The Structure of Zwitterionic Liquids

In this work, we analyzed the structures of a liquid zwitterion (zwitterionic liquid), 1-[2-(2-methoxyethoxy)ethyl]-3-(3-carboxypropyl)-imidazolium (OE2ImC3C), and its analogous ionic liquid, 1-ethyl-3-methylimidazolium acetate (OE2eim+/OAc-). The molecular structures two ionic liquids are very similar, except that in OE2ImC3C, the positively charged part is connected to the negatively charged part with a covalent bond.

Their structure factors S(q) are obtained using high-energy X-ray scattering method and Molecular Dynamics Simulations. Peaks are observed at q = 0.42 Å-1 and q = 0.67 Å-1 are observed in the S(q) of OE2ImC3C and OE2eim+/OAc-, respectively, indicating that intermediate range ordering exhibits in both liquids.

An improved interpretation of liquid structure can be obtained by partitioning the total structure factor into ionic contributions, as shown in the figure above, where S(q) = SC-C(q)+SA-C(q)+SA-C(q)+SA-A(q) (A stands for anion and C stands for cation). The strong peaks and anti-peaks in OE2ImC3C at 0.8 Å-1 tell us that even though weaker than in conventional ionic liquids, the charge ordering is still a significant feature for zwitterionic liquids. In OE2ImC3C, the low q peak is contributed by anion-anion interaction, this is a sign that in OE2ImC3C, the intermediate range ordering arises from nano-domain aggregations. Lack of peak in SA-A(q) of OE2eim+/OAc- indicates that the intermediate range in OE2eim+/OAc- is not caused by nano-domain aggregations. We hypothesize that the low q peak in OE2eim+/OAc- is caused by hydrogen bonding network, since analyzing the molecular dynamics reveals that the number of hydrogen bonds in OE2eim+/OAc- is greater than in OE2ImC3C.

The structure of another zwitterionic liquid, OE2ImC5C, is also investigated. We found that structure features of OE2ImC5C, including S(q) and hydrogen bonding, is between OE2ImC3C and OE2eim+/OAc-. This tells us that increasing positive and negative charge separation has a similar effect to changing the carbon-carbon bond in the zwitterionic OE2ImC3C to obtain the standard ionic liquid.

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

Boning Wu, Kosuke Kuroda, Kenji Takahashi, and Edward W. Castner, Jr., “Structural Analysis of Zwitterionic Liquids vs. Homologous Ionic Liquids.”, J. Chem. Phys., 2018, 148, 193807, doi: 10.1063/1.5010983.


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