Solute Redistribution in Some Multicomponent Aqueous Systems on Freezing

Aqueous solutions of HCl, inorganic salts, and neutral and ionic organic compounds were frozen, and the solute distribution in the frozen systems was assessed in terms of its dependence on composition and initial solute concentration. The distribution may be uniform or uneven, depending on compositional aspects of the solutions. The degree of non-uniformity of the distribution of a given solute is a function not only of its own initial concentration but also of the concentration of any additional solute. With a variety of two-component solutions, maximally uneven solute distribution was found to be associated with approximately the same initial concentration (about 10-4 m, under the conditions of these experiments). As the concentration was raised above this level, the distribution of solute became increasingly more uniform; this was similar for all solutes. Upon lowering the concentration below this level, the tendency toward uniformity of distribution varied with the solute. With multicomponent systems, reciprocal effects of the concentration of one solute on the distribution of another were noted. In producing these effects, the absence or presence and sign of electric charge borne by the solutes seems to play no major role at concentrations higher than the optimal concentration for maximal nonuniformity of distribution. Electric charge and the specific nature of the solute may be important determinants of solute redistribution at very low concentrations.