Effect of Sodium Chloride Adsorption on the Surface Premelting of Ice

TL;DR

This study uses molecular dynamics simulations to show how sodium chloride ions influence the surface melting behavior and structure of ice, affecting its physical and chemical surface properties.

Contribution

It reveals that Na$^+/$Cl$^-$ ions alter ice surface melting from step-wise to gradual, increasing QLL thickness and disorder, and affecting crystal growth and surface reactions.

Findings

Na$^+/$Cl$^-$ ions cause a transition from step-wise to gradual melting.

Presence of ions increases the thickness and disorder of the quasi-liquid layer.

Ions attenuate differences between ice surface facets and influence environmental interactions.

Abstract

We characterise the structural properties of the quasi-liquid layer (QLL) at two low-index ice surfaces in the presence of sodium chloride (Na$^+/$Cl$^-$) ions by molecular dynamics simulations. We find that the presence of a high surface density of Na$^+/$Cl$^-$ pairs changes the surface melting behaviour from step-wise to gradual melting. The ions lead to an overall increase of the thickness and the disorder of the QLL, and to a low-temperature roughening transition of the air-ice interface. The local molecular structure of the QLL is similar to that of liquid water, and the differences between the basal and primary prismatic surface are attenuated by the presence of Na$^+/$Cl$^-$ pairs. These changes modify the crystal growth rates of different facets and the solvation environment at the surface of sea-water ice with a potential impact on light scattering and environmental chemical reactions.