Impact of sodium and calcium chloride uptake on the interfacial behavior of ice: Premelting, structure, and dynamics

Seawater ice and frozen aqueous solutions in contact with air exhibit a thin quasi-brine surface layer intruding between ice and vapor. This layer hosts important atmospheric chemical reactions such as photolysis and halogen activation, and is often used as a template for the synthesis of two dimensional materials. However, a detailed characterization of surface properties lacks completely, while the relation of the surface adsorbed film with the bulk aqueous solution is poorly understood. Using thermodynamic arguments we show how it is possible to characterize the surface layers by comparison to the three phase ice-brine-air bulk phase diagram, despite the difficulty to control or monitor all the relevant thermodynamic fields of the two component system. Simulations: We performed computer simulations of surface briny layers of sodium and calcium chloride adsorbed on ice. Using suitable order parameters and a rigorous geometrical dividing surface, we are able to characterize the layer's thermodynamic state, measure its properties and relate them to the corresponding properties of the bulk solution.

Our results confirm that undersaturated briny surface layers can form down to the eutectic point, with a maximum concentration that is bound by the liquidus line of the ice-brine phase diagram. Such layers are distinct from finite size realizations of three phase coexistence, and can be regarded as genuine surface states, but their salt content can increase the premelting layer thickness by a factor of two or more. Owing to this significant thickness, these layers can be related to bulk electrolyte solutions of similar concentration, both as regards the structural organization of ions and the dynamical properties of the quasi-liquid film.