Number Density of Liquid Inclusions Formed in Frozen Aqueous Electrolyte

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Abstract

Frozen aqueous chlorides (≤50 mM) are characterized by using confocal fluorescence microscopy and small angel X-ray scattering (SAXS). The former method allows us to determine the size of a liquid inclusion formed in the ice matrix at temperatures above the eutectic point of the system (teu). Isolated liquid inclusions of a uniform size are formed when the temperature of a frozen electrolyte increases past teu. The size of the liquid inclusions depends on the observation temperature as well as on the concentration (csalt) and type of salt dissolved in the original unfrozen solution. However, the number density of liquid inclusions is almost constant and independent of these experimental parameters, particularly when an electrolyte is frozen in liquid nitrogen. Salt accumulation can then occur at the imperfections of the ice crystals. The occurrence probability of the imperfections is independent of the nature of an incorporated salt. The amount of a salt confined in each inclusion ranges from 7 to 240 fmol, depending on csalt. SAXS measurements provide information on the size of individual salt crystals formed at temperatures below teu. The radius of gyration of a salt crystal ranges from 2 to 2.8 nm, and does not depend significantly on csalt. Thus, each inclusion is formed from 106–109 nanocrystals, which can act as seeds. When doped ice is prepared at higher temperatures, for example −16 °C, the isolation of liquid inclusions is not sufficient and coalescence occurs more easily upon an increase in temperature or csalt. However, when csalt is lower than 10 mM, the number density of liquid inclusions is almost constant, irrespective of the freezing temperature.

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