Local composition structured fluid model for the excess Gibbs energy of liquid mixtures

An n-fluid local composition model is derived for the excess free energy of multicomponent liquid mixtures. Taking into account that differences in fl uid structure affect the intermolecular interactions leads to the following expression for a binary mixture: G(E)/RT=x(1)x(2)[A(12)x(1)/x(1)+alpha (12)x(2)+A(21)x(2)/alpha (21)x(1)+x(2 )] The parameters A(ij) = In gamma (infinity)(j) and alpha (ij) can be express ed in terms of molecular properties, and the model should therefore have ge neral validity. For most systems tested, the approximation alpha (ij) = 1/a lpha (ji) is valid. This leads to a three-parameter version that, with the exception of the alcohol-alkane systems, shows greater flexibility than the nonrandom two-liquid equation in correlating vapor-liquid equilibrium (VLE ) behavior. As is the case for other local composition models, multicompone nt behavior can be predicted from knowledge of the binary VLE alone. Becaus e they can be shown to be special forms, the local composition structured f luid model also provides a proper framework for the extension of the Porter , Van Laar, Margules, and Scatchard-Hildebrand equations to multicomponent systems.