Two-component dynamics and the liquid-like to gas-like crossover in supercritical water

TL;DR

This study investigates the microscopic dynamics of supercritical water, revealing a transition from liquid-like to gas-like behavior characterized by a shift in spectral components as temperature increases.

Contribution

It combines inelastic X-ray scattering and molecular dynamics simulations to identify two main dynamic components in supercritical water and links spectral changes to the liquid-gas crossover.

Findings

Spectral weight shifts from high-frequency to low-frequency components with temperature.

Identification of a two-component dynamic spectrum in supercritical water.

Rapid dynamic changes occur around the Widom line.

Abstract

Molecular-scale dynamics in sub- to super-critical water is studied with inelastic X-ray scattering and molecular dynamics simulations. The obtained longitudinal current correlation spectra can be decomposed into two main components: a low-frequency (LF), gas-like component and a high-frequency (HF) component arising from the O--O stretching mode between hydrogen-bonded molecules, reminiscent of the longitudinal acoustic mode in ambient water. With increasing temperature, the hydrogen-bond network diminishes and the spectral weight shifts from HF to LF, leading to a transition from liquid-like to gas-like dynamics with rapid changes around the Widom line.