Those who like to chug a little ouzo, or perhaps absinthe, will know how the liquor clouds up when water is added to it. What causes two clear liquids to form a cloud in the glass? Chemistry gets opaque with the Ouzo Effect.

If you are starting this year just the way you finished the last one, you'll be going to a bar tonight. Just for fun, ask the bartender for an ouzo and water, or perhaps an absinthe and water. Of the two, ouzo is a bit more impressive. It's a clear liquid, and so when water, another clear liquid, is added to it, the burst of cloudiness in the resulting liquor is dramatic.

The change from perfectly clear to cloudy and opaque is counter-intuitive, as if something has quietly been added to the mixture without us noticing. Something has been added - just not during the mixing of the two liquids. That something is an essential oil made from anise. Inside the oil made from anise is anethole, an organic compound. Anethole made with a few hydrogen atoms thrown together with a carbon atom and oxygen. There's nothing particularly unusual about it, except for the fact that it's highly hydrophobic. Put it near water and it scrambles onto a chair, gathers up its skirts, and screams. It wants as little to do with water as possible. This is the reason for all that cloudiness.

The fact that oil is repelled by the water means that the oil will be driven into shapes that expose as little oil to water as possible. At first, when the oil particles get kicked out of the solution, they're on their own. Then Ostwald ripening kicks in. A large droplet of oil will expose a thin skin of oil to the water, but most of its molecules will stay tucked inside that skin. On the other hand, a small droplet of oil will have almost all of its molecules exposed, with no space inside for molecules to hide. So small particles of oil will be driven apart by the water, while large particles will add and add molecules over time. Those particles will get big enough to see, and the entire drink clouds up.

Via Spectroscopy Now and Langmuir.