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

A two-dimensional lattice gas model of network-forming particles on a. tria ngular lattice is studied by the grand canonical Monte Carlo method. The ef fects of association and cooperative interactions, which lead to a weakenin g of the association interaction, are considered. Phase diagrams for severa l systems are presented. It is shown that the molecular association leads t o the formation of two different condensed phases characterized by differen t densities and internal structures. The transition between those two conde nsed phases appears to be a seldom-observed truly second-order transition a t finite temperatures.