What is a surfactant?

A surfactant, also called surfactant or surfactant, is a chemical substance capable of reducing the surface tension of a liquid, which improves its ability to spread over a larger surface area . They do this by creating a layer of molecules on the surface of the liquid or at the interface between two liquids.

Due to their ability to reduce surface tension, surfactants facilitate the dispersion of one liquid phase within another with which it is immiscible. As a consequence, they allow the formation and stabilization of emulsions between two liquids, which is why they are also called emulsifiers.

Structure of surfactants

Surfactants are characterized by being amphiphilic molecules, which means that one part of the molecule is hydrophilic (has an affinity for water) while the other is hydrophobic or lipophilic (has an affinity for fats).

polar head

The hydrophilic part of the molecule always contains one or more polar functional groups concentrated in a relatively small portion of the molecule, often referred to as the polar head.

The polar head is always capable of forming one or more hydrogen bonds with water and with other similar polar protic solvents.

In some cases, the polar head has neutral polar functional groups such as hydroxyl groups, carboxylic acid esters, ethers, or a combination thereof. In others, it has ionizable groups such as carboxylates and even inorganic acid esters such as sulfuric acid (sulfates) and phosphoric acid (phosphates) that are ionized. Some polar heads also possess groups with quaternary amines (ammonium cations).

the hydrophobic tail

The hydrophobic or lipophilic part of the molecule generally consists of a long hydrocarbon chain that may or may not contain unsaturations, as it may or may not contain aromatic groups and other nonpolar organic functional groups.

Due to the fact that they are long nonpolar carbon chains, this part of the molecule is often called the nonpolar tail.

How surfactants work

As mentioned above, surfactants act by partitioning at the interface between a liquid (commonly water) and another phase, which can be a nonpolar liquid such as oil or a gas such as air. This decreases the surface tension at the interface, facilitating the mixing of both phases and the formation of an emulsion. In the event that the phases are water and oil, the emulsion can be formed in two different ways:

• The oily phase is dispersed in the form of droplets within an aqueous matrix, in which case there is an oil-in-water emulsion (as occurs, for example, in mayonnaise).
• The aqueous phase is dispersed in an oil matrix, in which case you have a water-in-oil emulsion (as in the case of butter).

The way in which surfactants facilitate the formation of either of these two types of emulsion is basically the same. The emulsifier molecules are distributed at the interface between water and oil, distributing in such a way that the hydrophilic polar head is dissolved in the aqueous phase, while the lipophilic tail is dissolved in the oil.

If water is present in a larger proportion than oil, then small surfactant-coated oil droplets will form with the polar heads pointing outward.

If, on the contrary, there is a higher proportion of oil, the opposite happens: small water droplets coated with surfactant molecules are formed with the apolar tails pointing outwards.

Types of Surfactants

Surfactants are classified into four groups based on the characteristics of the polar head: nonionic, anionic, cationic, and switerionic or amphoteric surfactants.

Nonionic surfactants

They are those whose polar head does not contain ionizable polar groups, such as hydroxyl groups in alcohols, ester groups and ethers.

anionic surfactants

These contain acid groups or their alkaline salts that dissociate when dissolved in water, generating a negatively charged ion or anion. These charged groups are strongly hydrophilic (and therefore strongly lipophobic), since they can accept multiple hydrogen bonds from water and also interact via ion-dipole forces with water and any other polar solvent.

Soaps prepared by saponification of fats are examples of these surfactants.

Cationic surfactants

They are the opposite of anionic surfactants. In this case, the polar head has a positive charge (ie, it is a cation). In most cases, these surfactants are quaternary derivatives of ammonia and are in the form of salts with a halide as a counterion.

An example of this type of cationic surfactant is DSDMAC or disteryldimethylammonium chloride.

Switterionic or amphoteric surfactants

A switterion, also called an internal salt, is a chemical species that has an equal number of formal positive and negative charges in its structure and therefore has no net charge. Switterionic surfactants provide much greater polarity than nonionic surfactants; they also provide many of the benefits of cationic and anionic surfactants, but without the addition of a free counterion that remains dissolved in the aqueous phase.

Typical examples of switterionic surfactants are N-alkylamino acids and sultaines.

Examples of surfactants

• Sodium stearate is an anionic surfactant.
• Taurine is an example of a switterionic or amphoteric surfactant.
• 4-(5-dodecyl)benzenesulfonate is also an anionic surfactant.
• Polysorbates such as Tween 20 and Tween 80 are examples of nonionic surfactants or emulsifiers used in the food and personal care industries.
• Docusate (Dioctyl Sodium Sulfosuccinate).
• Alkyl ether phosphates.
• Benzalkaonium Chloride (BAC).
• Perfluorooctane (PFOS).

References

Piorr R. (1987) Structure and Application of Surfactants . In: Falbe J. (eds) Surfactants in Consumer Products. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-71545-7_2

Chang, Q. (2016). Surface of Solution . Colloid and Interface Chemistry for Water Quality Control, 161–174. https://doi.org/10.1016/b978-0-12-809315-3.00009-8

Free, ML (2016). The Use of Surfactants to Enhance Particle Removal from Surfaces . Developments in Surface Contamination and Cleaning, 595–626. https://doi.org/10.1016/b978-0-323-29960-2.00013-7

Guill, I. (2019, February 17). Surfactant and emulsifier, are they the same? Are they bad in cosmetics? Retrieved July 24, 2021, from https://mismimos.es/tensioactivo-y-emulsionante-es-lo-mismo/

Nakama, Y. (2017). Surfactants . Cosmetic Science and Technology, 231–244. https://doi.org/10.1016/b978-0-12-802005-0.00015-x