Novel phase behavior in a two-dimensional network-forming lattice fluid

A two-dimensional lattice gas model of network-forming particles on a. triangular lattice is studied by the grand canonical Monte Carlo method. The effects of association and cooperative interactions, which lead to a weakening of the association interaction, are considered. Phase diagrams for several systems are presented. It is shown that the molecular association leads to the formation of two different condensed phases characterized by different densities and internal structures. The transition between those two condensed phases appears to be a seldom-observed truly second-order transition at finite temperatures.