Crystallization crystal habit

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... 

The natural shape or habit of a crystal obtained in the medium from which it is ciystallized is of utmost importance in designing an industrial-scale crystallizer. Crystal habit is also of extreme interest to crystallization theorists. Many articles have been written describing variables that affect natural habit, such... 

Referring to Fig. VII-2, assume the surface tension of (10) type planes to be 400 ergs/cm. (a) For what surface tension value of (11) type planes should the stable crystal habit just be that of Fig. Vll-2a and (b) for what surface tension value of (11) type planes should the stable crystal habit be just that of Fig. VII-2i> Explain your work. 

An enlarged view of a crystal is shown in Fig. VII-11 assume for simplicity that the crystal is two-dimensional. Assuming equilibrium shape, calculate 711 if 710 is 275 dyn/cm. Crystal habit may be changed by selective adsorption. What percentage of reduction in the value of 710 must be effected (by, say, dye adsorption selective to the face) in order that the equilibrium crystal exhibit only (10) faces Show your calculation. 

In Section 1.3 it was noted that the energy of adsorption even for a perfect crystal differs from one face to another. An actual specimen of solid will tend to be microcrystalline, and the proportion of the various faces exposed will depend not only on the lattice itself but also on the crystal habit this may well vary amongst the crystallites, since it is highly sensitive to the conditions prevailing during the preparation of the specimen. Thus the overall behaviour of the solid as an adsorbent will be determined not only by its chemical nature but also by the way in which it was prepared. 

The number and kind of defects in a given specimen, as well as the crystal habit and with it the proportion of different crystal faces exposed, will in general depend in considerable degree on the details of preparation. The production of a standard sample of a given chemical substance, having reproducible adsorptive behaviour, remains therefore as much an art as a science. 

The properties of fillers which induence a given end use are many. The overall value of a filler is a complex function of intrinsic material characteristics, eg, tme density, melting point, crystal habit, and chemical composition and of process-dependent factors, eg, particle-si2e distribution, surface chemistry, purity, and bulk density. Fillers impart performance or economic value to the compositions of which they are part. These values, often called functional properties, vary according to the nature of the appHcation. A quantification of the functional properties per unit cost in many cases provides a vaUd criterion for filler comparison and selection. The following are summaries of key filler properties and values. 

Ammonium sulfate [7783-20-2], (NH 2 U4, is a white, soluble, crystalline salt having a formula wt of 132.14. The crystals have a rhombic stmcture d is 1.769. An important factor in the crystallization of ammonium sulfate is the sensitivity of its crystal habit and size to the presence of other components in the crystallizing solution. If heated in a closed system ammonium sulfate melts at 513 2° C (14) if heated in an open system, the salt begins to decompose at 100°C, giving ammonia and ammonium bisulfate [7803-63-6], NH HSO, which melts at 146.9°C. Above 300°C, decomposition becomes more extensive giving sulfur dioxide, sulfur trioxide, water, and nitrogen, in addition to ammonia. 

Both supersaturation and temperature can have different effects on the growth rates of different faces of the same crystal. Such occurrences have implications with respect to crystal habit, and these are dealt with in a later section. 

Another area of TEM application to energetic materials is the work of S.M. Kaye at PicArsn on expls and propints. He used TEM. to establish a procedure for detg the particle size distribution of LA batches of different crystal habits from various manufacturers (Ref 25. ... 

Moisture can also result in undesirable dimensional changes occurring in AN—K nitrate based expls, caused by changing crystal habit on thermal cycling. The physical stresses produced may cause stress failures of their containers (Refs 26 32)... 

Rubinsky, B. DeVries, A.L. (1989). Effect of ice crystal habit on the viability of red blood cells. Cryobiol. 26, 580 (abstract). 

Therefore we have two extremes in crystal habit in sodium chloride, the octahedron and the skeleton, the prevailing shape depending upon the relative rates of growth in different directions in the crystal lattice. The common cube shape is formed when a balance in the two rates of growth prevails. 

Takiyama, K. (1959a) Formation and aging of precipitates. X. An electron micro-diffraction study on crystal habit of barium sulphate precipitates. Bull Chem. Soc. Japan, 32, 68-70. 

Selection of the most suitable chemical form of the active principle for a tablet, while not strictly within our terms of reference here, must be considered. For example, some chloramphenicol esters produce little clinical response [13], There is also a significant difference in the bioavailability of anhydrous and hydrated forms of ampicillin [14], Furthermore, different polymorphic forms, and even crystal habits, may have a pronounced influence on the bioavailability of some drugs due to the different dissolution rates they exhibit. Such changes can also give rise to manufacturing problems. Polymorphism is, of course, not restricted to active ingredients, as shown, for example, in an evaluation of the tableting characteristics of five forms or sorbitol [15]. 

Many drugs have definite and stable crystal habits. Morphological changes rarely occur in such drugs as... 

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