Evidence of phase separation in mixtures of Lennard-Jones and Stockmayer fluids

Mixtures of Lennard-Jones and Stockmayer molecules provide a convenient model system which can be used to study the effects of composition and polarity in polar/non-polar mixtures. The structure and phase behaviour of such mixtures are currently topics of great interest Eli. Since the Stockmayer potential, qSs, is obtained from the usual Lennard-Jones potential, ~buj, by the sole addition of a point dipole interaction, the free energy of mixing can be determined in a particularly simple and straightforward manner. Moreover, knowledge of the free energy of the pure components is not required, as can be seen by considering the following two-stage process. Firstly a fraction of atoms, x, in a pure Lennard-Jones liquid is changed into molecules with a moment of inertia I. One may think of these as Stockmayer molecules with dipole moment # = 0. The free energy per particle (in units of the LJ well depth ~) of this ideal mixture is given by