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Two-Component Dynamics and the Liquidlike to Gaslike Crossover in Supercritical Water
Peihao Sun1, J B Hastings1, Daisuke Ishikawa2
1SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
Physical Review Letters
|January 8, 2021
Summary
Molecular dynamics in supercritical water transition from liquid- to gaslike behavior with increasing temperature. This shift is driven by a diminishing hydrogen-bond network, observed via scattering and simulation.
Area of Science:
- Physical Chemistry
- Materials Science
Background:
- Understanding molecular dynamics in water under extreme conditions is crucial for various chemical and physical processes.
- Supercritical water exhibits unique properties relevant to energy and environmental applications.
Purpose of the Study:
- To investigate the molecular-scale dynamics of water in sub- to supercritical states.
- To elucidate the transition in dynamics from liquid-like to gas-like behavior with increasing temperature.
Main Methods:
- Inelastic X-ray scattering (IXS) was employed to probe dynamics.
- Molecular dynamics (MD) simulations were used to complement experimental data.
Main Results:
- Longitudinal current correlation spectra revealed two main components: a low-frequency (LF) gas-like component and a high-frequency (HF) component related to O-O stretching in hydrogen bonds.
- Increasing temperature caused a shift in spectral weight from HF to LF, indicating a weakening of the hydrogen-bond network.
- A transition from liquid-like to gas-like dynamics was observed, with significant changes near the Widom line.
Conclusions:
- The study successfully characterized the molecular dynamics transition in supercritical water.
- The findings highlight the role of hydrogen bonding in dictating water's dynamic behavior across different states.
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