Enantiotropic transition, definition

Mesophase that is thermodynamically stable over a definite temperature or pressure range. Note The range of thermal stability of an enantiotropic mesophase is limited by the melting point and the clearing point of an LC compound or by any two successive mesophase transitions. 

An interesting test of the third law is possible when a solid is capable of existing in two or more modifications, i.e., enantiotropic forms, with a definite transition point. The entropy of the high temperature form (a) at some temperature above the transition point may, in some cases, be obtained in two independent ways. First, heat capacity measurements can be made on the form (/3) stable below the transition point, and the entropy at this temperature may then be determined in the usual manner. To this is then added the entropy of transition, thus giving the entropy of the o-f orm at the transition point (cf. first three lines of Table XVI). The entropy contribution of the a-form from the transition temperature to the chosen temperature is then obtained from heat capacity measurements on the o-form. The second procedure is to cool the ot-form rapidly below the transition point so that it remains in a metastable state. Its heat capacity can then be determined from very low temperatures up to temperatures above the normal transition point, and the entropy of the a-form is then obtained directly from these data. Measurements of this kind have been made with a number of substances, e.g., sulfur, tin, cyclohexanol and phosphine, and the entropies obtained by the two method have been found to be in close agreement. ... 

Polymorphism can be detected by the differences in physical properties due to individual characteristics. Based on the fugacity, which relates to the thermodynamic term, entropy of the solid molecule, polymorphism may be defined as monotropic or enantiotropic. Furthermore, combination of these two systems, monotropic and enantiotropic, may yield a third system. The definition of these categories may best be illustrated by the solubility-temperature plots, based on the van t Hoff equation. In a monotropic category as shown in Figure 9, the solubility of form I (the stable form) and that of form II (the metastable form) will not intersect each other at the transition temperature calculated only from the extrapolation of the two curves. In the enantiotropic category (Fig. 10), the solubility of form I (the stable form) and that of form II (the metastable form) will intersect each other at the transition temperature. In the combined category (Fig. 11), for which there are two transition temperatures, the solubility of form III will not intersect any other curves. 

Equilibrium Relations in the Case of Liquid Crystals.— Whether we are dealing here with substances in two strictly crystalline and enantiotropic forms (which we may call the solid and liquid crystalline forms), possessing a definite transition point, or whether we... 

If as shown in Fig. 10 one polymorph is stable (i.e., has the lower free energy content and solubility over a certain temperature range and pressure), while another polymorph is stable (has a lower free energy and solubility over a different temperature range and pressure), the two polymorphs are said to be enantiotropes, and the system of the two solid phases is said to be enantiotropic. For an enantiotropic system a reversible transition can be observed at a definite transition temperature, at which the free energy curves cross before the melting point is reached. Examples showing such behavior include acetazolamide, carbamazepine, metochlopramide, and tolbutamide [9,14,15]. 

It should be noted that the ordinary transition point of enantiotropic systems (which is measured at atmospheric pressure) will be less than the melting point of either solid phase. Each polymorph will therefore be characterized by a definite range of conditions under which it will be the most stable phase, and each form is capable of undergoing a reversible transformation into the other. 

Two types of polymorphism occur in lipids. The forms are enantiotropic. When each form is thermodynamically stable in a definite range of temperature and pressure, the forms are enantiotropic. Thus one enantiotropic form changes to another at a certain transition temperature. If, on the other hand, one of two forms is thermodynamically unstable, the two forms are said to be monotropic. The phase transitions in lipids are usually of the monotropic type, for example in glycerides. 

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