Cölfen, Helmut
0000-0002-1148-0308 Berufsbeschreibung
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Analytical ultracentrifugation in colloid and polymer science : new possibilities and perspectives after 100 years
Analytical ultracentrifugation (AUC) is a classical polymer and colloid analysis technique invented by Theodor Svedberg 100 years ago. Modern hard- and software and powerful computers make it now possible to develop the methodology beyond what was possible with this technique before. This perspective aims to describe new possibilities, which extend the possibilities of AUC beyond the classical repertoire of the determination of distributions of sedimentation coefficient, particle size, and molar mass as well as stoichiometries and interaction constants of interacting systems. High-resolution simultaneous characterization of particle size and optical property distributions, investigation of nucleation by reaction in the AUC cell, characterization of particle interactions at a very high concentration, and characterization of complex fluids or osmotic pressures over large concentration ranges even crossing phase boundaries are among the discussed topics. They show that even after 100 years of successful application, AUC still has much yet unexplored potential in colloid and polymer science. Graphical Abstract This perspective paper spans from the days of invention of analytical ultracentrifugation to now including nonmainstream methodology and instrumentation, which has a huge potential for the future. This includes multiwavelength detectors, high-resolution particle size distributions, chemical reactions in the ultracentrifuge, high-concentration work, osmotic pressure distributions, and characterization of complex fluids.
A Facile and Rational Method to Tailor the Symmetry of Au@Ag Nanoparticles
Precisely controlling the morphologies of plasmonic metal nanoparticles (NPs) is of great importance for many applications. Here, a facile seed-mediated growth method is demonstrated that tailors the morphologies of Au@Ag NPs from cubes/cuboids to chiral truncated cuboids/octahedra, well-defined octahedra, and tetrahedra, via simply increasing the concentrations of AgNO3 and cysteine in the halide surfactant systems. Accordingly, the particle symmetries are also tuned. The method is quite robust where seeds with distinct shapes including irregular ones can all lead to uniform Au@Ag NPs. The evolution of these shapes can be illustrated by a recently proposed symmetry-based kinematic theory (SBKT). Furthermore, SBKT shows a strategy to optimize the preparation of chiral/dissymmetric NPs, and the experimental results confirm such a dissymmetric synthesis strategy. Cuboids and octahedra with corners differently truncated are identified as two different chiral forms. The chirality of the NPs is additionally probed by electrochemistry, where the chiral NPs show enantioselectivity in the oxidation of d- and l-glucose. Altogether, the results gain fundamental insights into tailoring the plasmonic NP morphologies, and also suggest strategies to obtain chiral NPs.
The Structure, Preparation, Characterization, and Intercalation Mechanism of Layered Hydroxides Intercalated with Guest Anions
Since the intercalation of anions into layered hydroxides (LHs) has a great impact not only on their nucleation and growth but also on their structure, composition, and size, the intercalation chemistry of LHs has aroused the strong interest of researchers. However, the progress in the fundamental understanding of LHs intercalated with guest anions have not been paralleled by a concomitant development of the preparation and performance improvement of such materials. Considering the guidance of a timely in-depth review for scientists in this area, a systematic introduction about the development that is made on the above-mentioned issues is highly needed but yet missing so far. Herein, recent advances in understanding the chemical composition and structure of LHs intercalated with guest anions are systematically summarized. Meanwhile, typical and emerging bottom-up synthesis methods of LHs intercalated with anions are reviewed, and the potential impact of external reaction parameters on the intercalation of anions into LHs are discussed . Besides, different analytical characterization techniques employed in the examination of guest anion-intercalated LHs are deliberated upon. Finally, although progress is slow in exploring the intercalation mechanism, as many examples as possible are included in this review and inferred the possible intercalation mechanism.
Growth strategy for solution-phase growth of two-dimensional nanomaterials via a unified model
Two-dimensional (2D) materials prepared by a solution-phase growth route exhibit many unique properties and are promising for use in various fields. However, simple, rational and green fabrication of target materials remains challenging due to the lack of guiding principles. Here we propose a universal qualitative model for 2D materials grown for layered and non-layered crystal structures by a solution-phase growth route; both theoretical simulation and experimental results confirm the model’s validity. This model demonstrates that 2D growth can be controlled by only tuning the reaction concentration and temperature, and has been applied to fabricate more than 30 different 2D nanomaterials in water at room temperature and in the absence of additives. Furthermore, the model shows promise for optimizing the experimental design of numerous other 2D nanomaterials.
