Application of simultaneous thermomicroscopy/DSC to the study of phase diagrams

Abstract

The correlation between morphological changes and DSC recordings gives valuable information on the mechanism of phase transformations. The present paper describes a new instrument for simultaneous DSC and thermomicroscopy in transmitted light, where a DSC device is placed in a commercially available hot-stage. The application of this DSC/thermomicroscopy is exemplified by study of the phase diagrams for KNO3--NaNO v diphenylamine---benzophenone and a liquid crystal system.Results obtained from thermoanalytical measurements such as DSC are usually reproducible and quantitative, but they cannot always readily be correlated with the respective phase transformation or chemical reaction occurring in the substance. Attempts have therefore been made to improve such thermoanalytical methods by a correlation between their results and the physical or chemical changes of the substances investigated.These combinations of methods have proved very successful. When performed simultaneously with DTA or DSC, for example, TG is a very powerful tool for studying the chemical stability and decomposition of a variety of materials. This paper demonstrates the usefulness of simultaneous DSC and hot-stage microscopy. Effects which do not show any significant enthalpy change in DSC can often be detected with microscopy. On the other hand, thermomicroscopic investigations often require estimates of heats of transformation, fusion or crystallization.Further, thermomicroscopy with DSC is a very good teaching method to introduce beginners to thermoanalytical investigations, for one can see changes in the investigated substance together with the course of the measured curve. However, the method can also be used for theoretical and practical considerations. For example, differences in the measurement of the same substance in various crucible materials, e.g. glass, aluminium and sapphire, become visible and can be studied.For the recognition of phase boundaries, it is mostly more simple if simultaneous thermomicroscopy/DSC can be used for the determination of phase diagrams, especially for very small concentrations of one of the components.

Dielectric and Calorimetric Study of Orientationally Disordered Phases in Two Unusual Two-Component Systems

“…To get some information about the solid−liquid phase diagram, DSC scans were taken for a heating rate of 2°/min, and the resultant curves were analyzed using the DSC instrument software by determining the onset temperature ( T sol ) and end temperature ( T liq ) of the endotherms as shown in Figures and . The transition temperatures thus determined were plotted as a function of the mole fraction of CHC ( x m ), as suggested in the literature, ,− to determine the solid−liquid phase diagram. The phase diagram is shown in Figure , which, although not clear throughout the concentration range, reveals an eutectic with solid solutions A(α) and B(β) rich in A (i.e., DMCH) and B (i.e., CHC), respectively, on the lower- and higher- concentration sides.…”

A combined approach of differential scanning calorimetry and hot-stage microscopy with image analysis in the investigation of sulfathiazole polymorphism

“…This is illustrated by a coupling of DSC and HSM, which have proved to be very successful in the study of phase diagrams and polymorphic transformation [20][21][22][23]. Based on their works, visual observation through HSM was found to be a more sensitive indicator of phase changes than the DSC measurements alone; the changes happening on the surface of the crystals could clearly be seen using HSM even when the DSC showed no thermal event [21].…”

Crystal Growth of Pharmaceuticals from Melt

“…This method allows a direct observation of the texture of anisotropic solids, phase transitions, existence of polymorphism, etc (Kuhnert-Brandstatter 1982). By combining PLTM with DSC, valuable information on the mechanism of phase transitions can be obtained as will be seen later in this chapter (Wiedemann and Bayer 1985). The equipment used consists of a cell holder hot stage platform (Linkam DSC 600) coupled to a Leika DMRB microscope to which a video camera was adapted.…”