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 their 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 behavior 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 into the 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 behavior of water.

Phase Transitions in Fluid State of Systems of Purely Repulsive Potentials~!2008-02-04~!2008-05-13~!2008-06-11~!

“…[7], it is shown that the HDL-LDL transition can happen unaccompanied by the density anomaly, considering that the density anomaly surely appears in the PRP fluids, therefore, one can conclude that a combinational influence from the soft repulsion at smaller interparticle distance and an attractive tail at larger interparticle distance can remove away the density anomaly and at the same time induce the HDL-LDL transition. The combinational influence from the soft repulsion and attractive tail makes the investigation of the CSA potential [7] a very interesting problem [26][27][28], but far away from solution. It may be a feasible route to uncover the mechanism for the rich phenomena displayed by the CSA potential by disentangling the CSA potential and selecting the hard sphere repulsion plus the soft repulsion for single investigation, the present report is a first step towards the aim.…”

Which Shape Characteristics of the Intermolecular Interaction of Liquid Water Determine Its Compressibility?

“…Toward this aim, a number of core-softened and simplified models have been put forward and used to perform numerical simulations. None of them [20][21][22][23][24][25][26][27][28][29][30][31][32]., however, has quantitatively reproduced any anomaly of liquid water or elucidated the thermodynamic mechanisms that cause the anomaly. The author believes that the self-consistent Ornstein-Zernike approximation (SCOZA) with hardcore repulsion plus Yukawa tails is presently the most useful.…”

Thermodynamic mechanism of the density anomaly of liquid water

“…Models from both groups have presented us with many fruitful results regarding the anomalous behavior of liquids. Although the shapes of the potential tails of both groups are very different, they exhibit water-like anomalous behaviors (see e.g., [22][23][24][25][26][27][28][29][30][31][32] and references quoted therein). It has recently been shown that even weakly softened potentials are able to yield water-like anomalies, i.e., in this case anomalies arise without ever invoking (as usually done) the interplay between two repulsive length scales [33,34].…”