Surfactants as Surface-Active Substances

Basic Concept: What Makes a Substance “Surface-Active”?

Surfactants (surface-active agents) are substances that accumulate at interfaces (for example, between water and air, or water and oil) and thereby change the properties of these interfaces.

A characteristic feature of surfactants is their amphiphilic structure:

• One part of the molecule is hydrophilic (“water-loving”).
• One part of the molecule is hydrophobic (“water-fearing,” usually lipophilic, “fat-loving”).

Because of this dual character, surfactant molecules:

• Dissolve to some extent in both polar (e.g. water) and nonpolar (e.g. oil) media.
• Preferentially position themselves at interfaces, with the hydrophilic part in the water phase and the hydrophobic part in the nonpolar phase or pointing toward air.

This interfacial accumulation is the origin of almost all surfactant effects.

Typical Structure of Surfactants

Hydrophobic Tail

The hydrophobic (nonpolar) part is most often:

• A hydrocarbon chain: e.g. a C$_{12}$–C$_{18}$ alkyl chain.
• Sometimes an aryl group (e.g. a phenyl ring), or a fluorocarbon chain in special-purpose surfactants.

Important characteristics of the hydrophobic tail:

• Length: Longer chains increase hydrophobicity and often increase the tendency to form aggregates (micelles), but can decrease water solubility if too long.
• Branching: Branched chains can affect packing at interfaces and biodegradability.
• Saturation: Unsaturated (with C=C double bonds) vs saturated chains influence flexibility and packing.

Hydrophilic Head

The hydrophilic part can be:

• Ionic (charged)
• Nonionic (uncharged, but strongly polar)
• Zwitterionic (carrying both positive and negative charges)

Its polarity allows interaction with water via charge–dipole or hydrogen-bonding interactions. The exact chemical nature of the head group determines:

• Solubility in water.
• Sensitivity to pH and water hardness.
• Compatibility with other ingredients (e.g. other surfactants, electrolytes).

This leads directly to the formal classification of surfactants.

Classification by Head Group: Types of Surfactants

Anionic Surfactants

These carry a negative charge on the hydrophilic head in aqueous solution.

Typical functional groups:

• Sulfates: e.g. sodium dodecyl sulfate (SDS), containing –OSO$_3^-$.
• Sulfonates: e.g. alkylbenzene sulfonates.
• Carboxylates: e.g. soaps (salts of fatty acids).

General structural motif (for a sulfate-type surfactant):
$$
\ce{R-OSO3^- M^+}
$$
where $R$ is a hydrophobic alkyl or aryl group, $M^+$ is a counterion (e.g. Na$^+$).

Key features:

• Good foaming and detergency.
• Often sensitive to hardness ions (Ca$^{2+}$, Mg$^{2+}$) if carboxylate-based (soaps).

Cationic Surfactants

These carry a positive charge on the head group in water.

Typical examples:

• Quaternary ammonium compounds:
  $$\ce{[R4N]^+ X^-}$$
  where at least one R is a long alkyl chain.

Key features:

• Often exhibit antimicrobial and fabric-softening properties.
• Can be incompatible with anionic surfactants due to charge neutralization and precipitation.

Nonionic Surfactants

These have no formal charge on the head group but are strongly polar and hydrogen-bonding.

Typical head groups:

• Polyether chains, e.g. poly(ethylene glycol) units:
  $$\ce{-O-CH2-CH2-O-}$$
• Sugar-based heads (e.g. glucosides).

General motif:
$$
\ce{R-(O-CH2-CH2)_n-OH}
$$

Key features:

• Less sensitive to water hardness.
• Often milder (less irritating) to skin.
• Solubility and behavior strongly dependent on temperature (cloud points).

Amphoteric (Zwitterionic) Surfactants

These contain both positive and negative charges within the same molecule, often depending on pH.

