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A Brief Description of Surfactants and How they Work
Surfactants are a class of chemical compounds that are the workhorse ingredients in many household and commercial products. Surfactants are found in laundry detergents, fabric softeners, hard surface cleaners, disinfectants, hand soaps, body washes, shampoos, creams and lotions just to name a few.
What distinguishes surfactants from other compounds is that they have sort of a split personality. Their molecules possess a water loving or hydrophilic end and a water fearing or hydrophobic end. The fact that the hydrophobic end is also oil loving or lipophilic is important to how they function. They are usually represented in pictures by a circle with a line sticking out from it like in Figure A. The circle represents the hydrophilic head of the surfactant molecule and the line represents its hydrophobic tail.
The word “surfactant” is a combination of parts of the words “SURFace ACTive ageNT”. They are considered surface active because if you slowly add a surfactant to water its molecules start to accumulate at the surface with their hydrophilic heads in the water and their hydrophobic tails sticking straight out into the air, like in Figure B. Because the molecules are small this is not visible to the eye, but a simple kitchen experiment will allow you to see the effect of this behavior. If you cover the surface of a bowl of water with ground pepper then add one drop of hand dishwashing detergent into the middle, the pepper floating on the surface will quickly retreat to the edges of the bowl as the surfactant molecules from the dish detergent accumulate on the surface and push them out.
If you continue adding surfactant to water, the surface will eventually become saturated with surfactant molecules. When there is no more room on the surface, any additional molecules added to the water are forced to enter the water below its surface. When this occurs the hydrophobic tails of the molecules tend to huddle together creating a water free environment. This structure is known as a micelle. Micelles are often spherical, as shown in Figure C, but can take other shapes, with the tails all huddled inside and the hydrophilic heads on the surface in contact with the water. The concentration at which micelle formation occurs is called the critical micelle concentration or the CMC.
Everyone knows that water and oil don’t mix, at least not for long, but micelles provide an oil friendly environment that changes that. This is because the hydrophobic tails inside the micelle are also oil loving or lipophilic. Water containing surfactant at a concentration above the CMC is effective at removing oily soils because the oil migrates into the oil friendly environment inside of the micelles and is then carried away when the water is removed. For example, when you add laundry detergent to your washing machine to clean your clothes the oily soils migrate into the interior of the micelles, created by the surfactants in the detergent, and are carried away from the clothes by the water when the washing machine drains.
Additionally, surfactants work by reducing the surface tension of water. If you’ve seen water droplets bead up on a hydrophobic surface, like the hood of a freshly waxed car, you’ve seen surface tension in action. Water molecules are attracted to each other through a force called hydrogen bonding. The water molecules in the interior of the droplet pull the molecules on the surface toward the center. This pulls the droplet into a tight sphere and creates a tension on its surface. When the water droplet contains surfactant, the hydrophobic tails sticking out from its surface can absorb onto the hydrophobic surface making it more hydrophilic. As more and more of the surfactant tails absorb onto the surface they pull the droplet out of its spherical shape and cause it to spread across the surface. This pull by the surfactant molecules overcomes the pull of the molecules in the center of the droplet, reducing the surface tension. This spreading out or “wetting” of the surface creates improved contact between the water and the soil on surface, making it a better cleaner.