Heterogeneous reactions Nucleation

In this chapter, the essential aspects of kinetics of heterogeneous reactions (nucleation, interface reaction, and mass/heat transfer) are first presented. Then one class of heterogeneous reactions, the dissolution and growth of crystals, bubbles, and droplets, is elaborated in great detail. Some other heterogeneous reactions are then discussed with examples. Many complex problems in heterogeneous reactions remain to be solved. 

Chemical reactions may be classified by the number of phases involved in the reaction. If the reaction takes place inside one single phase, it is said to be a homogeneous reaction. Otherwise, it is a heterogeneous reaction. For homogeneous reactions, there are no surface effects and mass transfer usually does not play a role. Heterogeneous reactions, on the other hand, often involve surface effects, formation of new phases (nucleation), and mass transfer diffusion and convection). Hence, the theories for the kinetics of homogeneous and heterogeneous reactions are different and are treated in different sections. 

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). 

The second type is simple phase transitions in which one phase transforms into another of identical composition, e.g., diamond graphite, quartz coe-site, and water ice. This type sounds simple, but it involves most steps of heterogeneous reactions, including nucleation, interface reaction, and coarsening. 

Heterogeneous reactions that do not require nucleation of a new phase. That is, all the phases involved in the reactions are initially present. Many of these reactions can be quantified well if the boundary conditions are simple. The following are some examples. 

Heterogeneous reactions that require nucleation. Quantitative prediction of the rates of these reactions is not available because nucleation has not been quantified well. Examples include the following. 

For a reaction to produce a new phase, the new phase must first form (nucleate) from an existing phase or existing phases. Nudeation theory deals with how the new phase nucleates and how to predict nudeation rates. The best characterization of the present status of our understanding on nudeation is that we do not have a quantitative understanding of nudeation. The theories provide a qualitative picture, but fail in quantitative aspects. We have to rely on experiments to estimate nudeation rates, but nudeation experiments are not numerous and often not well controlled. In discussion of heterogeneous reaction kinetics and dynamics, the inability to predict nudeation rate is often the main obstacle to a quantitative understanding and prediction. The nudeation theories are... 

The low temperature ( 140°C) anionic ring opening polymerization is further complicated by the crystallinity in nylon 6. Magill [66] has reported that the temperature for maximum crystallization rate in nylon 6 is about 140-145°C. The nucleation rate is low above 145°C, whereas viscous effects hinder crystal growth below this temperature. As a result, at about 140-145°C, heterogeneous reaction conditions can be encountered (as we have seen in our studies) if there is simultaneous polymerization of caprolactam and crystallization of the nylon 6 formed. 

A heterogeneous reaction of the type A + B = AB necessarily begins with the nucleation of AB. Nucleation and early growth are different from the later stages of reaction as long as the number of atomic particles in the boundary region is similar to the number of those in the bulk. This means that the chemical potential of the components and the growth kinetics depend explicitly on the size and form of the nuclei. 

Figure 6-4. Nucleation and early growth stages of the heterogeneous reaction a+fi = y, in accordance with Figure 6-3.

These brief remarks on Ostwald ripening conclude the discussion of nucleation and early growth stages of heterogeneous reactions at this point. Some of the concepts are deepened in Chapter 12 on phase transformations [see also R. Wagner, R. Kampmann (1991)]. 

Metal oxidation is a heterogeneous solid state reaction and starts in the same way as other heterogeneous reactions with nucleation and initial growth. This was discussed in Chapter 6. A time-dependent nucleation rate may dominate the overall growth kinetics of thin Films. Even under an optical microscope (i.e., in macroscopic dimensions), preferential sites of growth can still be discerned [J. Benard (1971)). This indicates that lateral transport on the surface (e.g., at sites where screw dislocations emerge) can possibly be more important for the initial reactive growth than transport across thin oxide layers. 

It is interesting to note that in contrast to these results the thermal polymerization of DCH always proceeds heterogeneously with nucleation of separate polymer domains. The thermal polymer is polycrystalline with a fibrous texture. Lattice parameters are identical with those of the polymer obtained by irradiation. Observation of thermally polymerizing DCH crystals shows that the reaction starts at crystal... 

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