We study 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 evaluate
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 address 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.Comment: 17 pages, 7 figures, to appear in J. Phys. Chem.