10.4.2 Applications of Surfactants

Overview: Why Surfactants Are So Widely Used

Because surfactants lower surface and interfacial tension and can form micelles, films, or adsorbed layers, they are used wherever one wants to:

• wet a surface better
• disperse or emulsify otherwise immiscible substances
• solubilize hydrophobic materials in water
• stabilize or destabilize dispersions and foams
• modify surface properties (e.g., lubrication, antistatic behavior)

In this chapter, the focus is on applications of these effects, not on the basic structure or classification of surfactants.

Household and Personal Care Applications

Laundry Detergents

In laundry detergents, surfactants perform several roles:

• Removal of hydrophobic dirt and oils
  Surfactant monomers adsorb at the surface of oily soil and at the fiber–water interface. When the concentration is above the critical micelle concentration (CMC), micelles form and can incorporate oil droplets in their hydrophobic core (solubilization). This allows oily dirt to be suspended and rinsed away.
• Prevention of redeposition
  After soil is detached, surfactants and additional polymers surround the particles, giving them a charge and/or steric barrier so they remain dispersed in the wash liquor and do not settle back on fibers.
• Wetting of textiles
  Surfactants decrease the contact angle between wash water and fabric. Better wetting allows water and cleaning agents to penetrate fiber bundles and reach embedded dirt more effectively.
• Foam formation and control
  Many household users associate foam with cleaning “power.” Surfactants stabilize foam films by forming elastic, adsorbed layers at the air–water interface. However, excessive foaming can hinder machines, so detergent formulations often balance high-foaming surfactants with low-foaming types and foam regulators.

Different surfactant types are combined: for instance, an anionic surfactant for strong detergency and a nonionic surfactant to improve performance in cold or hard water.

Dishwashing and Surface Cleaners

• Hand dishwashing liquids
  These rely heavily on surfactants to:
• emulsify fats and oils from food residues
• wet hydrophobic plastic or ceramic surfaces
• suspend soil so it can be rinsed away
  Mildness to skin is important, so less aggressive surfactants and skin-conditioning additives are used.
• Automatic dishwashing detergents
  Too much foam is problematic in machines. Modern formulations typically use low-foaming nonionic surfactants that clean well but do not produce stable foam. They assist in:
• wetting dishes
• removing protein and fat residues (often with enzymes)
• preventing spots and filming (in combination with “rinse-aid” surfactants)
• Household surface cleaners
  For floors, bathrooms, kitchens, surfactants:
• lower surface tension so the cleaner spreads into cracks and pores
• loosen hydrophobic grime and greasy films
• help lift particulate dirt from surfaces
  Some cleaners are designed to leave a thin, even film after drying; surfactants help control how water evaporates and how residues are deposited.

Personal Care Products

Shampoos and Body Washes

• Cleansing: Surfactants solubilize skin and hair sebum and cosmetic residues into micelles that are rinsed away.
• Foam formation: High foam volume and fine bubbles are often desired for consumer acceptance; surfactants form and stabilize these foams.
• Mildness and irritation: Harsh surfactants can strip too many lipids or denature proteins, causing irritation. Formulators often blend anionic surfactants with amphoteric and nonionic surfactants to reduce irritation while maintaining cleaning performance.

Toothpastes and Mouthwashes

• Dispersing and wetting: Surfactants spread the paste in the mouth and help distribute active agents and abrasives.
• Foam: Foam visually signals cleaning and helps suspend and remove debris.
• Interaction with biofilms: Surfactants help loosen plaque and can improve contact of antimicrobial components with tooth and gum surfaces.

Cosmetics and Skin Care

• Emulsifiers: Surfactants stabilize oil-in-water or water-in-oil emulsions in creams and lotions (e.g., oil droplets in an aqueous phase).
• Solubilizers: Certain surfactants can solubilize fragrances or poorly water-soluble actives into clear aqueous products (e.g., toners, micellar waters).
• Make-up removers: Specialized surfactants dissolve hydrophobic pigments and sunscreens and keep them dispersed during rinsing.

