An Experimental Investigation on Ice-Nucleating Properties of Some Chemical Substances

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Abstract
A cold-box of the diffusion cloud chamber type has been constructed, and about 115 substances, inorganic as well as organic, after dispersed either by condensation method to smokes of particles ranging from 0.1μ to 0.8μ in diameter or by dispersion method to those of 0.5μ to 2.0μ diameter, have been put to test of the nucleating ability; the temperature of nucleation at which ice-crystals make their hasty appearance, and the vividness of the phenomenon have been observed by naked eye. The results show that, among others tested, NiO, CdCl2, BiI3, MgO, HgI2; etc., all of which are not of hexagonal type of crystal systems, behave as active ice-nuclei, though silver iodide is found, in fact, to be most effective. The runs of experiment on the particle size dependence of the ice-nucleating activity have been, next, carried out using iodides of silver, mercury and lead, oxides of nickel, magnesium, aluminium and zinc and besides, urea, each of them being of the particle size smaller than several microns in diameter. It was found in all these cases that the temperature of nucleation falls down rapidly as the particle size goes finer over 0.2μ or thereabout in diameter. A thermodynamical consideration on the relation between the nucleation temperature Tc (°K) and the particle radius γ(μ) leads to the following equation 1/(γ+Δ)=a-bTc, a=b(T0-x) wherein Δ, b, x are empirical constants and T0=273; Δ denotes the thickness of a quasi-ice structure formed by water molecules adsorbed from the vapor onto the nucleating particle, b is comprehensive of the surface tension of the outermost layer of the structure and x refers to a measure of the magnitude depending upon the accuracy of the determination of Tc. The equation has proved to agree with the experimental results.

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