The Effect of Anion Size on the Structure of Ionic Liquids
When a sufficiently long side chain is present in an ionic liquid cation, immediate range ordering in ionic liquid structure can be observed, due to the nano-domain segregation between the polar cation/anion head and non-polar cation tail. But how will the liquid structure be affected when the anion also contains a non-polar chain? To answer the question, we studied three ionic liquids with anions that have different non-polar chain lengths. The anions are bis(fluorosulfonyl)imide (FSI-), bis(trifluoromethanesulfonyl) imide (NTf2-), and bis(pentafluoroethanesulfonyl) imide (BETI-). They are pair with the same cation 1- methyl-3-trimethylsilylpropylimidazolium to form ionic liquids. The cation is large enough to introduce nano-domain segarations in the ionic liquid structure.
The structure factors of these ionic liquids are obtained using high energy X-ray scattering and Molecular Dynamics simulations. Each of these ionic liquids has a "first sharp diffraction peak" (FSDP) at q ≈ 0.4 Å, which is the sign of intermediate range ordering. When the fluorinated chain in the anion gets longer, the FSDP shifts to a higher q range, indicating smaller intermediate-range-domain in the IL structure. Therefore, we found that in this case a longer non-polar chain in the anion will make the non-polar domain shrink.
Please read our papers for more details if you are interested in this work:
Boning Wu, Yuki Yamashita, Takatsugu Endo, Kenji Takahashi and Edward W. Castner, Jr., “Structure and Dynamics of Ionic Liquids: Trimethylsilylpropyl-Substituted Cations and Bis(sulfonyl)amide Anions.”, J. Chem. Phys., 2016, 145, 244506, doi: 10.1063/1.4972410.
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