Formation and Growth of Crystals

Carbamazepine tablets containing stearic acid formed column-shaped crystals on the tablet surface during storage at high temperature.594 This ciystallization was ascribed to the recrystallization of carbamazepine promoted by the solvent, melted stearic acid. 

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For example, normal urine is supersaturated with calcium oxalate. To prevent formation of renal calculi (stones)719 an inhibitory glycoprotein is present and slows the formation and growth of crystals.720 Under some disease conditions calcium carbonate stones may form in pancreatic ducts. A17 kDa lectinlike glycoprotein called lithostatine has been proposed to inhibit stone formation by binding to certain planes on CaC03 microcrystals just as antifreeze proteins (Box 4-D) inhibit ice formation.721 However, this proposed function for lithostatine is doubtful.722 723 Pathological deposits of crystalline calcium pyrophosphate and basic calcium phosphates are sometimes present in joints,724 even in Neanderthal skeletons.725... 

The solid state reaction kinetic model mentioned above is based on such an assumption that the rate determined step in chemical interaction between solid states is the formation and growth of crystal nucleus of products on the active sites. The active site can be the defects of surface and point or the outcrop moved to the crystal surface. As the volume of the original material and products varies, the generation process of crystal nuclei will be accompanied by the lattice deformation, that is, the speed of the process depends not only on chemical factor but also depends on the nucleation chemistry. 

The gas product derived from reduction should be expelled from the reaction region along the same path. Here, the reducing agent contacts with iron oxide, namely, it is adsorbed on the surface of the solid and starts surface reactions. The reduction reaction includes seizure of oxygen from oxides, the formation and growth of crystal nuclei of the product. The continuous growth of product layer is maintained by solid state reaction and the diffusion in solid state. 

Precipitation can occur if a water is supersaturated with respect to a solid phase however, if the growth of a thermodynamically stable phase is slow, a metastable phase may form. Disordered, amorphous phases such as ferric hydroxide, aluminum hydroxide, and allophane are thermodynamically unstable with respect to crystalline phases nonetheless, these disordered phases are frequently found in nature. The rates of crystallization of these phases are strongly controlled by the presence of adsorbed ions on the surfaces of precipitates (99). Zawacki et al. (Chapter 32) present evidence that adsorption of alkaline earth ions greatly influences the formation and growth of calcium phosphates. While hydroxyapatite was the thermodynamically stable phase under the conditions studied by these authors, it is shown that several different metastable phases may form, depending upon the degree of supersaturation and the initiating surface phase. 

In the formation and growth of adion clusters, two processes are of fundamental importance (1) the arrival and adsorption of ions (atoms) at the surface, and (2) the motion of these adsorbed ions (adions, adatoms) on the surface. An adion deposited on the surface of a perfect crystal stays on the surface as an adion only temporarily since its binding energy to the crystal is small. It is not a stable entity on the surface, but it can increase its stability by the formation of clusters. The free energy of formation of a cluster of N ions, AG(N), has two components (terms) ... 

Ohmura, R. Matsuda, S. Uchida, T. Ebinuma, T. Narita, H. (2005b). Formation and Growth of Structure-H Hydrate Crystals on a Water Droplet. Crystal Growth Design, 5 (5), 1821-1824. 

Agl crystals grow in aqueous solution via four steps (I) formation of molecules and complexes, (2) formation and growth of nuclei, (3) aggregation, and (4) ripening. These reaction scheme can be written as... 

Formation and growth of nuclei have been observed microscopically in single crystals in several decomposition and dehydration reactions. 

Zhdanov (1) describes the mechanism of zeolite crystallization in terms of a quasiequilibrium between the solid and liquid phase in gels and emphasizes that the formation and growth of nuclei occurs in the liquid phase. 

Freeze-Avoidance Strategies Preventing Formation and Growth of Ice Crystals... 

It is convenient to divide the synthesis of organic molecular conductors into two basic processes (1) the synthesis of donor and acceptor molecules, and (2) the oxidation and reduction of the donors and acceptors to radical cations and radical anions. The chapter is organized along these lines. Donor and acceptor synthesis is discussed first, followed by a brief coverage of the formation and growth of radical cation- and radical anion-based crystals. 

The formation of glass is a kinetically controlled phenomenon and is directly associated with atomic or ioiric diffusion within the melt. This controls the formation and growth of microcrystals when the temperature of the liquid reaches the solidus line (i.e. the crystallization temperature). 

The initial stage of sintering is characterized by formation and growth of contact areas (necks) between the particles neither formation of closed pores nor crystal growth occur. The linear shrinkage due to sintering does not exceed about 5%. 

This indicates that the UPD-OPD transition obviously proceeds via the Stranski-Krastanov mechanism (cf. Fig. Ic) involving the formation and growth of 3D Pb crystallites on top of the 2D internally strained Pb UPD adlayers which act as precursors for the nucleation and growth process in the OPD range. The unstrained 2D hep surface structure of a 3D Pb(lll) crystal face is reached after deposition of about 10 Pb monolayers as shown in Fig. 4.18. The interatomic distance corresponds to [Pg.194]

The sigmoidal decomposition curves can be interpreted using the Prout and Tompkins model. This model assumes that the decomposition is governed by the formation and growth of active nuclei which occur on the surface as well as inside the crystals. The formation of product molecules sets up further strains in the crystal since the surface array of product molecules has a different unit cell from the original substance. The strains are relieved by the formation of cracks. Reaction takes place at the mouth of these cracks owing to lattice imperfections and spreads down into the crevices. Decomposition on these surfaces produces further cracking and so the chain reaction spreads. 

The formation and growth of precipitates and crystals are very important in analytical chemistry and in other areas of science. Shown in the photo is the growth of sodium acetate crystals from a supersaturated solution. Because supersaturation leads to small particles that are difficult to filter, it is desirable in gravimetric analysis to minimize the supersaturation and thus increase the particle size of the solid that is formed. 

When coal is burnt in pulverized fuel (p. f.). utility boilers the inorganic material inherent to the fuel may form deposits on the heat absorbing surfaces causing significant reduction in thermal efficiency. This study has involved the investigation of the crystallization of coal ash melts and the relationship to the formation and growth of these troublesome deposits. 

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