Soaps & Detergents: Chemistry, Types & Uses

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Soaps and detergents seem pretty straightforward, right? But they're really not. This lesson will examine how soaps and detergents work, as well as how they are similar and different.

Soap vs. Detergent

Sure, life may be a little stinkier without it, but is soap really that intriguing? I'll let you be the judge.

  • The next time you're scrubbing yourself with some soap, consider this: soap is made from animal fats or vegetable oil!
  • Soap operas got their name because soap companies would advertise during the show.
  • There's evidence that people have been making soap since 2800 B.C.
  • There's a lot of money in soap. In fact, in 2010, soap manufacturing was a $50.75 million moneymaker.

Sure soap is pretty interesting, but it's not going to get all of the glory. While this lesson is going to examine soap, it's also going to check out detergents. And you probably think you're washing yourself with 'soap', but in reality, you are probably using a detergent! More on all of that later.

Before we see how soap works, check out the definitions for soap versus detergent:

  • Soap: a substance used for cleaning that is made when animal fats or vegetable oils are treated with an alkaline solution.
  • Detergent: a substance used for cleaning that is often made from synthetic or man-made materials and can perform better than soap under certain conditions.

How Soap Works

Now, on to how soap works. Imagine a soap molecule as a tadpole with a tail and a head. Soap is formed when a fatty acid combines with an alkaline solution, usually one with sodium or a potassium atom.

The black balls are carbon atoms, the red balls are oxygen atoms, and the grey balls are hydrogen atoms. This is an example of a fatty acid.

The 'tail' of the soap is made up of a hydrocarbon chain (which just means carbon and hydrogen atoms bonded together). This hydrocarbon chain is hydrophobic, meaning it doesn't like water, but it does love oil. 'Hydro' means water and 'phobic' means fear. Claustrophobic means you're fearful of small spaces. Or arachnophobic means you're scared of spiders. You get the idea.

The 'head' of the soap is made up from either the sodium or the potassium and the carboxylic group in which it combined with (don't worry too much on the vocabulary here, just realize that this end is different from the 'tail'). This end is hydrophilic, meaning it loves water, but isn't a fan of oil. 'Philic' means to 'love'.

The oil on your hands, clothes, or hair and the water you use to wash them won't mix. This is because the oil is hydrophobic (just like the soap's 'tail'). You've probably noticed that oil and water don't mix, right?

The two ends of soap act as a mediator, bringing the oil into the water. Soap's hydrophobic tail can hook up with the oil and soap's hydrophilic head can hook up with the water. Imagine you have an oily shirt. You throw it into some water, and not much will happen. But, if you add soap, a bunch of soap molecules will surround the oil (with their hydrophobic tails), and the hydrophilic ends of the soap will face out, towards the water. This allows the oil to be suspended in the water, away from the shirt.

Water surrounds the hydrophilic end of the soap, whereas the hydrophobic end of the soap surrounds the oil

Soap is a type of emulsifier, which means it separates the oil out, thus causing an emulsion. In other words, the oil is removed from the material being cleaned and then is suspended in the water via the soap. An emulsion is when two substances that normally can't mix, mix.

The image on the left shows oil and water not mixing. The image on the right shows what happens when soap is added and suspends the oil in the water, causing an emulsion


Soap sounds like a pretty good product, right? It almost seems magical with its ability to remove oils and suspend them in water. But, alas, it isn't perfect. When hard water (water that contains a lot of minerals) is used, parts of the soap combine with the minerals in the hard water and make soap scum. In addition, the minerals make soap less effective at removing dirt and oils. Soap also doesn't perform well under acidic conditions and requires animal fats and/or vegetable oil, which are sometimes in short supply.

So, some genius scientists invented detergents, which work in a similar fashion to soaps, but have synthetic ingredients that prevent soap scum and can perform under acidic conditions. And when you're using soap, chances are you're really using a detergent. In fact most commercial soaps are technically detergents.

Detergents are on top and a soap is on bottom. The zigzag lines show the hydrocarbons. Note that some detergents have rings.

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