Some relations between the structure of the ice-solution interface and the free growth rate of ice crystals in supercooled aqueous solutions

Abstract

An experimental method was devised to determine the rate at which ice crystals grow freely in supercooled water and in dilute aqueous solutions of various salts. The solutions had concentrations between 10 −5 and 10 −1 moles liter −1 and were investigated at bath supercoolings between 0.5° and 20°C. It was found that the growth rate of ice crystals varied approximately with the square of the bath supercooling. Dissolved salts affected the growth rate of ice crystals in a systematic manner. At concentrations larger than 10 −2 moles liter −1 all salts reduced the growth rate below the one in pure water by an amount which depended on the type of salt in solution. At concentrations smaller than 10 −2 moles liter −1 some salts did not affect the growth rate whereas others increased it over and above the one in pure water. It was concluded that at low solute concentration the growth rate of ice in aqueous solutions is limited by the rate of dissipation of latent heat. As the solute concentration increases, the rate of diffusion of solute atoms away from the interface becomes increasingly important. At concentrations larger than 10 −2 moles liter −1 the growth rate reflects in a systematic manner the freezing point lowering effects associated with the poor mass transfer conditions which prevail in a stagnant system. Some of the observations are discussed in terms of the effect of the dissolved salts on the growth kinetics.