Thermodynamic response functions of the Curie-Weiss cell fluid model. I. Supercritical region
In this work, we present an analytical study of the thermodynamic response functions of a multiple-occupancy cell fluid model with competing Curie-Weiss attraction and local repulsion. In the grand canonical ensemble, explicit expressions are derived for the three independent response functions: the isothermal compressibility, the thermal pressure coefficient, and the isochoric heat capacity. The thermal expansion coefficient and the isobaric heat capacity are subsequently obtained from exact thermodynamic identities. The analytical formulas are used to investigate the behavior of the response functions in the supercritical region. The isothermal compressibility, the thermal expansion coefficient, and the isobaric heat capacity exhibit critical divergences, whereas the thermal pressure coefficient and the isochoric heat capacity remain finite at the critical points despite developing pronounced extrema. The thermal pressure coefficient displays a non-monotonic oscillatory dependence on density and becomes negative over a limited range of temperatures and densities, leading to negative values of the thermal expansion coefficient. The obtained analytical expressions provide a consistent theoretical framework for investigating the thermodynamic properties of the model and establish a basis for future analysis of the subcritical regime and multiphase equilibrium.