Monday February 6, 2017 Q&A

What gives soap its cleaning power?


1 the lather it forms

2 additives

3 micelles

Question related image

Answer 3: micelles, spherical aggregates suspended in water composed of molecules with hydrophilic (water-loving) heads and hydrophobic (water-repelling) tails. We all know that oil and water don’t mix. This is why it’s pointless to try and remove raclette stains or dirty napkins using clear water. To properly clean things—spring is on its way, after all—what’s needed is a diplomatic agent that can lower the tension between the two antagonists. This facilitator, called a surfactant, can be egg yolk lecithin when it’s mayo that needs removing, or soap when dealing with oily stains and residues. Soap, in its simple, perfume-free state, is made up of an animal or vegetable fat and a base such as hydoxyde of sodium or potassium. The molecules that result from this combination are known as amphiphilic. That is, one end of them prefers water and repels oil, and the other end is attracted to oil and repels water. This is what gives soap its cleaning power.

Because of their hydrophobic tails, soap molecules prefer to remain on the surface of the water — until, that is, they are forced beneath it. At that point, they aggregate into micelles, a sort of bubble, in which the hydrophilic heads remain in contact with the water and the hydrophobic hydrophobic tails can remain more or less dry. This entire process occurs on a scale of nanometers. If the micelle should happen to encounter any oil, its molecules will break apart, their hydrophobic ends firmly attaching to the oil molecules. The oil droplets, now surrounded by hydrophilic molecules, become suspended in the water. A quick rinse, this time with clear water, and they are are easily eliminated. Our thanks to Libero Zuppiroli, emeritus professor at EPFL and author of “Traité de la matière”. Illustration: wikipedia/superManu