Crystals are masters of order – yet they arise from chaos. From the chaos of a liquid solution, in which the most diverse molecules float aimlessly around each other, under suitable conditions, rigidly ordered solid forms are formed, in which every atom has its fixed place. Building such a structure, particle by particle and layer by layer, follows fixed rules. Experts have been investigating crystal growth mechanisms in more detail for decades, but one central question has remained unanswered until now: What exactly happens when a new particle joins a crystal? A group led by chemist Rajshree Chakrabarty of the University of Houston The puzzle has now been solved. Contrary to popular belief, the process takes place in two steps.
A new molecule or atom cannot bond to an existing crystal at any time. Growth occurs in layers, and the next particle always grows at the point that represents the greatest energy gain for the crystal. These are, in practical terms, kinks, edges, corners or protrusions. At these docking points (known in technical language as “kinks”), the added particle can form most of the bonds with the particles in the crystal.
It was not previously clear exactly how this process works. On the one hand, it was not known whether the new molecules arrived at the correct position directly from the solvent or whether they first stuck themselves to a flat spot on the crystal and then “migrated” to the correct place. On the other hand, it was not clear how the interaction between the solvent, the added molecule, and the docking point occurred. This is because the solute particle is surrounded by a shell of solvent molecules that it must first shed before it can form new bonds.
Crystals like tree slices
So experts from Houston took a closer look at what happens when molecules approach the nodes. They studied the crystallization of etoporphyrin I, an organic ring system similar to that found in hemoglobin or chlorophyll. The material crystallizes in flat layers, forming steps that resemble slices of a tree – meaning you can watch the crystal grow directly with the right tools.
The team used atomic force microscopy to observe how etoporphyrin I crystals gradually form from different solutions. Experts discovered something important: crystals grow the same way in all solvents. The fact that this occurs at different speeds was taken into account when they took into account the viscosities of different solvents. So the speed was due to the viscosity involved. When the team calculated this, they found that the solute molecules reacted equally quickly with the docking sites in all cases. The speed of crystal growth depends only on how much energy the molecule has to generate to react at the point of coalescence.
Until now, it has generally been assumed that the strength of the bond between the solute molecule and the solvent determines this activation energy. But this is not true, as the team's experiences show. Otherwise, it was assumed that etoporphyrin I molecules in different solvents would react at different rates on the crystal, depending on the strength of the bond between the solvent and etoporphyrin I.
The common assumption has been refuted
Because the strength of this bond does not determine the rate of the reaction, breaking the bond cannot be the decisive step in the reaction. The team concluded that fusion into the crystal occurs in two steps: First, the solute molecule breaks some of its bonds with the solvent and forms bonds to the target site in the crystal. But this is not yet the bond he will have in his final state, but rather a kind of intermediate stage. Only in the second step do these temporary bonds break again and the molecule inserts itself into the crystal in the intended manner. The second step requires more energy, which is why it is slower than the first and determines the speed of crystal growth. The team also gained these insights from observed reaction speeds; Molecular dynamics simulations support the result.
The fact that there is a molecular intermediate in which an additional molecule is temporarily attached to the crystal could inspire experts to conduct new research. How stable the medium is depends on the solvent – and what additives are still present in the solution. With the right addition, crystals can grow in a more targeted manner or even prevent their formation. This may be useful for materials research as well as for the development of pharmaceuticals or other chemicals.
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