How soap and detergent lift dirt: surfactants

Why water alone won't shift grease

Grime on a shirt collar, oil in a frying pan and the natural oils on your hands don't come off well with plain water. Water molecules are polar, slightly positive on one side and slightly negative on the other, so they cling tightly to each other. Oil molecules have their charge spread evenly, giving water nothing to grab onto. So water sticks with itself instead of mixing, and oil floats off separately or stays stuck to the surface. On top of that, much everyday dirt is dust and dead skin glued together by oil, so if the oil stays, the dirt stays too.

A molecule with a head and a tail

Surfactants, the main ingredient in soap and detergent, have two parts with opposite personalities in a single molecule. One end is a hydrophilic head that gets along with water; the other is a long carbon-chain tail that is lipophilic and dissolves readily in oil. The name is short for surface-active agent: they gather at the boundary where two different substances meet, such as water and oil or water and air. There they lower the surface tension of water, so instead of beading up, water spreads out and soaks between fibers and into the creases of your skin, wetting the dirt.

Wrapping oil up and carrying it off

When oily dirt meets water with surfactant in it, the tails burrow into the oil while the heads line up facing the water. Add some scrubbing and the oil breaks into small blobs, each enclosed in a ball with heads packed tightly around the outside. These clusters are called micelles. Because their surface is all water-friendly heads, micelles disperse evenly through the water and get rinsed away without clumping together or sticking back onto the fabric.

• Lower surface tension lets water soak into the dirt and fibers
• Tails dig into the oil and lift it off the surface
• Scrubbing breaks the oil into small pieces trapped in micelles
• Rinsing carries the micelles away in the water

How soap is made

Soap is made by reacting animal fat or vegetable oil with a strong alkali. The reaction is called saponification, and the fatty acid salt it produces is the soap molecule itself. Sodium hydroxide gives a hard bar soap, while potassium hydroxide gives a soft or liquid soap. Soap is mildly alkaline when dissolved in water. Most laundry and dish detergents on the market, meanwhile, use synthetic surfactants made from petroleum or plant-based materials. They work on the same principle, but many are designed to be less affected by water quality. If you make soap at home, always wear gloves and eye protection, since strong alkali can seriously damage skin and eyes.

Hard water and soap scum

Water with a lot of dissolved calcium and magnesium is called hard water. In hard water those ions bind with soap molecules to form a scum that won't dissolve. The gray ring around a bathtub or sink and soap that barely lathers are the result, and that soap is wasted. Synthetic detergents are less affected, and laundry detergents often contain ingredients that capture those ions and effectively soften the water. It also helps to know that foam is not the same as cleaning power. Detergents for front-loading washers are deliberately made to produce little foam.

Other helpers in detergent

Surfactants are great on grease, but they can't handle every stain alone. That's why laundry detergents also include enzymes or bleaching agents aimed at particular stains. Enzymes chop the large molecules that make up a stain into smaller pieces that wash away more easily. Bleach oxidizes the substances that give a stain its color. Checking the ingredient list gives you a good idea of what a product is strong against.

• Protease: protein stains like blood, sweat and egg
• Amylase: starch stains like rice and sauces
• Lipase: oil and body oil stains
• Oxygen bleach: a relatively gentle bleaching agent that can be used on many colors

Laundry and handwashing tips from the science

Warm water softens hardened grease and makes the surfactant's job easier, but protein stains like blood or egg set into fibers when hot water cooks them, so rinse those in cold water first. Micelles need scrubbing to form, which is why taking your time and rubbing every part of your hands matters when washing. Soap can also break apart the oily outer membrane that surrounds some viruses and bacteria. Using more detergent doesn't make things cleaner; it just leaves residue after rinsing. Wool and silk are protein fibers that alkali can damage, so use a neutral detergent.

• Greasy dirt: lukewarm or warm water
• Protein stains: pre-rinse in cold water
• Detergent: only the amount on the label
• Wool and silk: neutral detergent, hand wash or delicate cycle

Using cleaners safely

Mixing chlorine bleach with acidic cleaners or products containing ammonia releases harmful gases. Avoid using products marked with a do-not-mix warning back to back on the same day, and open a window for ventilation while cleaning. Keep detergents and bleach out of children's reach in their original containers, and never pour them into drink bottles. If any gets in your eyes, rinse with running water for a long time right away, and if someone swallows some or pain and symptoms persist, take the product with you and contact a medical provider. If your skin stings or itches, stop using the product and wear gloves.