An analytic equation-of-state for mixture of square-well chain fluids of variable well width

An analytic perturbation theory equation of state for a mixture of freely-jointed square-well fluids of variable well width (1≤λ≤2) is developed. The equation of state is based on second-order Barker and Henderson perturbation theory to calculate the thermodynamic properties of the reference sphere fluid, and on first-order Wertheim thermodynamic perturbation theory to account for the connectivity of spheres to form chains. A real function expression for the radial distribution function of hard spheres and a one-fluid type mixing rule are used to obtain an analytic, closed-form expression, for the Helmohltz free energy of mixtures of square-well spheres. Good results were obtained when this equation of state was used with temperature-independent parameters to correlate vapor–liquid equilibrium data of pure substances and mixtures.