Definition
A surfactant is a substance that changes how two different materials behave where they meet. One end is a head that loves water and the other is a tail that loves oil, so it can join water and oil, which normally never mix, into one.
The peace envoy between water and oil
Try washing a greasy frying pan with plain cold water, and the oil just floats around and won't come off. Water molecules pull very hard toward each other, and oil pushes water away. The boundary where two different substances touch like this is called an "interface."
Now drop a bit of dish soap in, and something surprising happens. The key ingredient, the surfactant molecule, has a "hydrophilic head" that loves water and a "lipophilic tail" that loves oil, both on the same molecule.
These molecules slip in between the water and the oil and take hold of both sides. The boundary that kept pushing them apart breaks down, and the grease stuck to the dish ends up peeling off and flowing away with the water.
The micelle trick that traps dirt in a ball
When you wash your hands or run the washing machine, surfactant molecules arrange themselves into a distinctive shape in the water. The tails, which hate water and love oil, avoid the water and huddle together toward the inside. The heads, which love water, stretch outward.
This round, ball-shaped structure is called a "micelle." The inner core of a micelle is friendly to oil. When it meets grease on your clothes or skin, the tails grab it and tuck it deep inside the ball.
The outside of the ball is wrapped in water-loving heads, so it can glide through the water anywhere. Once you pour the rinse water over it, the micelles, with the dirt locked inside, wash away with the water.
A closer look: the power to lower surface tension
To be more precise, the most basic ability of a surfactant is to lower a liquid's "surface tension." Surface tension is the force that makes a liquid pull tightly on itself to shrink its surface area as much as possible. It's why water beads up into round drops.
Pure water has high surface tension, so it can't get deep into the gaps between fibers and tends to just sit on top. When a surfactant is added, its molecules line up along the water's surface and lower the surface tension, so the water spreads out wide and thin. That's why it can soak into even the tiniest gaps in fabric and loosen the dirt.
Surfactants work all over our daily lives, not just in laundry. Lecithin in egg yolk, which helps blend vinegar and oil into smooth mayonnaise, and the ingredients in a face cream that keep water and oil from separating all use the same trick: lowering the tension between two substances so they stay mixed and stable.
Common misconceptions
Surfactants are only found in man-made, synthetic chemical detergents.
Plenty of natural surfactants exist in nature and even inside our own bodies, such as the lecithin in egg yolk, the saponin in beans, and even the substance that keeps the tiny air sacs in your lungs from sticking together when you breathe.
Where you meet it
When you scrub a greasy frying pan with dish soap, the grease washes away with the water.
Mixing in egg yolk keeps oil and vinegar from separating and turns them into a smooth sauce, which is how you make mayonnaise.
A thin film of soapy water on eyeglass lenses can keep them from fogging up into water droplets.
In one sentence
A surfactant is a molecule with a water-loving head and an oil-loving tail that breaks down the boundary between two substances so they mix evenly.