Gas–liquid coexistence in asymmetric primitive models of ionic fluids

Abstract

The gas–liquid coexistence of z:1 asymmetric primitive models is studied using the collective variable based theory. Coexistence-curve data are examined in terms of the corresponding-state variables τ ∗ = | T* − T c*|/ T c* and Δρ ∗ = | ρ ∗ − ρ c*|/ ρ c*. Calculations are made for 1:1, 2:1, 3:1 valences and for different values of ion size ratio λ = σ +/ σ − ( λ ≥ 1 and λ ≤ 1). We analyze a dependence of the critical amplitude of the coexistence curve and the asymptotic slope of the coexistence-curve diameter on the ion size ratio at a fixed valence. It is shown that the both characteristics take maximum values in the equisize case and demonstrate a nonmonotonous behavior with λ at z ≥ 2. Besides, the critical amplitude depends on λ very weakly. As expected, the approximation considered in this work predicts the mean-field critical behavior for size- and charge-asymmetric primitive models in the vicinity of the critical point. A comparison with the available theoretical and simulation results is given.