Kluthe Magazine
Water beads up on many surfaces. After a rain shower, tiny droplets can still be seen on windows for a long time. This occurs because thin water films contract into droplets. All these are effects of surface tension, which becomes a problem when objects need to be uniformly wetted. In surface technology, this occurs during cleaning or coating of parts. Chemistry provides a solution through surfactants that are used as chemical additives. This article explains how they work.
What Causes Surface Tension?
Water consists of molecules where an oxygen atom holds two hydrogen atoms. The centers of these atoms form a triangle. Because the electrons from the hydrogen atom’s shells are pulled toward the oxygen, the molecules develop a positively charged side and a negatively charged side. Opposite charges attract each other. As a result, water molecules stay in close contact.
At the boundary between water and air, oil, or a solid, the water molecules on the outer side lack neighbors. Inside, the attractive forces act, causing the water molecules at the surface to form a film. Small amounts of water then form droplets. In the air or a water-repellent liquid, the droplets become perfectly round. How far they spread on solid surfaces depends on the surface type. To ensure that the droplets form an even layer, chemistry must provide substances that neutralize these attractive forces. These substances are called surfactants.
How Do Surfactants Work?
Scientists have developed substances whose molecules consist of an electrically charged and an electrically neutral part. The first part attracts water (hydrophilic), and the second repels water (hydrophobic). The water-attracting part replaces the neighboring molecules of the liquid on the outside. The water-repelling part interacts with the environment. The surfactant molecules break up the film that holds droplets together. The first substances to do this were simple soaps, created when fats and bases mix. Fatty waste reacts with sodium hydroxide to form molecules that repel water on the fatty side. Where the sodium atom from the base attaches to the molecule, the molecules become water-attracting.
Chemistry has developed many substances that act like soap, known as surfactants. You can see how quickly surfactant molecules break up the water surface film with a simple kitchen experiment. Fill a plate or bowl with water. Sprinkle pepper on top to show where surface tension exists. Dip your fingertip in dish soap. Touch the water surface and watch the pepper scatter.
Where Are Surfactants Used in Industry?
As an Additive in Surface Technology
Surface technology uses surfactants as additives for water-based process fluids. Key applications include surface preparation, electroplating, protective coating, and dip coating. The surfactant molecules ensure chemicals reach all surface areas evenly and work effectively. The level of surfactants in the fluid is quite low, typically less than one percent.
For Parts Cleaning
At higher concentrations, surfactants work as cleaning agents. They act as dissolvers for oil and grease contamination. In the cleaning bath, they surround dirt particles, allowing water to rinse them away from surfaces. For effective cleaning chemistry, users must maintain the concentrations specified by manufacturers. Too much causes excessive foam; too little results in poor cleaning. Cleaners can only trap a limited amount of dirt before becoming ineffective. Regular monitoring and adjustment of concentration is necessary.
As Emulsifiers for Coolant Lubricants
Just as surfactants hold dirt in water, they can also disperse tiny oil droplets throughout it. This property is used in metalworking, where water-mixed coolants reduce friction and remove heat.
Other Applications for Surfactants
Horticulture
Just as surfactants hold dirt in water, they can also disperse tiny oil droplets throughout it. This property is used in metalworking, where water-mixed coolants reduce friction and remove heat.
Plant Protection
Plant leaves naturally repel water, which helps rainwater run off quickly, thus drying the surface. The lotus effect demonstrates this clearly. For pesticides to stick to leaves and work properly, they include substances that reduce surface tension.
Firefighting
Firefighting often requires getting water to the fire’s source, where it evaporates and removes heat from burning materials. Water-repellent materials like coal, many fabrics, or certain building materials cause water to roll off unused and create more embers inside. Adding a surfactant helps water soak into the embers. A special fitting called an injector goes attaches to the hose line. The flowing water creates suction there, pulling in surfactant and mixing it with water. The same principle creates foam for firefighting. In this case, more surfactant, called foam concentrate, gets pulled in. Foam tubes at the end of the water line mix in air through suction.
