Lesson Video: Detergents Chemistry

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

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Video Transcript

In this video, we will learn how to describe the structure of anionic detergents and explain how they remove stains from fabrics. When we want to wash our hands or clean our clothes, we frequently reach for a bottle of soap or a detergent rather than just use water alone. So what exactly is a detergent?

Detergents are a class of substances with cleaning properties that have a hydrophobic tail and a hydrophilic head. We can use a drawing like this to represent a detergent molecule. The circle represents the head, and the wavy line is the tail. The tail is hydrophobic, which literally translates to water fearing. We could also say that the tail of a detergent is nonpolar. The head is hydrophilic, which literally translates to water loving. We might also describe the head of a detergent as polar. So detergents have both a nonpolar and polar region. This difference in polarity is what makes detergents effective cleaning agents as we will see later in this video.

Let’s examine some sample detergents. The tail region of each detergent is shown in blue. We can see that the tail of a detergent is often a long nonpolar alkyl chain, although branch chains are also possible. The head region of each detergent shown in orange can be used to classify the type of detergent. The first three detergents are ionic. They contain a charged head region and a counterion. The head of the first detergent is a carboxylate group. This type of detergent is a soap, a detergent formed via the saponification of fats and oils. The head of the second detergent is a negatively charged sulphonate group.

Detergents with a negatively charged anion head region are anionic. As a carboxylate group is negatively charged, soaps are sometimes classified as anionic detergents. The head of the third detergent is trimethyl ammonium, a cation. Detergents with a positively charged cation head region are cationic. The head of the last detergent contains ethoxy and hydroxy groups and is not ionic. This type of detergent is called nonionic.

While each type of detergent has its uses, let’s take a closer look at soaps and anionic detergents. Soaps have been used for thousands of years, but they may not be the most ideal for cleaning. The carboxylate group can react with the ions that are present in tap water, such as calcium and magnesium, to produce an insoluble product. This product can build up on clothing and sinks. The gray or white residue is known as soap scum.

Anionic detergents can also react with calcium and magnesium ions to form an insoluble product. However, this product is less likely to form a soap scum buildup for reasons which are beyond the scope of this video. As such, anionic detergents have been the preferred cleansing agent for most applications since their invention in the 1930s.

Some of the first anionic detergents introduced were branched alkylbenzene sulfonates like this one, where the tail has a number of branches or substituents. The problem with these substances is that they do not easily biodegrade. Their use caused foaming in sewage systems and in rivers, on coasts, and in lakes, which contributed to the contamination of the drinking water supply. As such, the use of branched alkylbenzene sulfonate detergents has been prohibited in many countries.

Around 1960, linear alkylbenzene sulfonates, like these, began to replace branched alkylbenzene sulfonates. The linear versions biodegrade far more easily and pose less of a threat to environmental systems. Linear alkylbenzene sulfonates are the main cleaning agent used in many laundry detergents, dish detergents, and other household cleaners.

Alkylbenzene sulfonates can be prepared via a two-step process. In the first step, an alkylbenzene is reacted with hot, concentrated sulfuric acid. This produces an alkylbenzene sulfonic acid and water. The actual species that reacts with the alkylbenzene is not sulfuric acid, but rather sulfur trioxide formed from the dissociation of sulfuric acid. So we might see this reaction abbreviated like this. In the second step, the alkylbenzene sulfonic acid is reacted with sodium hydroxide, also called caustic soda. An acid–base neutralization reaction occurs, and an alkylbenzene sulfonate salt and water are produced.

There are other common anionic detergents in addition to alkylbenzene sulfonates. These include alkyl sulfates like sodium lauryl sulfate and sodium laureth sulfate. These detergents are cheap and very effective foaming agents. They are frequently found in shampoos, body washes, bubble bath products, and toothpastes. Now that we can recognize some common anionic detergents, let’s examine how detergents work.

Here, we have a container of water that contains H2O molecules. At the bottom is a piece of fabric that has an oily stain. This diagram is not drawn to scale, and only a few water molecules are shown for simplicity. The polar water molecules are not attracted to the nonpolar oil and are unable to remove the stain. So let’s add a detergent.

We will use this figure to represent a generic ionic detergent. The yellow circle represents the hydrophilic head, the wavy blue line represents the hydrophobic tail, and the pink circle represents the counterion. When we add the detergent to water, the counterion will dissolve and disperse in the water. The head of the detergent is hydrophilic or water loving. So the heads will be attracted to the water molecules of the surface. But the tails are hydrophobic or water fearing, so they will stick up into the air.

We may recall that water molecules are held together by strong hydrogen bonds. But the introduction of a detergent interferes with the hydrogen bonding between the water molecules on the surface. This interference decreases the surface tension. Substances that decrease the surface tension of a medium are called surfactants or surface acting agents. So we may hear the term surfactant used to describe or refer to a detergent.

