Rates and Mechanisms of Heterogeneous Ice Nucleation on Silver Iodide and Silver Chloroiodide Particulate Substrates

✍ Scribed by William G. Finnegan

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 143 KB
• Volume: 202
• Category: Article
• ISSN: 0021-9797
• DOI: 10.1006/jcis.1998.5514

✦ Synopsis

assumed that the mechanism of heterogeneous ice nucleation Information on two molecular mechanisms of ice nucleation on was completely understood at the time.

silver iodide and silver chloroiodide particulates has been gained

The discovery that silver iodide has a bulk crystal structure by a chemical kinetics interpretation of the rates and mechanisms close to that of ice and is an active nucleant substrate for of ice crystal formation from experiments conducted in a 1 m 3 ice (2) was important for cloud seeding efforts, but the isothermal cloud chamber. The interpretation required the considstructure match of AgI with that of ice does not provide a eration of the Langmuir surface adsorption of water molecules by mechanistic explanation of the nucleation step. Moreover,

AgI from the vapor and the chemistry of AgI colloids and suspen-

the relationship between ice nucleation and the structure sions in water with and without the influences of supporting elecmatch of the substrate with the ice structure has been questrolytes. One mechanism is a slow rearrangement of three-dimensional water molecule clusters, adsorbed at water saturation and tioned in view of the reported observations of ice crystal 020ЊC and lower on 500 A ˚and larger substrate particles, to ice formation at surface defects and steps on substrates. Simiembryos. The second occurs rapidly at 05 to 020ЊC on substrate larly, a study of the epitaxial overgrowth of ice crystals on particles wetted by liquid water. This mechanism is hypothesized nucleant substrates appeared to equate epitaxial overgrowth to involve the rearrangement of ordered water molecule clusters ability of the substrate with nucleation ability, as proposed associated with hydrated Ag / ion patches on the substrate surface. by Bryant et al. (3). This apparent connection was dismissed The rate of nucleation is slowed by the presence of alkali and as only interesting speculation on the basis of an insufficient alkaline earth halides. Crystal structure match of the substrate sum of evidence by Strickland-Constable (4).

with that of ice may help the first mechanism but, perhaps, has Almost completely independent of these efforts have been no effect on the second.