Amorphous polymorphism

Koza MM, Geil B, Winkel K, Kohler C, Czeschka F, Scheuermann M, Schober H, Hansen T. Nature of amorphous polymorphism of water. Phys. Rev. Lett. 2005 94 125506. 

The bulk structures of silicas are classified as crystalline and amorphous polymorphs. More than 35 well-defined crystalline silicas are known, which are well-characterized by the Si-O length, the Si-O-Si bond angle, and the Si-O bond topology and coordination (10). Some of the crystalline polymorphs are collected in Table I. Because of the lack of sufficiently precise methods to assess the long-range structural order, amorphous silicas remain poorly characterized. They can be loosely discriminated according to their dispersity, bulk density, and type of pore structure. 

Influence on the structure of the nucleated phase (amorphous polymorph). 

Independently of the definition of amorphous polymorphism chosen, experiments suggest that the relationship between LDA and HDA ( first-order like ) is different from the relationship between HDA and VHDA ( continuous ), at least at 140K [65,66]. Therefore, the possibility that there exists a second critical point in metastable water within the No man s land is still open and remains the topic for future works. However, the continuous nature of the HDA-VHDA transformation strongly questions the possibility of a third critical point in metastable water. Experiments at slower compression/decompression rates and at higher temperatures (>140K) may help to elucidate this issue. 

While many of the above systems form orientational intermolecular bonds, other are characterized by weakly directional interactions [53,54], and in some cases, such as in triphenyl phosphite, the dominating interaction is expected to be nondirectional [55]. Hence it is possible that liquid polymorphism may also occurs in materials characterized by nondirectional interactions. This possibility is supported by a recent observation of a transition between two amorphous polymorphs in CessAUs, a metallic glass with nondirectional bonds, in which the transition is caused by pressure-induced f-electron delocalization [56]. 

P. F. McMillan, M. Wilson, D. Daisenberger, and D. Machon, A density-driven phase transition between semiconducting and metallic amorphous polymorphs of silicon, Nat. Mater. 4, 680-684 (2005). 

Incidentally, there was a natural expectation that the disordered structure of liquid, or glass, would change gradually when pressure and temperature were changed. In other words, it was commonly thought that the one component material would have only one liquid phase. The possibility of a discontinuous change in volume of liquid was doubted and rarely discussed because there was no clear experimental evidence for the amorphous polymorphism (or poly a morphism). 

H. Schober, M. Koza, A.Toelle, F. Fujara, C. A. Angell, R. Boehmer, Amorphous polymorphism in ice investigated by inelastic neutron scattering, Physica B 241-243 (1998) 897-902. 

M. M. Koza, R. R May, H. Schober, On the heterogeneous character of water s amorphous polymorphism, J. Appl. Crystal-logr. 40 (2007) S517-S521. 

Highly densified vitreous silica may be an example of an amorphous polymorph. Recently the amorphous to amorphous (pressure induced high-density) reversible phase transition has received much attention in connection with the development of bulk metal glasses such as La68Al2oCu2oC 02 [Liu and Hong, 2007[. Amorphous fluid phases are also possible. 

Unlike other synthetic polymers, PVDF has a wealth of polymorphs at least four chain conformations are known and a fifth has been suggested (119). The four known distinct forms or phases are alpha (II), beta (I), gamma (III), and delta (IV). The most common a-phase is the trans-gauche (tgtg ) chain conformation placing hydrogen and fluorine atoms alternately on each side of the chain (120,121). It forms during polymerization and crystallizes from the melt at all temperatures (122,123). The other forms have also been well characterized (124—128). The density of the a polymorph crystals is 1.92 g/cm and that of the P polymorph crystals 1.97 g/cm (129) the density of amorphous PVDF is 1.68 g/cm (130). 

Properties and Structure. Phosphoms(V) oxide, the extremely hygroscopic acid anhydride of the phosphoric acids, exists in several forms but is often referred to by its empirical formula, P2O3. Three crystalline polymorphs, two distinct Hquids, and several amorphous or glassy soHds are recogni2ed. Some properties of the various forms of phosphoric oxide are Hsted in Table 10. 

