Water vapor nuclei formation

The burning of fossil fuel is a chemical reaction, which, as you recall from Section 2.1, is a reaction that involves changes in the way atoms are bonded and results in the formation of new materials. For fossil fuels, these new materials are mostly carbon dioxide and water vapor. As we explore in future chapters, the only thing that determines the ability of atoms to form new materials in a chemical reaction is the atoms ability to share or exchange electrons—the atomic nuclei are not directly involved. The chemistry of an atom is therefore more a function of its electrons than of its nucleus. Nuclear fission, by contrast, involves nuclear reactions, which, as shown in the chapter-opening photograph, involve the atomic nucleus. In this sense, the study of the atomic nucleus is not a primary focus of chemistry. 

As a particular example, one can consider the homogeneous nucleation in the pure water vapor at 25° C. The surface tension coefficient of water is a = 71.96 N/m at this temperature. Table 5.1 shows some characteristics of the new phase. When the oversaturation is p/p =8.1, the critical nucleus of 0.5 nm radius is seen to comprise 18 water molecules. The equihbrium pressure of such nuclei is not high (approximately 10 bar). Since the water vapor pressure in real clouds is usually no more than 0.1% over that of the saturated vapor, it is unrealistic to expect in the rea sonable time scale the homogeneous formation of water drops in Earth s atmosphere. 

Processes similar to those in liquid-water clouds govern the formation of ice particles from water vapor, except that only a limited subset of aerosol particles are effective at nucleating ice particles (ice-forming nuclei, IFN Demott, 2002). For this process to occur, there must be a match between the crystal structure of the particle nucleus and that of ice. Liquid-water clouds commonly form well below 0 °C because of the abundance of CCN and the paucity of IFN, despite the formation of ice particles being thermodynamically favored (Demott, 2002). 

Ice Nuclei Ice particles can be formed through a variety of mechanisms. All of these require the presence of a particle, which is called an ice nucleus (IN). These mechanisms are (1) water vapor adsorption onto the IN surface and transformation to ice (deposition mode), (2) transformation of a supercooled droplet to an ice particle (freezing mode), and (3) collision of a supercooled droplet with an IN and initiation of ice formation (contact mode). 

It goes without saying that suspended transformations may also accompany the phase interconversions of anhydrates and solvates. Efflorescence may not occur immediately once the pressure is reduced below the dissociation pressure, but such reactions will always take place upon formation of a suitable nucleus. For instance, it has been known since the time of Michael Faraday that the decahydrate phase of sodium sulfate is unstable with respect to open air, since the vapor pressure of the salt exceeds the vapor pressure of water vapor at room temperature. However, the system only dehydrates upon contact with the anhydrate phase, demonstrating the metastable nature of the decahydrate phase. 

Fibers of amorphous anhydrous silica, 1-50 microns in diameter, grow outward from an electrically heated platinum surface exposed to nitrogen-diluted SiF and water vapor at 1100 C, according to Haller (104). The mechanism of formation is unknown. It may be that the silica is being deposited at the base of the fiber simply because the temperature is highest at the platinum surface under the end of the fiber around which silica vapor is condensing. Once a nucleus of viscous silica is formed it probably does not wet the platinum, so that surface tension pulls it up into a drop, the outer, cooler portion of which solidifies and moves away, while more silica is added at the hotter base. This suggested mechanism is consistent with the observe... 

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. 

In a cloud chamber detector, ions at atmospheric pressure are electrically focused into a cloud chamber filled with cold water or octane vapors. The presence of the ion serves as the nucleus for the formation of small droplets that can scatter light from a laser beam passing through the cloud chamber. When mobility-separated ions entered the cloud chamber, perturbation in the laser light due to the formation of ion-nucleated particles was detected by a PMT. When the chamber was supersaturated with water, the scattered light intensity increased in the presence of ions, but when the chamber was supersaturated with octane, the intensity decreased in the presence of ions. Mobility spectra of difluorodibromomethane have been reported using cloud chamber detection." ... 

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