Liquid–vapor asymmetries in pure fluids

Abstract

The coexistence curve and the compressibility‐like quantity (∂ρ/∂μ)_T at the coexistence curve are calculated for Mermin’s decorated lattice gas. We tabulate the Padé approximants to the Ising model properties which were used in these calculations. It is shown that for the proper choice of the adjustable parameter in the model the asymmetries in the coexistence curve and in (∂ρ/∂μ)_T over a large temperature range, T_c/2 < T < T_c, are qualitatively similar to those in real fluids. Also, similar correlations between the overall coexistence curve asymmetry and the amplitudes of divergences for the critical region occur in the model and in real fluids. Based on our model calculations, several predictions for fluid behavior near the critical point are listed: (1) The asymptotic asymmetry in the coexistence curve and in (∂ρ/∂μ)_T at the coexistence curve should be most observable for fluids with large overall asymmetry such as SF₆, NH₃, and H₂O; (2) the range of the asymptotic asymmetry for (∂ρ/∂μ)_T is expected to extend somewhat beyond t = 1 − (T/T_c) = 0.1 for such fluids; (3) in order to obtain an estimate for the compressibility exponent γ′ that is within a few percent of the true asymptotic value, experiments will have to be confined to the range t < 10⁻². Recent experimental data for (∂ρ/∂μ)_T at the coexistence curve are found to be consistent with the model for t ≳ 10⁻², but are inconclusive closer to the critical point.