Molecular thermal energy transfer in binary mixture of simple liquids

Molecular thermal energy transfer in liquid mixtures was investigated using non-equilibrium molecular dynamics (NEMD) simulations of heat conduction in a series of binary mixture liquids with various compositions. Mixtures of argon and krypton were selected as simple liquids which were modeled using the Lennard-Jones (LJ) interaction potential. Thermal conductivities for these mixtures in a saturated state at a constant temperature (127 K) were compared with respect to mixture composition, i.e., molar fraction of each component. According to the molecular energy transfer mechanism, microscopic heat conduction flux can be decomposed into two molecular-scale contributions: one from self-transportation, which includes transport of kinetic energy and potential energy by molecular motion, and the other via the contribution of intermolecular energy transfer based on intermolecular interaction. The intermolecular energy transfer, which is the dominant source of macroscopic heat conduction flux in liquids, can be decomposed into contributions from various pair interactions, which in the present study are energy transfers via the paths ArAr, ArKr, and KrKr. The above contributions to heat conduction were evaluated quantitatively. It was found that the composition dependence of the first self-transportation contribution is mostly caused by the differences of their thermodynamic states, and the variation of total thermal conductivity with the composition is dominated by the second intermolecular energy transfer contribution, which accounts for about 80% of the total. The contributions of individual interaction paths were analyzed with our new concept of atomistic heat path [Matsubara et al., J. Chem. Phys., 142 (2015), 164509]. It was found that these contributions are mostly affected by the number densities of the interaction paths in the first neighbor shell. The molecular-scale internal structure of the liquid is changed by changes in composition, which affects the efficiency of the energy transfer by a single intermolecular interaction. A sensitivity analysis of LJ cross-molecular parameters was done using various combining rules. We found that the decomposed energy transfer contribution of interaction paths, ArAr and KrKr may be affected by changing the LJ parameters of ArKr.