Entropy-driven liquid-liquid separation in supercooled water

TL;DR

This paper introduces an entropy-driven thermodynamic model for supercooled water that accurately describes experimental data and suggests liquid-liquid separation is driven by entropy, not energy, with implications for understanding water's anomalies.

Contribution

The model provides a simpler, more accurate description of supercooled water's properties using fewer parameters, emphasizing entropy-driven phase separation.

Findings

Model fits experimental data better than previous models.

Predicts density maxima at a constant fraction of low-density structure.

Identifies entropy as the driving force for liquid-liquid separation.

Abstract

Twenty years ago Poole et al. (Nature 360, 324, 1992) suggested that the anomalous properties of supercooled water may be caused by a critical point that terminates a line of liquid-liquid separation of lower-density and higher-density water. Here we present an explicit thermodynamic model based on this hypothesis, which describes all available experimental data for supercooled water with better quality and with fewer adjustable parameters than any other model suggested so far. Liquid water at low temperatures is viewed as an 'athermal solution' of two molecular structures with different entropies and densities. Alternatively to popular models for water, in which the liquid-liquid separation is driven by energy, the phase separation in the athermal two-state water is driven by entropy upon increasing the pressure, while the critical temperature is defined by the 'reaction' equilibrium constant. In particular, the model predicts the location of density maxima at the locus of a near-constant fraction (about 0.12) of the lower-density structure.