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 f or mixtures composed of simple molecules, this behavior call be rather comp licated at high pressures, where tile system may exhibit gas-gas equilibriu m, a critical double point and density inversions. A few mixtures such as H e-H-2 and He-N-2 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 pres sures. The frequency shift, the linewidth, and the line shape provide infor mation about the solubility. the state of aggregation, and other conditions of the system. Recently, a new type of compound has been found, the so-cal led van del Waals compound. The results obtained in simple mixtures, such a s Ne-N-2 and Xe-N-2. are compared with the results of molecular dynamics (M D) simulations of hard sphere mixtures. It is pointed out that energy, and not a geometrical effect (or entropy), is the driving force for compound fo rmation.