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

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.

7 pages, 8 figures

References in corpus (2)

• Structural Order for One-Scale and Two-Scale Potentials
• A Family of Tunable Spherically-Symmetric Potentials that Span the Range from Hard Spheres to Water-like Behavior

Cited by in corpus (9)

• Representability problems for coarse-grained water potentials
• Phase behavior of a fluid with competing attractive and repulsive interactions
• Excess entropy, Diffusivity and Structural Order in liquids with water-like anomalies
• Thermodynamics and Dynamics of the Two-Scale Spherically-Symmetric Jagla Model of Anomalous Liquids
• Water-like hierarchy of anomalies in a continuous spherical shouldered potential
• Structural anomalies for a three dimensional isotropic core-softened potential
• Insights into glass formation and glass transition in supercooled liquids, by study of related phenomena in crystals
• Phase behaviour of attractive and repulsive ramp fluids: integral equation and computer simulation studies
• Anomalous phase behavior of a soft-repulsive potential with a strictly monotonic force