Cölfen, Helmut

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Helmut
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Monitoring the crystallization of tartaric acid with THz spectroscopy

2017-08, Soltani, Amin, Gebauer, Denis, Fischer, Bernd M., Cölfen, Helmut, Koch, Martin

We study the crystallization of L-(+)-Tartaric acid by employing attenuated total reflection THz time-domain spectroscopy (TDS). In the so-called nucleation stage, the absorption spectrum shows a broadband transient increase. Besides, we observed a transient resonance at 750 GHz. This peak can be assigned to aggregation of L-(+)-Tartaric acid molecules including hydrated water molecules. Furthermore, the THz data reveal the vibration of clusters in the solution phase which is confirmed by analytical ultracentrifugation technique.

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Hydration dynamics in CaCO3 nucleation by THz spectroscopy

2017-08, Sebastiani, Federico, Wolf, Stefan L. P., Born, Benjamin, Luong, Trung Quan, Cölfen, Helmut, Gebauer, Denis, Havenith, Martina

In CaCO3 nucleation, the role of water remains enigmatic. Changes in THz absorption during the early stages of CaCO3 nucleation evidence altered coupled motions of hydrated calcium and carbonate ions. By high precision THz absorption measurements we were able to follow the changes in the hydration bond dynamics during nucleation. Our THz data strongly suggest that amorphous CaCO3 forms through solidification of initially liquid precursors. Polycarboxylates, which stabilize CaCO3 liquid precursors, significantly enhance the kinetic stability of the metastable liquid-liquid state. The importance of water network dynamics in phase separation mechanisms as tested by THz absorption measurements using a p-Ge spectrometer is likely to be more general for aqueous systems.