Intermolecular energy transfer in liquid water and its contribution to heat conduction: A molecular dynamics study

Intermolecular energy transfer (IET) is a dominant factor in heat conduction in liquid. The IET in liquid water and its contribution to macroscopic heat conduction under a temperature gradient were analyzed by a molecular dynamics simulation utilizing the extended simple point charge (SPC/E) potential model. Intermolecular energy exchange rates (IEERs) for both the translational and rotational motion of molecules were defined and their characteristics examined. The IEER of hydrogen-bonded molecules and nonbonded molecules have different characteristics. The IEER oscillates with a high amplitude and its time average, which is much smaller than the magnitude of the IEER, gives the effective rate of the IET that contributes to macroscopic heat conduction. In the present study, the effective rate of the IET was assumed to be proportional to the magnitude of the IEER. Based on the characteristics of the IEER and the above supposition, contributions of the translational and rotational IET between hydrogen-bonded molecules and nonbonded molecules to macroscopic heat conduction were evaluated. The evaluated results were compared with the results of a molecular dynamics (MD) simulation of heat conduction under a constant temperature gradient, and good qualitative agreement between the predicted value and the simulated result was found. The rotational IET was found to be dominant as compared with the translational IET, and the contribution of hydrogen-bonded molecules to heat conduction was found to be relatively small. The possibility of a mechanism that cancels the IET between distant molecules and the development of a precise model for this mechanism were also discussed.