The shape of the van der Waals loop and universal critical amplitude ratios

Assuming that a van der Waals loop for a fluid, ferromagnet, etc, can be defined, its shape is analysed on the basis of universal critical amplitude ratios for three-dimensional Ising-type systems. Selected estimates for these ratios are tabulated. Near criticality, the loop attains a. universal scaled form given, optimally. by a new parametric extended sine modal for the bulk asymptotic equation of state; but a (particular) low-order interpolating Pade approximant provides a reasonable fit. Among other universal features, the reduced spinodal magnetization/density and conjugate field are found to be ((m) over bar(s), (h) over bar(s)) similar or equal to (0.697, -0.51(3)) in contrast to the van der Waals values (0.577..., -0.384...): thus the spinodal lies appreciably closer to the binodal (or coexistence curve) than predicted classically.