Development of Continuous Wertheim-like Association Site Models Suitable for Molecular Dynamics Simulations

One of the most recent versions of the Statistical Associating Fluid Theory for variable range potentials (SAFT-VR) is based on the Mie potentials (SAFT-g Mie) [1,2]. Apart from its capability to model complex fluid phase... [ view full abstract ]

One of the most recent versions of the Statistical Associating Fluid Theory for variable range potentials (SAFT-VR) is based on the Mie potentials (SAFT-g Mie) [1,2]. Apart from its capability to model complex fluid phase equilibria, one of its unique features is the very accurate representation of the underlying intermolecular potential. This close correspondence between the SAFT-γ Mie equation of state parameters and the parameters of the implied potential have allowed the development of coarse-grained force fields for describing fluid properties through molecular simulations [3]. Current implementations of this top-down parameterisation technique are limited to non-associating fluids, although, in principle nothing precludes its use for deriving potentials for associating fluids.

Wertheim-like [4,5] models, which are at the heart of the SAFT models, have proven highly successful in thermodynamic property prediction and in model parameterisation of associating fluids. These models utilise short range square well potentials, which are off-centred from the locus of reference isotropic fluid potentials. The discontinuous nature of the embedded association sites, make them difficult to employ in molecular dynamics simulations. Alternatively smoothly varying treatments of association are required.

In this study a formal procedure for the transmutation of Wertheim-like models into continuous analogues is presented. The new models are validated by comparing vapour-liquid equilibrium properties determined by molecular dynamics (MD) with Monte Carlo (MC) and with the corresponding SAFT predictions. The ultimate goal of this work is to develop continuous association models for molecules of interest, such as H₂S and H₂O, and the investigation of the dynamical properties.

References

[1] T. Lafitte, A. Apostolakou, C. Avendaño, A. Galindo, C. Adjiman, E. A. Müller, and G. Jackson, J. Chem. Phys., 139:154504, 2013.

[2] V. Papaioannou, T. Lafitte, C. Avendaño, C. S. Adjiman, G. Jackson, E. A. Müller, A. Galindo, J. Chem. Phys., 140:054107, 2014.

[3] E. A. Müller and G. Jackson, Annu. Rev. Chem. Biochem. Eng. 5: 405–427, 2014.

[4] M. S. Wertheim, J. Stat. Phys., 42:459-476, 1986.

[5] S. Dufal, T. Lafitte, A. J. Haslam, A. Galindo, G. N. I. Clark, C. Vega and G. Jackson, Mol. Phys., 113:948-984, 2015.