Differential thermal analysis processes observed

Differential thermal analysis (DTA) consists of the monitoring of the differences in temperature existing between a solid sample and a reference as a function of temperature. Differences in temperature between the sample and reference are observed when a process takes place that requires a finite heat of reaction. Typical solid state changes of this type include phase transformations, structural conversions, decomposition reactions, and desolvation processes. These processes may require either the input or release of energy in the form of heat, which in turn translates into events that affect the temperature of the sample relative to a nonreactive reference. 

Differential thermal analysis (DTA) is based upon the measurement of the temperature difference (AT) between the sample and an inert reference such as glass or AIjO as they are both subjected to the same heating programme. The temperature of the reference will thus rise at a steady rate determined by its specific heat, and the programmed rate of heating. Similarly with the sample, except that when an exothermic or endothermic process. occurs a peak or trough will be observed. Typical behaviour is shown schematically in Figure 11.7. 

Singh et al. (112) studied the degradation behavior in air by ther-mogravimetry, differential thermal analysis and wide angle X-ray diffraction. It is observed that crystallinity and degradation were affected by thermal exposure and time of polychromatic irradiation. It was concluded that the processing of PCT must be done carefully, since it melts near its decomposition temperature. So, an exhaustive drying process at about 125 °C is commonly performed before its use (14). 

Other techniques involving attempts to utilize the properties of surfactants have included crystallization of poorly soluble drugs such as sulphathiazole, prednisone and chloramphenicol in the presence of small amounts of surfactants [36]. Increases in the rate of solution were observed in each case when polysorbate was used as a 2.5 % solution as the crystallization medium. While the result might be partly ascribed to adsorption of surfactant molecules on to the hydrophobic crystal surface, differential thermal analysis also suggests that some surfactant is incorporated into the crystal structure. Interference of a surfactant in the crystallization process could lead to defect formation. Model studies with... 

For the [Fe(bpp)2]2+ system, spin transition behaviour is also observed in acetone solution. For the three salts examined, the tetrafluoroborate, iodide and hexafluorophosphate, the behaviour is virtually independent of the associated anion, unlike the situation in solid samples, and in this instance the molecular process occurs essentially independently of cooperative effects [86]. Analysis of the systems in terms of a simple low spin high spin thermal equilibrium gives AH=20 1 kj mol-1 and AS=80 4 J K-1 mol-1 for the forward process, values typical for iron(II) spin crossover systems and similar to those obtained for solid [Fe 592][BF4]2 (AH=24 kj mol-1 and AS=100 J K-1 mol-1) from differential scannning calorimetry measurements [94],... 

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