Precipitation nucleation

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

In this chapter we discuss the rates of adsorption, paying special attention to those few cases where information on the rate of specific adsorption (reaction of an adsorbate in the adsorption layer) is available. Furthermore, we elaborate on the chemical processes involved in the dissolution of minerals and concentrate on the dissolution of oxides, silicates, and carbonates, which play an enormous rx)le in the chemical weathering and erosion. We try to demonstrate that in most cases the rate-determining step in the dissolution is a chemical reaction at the surface of the mineral. Thus we have here an excellent example of the relationship between surface stracture and reactivity. Surface chemistry plays an equally important role in the formation of the solid phase (precipitation, nucleation, and crystal growth). Nature s selectivity is reflected in the creation of a crystal and its growth. 

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). 

Note that this distribution consists of specific (discrete) sizes of particles. In NATURE, we always have a continuous distribution particles. This means that we have all sizes, even those of fractional parentage, i.e.-18.56p, 18.57p, 18.58 p, etc. (supposing that we can measure 0.01 p differences). The reason for this is that the mechanisms for particle formation, i.e.- precipitation, nucleation and growth, Ostwald ripening, sintering, are random processes. Thus, while we may speak of the "statistical variation of diameters" and we use whole numbers for the diameters, the actuality is that the real diameters are fractional in nature. 

The classic example of precipitate nucleation in metals is the formation of GP zones in Al-Cu alloys. In ceramics, analogous examples include spinel in NiO, rutile in sapphire, or platelets of nitrogen in diamond. When particles are very small, the surface energy dominates. The calculation in Eqs. Box 15.1-Box 15.4 is instructive. Remember that the calculation is for a spherical nucleus and it ignores kinetics kinetics are actually important as we saw in Figure 15.5. 

The slow metal sorption step on many minerals and soils occurs over time scales of days and longer. This slow sorption has been ascribed to several mechanisms including interparticle or intraparticle diffusion in pores and solids, sites of low energy or reactivity, and surface precipitation/nucleation (49-51). 

Sek] Seko, A., Nishitani, S.R., Tanaka, I., Adaehi, H., Fujita, E.F., First-prineiples Calculation on Free Energy of Precipitate Nucleation , Calphad, 28(2), 173—176 (2004) (Calculation, Electronic Stracture, Phase Relations, Theory, Thermodyn., 19)... 

Due to the composition of the weld consumables used, the SAWs had a higher level of impurity Cu than the other materials. In the USA, Cu had been recognised as an impurity element promoting greater irradiation susceptibility in steels and welds exposed in PWRs at higher irradiation temperatures and doses. In the UK, Fisher, Harbottle and Aldridge showed that the process of Cu precipitate nucleation and growth, which was... 

Recently several types of homogeneous catalyst systems have been developed (usually metal-organic complexes, e.g., "metallocenes") that are used for polymerizations under conditions where the polymer precipitates from the solution. The active chains often have a very long life ("living polymers). In principle, the polymerization starts in the solution. But as the polymer chains grow, they become insoluble and start to precipitate. Nucleation, surface growth an agglomeration may... 

With soda ash alone, turbidity instantly appears, revealing precipitation (nucleation and growth), as seen in Fig. 6. 

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