To find out how mechanochemical reactions proceed and what really happens at the moment of collision, the research team led by Dr Ana Sunčana Smith and Dr Ivan Halasz from RBI developed a protocol for computational molecular dynamics simulations that makes it possible to observe the collision between a ball and a particle like an extremely slow-motion recording. Using calculations performed on the Supek supercomputer at SRCE, they were able to watch how a mechanical удар transforms a solid into chemically reactive “building blocks”, and they also determined why the presence of a small amount of water can help the reaction, while too much water can have the opposite effect.
These results are important because they open the way to more precise planning of mechanochemical reactions, from the strength and geometry of collisions to the targeted use of liquid additives. In the long term, this means higher efficiency, lower energy consumption, and less waste in industrial and research processes. The fact that these are exceptionally significant results is also confirmed by the editors of Angewandte Chemie International Edition, who featured this work on the journal’s cover.
A collision that “wakes up” chemistry
In the study, the scientists chose a simple “model pair”, ordinary table salt of potassium, potassium chloride (KCl), and 18-crown-6 ether, known as a kind of “molecular ring” because it captures a potassium ion in its cavity, forming a stable complex.
“Using the model system of KCl and 18-crown-6 ether, we showed that the reaction starts only when the collision transfers enough energy to break the ionic crystal lattice. When the energy absorbed in a collision, per ion pair, exceeds the crystal’s cohesive energy, fragmentation occurs and individual ions are formed. Our results clearly show that mechanical activation is not a continuous process, but depends on whether, during the collision, the energy threshold required for fragmentation has been reached,” explains Dr Ana Sunčana Smith, corresponding author of the paper from RBI.
In other words, the study clearly shows that a mechanochemical reaction does not begin gradually, but only when an impact reaches a certain threshold, when a strong удар breaks a crystal, like salt, into very small pieces. Such fragmentation creates a short-lived but highly reactive state in which ions interact with surrounding molecules before the system stabilizes. A milling reaction therefore does not proceed continuously, but “in jumps”, so that each new sufficiently strong collision enables another small advance of the reaction. This explains why successfully carrying out a milling reaction requires continuous and prolonged impacts of the balls.
A particularly interesting part of the study concerns water as a liquid additive. Chemists have long known that a small amount of liquid in a mill often accelerates mechanochemical reactions, but the explanation for why this is so has not always been clear.
“The simulations in this work show a nonlinear effect of the amount of water, namely that a small amount facilitates the formation of a complex between potassium ions and 18-crown-6, while too much stabilizes the reactants and prevents the complex from forming,” says Dr Ivan Halasz, corresponding author of the paper and head of the Laboratory for Solid-State Synthesis and Catalysis at RBI.
A new tool for rational design in mechanochemistry
The scientists emphasize that their approach does not attempt to literally mimic the entire milling process in a laboratory mill, but focuses on what is crucial, a single collision and the consequences it leaves at the molecular level. The authors believe that this is precisely what creates room for rational optimization and better “control” of a reaction that until now has often relied on experience and a trial-and-error approach.
“This is a new and, so far, unique approach that helps us understand the short-lived, unusual conditions created by ball impacts in a mill, and it will serve both for a deeper understanding of mechanochemistry and as a guide for smarter selection and dosing of liquid additives in milling reactions,” the authors conclude.
The paper, “Deciphering Ball Milling Mechanochemistry via Molecular Simulations of Collision-Driven and Liquid-Assisted Reactivity”, was published in Angewandte Chemie International Edition. In addition to Dr Ana Sunčana Smith and Dr Ivan Halasz, the team also includes Dr Leonarda Vugrin from RBI, and Rupam Gayen, Dr Zehua Zhang, and Dr György Hantal from Friedrich-Alexander University Erlangen–Nürnberg.
The research was carried out with financial support from the German Research Foundation and the Croatian Science Foundation.
