Expert: Profile Closed - 9/27/2009

Question
QUESTION: I just want to ask is it possible to have a substance to dissolve in oil to make it so that its more dense than water.

ANSWER: Hi, and thank you for your question.

The basic answer is that it is not possible to change the density of oil significantly by dissolving something in it. This is because a substance can only dissolve a relatively small amount of "solute" (the stuff being dissolved), and this is not normally enough to affect density significantly. To take the simplest example; salt-water is more dense than fresh-water, and so fresh-water floats on top of salt-water, but in interactions with other substances, both types behave pretty much the same (e.g. both are heavy enough to sink below oil).

In summary, the density of a liquid can be changed by adding a solute, but the change is too small to significantly affect the density of that liquid compared with other liquids. This is because any liquid can only dissolve a certain amount of solute before it becomes "saturated". For oil to become more dense than water, you'd have to dissolve a very dense material indeed in it, and the range of substances that do dissolve in oil is rather small (because of the type of forces that hold oil molecules together). Certainly there's no substance I know of that would achieve this.


The following link lists a variety of materials, including several oils, and their densities;
http://www.simetric.co.uk/si_liquids.htm

The best (theoretical) route to getting water to float on top of oil would be to create some sort of interface layer using the surface tension of the water; the surface tension of the water being used to hold it in place above the oil. This is the same effect that allows a denser (steel) needle to float on water, as in: http://www.funsci.com/fun3_en/exper2/exper2.htm (First experiment down). But such a system would be very unstable, and would not stay intact long.


Hope this helps, let me know if you want me to go in to any of these points in more detail. And many thanks for an interesting and though-provoking question.


---------- FOLLOW-UP ----------

QUESTION: Can you please go in more detail about how to use the surface tension of water to make water to be held above oil? And I think instead of trying to dissolve a solute (which are normally solid) is it possible to use something like another liquid that is more dense than water to mix with oil to make it more dense?
Thanks a lot.

Answer
Hi, and thanks for the follow-up.

Firstly, it makes no difference if the solute is solid or liquid; the same principle of a limited power to dissolve still stands. And if you just mix oil with something else, you don't create a new compound - it's just a closely-packed mixture (again a simpler example; if I get a cup of sand and a cup of salt and mix them together, I don't get a new substance, just finely mixed sand and salt). So even though (let's say) the sand is heavier than the salt, if you mix the two together, you don't create "dense salt"; they're still separate substances.

As I said, the instability of a water-oil mix with water on the top means the surface tension "method" of making dense things float on top of less dense things won't work. The principle of "surface interaction buoyancy" is that water molecules are very good at bonding to other water molecules, so water always behaves as a large mass, not a collection of little molecules. Get an eye dropper and splash a single drop of water on to a hard surface, and you should see the classic round drop shape of water; that's because the molecules of water are pulling themselves towards each other as closely as possible. The same force that does this (surface tension) resists the force of an object trying to break through the surface of the water; hence you can float a needle that's denser than water on the surface.

However, this approach is messed up when you are dealing with oil and water, because of what are known as hydrophilic / hydrophobic ("water-loving" / "water hating") interactions. Oils are usually hydrocarbons - long chains of carbon atoms joined together, and surrounded by lots of hydrogen atoms. The long carbon chain is water-hating, and so if you put an oil in water, there is an active force pushing the two different chemicals (oil and water apart). So it's not as simple as trying to just balance a dense fluid on a lighter one - how do you cope with the fact that there's a sizeable force doing all it can to throw the system off-balance and return it to the stage of "two separate fluids, dense under less-dense". If one could control the direction of the hydrophilic / hydrophobic interactions I suppose that might be a way to keep the liquids balanced with the water on top.....but sadly the individual molecules are moving fast and randomly, and so therefore is the repulsive force.

Hope that helps; again, any further questions, please don't hesitate to ask. Thanks!