Industrial and Technical Applications

Emulsification and Emulsion Polymerization

In many industries, surfactants are used to create and stabilize emulsions:

• Paints and coatings:
• Surfactants emulsify binders (often polymer dispersions) and improve pigment dispersion.
• They influence gloss, leveling, and stability against phase separation or flocculation.
• Food industry (application overlap with separate food chemistry, concept only here):
  Emulsifiers (a special class of surfactants) help form and stabilize oil-in-water or water-in-oil emulsions such as creams or sauces, often in combination with proteins or polysaccharides.
• Emulsion polymerization:
  Surfactants form micelles into which monomer molecules (e.g., styrene, acrylates) partition. Polymerization initiated in these micelles leads to polymer latex particles. Surfactants:
• define particle size and size distribution
• stabilize latex particles against coagulation
• influence final polymer properties and processability

Detergency in Industry

Surfactants are essential in industrial cleaning:

• Metal cleaning and degreasing
  Before painting, plating, or other surface treatment, oil and grease must be removed. Surfactants:
• emulsify cutting oils and lubricants
• disperse metal fines and particulates
• assist in rust and oxide removal (often combined with alkaline builders or acids)
• Textile processing
  Surfactants are used during:
• scouring (removing natural waxes and fats from fibers)
• dyeing (wetting fibers, improving dye penetration, dispersing hydrophobic dyes)
• finishing (controlling softness, antistatic behavior, or hydrophilicity/hydrophobicity of fabrics)
• Hard surface and institutional cleaning
  In hospitals, restaurants, and industry, surfactants:
• aid disinfection by removing soil that shelters microorganisms
• improve the spread of disinfectant solutions on surfaces
• help rinse away residues to avoid build-up.

Lubricants, Corrosion Inhibitors, and Antistatic Agents

• Lubricants
  Surfactants can act as friction modifiers in oils and greases, creating boundary layers that reduce metal–metal contact. They may also disperse solid lubricants (e.g., graphite) in oils.
• Corrosion inhibition
  Certain surfactants adsorb onto metal surfaces, forming a protective film that:
• limits contact with water and corrosive ions (e.g., $ \text{Cl}^- $)
• sometimes provides hydrophobic shielding
  Many corrosion inhibitors have amphiphilic characteristics that favor surface adsorption.
• Antistatic and antifoaming agents
• Antistatic: In plastics and fibers, surfactants increase surface conductivity by attracting a thin layer of moisture or mobile ions, allowing charge dissipation.
• Antifoam/Defoamers: Some surfactants are deliberately chosen to destabilize foam, especially in industrial processes where foam is harmful. They spread over foam films and weaken them, causing bubbles to break.

Surfactants in Coatings, Inks, and Pigment Dispersions

• Pigment dispersion
  Surfactants adsorb on pigment particle surfaces, reducing attractive forces between particles (electrostatic and/or steric repulsion), leading to:
• smaller particle size
• higher color strength
• better stability against sedimentation and flocculation
• Wet-out and leveling
  In coatings and inks, surfactants:
• promote wetting of substrates to avoid craters and pinholes
• improve “leveling,” i.e., the ability of a liquid film to flow out and remove brush marks or unevenness
• prevent defects like “fish-eyes” caused by local contamination with low-surface-tension substances (e.g., lubricants, silicones).

Surfactants in Environmental and Geotechnical Uses

Oil Spill Dispersants

For oil spills on water, dispersant formulations containing surfactants are sprayed onto the oil layer:

• Surfactants accumulate at the oil–water interface and greatly reduce interfacial tension.
• Wave action and mechanical agitation then break up the oil into small droplets that are stabilized by adsorbed surfactant layers.
• The resulting oil-in-water dispersion has a much larger surface area and can be more readily biodegraded by microorganisms.

This application is effective but controversial because it alters the distribution of oil (and toxicity) between surface, water column, and shorelines.

Enhanced Oil Recovery

In some oil reservoirs, surfactant flooding is used:

• Surfactant solutions are injected into porous rock.
• They reduce interfacial tension between trapped oil and reservoir water and improve wetting properties.
• This allows previously trapped oil to be mobilized and moved toward production wells.

Sometimes surfactants are combined with polymers (for viscosity control) and alkali (to help form in situ surfactants from crude components).

Soil and Groundwater Remediation

For contaminated soils or aquifers:

• Surfactant flushing:
  A surfactant solution is injected into contaminated zones. It:
• solubilizes or mobilizes hydrophobic pollutants (e.g., hydrocarbons, some chlorinated solvents)
• forms emulsions or micellar solutions that can be pumped out and treated above ground.
• In situ treatment assistance:
  By increasing pollutant availability in water, surfactants can enhance biodegradation by microorganisms, though careful selection is needed to avoid unwanted transport of contaminants.

