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

Through computer simulations and thermodynamic analysis, this study demonstrates that undersaturated sodium and calcium chloride surface layers on ice form genuine quasi-brine states distinct from bulk three-phase coexistence, which significantly increase premelting thickness while retaining structural and dynamical properties similar to bulk electrolyte solutions.

The Gist

Imagine a block of ice not as a perfectly solid, frozen rock, but as a surface that is always slightly "sweating," even when it's below freezing. Scientists call this a quasi-liquid layer (QLL). It's a thin, slippery film of water that exists right on the surface of the ice, acting like a secret lubricant that allows glaciers to slide or ice skates to glide.

This paper investigates what happens when you sprinkle salt (specifically sodium chloride, like table salt, and calcium chloride) onto this icy surface. The researchers wanted to know: Does the salt make this slippery film thicker? Does it change how the water molecules move inside it?

Here is the story of their findings, explained simply:

1. The "Goldilocks" Problem of Ice and Salt

Usually, when you mix salt and ice, the salt lowers the freezing point, causing the ice to melt. But on a surface, things get tricky. The scientists faced a puzzle: How do you tell the difference between a thin, special "surface film" and a tiny puddle of salty water that has formed because the whole system is about to melt?

Think of it like this: If you see a wet spot on a sidewalk, is it just a thin layer of condensation (a surface effect), or is it a small puddle of rainwater (a bulk effect)? The researchers developed a clever way to measure the "thickness" and "saltiness" of this layer to prove it was a genuine surface phenomenon, not just a tiny puddle.

2. The Salt Makes the "Sweat" Thicker

The study found that when salt sits on the ice, it acts like a magnifying glass for melting.

• Pure Ice: Has a very thin layer of "sweat" (maybe a few nanometers thick).
• Salty Ice: The layer becomes twice as thick or even more.

It's as if the salt tells the ice, "Hey, you don't need to be so solid right here; you can be a bit more liquid." This happens even at temperatures well below where the ice would normally melt completely.

3. Two Types of Salt, Two Different Personalities

The researchers tested two types of salt: Sodium Chloride (NaCl) and Calcium Chloride (CaCl2).

• Sodium Chloride (Table Salt): This is the main salt in seawater. It makes the ice surface wetter and thicker, behaving somewhat like the salt found in our oceans.
• Calcium Chloride: This is a "stronger" salt (used for de-icing roads in very cold places). It was even more aggressive. At certain temperatures, it made the ice melt so much that the entire block of ice in the simulation turned into water! It created a much thicker, stickier liquid layer than the table salt did.

4. The "Crowded Dance Floor" Analogy

Inside this thin, salty liquid layer, the water molecules and salt ions are dancing. The researchers looked at how fast they moved (diffusion) and how sticky the layer was (viscosity).

• The Crowd Effect: When salt is added, the water molecules move slower. Imagine a dance floor where people are holding hands (hydrogen bonds). Adding salt is like adding more people to the floor; it gets crowded, and everyone moves slower.
• The Calcium Effect: Calcium ions are "divalent" (they have a double charge), so they grab onto water molecules much tighter than sodium ions. This makes the calcium-salty layer move even slower and feel "thicker" or more viscous, almost like honey compared to the table salt layer.

5. The Secret Arrangement of Ions

The researchers also looked at where the salt ions stood in this thin layer.

• The Anions (Negative ions): These liked to hang out near the edges of the layer—both where the ice meets the liquid and where the liquid meets the air. It's like they were the bouncers standing at the doors.
• The Cations (Positive ions): These preferred to stay in the middle of the layer, away from the edges.
• The Ice Invasion: Interestingly, the negative chloride ions were brave enough to sneak into the solid ice lattice itself, replacing a couple of water molecules, while the positive ions stayed strictly outside.

6. The Big Takeaway

The most important discovery is that even though this salty layer is incredibly thin (only a few nanometers thick—thinner than a human hair), it behaves just like a big bucket of salty water in terms of how the molecules move and interact.

The researchers proved that you can treat this microscopic surface film as if it were a "miniature ocean." This helps us understand how ice interacts with the atmosphere, how glaciers slide, and how sea ice forms, by using the rules of big, bulk liquids to explain tiny, surface phenomena.

In short: Salt doesn't just melt ice; it creates a thicker, stickier, and more organized "sweat" layer on the surface that behaves like a tiny drop of liquid brine, even when the rest of the world is frozen solid.

Technical Summary

Technical Summary: Impact of Sodium and Calcium Chlorides Uptake on the Interfacial Behavior of Ice

Problem Statement
The interfacial properties of ice, particularly the existence and nature of a quasi-liquid layer (QLL) or premelting layer below the bulk melting point, remain a subject of debate. While pure water ice is known to exhibit a disordered surface layer, the behavior of this layer in the presence of impurities, specifically salts found in seawater and hypersaline environments, is less understood. A primary challenge in characterizing these systems is the thermodynamic difficulty of distinguishing between genuine surface premelting (a two-phase ice-vapor system with adsorbed ions) and a finite-size realization of three-phase bulk coexistence (ice-brine-vapor). Furthermore, the lack of control over all relevant thermodynamic fields in binary systems complicates the determination of whether observed surface layers are true surface states or merely thin films of bulk brine.

