Requirements of an Ice Nucleus

Abstract

THE striking similarity between the crystal lattices of silver iodide and ice led Vonnegut¹ to the conclusion that the efficiency of a substance as an ice nucleator depended on the closeness of fit between the ice lattice and the lattice of the nucleating substance. This view persisted for some years, but it has become increasingly apparent² that nucleation is not governed by lattice spacing only and that a good nucleator must also possess some other characteristic. By taking into account the changes in entropy involved in growing ice on a polar substrate, Fletcher³ has predicted that the second necessary characteristic of a good nucleator is that exactly half the water molecules comprising the layer immediately adjacent to the nucleating surface should have a proton directed towards this surface. As a first approximation this requires that a uni-univalent nucleator such as silver iodide should present a surface comprised of equal numbers of positive and negative ions; thus the surface should be at its isoelectric point, and any departure from this point due to the adsorption of potential-determining ions would be expected to decrease the nucleating efficiency.