Monotropes temperature

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. 

The variation of the transition temperatures of these polybibenzoates with the number of methylene units in the spacer is shown in the lower part of Fig. 5. Melting temperatures, Tm, (crystal-isotropic melt transition) are obtained [9] for m > 7 and m = 3 (monotropic behavior), while for the other members, Tm really represents the... 

In the case of monotropic behavior, the isotropiza-tion endotherm and the corresponding thermodynamic parameters for the mesophase-isotropic transition can be obtained by isolating the mesophase when cooling from the melt and holding the temperature in a region where the transformation into the crystal is very slow... 

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]. 

The first report on the liquid crystalline properties of these compounds was published by Gray and Mosley [44] in 1976. The series of 4 -n-alkyl-4-cyanobiphenyls (CBn) have been widely studied by different methods due to their readily accessible nematic ranges around room temperature. The compounds have the phase sequences crystal-nematic-isotropic for CBS, CBIO, and monotropic nematic for CBS, CB4 crystal-smectic A-nematic-isotropic for CB9 crystal-smectic A-isotropic for CBll. The lower homologous CB2 is nonmesogenic. The general chemical structure of the compounds CBn is presented in Fig. 1. 

In this section we will report on the crystal structure analyses of mesogenic 2,5-diphenyl pyrimidines. The crystal structure of 5-phenyl-2-(4 -n-butoxy-phenyl)-pyrimidine (5-PBuPP) and 2-phenyl-5-(4 -n-pentoxyphenyl)-pyrimi-dine (2-PPePP) were determined by Winter et al. [83, 84]. Compound 5-PBuPP forms a monotropic nematic phase, whereas compound 2-PPePP exhibits a smectic A mesophase within a wide temperature range. The chemical structure of the mesogenic 2,5-diphenyl pyrimidines is shown in Fig. 13. 

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). 

Derivatives of aliphatic alkynes (14 and 15) are more thermally unstable than 12, but they show SmA and N phases at low temperatures (below 130 °C). The type of phase and the mesophase stability depend on the length of both the terminal and the lateral chains. When both chains are elongated, the mesomorphism becomes metastable and compounds 14 display monotropic N and SmA transitions. Complexes IS, which contains an ester group with an opposite direction to that of complexes 14, display less stable nematic mesophases. 

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. 

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 monotropic case the relative solubility of the two polymorphs does not change with temperature if Form I is the most stable and least soluble at low temperatures then it will also be the same at all other temperatures, Figure 3. An example of this behavior is the Cimetidine Form A and B relationship presented in the case study of section 6. 

Forms A and B have a monotropic relationship, with Form B the most thermodynamically stable at all temperatures. 

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,... 

Note Monotropic transition temperatures are indicated by placing parentheses, (), around the values. 

It has been reported that the y-form, which melts at approximately 101°C, is the thermodynamically most stable form at room temperature and that the sorbitol system is monotropic [8]. Studies of the various forms show that upon standing or upon stress conditions, sorbitol will convert to the y-form [7]. However, a solution calorimetry study performed on selected crystal forms of sorbitol reports that sorbitol hydrate is the most stable form followed by the y-form [18]. 

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. 

The relation between the octahedral and prismatic modifications has not yet been satisfactorily elucidated. The former is the stable form at ordinary temperatures and the latter at higher temperatures the transition point according to Rushton and Daniels is 250° C. and according to Smits and Beljaars 200° C., but the prismatic form is persistent at much lower temperatures and the change from octahedral to prismatic may be monotropic. Interesting information has been obtained from measurements of the vapour pressure of the oxide. The following values have been obtained ... 

AUatropes. Some or the elements exist in two or more modifications distinct in physical properties, and usually in some chemical properties. Allotropy in solid elements is attributed to differences in the bonding of the atoms in the solid. Various types of allotropy are known. In ertuntiomorphic allotropy, the transition from one form to another is reversible and takes place at a definite temperature, above or below which only one form is stable, e.g., the alpha and beta forms of sulfur. In dynamic alloimpy. the transition from one form to another is reversible, but with no definite transition temperature. The proportions of the allotropcs depend upon the temperature. In monotropic allotropy, the transition is irreversible. One allotrope is mctastable at all temperatures, e.g.. explosive antimony. 

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