Application of the generalised SAFT-VR approach for long-ranged square-well potentials to model the phase behaviour of real fluids

Abstract

In a recent generalisation of the SAFT-VR equation of state the method was extended so as to deal with wide square-well ranges, namely, 1.2 ≤ λ ≤ 3.0 [B. H. Patel, H. Docherty, S. Varga, A. Galindo, and G. C. Maitland. Mol. Phys., 103(1), 129–139, 2005.]. In this work, this equation is used to revisit the adjustment of intermolecular model parameters, with special emphasis on substances where the upper boundary of the potential range (λ = 1.8) has been previously reported or may be expected on grounds of the polar nature of the molecules. For this purpose, we follow the work of Clark et al. [G. N. I. Clark, A. J. Haslam, A. Galindo, and G. Jackson. Mol. Phys., 104(22-24), 3561–3581, 2006] and study a relative least squares objective function and the percentage absolute average deviation (%AAD) to determine the intermolecular model parameters (m, λ, σ, ε/kB, εhb/kB and rc) by comparison to experimental vapour-pressure and saturated liquid density data. In order to ensure in each case that the global minimum is identified, the dimensionality of the problem is reduced by discretising the parameter-space. Applying this method to the study of argon, nitrogen, benzene, carbon dioxide, carbon monoxide, n-alkanes, the refrigerant R1270, water, hydrogen chloride and hydrogen bromide, we find that the optimal models always present square-well ranges λ < 1.8, meaning that an upper bound value of λ = 1.8 (as set in the original approach) for the square-well range is sufficient to model real fluids. Accurate intermolecular potential models with ranges higher than 1.8 are also identified, but we find that these do not usually correspond to the global minimum of the objective function considered.