Monotropic conversion

Physical Properties. Of the three modifications of TiOz, rutile is the most thermodynamically stable. Nevertheless, the lattice energies of the other phases are similar and hence are stable over long periods. Above 700 °C, the monotropic conversion of anatase to rutile takes place rapidly. Brookite is difficult to produce, and therefore has no value in the TiOz pigment industry. 

Formula weight 187.44. Very slightly soluble in water, slowly reacts with dilute mineral acids. Corundum structure (D51 type). Heating at 600°C in air produces slow monotropic conversion to iS-GasOa. On heating in an autoclave under water vapor above 305°C converts to /S-GaaOg, below305°C, to GaO(OH) (see above), d 6.44. 

Metastable over long periods of time despite being less thermodynamically stable than rutile. However, above 700°C, the irreversible and rapid monotropic conversion of anatase to rutile occurs. It exhibits a greater transparency in the near-UV than rutile. With an absorption edge at 385 nm, anatase absorbs less light at the blue end of the visible spectrum and has a blue tone... 

If a modification is unstable at every temperature and every pressure, then its conversion into another modification is irreversible such phase transitions are called monotropic. Enantiotropic phase transitions are reversible they proceed under equilibrium conditions (AG = 0). The following considerations are valid for enantiotropic phase transitions that are induced by a variation of temperature or pressure. 

In some cases, new phases that may not be detectable by other methods may be detected optically (Chang et al. 1995). Solid state conversions and their monotropic (Burger et al. 1997) or enantiotropic nature (Henck et al. 2000), or the products of desolvations may be easily recognized (Schinzer et al. 1997). Intimate processes of polymorphic behaviour, such as nucleation, crystal growth, habit transformation, sublimation and properties of the melt (e.g. degradation) may be readily observed and video recorded (de Wet et al. 1998). 

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. 

To help decide whether two polymorphs are enantiotropes or monotropes. Burger and Ramberger developed four thermodynamic rules [14]. The application of these rules was extended by Yu [15]. The most useful and applicable of the thermodynamic rules of Burger and Ramberger are the heat of transition rule and the heat of fusion rule. Figure 11, which includes the liquid phase as well as the two polymorphs, illustrates the use of these rules. The heat of fusion rule states that, if an endothermic polymorphic transition is observed, the two forms are enantiotropes. Conversely, if an exothermic polymorphic transition is observed, the two forms are monotropes. 

Cyclooxygenase (COX) is an enzyme that catalyzes the rate-limiting step in the arachidonic acid cascade involving the conversion of arachidonic acid to prostaglandin H2 (PGH2), the common biosynthetic precur-sortoprostaglandinsand thromboxane. Cyclooxygenase exists in two isoforms COX-1 and COX-2. COX -1 and COX-2 are monotropic enzymes bound to cellular membranes of the endoplasmic reticulum and nuclear envelope [2]. 

A melt to recrystallisation to melt phenomenon on a DSC curve may represent either monotropic or enantiotropic conversion. Small amounts of decomposition may favour transitions or the apparent production of a polymorph. Frequently thermogravimetry will detect this decomposition. An example of monotropic polymorphism is displayed by temazepam [11]. 

---