Nucleation, crystal heterogeneous

As we have seen in Chapter 6, the crystal growth has to be preceded by nucleation (Fig. 6.7). In fresh waters this nucleation occurs heterogeneously on particle surfaces. In seawater nucleation occurs primarily upon the templates of calcareous organisms. 

The geochemical fate of most reactive substances (trace metals, pollutants) is controlled by the reaction of solutes with solid surfaces. Simple chemical models for the residence time of reactive elements in oceans, lakes, sediment, and soil systems are based on the partitioning of chemical species between the aqueous solution and the particle surface. The rates of processes involved in precipitation (heterogeneous nucleation, crystal growth) and dissolution of mineral phases, of importance in the weathering of rocks, in the formation of soils, and sediment diagenesis, are critically dependent on surface species and their structural identity. 

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). 

Turnbull has extended the classical theory of homogeneous nucleation to heterogeneous processes. In doing so he relied on the existence of an equilibrium contact angle 6 when two phases (crystal and liquid) are in contact with a solid substrate (see Fig. 1). In such a situation three different surface free energies play a role. There is first of all the crystal-liquid surface free energy a, which we have considered already. In addition, there are the surface... 

Mam heterogeneous processes such as dissolution of minerals, formation of he solid phase (precipitation, nucleation, crystal growth, and biomineraliza-r.on. redox processes at the solid-water interface (including light-induced reactions), and reductive and oxidative dissolutions are rate-controlled at the surface (and not by transport) (10). Because surfaces can adsorb oxidants and reductants and modify redox intensity, the solid-solution interface can catalyze rumv redox reactions. Surfaces can accelerate many organic reactions such as ester hvdrolysis (11). 

When the blend is now further cooled, two possible ways of primary nucleation are possible. In a first case, the matrix phase is nucleated by heterogeneous species present in this phase and instantly, newly created crystals appear. Hence, the crystallization temperature of the matrix will be situated at its bulk T. A second possibility for coincident crystallization occurs in the case one finds again a single crystallization peak for the matrix phase, which however takes place above its bulk T. Some novel mutual nucleating mechanism was suggested in such blends a molten component (minor phase) acts as nucleating substrate for the matrix, which instantaneously crystallizes [Erensch and Jungnickel, 1989]. 

It is desirable to classify the various mechanisms of nucleation as shown in Figure 2.19. Primary nucleation occurs in the absence of crystalline surfaces, whereas secondary nucleation involves the active participation of these surfaces. Homogeneous nucleation rarely occurs in practice, however, it forms the basis of several nucleation theories. Heterogeneous nucleation is usually induced by the presence of dissolved impurities. Secondary nucleation involves the presence of crystals and its interaction with the environment (crystallizer walls, impellers, etc.). 

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. 

A nucleus is a fine particle on which the spontaneous formation or precipitation of a solid phase can take place. Nuclei can be formed from clusters of a few molecules or ion pairs of component ions of the precipitate, or they may be fine particles unrelated chemically to the precipitate but with some similarity of crystal lattice structure. Precipitation from homogeneous solution (i.e., a solution with no solid phase present) requires that nuclei be formed from ions in solution. If the nuclei are formed from the component ions of the precipitate, the initial phase of precipitation is referred to as homogeneous nucleation if the foreign particles are the nuclei, the nucleation is said to be heterogeneous. Because virtually all aqueous solutions contain fine particles of various types, most nucleation is heterogeneous. 

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