Cocktail Chatter: Oil and Water..Why They Don’t Mix

“Like mixing oil and water” is a common phrase when one means that two objects don’t mix at all. After contemplating a beautiful video shot by Jesse Zanzinger who took these images of oil and water not mixing, I had to get revisit my chemistry 101 class and get reacquainted with polar (think: fiesty, emotionally charged) and nonpolar (neutral, satiated, less reactive) molecules. While we could get into an emulsified slippery slope explaining the various properties of oil and water: here’s what you need to know next time the conversation comes up at a cocktail party.

Water: (H2O) is a polar molecule made up of two hydrogen atoms covalently bonded to one oxygen atom. Polarity means one end is positively charged while the other end is negatively charged.

This polarity generally makes water an excellent solvent (other atoms such as salts dissolve easily), except in the case of oil or lipids.

Oil or lipids are non-polar molecules and are not attracted (or attractive to) polar molecules. They don’t have an open charge on either end which would attract another atom. I like to think of these as larger, lazier molecules, who just don’t need to make friends (bonds).
Hence in the case of pure water and a pure oil, the excitable, highly charged water molecules are only attracted to eachother, leaving the oil molecules to clump together. Sorta sounds like a few people we know 😉

Thanks to this diagram from San Francisco’s Exploratorium, we can see that it has to do with the molecules that make up oil, water.

Another important factor due to which water and oil do not mix, is surface tension. When water molecules come together, they form a network of hydrogen bonds which results in a very high surface tension. Both oil and water have high surface tensions, due to which the adhesion between them is weak and they do not mix. Why are patches of oil floating on water circular in shape? It’s because of surface tension. The oil molecules cling to each other to form an elastic layer with the minimum possible surface area. Due to this interplay in the oil molecules, they form circular droplets over the surface of water.


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