11 Critical Region

Abstract

Publisher Summary This chapter reviews the current status of equations of state for fluids and fluid mixtures in the critical region. Critical phenomena in fluids have been the subject of many theoretical and experimental studies during the past thirty years. The most striking result of these studies has been the discovery of critical-point universality: the microscopic structure of fluids becomes unimportant near a critical point. Moreover, the principle of critical-point universality has been extended to near-critical binary mixtures (isomorphism approach). These discoveries make it possible to develop universal equations of state for fluids in the critical region and enable us to look into the problem of formulating global equations of state for dense fluids and fluid mixtures from an entirely new point of view. The principle of critical-point universality finds its physical origin in the phenomenon that long-range fluctuations of the order parameter (density in one-component fluids or/and concentration in fluid mixtures) dominate in the critical region and that the range of these fluctuations becomes much larger than any other microscopic scale. The spatial extent of these critical fluctuations is determined by a correlation length ξ, which diverges at the critical point. Therefore, the behavior of the thermodynamic properties becomes singular at the critical point. The mathematical nature of the asymptotic, singular, critical behavior is now well understood: it can be characterized by scaling laws with universal critical exponents.