Deviations from corresponding-states behavior in the vapor-liquid equilibrium of the square-well fluid

The vapor-liquid equilibrium of the square-well (SW) fluid of variable range is studied. The analysis focuses on the dependence on the SW range, which exhibits deviations from corresponding-states behavior. The study is based on a new, compact and accurate equation for the SW Helmholtz free-energy. This equation relies on the mean-field approximation and a scaled-particle theory of Boublik, and agrees well with available computer simulations. The position of the critical point, the vapor pressures and the width of the orthobaric curve are obtained in terms of the width of the well. They show non-trivial oscillatory departures from corresponding-states behavior as given by the augmented van der Waals theory.