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Category: Selected Topics

The excess functions constitute the usual way to express the extent to which real liquid mixtures deviate from ideality. These properties are used extensively in a wide variety of scientific and technical fields, including chemistry, spectroscopy and chemical engineering. From a theoretical point of view, the excess functions are also a valuable information since equations of state, particularly those based in statistical mechanics, lead naturally to the prediction of excess properties. Since they are more sensitive than phase equilibria to the molecular details, the prediction of the excess properties provides an excellent way to check if theoretical approaches are suitable for describing accurately the behaviour of a given system.

A great advance in the field of the equations of state has been made in last years, motivated partially by the industrial interest, and also by the rapid development of modern molecular theories. These approaches provide a realistic description of the free energy of the system, as they are able to make quantitative predictions for the phase behaviour of complex systems. Most of the thermodynamic studies undertaken during last years concentrate in obtaining the phase equilibria, including the high-pressure phase behaviour and the critical properties, of systems of industrial interest. However, descriptions of the excess thermodynamic properties of these mixtures, such as excess volume, heat, and Gibbs free energy are less common.

Our group has applied the well known Soft-SAFT equation of state to predict the excess thermodynamic properties of some model binary mixtures. The key point to succesfully determine the excess properties of complex mixtures is to include the most important microscopic features of the system, such as chain connectivity, association, etc. Prelimiar results indicate that the SAFT approach is able to capture all the essential features of systems considered (see the list below).

In particular, we have addressed the following problems:

• Excess thermodynamic properties of binary mixtures of spherical Lennard-Jones molecules.
• Excess bahvior of binary mixtures of non-associating Lennard-Jones chains (n-alkane + n-alkane model).
• Excess properties of self-associating binary mixtures of Lennard-Jones chains (n-alkane + 1-alcohol model).

Currently, we are working in collaboration with several experimental groups (Platon Group, University of Zaragoza, Spain, and Eduardo Filipe's Group, Instituto Superior Tecnico, Lisbon, Portugal) in order to apply the SAFT approach to deal with thermodynamic properties, and in particular excess thermodynamic functions, of complex mixtures.