Kinetics, crystal growth mechanisms

Classification and Nomenclature of Supramolecular Compounds, p. 267 Clathrate Hydrates, p. 274 Crystal Growth Mechanisms, p. 364 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 Self-Assembly in Biochemistry , p. 1257 Supramolecular Polymers, p. 1443... 

In a parallel study, a biocompatible composite made of sulphonated PSU and HAp was prepared and characterized. The incorporation of HAp was achieved by spontaneous precipitation of HAp at controlled conditions of pH and temperature. The time of the precipitation process was used to regulate the content of inorganic component in the composite. The composite material prepared in this experiment contained 9 wt% of HAp and 91 wt% of polymer. Analysis of the kinetic results of the crystallization process showed that the growth of HAp on the film-shaped composite material followed a polynuclear (nuclei above nuclei) crystal growth mechanism [103],... 

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). 

Nielsen, A.E. (1959a) The kinetics of crystal growth in barium sulphate precipitation. II. Temperature dependence and mechanism. Acta Chem. Scand., 12, 951-958. 

A better insight into the mechanisms of the individual steps in the formation of crystals would be of great help in explaining the creation and transformation of sedimentary deposits and biological precipitates. Valuable reviews are available on the principles of nucleation of crystals and the kinetics of precipitation and crystal growth (Zhang and Nancollas, 1990 Steefel and Van Cappellen, 1990 Van Cappellen, 1991). Only a few important considerations are summarized here to illustrate the wide scope of questions to be answered in order to predict rates and mechanisms of precipitation in natural systems. 

The growth rates of crystals with partially broken surfaces or with fines adhered to it were larger in both directions than those of smooth surface. Further, the optical purities of crystals with broken surfaces and with fines adhered on the surfaces were lower than those with smooth surfaces. From these results, the kinetics of growth of D-SCMC crystals and the mechanism of inclusion of impurity during their growth process was considered. 

The scope of kinetics includes (i) the rates and mechanisms of homogeneous chemical reactions (reactions that occur in one single phase, such as ionic and molecular reactions in aqueous solutions, radioactive decay, many reactions in silicate melts, and cation distribution reactions in minerals), (ii) diffusion (owing to random motion of particles) and convection (both are parts of mass transport diffusion is often referred to as kinetics and convection and other motions are often referred to as dynamics), and (iii) the kinetics of phase transformations and heterogeneous reactions (including nucleation, crystal growth, crystal dissolution, and bubble growth). 

By use of the proper experimental conditions and Ltting the four models described above, it may be possible to arrive at a reasonable mechanistic interpretation of the experimental data. As an example, the crystal growth kinetics of theophylline monohydrate was studied by Rodriguez-Hornedo and Wu (1991). Their conclusion was that the crystal growth of theophylline monohydrate is controlled by a surface reaction mechanism rather than by solute diffusion in the bulk. Further, they found that the data was described by the screw-dislocation model and by the parabolic law, and they concluded that a defect-mediated growth mechanism occurred rather than a surface nucleation mechanism. 

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