Possible errors made due to incorrect evaluation of time- or temperature-resolved X-ray data (TXRD) are described. The extraction
of compositional information, e.g., for kinetic purposes, can be highly erroneous when neglecting the influence of the changing
mass absorption coefficient and a proper conversion of mass fractions to molar fractions. When structural data are computed,
care should be given to a possible volume change of the sample. The correct way of evaluation and the extent of these errors
is demonstrated for the thermal decarboxylation of calcium carbonate.
Fundamentals and fields of application of time- and temperature-resolved diffraction methods are presented. X-ray diffraction
and neutron diffraction will be considered. Dynamic diffraction methods are increasingly applied in different fields like
solid state reactions, heterogeneous catalysis and biological sciences. New methods like synchrotron radiation and position-sensitive
detectors permit a considerable expansion of potential research areas. The dynamic diffraction methods are compared with the
classical thermoanalytical methods thermogravimetry and DSC.
The dehydrated lactose forms αH and αS were investigated by time- and temperature-resolved X-ray powder diffractometry and differential scanning calorimetry. We
found different X-ray structures for these two forms, which is probably related to the different dehydration processes. The
rapidly dehydrated form αH obviously has the same X-ray structure as the starting material α-lactose monohydrate, although the crystallinity is reduced.
A thermally induced transition of the αH-form into the αS-form was observed. This transition should allow one to “switch” between the physicochemical properties of the excipient,
which may be important for applications in pharmaceutical and food industries.
Halogenoacetates are known to undergo a solid-state elimination reaction to metal halide and poly(hydroxyacetic acid), polyglycolide. Earlier studies have shown that the reaction takes place exclusively in the solid-state without the occurrence of liquid intermediates. Single crystals of sodium chloroacetate and silver chloroacetate were reacted and studied with X-ray diffractometry, scanning electron microscopy and thermomicroscopy. The results show that the reaction leads from single crystals to a composite of polyglycolide and metal halide. Neither the salt nor the polymer exhibit a preferred crystallographic orientation, therefore it must be concluded that the crystal lattice is not preserved during the reaction.
The solvent-free reduction of benzophenone and five substituted benzophenones with sodium borohydride to the corresponding alcohols was studied by thermal analysis, X-ray powder diffractometry, NMR spectroscopy, and scanning electron microscopy. In most cases, the reaction occurs via liquid eutectic phases that are formed between the benzophenone and the resulting benzohydrol. Nevertheless, this reaction can be carried out without the need for a solvent, leading to pure alcohol without side products. In some cases, heating may be necessary to achieve a reasonably short reaction time. In conclusion, this reaction type appears to be feasible as a preparative organic reaction that avoids a solvent.
Development and experimental setup of the time-, and temperature -resolved X-ray powder diffractometry are described. This method allows far deeper insight into solid state reactions than conventional thermoanalytical methods like differential scanning calorimetry (DSC) or thermogravimetry. As an example, the dehydration of caffeine hydrate was investigated. We found that in earlier stages the reaction is nucleation controlled, whereas for higher extent of reaction diffusion limitation becomes rate-controlling.
The thermal elimination of NaCl from sodium chloroacetate, a polymerization reaction that takes place between 150 and 200‡C in the solid state, leads quantitatively to the simplest polyester, polyglycolide. Byin situ IR-spectroscopy, we have shown that the reaction proceeds smoothly and directly without intermediates or by-products. The endgroups of the polymeric product — ionised carboxylate groups (-COONa) and hydrogen-bonded alcohol groups (−COH) — are clearly detectable. It is therefore concluded that the polymer forms extended chains, not rings, during the course of this solid-state reaction. That corresponds well with the idea of a polymerization reaction in the solid state. However, this experiment does not exclude the formation of polyglycolide rings as further product because they do not contain any terminating groups.