A simple model for size non-additive mixtures

A simple and general mixture model based on the exact third virial coefficient of a non-additive hard spheres mixture has been developed. It reproduces the correct composition dependence of all virial coefficients. The new model, together with a model proposed recently, was tested by comparing the prediction of fourth virial coefficients with the exact values. More extensive tests of these models were performed at high densities by comparing their predictions of compressibility factors, for mixtures of additive hard spheres, non-additive hard spheres and Lennard-Jones molecules, with simulation data. For real molecules, predictions of molar densities of binary mixtures of argon and krypton and of carbon dioxide and ethane were compared with experimental data. The two models are more accurate than the van der Waals one-fluid model. One is even more accurate than the Weeks-Chandler-Andersen-Lee-Levesque-Grundke-Henderson (WCA-LL-GH) perturbation theory model. Application of the new models to real mixtures shows the importance of including size non-additivity in mixture models.