Continuous and First-Order Liquid-Solid Phase Transitions in Two-Dimensional Water.

Area of Science:

• Physical Chemistry
• Materials Science
• Nanotechnology

Background:

• Understanding water's phase behavior in nanoscale confinement is crucial for fundamental science and nanofluidics.
• Previous studies explored water freezing in nanoslits, but pressure-dependent transitions require further investigation.

Purpose of the Study:

• To investigate the pressure-dependent phase transitions of water confined between smooth walls.
• To identify novel ice phases formed under varying lateral pressures.
• To elucidate the nature of continuous and first-order phase transitions in nanoconfined water.

Main Methods:

• Molecular-dynamics (MD) simulations were conducted over sub-microsecond to microsecond timescales.
• Water was confined between two smooth walls with a separation of 1.0 nm.
• Simulations explored a range of lateral pressures from low (≤10 MPa) to high (≥400 MPa) and moderate (100–300 MPa).

Main Results:

• First-order phase transitions occurred at low (≤10 MPa) and high (≥400 MPa) lateral pressures, forming bilayer low-density amorphous (BL-LDA) ice and trilayer puckered high-density ice (TL-pHDI), respectively.
• A continuous phase transition was observed within moderate lateral pressures (100–300 MPa), resulting in a new phase: bilayer puckered high-density amorphous (BL-pHDA) ice.
• These findings extend previous observations of continuous phase transitions in nanoconfined water.

Conclusions:

• Nanoconfined water exhibits distinct phase behaviors, including continuous and first-order transitions, dependent on lateral pressure.
• The formation of novel ice phases like BL-pHDA ice under moderate pressures offers new insights into low-dimensional water thermodynamics.
• This research contributes to a deeper understanding of water's complex behavior in nanoscale environments.