Surfactants are surface active agents, which means “active on the surface.” Surfactant reduces surface and interfacial tension by adsorbing to surfaces and interfaces. “interface” refers to the link between two immiscible phases, while “surface” refers to a gas, such as air.
Surfactants offer significant benefits in various textile wet processes. A surface active chemical accumulates at the surface or interface. An interface is the point of contact between two substances. Surfaces refer to the interface between two substances that are not in the same phase. Surface active compounds reduce a substance’s surface tension by limiting intermolecular interactions. Surfactants used in industrial applications significantly reduce surface tension at low concentrations.
Chemically, surfactants are amphipathic compounds. The molecule exhibits both polar and non-polar properties in different regions. A surfactant molecule exhibits both hydrophilic and hydrophobic properties. Surfactant molecules have a polar “head” and a non-polar “tail” structure. Surfactants used in an aqueous medium often have a hydrophobic group, which can be a hydrocarbon, fluorocarbon, or siloxane chain with a suitable length. The hydrophilic group is polar and can be ionic or non-ionic.
The hydrophobic end of the molecule moves away from the water, while the hydrophilic end remains near the water. Surfactants surround and remove hydrophobic dirt and oil from surfaces.
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How does surfactant work?
When an adequate amount of surfactant molecules are added to a solution, they begin to combine. Surfactants first gather on the water’s surface to create a layer at the water-air interface, with water-loving (hydrophilic) heads towards the water and water-repelling (hydrophobic) tails in the air. This leads to decreased water surface tension, one of the surfactants’ major features.
Different types of aggregates can be created, including spherical or cylindrical micelles and lipid bilayers. Furthermore, the shape of the aggregates is mostly determined by the chemical structure of the surfactants (a balanced size between the hydrophilic head and hydrophobic tail).
Surfactants, in general, function by breaking down the contact between oils, water, and dirt. Oils and dirt are also suspended, making them easy to remove.
For more information watch this video:
https://youtu.be/UU9ij_z6jXQ?si=tdBiIzFMqkIyTiYy
Natural Surfactants
Polar lipids are naturally occurring surfactants. These are abundant in all living things. Surface active agents in biology provide similar functions as surfactants in technology, including overcoming solubility issues and emulsifying, dispersing, and modifying surfaces.
Examples of this in biological systems involve: Bile salts effectively dissolving hydrophobic components in the blood, whereas phospholipids form ordered bilayers of surfactants, which serve as cell membranes. A prime example of a phospholipid is lecithin.
Types of surfactants
Cationic
Their hydrophilic component is positively charged. In laundry detergents, cationic surfactants (positive charge) enhance the packing of anionic surfactants (negative charge) at the stain/water interface. This effectively reduces the interfacial tension between dirt and water, resulting in an enhanced dirt removal mechanism. They are very effective at removing oily stains.
They can also provide softening, antistatic, soil-repellent, antimicrobial, and corrosion-inhibitory properties when applied to surfaces. Typical applications include fabric softeners and antistatics. Cationic surfactants typically have a halide or methyl sulfate as their counter ion.
This class mostly consists of nitrogen compounds, including fatty amine salts and quaternary ammoniums, with long alkyl chains derived from natural fatty acids. Surfactants are often more expensive than anionics due to the high-pressure hydrogenation reaction required for manufacture. As a result, they are only used in two cases where there is no cheaper substitute: (1) as a bactericide, and (2) as a positively charged substance that can adsorb on negatively charged substrates to produce antistatic and hydrophobic effects, which are often commercially important, such as in corrosion inhibition.
Anionic
In aqueous solutions, anionic surfactants’ hydrophilic groups dissociate to form anions. These are the most widely used surfactants. Anionic surfactants account for approximately 50% of global output. In solution, the head is charged negatively. This type of surfactant is commonly used in laundry, dishwashing products, and shampoos due to its effective cleaning characteristics. The surfactant effectively repels dirt and removes fabric softener residues.
They are highly sensitive to water hardness. Common counter ions include sodium, potassium, ammonium, calcium, and protonated alkylamines. Sodium and potassium have water solubility, while calcium and magnesium have oil solubility. Amine/alkanol amine salts produce compounds that are both oil and water-soluble.
Nonionic
Nonionic surfactants are those that do not break down into ions in aqueous solutions. Non-ionic surfactants have a neutral head (neither positive nor negative charge). The relative intensity of the head and tail determines whether this class of surfactants is hydrophilic (more water-loving) or lipophilic (more oil-loving). This is known as the Hydrophilic-Lipophilic Balance (HLB), and it is an important feature of non-ionic surfactants whose optimal behavior varies greatly depending on the application.
These surfactants lack an electrical charge, making them resistant to water hardness deactivation. They effectively remove oil from laundry, home cleaners, and hand dishwashing liquids. Cleaning detergents commonly use fatty alcohol polyglycosides and alcohol ethoxylates, among other groups. The long-chain alcohols have some surfactant characteristics. The most common alcohols are fatty alcohols, cetyl alcohol, stearyl alcohol, cetostearyl alcohol (a mixture of cetyl and stearyl alcohols), and oleyl alcohol.
