Enantiotropic phase transition

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. 

Enantioseparation, 14 180 Enantiotropic phase transitions, 15 101 Enargite, 3 263t Enbrel, 2 824... 

MDSC is particularly useful for the study of reversible (related to the heat capacity) thermal reactions, and is less useful for non-reversing (kinetically controlled) reactions. Examples of reversible thermal events include glass transitions, heat capacity, melting, and enantiotropic phase transitions. Examples of non-reversible events include vaporization,... 

In the second part we have illustrated, for the prototypical organic semiconductor diindenoperylene, how structural changes, in this case the enantiotropic phase transition, can influence the temperature depend transport. In this case, the mobility measured by different techniques acts as a sensitive probe for the electronic properties at the surface and in the bulk. This approach enables detailed insights into the underlying mechanisms relevant for the conduction at various temperature regimes. 

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...

As already introduced, phase transitions between different solvates or between a solvate and the unsolvated compound as well as polymorphic transformations can be recognized by discontinuities in the solubility curves. Examples are shown in Figure 3.21 with several hydrates and a couple of enantiotropic phase transitions occurring in the Mg(N03)2/H20 and NH4NO3/H2O systems, respectively. 

An example of monotropic behavior consists of the system formed by anhydrous ibuprofen lysinate [41,42], Figure 4.12 shows the DSC thermogram of this compound over the temperature range of 20-200°C, where two different endothermic transitions were noted for the substance (one at 63.7°C and the other at 180.1°C). A second cyclical DSC scan from 25 to 75°C demonstrated that the 64°C endotherm, generated on heating, had a complementary 62°C exotherm, formed on cooling (see Fig. 4.13). The superimposable character of the traces in the thermograms demonstrates that both these processes were reversible, and indicates that the observed transition is associated with an enantiotropic phase interconversion [41]. X-ray powder (XRPD) diffraction patterns acquired at room temperature, 70°C, and... 

Herbstein EK (2006) On the mechanism of some first-order enantiotropic solid-state phase transitions from Simon through Ubbelohde to Mnyukh. Acta Crystallogr B 62 341-383... 

However, diffraction methods have severe drawbacks. Disordered crystals are often difficult to tackle. If the disorder is of dynamic nature, e.g. arising from small-or large-amplitude motions in the crystal, the use of devices for variable-temperature measurements is compulsory and can also yield very useful information (see below for some examples) on the existence of enantiotropic systems related by phase transitions. In some, not frequent, cases the crystals are sufficiently robust to be used for direct phase transition measurements on the diffractometer. Figure 3 shows an example of multiple diffraction data sets collected on the same specimen... 

Order-to-Order Phase Transitions Between Enantiotropic Systems... 

A degree of polymerization of about ten was determined by end-group analysis via H-NMR. Glass transitions were found to occur below room temperature. In each case, the material was smectic at 80 The polymers exhibited a broad enantiotropic phase range. A uniaxial teflon monolayer gave well-aligned parallel samples which were used to determine the smectic tilt angle as a function of temperature. The materials were well-behaved... 

Fig. 4.3 (See also colour plate section.) An enantiotropic phase transformation in 2,4,6-trinitro-5-fert-butyl-ra-xylene as observed on the hot stage microscope. Form 11 is stable at room temperature and the thermodynamic transition point is at 84 °C. (a) Room temperature stable Form 11, the coarse crystals at upper right embedded in aggregate of Form 1. (b) On heating Form n grows at the expense of Form 1. (c) At 84 °C the transformation can be halted, (d) Above 84 °C Form 1 is stable and has grown at the expense of Form 11. (From Kuhnert-Brandstatter 1971, with permission.)...

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

A further interesting aspect of diindenoperylene is the solid-to-solid phase transition which occurs at around 403 K [56, 57]. Figure 25.9 shows the characteristics of this enantiotropic transition measured by X-ray diffraction in Bragg-Brentano geometry along the [001] direction. The transition comprises... 

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