Working out which one goes on top

ed-2011-008262_0006We all know that oil and water do not mix. But they are not alone, oil, water and mercury also don’t mix, and so water, oil and mercury form three separate liquids, one on top of the other. And if you are really determined you can find not three but six different liquids, none of which mix with any of the others. This is what Ecklemann and Luning did*, to produce the test-tube shown to  the lefy, containing six layers, each of a different liquid.

The top layer is a type of oil (petroleum ether), the next is alcohol’s slightly smaller cousin, methanol. Below those are a silicone oil, and then water (with potassium carbonate added so it won’t mix with methanol). Finally, at the bottom is a fluorinated molecule, and then mercury. The mercury is obvious as it is the only metal, the water, methanol and oil are dyed blue, yellow and red, respectively, so we can tell them apart.

I love this picture, there is so much fun science in it. Quantum physics means that elements of different atoms are different, and means that they can form different molecules. These atoms and molecules then attract other similar atoms or molecules more than they do different ones. Then as thermodynamics tells us the liquids will separate. And they do.

The final ingredient is gravity. Gravity has absolutely no role in why oil and water, water and mercury, etc form separate layers, but it does determine which liquid is on top, and which one is at the bottom. The densest liquid (here mercury) is at the bottom, the next densest is next, and so on, until we reach the lightest liquid, which is at the top.

The fact that gravity determines which liquid ends up on top, in systems at equilibrium, like the one above, is well known.

I am interested in what happens when the set of liquids is not at equilibrium, for example due to the liquids evaporating as they separate into layers. Like gravity, evaporation also exerts weak forces on molecules, so then maybe the densest liquid is not at the bottom. Maybe. At the moment I don’t know, but I will try and find out.

* Image is from Mixing Liquids—Mission Impossible? A Colorful Demonstration on Immiscible Systems, by Ecklemann and Luning  J. Chemical Education 90, 224 (2013). It is copyright the American Chemical Society.

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