While reducing the surface tension is not necessarily the primary action of a detergent, it is useful. Decreased surface tension allows water to wet an item like clothing more easily. Nonionic detergents are often applied to the grass at golf courses for this reason, as the increased wetting ability of water allows the water to more evenly penetrate the green.

Another result of the decreased surface tension is foaming. If we placed a straw into a container of water and blew through it, bubbles would rise to the surface and quickly pop because the surface tension of water is high. But if we did the same to a container of water and detergent, the bubbles would rise to the surface and create a foam due to the decrease to surface tension.

Let’s go back to trying to remove the oily stain from the piece of fabric. We’ll go ahead and remove the counterions and water molecules from the drawing so that we can focus solely on the behavior of the detergent. If the concentration of the detergent is increased or the detergent is mixed into the water, the detergent molecules will group together to form spheres, where the hydrophobic tails are inside of the sphere and the hydrophilic heads form the outside of the sphere.

These spheres are called micelles. If a micelle or any free floating detergent molecules approach the stain, the nonpolar hydrophobic tails will be attracted to the nonpolar oil. Eventually, the molecules that make up the stain will lift from the surface. Mechanical action, such as agitation in a washing machine, can help to break up the stain. This makes it easier for the detergent to interact with and remove the stain. Once the oil molecules have lifted from the surface, the detergent molecules will once again come together to form a micelle with the oil molecules encapsulated inside. The micelle with the trapped oil molecules can then be rinsed away.

Before we summarize what we’ve learned about detergents in this video, let’s take a look at a question.

Which of the following is the most correct statement about anionic detergents? (A) The detergent molecule is composed of a hydrophilic head, which is a long carbon chain, and an ionized hydrophobic tail. (B) The detergent molecule is composed of a hydrophobic head, which is a long carbon chain, and an ionized hydrophilic tail. (C) The detergent molecule is composed of a hydrophilic tail, which is a long carbon chain, and an ionized hydrophobic head. (D) The detergent molecule is composed of a hydrophobic tail, which is a long carbon chain, and an ionized hydrophilic head.

Detergents are a class of substances with cleaning properties that have a head and tail region. We can represent a generic detergent with a figure like this, where the orange circle represents the head and the wavy blue line represents the tail. To answer this question, we need to determine the properties of the head and tail.

Looking at the answer choices, we can see that the words “hydrophilic” and “hydrophobic” are used to describe the head and tail. Hydrophobic literally translates to water fearing, and hydrophilic literally translates to water loving. To determine which portion of a detergent molecule is which, let’s consider what happens when a detergent is added to water.

First, the detergent molecules accumulate at the surface of the water with the heads in the water and the tails in the air. If the concentration of the detergent is increased or the detergent is mixed with the water, the detergent molecules will come together to form spheres called micelles. The heads form the outside of the sphere, and the tails are located within the sphere. As we can see from the diagram, the head of a detergent is often in contact with water, while the tail is often kept outside of the water or shielded from the water. This indicates that the tail is water fearing or hydrophobic and the head is water loving or hydrophilic. Based on this information, we can eliminate answer choices (B) and (C).

Now, we need to determine which portion of a detergent is a long carbon chain and which portion may be ionized. Detergents are classified by the composition of the head region. In this question, we are told that the detergent is anionic. This means that the head of this detergent is a negatively charged anion.

Here is an example of an anionic detergent. We can see that the head region shown in orange is a negatively charged anion and the tail region shown in blue is a long carbon chain. Looking at the remaining answer choices, we can see that the most correct statement about anionic detergents is answer choice (D), the detergent molecule is composed of a hydrophobic tail, which is a long carbon chain, and an ionized hydrophilic head.

Now, let’s summarize what we’ve learned. Detergents have a hydrophobic tail, typically a long carbon chain, and a hydrophilic head. Detergents are classified by the composition of the head region into anionic, cationic, nonionic, and soaps. Soaps are sometimes classified as an anionic detergent, as the head of a soap is a carboxylate anion. Alkylbenzene sulfonates and alkyl sulfates are common anionic detergents. Alkylbenzene sulfonates may be linear or branched. Linear alkylbenzene sulfonates are more biodegradable than branched alkylbenzene sulfonates.

Alkylbenzene sulfonates can be prepared in two steps. First, an alkylbenzene is sulfonated using sulfuric acid. Then, the resulting alkylbenzene sulfonic acid is reacted with sodium hydroxide to produce the detergent. Detergents are able to remove oils and stains because the hydrophobic tail is attracted to the stain, while the hydrophilic head is attracted to water.

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