Cölfen, Helmut

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Infiltration and crystallization behavior of calcium carbonate precursor formulations in porous materials

2018, Hoyt, Andra-Lisa M., Cölfen, Helmut

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Evidence for phase transitions of aqueous gelatin gels in a centrifugal field

2002, Borchard, W., Cölfen, Helmut, Kisters, D., Straatmann, A.

Experimental findings of turbid zones in the schlieren patterns of gels in a centrifugal field are explained by a demixing of gels into a highly swollen gel coexisting with a collapsed gel. The thermodynamic analysis of a slightly cross-linked aqueous gel at relatively low centrifugal field, which remains clear, leads to the conclusion that the system is close to its stability limit. A procedure is proposed to show how the stability limits may be extrapo lated by means of ultracentrifugal measurements. The phenomena are separated into a transient and a permanent demixing. The permanent demixing has been qualitatively explained by use of state diagrams predicted recently by Khokhlov and coworkers, Ilavsky and also by Moerkerke et al.

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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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Physicochemical and chemical characterisation of chitosan in dilute aqueous solution

2002, Berth, Gisela, Cölfen, Helmut, Dautzenberg, Herbert

Static and dynamic light scattering as well as analytical ultracentrifugation and viscosity measurements were used to investigate the chain conformation of chitosans in salt-containing solutions (pH 4.5; ionic strength about 0.12 M). The samples of various degrees of acetylation were chemically homogeneous. The molecular-weight dependence of the radius of gyration has given clear evidence for a relatively flexible wormlike chain with a persistence length of about 6 nm irrespective of the degree of acetylation, where excluded-volume effects and the polydispersity of the samples were taken into account. In contrast, the interpretation of the hydrodynamic data via a “whole-body approach” according to the Wales-van Holde ratio suggested a strongly elongated chain conformation. The failure of the latter to properly reflect the chain conformation was ascribed to the high extent of draining. A nearly free-draining case can also account for the high scaling exponent of the relationship between intrinsic viscosity and molecular mass.

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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.