What is Different in Mixtures? From Critical Point to High Pressures

It is well known that the properties of mixtures may be very different from those of the pure constituent components, due to the unlike forces. Even for mixtures composed of simple molecules, this behavior can be rather complicated at high pressures, where the system may exhibit gas gas equilibrium, a critical double point and density inversions. A few mixtures such as He H2 and He N2 are discussed. It is demonstrated that Raman spectroscopy is an important tool not only for obtaining knowledge about the dynamical behavior of mixtures but also for determining phase equilibria at high pressures. The frequency shift, the linewidth, and the line shape provide information about the solubility, the state of aggregation, and other conditions of the system. Recently, a new type of compound has been found, the so-called van der Waals compound. The results obtained in simple mixtures, such as Ne N2 and Xe N2 , are compared with the results of molecular dynamics (MD) simulations of hard sphere mixtures. It is poiInted out that energy, and not a geometrical effect (or entropy), is the driving force for compound formation.