Critical nucleus classical definition

The resistance to nucleation is associated with the surface energy of forming small clusters. Once beyond a critical size, the growth proceeds with the considerable driving force due to the supersaturation or subcooling. It is the definition of this critical nucleus size that has consumed much theoretical and experimental research. We present a brief description of the classic nucleation theory along with some examples of crystal nucleation and growth studies. 

We speak about unstable equilibrium because attaching more atoms from the parent phase the critical nucleus turns into a stable cluster and grows irreversibly. On the contrary, detachment of atoms from the critical nucleus leads to its irreversible decay. In the classical nucleation theory the definitions (1) and (2) are fully identical. However, we shall show in Chapter 1.4 that the first definition is more general. 

Similarly to the classical theory the Walton s treatment is based on the idea for a critical nucleus the formation of which is the rate-determining step of the phase transition. However, the definition of the critical nucleus is conformed to the specificity of the small clusters and the high supersaturations. Thus it is assumed that the critical nucleus may consist of a very low number of atoms - two, one and even zero atoms in the case of very active substrates. The c-atomic critical nucleus is considered as a cluster having a probability of decay higher than or equal to the probability... 

There are several other points of view regarding the definition of the nucleus of zeolite. For example, it was suggested that some primary structural units of the framework, such as rings and basic cages, could be defined as the nucleus of zeolites and other microporous crystals. It was also proposed that the nucleus of zeolite could be defined as particles with critical size. These particles should be stable under crystallization conditions. Compared with the classical theory of nucleation from homogeneous solution, the theory developed by Pope could well explain the significant decrease of the free-energy barrier of nucleation for zeolites and other microporous compounds.[43] This... 

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