Cölfen, Helmut


Suchergebnisse Publikationen

Gerade angezeigt 1 - 7 von 7
Vorschaubild nicht verfügbar

Potentiometric Titration Method for the Determination of Solubility Limits and pKa Values of Weak Organic Acids in Water

2020-07-21, Wiedenbeck, Eduard, Gebauer, Denis, Cölfen, Helmut

The determination of solubility limits of compounds in water is unprecise and relies on certain prerequisites such as UV-Vis absorption activity. In this study we designed an experimental approach based on potentiometric titrations to determine solubility limits of various organic compounds by exploiting their pH-active carboxylic acid groups. By applying the law of mass action, utilizing a double-dosing method ensuring a constant compound concentration, it is possible to determine the intrinsic solubility limits, which are independent of the pH value. The derived equations enable the precise and fast determination of intrinsic solubility limits of organic compounds in aqueous solutions within 2 - 4 hours. Moreover, it is shown how the pKa value can be determined based on titrations carried out at two different compound concentrations.

Vorschaubild nicht verfügbar

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.

Vorschaubild nicht verfügbar

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.

Vorschaubild nicht verfügbar

On classical and non-classical views on nucleation

2018-11-27, Gebauer, Denis, Raiteri, Paolo, Gale, Julian D., Cölfen, Helmut

Classical nucleation theory (CNT) is based on the notion of critical nuclei serving as transition states between supersaturated solutions and growing particles. Their excess standard free energy depends on supersaturation, and determines the height of the barrier for phase separation. However, predictions of CNT nucleation rates can deviate from experimental observations by many orders of magnitude. We argue that this is due to oversimplifications within CNT, rendering the critical nucleus essentially a conceptual notion, rather than a truly existing physical entity. Still, given adequate parametrization, CNT is useful for predicting and explaining nucleation phenomena, since it is currently the only quantitative framework at hand. In the recent years, we have been introducing an alternative theory, the so-called pre-nucleation cluster (PNC) pathway. The truly “non-classical” aspect of the PNC pathway is the realization that critical nuclei, as defined within CNT, are not the key to nucleation, but that the transition state relevant for phase separation is based on a change in dynamics of PNCs rather than their size. We provide a summary of CNT and the PNC pathway, thereby highlighting this major difference. The discussion of recent works claiming to provide scientific evidence against the existence of PNCs reveals that such claims are indeed void. Moreover, we illustrate that an erroneous interpretation of the concentration dependence of the free energy has led to a postulated rationalization of the standard free energy of ion pairs and stable ion associates within CNT, which is not sustainable. In fact, stable ion associates are stuck in a free energy trap from the viewpoint of CNT and cannot be considered in a straightforward manner. On the other hand, the notions of the PNC pathway, by dismissing the idea of the CNT-type critical nucleus as a required transition state, overcome this issue. While a quantitative theory of the PNC pathway is eagerly anticipated, the rationalization of experimental observations that are inconsistent with CNT proves its qualitative explanatory power, underpinning great promise towards a better understanding of, for instance, polymorph selection and crystallization control by additives.

Vorschaubild nicht verfügbar

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.

Vorschaubild nicht verfügbar

Mineral Nucleation : Stabilization of Mineral Precursors by Intrinsically Disordered Proteins

2018-09-10, Rao, Ashit, Drechsler, Markus, Schiller, Stefan, Scheffner, Martin, Gebauer, Denis, Cölfen, Helmut

Biogenic crystallization reactions produce hybrid nanostructured materials under physiological conditions. In article 1802063, Helmut Cölfen and co‐workers identify the disorder to order transitions of biomacromolecules as a regulatory feature of additive‐controlled mineralization. This molecular conditioning generates conformational sub‐ensembles and supramolecular assemblies adept at controlling the pathways of nucleation and crystallization.

Vorschaubild nicht verfügbar

pH-Dependent Schemes of Calcium Carbonate Formation in the Presence of Alginates

2016-03-02, Rao, Ashit, Vásquez-Quitral, Patricio, Fernández, María S., Berg, John K., Sánchez, Marianela, Drechsler, Markus, Neira-Carrillo, Andrónico, Arias, José L., Gebauer, Denis, Cölfen, Helmut

From recent studies on bone and shell formation, the importance of polysaccharides in biomineralization processes is gradually being recognized. Through ion-complexation and self-assembly properties, such macromolecules have remarkable effects on mineralization. However, their influences on the different regimes of crystallization including the interactions with precursor species are unclear. The present study therefore addresses calcium carbonate mineralization in the presence of alginates, a class of linear copolymeric saccharides composed of β-1,4 linked d-mannuronic and l-guluronic acid. During mineralization, this biopolymer is found to exert pH-dependent control over mineralization pathways in terms of the stability of prenucleation clusters, inhibitory effect toward nucleation and initially formed postnucleation products. Remarkably in the presence of this macromolecular additive, either amorphous or crystalline vaterite particles can be selectively nucleated in a pH-dependent manner. This is validated by electron microscopy wherein vaterite particles are intimately associated with alginate assemblies after nucleation at pH 9.75. At lower pH, aggregates of amorphous particles are formed. Thus, in addition to the general focus on biochemical properties of additives, solution pH, a physiologically fundamental parameter significantly alters the scheme of mineralization.