The Effect of Water Soluble Substances on the Supercooling of Water Drops

Abstract

While the temperature at which ice is in equilibrium with a solution (equilibrium freezing point of the solution) is invariably lower than the corresponding value for pure water, in actual experiments in which the temperature of a sample of solution is reduced until freezing starts (non-equilibrium freezing point of the solution) the supercooling is frequently less than that for pure water, in spite of theoretical expectations to the contrary. New experiments on the supercooling of aqueous solutions were carried out with a special experimental technique. Reproducible, non-contradictory results were obtained only if the solutions were filtered through millipore filters of 0.01 micron pore-size. All solutes investigated enhanced the supercooling of pure water. The non-equilibrium freezing point depression (?t)f increased with increasing concentration of the solute. Comparison of (?t)f with the sum of the equilibrium freezing point depression (?t)e and the supercooling of water (?t)w showed that there was in general a small difference δ which expressed the additional effect of the dissolved substance on the formation of ice-like clusters over and above the usual vapor pressure effects which are contained in (?t)e. This comparison can he expressed by the equation: The increment δ was found to be a direct measure of the effect of the ion size on the formation of ice embryos in the supercooled solution. The results are compared with those reported by various authors in recent literature and are used to discuss the ice-forming mechanism in the atmosphere.