Re-examination of collinearity tests for density-dependent mixing rules

This article corrects a mistaken conclusion reached by one of us (PTE) in an earlier article in this journal [Fluid Phase Equilibria 48 (1989) 209–222]. We had found that density dependent mixing rules, when used with the Redlich-Kwong equation of state (RK/EoS), violated the isochoric collinearity constraint at the cricondentherm (CT), while use of the more familiar van der Waals one-fluid mixing rules do not. Here we repeat the collinearity tests but with the simpler equation of state of van der Waals. Collinearity at the CT is found for both density dependent and quadratic mixing rules. Because the exact form of the cubic EoS should not affect collinearity, we conclude that numerical errors in our earlier RK/EoS calculations are the culprit for the mistaken conclusion involving density dependent mixing rules. All terms but one can be found analytically with the vdW/EoS, increasing the accuracy of the test by more than one order-of-magnitude.