The generalized van der waals partition function—IV. Local composition models for mixtures of unequal-size molecules

Abstract

In this study, we present the results of Monte Carlo computer simulations for binary mixtures of square-well molecules of equal energy but differing size ratios at a variety of temperatures and densities. These data are used to determine the extent of the local composition effects in these mixtures, and to test various local composition models which had been proposed. We then show how, by use of the most successful of the local composition models and the generalized van der Waals partition function, a new two-parameter activity coefficient model and a density-dependent mixing rule can be derived. The new activity coefficient model is found to be quite successful in correlating data for the aqueous nonelectrolyte solutions on which it was tested, with the important advantages of having two parameters which are not intercorrelated and only slightly temperature-dependent. The density-dependent equation of state mixing rule we derived was found, when compared to simulation data, to be of some improvement over the van der Waals one-fluid mixing rule for mixtures of molecules of different square-well depths, but of no advantage for molecules of similar energies but different size.