Self-destructing droplets


Olive oil with Balsamic Vinegar
Above are droplets of balsamic vinegar (mainly water, but tastier of course) in olive oil. Water and oil phase separate, hence the droplets. The droplets above are maybe a few millimetres across, and they won’t move unless you stir the oil. This blog post is about much smaller droplets, too small to be seen with the eye, so the picture above of much larger droplets, will have to do. Smaller droplets can move. And two scientists working in Darmstadt in Germany, Hajian and Hardt, have seen small droplets move, which is not so surprising. But what is surprising is that they then dissolved.

Spontaneous movement of the droplets has to decrease the free energy of the system — this is the Second Law of Thermodynamics. Hajian and Hardt assume that the free energy that matters is that of the surface of their droplets, and this is probably right although it is a bit of a delicate assumption.

Anyway, let us go with this assumption, then the droplets will move downhill in surface free energy, i.e., they will move towards regions where the surface free energy is lower.  Now the surface free energy of a droplet depends on its surroundings. The oil surrounding water droplets is never perfectly pure oil, it always contains some water. And the more water the oil contains, the more similar the surroundings are to the water droplet, and so the lower the surface free energy.

So, water droplets in mostly-oil will head towards regions where the mostly-oil contains more water, as this lowers the free energy, which is what the Second Law tells us must happen. Then the water droplet will be in a region where there is more water, and so it will tend to pick up more water, and so it will grow. So mostly-water droplets in mostly-oil do the opposite of what Hajian and Hardt see, the droplets here will move and grow, not move and dissolve.

But Hajian and Hardt did not study a simple oil and water system, they studied a mixture with three components: oil, ethanol (=alcohol) and water, where the oil and water don’t mix, but the ethanol and oil do (ethanol and water also mix of course). They create droplets by mixing water into their oil in ethanol system. Water here is acting as an anti-solvent — a solvent dissolves something, but an anti-solvent does the opposite. When you add an anti-solvent it pushes a dissolved something out of solution to form droplets.

And I think systems with a solvent (here ethanol) and an anti-solvent (water) are different here. The anti-solvent tends to increase the surface free energy, so droplets will head towards regions with lots of solvent, and little anti-solvent. But these regions with lots of solvent will dissolve the droplets, just as Hajian and Hardt see.

So it looks like the systems with two components, such as oil and water, form droplets that head towards safety, while with three components, oil, ethanol and water, the droplets have a tendency towards self-destruction. All this is a little speculative, as observing these droplets in detail is not yet possible, but although it is not yet possible for scientists to observe this in detail, to have this happen in front of your eyes is easy.

You just have to buy ouzo, or pastis, or any of the many other aniseed-flavoured spirits, and add water. The aniseed flavour comes from a molecule called anethole. Anethole is highly soluble in ethanol, but water is an anti-solvent. Diluted ouzo turns cloudy because of the anethole droplets that form when its anti-solvent, water, is added.

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