Enantiotropic phenomenon

Fig. 10.5 Illustration of enantiotropic phenomenon on the cooling and heating curves of the stable mesophases. (a) Free energy curves (b) DSC heating and cooling curves...

The mesophase state of liquid crystals is normally opaque due to relatively large sizes of ordered domains. Its transition point to the isotropic melt state is called the clear point T,. The DSC scanning curves of liquid crystals can exhibit either enantiotropic or monotropic phenomena. For the thermodynamically stable mesophases of liquid crystals, they occur between the melt and the crystal states during both cooling and heating processes, as illustrated in Fig. 10.5. When both the cooling and heating curves show two symmetric consecutive phase transitions, it is known as the enantiotropic phenomenon. In contrast, for the metastable mesophase... 

Polymorphism is the ability of the same chemical substance to exist in different crystalline structures that have the same empirical composition [39,40]. It is now well established that DSC is one of the core technologies used to study the phenomenon. Polymorphic systems are often distinguished on the basis of the type of interconversion between the different forms, being classified as either enantiotropic or monotropic in nature. 

Phase diagrams with monotropic transformation or enantiotropic transformation have been discussed. " Quite interesting is the transformation of the racemic compound into a conglomerate since this phenomenon can be used for purification via crystallization, as described for nimodipine. DSC is applied for the establishment of phase diagrams, for the determination of thermodynamic data, for the purity determination, or for the monitoring of industrial resolutions. For the establishment of phase diagrams it is suitable to add spectroscopic or crystallographic methods. ... 

Polymorphism is a common phenomenon in the chemical and pharmaceutical industries. Figure 2-15 illustrates the solubility of Forms 1 and 111 polymorphs of a reverse transcriptase inhibitor drug candidate which exhibit enantiotropic behavior. Figure 2-16... 

Polymorphism. Polymorphic literally means multiform, but the term does not refer to variation in external shape. It indicates that crystals of the same molecules have different unit cells, be it of the same or of a different crystal system. The phenomenon is quite common. There are two types of polymorphism. Enantiotropic polymorphs each are stable within a certain range of temperature and pressure. Consequently, a phase diagram of the various polymorphs can be made. The prime example is ice (Section 15.3.1). If monotropic polymorphs exist, all but one of these are unstable. There is no phase diagram and, given time, only the most stable form will remain. The prime examples are compounds with long paraffinic chains, including most lipids (especially acylglycerols), where three main polymorphs exist (a, (F, and (3). 

A substance is said to be polymorphous when it is capable of existing in two or more forms with different crystal structures. We have already encountered numerous instances of this phenomenon as, for example, carbon, selenium, some of the metallic elements, zinc sulphide, ferric oxide, silica, and many others. In some of these examples one form alone is found under a given set of physical conditions and a reversible transition between forms is brought about by a change in these conditions, in which case the forms are said to be enantiotropic. Thus iron has the cubic close-packed structure between the temperatures 906 and 1401 °C, and the cubic body-centred structure at temperatures outside this range. Even so, the rate at which the transition takes place may vary between wide limits at the one extreme it may be virtually instantaneous and at the other it may be so slow that a form is capable of indefinite existence under conditions in which it is, strictly speaking,... 

Another interesting phenomenon found by Stevens et al. is the monotropic mesophase formed by the dimer and trimer of the polymer with m = 6. The dimer has a monotropic nematic phase, the trimer has a monotropic smectic phase. These metastable monotropic phases become stable enantiotropic phases with the increase of n by 1. At about the same time, Blumstein et al. (1984) found the low mass model compound of a main-chain type liquid crystalline polymer was monotropic while the mesophase of the polymer was enantiotropic. 

In the series of complexes with twelve chains ((55) M = Cu, Pd, VO R = OC H2 +i, = 6, 8, 10, 12, 14), an unusual phase sequence was observed (Table 34). The copper and palladium complexes displayed enantiotropic, disordered Coh and Coin phases, identified by their optical texture, and by X-ray diffraction. Quite uniquely, the Coin phase was systematically the lower-temperature mesophase, observed below the Coh phase. For the copper complexes, a Coir phase was seen for short-chain compounds with a transition to a Colh phase for n > 10. The intermediate octyloxy derivative displayed an unusual, but reproducible, I-Coh-Colh-Colr-Colh-Colh phase sequence on cooling from the isotropic liquid, several of these transitions being weakly first order. The decyloxy analog displayed a Colh-Colh-Colh-Colh phase sequence, and the two next homologous compounds showed a single Coin phase. To have, supposedly, so many phases of apparently the same symmetry in a single material is most unusual and no explanation was offered to help understand the phenomenon. It is to be hoped that at some stage they may be subject to re-examination so that this exotic behavior may be properly understood. 

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]. 

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