Wiedenbeck, Eduard

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Organic Nucleation : Water Rearrangement Reveals the Pathway of Ibuprofen

2024, Lu, Hao, Macht, Moritz, Rosenberg, Rose, Wiedenbeck, Eduard, Lukas, Max, Qi, Daizong, Maltseva, Daria, Zahn, Dirk, Cölfen, Helmut, Bonn, Mischa

The organic nucleation of the pharmaceutical ibuprofen is investigated, as triggered by the protonation of ibuprofen sodium salt at elevated pH. The growth and aggregation of nanoscale solution species by Analytical Ultracentrifugation and Molecular Dynamics (MD) simulations is tracked. Both approaches reveal solvated molecules, oligomers, and prenucleation clusters, their size as well as their hydration at different reaction stages. By combining surface-specific vibrational spectroscopy and MD simulations, water interacting with ibuprofen at the air–water interface during nucleation is probed. The results show the structure of water changes upon ibuprofen protonation in response to the charge neutralization. Remarkably, the water structure continues to evolve despite the saturation of protonated ibuprofen at the hydrophobic interface. This further water rearrangement is associated with the formation of larger aggregates of ibuprofen molecules at a late prenucleation stage. The nucleation of ibuprofen involves ibuprofen protonation and their hydrophobic assembly. The results highlight that these processes are accompanied by substantial water reorganization. The critical role of water is possibly relevant for organic nucleation in aqueous environments in general.

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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.