Unified description for the temperature dependence of mobility in liquids

JC Yungbluth and GA Medvedev and BM Savoie and JM Caruthers, JOURNAL OF CHEMICAL PHYSICS, 158, 164503 (2023).

DOI: 10.1063/5.0145794

The physical mechanisms governing molecular mobility in liquids remain unresolved. Whether distinct mechanisms govern mobility in the Arrhenian and super-cooled regions, and whether the mobility in these regions can be unified remain open questions. Here, molecular dynamics simulations were used to search for a structural property with a temperature dependence related by a simple functional form to the temperature dependent translational diffusion coefficient, D-trans. The logarithm of D-trans was found to be a two-parameter function of F-2, where F-2 is the mean squared-force per molecule. The relationship is demonstrated for all systems investigated: a three-bead model of ortho-terphenyl, an 80:20 binary mixture of Lennard-Jones spheres, and a system of Lennard- Jones dumbbells. For each system, the relationship holds for the entire range of temperatures under both constant pressure and constant density conditions. The same F-2-based expression describes the translational diffusion coefficient in the Arrhenian, crossover, and super-Arrhenian regions.

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