Differential scanning calorimetry recrystallization

The crystal stmcture of glycerides may be unambiguously determined by x-ray diffraction of powdered samples. However, the dynamic crystallization may also be readily studied by differential scanning calorimetry (dsc). Crystallization, remelting, and recrystallization to a more stable form may be observed when Hquid fat is solidified at a carefully controlled rate ia the iastmment. Enthalpy values and melting poiats for the various crystal forms are shown ia Table 3 (52). 

Triethanolamine salts of alcohol sulfates form white crystals when obtained in pure form after recrystallization. At their melting point they are semisolid with gelatinous appearance and the transition is difficult to detect. Melting points, determined through thermograms obtained by differential scanning calorimetry, gave 72, 76, 80, and 86°C for dodecyl, tetradecyl, hexadecyl, and octadecyl sulfates, respectively [63]. 

Fig. 6 Differential scanning calorimetry thermogram of 2,4-dinitrophenyl-2,4-dinitro-benzoate, illustrating the recrystallization of form IV into form III (Tl), the melting of forms III and II (T2 and T4), the solidification of the melts produced by T2 and T4 (T3 and T5), and the melting of form I (T6). (Data adapted from Ref. 46.)...

X-ray diffraction studies are usually carried out at room temperature under ambient conditions. It is possible, however, to perform variable-temperature XPD, wherein powder patterns are obtained while the sample is heated or cooled. Such studies are invaluable for identifying thermally induced or subambient phase transitions. Variable-temperature XPD was used to study the solid state properties of lactose [20], Fawcett et al. have developed an instrument that permits simultaneous XPD and differential scanning calorimetry on the same sample [21], The instrument was used to characterize a compound that was capable of existing in two polymorphic forms, whose melting points were 146°C (form II) and 150°C (form I). Form II was heated, and x-ray powder patterns were obtained at room temperature, at 145°C (form II had just started to melt), and at 148°C (Fig. 2 one characteristic peak each of form I and form II are identified). The x-ray pattern obtained at 148°C revealed melting of form II but partial recrystallization of form I. When the sample was cooled to 110°C and reheated to 146°C, only crystalline form I was observed. Through these experiments, the authors established that melting of form II was accompanied by recrystallization of form I. 

Fig. 12 Differential scanning calorimetry thermograms obtained for piretanide, as recrystallized from (a) f-butanol, (b) n-propanol, (c) i-propanol, and (d) IV V-dimethyl-formamide. (Data adapted from Ref. 34.)...

The different recrystallization behavior as revealed by differential scanning calorimetry (DSC) complicates the understanding of structure formation as a dependence of take-up speed and seems to particularly correspond to the... 

The continuous change of the thermodiagram in polymerization has mainly been studied for DSP by means of differentially scanning calorimetry (DSC)10). DSC curves obtained in the course of photopolymerization of DSP crystals at the irradiation times of 20, 40, 50, and 70 min with a xenon lamp are shown in Fig, 15 together with those of DSP, as-polymerized, recrystallized, and amorphous poly-DSP s. 

As discussed above, linear alkyl chains such as the fatty acid moiety of monoglycerides can form a complex with helical segments of the solubilized starch molecules. The recrystallization of amylose is apparently little affected by complexation with mono glycerides. Differential scanning calorimetry (DSC) studies of day-old bread crumb show that all the amylose seems to be present in a crystalline state. Complex formation with monoglyceride by the short side chains of amylopectin, however, markedly slow the rate of retrogradation. The half-life for reformation of amylopectin crystals (measured by DSC) is increased twofold to threefold when... 

The diuretic fmsemide illustrates how solvent recrystallization experiments can be used to discover new solid-state forms of older drugs. Fmsemide was marketed for many years with no reports of polymorphism. Doherty and York [14], however, noted some features of the differential scanning calorimetry (DSC) thermogram of fmsemide that suggested the possibility of polymorphs. They performed a number of solvent recrystallization experiments and discovered a new polymorph which they characterized analytically. At nearly the same time, Matsuda and Tatsumi [15] published the results of their experiments with fmsemide with the conclusion that there are five polymorphs and two solvates, illustrating the principle that the number of known forms is related to... 

As was pointed out earlier, the determination of the melting temperature of solution formed crystals of linear polyethylene by differential scanning calorimetry is complicated by melting, or partial melting followed by recrystallization. In essence, what happens is that, because of the very small crystallite thicknesses... 

The PCN monomer was prepared by reaction of cyanamide with phenylisocyanate in alkaline solution at 15-18°C according to the literature [40]. The crude white powdered product was purified by recrystallization from an acetone-petroleum ether mixed solvent. The resulting colorless fine needles of phenylcarbamidonitrile melted at 134-136°C. Figure 5 shows its thermal behavior in differential scanning calorimetry (DSC, Dupout 1090) a sharp endothermic melting point peak appears... 

Similarly, the reactions between solid 1-3 dimethylbarbituric acid (dmb) with vapours of NH3 and of the volatile amines NH2(CH3), and NH(CH3)2 have been investigated [69]. The barbiturate salts [NH4]dmb, [NH3(CH3)]dmb and [NH2(CH3)2]dmb have been characterized by X-ray powder diffraction (XRPD), differential scanning calorimetry and thermogravimetric analysis. The solid-gas reactions were monitored by UV-Vis spectroscopy in the solid state. In the case of [NH2(CH3)2]dmb, recrystallization from methanol yields the salt... 

Fillon B, Wittman JC, Lotz B, Thierry A. Self-nucleation and recrystallization of isotactic polypropylene (a phase) investigated by differential scanning calorimetry. J Polym Sci B Polym Phys... 

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