Typical structures:

• Betaine-type surfactants, e.g.:
  $$\ce{R-N^+(CH3)2-CH2-COO^-}$$

Key features:

• pH-dependent charge state.
• Frequently used as co-surfactants in formulations for mildness (e.g. shampoos).
• Can behave anionic, cationic, or zwitterionic depending on environment.

Action at Interfaces: Surface and Interfacial Tension

Surface Tension of Pure Liquids

At an interface (e.g. water–air), molecules at the surface experience an imbalance of attractive forces compared to those in the bulk. This leads to a tendency to minimize surface area and manifests macroscopically as surface tension $\gamma$.

Water has a relatively high surface tension due to strong hydrogen bonding between water molecules.

How Surfactants Lower Surface and Interfacial Tension

When a surfactant is added to water:

• Its hydrophilic heads interact with water.
• Its hydrophobic tails avoid water and orient away from the aqueous phase.

At the water–air interface:

• Surfactant molecules align with their hydrophobic part toward the air and hydrophilic part in the water.
• This disrupts the strong water–water interactions at the surface.
• As a result, the surface tension decreases.

At a water–oil interface:

• The hydrophobic tail prefers the oil phase, while the hydrophilic head prefers the water phase.
• Accumulation at the interface reduces the interfacial tension between water and oil.

Key consequences:

• Easier spreading of water on surfaces (improved wetting).
• Facilitated dispersion of one liquid in another (e.g. oil in water).

The relationship between surfactant concentration and surface tension typically shows:

• Strong decrease at low concentrations.
• Leveling off as the interface becomes saturated with surfactant.

Micelle Formation and the Critical Micelle Concentration (CMC)

From Monomers to Aggregates

In dilute solution, surfactant molecules are mainly present as monomers. As concentration increases:

• More surfactant molecules accumulate at interfaces.
• Once the interface is saturated, further addition leads to formation of aggregates in the bulk solution.

The most common aggregates are micelles:

• Spherical or elongated structures.
• Hydrophobic tails packed in the interior.
• Hydrophilic heads exposed to water.

Critical Micelle Concentration (CMC)

The critical micelle concentration (CMC) is the surfactant concentration at which micelles start to form in significant amounts.

Below CMC:

• Surfactants exist primarily as monomeric species.
• Surface tension decreases with increasing concentration.

At and above CMC:

• Surface tension approaches a plateau.
• Additional surfactant mainly forms micelles rather than further lowering surface tension.

The CMC depends on:

• Structure of the surfactant (head group, tail length, branching).
• Temperature.
• Presence of salts and other solutes.

General trends:

• Longer hydrophobic chains → lower CMC (stronger hydrophobic effect).
• Added electrolytes often lower the CMC of ionic surfactants by screening charges on the head groups.

Micelle Structure and Variants

Common micelle types:

• Normal (oil-in-water) micelles:
• Hydrophilic heads outside, in contact with water.
• Hydrophobic tails inside, creating a nonpolar core.
• Typical for surfactants in water.
• Reverse (water-in-oil) micelles:
• Hydrophilic heads inside, surrounding a small water pool.
• Hydrophobic tails outside, in contact with a nonpolar solvent.
• Formed in nonpolar solvents with appropriate surfactants.

Other possible aggregates (depending on concentration and conditions):

• Cylindrical micelles.
• Lamellar phases (layered structures).
• Vesicles and bilayers (especially for surfactants with two tails, like many lipids).

These self-assembled structures are central to many applications, including detergency, emulsions, and biological membranes.

Surfactants and Wetting of Solids

Contact Angle and Wetting

When a liquid droplet rests on a solid surface, it forms a certain contact angle $\theta$ with the surface. This angle reflects the balance of interfacial tensions:

• Solid–liquid.
• Solid–gas.
• Liquid–gas.

Qualitatively:

• Small contact angle (approaching 0°): good wetting, the droplet spreads.
• Large contact angle: poor wetting, the droplet beads up.

