Impact of the sodium and calcium chlorides uptake on the interfacial behavior of ice: premelting, structure, and dynamics

TL;DR

This paper develops a thermodynamic-assisted, atomistic framework to characterize briny premelting at ice–vapor interfaces and to distinguish surface states from bulk three-phase coexistence. Using TIP4P/Ice water and Madrid-2019 ions, it investigates NaCl and CaCl$_2$ adsorption at multiple surface coverages, linking interfacial film thickness, composition, and dynamics to bulk brine phase behavior via the liquidus line. The results show that briny films are thicker than those on pure ice, with strong chloride enrichment near interfaces and notable ion–water structuring; CaCl$_2$ in particular yields thicker, more bulk-like films and higher viscosities, while NaCl remains more influenced by the ice–water coupling at thinner films. Overall, the work provides a practical approach to interpret interfacial brine films by anchoring them to bulk phase diagrams, with implications for sea ice physics, frost heave, and atmospheric chemistry at icy interfaces.

Abstract

Hypothesis: Seawater ice and frozen aqueous solutions in contact with air can exhibit a thin quasi-brine surface layer intruding between ice and vapor, but a detailed characterization of surface properties and its relation to three phase coexistence has been lacking. 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 of 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. Results: 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.

Figures (10)

• Figure 1: Schematic phase diagram of salt-water solutions in the proximity of the triple point. Part (a): Full black lines denote the phase boundaries of pure water, showing the melting, condensation and sublimation boundaries. The black dotted line denotes the metastable prolongation of the liquid-vapor condensation line. Similar pseudo one-component phase diagrams can be drawn for the phase coexistence of brines of fixed composition, denoted here in blue, orange and green in order of increasing concentration. Filled circles denote the corresponding ice-brine-vapor triple points. Joining this points together produces a line of ice-brine-vapor triple points. Part (b): Binary phase diagram for the ice-salt phase coexistence. The shaded area between the pure-component sublimation line and the triple-point line indicates the loci of thermodynamic states where the equilibrium ice-vapor coexistence is possible. Parts (c) and (d): Examples of equilibrium phase diagram for a two-component system consisting of water and a soluble salt - "ideal" (as in NaCl) (c) and "non-ideal" (as in CaCl$_2$) salts (d). The marked points on the liquidus line correspond to the triple points shown in parts (a) and (b). The abbreviations are: ice (I), brine (B), and vapor (V).
• Figure 2: Snapshot of the simulation setup for $\sigma=2.2$ at $T=260$ K. Left panel: an initial configuration of a perfect ice crystal (red sticks) with the basal plane exposed to the vapor with the Na$^+$ (blue spheres) and Cl$^-$ (green spheres) ions deposited slightly above the interface. Right panel: equilibrated configuration after 400 ns of simulation time.
• Figure 3: Premelting structure of ice with adsorbed NaCl. Figure shows the number density profiles $\rho(z)$ at four different temperatures. Results for films with $\sigma=0.55$ and $\sigma=2.2$ are compared with those of pure ice premelting.
• Figure 4: Premelting structure of ice with adsorbed CaCl$_2$. Figure shows the number density profiles $\rho(z)$ at three different temperatures. Results for films with surface coverage $\sigma=0.55$, $\sigma=1.1$ and $\sigma=2.2$ are compared with those of pure ice premelting.
• Figure 5: Premelting film heights as a function of temperature for the ice-vapor (i-v) reference system and three examined surface coverages for NaCl (a) and CaCl$_2$ (c). Equilibrium concentrations formed at the ice-vapor interface with a liquidus line from the bulk phase diagram for NaCl (b) and CaCl$_2$ (d). The reference black solid line is taken from Ref. blazquez24 for NaCl and from Ref.oakes90 for CaCl$_2$.