What are surfactants



Surfactants (from lat. tensus, van tenderetension, tighten), are substances that increase the surface tension of a liquid or the interfacial tension between two phasesreduce and enable or support the formation of dispersions.

Surfactants mean that two liquids that are actually immiscible with one another, such as oil and water, can be finely mixed. Surfactants are generally understood to mean washing-active substances (detergents) that are contained in detergents, dishwashing detergents and shampoos. They were developed in the first half of the 20th century and have largely replaced the traditional surfactant soap. When used in food technology, surfactants are called emulsifiers.

properties

         

The function of surfactants can be explained by their molecular structure. Surfactants generally consist of one hydrophobic, water-repellent hydrocarbon residue and a hydrophilic, "water-loving" part of the molecule; they are said to be amphiphilic. In the following figures, the "water-loving" parts of the molecule are marked with a minus sign or a red dot.

If surfactants are added to water, the individual surfactant molecules arrange themselves at a critical concentration and usually form very small droplets called micelles within the water. The surfactant molecules align themselves in such a way that the hydrophobic ends collect inside the droplets and the hydrophilic ends are arranged in the direction of the water. However, mostly depending on the concentration, the formation of so-called worm-like micelles is also possible, and water can also be encapsulated by a surfactant double layer.

The surfactants form a thin layer on the surface of the water and thus lower the surface tension of the water. Here, too, the surfactant molecules arrange themselves. The hydrophilic ends protrude in the direction of the water, the hydrophobic ends protrude in the direction of the air.

  • The influence of surfactants on surface tension can be demonstrated by a simple experiment: a light object (for example a pin) is placed on a water surface (without any surfactants contained in it). Normally this will not go under, but rather is carried by water due to the high surface tension. If you then add small amounts of a surfactant (e.g. washing-up liquid), the surface tension is reduced so much that it can no longer counteract the weight acting on the surface of the water due to the higher density of the object and the object goes under.

Effect as Emulsifier: Surfactants have the effect that two immiscible liquids (e.g. oil in water) can mix together to form an emulsion. Due to the amphiphilic character of the surfactant, they penetrate the oil with their fat-soluble part. Due to the hydrophilic part, the oil droplet, which has now been created by stirring, can be "kept in solution" in the aqueous environment.

Of Wetting agents one speaks when the aim of using surfactants is not to mix two phases, but to reduce the interfacial tension between a solid surface and a liquid. Instead of forming droplets, water flows more easily from a surface. In the photo laboratory, surfactants are used as wetting agents to prevent dry spots on photo materials after the final wash.

Surfactants support this Detachment of small solid particles of solid surfaces, e.g. B. the removal of dirt particles on clothing. The Solid particles are "held in suspension" in the water. It supports the formation and maintenance of a so-called suspension. The surfactants accumulate around the solid particles in a similar way to an emulsion and prevent them from clumping together, sinking (= sedimentation) and re-adherence to other solid surfaces that are themselves covered with a "surfactant layer". The solid particles coated with the surfactant form a so-called colloid with the water. Dispersants are surfactants that keep the solid pigments suspended in a (still) liquid paint.

The Formation of foam is due to the properties of surfactants. The surfactant molecules form a two-layer film with the hydrophobic ends of the surfactants forming the two surfaces. The hydrophilic ends point into the film. A strong foam development can be disturbing when using or in the presence of surfactants, which is why defoamers are used.

Anionic surfactants form insoluble precipitates with alkaline earth metal cations, which are generally referred to as lime soaps. Lime soaps no longer have the properties of "soluble" surfactants described above. The Formation of lime soaps is due to the hardness of the water. If surfactants are used as detergents, a softener is included in the detergent.

Structure of surfactants

All surfactants are made up of a non-polar and a polar part (functional groups) (→ polarity). An alkyl group always serves as the non-polar part. The polar part can have different structures and is summarized in the table.

Often an attempt is made between natural and synthetic To distinguish between surfactants. This distinction is not easy and does not always make sense. One example of a naturally occurring surfactant is lecithin. Surfactants of natural origin are for example soaps that are made from natural raw materials (e.g. animal fats) by saponification. However, its production requires a profound chemical reaction. Synthetic surfactants are made from raw materials such as petroleum, but also sunflower oil, through numerous chemical reactions. However, more important than the origin of a surfactant is its biodegradability. Surfactants are regularly checked for their occurrence in drinking water, but not their degradation products, some of which are considerably more toxic. The health effects of surfactants have so far hardly been researched.

Use of surfactants

Surfactants in the food industry

Surfactants are mainly used in the food industry for ready-made baking mixes.

Surfactants in detergents and cosmetics

In detergents, dishwashing detergents, shampoos, shower gels, etc., surfactants are used to increase the "solubility" of fat and dirt particles that adhere to the laundry or to the body in water. They are the most important component in cosmetics to make oily skin cream.

Surfactants in biochemistry

In biochemistry, surfactants are used, among other things, to denature proteins and to solubilize membrane proteins:[1]

Surfactants in technology

Plastics technology: Surfactants have a special application in plastics technology. Aqueous surfactant solutions are used to test the susceptibility of polymer materials to stress cracking. Surfactants are also used to shorten the failure time of long-term tests; This is particularly useful for crack growth tests on polyethylene (e.g. full notch creep test for testing PE types that are used for pipelines).

The following wetting agents are mainly used in plastics technology:

  • Arkopal N100, N110 and N150
  • Igepal CO630
  • Teepol

Metalworking: Component in water-mixed cooling lubricants

photography: Prevention of drying spots and streaks during film development

Paper recycling: Detachment of the printing ink particles from the paper fibers and transport of the printing ink to the surface during deinking

They also play an important role in tool cleaning.

Surfactants in fire fighting

One method of fighting a fire is to extinguish it with "relaxed water", also known as "net water". H. Water with a greatly reduced surface tension. On the one hand, this has the advantage that the extinguishing water can penetrate better into burning materials such as wood or fabric and thus has an even better cooling effect. On the other hand, extinguishing water mixed with surface-active substances can be sprayed over a greater distance with the same pumping power due to its effect as a flow improver. However, the latter effect is not used consciously. Special foam concentrates (AFFF) for fighting liquid fires contain perfluorinated surfactants, which form a gas-tight liquid film between the material to be burned and the foam, which at the same time gives the foam carpet better sliding properties and thus enables larger liquid fires to be extinguished in the first place.

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  1. Calbiochem Booklet: Detergents (PDF)

See also

Categories: Dispersion (Chemistry) | Surfactant | cleaning supplies