On the origin of excess thermodynamic quantities in liquid mixtures

The nature of the interactions between components in liquid mixtures has been investigated by means of Brillouin light scattering. Several binary systems, able of mixing over the whole concentration range, have been studied. In each case deviations of the adiabatic compressibility from ideal behaviour has been observed. This result has been interpreted within the framework of a well established theoretical model, namely the non additive hard sphere mixture. Despite similar findings were rationalized in terms of enhanced interactions between molecules, the analysis of excess thermodynamic functions indicates that they are mainly due to excluded volume effects and that the differences in the local intermolecular interaction act as higher order contributions: this can be a general consideration for liquid mixtures. A discussion is proposed on the re evaluation of excess thermodynamic data and on their ability in providing direct information on intermolecular interactions.