A modified Van der Waals model for the coexistence curve of expanded metals

Recent measurements for expanded Hg, Cs and Rb have shown that the liquid-vapour coexistence curves for these metals do not obey the 'law of rectilinear diameters'. It is shown here that a two-state Van der Waals equation with a density-dependent cohesive energy will lead to compositional fluctuations that result in a breakdown of the 'law'. Calculations predict that Hg and the alkali metals will exhibit similar behaviour near the critical point. However, in the case of Hg the 6p-6s band-gap closure, which occurs at higher than critical densities, leads to an anomalous behaviour not observed in the alkali metals. The influence of this transition on the rectilinear behaviour is treated by introducing a further modification of the two-state model.