Crystal growth molecular concepts

In the figure, step growth of the hydrate crystal is depicted with the following components  

With Avogadro s number of molecules participating in the above process, it would be a mistake to suppose all molecules progress through the above steps in a deterministic manner. With so many particles in motion, every possible combination of attachment is tried. For example, some clusters adsorb directly at a kink without significant diffusion. Other clusters detach from the surface and diffuse away in contrast to our macroscopic observations of growth. However, 

If all possible combinations were equally probably, we would observe stochastic behavior like primary nucleation, so that crystal growth kinetics would be virtually unpredictable. However, a few molecular paths for crystal growth are highly preferred over others, these paths combine in an ensemble to provide the macroscopic observations of crystal growth described in the next section. 

The reader should be warned that the above conceptual picture has little supporting evidence from hydrate growth experiments, other than the few single crystal growth studies in Section 3.2.2.1. Nevertheless, it is hoped that such a conceptual picture can promote some understanding of the phenomena involved, if only to serve as a basis for improvement. 

All seven steps require time, resulting in a rate of incorporating clusters into the growing crystal surface, which is called crystal growth kinetics. The following two sections consider translation of such a rate into a macroscopic equation for correlation and prediction. It is difficult to say which of the steps control the process, or even if the conceptual picture is valid. However, the first step—species transport to the solid surface—is well established and a brief description is given in Section 3.2.1.2. 

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The rate of growth of polymer-salt complexes can provide fundamentally important information that is difficult to determine otherwise. The rate of crystal growth of (PEO)3 NaSCN from its undercooled liquid was measured and used to determine values for the diffusion coefficients of Na" " and SCN (Lee, Sudarsana and Crist, 1991). Also it was shown that the rate of the salt diffusion is independent of the molecular weight of the polymer for PEO molecular weights above 10. This result is fully consistent with the concept that ion motion is due to local segmental motion of the polymer. 

Molecular forms bring concepts back developed during the nineteenth century in which crystal growth forms were systematically investigated. At that time, atoms were not considered to be real but only as a way of expressing chemical laws [4], and lattice periodicity an hypothesis compatible with the empirical law of rational indices with no consequence for the physical nature of crystals [5]. The present molecular situation is reversed one knows that there are atoms and where the atoms are. A molecular lattice allows an interpretation of the molecular morphology, also expressible in terms of rational indices, but without a theoretical basis, even if one can speak of molecular crystallography [6]. 

This concludes the discussion of the nucleation dynamics of macromolecules. It shows that the usually assumed constant number of heterogeneous nuclei and linearly increasing number of homogeneous nuclei is a simplification, and secondary nucleation as the basis for crystal growth is a doubtful concept. Finally, molecular nucleation is not a well enough understood concept to quantitatively explain facts such as the molar mass dependence of crystallization. The further work needed to understand the basis of nucleation in polymers is a big challenge for new research in solid-state polymer science. 

Abstract. We review how the nucleation mechanism of polymer crystallization could be assigned to intramolecular processes and what are the preliminary benefits for understanding some fundamental crystallization behaviors. The speculative concept of molecular nucleation and the theoretical model of intramolecular nucleation have been elucidated in a broad context of classical nucleation theory. The focus is on explaining the phenomenon of molecular segregation caused by polymer crystal growth. 

In most cases, separation and purification via crystallization are highly selective due to the fact that molecular recognition process at the crystal-solution interface acts in such a way as to select the host molecules and reject impurities. However, sometimes the solute and impurity molecules are not discriminated at certain crystal faces, especially when the impurity has many of the structural and chemical characteristics of the primary solute but differs only in some specific way. A systematic approach toward understanding the effects of such impurities on crystal growth has been developed using the concept of tailor-made additives (Weissbuch et al. 2003). These additives are structurally similar to the solute molecules and are basically composed of two moieties. The first, known as the binder, has a similar structure (and stereochemistry) to that of the substrate molecule on the crystal surface where it adsorbs. The second, referred to as the perturber, is modified when compared with the substrate molecule and thus hinders the attachment of the oncoming solute molecules to the crystal surface. Several classic examples in the literature highlight this type of interaction mechanism in molecular crystals. 

The concept of this widely used technique consists in the decoupling of (i) nucleation and seed formation at high supersaturation, and (ii) growth of the seed crystals to a continuous layer at low supersaturation. Usually, in the crystal growth step a new nucleation can be avoided and only the seeds grow to the molecular sieve layer. Therefore, the layers obtained by secondary growth are less polycrystalline. If the support surface was covered by a homogeneous and dense seed layer, relative thin zeolite and MOF layers can be obtained. 

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