Excess compressibility in binary liquid mixtures

Brillouin scattering experiments have been carried out on some mixtures of molecular liquids. From the measurement of the hypersonic velocities we have evaluated the adiabatic compressibility as a function of the volume fraction. We show how the quadratic form of the excess compressibility dependence on the solute volume fraction can be derived by simple statistical effects and does not imply any interaction among the components of the system other than excluded volume effects. This idea is supported by the comparison of the experimental results with a well-established prototype model, consisting of a binary mixture of hard spheres with a nonadditive interaction potential. This naive model turns out to be able to produce a very wide spectrum of structural and thermodynamic features depending on values of its parameters. An attempt has made to understand what kind of structural information can be gained through the analysis of the volume fraction dependence of the compressibility.