Intrinsic structure and interfacial tension of the HDL/LDL water interface

Area of Science:

• Physical Chemistry
• Condensed Matter Physics
• Materials Science

Background:

• Water exhibits distinct high-density liquid (HDL) and low-density liquid (LDL) phases in its supercooled region.
• The interface between these liquid phases is challenging to study due to metastability and slow relaxation dynamics.
• Complex interfacial morphologies arise from low free-energy costs, hindering experimental and simulation-based analysis.

Purpose of the Study:

• To investigate the structure and thermodynamics of the HDL/LDL water interface.
• To determine the interfacial tension between HDL and LDL water phases.
• To characterize the complex morphology and fluctuations at the coexistence line.

Main Methods:

• Utilized long-term molecular dynamics simulations (approx. 1.35 μs) with explicit water molecules.
• Employed elongated simulation cells with a high 1:1:8 aspect ratio to stabilize planar interfaces.
• Applied capillary wave theory, accounting for intrinsic interfacial broadening, to analyze interfacial properties.

Main Results:

• Achieved stable planar coexistence of HDL-rich and LDL-rich water phases for the first time in simulations.
• Observed complex morphology including HDL-rich slabs, LDL-rich droplets, and fluctuating ice-like crystallites.
• Determined a very soft HDL/LDL interface with an interfacial tension (γ) of 4.25 ± 0.25 mN/m.

Conclusions:

• The HDL/LDL interface in supercooled water is weak, structured, and highly susceptible to distortions.
• Competing liquid and crystalline fluctuations significantly influence the interface's properties.
• The findings provide crucial insights into the complex phase behavior of water under metastable conditions.