Augmented van der Waals Theory for Mixtures

The excess mixing properties and the critical line for mixtures of square-well molecules of equal core sizes and equal range of the forces are exactly evaluated in the high temperature limit. This model, when used at any temperature, can predict all the qualitative features, including immiscibility, observed in simple gas mixtures. This confirms the conclusion of the van der Waals theory of mixtures that differences in core size are not essential for these phenomena. On the other hand, such differences have a large quantitative effect, and if they are too great immiscibility disappears, as shown here for a mixture of square-well and perfect gas particles.