Structure and Composition of the Solid Product

Product Structure A feature frequently observed in the decomposition of crystalline hydrates, which has not yet been given a convincing interpretation in the framework of universally accepted ideas, is the formation of solid products in either an amorphous or a crystalline state, depending on the actual water vapour pressure in the reactor. This phenomenon was observed by Kohlschiitter and Nitschmann in 1931 [35] and has been the subject of numerous publications, including the study of Volmer and Seydel [36], who used it as a basis for explaining the Topley-Smith (T-S) effect, and a series of articles by Frost et al. [37-39]. Dehydration of many crystalline hydrates in vacuum entails formation of an X-ray amorphous (finely dispersed) residue and, in the presence of water vapour, formation of a crystalline product. The highest H2O pressure at which an amorphous product can still form varies for different hydrates from a few tenths to a few Torr (Table 2.4). As the decomposition temperature increases, the boundary of formation of the crystalline product shifts towards higher H2O pressures. 

Most of the researchers [36-39] attribute this effect to accelerated recrystallization of the dehydrated amorphous product initiated by the presence of water vapour, although the mechanism responsible for this influence is far from being obvious. 

It appears much simpler to explain the differences in formation of the X-ray amorphous and crystalline products as due to a change of the real temperature 

We assume that the differences in formation of the condensates with different structures (or, to be exact, of crystallites differing in size) are determined by the degree of oversaturation of the vapour of the condensing substance, which can be defined as the ratio of the real pressure of a substance, P, to that of the saturated pressure, Poo T), for this substance  

In connection with this assumption, one can refer to the Gibbs-Thomson 

---