Gas-Liquid Coexistence in the Primitive Model for Water

TL;DR

This paper investigates the gas-liquid phase coexistence and critical point in the primitive model for water, revealing metastability and differences from spherical potentials, with implications for colloidal and protein systems.

Contribution

It provides the first detailed evaluation of the gas-liquid coexistence line and critical point for the primitive model for water, highlighting its metastability and unique phase behavior.

Findings

Gas-liquid coexistence is metastable and occurs where the crystal phase is favored.

No crystallization observed near the critical point unlike spherical potentials.

The gas-liquid instability region is significantly smaller than in spherical models.

Abstract

We evaluate the location of the gas-liquid coexistence line and of the associated critical point for the primitive model for water (PMW), introduced by Kolafa and Nezbeda [J. Kolafa and I. Nezbeda, Mol. Phys. 61, 161 (1987)]. Besides being a simple model for a molecular network forming liquid, the PMW is representative of patchy proteins and novel colloidal particles interacting with localized directional short-range attractions. We show that the gas-liquid phase separation is metastable, i.e. it takes place in the region of the phase diagram where the crystal phase is thermodynamically favored, as in the case of articles interacting via short-range attractive spherical potentials. Differently from spherical potentials, we do not observe crystallization close to the critical point. The region of gas-liquid instability of this patchy model is significantly reduced as compared to equivalent models of spherically interacting particles, confirming the possibility of observing kinetic arrest in an homogeneous sample driven by bonding as opposed to packing.