Singular coexistence-curve diameters: Experiments and simulations

Abstract 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 − T c |/ T c ≲ 10 −3 ; this compares favorably with extensive data for SF 6 , also reflected in C 2 H 6 , N 2 , 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.