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

Abstract

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.