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Crystallization mechanisms of acicular crystals

机译：针状晶体的结晶机理

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In this contribution, we present an experimental investigation of the growth of four different organic molecules produced at industrial scale with a view to understand the crystallization mechanism of acicular or needle-like crystals. For all organic crystals studied in this article, layer-by-layer growth of the lateral faces is very slow and clear, as soon as the supersaturation is high enough, there is competition between growth and surface-activated secondary nucleation. This gives rise to pseudo-twinned crystals composed of several needle individuals aligned along a crystallographic axis; this is explained by regular over- and inter-growths as in the case of twinning. And when supersaturation is even higher, nucleation is fast and random. In an industrial continuous crystallization, the rapid growth of needle-like crystals is to be avoided as it leads to fragile crystals or needles, which can be partly broken or totally detached from the parent crystals especially along structural anisotropic axis corresponding to weaker chemical bonds, thus leading to slower growing faces. When an activated mechanism is involved such as a secondary surface nucleation, it is no longer possible to obtain a steady state. Therefore, the crystal number, size and habit vary significantly with time, leading to troubles in the downstream processing operations and to modifications of the final solid-specific properties. These results provide valuable information on the unique crystallization mechanisms of acicular crystals, and show that it is important to know these threshold and critical values when running a crystallizer in order to obtain easy-to-handle crystals.
机译：在这项贡献中，我们提出了工业规模生产的四种不同有机分子生长的实验研究，以了解针状或针状晶体的结晶机理。对于本文研究的所有有机晶体，侧面的逐层生长非常缓慢且清晰，一旦过饱和度足够高，就在生长和表面活化的二次成核之间存在竞争。这产生了由几个沿结晶轴排列的针头个体组成的伪孪晶。与孪生一样，这可以通过定期的过度生长和相互生长来解释。而当过饱和度更高时，成核又快又随机。在工业连续结晶中，应避免针状晶体的快速生长，因为它会导致脆弱的晶体或针状体，尤其是沿着结构各向异性轴（对应于较弱的化学键），它们可能会部分破碎或完全脱离母体晶体，因此导致脸部成长变慢。当涉及诸如次级表面成核之类的激活机制时，不再可能获得稳态。因此，晶体的数目，大小和习性随时间而显着变化，从而导致下游加工操作中的麻烦以及最终的固体比性质的改变。这些结果提供了有关针状晶体独特的结晶机理的有价值的信息，并且表明，在运行结晶器时，了解这些阈值和临界值对于获得易于处理的晶体非常重要。

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