Excess Thermodynamic Properties in Liquid Binary Mixtures

Excess volumes and adiabatic compressibility have been measured in several binary liquid mixtures to answer the question if structural information can be gained through the analysis of the concentration dependence of excess quantities. The obtained results are compared with independent indications from Raman spectroscopy, able to probe directly the occurrence and the nature of effective intermolecular interaction. Some doubts are arisen against the usual approach adopted for estimating the excess quantities and about the adequacy of the usual assumptions for the reference ideal behaviors. In particular, it is shown how the excess part of the compressibility can be just originated from statistical effects also in absence of any excess volume contribution. The leading idea is supported by the comparison of the experimental data with the results from a naïve model for binary mixtures of hard spheres. The model turns out able to produce a very wide spectrum of structural and thermodynamic behaviours depending on the values of its parameters and on the nature (additive or non additive) of the hard sphere potential. A discussion is proposed on the re evaluation of excess thermodynamic data and on their ability in providing direct information on intermolecular interactions.