An Explanation for the Unusual Nucleating Ability of Aerosols Produced from the AgI-NH4I-Acetone System

Abstract

The exceptional ice nucleating ability of aersols obtained from combustion of AgI-NH4I-acetone solution is now well known. The high nucleating ability has been determined to come from the existence of a complex compound having a better epitaxial fit with respect to ice than has silver iodide. The compound has a stability region which includes the temperature interval of ?20 to +9°C at water saturation and has been observed to be present on AgI aerosol particles produced from standard aircraft seeding generators in a wind tunnel dilution system. Its presence is presumed to occur as a result of incomplete destruction of the NH4+ of the original solution. Although the unit cell of the phase is monoclinic, c-centered, it has a close packed structure nearly identical to silver iodide in the a0-b0 crystallographic plane. In this plane the phase has a misfit with respect to the basal plane of ice of 1.3% at ?7°C as compared to 1.5% for silver iodide. The composition of the phase is 3AgI·NH4I·6H2O but with 25% of the silver positions being vacant on the average. Compositions of 2:1 mole ratio AgI:NH4I can also exist in an apparent metastable state. The threshold of ice nucleation for the pure Bn phase was found to be ?1°C as contrasted to the recognized ?4°C threshold for silver iodide.