Theory and simulation of associating fluids: Lennard-Jones chains with association sites

Abstract

A perturbation approach based on Wertheim's first order theory for associating fluids is applied to predict the thermodynamic properties of chain molecules with association sites. A Lennard-Jones chain is used as a reference to account for the repulsive, dispersive and anisotropic (shape) contributions, while the short range directional attraction peculiar to associating fluids, modelled as spherical square-well sites, are treated as a perturbation over the chain reference fluid. Isobaric-isothermal Monte Carlo simulations were performed for model fluids composed of a chain of four tangently bonded Lennard-Jones spheres. Three cases are considered: one-site models with the association site placed either in an end sphere or in a central sphere, and a two-site model with sites placed at both ends. Subcritical and supercritical temperatures and several association strengths were studied. While the theory is seen to predict accurately the simulation results, its practical accuracy is subject to the limitations of the reference equation of state and the approximations made regarding the reference fluid.