Amorphous form formation

The red, amorphous form of vanadium pentoxide is the form most frequently met with in the laboratory. Its preparation has been described above. It melts at 658°10 or 675° C.u to a dark red liquid, but is not volatile even at high temperatures it can be vaporised only in the electric furnace.18 The fused solid conducts electricity, with formation of hypovanadic oxide, V02 18 the electrical conductivity has been measured.14 The oxide absorbs water on exposure to the air, the... 

Laschtschenko therefore suggested that the amorphous forms may be of colloidal origin and may thus represent stages in the continuous passage from the colloidal to the crystalline state. The mode of preparation of the brown form by reduction of arsenic compounds in solution is favourable to sol formation, and the view that the various forms differed only in degree of dispersion of the particles has already been mentioned (p. 30). X-ray investigations6 show that the precipitated... 

Similarly, a number of amorphous alloys based on Fe-Zr, Ni-Zr, Co-Zr, Ni-Nb, have not shown any increase in activity over that expected for the mechanical mixture of the crystalline components [571]. For Ni-Nb the overpotential has even increased. Only Cu-Ti alloys have shown apparent synergetic effects, but the results of Machida et al. [89] (cf. Fig. 32) should also be taken into account. Jorge et al. [152] have observed higher activity for the amorphous form of Cu-Ti alloys, but they have attributed it to the preferential dissolution of Ti in the amorphous sample under cathodic load, with formation of a relatively porous Cu layer. The same effect was obtained more rapidly by means of HF etching [89,152]. 

Similar analyses for individual degradation products reveal interesting behaviors. For example, the pattern for peak B for the amorphous form (Fig. 7) is similar to the total related substances results. In contrast, the rate of peak F formation for the crystalline form is almost independent of water content (Fig. 8). 

Figure 9 Arrhenius plot for peak B formation in amorphous form sample.

Differential thermal analysis (DTA) thermograms of ( )-etodolac sodium salt exhibited endothermic transitions around 80,120, and 297°C and an exothermic transition around 83°C [12]. The exothermic phase change was observed after exposure of the sample to moisture, indicating conversion of the amorphous form of ( )-etodolac sodium salt to a crystalline phase. In contrast, the thermogram of (+)-etodolac sodium salt, after exposure to moisture, showed endotherms at 60, 80, 120, and 297°C, indicating that the salt contained methanol, acetonitrile, and water. There was no sign of degradation product formation. 

It is now clear that if the hydrothermal formation of boehmite is avoided (e g. by using low-pressure CRTA and small crystals), then the three hydroxides lose their structural water at quite low temperature ( 200°C) to give the almost amorphous form, p-Al203. Complex changes occur as the temperature is increased to 250-800°C with the formation of certain members of the y-series aluminas y or rj and 9. At temperatures 1200°C the conversion to the dense, low-area a-Al203 normally takes place. 

When amorphous forms convert to crystalline, and finely dispersed materials to larger particles, the solubility should decrease and the isobaric-isothermal potential of mineral formation should increase (by an absolute value). 

P.S. DeCarli and J.C. Jamieson, Formation of an amorphous form of quartz under shock conditions. J. Chem. Phys. 31, pp. 1675-1676 (1959). 

Exothermic Crystallization Precipitation Solidification Adsorption Solvent vapor induced crystallization of amorphous excipients Formation of salt forms of drug substances Melt granulation with semisolid excipients Solvent vapor sorption by drug substances... 

Note Calculations have been made using the MINTEQA2 thermodynamic data base modified by Turner et al. (1993), with revised thermodynamic data for Am from Silva et al. (1995) and revised thermodynamic data for U from this chapter. Log K for formation of AmOH(COj)(c) in MINTEQA2 has been changed from 8.605 to 7.20 (Silva et al. 1995). Np(OH)j and Pu(OH)j complexes have been eliminated from the data base by setting their log K values to -30. The hydroxides of Np, Pu, ni, and U are their most soluble, relatively amorphous forms. 

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