Surfactant
- Also known as
- surface-active agent, tenside
A surfactant, short for surface-active agent, is a compound that lowers the surface tension of a liquid, or the interfacial tension between two phases, by adsorbing at the boundary between them. Each molecule has a water-attracting head and a water-repelling tail.
What is a surfactant?
A surfactant is a substance that concentrates at surfaces and interfaces and changes their properties, most obviously by lowering surface tension and interfacial tension. The word is a contraction of surface-active agent.
Every surfactant molecule is amphiphilic: one part, the head group, is attracted to water, and another part, usually a hydrocarbon chain called the tail, is attracted to oil and repelled by water. That split personality is what drives a surfactant to the boundary between water and air, water and oil, or water and a solid, where each end can sit in the environment it prefers.
How surfactants work
At low concentration, surfactant molecules gather at interfaces with their heads in the water and their tails pointing away from it. Replacing water molecules at the interface lowers the energy of that interface, so the surface tension falls as more surfactant is added.
Once the interfaces are crowded, additional molecules associate with one another in the bulk liquid, forming micelles. The concentration at which this begins is the critical micelle concentration. Above it, surface tension stays roughly constant and the extra surfactant goes into micelles, which can hold oily material in their cores.
The four main types
Surfactants are usually classified by the electrical charge of the head group in water.
| Type | Head group charge | Common examples |
|---|---|---|
| Anionic | Negative | Soaps (fatty acid salts), sodium lauryl sulfate, sodium laureth sulfate |
| Cationic | Positive | Quaternary ammonium compounds |
| Nonionic | None | Alcohol ethoxylates, alkyl polyglucosides |
| Amphoteric | Positive and negative, depending on pH | Betaines such as cocamidopropyl betaine |
Surfactants can also be grouped by origin. Most are made by chemical synthesis from petroleum or plant oils; a bio-based surfactant is made from renewable feedstocks, and a biosurfactant is produced directly by microorganisms.
What surfactants do
The same basic behavior at interfaces produces several practical effects, and a surfactant is usually chosen for the one that matters most in the application:
- Wetting: helping a liquid spread over a surface instead of beading up.
- Emulsification: stabilizing droplets of one liquid in another, as an emulsifier.
- Dispersion: keeping solid particles separated in a liquid, as a dispersant.
- Detergency: lifting oily soil from a surface and holding it in the wash solution.
- Foaming: stabilizing gas bubbles, or, with the right choice, avoiding foam.
- Solubilization: carrying poorly water-soluble substances inside micelles.
How surfactant performance is compared
Three measurements come up constantly. The critical micelle concentration shows how much surfactant is needed before interfaces are saturated. The surface tension reached at the CMC shows how far the surfactant can lower it. The hydrophilic-lipophilic balance (HLB) indicates whether a nonionic surfactant favors water or oil, which guides the choice of emulsifier. None of these replaces testing in the actual formulation, where water hardness, salinity, temperature and other ingredients all change how a surfactant behaves.
Where surfactants are used
Surfactants are among the most widely used chemical ingredients. They are the active cleaning agents in household and industrial and institutional cleaning products; the basis of agricultural spray adjuvants; additives in paints and coatings; components of fracturing fluids and enhanced oil recovery formulations; and, as collectors and frothers, part of the reagent scheme in froth flotation.
Frequently asked questions
Yes. Soap is the sodium or potassium salt of a fatty acid, and it is an anionic surfactant. It is one of the oldest surfactants in use. Its main drawback is that it reacts with the calcium and magnesium in hard water to form insoluble scum, which is one reason synthetic surfactants replaced it in most cleaning products.
A surfactant is an ingredient; a detergent is a formulated cleaning product. A detergent contains one or more surfactants together with other ingredients such as builders, chelating agents, enzymes and solvents.
Emulsifier describes a job, not a type of molecule. Most emulsifiers are surfactants, chosen because they stabilize the boundary between oil and water droplets. The same surfactant might be called a wetting agent in one product and an emulsifier in another.
Sources
- surfactant, IUPAC Compendium of Chemical Terminology (Gold Book)
- Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012