Fluid-fluid phase separation of nonadditive hard-sphere mixtures as predicted by integral-equation theories

We investigate the existence of a fluid-fluid phase separation in binary mixtures of equal-size hard spheres with positively nonadditive diameters [i.e., d11 = d22 ≡d, d12 = (1 + Δ)d with Δ > 0]. An integral-equation approach is used to evaluate both thermodynamics and structure of many symmetric (equal to equimolar) mixtures (with Δ=0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1) and some asymmetric cases. We present the results obtained via the Percus-Yevick, the Martynov-Sarkisov, and the Ballone-Pastore-Galli-Gazzillo closures; the thermodynamic consistency of these approximations is discussed and some possible ways to get further improvements are proposed too. The integral-equation results are then compared with the available "exact" simulation data, a first-order perturbation approach, and a scaled particle theory. Our study predicts that there exists a demixing for each considered value of the nonadditivity parameter Δ. © 1991 American Institute of Physics.