Competing interactions near the liquid-liquid phase transition of core-softened water/methanol mixtures

TL;DR

This study uses molecular dynamics simulations to explore how competing interactions influence the liquid-liquid phase transition and density anomalies in water/methanol mixtures, revealing how fine-tuning these interactions affects anomalous behaviors.

Contribution

It demonstrates the connection between spontaneous crystallization, density anomalies, and the critical temperature in water/methanol mixtures, highlighting the role of competing interactions.

Findings

Pure water-like fluid shows LLPT and density anomaly.

Pure methanol-like fluid has LLPT but no density anomaly.

Increasing crystallization temperature correlates with the disappearance of density anomaly.

Abstract

Water is an unique material with a long list of thermodynamic, dynamic and structural anomalies, which are usually attributed to the competition between two characteristic length scales in the intermolecular interaction. It has been argued that a potential liquid-liquid phase transition (LLPT) ending at a liquid-liquid critical point (LLCP) lies at the core of the anomalous behavior of water. This transition which has been evidenced in multiple simulation studies seems to be preempted experimentally by spontaneous crystallization. Here, in order to expose the connection between the spontaneous crystallization observed in the supercooled regime in the vicinity of the LLPT, and the density anomaly, we perform extensive Molecular Dynamics simulations of a model mixture of core-softened water and methanol. The pure water-like fluid exhibits a LLPT and a density anomaly. In contrast, our pure methanol-like model does have a LLPT but lacks the density anomaly. Our results illustrate the relation between the vanishing of the density anomaly and an increase in the temperature of the spontaneous crystallization: once this temperature surpasses the LLCP critical temperature, no density anomaly is observed. This peculiar feature illustrates how fine tuning the competitive interactions determine the anomalous behavior of water/alcohol mixtures.