61st Annual Report on Research 2016

As a compelling case of sensitive structure-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. We discovered 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.

Reference: K. Yang, Z. Cai, M. Tyagi, M. Feygenson, J. C. Neuefeind, J. S. Moore, Y. Zhang*, "Odd-Even Structural Sensitivity on Dynamics in Network-Forming Ionic Liquids", Chem. Mater. 28, 3227 (2016).

2. Discovery of the Dynamic Odd-even Effect in Liquid n-Alkanes near Melting Points

n-Alkanes are the textbook examples of the odd-even effect, i.e., the difference in the periodic packing of odd- and even-numbered n-alkane solids results in the odd-even variation of their melting points. However, in the liquid state, where this packing difference is not obvious, it seems natural to assume that the odd-even effect does not exist, which is further supported by the fact that the boiling points of n-alkanes show a monotonic dependence on the chain length. We discovered a surprising odd-even effect in the translational diffusional dynamic properties of n-alkanes in their liquid states. To measure the molecules’ dynamics, we performed quasi-elastic neutron scattering measurements on n-alkanes near their melting points. We found that odd-numbered n-alkanes exhibit up to 30 times slower dynamics than the even-numbered ones near their respective melting points. Our results suggest that the dynamic properties of n-alkanes are extremely sensitive to number of carbons despite the fact that n-alkanes are the simplest hydrocarbon molecules.

The result is appealing because it shows a clear violation of the classic Kauzmann-Eyring theory of molecular viscous flow. We might have thought that no structural memory may carry over from the solids to the liquids. But clearly, the liquids already have the origins of the odd even effect built into its diffusion! Furthermore, on the practical side, the result is important to the petroleum industry as we know gasolines and crude oils are mainly alkanes. The studied dynamics of alkanes is directly relevant to the understanding and control of the viscosity of gasolines and crude oils. The result of the transport properties of n-alkanes are fundamental knowledge in petroleum science and are essential to a wide spectrum of chemical processes, such as lubrication, diffusion through porous media, heat transfer, etc.

Reference: K. Yang, Z. Cai, A. Jaiswal, M. Tyagi, J. S. Moore, Y. Zhang*, "Dynamic Odd-even Effect in Liquid n-Alkanes near Melting Points", Angew. Chem. Int. Ed. in press (2016)