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 a mplitude ratios for three-dimensional Ising-type systems. Selected est imates for these ratios are tabulated. Near criticality, the loop atta ins a. universal scaled form given, optimally. by a new parametric ext ended sine modal for the bulk asymptotic equation of state; but a (par ticular) low-order interpolating Pade approximant provides a reasonabl e fit. Among other universal features, the reduced spinodal magnetizat ion/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 app reciably closer to the binodal (or coexistence curve) than predicted c lassically.