Temperature enantiotropes

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

Focusing attention on PTEB, it has been found that, similar to the case of PDTMB, the mesophase experiences a very slow transformation into the crystal. Thus, only the isotropization is observed in a sample freshly cooled from the melt [27]. However, after a long time at room temperature, the transformation mesophase-crystal is produced, owing to a glass transition temperature of about 14°C. Moreover, several endotherms were obtained before the final isotropization for a sample of PTEB annealed at 85°C for 12 days, i.e., PTEB shows enantiotropic behavior. The different endotherms may arise from polymorphism or melting-recrystallization phenomena [30]. 

The presence of three oxyethylene units in the spacer of PTEB slows down the crystallization from the meso-phase, which is a very rapid process in the analogous polybibenzoate with an all-methylene spacer, P8MB [13]. Other effects of the presence of ether groups in the spacer are the change from a monotropic behavior in P8MB to an enantiotropic one in PTEB, as well as the reduction in the glass transition temperature. This rather interesting behavior led us to perform a detailed study of the dynamic mechanical properties of copolymers of these two poly bibenzoates [41]. 

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

As indicated above in chiral mesophases, the introduction of a functional group in mesogenic stmctures offers the opportunity to achieve functional LCs. With this aim, mesomorphic crown-ether-isocyanide-gold(I) complexes (26) have been prepared recently [38]. The derivatives with one alkoxy chain show monotropic SmC mesophases at or close to room temperature. In contrast, the complexes with three alkoxy chains behave as monotropic (n = 4) or enantiotropic (n > 4) LCs. The structure of the mesophases could not be fully eluddated because X-ray diffraction studies in the mesophase were unsuccessful and mesophase characterization was made only on the basis of polarized optical microscopy. These complexes are luminescent not only in the solid state and in solution, but also in the mesophase and in the isotropic liquid state at moderate temperatures. The emission spectra of 26a with n=12 were... 

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. 

The importance of temperature-controlled scanning calorimetry for measurements of heat capacity and of scanning transitiometry for simultaneous caloric and pVT analysis has been demonstrated for polymorphic systems [9]. This approach was used to study an enantiotropic system characterized by multiphase (and hindered) transitions, the role of heat capacity as a means to understand homogeneous nucleation, and the creation of (p, T) phase diagrams. The methodology was shown to possess distinct advantages over the more commonly used combination of characterization techniques. 

An enantiotropically related pair of polymorphs was obtained for p-aminoben-zoic acid, with the system being characterized by a transition temperature of 25 °C [31]. The a-form was obtained as fibrous needles, while the /M orm was obtained in the form of prisms. The solubilities of the two forms are almost the same, indicating the existence of comparable values for AG, which in turn explained the slow transformation of the a-form into the [>-form. Nucleation of the a-form was found to be favored, which is reasonable considering that the structural motif of the a-form consists of carboxylic acid dimmers that would be expected to be stable association species in solution. 

The structures of two polymorphs of pleconaril, enantiotropically related with a transition temperature of 35.7°C, have been reported [36], Form I was described as consisting of a network of dimers, while Form III was described as a three-dimensional network of monomers. The two forms contradicted the density rule, and the solid solid transition could occur only through a destructive-reconstructive mechanism. A quantitative differential scanning calorimetry method was also described that enabled the quantitative determination of Form I in bulk Form III to be made at levels as low as 0.1%. 

Based on the reversibility of their phase transformation behavior, polymorphs can easily be classified as being either enantiotropic (interchange reversibly with temperature) or monotropic (irreversible phase transformation). Enantiotropic polymorphs are each characterized by phase stability over well-defined temperature ranges. In the monotropic system, one polymorph will be stable at all temperatures, and the other is only metastable. Ostwald formulated the rule of successive reactions, which states that the phase that will crystallize out of a melt will be the state that can be reached with the minimum loss of free... 

Differential thermal analysis proved to be a powerful tool in the study of compound polymorphism, and in the characterization of solvate species of drug compounds. In addition, it can be used to deduce the ability of polymorphs to thermally interconvert, thus establishing the system to be monotropic or enantiotropic in nature. For instance, form I of chloroquine diphosphate melts at 216°C, while form II melts at 196°C [18]. The DTA thermogram of form I consists of a simple endotherm, while the thermogram of form II is complicated (see Fig. 4). The first endotherm at 196°C is associated with the melting of form II, but this is immediately followed by an exotherm corresponding to the crystallization of form I. This species is then observed to melt at 216°C, establishing it as the thermodynamically more stable form at the elevated temperature. 

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

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. 

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... 

In the enantiotropic case the relative solubility and stability changes at a specific transition temperature where the two solubility curves cross. A given polymorph may be more soluble (less stable) above the transition point, but less soluble below and vice versa, Figure 4. An example of this relationship is the Form C and A or B relationships for Cimetidine in the case study. 

Enantiotropic behaviour may constrain the temperature envelope over which a crystallization process could operate. Transition temperatures can be predicted with DSC derived AHm and Tm values together with Eq. 2. Sluny experiments on either side of the transition temperature can then be used to validate the calculations. 

A plot of the ideal solubility curves can be used to identify the transition temperatures for the enantiotropic relationships. Form C is the most stable polymorph at high temperature, with a transition to Form B at 20 °C. The Form C to A transition occurs at 11 °C. 

Metastable crystalline phases frequently crystallise to a more stable phase in accordance with Ostwald s rule of stages, and the more common types of phase transformation that occur in crystallising and precipitating systems include those between polymorphs and solvates. Transformations can occur in the solid state, particularly at temperatures near the melting point of the crystalline solid, and because of the intervention of a solvent. A stable phase has a lower solubility than a metastable phase, as indicated by the solubility curves in Figures 15.7a and 15.7/ for enantiotropic and monotropic systems respectively and,... 

Y. Hu, H. Wikstrom, S.R. Bym and L.S. Taylor, Estimation of the transition temperature for an enantiotropic polymorphic system from the transformation kinetics monitored using Raman spectroscopy, J. Pharm. Biomed. Anal, 45, 546-551 (2007). 

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. 

Metastable mesophase that can be formed by supercooling an isotropic liquid or an enantiotropic mesophase at a given pressure to a temperature below the melting point of the crystal. 

Ammonium nitrate exists in four different forms, all of which are enantiotropic the change of white phosphorus into the red (or violet) variety is monotropic. Mercuric iodide exhibits a striking example of an enantiotropic transition. Above 126.3°, it is obtained in yellow rhombic crystals while below that temperature, a scarlet tetragonal modification appears. 

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

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