Liquid-Liquid Crossover in Water Model: Local Structure vs Kinetics of Hydrogen Bonds.

Area of Science:

• Thermodynamics
• Physical Chemistry
• Computational Fluid Dynamics

Background:

• Polymorphism in liquids, particularly water, is characterized by distinct local structures within thermodynamic regions of the phase diagram.
• Understanding liquid-state polymorphism is crucial for explaining water's anomalous properties.

Purpose of the Study:

• To investigate the presence and characteristics of a liquid-liquid crossover in the TIP4P/2005 water model.
• To analyze the impact of this crossover on water's structural, dynamic, and hydrogen bonding properties.

Main Methods:

• Utilized molecular dynamics simulations of the TIP4P/2005 water model.
• Analyzed local order parameters, self-diffusion coefficients, and hydrogen bond kinetics.

Main Results:

• Identified a smooth liquid-liquid crossover line in the phase diagram of the TIP4P/2005 water model at approximately 3150 ± 350 atm.
• Observed significant changes in molecular dynamics, with self-diffusion coefficients peaking near the crossover.
• Detected alterations in hydrogen bond kinetics, including changes in average lifetimes and coordination number dynamics.

Conclusions:

• The TIP4P/2005 water model exhibits a liquid-liquid crossover, indicating distinct thermodynamic regions with different local ordering.
• This crossover profoundly influences water's dynamic and structural properties, including hydrogen bond behavior.
• The free energy landscape concept provides a framework for interpreting hydrogen bond kinetics in relation to coordination numbers.