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Crystal growth needed supersaturation

Cryst ization needs a supersaturated solution in order to allow nucleation and crystal growth. The supersaturation, however, should take dace be between limits [67]. Indeed, according to dieir structure and composition, microporous solids have different thermodynamic stabilities and, therefore, different solubilities. Thus, the supersaturation domains for the possible phases are generally not completely superimposed. (Figure 22.).The art of the synthesis consists then to reach the supersaturation domain corresponding to the phase required, by using, for instance, an amoiphous gel, with a solubility level in the former domain, as a source of the polycondensable species. [Pg.70]

As with nucleation, classical theories of crystal growth 3 20 2135 40-421 have not led to working relationships, and rates of crystallisation are usually expressed in terms of the supersaturation by empirical relationships. In essence, overall mass deposition rates, which can be measured in laboratory fluidised beds or agitated vessels, are needed for crystalliser design, and growth rates of individual crystal faces under different conditions are required for the specification of operating conditions. [Pg.844]

The three-dimensional aggregation of target molecules into a critical nudeus from which crystal growth may proceed, is a process that requires a higher activation energy, and hence higher supersaturation, than the subsequent one- or two-dimensional nudeation needed for crystal facet growth. For this reason, an opti-... [Pg.245]

The objective of most protein crystallization experiments is to obtain a few large crystals. As outlined in Sections IV,C and VI, two of the major obstacles to controlled protein crystal growth are the extreme sensitivity of nucleation rate to supersaturation conditions and the necessity for higher supersaturations to promote nucleation than are needed for growth (Fig. 2). An inherent shortcoming of many crystallization methods is that they depend on similar conditions both to promote nucleation and to support growth. A frequent result is either no crystals or the formation of many small crystals. However, alternative approaches have been developed that attempt to individually optimize nucleation and growth conditions. [Pg.22]

Although a certain degree of supersaturation is needed for crystal growth, it is much less than that required for nucleation. Crystal growth involves a number of steps including diffusion of the growth units to the erystal surface and diffusion and adsorption processes at the surfaee itself The overall rate of growth is detemiined by the slowest of these steps. [Pg.57]

There are a number of experimental methods that can be used to obtain the crystal growth rate data needed to obtain kinetics. Unfortunately, unless great care is taken, these methods can provide very different results for the same system at the same supersaturation. We will first review the measurement techniques and then describe the various problems and pitfalls that may be encountered. [Pg.58]

Polymorphs and solvated crystals is generally observed in pharmacentical indnstry [1], The bioavailability, stability, solnbility, and morphology of the pharmacentical products are very influenced by polymorphs [2-7], therefore the control of the polymorphic crystallization is very important. The crystallization process of polymorphs and solvated crystals is composed of competitive nucleation, growth, and transformation from a meta-stable form to a stable form [4], Furthermore, the crystallization behavior is influenced by various controlling factors such as temperature, supersaturation, additives and solvents [8], In order to perform the selective crystallization of the polymorphs, the mechanism of each elementary step in the crystallization process and the key controlling factor needs to be elucidated [8], On the other hand, we reported for L-Glutamic acid and L-Histidine system previously [4] that the nucleation and transformation behaviors of polymorphs depend on the molecular stractures. If the relationship between molecular stmcture and polymorphic crystallization behavior is known, the prediction of the polymorphism may become to be possible for the related compound. However, detail in such relationship is not clearly understood. [Pg.125]

The growth of a crystal in solution probably proceeds similarly to the growth in vapour but growth from supercooled melts needs further study. In all instances a very high supersaturation (1.5 times) is normally needed to initiate growth. [Pg.161]

In the case of an isotropic advancement rate of the step, a growth spiral with circular symmetry" is formed. Due to the character of its origin the spiral step can never disappear and the crystal face grows perpetually even at relatively lov supersaturations without the need of a two-dimensional nucleation. [Pg.238]


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Growth supersaturation

Supersaturation

Supersaturations

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