Abstract. 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.

Print publication: Issue 42 (21 October 2009)
Received 22 May 2009, in final form 15 September 2009
Published 29 September 2009