Liquid-liquid phase transitions in supercooled water studied by computer simulations of various water models

TL;DR

This study uses computer simulations to explore multiple liquid-liquid phase transitions in supercooled water models, revealing their locations, sensitivities, and implications for understanding real water's anomalies.

Contribution

It demonstrates the presence of multiple liquid-liquid phase transitions in various water models and discusses their relation to real water's thermodynamic behavior.

Findings

Multiple liquid-liquid phase transitions observed in all studied models.

Transitions' locations are highly sensitive to water model specifics.

The apparent singularity in real water may be linked to the spinodal of the first transition.

Abstract

Liquid-liquid and liquid-vapor coexistence regions of various water models were determined by MC simulations of isotherms of density fluctuation restricted systems and by Gibbs ensemble MC simulations. All studied water models show multiple liquid-liquid phase transitions in the supercooled region: we observe two transitions of the TIP4P, TIP5P and SPCE model and three transitions of the ST2 model. The location of these phase transitions with respect to the liquid-vapor coexistence curve and the glass temperature is highly sensitive to the water model and its implementation. We suggest, that the apparent thermodynamic singularity of real liquid water in the supercooled region at about 228 K is caused by an approach to the spinodal of the first (lowest density) liquid-liquid phase transition. The well known density maximum of liquid water at 277 K is related to the second liquid-liquid phase transition, which is located at positive pressures with a critical point close to the maximum. A possible order parameter and the universality class of liquid-liquid phase transitions in one-component fluids is discussed.