Liquid-Liquid Phase Transitions for Soft-Core Attractive Potentials

TL;DR

This study uses molecular dynamics simulations to explore how a soft-core attractive potential influences the phase diagram of a system, revealing conditions for both gas-liquid and liquid-liquid critical points and their dependence on potential parameters.

Contribution

It systematically investigates the impact of potential parameters on phase behavior, identifying conditions for dual critical points and proposing a modified van der Waals model to explain these phenomena.

Findings

Both gas-liquid and liquid-liquid critical points can coexist for certain parameters.

Critical temperature and pressure derivatives behave similarly to standard liquids for the gas-liquid transition.

A modified van der Waals equation qualitatively captures the critical behavior observed.

Abstract

Using event driven molecular dynamics simulations, we study a three dimensional one-component system of spherical particles interacting via a discontinuous potential combining a repulsive square soft core and an attractive square well. In the case of a narrow attractive well, it has been shown that this potential has two metastable gas-liquid critical points. Here we systematically investigate how the changes of the parameters of this potential affect the phase diagram of the system. We find a broad range of potential parameters for which the system has both a gas-liquid critical point and a liquid-liquid critical point. For the liquid-gas critical point we find that the derivatives of the critical temperature and pressure, with respect to the parameters of the potential, have the same signs: they are positive for increasing width of the attractive well and negative for increasing width and repulsive energy of the soft core. This result resembles the behavior of the liquid-gas critical point for standard liquids. In contrast, for the liquid-liquid critical point the critical pressure decreases as the critical temperature increases. As a consequence, the liquid-liquid critical point exists at positive pressures only in a finite range of parameters. We present a modified van der Waals equation which qualitatively reproduces the behavior of both critical points within some range of parameters, and give us insight on the mechanisms ruling the dependence of the two critical points on the potential's parameters. The soft core potential studied here resembles model potentials used for colloids, proteins, and potentials that have been related to liquid metals, raising an interesting possibility that a liquid-liquid phase transition may be present in some systems where it has not yet been observed.