Biosurfactants or amphiphilic compounds are a diverse group of molecules which consist of a polar (hydrophilic) head and a nonpolar (hydrophobic) tail.
This unique structure makes these compounds surface (interfacial) tension reducers, which is why they are called biosurfactants (biological surface active agents). Surface tension can be defined as the property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of the water molecules.
For example, washing dirty clothes with only water would not be very effective, while adding a little bit of soap will help make the clothes cleaner. That is because soap is also a surfactant (which initially was made from amphiphilic saponins derived from plants). Soap allows both better absorption of particles from the liquid and better removal of particles from the surface.
The surface tension of water is 72 mN/m; a good biosurfactant can reduce this value to ≤ 35 mN/m, more than twice.
Figure A
The surface and interfacial tension reduction, and other properties, are based on these molecules’ ability to form micelles: aggregations of biosurfactant molecules. The concentration at which micelles start forming is called the critical micelle concentration (CMC). When surfactants reach CMC, they are able to greatly reduce surface (interfacial) tension and therefore change the characteristics of the products containing these amphiphilic compounds (see figure on the right).
The benefit of such a mechanism is that once CMC is reached (for good biosurfactants this value is about 100 mg/L), the maximum effect is already reached and there is no necessity to use large amounts of a biosurfactant.
Although named ”biosurfactants”, unlike many chemically synthesized surfactants, their surface activity is not the only property of the molecules.
Figure B
They, depending on the type, have solubilizing, emulsifying, dispersing, detergent and stabilizing properties, opening a wide spectrum for potential applications.
For example, emulsifying properties are determined by the ratio between the hydrophilic and hydrophobic parts (HLB – hydrophilic-lipophilic ratio). The larger the hydrophilic portion is in comparison to the hydrophobic part, the higher the HLB is. Depending on HLB value, the compound would be effective in different applications (see figure B).
Biosurfactants also have a wide range of biological, physical and chemical functions including antibacterial, antiviral, antifungal, anti-inflammatory, anti-tumor, immunomodulating, chelating and many others.