Theory and simulation for associating hard chain fluids

The effect of intermolecular association on the thermodynamic properties of chain molecules with 2, 3, 4, and 8 hard sphere segments is studied using Metropolis Monte Carlo simulation and theory. The purpose of the study is to test the extension of Wertheim's theory for associating fluids by Chapman et al. (1988, Molec. Phys., 65, 1057) to associating polyatomic chains. Since the first order theory predicts that the properties of associating chains are independent of the position of the association site, we consider rigid triatomics with two positions of the association site, along the molecular axis on the terminal segment and perpendicular to the molecular axis on the middle segment. The association site is modelled as a short ranged, orientation dependent square well. For tetraspheres and octaspheres we consider flexible chains with one association site placed on the terminal segment. We have also studied the case of diatomics and tetraspheres with three association sites on the terminal segment. The agreement between molecular simulation results and predictions from theory is good. We explain approximations used to simplify the theory, and show how a spherical reference can be used to predict the properties of an associating chain fluid.