The Effect on Ionic Liquid Structure When a Carbon Atom in the Cation is Replaced by Silicon

In an ionic liquid, when a carbon atom in the cation is replaced by silicon, its viscosity may drop up to 7.4 times. We studied the structure of ionic liquids before and after this substitution. The molecular structures of the two ionic liquids that we studied, C-mim+ / NTf2- and Si-mim+ / NTf2- are shown in the figure below.

In the S(q) of C-mim+ / NTf2- , the charge-charge correlation peak (which arises from the charge ordering in ionic liquids) at q ≈ 0.4 Å-1 is stronger than that in Si-mim+ / NTf2-, indicating the charge ordering is weakened when the atom substitution takes place.

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The two graphs above show the spatial distribution of the cations (red blocks)  and anions (blue) around another cation/anion. The orange block shows the intermolecular distribution of cation tails regarding to cation head. The probability distribution in the two ionic liquids are plotted at the same isodensity value. It is clear that the intermolecular interactions in C-mim+ / NTf2- is stronger than that in Si-mim+ / NTf2-.

Please read our papers for more details if you are interested in this work.

Boning Wu, Hideaki Shirota, Sharon Lall-Ramnarine and Edward W. Castner, Jr., “Structure of ionic liquids with cationic silicon-substitutions.”, J. Chem. Phys., 2016, 145, 114501, doi: 10.1063/1.4962257.