Zwitterionic
Zwitterionic, or amphoteric, surfactants have both positive and negative charges on their hydrophilic end. They contain both cationic and anionic centers inside the same molecule. It has a net charge of zero.
The acidity or pH of the water determines whether they are anionic (negatively charged), cationic (positively charged), or non-ionic (no charge) in solution. They are compatible with all surfactant classes and remain soluble and effective at high concentrations of electrolytes, acids, and alkalis.
Zwitterion is a group that is distinguished by its superior dermatological qualities. They produce minimal eye and skin discomfort. These are ideal for use in shampoos and other personal care products.
Zwitterionic (amphoteric) surfactant contains both cation and anion centers within the same molecule. The cation component involves primary, secondary, or tertiary amines or quaternary ammonium cations. The anion component might vary and may include sulfonates.
Dynamics of surfactants in solution
In an aqueous solution, surfactant behaves similarly to electrolytes, but at larger concentrations, their behavior changes significantly. Micelles, organized aggregates of many molecules, are responsible for this activity.
Surfactant’s lipophilic components stay within the aggregate, whereas hydrophilic components face the watery medium. Micelles in aqueous solution are thought to constitute a compromise between alkyl chains avoiding energy-inefficient contact with water and polar sections maintaining touch with the environment.
Surfactant often dissolves in water and adsorb at air-water interfaces or oil-water interfaces. The water-insoluble hydrophobic group may stretch out of the water phase that is in bulk with the air or oil phase. The water-soluble head group is associated with the water phase. The surfactant’s surface alignment enhances the water’s characteristics at the water-oil or air interface.
Surfactants have the unique ability to create micelles, which are aggregates of monomers in solution. Micelle production, also known as micellization, is an alternate adsorption mechanism in which the hydrophobic group is removed from the water at the interface, lowering the system’s free energy. Surfactant molecules in micelles behave differently than free monomers in solution, making this an important process. Surfactant monomers are the only ones present on the surface, while the concentration of free monomers in the solution regulates interfacial tension, wetting, and foaming.
Micelles operate as a reservoir for monomers and surfactants. The rate at which a surfactant molecule exchanges between micelle and bulk solution varies significantly based on its size and shape.
The micelle is a polar aggregation with excellent water solubility and minimal surface activity. When a surfactant adsorbs from an aqueous solution to a hydrophobic surface, its hydrophobic group aligns with the surface while exposing the polar group to water. The surface becomes hydrophilic, reducing interfacial tension with water. Adsorption at hydrophilic surfaces generally leads to more complex surfactant complexes.
Surfactant and detergency
Detergency can be defined as “the surfactant action that adsorbs at interfaces and reduces the energy needed to effect the removal, thereby causing or aiding in the removal of foreign material from solid surfaces.” Wetting agents with fast diffusion and adsorption at appropriate surfaces are typically the most effective. The majority of surfactant manufacture is focused on ingredients that are mixed into commercial detergent formulas.
The greatest number of surfactants are still made from alkyl sulfates, alkyl-aryl sulfonates, and non-ionic polyethylene oxides. Not all surfactants produce suitable detergents. To be called a good detergent, a surfactant must be a good wetting agent, able to displace soil elements into the washing solution, a good solubilizing agent, and an effective anti-reposition agent.
During the detergency process, surfactant molecules are deposited on both the soil and fabric surfaces. Surfactant adsorption has two purposes in soil removal. They reduce the attraction between soil and fabric by adhering to both.
This method simultaneously loosens soil from the cloth and deflocculates particles, resulting in colloidal particles and stabilized aqueous dispersion. A fine and steady dispersion of soil in the wash liquid is less likely to adhere to the cloth during the wash cycle compared to a coarse and unstable dispersion. Enhancing surfactant adsorption can improve its detergency without an electrolyte.
Applications of surfactants
- Surfactant replacement treatment can cure several lung diseases, including meconium aspiration syndrome, child pneumonia, and congenital diaphragmatic hernias.
- Quaternary ammonium surfactants (quats) are cationic surfactants with strong germicidal properties. They are also used as fabric softeners in detergents.
- They are used in various food manufacturing processes, including cholesterol extraction, oil solubilization, emulsification, component separation prevention, and nutrient solubilization.
- They are utilized in firefighting and pipelines as liquid drag reduction agents.
- Alkali surfactant polymers are also employed for transporting oil in oil wells.
- They are sometimes added to car engine lubricants to help keep particles from sticking to engine components.
- They are also often employed to reduce corrosion during ore flotation.
- They are also employed to prevent corrosion, enhance oil flow through porous rocks, and produce aerosols.
References
- https://byjus.com/jee/surfactants/
- https://www.sciencedirect.com/topics/chemistry/surfactant
- https://www.dispersa.ca/blog/what-are-surfactants-and-how-do-they-work/
- https://colloidmueg.weebly.com/uploads/2/5/7/3/25736627/puu-0_3110_surfactants_types_and_uses.pdf
- https://www.lcpe.uni-sofia.bg/eufoam2010/downloads/ts_lectures/Surfactants_ND.pdf
- file:///D:/C%20Data/Downloads/RSC-2001-Marangoni-Schramm.pdf
