Detergents

What is a detergent?

A detergent is a chemical cleaning agent designed to remove soil from clothing. It is a specially formulated mixture of surfactants, builders, enzymes, and fragrances and dyes.

At a surface level, these components serve the following roles:

• Surfactants (SURFace ACTive AgeNTS) are substances that allow water to interact with oil and grease. Water can’t wash away oil on its own, but when a surfactant is present, it can.
• Builders help soften water. If water is hard (meaning it contains a high concentration of dissolved minerals), surfactants don’t work as effectively. Builders also raise pH, which helps with soil and stain removal.
• Enzymes fight stains. They break large stain molecules into smaller ones, making them easier to remove from clothing fibers.

Wait, so… what’s a surfactant?

All surfactants have a hydrophilic head and a hydrophobic tail.

Water is polar because the electrons in the molecule are not shared equally. Oxygen is much more electronegative than hydrogen, meaning it pulls electrons toward itself more strongly. One reason for this is effective nuclear charge: oxygen has 8 protons pulling on its electrons, and although inner electrons shield some of that pull, the outer electrons still feel a strong attraction. Hydrogen, with only 1 proton and no inner electrons, has a much weaker pull on shared electrons.

As a result, the shared electrons spend more time near oxygen than hydrogen. This gives the oxygen a slight negative charge (δ–) and the hydrogens a slight positive charge (δ+). This separation of charge makes water a dipole, with a partially negative end (oxygen) and partially positive ends (hydrogens).

When polar molecules like water come near each other, their dipoles align so that opposite charges attract, creating dipole-dipole interactions, which are weak attractions between molecules.

Another type of weak attraction is London Dispersion Forces (LDFs). Electrons are constantly moving around atoms or molecules, so they are sometimes unevenly distributed. This creates a temporary dipole, which can induce a similar dipole in a neighboring molecule. These induced dipoles attract each other, resulting in weak LDFs. Larger molecules have more electrons, which can produce stronger temporary dipoles, so LDFs are stronger in larger molecules.

It is important to note that LDFs do not only occur in nonpolar molecules — polar molecules can also experience them. However, in small molecules like water, LDFs are much weaker than dipole–dipole forces, which dominate.

Surfactants’ nonpolar tails cling to oil via LDFs, clustering together to form spherical structures called micelles. The polar heads face the water, attracted by dipole–dipole interactions. These micelles trap the oil inside, allowing it to be carried away by water and leaving the surface oil-free.

Hey… surfactants kinda sound like soap

Yeah, that’s because soaps are surfactants! Soaps are defined as salts of fatty acids and are one of the most straightforward ways to produce surfactants from natural ingredients.

Check out this article on saponification, the science of soap making, to understand the full process!

How do surfactants clean?

There are two core phenomena that make cleaning with soap possible: emulsification and micelle formation.

When a surfactant is applied to oil or grease, its hydrophobic tail clings to the oil via London dispersion forces, while its hydrophilic head is attracted to water. This makes it more favorable for the oil to break up into small droplets surrounded by water, rather than staying in a single large blob.

When enough surfactant is present, the molecules surround the grease, forming spherical structures called micelles. The outside of these micelles is made up of the hydrophilic heads, which interact with water. Once the grease is fully encapsulated inside a micelle, water can wash it away, leaving the surface clean.

Why does scrubbing soap into oil help?

Oily substances are easier for surfactants to remove when they have more surface area for the molecules to attach to. Agitating the oil breaks it into smaller droplets, increasing its surface area and making it easier for micelles to form around the grease.

What is an HE detergent?

An HE detergent, short for High Efficiency detergent, is a specially formulated detergent introduced in the 1990s. It is designed to work in high efficiency (HE) laundry machines, though it also works in traditional washers. You can identify it by the HE logo on the container.

HE detergent differs from traditional detergent in two main ways:

• Concentration: HE detergents are more concentrated than traditional detergents, so less is needed to achieve the same level of cleaning.
• Low sudsing: HE detergents are formulated to produce fewer suds.

When detergents are agitated in water, they generate foam. Standard washing machines use a lot of water, so traditional detergent produces a reasonable amount of foam. However, in the 1990s, HE washing machines were introduced, using significantly less water. Traditional detergents in these machines would create excessive suds, which could impair cleaning performance.

To solve this, a special low-sudding producing detergent was developed. This new detergent advancement, HE detergent, kept foam level manageable in the low-water environment of HE washing machines.

Wait… Suds are bad?

Although suds are often seen as a symbol of cleaning, they are actually just a visual phenomenon — they don’t directly remove soil from a surface.

Suds are bubbles stabilized by surfactants. Each bubble is essentially a thin film of water with surfactant molecules on both sides, trapping air inside. These surfactants sit parallel to the surface they are on — it is the hydrophilic head of the surfactant that touches the surface. Suds may adhere to nonpolar surfaces via London dispersion forces, but they are more stable on polar surfaces through hydrogen bonding. Suds tend to sit on static surfaces as opposed to water because they can form more hydrogen bonds with a stable surface than with a moving one.

Suds do provide some indirect help with cleaning — they create a cushioning layer that can enhance mechanical agitation, helping to break up grease blobs. They are also useful as a visual cue, showing the cleaner where surfactant has been applied.

However, excessive foaming can hinder cleaning. Dirt and grease can become trapped within the surfactant film, and because suds don’t flow like water, they can be difficult to rinse away.

How much detergent should be used to adequately clean clothing?

Only 1 to 2 tablespoons of detergent are necessary, which is about half of what is typically recommended on the back of detergent containers. Most clothing isn’t as dirty as people assume. Once all the oil and grime have been removed, any extra detergent simply sits in the machine and gets rinsed away.

Using more detergent than needed can also have other downsides. Machines may run longer to process the excess, wasting time and energy. Additionally, leftover detergent can stick to clothing even after the cycle is finished, potentially causing stains or irritating skin.