Cölfen, Helmut

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Cölfen
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Helmut
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The multiple roles of additives in CaCO3 crystallization : a quantitative case study

2009, Gebauer, Denis, Cölfen, Helmut, Verch, Andreas, Antonietti, Markus

To date, the knowledge of the multiple roles of additives in calcium carbonate crystallization results from empirical control of crystal growth. We present a crystallization assay generating controlled supersaturation, which allows the categorization of different modes of additive action. This facilitates a novel understanding of the action of crystallization additives.

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Influence of Conducting Polymers Based on Carboxylated Polyaniline on In Vitro CaCO3 Crystallization

2008-11-04, Neira-Carrillo, Andronico, Acevedo, Diego F., Miras, Maria C., Barbero, Cesar A., Gebauer, Denis, Cölfen, Helmut, Arias, Jose L.

Conducting polymers are interesting materials of technological applications, while the use of polymers as additives controlling crystal nucleation and growth is a fast growing research field. In the present article, we make a first step in combining both topics and report the effect of conducting polymer derivatives, which are based on carboxylated polyanilines (c-PANIs), on in vitro CaCO3 crystallization by the Kitano and gas diffusion method. This is the first example of the mineralization control of CaCO3 by a rigid carboxylated polymer. Both the concentration of c-PANI and the presence of carboxylate groups have a strong influence on the CaCO3 crystallization behavior and crystal morphology. X-ray diffraction (XRD) analysis shows crystalline calcite particles confirmed by FTIR spectra. pH and Ca2+ measurements during CaCO3 crystallization utilizing the Kitano and a constant-pH approach show a defined nucleation period of CaCO3 particles. The measurements allow for the calculation of the supersaturation time development, and the kinetic data can be combined with time-dependent light microscopy. The presence of c-PANIs delays the time of nucleation indicative of calcite nucleation inhibition. Microscopy illustrates the morphologies of CaCO3 crystals at all crystallization stages, from homogeneous spherical amorphous CaCO3 (ACC) particles corresponding to the first steps of crystallization to transition stage calcite crystals also involving a dissolution−recrystallization process in a late stage of crystallization. The data show that it is not possible to conclude the crystallization mechanism even for a very simple additive controlled crystallization process without time-resolved microscopic data supplemented by the analysis of the species present in the solution. Finally, fluorescence analysis indicates that conducting polymer derivatives can be incorporated into precipitated calcite particles. This gives rise to CaCO3 particles with novel and interesting optical properties.

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Influence of Selected Artificial Peptides on Calcium Carbonate Precipitation : A Quantitative Study

2009, Gebauer, Denis, Verch, Andreas, Börner, Hans G., Cölfen, Helmut

Calcium carbonate is an abundant biomineral with fascinating shapes and properties. Much effort is spent to study how creatures can control mineral formation. We present a quantitative study of the early stage of calcium carbonate precipitation in the presence of artificial peptide additives, the sequences of which were derived in phage assays to have aragonite binding affinity. A novel crystallization assay shows that the peptide additives inhibit nucleation of calcite. Analysis of the precipitated particles and comparison with nucleation inhibition confirm our recent findings, which suggest that calcitic and vateritic short-range order is already preformed in stable prenucleation clusters, which form amorphous intermediates after nucleation reflecting similar structures and finally become crystalline. In the long run, this process facilitates the control of polymorph formation by the design of the binding affinity of additives to different polymorphs (i.e., the polymorph bound weakest by the additive is to be formed as its formation is least inhibited). These findings facilitate a novel understanding of mineralization control and provide a basis for the analysis of biological peptide sequences and for the analysis of their role in biomineralization processes.

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Stable prenucleation calcium carbonate clusters

2008-12-19, Gebauer, Denis, Völkel, Antje, Cölfen, Helmut

Calcium carbonate forms scales, geological deposits, biominerals, and ocean sediments. Huge amounts of carbon dioxide are retained as carbonate ions, and calcium ions represent a major contribution to water hardness. Despite its relevance, little is known about the precipitation mechanism of calcium carbonate, and specified complex crystal structures challenge the classical view on nucleation considering the formation of metastable ion clusters. We demonstrate that dissolved calcium carbonate in fact contains stable prenucleation ion clusters forming even in undersaturated solution. The cluster formation can be characterized by means of equilibrium thermodynamics, applying a multiple-binding model, which allows for structural preformation. Stable clusters are the relevant species in calcium carbonate nucleation. Such mechanisms may also be important for the crystallization of other minerals.