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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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.