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Oil and water don’t mix. Except when…

closeup to white foam on blue water
In a previous article, we discussed the scientific reason behind why oil and water don’t mix. At the end of that same article, we noted that there is one special circumstance under which you can actually get water and oil to mix well with each other. In this article, we’ll explore the science behind this special circumstance that facilitates the mixing of oil and water… two substances that we all know typically don’t mix well with each other.

At some point in your life, you’ve no doubt come in contact with oil. You might have put some baby oil on your child, niece, or nephew, or you might have used some sort of oil to dress a salad, to make an omelette, fried chicken, etc. While dealing with oil, you’ve probably gotten some on your hands at some point and may have noticed that trying to get oil off your hands by using a thorough water rinse doesn’t work. The reason for this phenomenon is because oil and water do not mix… and the reason why they don’t mix is because water is a polar molecule and oils are generally nonpolar molecules. As nature will have it, polar molecules don’t mix with nonpolar molecules. So when you try to rinse your oily hands with just water, the water simply beads up and falls right off your hands still leaving them super oily.

Attempting to rinse oil off your hands using water by itself is generally a waste of time until you add soap to the mix. You might not have taken conscious note of it till now, but if you think back to various experiences in your life, I think you will see that it is true. I’m betting that all those times you tried to get grease off your skillet by rinsing it with only water didn’t seem to do anything until you added dishwashing soap and used a sponge to work it all out of there.

The intellectually curious amongst us whose practical experiences match the general theme of what we discussed above have probably wondered about the scientific reason why soap is able to get two otherwise immiscible substances to mix. I wondered about this too until I took a few chemistry classes so if you’re interested in understanding this scientific concept in plain english, keep reading.

OK, so if polar molecules such as water only mix with other polar molecules, and nonpolar molecules such as oils only mix with other nonpolar molecules, then the only logical reason why soap would be able to mix with both oil and water would be if it were somehow polar and nonpolar at the same time. Truth be told, this is in fact the case. By the way, you can loosely assume that the word hydrophobic is equivalent to nonpolar, and that the word hydrophilic is equivalent to polar for the purposes of this article. Notice that the generic chemical structure of soap shown in the image below shows that soap has a hydrophilic (water loving) or polar head, while also possessing a hydrophobic or (water fearing) tail. Indeed, it is this hybrid chemical structure of soap that allows it to facilitate the mixing of oil and water.

When soap is added to oil and water, the soap molecules orient themselves in such a way as to ensure that their hydrophobic tails interact with the oil molecules, while their hydrophilic heads interact with the water molecules. In this way, soap facilitates an indirect connection between oil and water, allowing both substances to mix. So the next time you use dish soap or hand soap in the bathroom, know that the hydrophobic tails of the soap molecules work to encapsulate the grease that you are trying to get rid of, leaving their hydrophilic heads exposed for water to interact with and wash away.

Notice that oil forms a distinct layer above water at time equals 0. As soon as we add soap at time t = 1, the soap molecules orient themselves in a way that results in an indirect connection between oil and water, causing them to mix “by proxy” if you will.

To conclude, oil and water don’t mix, except when you add soap. From all of us here at chubaoyolu.org, take care of yourselves and each other.

 
 
Without Wax
Oyolu B.C. Ph.D.
chubaoyolu.org

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