Thermodynamic stability of fluid-fluid phase separation in binary athermal mixtures: The role of nonadditivity

TL;DR

This study investigates how small nonadditive effects influence the stability of fluid-fluid phase separation in binary mixtures of hard spheres, revealing that even modest nonadditivity can stabilize fluid-fluid coexistence.

Contribution

The paper demonstrates that minor nonadditivity in binary hard-sphere mixtures can significantly affect phase stability, using multiple theoretical approaches for validation.

Findings

Small nonadditivity can stabilize fluid-fluid phase separation.

Qualitative agreement between integral equation and perturbation theories.

Implications for rare-gas mixtures under extreme conditions.

Abstract

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.