Singular coexistence-curve diameters: Experiments and simulations

Precise calculations of the coexistence-curve diameters of a hard-core square-well (HCSW) fluid and the restricted primitive model (RPM) electrolyte exhibit marked deviations from rectilinear behavior. The HCSW diameter displays a |t|1−α singularity that sets in sharply for |t| ≡ |T − Tc|/Tc ≲ 10−3; this compares favorably with extensive data for SF6, also reflected in C2H6, N2, etc. By contrast, the curvature of the RPM diameter varies slowly over a wide range |t| ≲ 0.1; this behavior mirrors observations for liquid alkali metals, specifically Rb and Cs. Amplitudes for the leading singular terms can be estimated numerically but their values cannot be taken literally.