The surface of ice features a disordered pre-melted layer known as the quasi-liquid layer (QLL). The unique molecular structure of the QLL is responsible for ice surfaces acting as catalysts for many atmospheric chemistry reactions in various environments. In recent years there have been many strides in characterizing the structure and dynamics of pure water ice surfaces. While there are still many unanswered questions regarding pure water ice, even more remains to unravel about how the structure of the pre-melting layer changes in the presence of adsorbed species. An adsorbed species of particular interest is sodium chloride, seeing as sodium and chloride are the most abundant ions in sea water and can be found naturally in the atmosphere. Using classical molecular dynamic simulations, we characterized the structural properties of the QLL at the surface of ice in the presence of sodium chloride (Na+/Cl-) ions. Simulations indicate that high concentration of Na+/Cl-, as in seawater ice, significantly alters the (pre)melting behavior of two low index ice surfaces – the basal and the primary prismatic facet. We observe a shift from a bilayer-by-bilayer melting to a gradual melting as a function of temperature, thus causing an overall increase in disorder of the QLL. Na+/Cl- increases the surface roughness and disrupts the long-range order at the surface. The structural changes caused by the presence of ion pairs at the ice surface affect the growth behavior of ice crystals and the solvation properties of ice surfaces with a deep impact on the chemistry of the environment.

Effect of sodium chloride adsorption on the surface premelting of ice

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