Selected topics in the statistical mechanics of fluids.

Abstract

The phase behaviour and thermodynamic properties of simple model mixtures are examined using the statistical associating fluid theory as extended to chain molecules interacting with potentials of variable range (SAFT-VR), and by computer simulation. The SAFT-VR approach is based on an accurate and compact representation of the free energy of chain molecules. We present the SA FT -VR methodology as applied to mixtures of non-conformal molecules. A series of mixing rules are presented, beginning with the van der Waals one-fluid prescription and including more complex treatments. The vapour-liquid equilibria of a mixture consisting of hard spheres and square-well monomers is examined with the SAFT-VR equation of state, together with the liquid-liquid equilibria of a symmetrical square-well mixture with no unlike interactions. Additionally, we examine the vapour-liquid equilibria of a square-well monomer-dimer mixture, composed of equal-sized segments, both with the SAFT-VR approach and by Gibbs ensemble Monte Carlo simulation. The simulation data are used to determine the vapourliquid critical line of the mixture. An extension of the SAFT-VR approach to describe the phase behaviour of chain molecules interacting with a soft repulsive potential and an attractive well of variable range is presented. We focus on the vapour-liquid properties of Lennard-Jones chains using a simple recipe for the evaluation of the chain free energy. We also perform a case study for a specific class of phase equilibria exhibited by binary mixtures, where systems are seen to posses a region of closed-loop immiscibility in their phase diagrams. We examine the nature of this type of pl1ase behaviour using the SA FT· VR equation of state and Gibbs ensemble simulation for a simple model system with an anisotropic bonding site, which is seen to be the governing factor in the appearance of the region of low-temperature miscibility for this system. The model is chosen in order to mimic the physical features of real systems which exhibit this type of re-entrant phase behaviour. The critical regions of this model are examined using a finite-size scaling analysis performed in the semigrand canonical ensemble