Aufdeckung des Entstehungsmechanismus und Optimierung der Synthesebedingungen von geschichteten Doppelhydroxiden für die Sauerstoffentwicklungsreaktion
Geschichtete Doppelhydroxide (LDHs), deren Bildung stark von der OH-Konzentration abhängt, haben in verschiedenen Bereichen großes Interesse geweckt. Die Auswirkung der Echtzeit-Änderung der OH− Konzentration auf die Bildung von LDHs wurde jedoch noch nicht vollständig erforscht, da die bestehenden Synthesemethoden für die in situ Charakterisierung nicht geeignet sind. Hier bietet die gezielt entwickelte Kombination aus NH3-Gasdiffusion und in situ pH-Messung eine Lösung für das oben genannte Problem. Die so erhaltenen Ergebnisse deckten den Bildungsmechanismus auf und führten dazu, dass wir eine Bibliothek von LDHs mit den gewünschten Eigenschaften in Wasser bei Raumtemperatur ohne jegliche Additive synthetisieren konnten. Nach der Bewertung ihrer Sauerstoffentwicklungsreaktionsleistung stellten wir fest, dass FeNi-LDH mit einem Fe/Ni-Verhältnis von 25/75 eine der besten, der bisher berichteten, Leistungen aufweist.
Biodegradable Mineral Plastics
Mineral plastics are a promising class of bio-inspired materials that offer exceptional properties, like self-heal ability, stretchability in the hydrogel state, and high hardness, toughness, transparency, and non-flammability in the dry state along with reversible transformation into the hydrogel by addition of water. This enables easy reshape-ability and recycling like the solubility in mild acids to subsequently form mineral plastics again by base addition. However, current mineral plastics rely on petrochemistry, are hardly biodegradable, and thus persistent in nature. This work presents the next generation of mineral plastics, which are bio-based and biodegradable, making them a promising, new class of polymers for the development of environmentally friendly materials. Physically cross-linked (poly)glutamic-acid (PGlu)-based mineral plastics are synthesized using various alcohol-water mixtures, metal ion ratios and molecular weights. The rheological properties are easily adjusted using these parameters. The general procedure involves addition of equimolar solution of CaCl2 to PGlu in equal volumes followed by addition of iPrOH (iPrOH:H2O = 1:1) under vigorous stirring conditions. The ready biodegradability of PGlu/CaFe mineral plastic is confirmed in this study where the elements N, Ca, and Fe present in it tend to act as additional nutrients, supporting the growth of microorganisms and consequently, promoting the biodegradation process.
The nucleation of C–S–H via prenucleation clusters
The nucleation and growth of calcium–silicate–hydrate (C–S–H) is of fundamental importance for the strength development and durability of the concrete. However, the nucleation process of C–S–H is still not fully understood. The present work investigates how C–S–H nucleates by analyzing the aqueous phase of hydrating tricalcium silicate (C3S) by applying inductively coupled plasma-optical emission spectroscopy as well as analytical ultracentrifugation. The results show that the C–S–H formation follows non-classical nucleation pathways associated with the formation of prenucleation clusters (PNCs) of two types. Those PNCs are detected with high accuracy and reproducibility and are two species of the 10 in total, from which the ions (with associated water molecules) are the majority of the species. The evaluation of the density and molar mass of the species shows that the PNCs are much larger than ions, but the nucleation of C–S–H starts with the formation of liquid precursor C–S–H (droplets) with low density and high water content. The growth of these C–S–H droplets is associated with a release of water molecules and a reduction in size. The study gives experimental data on the size, density, molecular mass, and shape and outlines possible aggregation processes of the detected species.
Tuning the growth morphology of gypsum crystals by polymers
True control over the morphology of gypsum crystals formed via homogeneous precipitation from solution has rarely been reported in the literature. In this work, we have tested a large number of dissolved additives (polymers as well as small molecules) with respect to their ability to alter the typical microscopic appearance of precipitated gypsum powders, which is usually characterized by a mixture of single-crystalline needles and twinned plates. Among the many additives studied, a copolymer of vinylpyrrolidone and acrylic acid (PVP-co-PAA) was identified as powerful growth modifier for gypsum already at low concentrations. In both slow titration and rapid mixing experiments, unconventional blocky crystals with tilted stacking edges as well as pseudo-hexagonal plates could be synthesized reproducibly with the help of the copolymer. Systematic characterization revealed the dynamic mode of action of the newly identified growth modifier, which seems to stabilize a highly reactive face of gypsum and promote the formation of macrosteps. The degree of morphological control achieved in this way is unprecedented in the case of calcium sulfate and may devise entirely new concepts for additive design in the areas of plasters and cementitious materials, gypsum wallboard production and/or scale prevention.