Scientists are especially interested in oscillating chemical reactions. These occur when reaction products periodically and repeatedly change. Their behaviour is of importance to many fields of study - including chaos research. That is because these reaction systems are always complex and far away from thermodynamic equilibrium. One particularly well-known example is the "Belousov-Zhabotinsky" reaction. In it, a coloured indicator is used to make the reaction products of a coupled redox reaction visible. They typically take on the pattern of concentric circles, spreading out, for example, across a petri dish.
Mathematically, spatially oscillating reactions can be described as "reaction-diffusion systems". This means that it is not just chemical reactions which influence the amount of material at a certain point in space. Diffusion also plays a role - the exchange of material with the surrounding area. In such simulations, we get the typical concentric circle pattern of a Belousov-Zhabotinsky reaction. In the picture above, it is indicated in red-violet.
Researchers from Potsdam have now proven that these oscillating reactions can also apply to multi-phase systems, and even to the self-organisation processes of nanoparticles. What is central is that in a multi-phase reaction system, it is possible to formulate either an autocatalyic or autoinhibiting reaction step. This leads an oscillating system to be constructed, and ultimately a pattern to be formed.
The researchers used a newly synthesized polymer to create the typical concentric circle pattern, via controlled barium carbonate crystallisation (see image). Such patterns correspond quite well to the calculations in a simulation. The researchers also were able to formulate a complex coupled reaction system including crystallisation, complexation, and precipitation reactions and identify the autocatalytic formation of a complex between barium and the polymer.
Notably, the elongated crystalline structures which made up the circular pattern are themselves created by superstructures of nanoparticles, which are themselves created by self-organisation (see image). In this way, Max Planck researchers have shown for the first time that the Belousov-Zhabotinsky reaction does not just take place in a solution, but also in multi-phase systems, and in nanoparticle self-organisation. This discovery is not only important to research into reactions far away from thermodynamic equilibrium. It can also help explain biological pattern formation. One example of biological self-organisation is mussel shell patterns. They are created via controlled crystallisation, just like the model systems of the researchers in Potsdam used. Interestingly, these patterns also mathematically duplicate reaction-diffusion systems exactly.
Angewandte Chemie