Crystalline Silica. Sihca exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a Hquid. The Hterature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of siHca, there are three main forms at atmospheric pressure quart2, stable below about 870°C tridymite, stable from about 870—1470°C and cristobaHte, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the stmctures are based on SiO tetrahedra linked in such a way that every oxygen atom is shared between two siHcon atoms. The stmctures, however, are quite different in detail. In addition, there are other forms of siHca that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of siHcon is six rather than four. 

Anatase and mtile are produced commercially, whereas brookite has been produced by heating amorphous titanium dioxide, which is prepared from an alkyl titanate or sodium titanate [12034-34-3] with sodium or potassium hydroxide in. an autoclave at 200—600°C for several days. Only mtile has been synthesized from melts in the form of large single crystals. More recentiy (57), a new polymorph of titanium dioxide, Ti02(B), has been demonstrated, which is formed by hydrolysis of K Ti O to form 20, followed by subsequent calcination/dehydration at 500°C. The relatively open stmcture... 

During precipitate ageing, a gradual transformation of an initially precipitated metastable phase into a final crystalline form often occurs. The metastable phase may be an amorphous precipitate, a polymorph of the final material, a hydrated species or some system-contaminated substance (Mullin, 2001). In 1896, Ostwald promulgated his rule of stages which states that an unstable... 

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

Although vitreous silica is nominally a homogeneous isotropic amorphous material, and should normally remain so during its service life, it is in fact in a metastable condition. The tendency to revert to crystalline forms with attendant deterioration in mechanical durability places severe limitations on the range of applications. Figure 18.2 illustrates the polymorphic forms of silica, and the dimensional changes accompanying each transition. 

Also the polymorphic behavior of s-PS can be altered by blending, in particular with poly-2,6-dimethyl-l,4-phenylene oxide (PPO), both for the case of crystallization from the melt [104] and for the case of crystallization from the quenched amorphous phase [105]. 

To produce amorphous VOPc, 5.0 g of crude VOPc was added into 250 ml of concentric H2SO4 solution, and then the mixture was stirred slowly at 5 °C for 2 h. After acid-treatmcait, cake was collected by filtration, washed with distilled water until washing solution became neutral, then dried at 70 °C over 24 h in a dry oven. To produce fine crystal VOPc, 5 g of amorphous VOPc was added into 90 ml of NMP/H2O solution, and then stirred slowly at 80 C for 1 h. After recrystallization, cake was collected by filtration, washed with methanol, and then dried. All polymorphs were assayed by XRD analysis. 

Solid compounds can have four morphic states polymorphic, pseudo-polymorphic (solvates), amorphous, and desolvated solvates. Crystals usually exhibit narrow melting point ranges and defract light under an optical microscope. When a change in the arrangement of... 

There are other soUd states which sometimes confuse the measurement and definition of solubiUty. The dmg may crystaUize as a hydrate, i.e. under inclusion of water molecules. If the hydrate form is more stable than the pure form it may be difficult to measure the intrinsic solubility of the drug at all. Often drugs tend to precipitate in an amorphous form, often under the inclusion of impurities. As with metastable polymorphs, such amorphous precipitates may lead to erroneously high solubility measurements. CommerciaUy, drugs are often crystallized in salt form, e.g. as the hydrochloride salt, a cation with a chloride anion. In these co-crystallized salts, a much lower solubility than the intrinsic solubility will typi-... 

The physicochemical characteristics of the active ingredient in relation to the dosage form and the suitability for its intended purpose was discussed in several EPARs, particularly relating to the solubility characteristics and absorption from the gut. The compression characteristics were also mentioned in some EPARs. The possible effects of different polymorphs or evidence that only a single polymorph is used are addressed as appropriate. Different amorphous or crystalline forms are also discussed. Where affecting the dosage form, selection properties such as unpleasant taste or smell are mentioned. 

This chapter describes some of the properties of solids that affect transport across phases and membranes, with an emphasis on biological membranes. Four aspects are addressed. They include a comparison of crystalline and amorphous forms of the drug, transitions between phases, polymorphism, and hydration. With respect to transport, the major effect of each of these properties is on the apparent solubility, which then affects dissolution and consequently transport. There is often an opposite effect on the stability of the material. Generally, highly crystalline substances are more stable but have lower free energy, solubility, and dissolution characteristics than less crystalline substances. In some situations, this lower solubility and consequent dissolution rate will result in reduced bioavailability. 

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