2.1 Phase Transitions in Thermodynamic Systems (continued)

Figure 2.2. Overview of the variety of demixing behavior in different binary systems. OKP: upper critical point; UKP: lower critical point.

One special property of the critical point is that the equilibrium and stability criteria are satisfied simultaneously. The thermodynamic properties of demixing are most readily examined in Fig. 2.1. Each curve is compared isothermally, at constant T; pressure p is constant throughout the diagram.

At temperature T₂, above the upper critical point, the ΔG(x) isotherm lies in the stable homogeneous region. In accordance with Eq. (2.11), it is convex over the entire concentration range. At T₁, the isotherm passes through both the stable homogeneous region and the unstable heterogeneous region.