Physical property, structure, and dynamics studies of ionic liquid systems
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Abstract Ionic liquids (ILs) are organic salts with melting points at or below 373 K. As the physicochemical properties of ionic liquids can be modulated according to need, they are also called “designer” solvents. A comparative study of the intermolecular dynamics of a nonpolar molecular solute CS2 in a series of monocationic ILs, 1-alkyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)amide ([CnC1im][NTf2] for n = 3-5) and their dicationic equivalents, 1,6-bis(3-methylimidazolium-1-yl) alkane bis(trifluoromethane-sulfonyl)amide ([(C1im)2Cn][NTf2]2 for n = 6, 8 and 10) was done as a function of concentration using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The OHD-RIKES data indicate that CS2 molecules experience a stiffer potential in dicationic ILs as compared to monocationic ILs. The effect of cation symmetry on the solvation of solute molecules in imidazolium-based ILs was investigated by comparing the OKE spectra of CS2 in asymmetric ILs, ([CnC1im][NTf2] for n = 3, 5 and 7) to those of CS2 in same number of alkyl chain carbon atoms, symmetric ILs ([(Cn)2im][NTf2] for n = 2-4) using OHD-RIKES. The OKE results indicate that the CS2 molecules experience stiffer potential in shorter chained symmetric IL C2C2, than in asymmetric IL, C3C1. But in longer alkyl chain length, CS2 molecules experience similar stiffness of potential irrespective of symmetry of the alkyl chains. Two series of aromatically functionalized ILs containing common cations 1,3-dibenylimidazolium and 1-(2-naphthylmethyl)-3-methylimidazolium, but with varying anions were successfully synthesized and crystallized. Determinations of single X-ray crystal structures indicate that variation of anions has an effect on hydrogen bonding and π-stacking interactions in the crystalline states of these ILs. In addition, these two series of ILs that differ in symmetry due to the position of the phenyl rings had different patterns of packing of ions in the crystal lattice. Understanding how cation structure determines physicochemical properties of ILs helps in further tuning and designing ILs for a specific application. The densities of the homologous series of branched ILs and their linear counterparts with a given number of alkyl carbon atoms N, were measured as functions of temperature. This study showed that branching of alkyl chains does not affect the densities of ILs.