Impurities habit modification

The term crystal habit is often used to describe the relative sizes of the faces of a crystal. Crystal habit is readily modified by conditions of nucleation and growth, and it is rather difficult to prepare ciystals with all faces of the same form equally developed (M2). Small amounts of soluble impurities, especially dyes, which may be adsorbed selectively on the different faces of a crystal, cause these faces to be suppressed in favor of others. This can alter the external geometry of a crystal completely, except for its interfacial angles. Many examples of crystal habit modification are reported in the literature (B8), and in some commercial... 

All of these changes in ciystal habit caused by kinetic factors are drastically effected by the presence of impurities that adsorb specifically to one or another face of a growing ciystal. The first example of crystal habit modification was described in 1783 by Rome de Lisle [77], in which urine was added to a saturated solution d NaCl changing the crystal habit from cubes to octahedra. A similar discovery was made by Leblanc [78] in 1788 when alum cubes were changed to octahedra by the addition of urine. Buckley [65] studied the effect of organic impurities on the growth of inoiganic crystals from aqueous solution, and in Mullin s book [66] he discusses the industrial importance of this practice. 

Whatever the details of the kinetic mechanism, impurities cause crystal habit modification. Buckley [65] has classified many impurity effects on different crystal habit modifications. In most cases, impurities decrease the growth rate of specific crystal faces, which lead to a change in the crystal habit because the slowest growing faces will dictate the crystal morphology. In some exceptional cases, impurities can increase the growth rate of a particular crystal face. For example, 1% Fe added... 

Buckley 1951), especially for inorganic systems. Known habit modifiers for inorganic materials are listed in Table 2.5. In recent years great strides have been made in developing a quantitative understanding of habit modification. More information relating to habit modification can be found in Chapter 3 of this volume, which deals with impurity crystal interactions. 

These few examples indicate the importance of understanding the molecular and crystal structure of a given system to understand impurity incorporation and habit modification in the crystallization of these complex molecules. 

The morphological development of a precipitate is a complex combination of a variety of processes, including nucleation, growth, habit modification, phase transformation, ripening, agglomeration, and so on. The dominant system parameters are supersaturation and the level of active impurities, although in some aqueous systems pH can also exert a profound effect. 

Another important effect associated with the presence of impurities is that they may change the crystal habit. Habit alteration is considered to result from unequal changes in the growth rates of diflerem crystal faces. Davey reviews the role of impurities in the general context of habit modification. Surfactants, especially, have been observed to modify growth rates of individual faces and thereby change the habit of a crystal. ... 

Kawamura F, Yasui I, Sunagawa I (2001a) Effects of supersaturation and impurity on step advancement on HOj (110) faces grown from high-temperature solution. J Cryst Growth 233 517-522 Kawamura F, Yasui I, Kamei M, Sunagawa I (2001b) Habit modifications of SnO crystals in SnOj-Cu O flux system in the presence of trivalent impurity cations. J Am Ceram Soc 84(6) 1341-1346 Klein A, Sauberlich F, Spath B, Schulmeyer T, Kraft D (2007) Non-stoichiometry and electronic properties of interfaces. J Mater Sci 42 1890-1900... 

Clydesdale, G., Hammond, R. B., and Roberts, K. J. 2003. Molecular modeling of bulk impurity segregation and impurity-mediated crystal habit modification of naphthalene and phenanthrene in the presence of heteroimpurity species. J. Phys. Chem. B 107 4826. 

Clydesdale, G., Thomson, G. B., Walker, E. M., Roberts, K. J., Meenan, P., and Docherty, R. 2005. A molecular modeling study of the crystal morphology of adipic acid and its habit modification by homologous impurities. Cryst. Growth Des. 5 2154. 

The feed preparation stage e.g. by mineral extraction and evaporation, chemical reaction etc. can give rise to both dissolved and suspended solid impurities, either of which may affect the crystallization step. Removal of suspended solids, e.g. by filtration, is usually the easier process. Dissolved impurities can have by far the more pronounced effect, however, and may have to be removed e.g. by chemical means or by adsorption. Such impurities may, of course, actually be beneficial to the process by inducing nucleation, habit modification etc. 

Crystals for one reason or another, are generally not perfect but contain various imperfections. A common defect is a "missing" unit from a lattice that leaves a "hole" in the structure. The vacant site may be occupied by a substitutional impurity as illustrated on Fig. 8.3. The spaces between the basic crystal units may also accommodate "foreign" bodies quite different from the crystalline material. It is possible under certain circumstances for individual ions to rearrange themselves within the lattice. It is also possible for an ion not consistent with the ions making up the general crystal structure, to be incorporated in the structure. All these modifications to the pure crystal habit are likely to lead to distortions that can have an influence on the stability of the crystalline structure. 

Berkovitch-Yellin, Z., Addadi, L., Idelson, M., Lahav, M. and Leiserowitz, L. (1982) Controlled modification of crystal habit by tailor made impurities application to benzamide. Angewandt Chemie Supplement, 1336-1345. 

A key issue in pharmaceutical manufacturing is the reproducibility of solid-state attributes of the crystalline product. Batch-to-batch variation in the crystal size, habit, polymorphism, or chemical purity could be caused when certain impurities—usually, reaction by-products from the upstream processes—are present in the crystallizing solution. The resulting modification of crystal properties can affect downstream processing and formulation of the final product. Sometimes tailor-made and polymeric additives are used to tune the crystallization process and, hence, crystal properties at the molecular level. For instance, a good crystal habit can facilitate better filtration and separation. A stable (or metastable) polymorph can influence the stability and formulation behavior of the drug product. 

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