Thermodynamic Stability of Fluid−Fluid Phase Separation in Binary Athermal Mixtures:  The Role of Nonadditivity

We studied the thermodynamic stability of fluid−fluid phase separation in binary nonadditive mixtures of hard-spheres for moderate size ratios. We are interested in elucidating the role played by small amounts of nonadditivity in determining the stability of fluid−fluid phase separation with respect to the fluid−solid phase transition. The demixing curves are built in the framework of the modified-hypernetted chain and of the Rogers−Young integral equation theories through the calculation of the Gibbs free energy. We also evaluated fluid−fluid phase equilibria within a first-order thermodynamic perturbation theory applied to an effective one-component potential obtained by integrating out the degrees of freedom of the small spheres. A qualitative agreement emerges between the two different approaches. We also addressed the determination of the freezing line by applying the first-order thermodynamic perturbation theory to the effective interaction between large spheres. Our results suggest that for intermediate size ratios a modest amount of nonadditivity, smaller than earlier thought, can be sufficient to drive the fluid−fluid critical point into the thermodinamically stable region of the phase diagram. These findings could be significant for rare-gas mixtures in extreme pressure and temperature conditions, where nonadditivity is expected to be rather small.