Translational and rotational diffusion in water in the Gigapascal range.

Area of Science:

• Condensed matter physics
• Physical chemistry
• Materials science

Background:

• Understanding water's behavior under extreme conditions is crucial for various scientific fields.
• Previous studies on water's self-dynamics at high pressures were limited.

Purpose of the Study:

• To measure the translational and rotational diffusion coefficients of liquid water at high pressures (up to 3 GPa).
• To compare experimental results with molecular dynamics simulations.
• To investigate the influence of pressure on water's self-dynamics and hydrogen bonding.

Main Methods:

• Utilized a novel setup for the Paris-Edinburgh press designed for quasielastic neutron scattering.
• Performed direct measurements of diffusion coefficients along the 400 K isotherm.
• Employed molecular dynamics simulations for comparative analysis.

Main Results:

• Translational diffusion significantly decreases with increasing pressure, with a slower variation above 1 GPa.
• Rotational diffusion remains largely insensitive to pressure changes.
• Observed decoupling of translational diffusion from shear viscosity at high pressures.

Conclusions:

• The Stokes-Einstein-Debye equations are inadequate for predicting water's self-diffusion at high temperatures and pressures.
• The rigidity of the first neighbor shell and stable hydrogen bond network explain the pressure insensitivity of rotational diffusion.
• Hot dense water exhibits complex behavior that challenges simple liquid models.