Nanostructural organization in acetonitrile/ionic liquid mixtures: molecular dynamics simulations and optical Kerr effect spectroscopy

In this paper, the nanostructural organization and subpicosecond intermolecular dynamics in mixtures of acetonitrile and the ionic liquid (IL) 1-pentyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C5mim][NTf2]) were studied as a function of concentration using molecular dynamics (MD) simulations and optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The MD simulations show the IL to be nanostructurally organized into an ionic network and nonpolar domains, with CH3CN molecules localized in the interfacial region between the ionic network and nonpolar domains, as found previously by other researchers. The MD simulations indicate strong interactions between CH3CN and the hydrogens on the imidazolium ring of the cation. The low-frequency (0-200 cm-1) intermolecular part of the reduced spectral densities (RSDs) of the mixtures narrows and shifts to lower frequency as the concentration of CH3CN increases. These spectral changes can be partly attributed to the increasing contribution of the low frequency intermolecular modes of CH3CN to the RSD. At a given composition, the RSD of a mixture is found to be broader and higher in frequency than the corresponding ideal RSD given by the volume-fraction weighted sum of the RSDs of the neat liquids. This difference is rationalized in terms of the competition between CH3CN-cation interactions and solute-induced disruption of the ionic networks.