Scaled particle theory for mixtures of nonadditive hard discs

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Abstract
A scaled particle theory for nonadditive hard discs is developed and solved which should have applications in the theory of partial localization of adsorbates. Solutions are given for positively as well as negatively nonadditive hard discs. For positive nonadditivity, the solutions obtained with the Gibbs–Duhem equation show better agreement with the single molecular dynamics calculation available. These solutions again show a demixing phase transition for large nonadditivity and coverage. As in three dimensions, the solutions from the virial equation are superior for negative nonadditivities. We believe that we have discovered a phase transition for large negative nonadditivity: Δcr=−0.365±0.005. For physical reasons, this should be an ordering of the fluid into a crystalline solid phase. The compressibility factor and the free energy have a minimum for equicomposition mixture. The free energy has a kink at the critical coverage ϑcr. This phase transition has also been found for negatively nonadditive hard spheres.