How does salt begin to melt ice?

Question

I can't seem to resolve how salt melts ice on a cold day.

Imagine starting with an equilibrated small bowl of ice in the kitchen freezer at -18C and a separate tablespoon of sodium chloride (also at -18C). The salt is then placed on the ice without ever leaving the freezer.

If you asked me for my naive prediction of what would happen, I would have said nothing. You have two solids well below their freezing points and there should be no liquid water that would be needed to solvate the Na+ Cl- ions.

Doing this experiment, I find a puddle of brine the next day. This shows there must've been some liquid water present to start solvating the ions, I believe this liquid water comes from the quasi-liquid phase at the surface of the ice (present at temperatures like -18C).

But some source seem to disagree, there is no mention of premelted ice surface here:

Energy is required to initiate the solution process and to continue it. The solution process, in the case of salt, will take place very slowly. A dry particle of salt placed on a dry surface will just sit there for a time until it can absorb enough thermal energy from the surrounding environment to a point where a liquid film is formed on the surface of the particle. This initial brine then triggers the solution of the rest of the salt. As the particle dissolves, it continues to absorb thermal energy from its surroundings. This type of absorption process is called an endothermic reaction.

This explanation also demands that some energy to initiate melting:

Salt that’s dumped on top of ice relies on the sun or the friction of car tires driving over it to initially melt the ice to a slush that can mix with the salt and then won’t refreeze.

This earlier SA article says that the quasi liquid layer does indeed dissolve the salt:

When added to ice, salt first dissolves in the film of liquid water that is always present on the surface, thereby lowering its freezing point below the ices temperature.

Is the freezer experiment sufficient to show that there is a quasi liquid layer on ice at -18C? It is hard to reconcile with some of the above articles.

Answer 1

This shows there must've been some liquid water present to start solvating the ions

The underlying assumption here seems to be that salt cannot diffuse into solid ice. This is incorrect; everything is soluble in everything else to some degree. (See the Kirkendall effect, for example, for experimental demonstration.)

The fundamental reason for universal solubility (again, to some degree) is the Second Law; the number of possible positions of the first Na and Cl ions within the pristine ice is tremendous, corresponding to a large entropic driving force for mixing. (Put another way, the segregation coefficient of any material pair is always greater than zero.)

One might object by saying that no ice region exists within the ice-salt phase diagram, only a seeming combination of pure ice and pure salt:

But thermodynamics tells us that there must exist a finite-sized region on the left side of the phase diagram—too small to be visible here—of ice (with dissolved salt); this rule is discussed in Section 2 here, for example:

"Here we recall that the extrapolated liquidus cannot cross the 0 at.% solute line, because the slope of the Gibbs energy function of the liquid phase always (except at 0 K) has a negative infinity value at 0 at.% due to the RTXIogX term, which derives from the contribution of the ideal entropy of mixing (Fig. 3)"

More complete phase diagrams will emphasize this point with an arrow (Figs. 3, 6, 8, etc.) labeling the region ("Ice").

If this region were larger, it would look like this (note: this is a mockup and not the actual ice-salt phase diagram):

The introduction of this impurity can then begin to melt the ice through freezing point depression. Thus, I don't see why it would be necessary to posit that some liquid must initially exist—although it would speed the process up substantially due to easier mixing.

Answer 2

If you leave an ice cube in the freezer for a long time, it will sublimate. Water molecules leave the surface and the ice cube gets smaller.

Maybe water molecules sublimate from the ice under a salt crystal directly into the salt. And maybe water molecules that have sublimated from the ice (and from elsewhere in the freezer) will get adsorbed to all the other surfaces of the salt crystal.

You might be able to detect that last possibility, with a video camera that can work inside the freezer. Record a magnified image of the salt crystal overnight. See whether you can tell about changes in the crystal structure. If it definitely melts from the bottom, or if it definitely loses structure on all surfaces at once, then you know.

There are various substances that are claimed to pass water vapor but not liquid water. Gore-tex, tyvek, etc. To the extent those claims are true, you could put a thin barrier between the ice and the salt. If the ice-salt interaction requires a layer of liquid water, then it will not happen with tyvek between them. But if water vapor is enough, then the salt should slowly get wet. I assume that salt can't get through the tyvek and still would not melt the ice.

If @tom10 is right and your freezer often heats, then I'm not sure what that implies. Sodium chloride appears to have a higher thermal conductivity than ice. Does that imply the salt crystals would absorb heat faster and transmit it to the ice as tiny little hot spots? Or would the surface of the ice melt quickly because it doesn't transmit heat quickly into the interior, so it gets a layer of water all over?

As usual, as soon as things get the slightest bit complicated my intuition about what to expect becomes worthless.

thermal conductivity of NaCl

thermal conductivity of ice