Crystallization recording heat effects

The presence of seed crystals generally reduces the induction period, but does not necessarily eliminate it. Even if the system is seeded at time t = 0, a measurable induction period tmd may elapse before new crystals are detected. By definition, these are secondary nuclei and they may appear in several bursts throughout the latent period, making it difficult to attach any real significance to the induction time itself For these reasons it may be preferable to record the latent period as the more practical characteristic of the system. Factors that can influence the induction and latent periods and the rate of desupersaturation are temperature, agitation, heat effects during crystallization, seed size, seed surface area and the presence of impurities. 

In DSC experiments only the heat effects of crystallization can be recorded. This means that only the crystal growth, not the nucleation, can be measured. However, indirect conclusions about the primary nucleation can be made by analyzing DSC data from isothermal crystallization as described later in this section. 

Differential thermal analysis (DTA) is a thermal technique in which the temperature of a sample, compared with the temperature of a thermally inert material, is recorded as a function of the sample, inert material, or furnace temperature as the sample is heated or cooled at a uniform rate. Temperature changes in- the sample are due to endothermic or exothermic enthalpic transitions or reactions such as those caused by phase changes, fusion, crystalline structure inversions, boiling, sublimation, and vaporization, dehydration reactions, dissociation or decomposition reactions, oxidation and reduction reactions, destruction of crystalline lattice structure, and other chemical reactions. Generally speaking, phase transitions, dehydration, reduction, and some decomposition reactions produce endothermic effects, whereas crystallization, oxidation, and some decomposition reactions produce exothermic effects. 

Crystalline samples sometimes produce spectra with distorted band shapes, an effect known as the Christiansen effect [see Potts (1963) and Table 1.4]. Also, polymorphic forms of the same substance frequently show differences in infrared spectra. An example is N-benzoyl-2,3,4,6-tetra-0-benzoyl- -D-glucosylamine, a compound that exists in a form with melting point 113-115°C which, when heated to 117-120°C and allowed to crystallize from the melt, gives a form with melting point 184°C having a somewhat different spectrum in Nujol (Tipson, 1968). Also, different crystal habits (same melting point) of a compound may display partially differing spectra, especially if examined as mulls, in which little pressure is applied. Shifts of up to 20 cm" for certain bands have been observed (Barker et al., 1956) for crystalline and amorphous forms of some carbohydrates. In all such instances, however, spectra of samples of each of the forms, recorded after dissolution in the same solvent, or as a molten substance, are identical. 

Perkin-Elmer DSC 2. The original samples were first fused at 473 K for 5 min to erase the effect of thermal and mechanical history, followed by complete crystallization under isothermal conditions at 401 K for 30 min. The samples were then fused again by heating them to different temperatures (Tf). Finally, the crystallization curves were recorded by cooling the samples to 383 K at a rate of 10 K min... 

A most interesting version of the liquid crystal method for the determination of the ir polarization has been proposed by Kelker et al. [22]. This method makes use of homeotropically ordered nematic phases first the spectrum is recorded (without polarizer) in a homeotropically oriented nematic layer, then the spectrum is taken after heating the sample above the isotropic state. The primary advantage of this method is that it does not need polarizers. Information on the polarization direction is obtained by comparing the optical densities measured in the randomly and in the uniformly ordered phase, respectively. Consider a rod-shaped molecule (rod axis z). Then = Syy = - 1/2 The effective molar extinction coefficient for unpolarized light that is traversing the homeotropically ordered system in a direction parallel to the optic axis is = 1/2 (e + 6y) (Fig. 12). For a band that is polarized parallel to the rod axis the ratio of the optical density measured in the ordered phase to the density measured in the isotropic state is (Equ. (18)) ... 

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