J. Mater. Chem., 19, 4343 (2009). In Chapter 5, molecular dynamics simulations of a series of bis(trifluoromethylsulfonyl)imide anion based room temperature ionic liquids have been carried out in order to identify the effects of the molecular symmetry of the cation on the structure and dynamics of the liquid. Simulations of ionic liquids with imidazolium cation containing varying lengths of alkyl groups were performed. The calculated density and total X-ray scattering function of the liquids agree well with experimental data. Liquids containing symmetric cations ([CnCnim][NTf2]) are found to be more structured than those with asymmetric ones ([CnC1im][NTf2]), manifested in greater intermolecular ordering and slower dynamics. This chapter is reproduced with permission from S. G. Raju and S. Balasubramanian, “Role of cation symmetry in intermolecular structure and dynamics of room temperature ionic liquids: Simulation studies,” J. Phys. Chem. B, 114, 6455 (2010) Copyright 2010 Americal Chemical Society. RTILs contain molecular ions which are in general, monovalent. In Chapter 6, we have explored the intermolecular structure, dynamics and intermediate range structure in a model ionic liquid whose cation and anion are mono- and divalent respectively. Charge compensation is met by doubling the mole fraction of the cations in the sample. Through coarse grained molecular dynamics simulations, we observe an enhancement of electrostatic interactions in the liquid which leads to greater ordering and sluggish dynamics, relative to traditional room temperature ionic liquids. The nanoscale heterogeneity inherent to RTILs is also further increased in these divalent systems. This chapter is reproduced with permission from S. G. Raju and S. Balasubramanian, “Molecular dynamics simulation of model room temperature ionic liquids with divalent anions” Indian J. Chem. A, 49, 721 (2010).