Unlike Binary Intermolecular Parameters

In the case of calculations of mixture properties and phase equilibria in mixtures, mixture-specific parameters have to be calculated based on the pure-component (like, h) and binary-interaction (unlike, ij) parameters with the use of mixing and combining rules. The simplest mixing rules for use in an equation of state are those presented by van der Waals, which use quadratic functions of composition involving like and unlike parameters e.g. in the one-fluid approximation ffx = X) Z) where the subscript x indicates a mixture 

The quadratic form of the mixing rule has a theoretical basis in the composition dependence of the second virial coefficient, but the Lorentz-Berthelot combining rules are well-known to fail in the case of highly non-ideal mix-tures. A correcting adjustable parameter ky that is mixture specific and typically determined by fitting to relevant experimental data for the mixture of interest is commonly introduced to improve agreement with experimental data. 

Given the failure of the Lorentz-Berthelot rules, in order to increase the predictability of mixture models (be it equations of state or computer simulations) different combining rules have been proposed and investigated. Schnabel et have recently provided a quantitative assessment of eleven combining rules in terms of their performance in describing binary mixtures classified by the molecular shape and polarity of the components. Of particular interest is a 

It is also worth noting that in the case of SAFT equations of state, the molecular basis of the theory means that it is possible to transfer binary-interaction parameters when studying mixtures within a homologous series very successfully. This idea has been used in numerous works to predict the phase behaviour of (alkanes - -water), (alkanes + perfluoroalkanes),(alkanes -I- carbon dioxide), (alkanes-I-hydrogen chloride) , and (alkanes-I-nitrogen), and aqueous solutions of surfactants, to name but a few. 

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