Surfactants in Agriculture

Pesticide Formulations

Many pesticides are poorly soluble in water and are applied as emulsions or suspensions:

• Emulsification and dispersion
  Surfactants emulsify oil-based pesticide concentrates into water or disperse solid active ingredients into fine particles.
• Wetting and spreading on leaves
  Leaf surfaces are often hydrophobic and waxy. Surfactants:
• decrease water contact angle
• allow spray droplets to spread over large areas
• help droplets adhere (sticker effect), reducing run-off.
• Penetration aids
  Some surfactants facilitate penetration of active ingredients through plant cuticles or insect exoskeletons, improving efficacy so lower doses may be sufficient.

Fertilizers and Soil Conditioners

• Foliar fertilizers
  Surfactants improve wetting of leaf surfaces and promote better absorption of nutrients when fertilizers are sprayed onto leaves.
• Soil wetting agents
  In hydrophobic or water-repellent soils (e.g., very dry sandy soils), surfactants:
• lower water–soil contact angle
• break water repellency
• promote uniform wetting and infiltration

Surfactants in Food and Pharmaceuticals (Application Focus)

Food Emulsifiers (Conceptual Overview)

Certain surfactants, often of natural or “food-grade” origin, are used as emulsifiers and stabilizers:

• Texture and stability:
  Emulsifiers stabilize small droplets in products such as creams, dressings, and ice cream, affecting mouthfeel, creaminess, and shelf life.
• Air incorporation:
  In whipped products (e.g., whipped cream, some desserts), surfactants stabilize air bubbles to maintain volume and texture.
• Crystallization control:
  Some emulsifiers interact with fats or starches, influencing crystallization and retrogradation, thereby affecting softness or staling (e.g., in baked goods).

Pharmaceutical Formulations

Surfactants appear in many dosage forms:

• Suspensions and emulsions
• Wetting agents to help water wet hydrophobic drug particles.
• Dispersants and emulsifiers to keep suspensions and emulsions physically stable.
• Solubilizers for poorly soluble drugs
  Surfactant micelles can solubilize hydrophobic drugs, making clear aqueous solutions for oral, topical, or parenteral use.
• Controlled release and targeting
  Specialized surfactants (including some polymeric ones) can be used:
• to modify drug release rates
• to enhance absorption through biological membranes
• as components of liposomes or other colloidal carriers.

Foam-Related Applications

Beneficial Foams

In some processes and products, foam is desired:

• Firefighting foams
  Surfactants stabilize foam films that form a thick layer over burning liquids:
• excluding oxygen from the fuel surface
• cooling and reducing vapor release
  Special fluorinated or high-performance surfactants have historically been used; more environmentally benign alternatives are being developed.
• Foamed construction materials
  In foamed concretes and gypsum, surfactants stabilize gas bubbles in the wet mixture, controlling density and thermal insulation properties.
• Food foams
  In products like whipped toppings or mousses, food-grade surfactants help proteins and fats stabilize air bubbles, maintaining structure over time.

Foam Suppression

In many industrial processes (fermentations, paper manufacture, wastewater treatment, cooling systems), foam can:

• reduce capacity
• cause spills
• interfere with sensors and process control

Surfactants with antifoam or defoaming action:

• spread rapidly over foam films
• displace foam-stabilizing surfactants
• thin and weaken films so bubbles burst
  Often, antifoams are formulated with hydrophobic oils and solids (e.g., silicone oils) that interact with these interfaces.

Environmental and Sustainability Considerations in Applications

Across many applications, similar questions arise:

• Biodegradability and persistence
  Surfactants used in detergents and other mass applications should degrade reasonably quickly in the environment to avoid accumulation.
• Toxicity and bioaccumulation
  Application-specific regulations may restrict certain surfactants (e.g., some older nonylphenol ethoxylates) due to endocrine or other toxic effects.
• Renewable resources
  There is growing use of surfactants derived from natural fats, oils, and sugars (e.g., alkyl polyglucosides, fatty alcohol sulfates), especially in household and personal care applications.

The choice of surfactant in any application is thus not only guided by technical performance (wetting, foaming, emulsifying) but also by safety, regulatory, and environmental requirements.

Summary of Application Principles

Across the wide range of uses, surfactants are selected and formulated based on:

• Required interfacial effect: wetting, emulsification, dispersion, solubilization, foam stabilization or destruction.
• Medium: aqueous vs. non-aqueous, pH, ionic strength, presence of electrolytes or other components.
• Substrate and interface: type of surface or interface (air–water, oil–water, solid–liquid) and desired modification.
• Compatibility: with other formulation components (enzymes, polymers, electrolytes, fragrances, active ingredients).
• Regulatory and environmental constraints: especially in high-volume consumer and environmental applications.

Understanding these application principles allows chemists to design surfactant systems optimized for specific tasks, from cleaning clothes to formulating medicines and remediating polluted sites.