Single Review on past events

What do mussels, lobsters, snails, algae, chalk and cement have in common? – This is one of the questions that physicochemist Denis Gebauer can answer in an easy but scientifically competent manner. The mentioned animals and materials all use calcium carbonate, the most abundant biomineral. Minerals are solid, mostly inorganic crystalline compounds, built-up of a regular arrangement of atoms, ions or molecules in a 3-dimensional lattice. In biologically formed minerals, crystal morphologies, structures and orientations are controlled in very sophisticated ways. In general, the resulting biominerals —like bones, teeth, or shells— show outstanding performance in terms of their material properties like superior fracture resistance. Studies into biomineralization are thus motivated by finding inspiration in nature, which can potentially be utilized for the development of novel functional materials. And this is what makes it that interesting for Denis Gebauer. He analyzes the crystalline structures, in particular the ways how they are formed via intermediates and precursors including so-called prenucleation clusters.

In his talk, he first explained the classical theory of nucleation, which deals with the formation of seed particles in solution, and then showed his research results on non-classical nucleation, which appears to be relevant for most biominerals. From the classical point of view, nucleation eventually happens when a chemical system becomes supersaturated, supercooled, or superheated. For example, when beer is chilled, it can stay in its liquid form even below the freezing point. At times, it can suddenly solidify when supercooled and taken from the freezer. The bulk energy of seeds randomly forming in solution drives nucleation, whereas the energetic costs due to the generation of internal surface inhibit the growth of seeds at small sizes. Hence, seeds will dissolve again when they are smaller than a certain critical size, and beer in the freezer may remain liquid even below its freezing point. To start the freezing process the barrier that arises from the energetic costs due to the surface of the seeds needs to be overcome. This can be achieved via cooling to even lower temperatures, or may happen spontaneoulsy when the bottle of beer is touched, and a small amount of energy transfered by the movement helps to overcome the energetic barrier to initiate the process of solidification.

To explain the importance of his research, Denis Gebauer illustrated the relevance of prenucleation clusters and non-classical nucleation to the early stages of the precipitation of calcium carbonate. In case of this prominent biomineral, the classical point of view as outlined above obviously does not apply. So-called prenucleation clusters are stable even at small sizes in solution before nucleation, and nucleation proceeds via cluster-cluster aggregation, initially yielding amorphous nanoparticles as intermediates. A better understanding of the mechanisms underlying nucleation of minerals is not only important for the development of bio-inspired materials, but can also help to avoid unwanted mineralization, for instance, the scaling (incrustration) of private or industrial appliances due to water hardness. Scientific fields that benefit from novel insights into crystallization and nucleation range from materials science and pharmacy to medicine and engineering.