Surfactants are essential ingredients in many everyday products, ranging from personal care items such as shampoos and shower gels to industrial products such as detergents, emulsifiers and lubricants. Their unique ability to reduce surface tension between different substances allows for increased product efficacy and improved overall performance.
What are surface and interfacial tensions?
All liquids – water, organic solvents, oils, etc. – are characterised by strong intermolecular cohesive forces. Molecules within the bulk are subject to this attractive force in all directions. However, the liquid molecules at the surface, situated between the liquid and a gas such as air, do not have neighbours of the same type on all sides; therefore, they are subject to a resultant attractive force directed towards the bulk of the liquid, which causes a surface ‘film’ to form. This functions like an elastic membrane on which a tensile force acts in all directions along the surface, i.e. surface tension. A similar situation occurs between two immiscible liquids or two different phases. A so-called phase boundary forms between them. In this case, it is called interfacial tension.
Surfactants
Surfactants are chemical substances that reduce the surface tension of liquids. They are amphiphilic molecules with distinct hydrophobic and hydrophilic components. Depending on the chemical nature of the hydrophilic group (and in particular on its charge), surfactants are classified as non-ionic, anionic, cationic and amphoteric.
The primary function of surfactants is to reduce surface and interfacial tension and to stabilise interfacial surfaces. Without surfactants, tasks such as washing would be difficult, and products such as mayonnaise and ice cream might not exist. Therefore, optimising surfactants for various applications is crucial, and measurements of surface and interfacial tension play a significant role in this process.
Mechanism of action of surfactants
Surface tension is a measure of the mutual attractive force between liquid molecules. Liquids such as water are characterised by high surface tension due to strong intermolecular forces.
When a surfactant is added to a multiphase system, its molecules begin to self-assemble, positioning themselves such that the hydrophobic part is in the non-polar phase and the hydrophilic part in the polar phase. This weakens the cohesive attraction between liquid molecules, reducing the energy required to expand the surface.
The adsorption of surfactants breaks the strong intermolecular interactions that generate surface tension.
Surfactants in industry
The ability of surfactants to reduce surface and interfacial tension is one of the most important parameters in industrial chemistry.
Agrochemicals
Thanks to their ability to reduce surface tension, surfactants play a key role as wetting agents in agriculture. Added as adjuvants, e.g. to pesticides, they significantly influence the effectiveness of spraying. By reducing the surface tension of aqueous formulations, surfactants improve wetting and the spread of the solution across the leaf surface. The contact angle between the pesticide solution and the leaf is reduced, and adhesion is improved, which consequently facilitates the penetration of the pesticide into the plant’s vascular tissue.
Read: Wetting agents in foliar fertilisers
Read: Wetting agents in pesticide formulations
Textile industry
In the dyeing process, surfactants act as non-foaming wetting agents, ensuring uniform impregnation, evening out the colour during dyeing and dispersing unfixed dye during subsequent soaping, thereby improving its fastness. Surfactants facilitate the use of very fine-grained colouring pigments. This allows for a more economical use of textile dyes, whilst reducing the amount of residues.
Detergents
Surfactants are one of the main components of detergents and significantly influence cleaning effectiveness. By reducing surface tension, they facilitate the interaction of water with dirt or oil. The effectiveness of detergents depends on the type and quantity of surfactants they contain, as this directly affects the wetting properties of the product. A larger contact surface area intensifies the cleaning process.
Automotive industry
In manufacturing environments, surfactants are frequently used in aqueous cleaning systems. They enable the removal of oils, greases and other contaminants from metal, plastic or composite surfaces prior to bonding, painting or coating.
