Evolution of Ion–Ion Interactions and Structures in Smectic Ionic Liquid Crystals

Journal Article · Fri Jul 26 00:00:00 EDT 2019 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/acs.jpcc.9b04056· OSTI ID:1728643

Park, Seo Kyung ^[1]; Han, Kee Sung ^[2]; Lee, Jin Hong ^[3]; Murugesan, Vijayakumar ^[2]; Lee, Seung Hyun ^[1]; Koo, Chong Min ^[4]; Lee, Je Seung ^[1]; Mueller, Karl T. ^[2]

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1. Kyung Hee Univ., Seoul (Korea, Republic of)
2. Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Argonne National Lab. (ANL), Argonne, IL (United States). Joint Center for Energy Storage Research (JCESR)
3. Pusan National Univ., Busan (Korea, Republic of)
4. Korea Inst. of Science and Technology, Seoul (Korea, Republic of)

Mixture of ionic liquid, 1,3-dimethylimidazolium 2-methoxy(2-ethoxy(2-ethoxy(2-ethoxy)))ethylphosphite ([DMIm][MPEGP]), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), which forms an ionic liquid crystal with a superior ionic conductivity, evolves a smectic structure through ion–ion interactions as a function of LiTFSI concentration. Here, nuclear magnetic resonance (NMR) spectroscopy and pulsed field gradient (PFG) NMR examinations showed that the morphology of the structures and the strength of ion–ion interactions are closely related to the ratio of Li⁺/[MPEGP]^–. The results revealed structures composed of Li⁺ : [MPEGP]^– : TFSI^– (≈ 3 : 4 : 1) mainly by the Coulombic interactions between Li⁺ cations and phosphite head groups in [MPEGP]^– anions. NMR diffraction on PFG-echo profiles revealed a cluster size of ~ 2 µm, inversely proportional to the number of mobile [MPEGP]^– anions, and an ion diffusion on the cluster surfaces that is 1000 times faster than that in the bulk liquid. These observations suggest that the superior ionic conductivity in the mixture is mainly due to the fast ion transport on the cluster surfaces of smectic ionic liquid crystals. The variations of diffusion ratios between mobile Li⁺, TFSI^–, [DMIm]⁺, and [MPEGP]^– ions indicate that these mobile ions remaining in the voids of structures preserve the bulk ionic liquid properties.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: National Research Foundation of Korea (NRF); USDOE Office of Science (SC)
• Grant/Contract Number: AC05-76RL01830
• OSTI ID: 1728643
• Report Number(s): PNNL-SA--146187
• Journal Information: Journal of Physical Chemistry. C, Journal Name: Journal of Physical Chemistry. C Journal Issue: 33 Vol. 123; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY