Nucleation of supersaturated vapors

Dillmann, A., and Meier, G. E. A. (1989) Homogeneous nucleation of supersaturated vapors, Chem. Phys. Lett. 160, 71-74. 

A. E ngton, C. S. Kiang, D. Stauffer, and G. H. Walker, Droplet model and nucleation of supersaturated vapors near the critical point, Phys. Rev. Lett. 26, 820-822 (1971). 

Nucleation of supersaturated vapors in nozzles. II. C0H0, CHCI3, CCI3F, and C2H5OH, J. Chem. Phys. 51 5389 (1969). 

The nucleation mode (ii ie < 0.1 pm) accounts for the majority of particles by number but because of their small size, these particles rarely account for more than a few percent of the total mass of atmospheric particles. These particles originate from condensation of supersaturated vapors from combustion processes and from the nucleation of atmospheric particles to form fresh particles (Seinfeld and Pandis, 1998 Horvath, 2000). 

The particle size corresponding to the maximum in p/p, does not in general corre.spond to a critical size at which nucleation takes place. Development of a more complete theory of nucleation by ions will require the use of fluctuation theory, introduced at the end of this chapter for equilibrium systems and in Chapter 10 for homogeneous nucleation in supersaturated vapors. 

In region BL of Fig. 13.2 the system is a supersaturated vapor and may begin to condense if nucleation can occur. This is a metastable state. Similarly, in region AJ we have a superheated liquid that will vaporize if there is nucleation of the vapor phase. The stable, metastable and unstable regions are indicated in Fig. 13.2. Finally, at the critical point C, both the first and second derivatives of p with respect to Vequal zero. Here the stability is determined by the higher-order derivatives. For stable mechanical equilibrium at the critical point, we have... 

A reduction in the magma density will generally increase nucleation and decrease the particle size. This technique has the disadvantage that crystal formation on the equipment surfaces increases because lower shiny densities create higher levels of supersaturation within the equipment, particularly at the critical boiling surface in a vaporization-type ciystaUizer. 

Condensation is the result of collisions between a gaseous molecule and an existing aerosol droplet when supersaturation exists. Condensation occurs at much lower values of supersaturation than nucleation. Thus, when particles already exist in sufficient quantities, condensation will be the dominant process occurring to relieve the supersaturated condition of the vapor-phase material. 

A difiiculty with this mechanism is the small nucleation rate predicted (1). Surfaces of a crystal with low vapor pressure have very few clusters and two-dimensional nucleation is almost impossible. Indeed, dislocation-free crystals can often remain in a metastable equilibrium with a supersaturated vapor for long periods of time. Nucleation can be induced by resorting to a vapor with a very large supersaturation, but this often has undesirable side effects. Instabilities in the interface shape result in a degradation of the quality and uniformity of crystalline material. 

Another thin film technology based nanoparticle preparation route is gas condensation, in which metal vapor is cooled to high levels of supersaturation in an inert gas ambient [126-128]. In these experiments particles necessarily nucleate in the gas phase. In a promising extension of this technique a pulsed laser beam replaces the conventionally used thermal metal vapor source [120,121,129-134]. 

This has important implications for nucleation in the atmosphere. Condensation of a vapor such as water to form a liquid starts when a small number of water molecules form a cluster upon which other gaseous molecules can condense. However, the size of this initial cluster is very small, and from the Kelvin equation, the vapor pressure over the cluster would be so large that it would essentially immediately evaporate at the relatively small supersaturations found in the atmosphere, up to 2% (Prup-pacher and Klett, 1997). As a result, clouds and fogs would not form unless there was a preexisting particle upon which the water could initially condense. Such particles are known as cloud condensation nuclei, or CCN. 

The Kelvin equation helps explain an assortment of supersaturation phenomena. All of these —supercooled vapors, supersaturated solutions, supercooled melts —involve the onset of phase separation. In each case the difficulty is the nucleation of the new phase Chemists are familiar with the use of seed crystals and the effectiveness of foreign nuclei to initiate the formation of the second phase. 

Example 2.9. The nucleation of water is analysed in an expansion chamber. A vapor at an initial pressure of 2330 Pa at 303 K is expanded to a final pressure of 1575 Pa. In this process it cools down to 260 K. At 260 K the equilibrium vapor pressure is 219 Pa. Thus, the supersaturation reaches P/Po = 7.2. What is the nucleation rate ... 

Nucleation Process of formation of new particles from a supersaturated vapor or a chemical reactive gas. 

If particles (or ions) are already present in a supersaturated vapor, nucleation will take place preferentially on these particles at supersaturations far smaller than for the homogeneous vapor. In this case, nucleation takes place heterogeneously on the existing nuclei at a rate dependent on the free energy of a condensate cap forming on or around the nucleus. Heterogeneous nuclei always occur in the earth s atmosphere. They are crucial to the formation of water clouds and to the formation of ice particles in supercooled clouds. 

Particle formation events from gaseous precursors are observed frequently almost everywhere in the troposphere, both in polluted cities and remote clean areas [4]. It is likely that different nucleation mechanisms are at work in different conditions, but no formation mechanism has been identified so far. It is, however, clear that particles are formed by nucleation of a multicomponent vapor mixture. Water vapor is the most abundant condensable gas in the atmosphere, but it can not form particles on its own homogeneous nucleation requires such a high supersaturation, that heterogeneous nucleation on omnipresent pre-existing particles always starts first and consumes the vapor. However, vapor that is un-... 

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