Lesson Explainer: Detergents Chemistry • Third Year of Secondary School

In this explainer, we will learn how to describe the structure of anionic detergents and explain how they remove stains from fabrics.

Detergents are a class of substances that have cleaning properties similar to soaps. The cleaning properties of both detergents and soaps are a result of their unique structure, represented in the cartoon below, which has a hydrophobic tail region and a hydrophilic head region.

The structure of detergents and soaps allows these molecules to remove stains that water alone cannot. This is because oils, fats, and many other stains contain molecules that are nonpolar, while molecules of water are polar. Detergents and soaps are able to remove these stains since the hydrophobic region of the molecule can attract the stain molecules, while the hydrophilic region can attract water molecules to wash the stain away.

There are different classes of detergents: anionic, cationic, and nonionic. These three classes can be distinguished by the identity of the head group, which gives the detergent slightly different properties and uses.

Anionic detergents are frequently used to make laundry detergents, dishwashing liquids, shampoos, bodywashes, and industrial cleaning products.

The chemical structure of an anionic detergent as well as a soap are shown below.

If we compare the structures of the anionic detergent and soap above, we can see that both are sodium salts containing a long carbon chain for the hydrophobic tail and a negatively charged hydrophilic head group. However, soap has a carboxylate () group, while anionic detergents have a sulfonate () group.

Soap and anionic detergents function in the same way to clean and remove stains. However, the carboxylate group in soaps is far more likely to bind to the ions that are present in tap water, such as calcium and magnesium, than the sulfonate group in anionic detergents. When this happens, the soap does not work as effectively and does not wash away cleanly, leading to a build up of “soap scum,” as shown in the photo below. This has made anionic detergents the preferred cleansing agent for most applications since their invention in the 1930s.

There are many different chemicals that are anionic detergents. The structure below is sodium 4-dodecylbenzenesulfonate, which is an example of a linear alkylbenzene sulfonate. Linear alkylbenzene sulfonates are by far the most common detergents for both household and industrial applications.

Linear alkylbenzene sulfonates are the main ingredient in most laundry detergents and other household cleaners.

In the past, branched alkylbenzene sulfonates, such as the one shown below, were the most common choice for detergents. However, branched alkylbenzene sulfonates do not biodegrade as easily as linear alkylbenzene sulfonates. This caused extensive foaming in rivers, coasts, lakes, and sewage systems as well as contamination of the drinking supply. Linear alkylbenzene sulfonates biodegrade far more easily, so they have largely replaced branched alkylbenzene sulfonates.

Alkyl sulfates are another frequently used type of anionic detergent. The structure below is an example of an alkyl sulfate, sodium lauryl sulfate. Sodium lauryl sulfate is frequently used in shampoos, bodywashes, toothpastes, and facial cleansers.

Example 1: Determining Which Substance Should the Preferred Commercial Detergent

The structures of two alkylbenzene sulfonates are shown below.

Why might structure B be preferred over structure A as a commercial detergent?

1. Structure B is more thermally stable.
2. Structure A has a higher melting point.
3. Structure B is more biodegradable.
4. Structure B has a more aromatic smell.
5. Structure A is more expensive.

Answer

Alkylbenzene sulfonates are the most common and widely used detergents in cleaning and personal care products. All detergents have two portions: a hydrophobic tail that is made of a hydrocarbon chain and a hydrophilic head. We can see below these two portions for the two alkylbenzene sulfonates in the question.

Both of these structures have the same head group (), but the tail of structure A has far more branching than the tail of structure B.

Alkylbenzene sulfonates with branched tails, like structure A, do not biodegrade easily. They can end up accumulating in large bodies of water where they can contaminate the water and cause excessive foaming to happen.

Alkylbenzene sulfonates with linear chains, like structure B, are more biodegradable. They do not tend to accumulate in large bodies of water. Chemical companies prefer to make commercial detergents with alkylbenzene sulfonates primarily because they are more biodegradable and they do not contaminate or disrupt natural bodies of water.

These statements can be used to determine that option C is the correct answer for this question.

No matter the chemical structure of the detergent, the way it removes stains is the same. Let’s break down how detergent molecules remove a fatty stain from an article of clothing.

