Morphology and intermolecular dynamics of 1-Alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ionic liquids: Structural and dynamic evidence of nanoscale segregation

Here we report on the structural and dynamical properties of a series of room temperature ionic liquids, namely 1-Alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C_nmim][NTf₂]), with varying alkyl chain lengths (1≤n≤10) at ambient temperature, where all the salts are stable liquids. Using small-wide angle x-ray scattering (SWAXS), three major diffraction peaks are found: two high- Q peaks that show little dependence on the alkyl chain length (n) and a low-Q peak that strongly depends both in amplitude and position on n. This low-Q peak is the signature of the occurrence of nanoscale structural heterogeneities whose sizes depend on the length of the alkyl chain and are related to chain segregation into nano-domains. Using optical heterodyne-detected Raman-induced Kerr effect spectroscopy, we access intermolecular dynamic features that suggest that chain aggregation only occurs for n≥3, in agreement with the SWAXS data. Moreover, the increase in the frequency and width of the main band of the optical Kerr effect spectra in going from n = 2 to 3 is consistent with stiffening of the intermolecular potential due to chain segregation. Multicomponent line shape analysis suggests that there are least three modes that underlie the main band in the 0-200 cm^-1 region of the optical Kerr effect spectra of these ionic liquids. Given the similarity of ionic liquids to other complex fluid systems, we assign the low-frequency component to a fast β-relaxation mode and the intermediate- and high-frequency components to librational modes.