Mixtures congruently melting compounds

McLaughlin and Messer noted the formation of 1-1 congruently melting compounds in the mixtures CaFa + naphthalene, perfluoronaphthalene + CaHa, and perfluorobiphenyl + CaHa. In contrast CaFa + biphenyl was found to form an incongruent 1-1 compound, this being the only example so far observed of a mixture of CaFa with an aromatic hydrocarbon that fails to form a congruently melting compound. 

Congruent melting points are commonly encountered both in metallic and organic systems. The former is exemplified by the binary Zn/Mg system (with MgZn2 compound), whereas the latter is exemplified by phenol/aniline mixtures (with 1 1 hydrogen-bonded complex). 

Figure 14.29 shows the (solid + liquid) phase diagram for (benzene + hexafluoro-benzene). A congruently melting solid molecular addition compound with the formula QFU-CeFe ) is evident in this system.26 The rounded top of the freezing curve (solid line) for the addition compound results from almost complete dissociation of the addition compound in the liquid mixture. In other words, benzene and hexafluorobenzene act as independent molecular species in the liquid state and combine together as the addition compound only in the solid state. 

Determination of the Composition of a Compound by Thermal Analysis.—It has already been pointed out that when one plots the duration of constant eutectic temperature on the cooling curve against the initial composition of the liquid mixture, a curve is obtained which shows a maximum when the initial composition is that of the eutectic point. When a compound possessing a congruent melting-point is formed, it behaves like a pure component. If, therefore, the initial composition of the solution is the same as >4 ... 

In addition to simple binding there are many examples where a low molecular weight species enters either the crystal interior or the interlamellar space with compound formation. These situations, although not uncommon, must obviously be very specific in nature and are termed inclusion compounds or clathrates. An example is given by the phase diagram of Fig. 3.25 for polyethylene-perhydrotriphenylene mixtures.(112) A compound is formed that melts congruently at 178.2 °C. This inclusion compound does not exist in the liquid phase and does not form mixed crystals with the pure species. 

Liquid mixtures freeze at lower temperatures than the pure components of which they are comprised. The lowest possible temperature in which we only have liquid is called the eutectic point. The presence of solid compounds of stoichiometry aj y increases the complexity of their phase diagrams. A compound with a definite melting point is termed congruent, while a compound that is not stable all the way up to a well-defined melting point is said to have an incongru-ent melting point. The state at which the latter case dissociates is called the peritectic point. 

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