The phase behaviour of interaction site fluids

Two simple models for polar molecular fluids, the polar hard dumb-bell (PHD) and the polar hard sphere (PHS), are shown to have a critical point for an elongation of l/σ = 0·5. The solution of the site-site Ornstein-Zernike (SSOZ) equation is used to obtain the energy as a function of density and reduced dipole moment from which the excess Helmholtz free energy ΔA is obtained analytically. The coexistence and spinodal curves are generated and the classical gas-liquid critical point for PHD located at kTσ 3 /μ 2 = 0·106 and πρσ 3 /3 = 0·05. For more complex molecular fluids either the analytic solution is unknown or its use becomes impractical, so we investigate the accuracy and reliability of obtaining the free energy from numerical solutions of the SSOZ equation.