On the theoretical description of the liquid-vapor coexistence of water-like models with square-well attraction and site-site chemical association

A liquid-vapor coexistence envelope for water-like models with square-well attraction between particles and with four attractive associating sites per particle has been investigated. The mean-field theory and the first-order mean spherical approximation for attractive interactions together with the first-order thermodynamic perturbation theory of Wertheim for association have been applied to construct the temperature - density and the temperature - chemical potential projections of the equation of state. In addition, the dependence of the fraction of non-bonded particles on the parameters of the models has been evaluated and analyzed. We explored a set of models with different square-well width and energy of associative interaction. In some cases a comparison with computer simulation data has been performed. For two specific parameter sets previously proposed to mimic water properties a comparison with experimental data for the vapor-liquid coexistence envelope and for vapor pressure has been performed. It is shown that the applied theoretical procedures are successful and can be used with confidence. On the other hand, they provide solid basis to apply in the theory of inhomogeneous associating fluids.