Impurities, effect habit

The work discussed in the previous paragraphs provides the framework for the prediction of crystal habit from internal structure. The challenge is to add realistic methods for the calculation of solvent and impurities effects on the attachment energies (hence the crystal habits) to allow this method to provide prediction of crystal habit. Initial attempts of including solvent effects have been recently described (71. 721. The combination of prediction of crystal habit from attachment energies (including solvent and impurity effects) and the development of tailor made additives (based on structural properties) hold promise that practical routine control and prediction of crystal habit in realistic industrial situations could eventually become a reality. 

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... 

The presence of a solvent, especially water, and/or other additives or impurities, often in nonstoichiometric proportions, may modify the physical properties of a solid, often through impurity defects, through changes in crystal habit (shape) or by lowering the glass transition temperature of an amorphous solid. The effects of water on the solid-state stability of proteins and peptides and the removal of water by lyophilization to produce materials of certain crystallinity are of great practical importance although still imperfectly understood. 

The use of tailor made additives holds great promise in the area of crystal growth and morphology control. The routine selection and use of these type of additives will require a fundamental understanding of the mechanism which the additives work on a molecular basis. At the same time, the effect of solvent molecules on the crystal growth process is another related and important problem. In both instances, the relationship between internal aystal structure, aystal growth rate, solvent and impurities are needed to predict the habit of a crystal and thus allow seleaion of the proper conditions and components required to obtain a desired habit... 

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. 

Increase adhesion tension. Maximize surface tension. Minimize contact angle. Alter surfactant concentration or type to maximize adhesion tension and minimize Marangoni effects. Precoat powder with wettable monolayers, e.g., coatings or steam. Control impurity levels in particle formation. Alter crystal habit in particle formation. Minimize surface roughness in milling. 

Effects of additives, impurities, and pH on the crystai habit, crystai size distribution, crystai purity, and crystal hardness. 

Additives or the level of impurities are only effective within a narrow range. Crystal habit can be poorer at too high a level of impurities and the habit may not be affected at all at too low a level of impurities. 

Impurities often have a profound effect on crystal habit. Adsorption of an impurity on faces of a crystal may retard the growth of certain crystal faces, and these will therefore become prominent in the final crystal, as described earlier (see Figure 2.7). For example, sodium chloride ... 

Changes in the solvent used or the presence of an impurity can also profoundly affect the crystal habit. Figure 2.17 shows the effect of aluminum fluoride on the habit of anhydrous calcium sulfate. The impurity transforms the needle-like habit to a cubic looking crystal. Figure 2.18 demonstrates the effect of urea on sodium chloride crystals. A large amount of qualitative information exists on the effect of impurities on crystal habit (Mullin 1993 ... 

Figure 3.1 Effect of impurities on crystal growth habit. (Reproduced with permission from Addadi et al. 1985.)...

There has been a series of systematic studies of the effect of impurities and tailor-made additives on the shape of crystals grown from solution (Addadi et al. 1982 Berkovitch-Yellin 1985 Weissbuch et al. 1999.) They used amino acids and amino acid derivatives as model solutes. By combining habit observations with crystal structure and molecular orientation, they were able to rationalize a number of effects, including resolution of steroisomers. Hopefully, further studies in this area will provide a rational basis for the many impurity and additive effects found during crystallization of biochemicals. 

The earlier discussion on the effects of additives or impurities on crystal growth (Section 11.3.2) suggests that impurity incorporation is often surface specific. Black and Davey (1988) have reviewed much of the available information on the crystallization of amino acids. Amino acids are interesting model systems because of their common zwitterion group coupled with a variety of side chains, which may be present on a particular crystal face. L-asparagine can accommodate some 15% of L-aspartie acid as a mixed crystal (solid solution). From the effects of aspartic acid on the habit of asparagine crystals it is believed that aspartic acid primarily is incorporated at the 010 face whose growth rate is considerably reduced. 

---