Tailor face-selective additives

An important class of additives are the so-called, tailor-made additives, which are designed to interact in very specific ways with selected faces of crystalline materials. These compounds are designed to contain a portion (e.g., chemical groups or moieties) that mimic the solute molecule and are thus readily adsorbed at growth sites on the crystal surface. The additives are designed to expose a reverse side, which chemically or structurally differs from the host molecule, thereby disrupting subsequent growth... 

Tailor-made additives can also be used to template a particular face by nucleation at the air-water interface modified with a carefully selected monolayer. An example, once again, is the crystallization of a-glycine. The hypothesis for this work was that the R-a-amino acids should induce the (010) face in the glycine crystals while the 5-isomer should induce the (010) face, and indeed this was found to be the case. Nucleation under monolayers (or Langmuir... 

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. 

In order to be effective, it is important that surface modifiers are strongly adsorbed onto an available crystal face of a particle. Nature uses such mechanisms in biomineralization, e.g. in the formation of mussel shells or in the protection of hardy plants against frost. In both cases, additives control the crystal growth of either calcium carbonate or ice through selective adsorption of tailor-made proteins on certain crystal faces . ... 

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