When detergents are first added to water, they tend to accumulate at the surface of the water with the head of the detergent in the water and the tail in the air. This reduces the surface tension of the water by disrupting the hydrogen bonding between water molecules. Though this is not the primary action of the detergent, the reduced surface tension allows water to wet the clothing more effectively.

Some detergents (typically nonionic detergents) are used specifically for the purpose of increasing the wetting ability of water. For example, detergents are applied to the grass at golf courses so that water will evenly penetrate the green.

Foaming is also a consequence of the decreased surface tension caused by adding detergents to water. This is because bubbles in the water are less likely to burst without the strain on the surface of the bubble caused by surface tension. The large number of stable bubbles produces foam in the water.

Example 2: Identifying the Effect Anionic Detergent Molecules Have When Added to Water

As well as removing stains, what other effect can anionic detergent molecules, such as those of alkylbenzene sulfonates, have when added to stained items in water?

1. Precipitating out metal cations
2. Repairing any structural damage on the surface of the stained item
3. Softening clothes and other fabrics
4. Reducing the surface tension of water
5. Penetrating and breaking down soils

Answer

Alkylbenzene sulfonates are a type of anionic detergents that are widely used for their cleaning properties. These molecules have a hydrophobic tail group and a negatively charged hydrophilic head group.

Without detergents in the water, there will be strong hydrogen bonding between all the water molecules. There is a “pull” on the surface of the water due to these attractions that creates surface tension.

When a detergent is added to water, the hydrophilic head will be attracted to the water molecules. These detergent molecules can accumulate on the surface of the water with heads in the water and the tails aligned in the air.

The addition of the detergent molecules will disrupt the hydrogen bonding between the water molecules, reducing the surface tension of the water.

The correct answer is therefore option D: Reducing the surface tension of water.

If the concentration of detergents in the water is high, the hydrophilic heads of the molecule will be attracted to the water, while the hydrophobic ends of the detergent molecules will begin to stick together forming spherical structures called micelles.

Example 3: Determining the Arrangement of Anionic Detergent Molecules When Placed in Water

The following image shows a cartoon structure of sodium 4-dodecylbenzenesulfonate.

What structural arrangement will these molecules initially form when placed into water?

Answer

Sodium 4-dodecylbenzenesulfonate is an example of an anionic detergent. Detergent molecules have two portions: a hydrophobic tail and a hydrophilic head. This has been labeled for sodium 4-dodecylbenzenesulfonate below.

When detergents are introduced to water, the hydrophilic head group will be attracted to the water molecules and the tail of the detergent molecules will be attracted to the tails of other detergent molecules. The detergents will end up forming spherical structures with the hydrophobic tails on the inside of the sphere and the hydrophilic heads near the water molecules on the outside of the sphere. We can use these statements to determine that option A is the correct answer for this question.

If these spheres of detergent molecules or any free-floating detergent molecules in the water approach the stain, the nonpolar tail of the detergent molecule will be attracted to the nonpolar stain and bind to it.

Eventually the stain will begin to lift from the surface. Mechanical action, such as rubbing your hands together or the agitation in a washing machine spin cycle, can break up the stain making it easier for the detergent molecules to remove it.

Once the stain is removed from the surface, the detergent molecules will reform the spherical structure, this time with the stain inside. The attraction between the water and the hydrophilic head of the molecule allows the structure to be easily washed away by water.

The synthesis of alkylbenzene sulfonates has two steps. In the first step, an alkylbenzene is reacted with either concentrated sulfuric acid or sulfur trioxide gas. This produces an alkyl benzene sulphonic acid.

In the second step, the alkylbenzene sulphonic acid is neutralized with (caustic soda) to create an alkylbenzene sulphonate salt and water.

Reaction: Synthesis of Alkylbenzene Sulfonate Detergents

Example 4: Identifying Which Substance Is Important for Creating Detergents

Which of the following plays a very important role in the detergent industry?

Answer

Most detergents are sodium salts of molecules with long carbon chains. The most common molecules used in detergents are alkylbenzene sulfonates, such as the one shown below.

This detergent is made by neutralizing an alkylbenzene sulphonic acid with sodium hydroxide, which produces alkylbenzene sulfonate and water.

Though not all detergents will be synthesized starting with an alkylbenzene sulphonic acid, sodium hydroxide is almost always used in the synthesis of detergents, which is why it plays a very important role in the detergent industry. The correct answer is D: .