Comparison of molecular heat transfer mechanisms between water and ammonia in the liquid states

Water and ammonia are both associated liquids with high thermal conductivity, but their degrees of molecular association are characterized differently by strong and weak hydrogen bonds, respectively. Here, we employed non-equilibrium molecular dynamics simulation to clarify and compare the molecular mechanisms of high thermal conductivity of water and ammonia. The molecular-scale heat transfer was analyzed in relation to molecular configuration using the atomistic heat path analysis [J. Heat Mass Transf. 108, 749 (2017)], where a single van der Waals (vdW) interaction and a single Coulomb interaction were considered as a heat path and their contributions to heat transfer were quantified. Both water and ammonia showed that the primary factor of high thermal conductivity is a large amount of heat transfer via Coulomb interaction, which was enabled by a high density of heat paths. These heat paths for outstanding Coulomb heat transfer included not only hydrogen bonds, but also more distant Coulomb interactions. On the other hand, an important role of hydrogen bond was to form a specific coordination structure of molecules that realizes the high heat path density. Such coordination structures differed between water and ammonia because of different strengths of hydrogen bond, which in turn led to different molecular pictures of heat transfer. Thus, water and ammonia achieve high thermal conductivity using somewhat different mechanisms at the molecular scale.