Heterogeneous nucleation affecting crystallization rate

Precipitation reactions tend to be sluggish if nucleation is very slow. At the scale of individual grains, cements are not distributed uniformly throughout the available pore spaces, but tend to be spatially localized. Cements are localized on pre-existing surfaces, probably because surface area and surface composition affect nucleation rates. Incipient crystals nucleated on a surface have a smaller total surface area than free crystals of the same volume in the pore fluid, leading to a free energy difference that favors nucleation on a substrate (heterogeneous nucleation, e.g., Kirkpatrick, 1981). In the absence of a suitable... 

Garbolino et al. (65) studied the effects of shear rate on crystallization of a confectionery coating fat (hydrogenated and fractionated mixture of soybean and cottonseed oils) using ultrasonic sensors. They hypothesized that primary nucleation is less likely to be affected by shear and suggested that crystal nuclei probably form from heterogeneous nucleation sites (dust particles or other suspended insoluble materials and imperfections in the container walls). They also suggested that... 

Heterogeneous nucleation mechanisms can significantly affect the dissolution of metastable solid phases, because this form of nucleation can occur at low driving forces. While the choice of a metastable solid phase with solubility higher than other crystalline modifications is motivated by the expectation of faster dissolution rates, achievement of faster dissolution rates and higher concentrations in solutions is jeopardized by surface-mediated nucleation. We have reported that the surface of the metastable phase of theophylline promoted the nucleation of the stable monohydrate crystals. The observed oriented growth of monohydrate crystals on the anhydrous surface is consistent with a close lattice match between the b and c... 

Nucleation. There is relatively little information available on nucleation rates of biochemicals from solution, yet the control of nucleation rates controls the number of particles produced and has a direct bearing on crystal size for a given yield of crystallizing product. Uncontrolled nucleation either heterogeneously from contaminants or as secondary particles produced from existing crystals can greatly affect the desired size distributions. The general principles of nucleation theory as discussed in Chapter 2 apply to biochemical systems. 

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