Interaction of NaF, NaCl, and NaBr with Amorphous Ice Films. Salt Dissolution and Ion Separation at the Ice Surface

Abstract

We have studied the adsorption and dissolution phenomena of sodium halide salts (NaF, NaCl, and NaBr) at the surface of amorphous D2O−ice films for the temperature range 105−140 K by the techniques of low-energy sputtering, reactive ion scattering, and temperature-programmed desorption mass spectrometry. These salts readily dissociate to ions at the ice surface. The dissociated Na+ and F- ions migrate from the surface to the interior at temperatures above 125 K, while Cl- and Br- ions stay at the surface, leading to the spatial separation of ions near the ice surface. Br- shows a slightly higher propensity for residing at the surface than Cl-, most likely due to its less efficient solvation by surface water molecules. The ion separation process is driven by thermodynamic forces specific to the ions. The speed of ion migration depends strongly on ice temperature, indicating that the ion motion is limited by the diffusion of water molecules.