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Title: Structural and Rotational Dynamics of Carbon Dioxide in 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids: The Effect of Chain Length

Journal Article · · Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry

Ionic liquids (ILs) have been proposed as possible carbon dioxide (CO2) carbon capture media, and so it is useful to understand the dynamics of both the dissolved gas and its IL environment as well as how altering the IL affects these dynamics. With increasing alkyl chain length, it is well established that ILs obtain a mesoscopic structural feature assigned to polarapolar segregation, and the change in structure with chain length affects dynamics. In this work, the dynamics of CO2 in a series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids (ILs), where alkyl is ethyl, butyl, hexyl, or decyl, were investigated using ultrafast infrared spectroscopy by measuring the reorientation and spectral diffusion of carbon dioxide in the ILs. It was found that reorientation of the carbon dioxide occurs on 3 timescales, which correspond to 2 different timescales of restricted wobbling-in-a-cone motions and a long time complete diffusive reorientation. The complete reorientation slows with increasing chain length but less than the increases in viscosity of the bulk liquids. Spectral diffusion, measured with 2D IR spectroscopy, is caused by a combination of the liquids’ structural fluctuations and reorientation of the CO2. The data were analyzed using recent theory that takes into account both contributions to spectral diffusion and extracts the structural spectral diffusion. Different components of the structural fluctuations have distinct dependences on the alkyl chain length. All of the dynamics are fast compared to the complete orientational randomization of the bulk ILs, as measured with optical heterodyne detected optical Kerr effect measurements. The results indicate a hierarchy of constraint releases in the liquids that give rise to increasingly slower dynamics.

Research Organization:
Stanford Univ., CA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)
Grant/Contract Number:
FG02-84ER13251; CHE-1461477
OSTI ID:
1804755
Journal Information:
Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry, Vol. 120, Issue 27; ISSN 1520-6106
Publisher:
American Chemical SocietyCopyright Statement
Country of Publication:
United States
Language:
English

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