Methodology
The authors employed molecular dynamics simulations to investigate the adsorption of sodium chloride (NaCl) and calcium chloride (CaCl₂) on the basal plane of hexagonal ice (Ih).

• Models: Water was modeled using the TIP4P/Ice force field, while ions were described using the Madrid-2019 force field (a scaled-charge model).
• Simulation Setup: Systems consisted of ice slabs (10,240 water molecules) exposed to vapor, with varying surface coverages ($\sigma$) of Na⁺/Cl⁻ or Ca²⁺/Cl⁻ ions deposited on the interface. Simulations were run in the canonical ensemble (NVT) at temperatures ranging from 250 K to 265 K.
• Thermodynamic Framework: To address the difficulty of characterizing two-component surface states, the authors developed a rigorous thermodynamic approach. They utilized the bulk ice-brine-vapor phase diagram (specifically the liquidus line) as a reference. By defining an effective molal concentration ($c$) based on the ratio of ions to liquid-like water molecules within the film, they could map surface states onto the bulk phase diagram.
• Analysis Tools:
  • Structure: The CHILL+ order parameter was used to distinguish solid-like (ice) from liquid-like molecules. Film thickness ($h$) was determined using a geometrical dividing surface based on local density profiles.
  • Dynamics: Parallel self-diffusion coefficients were calculated via mean-squared displacement (MSD). Shear viscosity was estimated using the Stokes-Einstein relation, employing hydrodynamic diameters derived from bulk solution properties.

Key Contributions

1. Thermodynamic Distinction: The paper establishes a method to distinguish genuine surface premelting states from bulk three-phase coexistence in binary systems. By comparing the effective concentration of the surface film to the liquidus line of the bulk phase diagram, the authors demonstrate that films with concentrations below the liquidus limit are distinct surface states, whereas those reaching the liquidus limit approach the triple point conditions.
2. Quantification of Salt-Induced Premelting: The study quantifies how NaCl and CaCl₂ adsorption enhances premelting, showing that salt uptake can increase the QLL thickness by a factor of two or more compared to pure ice.
3. Correlation with Bulk Properties: The work demonstrates that despite the nanoscale confinement, the structural organization and rheological properties of these briny surface layers can be meaningfully related to bulk electrolyte solutions of similar concentration.

Results

• Film Thickness and Concentration:
  • Premelting film thickness increases with both temperature and surface coverage ($\sigma$).
  • CaCl₂ induces significantly thicker premelting layers than NaCl at equivalent coverages. At high coverage ($\sigma = 2.2$) and higher temperatures, CaCl₂ films can lead to complete melting of the ice stack in the simulation box.
  • The effective concentration of the films is bounded by the liquidus line of the respective salt-water phase diagram. Films with nominal concentrations lower than the liquidus value represent genuine surface states. As the system approaches the triple point, film concentrations converge tangentially to the liquidus line.
• Structural Organization:
  • Ions within the premelting film exhibit strong stratification. Anions (Cl⁻) preferentially accumulate at both the ice-liquid and liquid-vapor interfaces, with some penetration into the second bilayer of the ice lattice (substituting water molecules). Cations (Na⁺, Ca²⁺) are depleted from the interfaces and concentrate in the film's center.
  • This stratification is more pronounced in thinner films (lower temperatures) and diminishes as the film thickens, approaching a uniform distribution in the film center for thicker layers.
• Dynamic and Rheological Properties:
  • The parallel self-diffusion coefficients of water in the premelting layers are generally lower than in bulk pure water, consistent with the "structure-making" behavior of these salts.
  • CaCl₂ films exhibit lower diffusion and higher shear viscosity compared to NaCl films, attributed to the stronger hydration shells of divalent Ca²⁺ ions.
  • Crucially, when compared to bulk electrolyte solutions of the same concentration and temperature, the rheological properties (diffusion and viscosity) of the premelting films show excellent agreement. This suggests that bulk solutions serve as a reliable proxy for the dynamics of these confined surface layers, provided the comparison is made using parallel diffusion components and strictly liquid-like molecules.

Significance and Claims
The paper claims to provide the first detailed characterization of briny premelting layers that rigorously distinguishes surface phenomena from bulk three-phase coexistence. By linking surface properties to the bulk phase diagram, the authors resolve the ambiguity regarding the thermodynamic state of these interfaces.

The study concludes that:

1. Undersaturated briny surface layers can exist down to the eutectic point, with their maximum concentration strictly bound by the liquidus line.
2. These layers are genuine surface states that can be significantly thicker than those on pure ice.
3. Despite their confinement, the structural and dynamical properties of these layers are well-described by bulk electrolyte solutions of equivalent concentration.
4. NaCl serves as a suitable proxy for seawater ice in environmental conditions (above the eutectic point), while CaCl₂ models hypersaline environments.

The authors emphasize that their findings clarify the conditions under which salt uptake enhances premelting and provide a framework for understanding interfacial processes relevant to glacier sliding, frost heaving, and atmospheric chemistry, without overextending claims to conditions below the eutectic point where salt precipitation would occur.