Gas-Liquid Coexistence in the Primitive Model for Water

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 particles 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.