Abstract-Crystallization Caught in the Act with Terahertz Spectroscopy: Non-Classical Pathway for l-(+)-Tartaric Acid

Amin Soltani, Denis Gebauer, Lennart Duschek, Bernd M. Fischer,  Helmut Cölfen, Martin Koch,

http://onlinelibrary.wiley.com/doi/10.1002/chem.201702218/full#references


Crystal formation is a highly debated problem. This report shows that the crystallization of l-(+)-tartaric acid from water follows a non-classical path involving intermediate hydrated states. Analytical ultracentrifugation indicates solution clusters of the initial stages aggregate to form an early intermediate. Terahertz spectroscopy performed during water evaporation highlights a transient increase in the absorption during nucleation; this indicates the recurrence of water molecules that are expelled from the intermediate phase. Besides, a transient resonance at 750 GHz, which can be assigned to a natural vibration of large hydrated aggregates, vanishes after the final crystal has formed. Furthermore, THz data reveal the vibration of nanosized clusters in the dilute solution indicated by analytical ultracentrifugation. Infrared spectroscopy and wide-angle X-ray scattering highlight that the intermediate is not a crystalline hydrate. These results demonstrate that nanoscopic intermediate units assemble to form the first solvent-free crystalline nuclei upon dehydration.

Abstract-Crystallization caught in the act with terahertz spectroscopy: non-classical pathway for L-(+)-tartaric acid

http://onlinelibrary.wiley.com/doi/10.1002/chem.201702218/abstract

Crystal formation is a highly debated problem. We show that the crystallization of L-(+)-tartaric acid from water follows a non-classical path involving intermediate hydrated states. Analytical ultracentrifugation indicates solution clusters of the initial stages aggregate to form an early intermediate. Terahertz spectroscopy performed during water evaporation highlights a transient increase in the absorption during nucleation. This indicates the recurrence of water molecules which are expelled from the intermediate phase. Besides, a transient resonance at 750 GHz that can be assigned to a natural vibration of large hydrated aggregates vanishes after the final crystal has formed. Furthermore, THz data reveal the vibration of nanosized clusters in the dilute solution indicated by analytical ultracentrifugation. Infrared spectroscopy and wide-angle X-ray scattering highlight that the intermediate is not a crystalline hydrate. Our results demonstrate that nanoscopic intermediate units assemble to form the first solvent-free crystalline nuclei upon dehydration.

Abstract-Fabry-Pérot interferometer for sensing polar liquids at terahertz frequencies

David Jahn1, Amin Soltani1, Jan C. Balzer1, Withawat Withayachumnankul2, and Martin Koch

http://aip.scitation.org/doi/abs/10.1063/1.4983780

We propose and validate a sensor for polar liquids that operates in conjunction with terahertz time-domain spectroscopy. The sensor is constructed from an optically thick silicon wafer and a ground plane, separated by a gap into which the liquid is injected. This arrangement represents a Fabry-Pérot interferometer that causes a sharp minimum in the reflection spectrum. Compared to resonance-based sensors, this sensor design can maintain its sharp spectral response when loaded with highly absorbing polar liquids. This overcomes an issue of damped resonance caused by material losses in resonance-based sensors. We report a reflection minimum shift of 8 GHz per percent ethanol in water. The sensor can be readily integrated with a microfluidic channel for real-time fluid monitoring.

Abstract-THz ATR Spectroscopy for Inline Monitoring of Highly Absorbing Liquids

Amin Soltani,  Stefan F. Busch, Patrick Plew, Jan C. Balzer, Martin Koch,
http://link.springer.com/article/10.1007%2Fs10762-016-0285-6

We present a THz attenuated total reflection (ATR) setup which allows for inline measurements of highly absorbing liquids. As a proof of principle, we investigate a mixture of water and ground calcium carbonate (GCC) from 5 to 40 wt%. Inline measurements prove that our THz ATR setup allows for the distinction of various concentrations. As an example, we show inline THz ATR measurements for 30 to 40 wt% for GCC watery solution, as this concentration range is of technical relevance. We obtain a sensitivity better than 2 wt%.