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Cleaning with Surfactants

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn what makes soaps much better at cleaning than water alone. We will also learn about different types of surfactants and what kinds of products they are found in.

Surfactants as Cleaners

If you try using only water (even hot water) to clean a sticky, oil pan, you will probably not be very successful. It will require a lot of scrubbing! But as soon as you put on some dish soap, it becomes a lot easier. Or you are sure you have permanently stained your favorite shirt with ketchup, but somehow that stain remover performs its magic, and the shirt comes out of the washer looking beautiful! How does dish soap and stain remover 'magically' loosen dirt and stains that would have otherwise taken hours of soaking or hard scrubbing, or possibly have been impossible to clean with water alone? The 'magic' comes from the surfactants in the cleaners.

Water alone will have a hard time properly cleaning dishes, but combined with soap it is a lot easier.
Dirty dishes

A surfactant is a compound in cleaners that interacts with both water and soil to help them mix together and have the soil wash away with the water. When referring to cleaning, soil refers to anything that is making the object dirty; this can be dirt, oil, food, etc.

Surfactant Chemistry

Water is strongly attracted to itself. Have you ever seen how water will form little drops? This is because the water is trying to collect as close together as possible. There are some things that will dissolve into water, such as other polar compounds. Yet many soils are not polar; water will not like to mix with these soils. So when you run water over an oily pan, the oil will stick to the pan and the water will just stick to itself. This does not make cleaning an oily pan very effective. So how do we get the oil to wash away with the water? We use dish soap. The dish soap contains a surfactant--surfactants are able to interact with both water and soils.

Molecular structure of a surfactant
Surfactant structures

Surfactants work by having a polar 'head' and a non-polar 'tail'. The polar head is able to interact with the water, while the polar tail tries to get away from the water and interacts with the soil. Since the surfactant is interacting with both, when the water runs off the surfactant will go with it, bringing the soil along as well.

Soaps versus Detergents

When we talk about cleaning, we often talk about two different types of cleaners: soaps and detergents. A soap is a combination of oil with a base. A soap is a sodium or potassium salt of a fatty acid. When a soap is made, the non-polar tail comes from the fatty acid while the polar head is the salt between the base and the fatty acid. Soaps have been made for centuries. It is as simple as combining animal fat (the fatty acid) with ashes (the base).

When oils became scarce during the first part of the 1900s (due to World War I and World War II), there became a need for a cleaner that didn't use oil. Instead, they created long carbon chains from petroleum and added an alkali salt. So the long carbon chain is the non-polar tail and the alkali salt is the polar head. These types of surfactants, with any non-fatty acid carbon chain and its salt, are known as detergents.

Detergents can have a complex tail that can more specifically interact with soils
Detergent structure

Although we may refer to 'dish soap' or 'laundry soap,' most of the commercially available products are typically detergents instead of actual soaps. There are actual soaps that we can use, but since detergents can be so much more versatile, they are more commonly used for cleaning.

Types of Surfaces

So if dish soap and laundry detergent both have surfactants, then why can't we just have one kind of cleaner and use it for everything? The reason is because detergents can be created to be attracted to certain surfaces more than others, making different detergents more useful on that specific surface than on others. So dish soap is created in such a way that the surfactant will combine with common soils found on dishes, such as food, while laundry detergent will be attracted to a larger variety of soils.

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