Thermodynamic Anomalies at Critical Points of Fluids

The liquid-vapor transition in a pure fluid and the liquid-liquid transition in a partially miscible liquid mixture are examples of phase transitions that have critical points. These systems exhibit striking macroscopic phe:. nomena near their critical points. Thermodynamic fluctuations within a single phase extend over larger regions of space and become increasingly long-lived as the critical point is approached. For example, near a liquid­ vapor critical point, liquid-like fluctuations within the vapor phase and vapor-like fluctuations within the liquid phase become large enough to scatter significant amounts of light. Thus, fluids which are transparent in all other thermodynamic states become turbid near the critical point, a phenomenon known as critical opalescence. A characteristic size of the fluctuating regions is called the and is given the symbol �. The correlation length diverges toward infinity as the critical point is approached. In the last decade we have come to understand that the divergence of the correlation length is com­ mon to critical points in a plethora of systems: fluids, ferromagnets, fer­ roelectrics, superconductors, etc. It has been exciting to see that this common feature forms the basis for a quantitative understanding of criti­ cal phenomena in such a wide variety of systems.