SURFACTANTS AND BIOSURFACTANTS: WHAT IT IS FOR, EXAMPLES AND USES - BIOLOGY - 2024

A surfactant is a chemical compound capable of reducing the surface tension of a liquid substance, acting at an interface or contact surface between two phases, for example water-air or water-oil.

The term surfactant comes from the English word surfactant, which in turn is derived from the acronym of the expression surf ace active agent, which means in Spanish agent with interfacial or surface activity.

Figure 1. Structure of surfactants. Source: Major measure, from Wikimedia Commons

In Spanish the word "surfactant" is used, referring to the ability of a chemical compound to act on surface or interfacial tension. Surface tension can be defined as a resistance that liquids have to increase their surface.

Water has a high surface tension because its molecules are very tightly bound and resist separating when pressure is exerted on their surface.

For example, some aquatic insects, such as “cobbler” (Gerris lacustris), can move on the water without sinking, thanks to the surface tension of the water, which allows the formation of a film on its surface.

Figure 2. Insect capable of moving on water. Source: TimVickers, from Wikimedia Commons

Also, a steel needle stays on the surface of the water and does not sink, due to the surface tension of the water.

Structure and function of surfactants

All surfactant or surfactant chemical agents are amphiphilic in nature, that is, they have a dual behavior, since they can dissolve polar and non-polar compounds. Surfactants have two main parts in their structure:

• A hydrophilic polar head, akin to water and polar compounds.
• A lipophilic, hydrophobic nonpolar tail, akin to nonpolar compounds.

The polar head can be non-ionic or ionic. The surfactant tail, or apolar part, can be an alkyl or alkylbenzene carbon and hydrogen chain.

This very particular structure gives surfactant chemical compounds a dual, amphiphilic behavior: affinity for polar compounds or phases, soluble in water and also affinity for nonpolar compounds, insoluble in water.

In general, surfactant agents reduce the surface tension of water, allowing this liquid to expand and flow to a greater degree, wetting neighboring surfaces and phases.

What are surfactants for?

Surfactant chemicals exert their activity on surfaces or interfaces.

When dissolved in water, they migrate to the water-oil or water-air interfaces for example, where they can function as:

• Dispersants and solubilizers of insoluble or poorly soluble compounds in water.
• Humectants, as they favor the passage of water to insoluble phases in it.
• Stabilizers for emulsions of compounds insoluble in water and water, such as oil and water from mayonnaise.
• Some surfactants promote and others prevent foaming.

Biosurfactants: surfactants of biological origin

When the surfactant comes from a living organism, it is called a biosurfactant.

In a more strict sense, biosurfactants are considered as amphiphilic biological compounds (with dual chemical behavior, soluble in water and fat), produced by microorganisms such as yeast, bacteria and filamentous fungi.

Biosurfactants are excreted or retained as part of the microbial cell membrane.

Also some biosurfactants are produced by biotechnological processes, using enzymes that act on a biological chemical compound or natural product.

Examples of biosurfactants

Natural biosurfactants include plant saponins such as cayenne flower (Hibiscus sp.), Lecithin, bile juices from mammals or human lung surfactant (with very important physiological functions).

Additionally, amino acids and their derivatives, betaines and phospholipids, all these natural products of biological origin, are biosurfactants.

Classification of biosurfactants and examples

-According to the nature of the electrical charge in the polar part or head

Biosurfactants can be grouped into the following categories, based on the electrical charge of their polar head:

Anionic biosurfactants

They have a negative charge at the polar end, frequently due to the presence of a sulfonate group –SO ₃ ^-.

Cationic biosurfactants

They have a positive charge on the head, usually a quaternary ammonium group NR ₄ ⁺, where R represents a chain of carbon and hydrogen.

Amphoteric biosurfactants

They have both positive and negative charges on the same molecule.

Non-ionic biosurfactants

They do not have ions or electrical charges in their heads.

-According to its chemical nature

According to their chemical nature, biosurfactants are classified into the following types:

Glycolipid biosurfactants

Glycolipids are molecules that have in their chemical structure a part of lipid or fat and a part of sugar. Most of the known biosurfactants are glycolipids. The latter consist of sulfates of sugars such as glucose, galactose, mannose, rhamnose, and galactose.

Among the glycolipids, the best known are the rhamnolipids, bioemulsifiers that have been extensively studied, with high emulsifying activity and high affinity for hydrophobic organic molecules (which do not dissolve in water).

These are considered the most effective surfactants for the removal of hydrophobic compounds in contaminated soils.

