Metastable liquid-liquid coexistence and density anomalies in a core-softened fluid

TL;DR

This study explores how ramp potentials with a liquid-liquid critical point exhibit water-like density anomalies and metastability, providing insights into water's anomalous properties through simulation of core-softened models.

Contribution

It introduces a series of ramp potentials interpolating between stable and metastable regimes, revealing phase behaviors similar to water and supporting the second critical point hypothesis.

Findings

LLCP becomes metastable near Lennard-Jones limit

Density maxima line resembles that of water

Metastable LDL-HDL coexistence line has negative slope

Abstract

Linearly-sloped or `ramp' potentials belong to a class of core-softened models which possess a liquid-liquid critical point (LLCP) in addition to the usual liquid-gas critical point. Furthermore they exhibit thermodynamic anomalies in the density and compressibility, the nature of which may be akin to those occurring in water. Previous simulation studies of ramp potentials have focused on just one functional form, for which the LLCP is thermodynamically stable. In this work we construct a series of ramp potentials, which interpolate between this previously studied form and a ramp-based approximation to the Lennard-Jones (LJ) potential. By means of Monte Carlo simulation, we locate the LLCP, the first order high density liquid (HDL)-low density liquid (LDL) coexistence line, and the line of density maxima for a selection of potentials in the series. We observe that as the LJ limit is approached, the LLCP becomes metastable with respect to freezing into a hexagonal close packed crystalline solid. The qualitative nature of the phase behaviour in this regime shows a remarkable resemblance to that seen in simulation studies of accurate water models. Specifically, the density of the liquid phase exceeds that of the solid; the gradient of the metastable LDL-HDL line is negative in the pressure (p)-temperature (T) plane; while the line of density maxima in the p-T plane has a shape similar to that seen in water and extends well into the {\em stable} liquid region of the phase diagram. As such, our results lend weight to the `second critical point' hypothesis as an explanation for the anomalous behaviour of water.