Coexistence and criticality of fluids with long-range potentials

Using mixed-field finite-size scaling simulations, we have investigated the liquid–vapor critical behavior of three-dimensional fluids with algebraically decaying attractive pair interactions, which vary like −1/r3+σ with σ=3, 1, and 0.1. The finite-size scaling analysis was carried out by matching the critical ordering operator distribution, pL(x), against the limiting Ising form, i.e., Ising criticality was assumed. When the potential is short-ranged (σ=3) the simulation results are entirely consistent with the expected Ising critical behavior. When the potential is long-ranged (σ=1, 0.1), however, marked deviations from Ising behavior are observed, particularly in the form of the critical ordering operator distribution, and in the estimated values of β/ν. The results are consistent with non-Ising criticality which is predicted theoretically in fluids with long-range interactions. Some results from Gibbs ensemble simulations are also provided in order to sketch the shape of the liquid–vapor coexistence envelope. We discuss the relevance of our results to the current issue of criticality in ionic fluids.