Dynamic Odd-Even Effect in Liquid n-Alkanes near Their Melting Points

• Published in: Angewandte Chemie International Edition 2016-11, Vol.55 (45), p.14090-14095
• Main Authors: Yang, Ke, Cai, Zhikun, Jaiswal, Abhishek, Tyagi, Madhusudan, Moore, Jeffrey S., Zhang, Yang
• Format: Article
• Language: English
• Subjects: alkanes; molecular dynamics; odd-even effect; quasi-elastic neutron scattering; relaxation times
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201607316

n‐Alkanes are the textbook examples of the odd–even effect: The difference in the periodic packing of odd‐ and even‐numbered n‐alkane solids results in odd–even variation of their melting points. However, in the liquid state, in which this packing difference is not obvious, it seems natural to assume that the odd–even effect does not exist, as supported by the monotonic dependence of the boiling points of n‐alkanes on the chain length. Herein, we report a surprising odd–even effect in the translational diffusional dynamic properties of n‐alkanes in their liquid states. To measure the dynamics of the molecules, we performed quasi‐elastic neutron scattering measurements near their melting points. We found that odd‐numbered n‐alkanes exhibit up to 30 times slower dynamics than even‐numbered n‐alkanes near their respective melting points. Our results suggest that, although n‐alkanes are the simplest hydrocarbons, their dynamic properties are extremely sensitive to the number of carbon atoms. Odd‐bods have much in common: Odd–even effects are well‐known for n‐alkanes in the solid state, but they are not expected in the liquid state. Quasi‐elastic neutron scattering uncovered a surprising odd–even effect in the dynamic properties of liquid n‐alkanes (see graph) that violates the Kauzmann–Eyring prediction. The result demands reexamination and extension of the classical theories of molecular viscous flow.