CHARACTERIZATION OF THE QUASI LIQUID LAYER (QLL) OF ICE USING F-19 NMR

The marginal but selective encapsulation of ions by growing ice induces transient polarizations. Electroneutrality is ultimately restored via H+/OH − interfacial migration producing fluid phases of altered acidity. The media in which solutes become confined upon freezing are, therefore, not only more concentrated but have different acidities than the precursor solutions. This largely unrecognized phenomenon should affect reaction rates and equilibria in frozen aqueous systems. We report, for the first time, measurements of the acidity of extensively frozen dilute solutions of various electrolytes based on the temperature dependence of the 19F chemical shifts of the 3-fluorobenzoic acid in situ probe. We find that acidity significantly increases or decreases upon freezing depending on the type of ions involved. In particular, NaCl, NaNO2 and KNO3 solutions become more basic, while NH4OH or Na2SO4 solutions become more acidic, upon freezing. The environmental implications of these results are briefly analyzed