Homogeneous-Homomolecular Nucleation

Homogeneous nucleation is the formation of the condensed phase (particles) from purely gaseous molecules. If only a single molecular species is involved, the process is termed homomolecular, while it is called heteromolecular when more than one such species participates. Aspects of homogeneous nucleation depend to a great extent upon collision rates this leads to highly mixed results upon treatment by kinetic theoretic means. Undoubtedly, any ultimate description will necessitate details not only of kinetics but also of dynamics and microparticle microphysics to account for the rates and structure of critical (i.e., stable) cluster formation. 

Nucleation processes, especially in nature, are rarely homomolecular and homogeneous usually two species are involved (binary heteromolecular nucleation), and foreign surfaces may also be present (heterogeneous... 

Homogeneous-homomolecular. self-nucleation of a single species. No foreign nuclei or surfaces involved. 

We have seen that in homogeneous-homomolecular nucleation, nucleation does not occur unless the vapor phase is supersaturated with respect to the species. When two or more vapor species are present, neither of which is supersaturated, nucleation can still take place... 

In classical homogeneous-homomolecular nucleation theory the rate of nucleation can be written in the form... 

We have seen that in homogeneous-homomolecular nucleation, nucleation does not occur unless the vapor phase is supersaturated with respect to the species. When two or more vapor species are present, neither of which is supersaturated, nucleation can still take place as long as the participating vapor species are supersaturated with respect to a liquid solution droplet. Thus heteromolecular nucleation can occur when a mixture of vapors is subsaturated with respect to the pure substances as long as there is supersaturation with respect to a solution of these substances. The theory of homogeneous-heteromolecular nucleation parallels that of homogeneous-homomolecular nucleation extended to include two or more... 

We find that the Kelvin equation for the critical cluster size depends on the ratio of the product of the partial pressures of NH3 and HCl to the equilibrium constant Kp. The product pNH,f>HCi must exceed Kp for a critical cluster to exist. Physically this condition merely means that the product of the gas-phase partial pressures must exceed the equilibrium partial pressure product at that temperature for a solid phase to exist. It is equivalent to the condition that 5 > 1 for a critical cluster to exist for homogeneous-homomolecular nucleation. 

The majority of the current theories of nucleation correspond to the macroscopic approach discussed above, and consequently have the characteristic deficiencies noted. The original macroscopic theory of homogeneous homomolecular... 

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