Some properties of the GHC equation of state

•Composition functionality of the GHC energy parameter is studied.•Mean value approximations of all non-quadratic terms are proposed.•Approximations result in small errors in predicted density.•GHC predictions of second virial coefficients are studied.•Many numerical examples are presented to support analyses.

The composition functionality of the mixture energy parameter, aM, used in the Gibbs-Helmholtz Constrained (GHC) equation of state is studied. An analysis is presented that shows that aM for liquid mixtures is approximately quadratic in composition. All non-quadratic behavior is due solely to ln(TcM) in the GHC up-scaling equation. If a mean value approximation of this term is used, then aML is quadratic, but non-symmetric, in composition. For vapors, non-quadratic behavior is coupled to the molar volume of the mixture through the term βM = (VM + bM)/VM. It is shown that the difference between βM and some average βM is very small and aMV is quadratic in composition. Sensitivity analyses of aM and resulting molar density to these approximations are also presented. Finally, the non-symmetric composition functionality of aM is discussed along with GHC predictions of mixture second virial coefficients. Numerical examples are presented to support all claims.