Dystectic melting

A special case is cryoscopy performed in solvents with dystectic melting, i.e. in those, which undergo at melting a thermal dissociation. This case will be discussed in Section 33.2.2. 

Let us consider the dissolution of the admixture X in the dystectically melting solvent AB. The course of the liquidus curve of substance AB in the AB-X system (Figure 3.48) is described by the Le Chatelier-Shreder equation, which in the limit for Xr(AB) -> 1 we can express in the differential form... 

Figure 3.48. Phase diagram of the AB-X system with dystectic melting of component AB and a chemical...

Due to the thermal dissociation of the complex compound AB, its liquidus curve exhibits a curvature at the temperature of fusion with a slope at the composition of AB equal to zero. Such a phenomenon is called the dystectic mode of melting (Figure 3.47). Thus at the temperature of fusion the following equation is fulfilled... 

Now we will examine the behavior of Xr(AB) with regard to Xw(AB) in the limiting region Xw(AB) -> 1, when X reacts with AB under the formation of new, foreign compounds and compounds identical with the products of thermal dissociation of AB, for example according to the scheme AB + X = AX + B. The increase in the amount of substance AB caused by the reaction of 1 mol of admixture X with substance AB is denoted by / (/ is non-zero only when AB melts dystectically) and the decrease in the amount of substance AB caused by the reaction of 1 mol of admixture X with substance AB is denoted as m. For Xr(AB) we get... 

ITiis brings us to an important part of our discussion of stoichiometry of a crystal formed from a melt. As we have shown above, we can sometimes obtain a dystectic composition but most likely we will obtain a crystal containing "ordinary" types of defects. 

Attractive interactions between the two co-crystal components in the liquid phase may significantly change the picture in Figure 12.1 in that the eutectic and dystectic temperatures are further decreased and the temperature differences between the eutectics and the melting points of the components will increase. However, the stabilities of a mixture of the individual soUd components and the co-crystal relative to the molten state are affected by these attractive interactions to the same extent, and the relative thermodynamie stability of the co-crystal and the individual components will still be refleeted correctly in the phase diagram. 

The ternary phase diagram will additionally change with temperature. Two examples in the form of a contour map are represented in Figure 12.5 based on the thermodynamic data from Figure 12.1(a) and (b). The solubility lines of the individual components and the co-crystal extend into curved surfaces and the invariant points become eutectic grooves on both sides of the co-crystal phase boundary surface. Relatively symmetric cross sections can be seen at all temperatures for the phase diagram derived from the system with a dystectic. Very asymmetric temperature cross sections are found in the example with a peri-tectic melting of the co-crystal. 

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