Solid-state polymorphism

In 1933, Bernal and Crowfoot [1] reported on the solid state polymorphism of p-azoxyanisole. They found two crystalline modifications of this compound, a stable yellow form and an unstable white polymorph. Krigbaum et al. [31 reexamined the crystal structure of the stable yellow form. The compound shows an imbricated structure which is the basic packing required for nematic behaviour according to Gray [132]. 

Many of the investigated mesogenic compounds show solid state polymorphism. In order to obtain useful information about the arrangement of the molecules in the mesophase from the X-ray data of the single crystals, it is important to investigate the crystal structure of those solid phase which transforms into the liquid crystalline phase. For instance, only the crystal structures of the low temperature solid phases of the compounds MBBA [138, 139], MHPOBC [159], and T15 [81] could be determined, but the... 

POLYMORPHISM. 1, A phenomenon in which a substance exhibits different forms. Dimorphic substances appear in two solid forms, whereas trimorphic exist in three, as sulfur., carbon, tin, silver iodide, and calcium carbonate. Polymorphism is usually restricted to the solid state, Polymorphs yield identical solutions and vapors (if vaporizable). The relation between them has been termed physical isomerism. See Allotropes under Chemical Elements, See also Mineralogy,... 

Wright noted IR spectroscopy may be used to differentiate between the solid-state polymorphs (64T159). The IR spectrum of /8-HMX was found to... 

Figure 8.8 The B- to A-type transition of B-type potato starch lintners (DP 15) at 35% water content (d.b.). The arrows on the DSC thermal curve specify the temperatures at which the lintners were heated prior to x-ray analysis.320 The model of Perez et al.320 for the solid state polymorphic transition B to A is also shown, illustrating the progressive removal ofwater molecules (small dots) and the change in packing of chain duplexes the chain duplexes marked with 0 and Vi indicate their relative translation along the c axis.

Sautel, M. Lepinasse, G. Leveiller, F. Study of the textural changes occurring during a solid state polymorphic transformation induced by temperature and relative humidity. Colloids Surf, A Physicochem. Eng. Asp. 2001, 187-188, 337-347. 

Additional preformulation and physicochemical characterization of the candidate compound are performed and stress stability studies may be initiated. Ideally, the optimal solid state (polymorphic) and chemical (salt) form of the molecule are identified as part of clinical candidate selection. Selection of the most stable and bioavailable form will expedite subsequent development. The methods for testing the drug substance are refined and additional methods may be developed. 

Figure 12. Solid-state polymorphism of Pt(II)-aIkynyl complexes.

Thus for a system capable of existing in two solid-state polymorphs, a total of four phases would be possible, which would then imply that a total of four triple points would be theoretically accessible. Denoting the liquid phase as L, the vapor phase as V, and the two solid phases as Sj and S2, the triple points correspond to... 

According to the definition of Section LA, each solid-state polymorphic form constitutes a separate phase of the component. The phase rule can be used to predict the conditions under which each form can coexist, either along or in the presence of the liquid or vapor phases. One immediate deduction is that since no stable equilibrium can exist when four phases are simultaneously present, two polymorphic forms cannot be in equilibrium with each other and be in equilibrium with both their solid and vapor phases. When the two crystalline forms (denoted S, and Sj) are in equilibrium with each other, then the two triple points (Sj-Sj-V and S,-S2-L) become exceedingly important. 

VA Fig. 16.17 The structures of solid state polymorphs of sulfur trioxide contains tetrahedral SO4 units (a) Y-SO3 consists of trimeric units and (b) a- and P-SO3 contain polymeric chains. Colour code S, yellow O, red. 

The phase diagram of the line investigated (Fig. 2B) was typical of systems exhibiting solid-state polymorphic transitions. 

Conformational Analysis in the Solution State and in the Solid State (Polymorphism)... 

The basic methods for the determination of phase transition temperatures (7 ) are DSC (differential scanning calorimetry), DTA (differential thermal analysis), and polarization microscopy. Every method has its advantages and restrictions. DSC allows one to determine the enthalpies of phase transitions (A// ). Microscopy allows one both to determine the phase transition temperatures and to identify the t)fpe of mesophase. DTA gives reliable results for melting temperatures of LCs that show solid-state polymorphism, and of LC mixtures. The differences in that can be found in publications by different authors arise from different measurement techniques and the presence of impurities. We have selected the data with the higher 7 values in such cases. 

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