Examples of rhamnolipids include surfactants produced by bacteria of the genus Pseudomonas.

There are other glycolipids, produced by Torulopsis sp., With biocidal activity and used in cosmetics, anti-dandruff products, bacteriostats and as body deodorants.

Lipoprotein and lipopeptide biosurfactants

Lipoproteins are chemical compounds that have in their structure a part of lipid or fat and another part of protein.

For example, Bacillus subtilis is a bacterium that produces lipopeptides called surfactins. These are among the most powerful surface tension reducing biosurfactants.

Surfactins have the ability to produce erythrocyte lysis (breakdown of red blood cells) in mammals. In addition, they can be used as biocides for pests such as small rodents.

Fatty acid biosurfactants

Some microorganisms can oxidize alkanes (carbon and hydrogen chains) to fatty acids that have surfactant properties.

Phospholipid biosurfactants

Phospholipids are chemical compounds that have phosphate groups (PO ₄ ^3-), attached to a part with a lipid structure. They are part of the membranes of microorganisms.

Certain bacteria and yeasts that feed on hydrocarbons, when growing on alkane substrates, increase the amount of phospholipids in their membrane. For example, Acinetobacter sp., Thiobacillus thioxidans, and Rhodococcus erythropolis.

Polymeric biosurfactants

Polymeric biosurfactants are high molecular weight macromolecules. The most studied biosurfactants in this group are: emulsifier, liposuction, mannoprotein and polysaccharide-protein complexes.

For example, the bacterium Acinetobacter calcoaceticus produces polyanionic emulsifier (with various negative charges), a very effective bioemulsifier for hydrocarbons in water. It is also one of the most powerful emulsion stabilizers known.

Liposan is a water-soluble, extracellular emulsifier, consisting of polysaccharides and Candida lipolytica protein.

Environmental sanitation

Biosurfactants are used in the bioremediation of soils contaminated by toxic metals, such as uranium, cadmium and lead (biosurfactants of Pseudomonas spp. And Rhodococcus spp.).

They are also used in bioremediation processes of soils and water contaminated by gasoline or oil spills.

Figure 3. Biosurfactants are used in environmental sanitation processes due to oil spills. Source: Ecuador Foreign Ministry, via Wikimedia Commons

For example, Aeromonas sp. produces biosurfactants that allow oil degradation or reduction of large molecules to smaller ones, which serve as nutrients for microorganisms, bacteria and fungi.

In industrial processes

Biosurfactants are used in the detergent and cleaner industry, since they enhance the cleaning action by dissolving the fats that dirty clothes or surfaces in the wash water.

They are also used as auxiliary chemical compounds in the textile, paper and tannery industries.

In the cosmetic and pharmaceutical industry

In the cosmetics industry, Bacillus licheniformis produces biosurfactants that are used as anti-dandruff, bacteriostatic and deodorant products.

Some biosurfactants are used in the pharmaceutical and biomedical industry for their antimicrobial and / or antifungal activity.

In the food industry

In the food industry, biosurfactants are used in the manufacture of mayonnaise (which is an emulsion of egg water and oil). These biosurfactants come from lectins and their derivatives, which improve quality and additionally taste.

In the agriculture

In agriculture, biosurfactants are used for the biological control of pathogens (fungi, bacteria, viruses) in crops.

Another use of biosurfactants in agriculture is to increase the availability of micronutrients from the soil.

References

1. Banat, IM, Makkar, RS and Cameotra, SS (2000). Potential commercial applications of microbial surfactants. Applied Microbiology Technology. 53 (5): 495-508.
2. Cameotra, SS and Makkar, RS (2004). Recent applications of biosurfactants as biological and immunological molecules. Current Opinions in Microbiology. 7 (3): 262-266.
3. Chen, SY, Wei, YH and Chang, JS (2007). Repeated pH-stat fed-batch fermentation for rhamnolipid production with indigenous Pseudomonas aeruginosa Applied Microbiology Biotechnology. 76 (1): 67-74.
4. Mulligan, CN (2005). Environmental applications for biosurfactants. Environmental Pollution. 133 (2): 183-198.doi: 10.1016 / j.env.pol.2004.06.009
5. Tang, J., He, J., Xin, X., Hu, H. and Liu, T. (2018). Biosurfactants enhanced heavy metals removal from sludge in the electrokinetical treatment. Chemical Engineering Journal. 334 (15): 2579-2592. doi: 10.1016 / j.cej.2017.12.010.