Binary homogeneous nucleation process

In remote marine areas and the free troposphere, H2SO4 is formed by oxidation of SO2 in the presence of hydroxyl (OH) radicals. Ultrafine snlfuric acid particles are produced in presence of water vapor (24,25). This binary homogeneous nucleation process is strongly dependent on factors snch as temperature. 

F.M. d Heurle evaluated a specific thickness of the layers (an analogue of the critical radius of nuclei in a homogeneous system for more detail, see Ref. 31) for compounds of the Ni-Si binary system. For Ni2Si, its value was found to be 0.15 nm, i.e. the nucleus does not contain even one lattice unit. Although higher values were obtained for other nickel silicides, they never exceeded 1 nm. Therefore, the nucleation process can hardly play any significant role in the formation of most transition-metal silicides, except in some special cases. This conclusion is likely to be valid for any other chemical compound layer. It should be noted, however, that there is also a different viewpoint.38 132... 

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... 

The few observations of nucleation in the free troposphere are consistent with binary sulfuric acid-water nucleation. In the boundary layer a third nucleating component or a totally different nucleation mechanism is clearly needed. Gaydos et al. (2005) showed that ternary sulfuric acid-ammonia-water nucleation can explain the new particle formation events in the northeastern United States through the year. These authors were able to reproduce the presence or lack of nucleation in practically all the days both during summer and winter that they examined (Figure 11.16). Ion-induced nucleation is expected to make a small contribution to the major nucleation events in the boundary layer because it is probably limited by the availability of ions (Laakso et al. 2002). Homogeneous nucleation of iodine oxide is the most likely explanation for the rapid formation of particles in coastal areas (Hoffmann et al. 2001). It appears that different nucleation processes are responsible for new particle formation in different parts of the atmosphere. Sulfuric acid is a major component of the nucleation growth process in most cases. 

To study the depletion effect, let us consider the simple model of an isolated binary nanoparticle. Let us also assume that there is no constraint on lattice rearrangement Then, the process of nucleation of the new phase in the initially homogeneous system is related to the concentration fluctuations. Let Co be the molar fraction of species B in the particle before nucleation, C is the molar fraction of species B in the new phase (the new phase nucleus will have a concentration different from the parent phase, and C Co, No, and N are the number of atoms in the parent and new phases, respectively. 

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