Phase Transitions in Two-Dimensional Lattice Gases of Hard-Core Molecules with Long-Range Attractions

We have investigated the phase behavior that results when an infinitely weak and long-range attractive potential is added to the following hard-core lattice gases: the triangular-lattice gases with exclusions up to first, second, third, and fourth neighbors, and the square-lattice gases with exclusions up to first, second and third neighbors. Three of the systems considered have realistic (i.e., argonlike) phase diagrams, complete with a first-order solid-fluid phase change, a first-order liquid-gas phase change (with a critical point), and a triple point. Three other systems have a first-order solid-fluid phase change. The remaining system has a first-order liquid-gas phase change along with a higher-order transition that is not of the typical solid-fluid type. We find that when the hard-core system has a second-order or first-order phase change to begin with, the addition of the attractive potential spreads the transition out into a first-order phase change with temperature-dependent coexistence densities that bracket the density at which (or density interval over which) the original transition takes place. We also find that the presence or absence of realistic phase behavior for the combined system appears to be dependent upon the shape of the hard core as well as its range.