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2.4 Phase diagrams

The mixing behavior of ternary systems of type 2a is illustrated in Fig.(2.7). This kind of particular class of ternary liquid systems has been first found by Francis [45] in 1953. However, at constant pressure one can follow the line of so-called plait points between the demixing points of two limiting binary liquid systems. In the the case of Fig.(2.7(a)) it is the binary system CB and the binary system AB. Each of these plait points represents the critical consolute point of a critical composition of the ternary system A, B and C. At these points criterions for critical behavior like the equal-volume criterion or critical opalescence are fulfilled. In certain temperature ranges, the hypothetical systems under consideration, have two separate binodal lines. On lowering the temperature the two lines coincide at both plait points. Consequently, a new significant point appears, the so-called col point or saddle point. This behavior emerges when two conditions are satisfied. First, in the range of temperatures considered two components are miscible and the third one is partially miscible with both others up to the respective binary critical solution temper- atures, as shown in Fig.(2.7(b)). Second, the critical consolute point must be quite close the critical point of the other binary system. In conclusion, in the triangular tempera- ture/composition prism of the ternary system, the binodal surface is concave upwards and shows the existence of a saddle point (col point) as an extremum (in this case minimum). This happens at temperatures lower then the critical temperatures of considered binary systems, as has been indicated in Fig.(2.7(b)). Saddle points, also named col points as plait points in general fulfil critical point criteria.

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