Controlled Solid State Reactions

Let us now examine some solid state reactions which are regulated by dlfliision mechanisms. One such is the rusting of iron exposed to the weather. Over a period of time, we have observed that iron-based parts of machinery become coated with a layer of iron oxide. 

---

We have already dealt with two of these. Section 2 dealt with formation of a phase boundary while we have just completed Section 4 concerning nuclei growth as related to a phase boundary. We will consider diffusion mechanisms in nuclei and diffusion-controlled solid state reactions at a later part of this chapter. 

The only conclusion that we can draw is that diffusion-controlled solid state reactions tend to produce mixtures of compounds, the relative ratio of which is related to their thermod5rnamic stability at the reaction temperature. Obviously then, if we change the temperature of reaction, we would expect to see somewhat different mixtures of compounds produced. 

It should be clear by comparing the examples for calcium silicate and barium silicate that one cannot predict how the diffusion-controlled solid state reactions will proceed since they are predicated upon the relative thermodsmamic stability of the compounds formed in each separate phase. 

The solid-state polymerization of diacetylenes is an example of a lattice-controlled solid-state reaction. Polydiacetylenes are synthesized via a 1,4-addition reaction of monomer crystals of the form R-C=C-CeC-R. The polymer backbone has a planar, fully conjugated structure. The electronic structure is essentially one dimensional with a lowest-energy optical transition of typically 16 000 cm-l. The polydiacetylenes are unique among organic polymers in that they may be obtained as large-dimension single crystals. 

In the last decade, progress has also been made with using superlattices as templates, or structure-directing agents, to kinetically control solid-state reactions. This is accomplished by allowing interdiffusion to reach completion before the occurrence of heterogeneous nucleation, thus trapping the system in the... 

These mechanisms should now be self-evident. We have now described the formation of a phase boundaiy and how it grows (Section I.) and have just completed Section II. concerning nuclei growth. Now, we will consider diffusion mechanisms and diffusion-controlled solid state reactions without concern about how nuclei form or grow in the following section. 

The two following examples will show how galvanic cells can be used in the direct study of diffusion-controlled solid state reactions and phase boundary reactions. These examples are illustrative of a large number of possible applications in the field of solid state reaction kinetics. Emphasis here will be placed upon the principles of the methodology. Further examples can be found in the literature cited above. 

Diffi sion-Controlled Solid State Reactions. Andriy M. Gusak Copyright 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-40884-9... 

Diffusion-Controlled Solid State Reactions. Andriy M. Gusak... 

We have thus far discussed in this section some of the factors associated with crystallization in chiral structures, and pointed out that the molecules in such structures must adopt conformations which are to some extent chiral, whether or not they do so in disperse phases. Because of the chirality of both the molecule and of its environment, if the molecule takes part in a topochemically controlled solid-state reaction it is conceivable that chiral non-racemic products would be produced. In fact two successful asymmetric syntheses of this sort have been carried out in this laboratory. 

---