Optical Kerr effect spectroscopy, physical properties, and molecular dynamics simulations of imidazolium-based ionic liquid systems
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Room temperature ionic liquids (RTILs) are defined as materials that are composed of cations and anions and melt at or below 373 K. RTILs are the subject of extensive study from researchers of different areas due to their advantages over conventional solvents to be applied in a variety of applications. An important feature of ILs is that their physicochemical properties can be tuned by the selection or the functionalization of their constituent ions. Thus gaining an understanding of structure-property relationship of RTILs is of great importance in developing them for practical applications. Physicochemical property measurements (i.e. density, viscosity, thermal transitions), molecular dynamics (MD) simulations, and optical heterodyne detected Raman induced Kerr effect spectroscopy (OHD-RIKES) of a homologous series of new prepared branched imidazolium based ionic liquids (ILs) were carried out and compared with a corresponding series of linear imidazolium based ILs. A molecular level understanding of structure-property relationship of these ILs were gained. The change in heterogeneity in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CnC1im][NTf2]) based ILs with increasing alkyl chain length was explored by studying the dynamics of a nonpolar molecular solute CS2 by means of OHD-RIKES. The OHD-RIKES results indicate a change in heterogeneity occurring at n=3, which is consistent with previous MD simulation results. The Local structure and intermolecular dynamic study of benzene/[C1C1im][NTf2] mixtures were studied by combining MD simulations with OHD-RIKES. The simulated density of states (DOS) results provide a means of interpreting the Kerr spectra in terms of the motions of benzene and the [C1C1im]+ cations. The Kerr spectra of mono-benzyl and di-benzyl substituted imidazolium bis[(trifluoromethane)sulfonyl]amide ILs indicate that the phenyl moieties see a stiffer intermolecular potential than benzene molecules in the corresponding benzene/[C1C1im][NTf2] mixture.