Mean field lattice equations of state

Abstract

Abstract In preceding publications in this series, the Mean Field Lattice Gas (MFLG) model has been shown to produce a useful description of the pVT behavior of both polymers and low molar mass materials. Here we introduce a method, referred to as use of the Bondi constraint, by which the number of adjustable parameters required to apply the model can be reduced from five to three. This is accomplished by taking advantage of the general observations that (1) the parameter which represents the number of segments per molecule (or polymer repeat unit), m1, varies linearly with the van der Waals volume as calculated by Bondi and (2) the ratio of the molecular surface areas of a compound and a reference fluid, calculated from the MFLG parameters m1 and γ1, can be equated to the analogous ratio of surface areas calculated using Bondi's group contribution methodology. We show that this predictive method for derivation of the parameters m1 and γ1, produces a pVT description for ethane and ethylene which is comparable to that produced using the standard five-parameter fit. Thus the Bondi constraint is a useful means of generating material parameters that are necessary to apply the model to compounds for which few volumetric data are available.