Critical Phenomena in Dilute Binary Mixtures

The various thermodynamic properties used by physical chemists and chemical engineers to describe mixtures exhibit unusual and unexpected behavior when a fluid mixture is near the critical point of one of its components. For example, the partial molar volume and the enthalpy 

H2 of the solute diverge while those of the solvent, and H, tend to 

Knowledge of these unusual phenomena is fimdamental for any solution, but for metallic systems with state-dependent electronic structure and interatomic forces, these properties become especially interesting. Consider, for example, solutions in which the solvent is fluid mercury. In the critical region, the tem evolves from an assembly of neutral species (electrons bound to their parent atoms) to a partially ionized fluid. At the same time the compressibility increases rapidly due to its 

There is a paucity of information on fluid metal mixtures under extreme conditions. This is perhaps not surprising given the remarkable variety and complexity of the phase diagrams of mixtures of even simple molecular liquids (Rowlinson and Swinton, 1982). In a singlecomponent system, equilibriiun between two fluid phases is normally limited to liquid-gas equilibrium, and above the critical temperature the system exhibits the unique feature that its density can be varied in a continuous manner without the occurrence of an abrupt liquid-gas phase transition. We have discussed in detail the continuous metal-nonmetal transitions that occur in pure fluid metals under these conditions. The situation is quite different, however, for a two-component system. The equilibrium region where two fluid phases coexist is not necessarily limited to temperatures below the critical temperature of the less volatile component. On the contrary, among the more remarkable features of binary mixtures is the continuous existence of phase separation above the critical points of the pure components. One now speaks ot gas-gas or fluid-fluid equilibria. We consider the latter term to be the more appropriate since the phases are more accurately characterized as dense fluids than as gases under the conditions at which these separations occur. 

Fluid-fluid phase separations have been observed in many binary mixtures at high pressures, including a large number of systems in which helium is one of the components (Rowlinson and Swinton, 1982). Fluid-fluid phase separation may actually be the rule rather than the exception in mixtures of unlike molecules at high pressures. Fig. 6.4 shows the three-dimensional phase behavior of a binary mixture in schematic form. This diagram includes the vapor pressure curves and liquid-vapor critical points of the less volatile component (1) and the more volatile component (2) in their respective constant-x planes. The critical lines are interrupted one branch remains open up to very high temperatures and pressures. Systems that can be represented by a diagram such as Fig. 6.4, those for which the critical lines always have positive slope in the p — T projection, have been called fluid-fluid mixtures of the first kind. A second class of system, in which the critical line first drops to temperatures below T (l) and then increases, exhibit fluid-fluid equilibrium of the second kind. There is, however, no fundamental distinction between these two classes of fluid mixtures. 

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