Nucleation control, crystal growth solution

Where crystalline materials are involved, a supersaturated solution is set up prior to the onset of crystallization, followed by controlled crystal nucleation, controlled crystal growth and finally controlled cessation of growth. 

Kinetics in crystallization means nucleation and crystal growth. Both effects are important in melt crystallization but not as important as in crystallization from solutions. In melt crystallization, the process is mainly controlled by the rate of heat transfer and not by mass transfer as it is in solution crystallization. 

A mathematical model of Silicaiite crystallization from clear solution was developed and solved numerically. Crystallization was considered to be solution-mediated, and to involve three categories of silica species and three steps activation, nucleation, and crystal growth. Nucleation was represented by a condensation reaction between the hydroxyl groups present on the surface of 10 nm amorphous silica particles and the soluble silica species, producing activated complexes, which transform to crystalline nuclei. Both nucleation and crystal growth were considered to be reaction controlled. The model simulated well the in situ experimental data. This suggests that the hypothesized nucleation mechanism can be used to qualitatively describe the nucleation event during clear solution Silicaiite crystallization. 

Microemulsions [191, 192] are transparent, optically isotropic and thermodynamically stable liquids. They contain dispersion of polar and nonpolar solvent, usually water or aqueous solutions and oils. Adding surfactants stabilizes droplets of 1-100 nm in size. Due to amphiphilic properties of the surface active substances containing lipophilic groups and one or two lyophobic C-H chains mainly collected at the interface of two liquid phases, they cannot be mixed under normal conditions. Unlike traditional macroemulsion, which is kinetically stabilized only by the external mechanical energy supply, nano-domains in the microemulsions are formed spontaneously. Their size depends on the microemulsion composition, temperature and elastic properties of the separating film of surfactant. In particular, in the case of water-oil microemulsions with spherical nanosized micelles of water dispersed in oil, water droplets can be used as nanoreactors and templates for the solid nanoparticles fabrication. Since the reaction is initiated by the spatially restricted water and micelle, heterogeneous nucleation and crystal growth can be controlled. 

Table 7-1 gives an overview of crystallization processes from a solution and from a melt, and describes the intermediate obtained and the final products. Possibilities to control crystallization processes are also given, particularly in the stages of seed formation (nucleation) and crystal growth. These are used to influence the number of crystals, crystal size, crystal size distribution, form and structure. 

The kinetics of crystal growth has been much studied Refs. 98-102 are representative. Often there is a time lag before crystallization starts, whose parametric dependence may be indicative of the nucleation mechanism. The crystal growth that follows may be controlled by diffusion or by surface or solution chemistry (see also Section XVI-2C). 

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