What Are Surfactants and How Do They Work?

Introduction to Surfactants

Surfactants (aka surface-active agents) are chemical multi-purposers that are used in industry and daily life. They are molecules that lower the surface tension of two things, like liquid and gas or liquid and liquid, and they mix more easily. Surfactants are as old as time and earliest uses were natural soaps from animal fats and vegetable oils. They are used all the time in products ranging from housecleaning to pharmaceuticals, and their uses and value are endless.

Chemical Structure of Surfactants

Surfactants are made up of two different molecules at the molecular level, one with a hydrophilic (water-attracting) head and one with a hydrophobic (water-repelling) tail. This bifunctional character makes it possible for surfactants to bond with water and oil in the same process, such as for emulsification and washing.

Classification by Head Group

Anionic Surfactants

Anionic surfactants are surfactants that entrain in water to release negative-charged active groups. These are found extensively in industrial, agricultural and household goods. From the hydrophilic head type, anionic surfactants are grouped into carboxylates, sulfonates, sulfates and phosphates, with sulfonates producing the most and being most widely used.

Cationic Surfactants

Cationic surfactants are positively charged surfactants. They typically have a long hydrocarbon chain and a hydrophilic group, which usually carries a positive charge (as carried by nitrogen, sulphur or phosphorus atoms). According to their chemical composition, there are different classes of cationic surfactants, including amine salts, quaternary ammonium salts and heterocyclic surfactants. The most widespread of these are quaternary ammonium salts, which are stable and antibacterial and suitable for acidic media.

Nonionic Surfactants

Non-ionic surfactants are the surfactants that do not ionize in water. Their molecular structure is often composed of hydrophilic and hydrophobic groups. The classification of non-ionic surfactants according to hydrophilic groups includes a broad range of polyethylene glycols, polyols, and nitrogen-containing ones.

Amphoteric (Zwitterionic) Surfactants

Amphoteric (zwitterionic) surfactants are surfactants whose molecular structure includes both cationic and anionic molecules. What's special about them is that they can have the properties of cationic surfactants in acidic conditions, anionic under alkaline conditions, and non-ionic at the isoelectric point.

The surfactants that are usually most useful are soaps, detergents and emulsifiers, which are all specialized to specific applications.

How Do Surfactants Work?

The activity of surfactants relies almost exclusively on their specialised molecular geometry, consisting of hydrophilic and hydrophobic elements. When surfactant molecules are absorbed at the liquid interface (gas-liquid, liquid-liquid interfaces), their hydrophilic groups engage water molecules while their hydrophobic groups face air or oil, reducing the interfacial tension significantly.

In particular, surfactants work as follows:

Reduction of Interfacial Tension

Surfactant molecules contain hydrophilic and hydrophobic groups that enable them to form oriented adsorption films on liquid-liquid, liquid-gas or liquid-solid interfaces that reduce the interfacial stress. For instance, when applied at the oil-water interface, surfactants decrease the interfacial tension between oil and water so that oil droplets dissolve into smaller particles to form stable emulsions.

Fig. 1. Surface tension curve versus surfactant concentration

Emulsification

When dissolved in water and oil, surfactant molecules sink to the oil-water interface; their polar atoms sink into the water and their non-polar atoms into the oil. This minimises the interfacial pressure so that oil droplets can disperse evenly in water and create stable emulsions.

Fig. 2. Effect of surfactant phase behavior on emulsification

Cleaning Action

The hydrophobic part of the surfactant bonds to dirt or grease during cleaning while the hydrophilic part is interacting with water to remove dirt from surfaces.

Fig. 3. Surfactants on cleaning oil contaminations

Wetting Action

The surfactants reduce the pressure between the liquid and the solid surfaces so that the liquids can wet all the solids.

Dispersion

Surfactants may cover solid particles so they don't clump and segregate, making uniform dispersion.

Foaming and Defoaming

Surfactants can dilute water so air can enter the liquid and create foam. Simultaneously, some surfactants act to break down foam and get rid of it.

Fig. 4. Schematic representation of oil globule at the entrance of the foam film surface

Solubilization

When the concentration of surfactants reaches the critical micelle concentration, they form micelles that encapsulate hydrophobic substances within, achieving solubilization effects.

Fig. 5. Micellar solubilization technique for solubility enhancement of poorly soluble drugs

References

1. Shahmarvand, S., et al. "Experimental investigation on the stability of foam using combination of anionic and zwitterionic surfactants: A screening scenario to obtain optimum compound." Journal of Dispersion Science and Technology 45.10 (2024): 2016-2027.
2. Kaizu, K., et al. "Effect of surfactant phase behavior on emulsification." Journal of colloid and interface science 466 (2016): 138-149.
3. Rakowska, J., et al. "Experimental study on surface activity of surfactants on their ability to cleaning oil contaminations." Journal of cleaner production 144 (2017): 437-447.
4. Deotale, S. M., et al. "Foaming and defoaming–concepts and their significance in food and allied industries: a review." Discover Chemical Engineering 3.1 (2023): 9.
5. Khan, K. U., et al. "Overview of nanoparticulate strategies for solubility enhancement of poorly soluble drugs." Life Sciences 291 (2022): 120301.