Polymorphism monotropy

When a solid system undergoing a thermal change in phase exhibits a reversible transition point at some temperature below the melting points of either of the polymorphic forms of the solid, the system is described as exhibiting enantiotropic polymorphism, or enantiotropy. On the other hand, when a solid system undergoing thermal change is characterized by the existence of only one stable form over the entire temperature range, then the system is said to display monotropic polymorphism, or monotropy. 

Henck, J.-O. and Kuhnert-Brandstatter, M. (1999). Demonstration of the terms enan-tiotropy and monotropy in polymorphism research exemplified by flurbiprofen. J. Pharm. ScL, 88, 103-8. [33]... 

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. 

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. 

Fig. 7 Possible DSC curves for two polymorphs (A) Enan-tiotropy A B, B is the highest melting form, A is the stable form below the transition point. (B) Monotropy B A, A is the highest melting form. (For explanations of the scans, see text.)...

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. 

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. 

In monotropy, one polymorph is always more stable than the other at all temperatures below their melting points (Fig. 7b). This definition is based on the assumption that the pressure remains eonstant. An alternative definition is that, if the pressure temperature phase diagram does not allow a polymorph to be in equilibrium with its vapor phase below the critical point, it is the unstable monotrope, otherwise it is an enantiotrope. This definition recognizes that some monotropes may be thermodynamically stable at elevated pressures and temperatures, e.g., diamond, which is the metastable polymorph of carbon under ambient conditions. 

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. 

Figure 3.19 Solubility curves in case of polymorph/. Enantiotropic phase transitions occur reversibly at the phase transition temperature leading to a kink in the solubility curve (a). In case of monotropy (b), one of the...

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

Monotropy represents irreversible transition between polymorphs. The higher melting point form is always the thermodynamically stable form. Additionally, if the melting points of two monotropic forms differ by 25-50°C, it is very difficult to crystallise the lower melting point form [11]. In effect, the transition... 

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