期刊
JOURNAL OF PHYSICS-CONDENSED MATTER
卷 21, 期 42, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.1088/0953-8984/21/42/424121
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资金
- National Science Foundation [CHE-0718678]
- American Chemical Society Petroleum Research Fund [47615-AC6]
- Welch Foundation [D-0775]
- Industrial Advisory Board of QUILL
- EPSRC [EP/D029538/1]
- Engineering and Physical Sciences Research Council [EP/D029538/1] Funding Source: researchfish
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(n)mim][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 beta-relaxation mode and the intermediate- and high-frequency components to librational modes.
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