Hydrothermal Synthesis and Growth of Zeolites - Hydrothermal Technology - page 306

6.5.7 Nucleation
This forms an important part of zeolite synthesis and it is one of the most compli-
cated aspects because the chemical precursors here are more complex and varied
compared to the chemical precursors in the formation of many ionic solids, like
M 1 Cl (M 1 5 Li, Na, K, Rb, Cs). Nevertheless, the stages of nucleation are more or

less the same in all cases, i.e., small aggregations of precursors give unstable germ
nuclei; some of these embryos become large enough to be stable nuclei and sponta-
neous deposits of more material on such nuclei result in larger crystallites.
The analysis of metastable solution structure is intimately related to nucleation
theory. There are several approaches for understanding the nucleation: the classical
theory and the modern nucleation theory [110,111] .
In the first case, if a solution is in a metastable state (supersaturated solution), it
will, sooner or later, enter another state, which is stable. Gibbs was the first to propose
that the work of the critical nucleus formation can be considered as a measure of the
metastable state stability. Gibbs' idea was further developed by Volmer Doring,

Frenkel, Zeldovich, and Lifshitz Spyozov using thermodynamic concepts. The mod-

ern theoretical models of nucleation are based on the field-theoretic approach to
nonequilibrium dynamics of the metastable state. These models provide an alternative
to the classical theories.
Nucleation, on the whole, occurs from supersaturation and it can be of two types:
Homogeneous and heterogeneous nucleation; the former occurs spontaneously, and
the latter is induced largely by the presence of impurities or foreign particles present
in the solution.
Barrer [40] has summarized the general properties of nucleation that can be
applied to zeolite synthesis also.

1. Rates of nucleation increase with the extent of undercooling, i.e., with increasing metasta-
bility; however, viscosity also tends to increase, often rapidly as temperature falls, so that
the effects of the degree of undercooling and of viscosity oppose one another in influenc-
ing nucleation rates that can then pass through a maximum as temperature falls.
2. An incubation period is observed, particularly in condensed phases, during which nucle-
ation cannot be detected. Even in seeded solutions, metastable regions of supersaturation
can occur within which nucleation is not detectable. In many phase studies involving
solutions, well-defined composition boundaries are found beyond which nucleation occurs
freely so that under appropriate conditions the rate of nucleation increases extremely rap-
idly with the degree of supersaturation.
3. The extent of the incubation time can be changed significantly by very small changes in
composition.
4. The onset of nucleation often depends on the previous history of the system.

The formation of nuclei takes place during the entire process of crystallization,
but the rate of nucleation increases only during its initial stage. During the forma-
tion of viable nuclei, different kinds of germ nuclei (embryos) will form by chemi-
cal aggregations of the precursor species mentioned above and disappear again
upon depolymerization [40] . As a result of such fluctuations, in time the germ
nuclei will grow and eventually form different kinds of nuclei with dimensions