Volume-explicit equation of state and phase behavior for mixtures of hard disks

In this study, a simple volume-explicit equation of state (EOS) for mixtures of hard disks is developed and tested using molecular dynamic simulation data for both additive and nonadditive hard disk mixtures over a wide range of size ratio, densities and compositions. The equation accurately represents the simulation data in most of the cases studied. The EOS is accurate for all pressures or packing fractions in the case of additive and slightly nonadditive (nonadditivity parameter, Δ=+0.05) mixtures (which is the region of interest for real systems). For large nonadditivity, (Δ≥+0.2), the EOS is accurate only at low pressures/packing fractions. The new equation also gives the exact second and third pressure virial coefficients. The phase behavior of some nonadditive hard disks mixture is predicted using the developed equation. Symmetric phase boundaries are obtained for equal size species and unequal size species result in asymmetric phase boundaries. The developed EOS can be used as the repulsive part in developing volume-explicit real fluid equations of state for molecules on surfaces.