(Beethoven was now feeling a little adventurous. You should too! Please excuse my lame attempts at humor. The crescendo is necessary to fully appreciate the descend-o)
(The maestro - Ludwig van Beethoven, now exploring other nucleation theories on his iPhone (Source))
Non-classical crystallization refers to the idea that crystallization does not simply occur through the nucleation of a structure-insensitive cluster and the subsequent attachment of individual building blocks. Non-classical crystallization pathways often accelerate the crystallization process, as they provide intermediate states and replace the nucleation barriers with multiple reduced ones — defining a mechanism similar to catalysis.
(Classical versus non-classical processes in the nucleation and crystal growth steps. Intermediate structures include dense liquid droplets, amorphous particles, structured clusters or metastable crystals (Source))
When more complexities are considered, such as crystal structure, the conformation of building blocks and interaction with the solvents, real-world crystallization may deviate beyond the scenarios classical theories can reasonably model. This is particularly evident in soft materials, in which factors not accounted for by classical crystallization theories are often notable [1].
(Free-energy evolution in different nucleation pathways. In the second case (blue), the metastable intermediates are unstable relative to the solution, whereas those in the third case (green) have a lower energy than the solution and can, thus, grow into macroscopic amount before transitioning to lower-energy structures. (Source))
In the regime of crystal growth that follows nucleation, the classical view ignores interparticle dynamics and only assumes monomer exchange between the crystals and the solution. The size redistribution among particles, known as Ostwald ripening (describing the dissolution of small particles in favour of the growth of larger ones), is also considered a monomer exchange process through the solution phase. Its kinetics follows the Lifshitz–Slyozov–Wagner model, which predicts that the mean particle volume increases linearly over time. This view, however, has been challenged by numerous examples showing crystallization by particle attachment (CPA) or, in other words, aggregative growth. Aggregative growth can usually be distinguished from Ostwald ripening by its sigmoidal kinetics.
(Growth kinetics of ZnS nanocrystals dominated by aggregative growth (stage 1, see also inset) or Ostwald ripening (stage 2) (Source))
References:
Du, J.S., Bae, Y. & De Yoreo, J.J. Non-classical crystallization in soft and organic materials. Nat Rev Mater 9, 229–248 (2024). https://doi.org/10.1038/s41578-023-00637-y
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