Microscopic picture of heat conduction in liquid ethylene glycol by molecular dynamics simulation: Difference from the monohydric case

The present study investigates the molecular-scale heat transfer in the liquid of ethylene glycol, which is widely used as heat transfer media. First, by combining existing molecular models, we developed a new united atom model of ethylene glycol, and showed that this model reasonably reproduces the experimental thermal conductivity. Using the non-equilibrium molecular dynamics simulations with this model, we characterized the heat transfers due to different kinds of inter- and intramolecular interactions on the basis of a picture that a single pair interaction is a path of heat transfer. These characteristics were compared with those of ethanol (Matsubara et al., 2017) to elucidate the molecular mechanism which realizes an enhanced thermal conductivity because of an additional hydroxylation on ethanol. The results indicate that the thermal conductivity enhancement occurs because the additional heat paths provided by the second hydroxyl group increases the amount of heat conduction owing to all of the van der Waals, Coulomb, and covalent interactions. In particular, the increase in the number of the paths associated with the intermolecular Coulomb interaction between the non-hydrogen bonding hydroxyl groups is prominent and consequently the Coulomb interaction, which is an efficient heat carrier, performs the largest amount of heat conduction in ethylene glycol. Although the second hydroxyl group also increases the number of hydrogen bonds, the direct heat transfer via the hydrogen bonds accounts for only a small portion of the total heat conduction. On the other hand, this augmentation of hydrogen bond, since it keeps a dense molecular packing against the increase in molecular volume, is indispensable in increasing the density of heat paths.