Confocal Fluorescence Microscopy of the Morphology and Composition of Interstitial Fluids in Freezing Electrolyte Solutions

Ice rheology, the integrity of polar ice core records, and ice−atmosphere interactions are among the phenomena controlled by the morphology and composition of interstitial fluids threading polycrystalline ice. Herein, we investigate how ionic impurities affect such features via time-resolved confocal fluorescence microscopy of freezing electrolyte solutions doped with a pH probe. We find that the 10 μM probe accumulates into 12 μm thick glassy channels in frozen water, but it is incorporated into randomly distributed <1 μm diameter inclusions in freezing 1 mM NaCl. We infer that morphology is largely determined by the dynamic instabilities generated upon advancing ice by the rejected solute, rather than by thermodynamics. The protracted alkalinization of the fluid inclusions reveals that the excess negative charge generated by the preferential incorporation of Cl^− over Na^+ in ice is neutralized by the seepage of the OH^− slowly produced via H_2O → H^+ + OH^− thermal dissociation.