Enantiotropy and monotropy

Univariant Systems.—Equilibrium between liquid and vapour. Vaporisation curve. Upper limit of vaporisation curve. Theorems of van t Hoff and of Le Chatelier. The Clausius-Clapeyron equation. Presence of complex molecules. Equilibrium between solid and vapour. Sublimation curve. Equilibrium between solid and liquid. Curve of fusion. Equilibrium between solid, liquid, and vapour. The triple point. Complexity of the solid state. Theory of allotropy. Bivariant systems. Changes at the triple point. Polymorphism. Triple point Sj—Sg— V. Transition point. Transition curve. Enantiotropy and monotropy. Enantiotropy combined with monotropy. Suspended transformation. Metastable equilibria. Pressure-temperature relations between stable and metastable forms. Velocity of transformation of metastable systems. Metastability in metals produced by mechanical stress. Law of successive reactions. 

Thermodynamic and kinetic aspects of polymorphism enantiotropy and monotropy... 

A number of empirical rules have been proposed to deduce the relative order of stability of polymorphs and the nature of the process that interconverts these (i.e., enantiotropy vs. monotropy). Among the better known are the Heat of Transition Rule, which states that if an endothermic transition is observed at some temperature, it may be assumed that there must be a transition point located at a lower temperature where the two forms bear an enantiotropic relationship. Conversely, if an exothermic transition is noted at some temperature, it may be assumed that there is no transition point located at a lower temperature. This in turn implies that either the two forms bear a monotropic relationship to each other or that the transition temperature is higher than the temperature of the exotherm. 

Knowing the relationship between the thermodynamic quantities H (enthalpy), G (free energy), S (entropy), and T (temperature), it is often simple to represent equilibrium states by plotting the free energy G as a function of the temperature for each form. If the two curves intersect before the melting point, there is reversibility, i.e., enantiotropy, and if the reverse is true, there is monotropy. 

Fig. 6 Energy diagrams showing plots of enthalpy H and Gibbs free energy G, against temperature T, for the solid and liquid phases of a single compound, showing (A) enantiotropy and (B) monotropy.

Fig. 7 illustrates the behavior of polymorphs A and B in case of enantiotropy (Fig. 7A) and monotropy (Fig. 7B) during heating. For all analysis where a temperature change is involved, kinetic factors have to be considered for proper interpretation of the results. The DSC scans will differ if the sample being analyzed is stable or metastable at ambient temperature. A is the stable form at ambient temperature in both cases. 

Solubility curves exhibiting (a) monotropy, (b) enantiotropy, and (c) enantiotropy with metastable phases. (Reprinted with permission of the copy right holder [15].)... 

Figure 8.2 The relationship between Gibbs energy (G) and temperature for two modifications in the cases of enantiotropy (reversible) and monotropy (irreversible) transition between forms.

Very often some substances have two melting points separated by an exotherm. Such a DSC scan can correspond to a monotropy or to an enantiotropy. The sample may be a pure form or a mixture. Using different heating rates and tempering in DSC, one... 

Generally speaking, the concepts of monotropy and enantiotropy in phase theory appear to coincide with the structural concepts of unrelated and related lattices. Nevertheless, one must avoid equating the two, for it is certainly possible that one of two related lattices of the same substance is less stable than the other under all conditions of temperature and pressure. This would indicate the existenced of monotropy in spite of the existence of related lattices. This situation becomes especially important for polymorphic organic compounds, which form molecular lattices. 

As well, the real behavior in the liquid and solid phase has an influence on the SLE behavior. In some cases one or more solid compounds are formed. At the same time miscibility gaps can appear. Furthermore, many substances occur in more than one crystalline form. This phenomenon is called polymorphism for compounds and allotropy in the case of elements. If the transition curve lies within the region of stable states the phenomenon is called enantiotropy. In the metastable region it is called monotropy. 

From the DSC trace shown in Figure 8.13, a low-temperature, solid-solid transition occurs prior to the main endotherm corresponding to the melting form. This transition can be distinguished from a low-temperatirre desolvation process since no mass loss is detected by TGA and the transition is reversible. (That is, the reversing transition will also be detected upon cooling.) For solid-solid transformations, this transition is exothermic for monotropy and endothermic for enantiotropy. 

Such a DSC scan can correspond to both monotropy and enantiotropy, with the sample being either a pure form or a mixture. Fast scanning rates can kinetically hinder this transformation, giving us detailed information about the actual composition of the sample. 

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