Ice zone melting method for probing ion-specific partitioning at frozen interface

Abstract

When water freezes, impurities are expelled from the ice crystals and accumulate in the grain boundary space. Salts are a major impurity common to aqueous systems and, upon freezing, are dissolved in the liquid phase at temperatures above the eutectic point. Some constituent ions remain near the ice interface, but some readily diffuse away from the interface. The behavior of ions depends on their properties and affects the progress of freezing. Therefore, although the equilibrium and dynamic behavior of ions at this interface is important for understanding water freezing, nothing is known because of the lack of a suitable approach to this aspect. We extend zone melting, which is used for the purification of metal and semiconductor materials, to frozen aqueous systems. The partition behaviors of anions (F−, Cl−, Br−, and NO3−) between frozen and thawed zones are evaluated using ice zone melting followed by ion chromatographic determination of ions. The effective partition coefficients of the ions are determined from the zone melting measurements of frozen aqueous solution. Differences in the effective partition coefficient between anions are observed. The ratios for anions reveal the ion-specific behaviors at the interface between the freeze concentrated solution (FCS) and melting zone. Poorly hydrated ions tend to be thermodynamically distributed well in the FCS. However, as zone melting proceeds, F− shows a different trend from that predicted by ion hydration. This characteristic behavior of F− is understood by considering its lower diffusivity than other halide anions. Thus, ice zone melting provides information on the equilibrium and dynamic features of ions at the frozen aqueous interface.