Odd-Even Structural Sensitivity on Dynamics in Network-Forming Ionic Liquids
Ke Yang, Zhikun Cai, Madhu Sudan Tyagi, Mikhail Feygenson, Joerg Neuefeind, Jeffrey S. Moore, Yang Zhang
As a compelling case of sensitive-property relationship, an odd-even effect refers to the alternating trend of physical or chemical properties on odd/even number of repeating structural units. In crystalline or semi-crystalline materials, such odd-even effect emerge as manifestations of differences in the periodic packing patterns of molecules. Therefore, due to the lack of long-range order, such odd-even phenomenon is not expected for dynamic properties in amorphous state. Herein, we report the discovery of a remarkable odd-even effect of the dynamical properties in the liquid phase. In a class of glass-forming diammounium citrate ionic liquids, using incoherent quasi-elastic neutron scattering measurements, we measured the dynamical properties including the diffusion coefficient and the rotational relaxation time. These directly measured molecular dynamics showed pronounced alternating trends with increased number of methylene (-CH2-) groups in the backbone. Meanwhile, the structure factor S(Q) showed no long-range periodic packing of molecules, while the pair distribution function G(r) revealed subtle differences in the local molecular morphology. The observed dynamical odd-even phenomenon in liquids showed that profound dynamical changes originate from subtle local structural differences.
, Cai, Z.
, Tyagi, M.
, Feygenson, M.
, Neuefeind, J.
, Moore, J.
and Zhang, Y.
Odd-Even Structural Sensitivity on Dynamics in Network-Forming Ionic Liquids, Chemistry of Materials, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=920853
(Accessed March 2, 2024)