Modelling the solid-liquid-vapour phase behaviour of n-alkanes in a TPT-1 framework

We study the global phase behaviour of n-alkanes applying Wertheim's first order thermodynamic perturbation theory (TPT1). The molecules are modelled as homonuclear chains comprised of m freely jointed spherical segments interacting via the Lennard-Jones potential. Vega et al. [J. Chem. Phys. 116, 17 (2002)] have shown that the TPT1 is suitable to treat solid phases as well as fluid phases when model chains are considered, but that the adoption of a fully flexible chain model leads to the under-prediction of triple point temperatures and overestimation of the fluid ranges in comparison to experiment. Here, we propose a model in which a different number of segments are used to treat the fluid and the solid phase. The number of segments used to model the molecules in the fluid phase mf, and the LJ monomer potential parameters s and e are taken from published soft-SAFT values, whereas in the case of the solid phase a reduced temperature-dependent effective chain length ms(T*) is determined through a minimisation between theoretical and experimental liquid-solid phase equilibrium data. We refer to this model as effective-solid TPT1 (es-TPT1). We use the model proposed to calculate the solid-liquid-vapour phase diagrams of several n-alkanes and com-pare with experimental data. In the approach proposed, the conformation of chains in the solid phase is decoupled from the fluid phase, and an excellent description of the melting properties, as well as accurate predictions of the triple point temperatures for the n-alkanes examined is obtained. This simple solution provides an avenue to model the solid-liquid-vapour phase behaviour of other real substances in a TPT1 framework, and hence within the SAFT family of equations.

Automated summary

A new model is proposed to study the phase behavior of n-alkanes, which provides accurate predictions of the triple point temperatures and phase diagrams. By adjusting the number of molecules and potential parameters for the fluid and solid phases, the model decouples the conformation of chains in the solid phase from the fluid phase, resulting in an excellent description of melting properties. This approach can be applied to model the phase behavior of other substances within the SAFT family of equations.