Habit modification

The term crystal habit is used to describe the geometric appearance of the crystal. Related to the relative width and length and the number of the crystal faces, the habit can be, for example, acicular, tabular, or striated. The term morphology describes the number of all possible combinations of crystal faces. Thus, two crystals of an identical morphology may have different habits. 

The crystal shape depends on the relative growth rate of the crystal faces. Major impact on the face growth rate is due to the effect of impurities (desired, for example, additives, and undesired) as well as solvents. 

An overview of additives acting as habit modifier for sodium chloride is given in Table 6.1. More information on habit-modifying agents for other substances is found in Ref. [6]. 

Copolymers of vinyl acetate and maleic anhydrite HoUow-faced cubes 

Sodium hexametaphosphate plus an aluminum salt Tetrakaidecahedra 

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Davey, R.J., Whiting, M.J.L., (1982). Habit modification of succinic acid crystals grown from different solvents. Journal of Crystal Growth., 58, 304-312. 

WHETSTONE, J. Trans. Faraday. Soc. 51 (1955) 973-80 and 1142-53. The crystal habit modification of inorganic salts with dyes (in two parts). 

Benzoic acid is ordinarily obtained as a feathery, light crystalline powder. Certain habit modifications can cause the material to exist as needles or as plates. 

Y.N. Tsuchiya, Crystal Habit Modification of Ammonium Nitrate. III. Application to Ammonium Nitrate-Fuel Oil Explosives , KKK 23, 78-83 (1962) (in Engl) CA 58,... 

In actual production, effective additive bisPEA is a by-product of the racemization step, and racemized PEA containing enough bisPEA to produce crystal habit modification is recycled to the next resolution step as part of the raw material (7 S)-PEA (Figure 17). 

Since this new methodology has been applied in an actual production process, optical purity of enantiopure PEA has been well stabilized. Detailed crystal habit modification mechanisms of the phenomenon35"38 and its application to other compounds39 are reported in the literature. 

Sakai, K., Yoshida, S., Hashimoto, Y., Kinbara, K., Saigo, K. and Nohira, H. (1998) Reciprocal resolution of l-(4-methylphenyl)ethylamine and 2-hydroxy-4-phenylbutyric acid, and habit modification of a less-soluble diastereomeric salt with a chiral additive, Enantiomer 3, 23-35. 

Sakai, K., Maekawa, Y., Saigo, K., Sukegawa, M., Murakami, H. and Nohira, H. (1992) Habit modification of a diastereomeric salt with an additive in optical resolution, Bull. Chem. Soc. Jpn. 65, 1747-1750. 

Sakai, K. (1999) Application of habit modification of diastereomeric salt crystals obtained from optical resolution via crystallization manufacture of enantiomerically pure 1-phenylethylamine,./. Org. Synth. Chem. Jpn, 57, 458-465. 

Crystal habit modification. Several crystal habits have been reported in the open and patent literature for zeolite omega. Elongated hexagonal rods (15,16) or fibres (17) have been reported when the zeolite resulted from the recrystallization of another zeolite, Y (15) or S (17), or of clays (16). Moreover the natural counterpart of zeolite omega, mazzite, appears as bundles of needle-shaped particles (18). All these solids have been grown at low supersaturation levels, hence under conditions in which our results show that the growth in the direction <001> prevails on the growth normal to the c-axis (Fig.8). 

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. 

Each face of the ciystal has a different structure and as a result will be different with respect to adsorption. For example, kaolin platelets have an edge that is predominantly AI2O3 and a face that is predominantly Si02. Consequently, each ciystal face will have its own adsorption isotherm, F,-. Kem [70] discusses the adsorption of ions like Cd, in the habit modification of a NaCl (and Pb in the habit modification of KCl). There is a similarity between the 111 planes of NaCl and the... 

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... 

It is important to consider the influence of interaction between functional groups of drugs that leads to their habit modification when formulated in suspension dosage form. Proton transfer from the N atom of sulfamethoxazole to the pyrimidine basic N1 atom of trimethoprim has been reported to occur in their equimolar complexes. Bettinetti et al. have reported nucleation of the complex of trimethoprim and sulfa-methoxypyridazine (1 1) to be accelerated by water or wet granulation. Our studies on cotrimoxazole (unpublished results) revealed immediate formation of fine needle-shaped crystals irrespective of the initial shape of sulfamethoxazole and trimethoprim crystals as a result of the interaction between the two drugs in suspension form. Small needles (Fig. 6A) were... 

Garekani, H.A. Sadeghi, F. Badiee, A. Mostafa, S.A. Rajabi-Siahboomi, A.R. Crystal habit modifications of ibuprofen and their physicochemical characteristics. Drug. Dev. Ind. Pharm. 2001, 27 (8), 803-809. 

Whetstone, J. Crystal habit modification of inorganic salts with dyes. II. Relationship between the structure of crystals and habit-modifying dyes. Trans. Faraday Soc. 1955, 51, 1142-1153. 

Venkataram, S. Khohlokwane, M. Microencapsulation of an iron chelator for sustained release and crystal habit modification. J. Microencapsulation 1996, 13 (5), 519-525. 

Garti, N. Tibika, F. Habit modifications of nitrofurantoin crystallized from formic- acid mixtures. Drug Dev. Ind. Pharm. 1980, 6 (4), 379-398. 

Table 2 Habit modification by addition of trace ions or chemicals...

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