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- Acoustic levitation (5) (entfernen)
For the investigation of small sample volumes, the use of an acoustic levitator was tested as a `sample holder' for hovering droplets in a synchrotron beam. It might be advantageous to use levitated droplets instead of samples confined in solid holders, especially for the study of crystallization processes where the influence of containing walls has to be minimized. In a first experiment, the crystallization of sodium chloride in a small droplet of aqueous solution has been followed with a time resolution of 30 s. The collected diffraction peaks are compared with data in the ICSD database.
Acoustic levitation is used as a newtool to study concentration-dependent processes influorescence
spectroscopy. With this technique, small amounts of liquid and solid samples can be measured
without the need for sample supports or containers, which often limits signal acquisition and
can even alter sample properties due to interactions with the support material. We demonstrate
that, because of the small sample volume, fluorescence measurements at high concentrations
of an organic dye are possible without the limitation of inner-filter effects, which hamper such
experiments in conventional, cuvette-based measurements. Furthermore, we show that acoustic
levitation of liquid samples provides an experimentally simple way to study distance-dependent
fluorescence modulations in semiconductor nanocrystals. The evaporation of the solvent during
levitation leads to a continuous increase of solute concentration and can easily be monitored by
laser-induced fluorescence.
As a model for spray drying the early stages of crystallization of complex molybdate catalyst precursors were monitored online in droplets levitated acoustically. Synchron X-ray scattering techniques were applied to study the drying process of a typical molybdate catalysts precursor prepared from Ni-,Fe-,Bi-nitrate, ammonium heptamolybdate and H3PO4. Comparison with diffraction patterns obtained from sessile droplets shows significant differences in the crystal growth, whereas the final crystal structure – including a Keggin-type anion – is identical in both cases. The levitated samples exhibit a decelerated growth with a large number of small crystallites at the beginning of the crystallization, whereas in case of the sessile droplets a small number of large crystallites were observed after a few minutes. The acoustic levitation using an ultrasonic trap proves to be an elegant tool to mimic spray drying and offers new possibilities in relation to the understanding of the drying process.
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.
A combination of two analytical methods, time-resolved X-ray diffraction (XRD) and Raman spectroscopy, is presented as a novel tool for crystallization studies. An acoustic levitator was employed as sample environment. This setup enables the acquisition of XRD and Raman data in situ simultaneously within a 20 s period and hence permits investigation of polymorphic phase transitions during the crystallization process in different solvents (methanol, ethanol, acetone, dichloromethane, acetonitrile). These real time measurements allow the determination of the phase content from the onset of the first crystalline molecular assemblies to the stable system. To evaluate the capability of this approach, the setup was applied to elucidate the crystallization process of the polymorphic compound nifedipine. The results indicate the existence of solvent-dependent transient phases during the crystallization process. The quality of the data allowed the assignment of the lattice constants of the hitherto unknown crystal structure of the β-polymorph.