Oil-Water-Salt - What's the force at work here?

[rxyzm]

Hi,
This is something I observed in the kitchen when I was cooking something.
I had put some water in a vessel, to which, I added couple of spoons of oil. The oil formed large droplets on water surface.

I then sprinkled normal table salt crystals on to the water surface and they sank to the bottom immediately.

When I sprinkled small quantity of table salt over oil droplets, the salt crystals sank through the top surface of oil droplet, but, stuck at the bottom surface which was the boundary surface for oil-water.

Since -

a. the salt crystals sank in water immediately, indicating that Salt density is higher than water density.
b. Oil droplets floated on the water surface, indicating that oil density is lower than water density.
c. Salt crystals sank through the top surface of oil droplet, because it's density is higher than oil.

I was expecting for salt crystals to fall through oil-water boundary layer same way because it's density is higher that both oil and water (pt. a and b). But, this is something that did not happen as expected.

I repeated sprinkling of salt crystals on other adjacent oil droplets and observed same behavior for salt crystals. Now, I am wondering why the salt crystals could not pass through the oil-water boundary layer?

FYI - as we know, the oil forms big droplets on water like ⊂⊃ thereby defining a top and bottom surface for oil droplet.

Warm Regards,

 

[stockzahn]

I assume that behaviour can be explained with surface tension/surface energy. Like the oil doesn't mix with the water, an oil film on the salt crystal impedes it to break the oil-water interface and the salt stays close above the interface. Maybe also a salt specific characteristic plays a role, like less energy is needed to generate free surfaces with oil than with water, but that's just a guess.

EDIT: Although in my understanding salt should be polar like water and therefore prefer to be surrounded by water than by oil ...

 

[davenn]

Now, I am wondering why the salt crystals could not pass through the oil-water boundary layer?

Think about what happens to the salt crystals whilst immersed in the oil

 

[sophiecentaur]

It is worth saying that this was an actually observed effect so we need to explain it, rather than judging whether or not it could occur. I make this comment because, at first sight, my opinion was that it couldn't happen.
I looked up the density of Sodium Chloride (solid) it's about 2.1gm/cm³.
Oils, in general have a density just over 0.9gm/cm³ and some oils are higher than that at room temperature. Take the low (pessimistic) option.
Ignoring the solubility issue, a 1cm³grain of salt will have an effective weight of 0.001X(2.1 -0.9) = 0.0012 N when 'resting' on the oil / water interface.
The block would have an effective weight of 0.0011N when it's under the water. It's that difference in density that would determine floating or sinking through the interface. It should sink.
Sinking through the interface (or not) requires that there is some small additional force (0.0001N) involved. That's less than 10% of the weight of the block. As the size decreases, the other forces would be more significant.
In the case of salt grains, there could be air trapped around the surface and that could cause a small decrease in effective density. But surface tension effects could easily account for the extra force. If the oil has wetted the surface of a grain then hydrophobic forces between the oil and water could be significant. I guess there will be a maximum grain size that could make this effect happen.

 

[LURCH]

I assume that behaviour can be explained with surface tension/surface energy.

I agree with this, but would add buoyancy. Think of what happens when the salt crystal is placed on the surface (of oil or water), why does it break the surface and sink in the first place? What is different when it is sitting submerged in oil? ( I think this is the same thing @davenn was referring to in post #3. )

 

[sophiecentaur]

I agree with this, but would add buoyancy.

Exactly. Without the presence of the dense oil (more dense than air, that is) the salt would sink, even if coated with a waterproof membrane of minimum thickness. The old supports 'most of the weight' of the salt grains.