Interaction between Neighboring Supercritical Water Molecules and Density Fluctuation by Molecular Dynamics Simulations

Y Wang and JL Xu and XJ Ma, JOURNAL OF THERMAL SCIENCE, 31, 907-922 (2022).

DOI: 10.1007/s11630-022-1574-0

Supercritical water (scW) is important for various engineering applications. The structure and distribution of scW is key to dominate the related processes and phenomena. Here, scW is investigated using molecular dynamics (MD) simulation with controlled pressure and temperature. Density oscillation is observed to occur in a 1 nm thickness bin, indicating mass exchange of particles across the bin interface. We show that the low density scW behaves strong heterogeneity. Quantitative analysis of system density fluctuations is performed by square root error and maximum structure factor, demonstrating the agreement between the two methods. The scW molecules are tightly gathered to form "liquid island" locally, but are very sparse in other regions, which are similar to the gas-liquid mixture in subcritical pressure. A target molecule is tracked to plot 3D displacements and rotating angles, with the former indicating large amplitude ballistic (diffusing) motion and small amplitude oscillation, and the latter displaying two scales of angle jumping. Both translation and rotating motion are related to hydrogen bond break up and reorganization. The low density scW behaves isolated molecules with few combinations of hydrogen bonds between molecules, while the high density scW behaves more combinations of molecules via hydrogen bonds. The two scales motion is expected to influence thermal/chemical process in supercritical state, deepening the fundamental understanding of scW structure.

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