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- Mechanochemistry (13)
- Polymorphism (6)
- Crystallization (5)
- SAXS (5)
- In situ (4)
- Synchrotron radiation (4)
- XRD (4)
- Cocrystal (3)
- Nanoparticle (3)
- Nifedipine (3)
Organisationseinheit der BAM
- 6 Materialchemie (3)
- 6.3 Strukturanalytik (3)
The effect of the ball to reactant ratio on mechanochemical reaction times studied by in situ PXRD
(2017)
The effect of the reactant powder mass on reaction times for the mechanochemical formation of a soft matter model system was studied by in situ PXRD. The syntheses were performed at a constant ball mass in a shaker mill with and without glassy SiO2 as an inert additive. Reaction times decreased with the increase of the ball to reactant ratio (BRR). The kinetic influence of the SiO2 powder was excluded. The decrease in the reaction time with decreasing mass of reactants was related to the rise in the stress energy transferred to the powder by a higher ball impact. The BRR had no effect on the induction time. But the product conversion was accelerated by raising the BRR. While a certain temperature is needed for the activation of reactants in the induction phase, the conversion of soft matter reactants is rather controlled by impact than temperature.
The use of a fibre-based light sensor for the calibration of Scanning Probe Microscopy piezos
(1999)
The crystallization of nifedipine was studied by means of synchrotron-X-ray diffraction, single-crystal X-ray structural analysis, and Raman spectroscopy. The results of slow evaporation (24 h in minimum) using dimethyl sulfoxide (DMSO) are presented. Since fast crystallization processes (typically minutes) in different solvents always led to the final formation of the thermodynamically most stable α-polymorph of nifedipine, we observed a novel pseudo-polymorph due to slow crystallization from DMSO. The single-crystal X-ray structure of the solvated species nifedipine·DMSO (1:1) is reported for the first time. In addition, the crystallization process on surfaces was followed by means of light microscopy and environmental scanning electron microscopy (ESEM) coupled with energy-dispersive X-ray spectroscopy (EDS) analysis. Different diffractions pattern and Raman spectra were observed for crystals grown from stock solution and those obtained by drying the solution on soda lime silicate surfaces.
Crystallization processes under different conditions are of fundamental interest in chemistry, pharmacy, and medicine. Therefore, we have studied the formation of micro- and nanosized crystals using water-caffeine (1,3,7-trimethyl-1H-purine-2,6(3H,7H)-dione) solutions under ambient conditions as a relevant model system. When droplets of an aqueous caffeine solution evaporate and eventually dry on surfaces (glass, polystyrene, and polyester), stable coffee tabletop rings with a perimeter of typically 3 mm are formed after 20 to 50 min. Using a micro focus X-ray beam available at the BESSY µSpot-beamline, the fine structure of different caffeine needles can be distinguished. Unexpectedly, both crystal modifications (α- and β-caffeine) are present, but locally separated in these rings. Furthermore, AFM studies reveal the presence of even smaller particles on a nanometer length scale. To eliminate influences of surface irregularities from the crystallization process, acoustic levitation of liquid samples was employed. Such levitated droplets are trapped in a stable position and only surrounded by air. The solvent in an ultrasonically levitated drop evaporates completely, and the resulting crystallization of caffeine was followed in situ by synchrotron X-ray diffraction. In this case, the diffraction pattern is in accordance with pure α-caffeine and does not indicate the formation of the room temperature polymorph β-caffeine. Hence, our investigations open new vistas that may lead to a controlled formation of cocrystals and novel polymorphs of micro- and nanocrystalline materials, which are of relevance for fundamental studies as well as for pharmaceutical and medical applications.