Extended Veytsman statistics for the solid-fluid transition in the framework of lattice fluid

Lattice fluid can describe a vapor-liquid transition but not a solid-fluid transition. In this work, we propose a simple and analytic term which yields a solid-fluid transition when coupled with a lattice based equation of state (EOS). The proposed term is derived based on the two assumptions that (1) solid can be considered as highly associated phase affected by strong attractive force and (2) this force is distinct from the conventional attractive forces yielding a vapor-liquid transition. To formulate these assumptions, we extend Veytsman statistics by modifying its density dependency. The derived term was combined with a quasi-chemical nonrandom lattice fluid theory (QLF) developed by the authors. The combined model was found to require only two parameters besides 3 QLF parameters for physical properties calculation of three phases. When tested against equilibrium properties of 8 components, the combined model was found to closely reproduce melting pressure, sublimation pressure, and vapor pressure, but underestimate solid density as well as heat of melting at the triple point temperature. It was found that the present approach can yield a solid-liquid transition at all temperatures.