Surfactants:

• Reduce the liquid–gas and often the liquid–solid interfacial tensions.
• Thus decrease the contact angle.
• Promote spreading of the liquid on surfaces (e.g. water on greasy dishes, fabric, or skin).

Improved wetting:

• Exposes more surface area to the liquid phase.
• Facilitates subsequent processes, such as dirt detachment in cleaning or penetration of active ingredients.

Adsorption on Solid Surfaces

Surfactant molecules can also adsorb directly onto solid surfaces:

• Hydrophobic regions may bind to hydrophobic substrates (e.g. plastics, fats).
• Charged head groups can interact electrostatically with charged surfaces (e.g. mineral particles, fibers).

This adsorption can:

• Change the surface from hydrophobic to more hydrophilic (or vice versa).
• Stabilize suspensions (dispersions) by preventing particle aggregation through electrostatic or steric repulsion.

Emulsification and Dispersion

Emulsions and the Role of Surfactants

An emulsion is a dispersion of droplets of one liquid in another immiscible liquid (e.g. oil-in-water or water-in-oil). Without surfactants, such emulsions are usually unstable and quickly separate.

Surfactants stabilize emulsions by:

• Adsorbing at the oil–water interface.
• Lowering the interfacial tension.
• Forming a protective layer around droplets that reduces coalescence.

The choice of surfactant influences:

• Whether an oil-in-water (O/W) or water-in-oil (W/O) emulsion is favored.
• The stability and droplet size distribution.

Suspensions and Solid Dispersions

When solid particles are dispersed in a liquid (suspensions), surfactants can:

• Improve wetting of solid surfaces.
• Prevent aggregation by:
• Electrostatic repulsion between similarly charged surfaces.
• Steric stabilization via adsorbed molecular layers.

This is essential for applications like:

• Pigment dispersions in paints and inks.
• Pharmaceutical suspensions.
• Soil removal in cleaning processes.

Foam and Foam Stability

Foam Formation

A foam is a dispersion of gas bubbles in a liquid, separated by thin liquid films (lamellae). Many surfactants, particularly anionic ones, promote foam formation.

Mechanism:

• Surfactants accumulate at the air–water interface of bubbles.
• This lowers surface tension, making bubble formation easier.
• The surfactant layer acts as a flexible “skin” stabilizing the bubble surface.

Foam Stability

Foam stability is influenced by:

• Surface viscosity and elasticity of the surfactant film.
• Rate of drainage of liquid from the films.
• Solubility and diffusion of gas between bubbles.

Surfactant properties that increase foam stability:

• Ability to form strong, elastic interfacial films.
• Optimal balance of solubility and interfacial adsorption.

Foams can be:

• Desired, e.g. in some detergents, fire-fighting foams, cosmetic products.
• Undesired, e.g. in industrial processes, where antifoaming agents (often hydrophobic particles or oils) are used to destabilize foam.

Physicochemical Parameters and Structure–Property Relationships

Hydrophilic–Lipophilic Balance (HLB)

A practical concept for nonionic surfactants is the hydrophilic–lipophilic balance (HLB):

• Numerical scale representing how hydrophilic or lipophilic a surfactant is.
• Low HLB: more lipophilic, suitable for W/O emulsions.
• High HLB: more hydrophilic, suitable for O/W emulsions.

Although different empirical calculation methods exist, the key idea is that:

• Molecular structure (head group size and type, tail length) systematically influences application behavior.

Krafft Temperature and Cloud Point

For ionic surfactants:

• The Krafft temperature is the minimum temperature at which the solubility of the surfactant equals the CMC.
• Below this temperature, micelles cannot form effectively; the surfactant is poorly soluble.

For many nonionic surfactants:

• The cloud point is the temperature at which the solution becomes turbid due to phase separation.
• Above the cloud point, decreased hydration of the head group leads to aggregation and separation.

These temperatures are important for:

• Designing surfactant systems that work effectively under specific conditions (e.g. cold-water detergents, high-temperature cleaners).

Influence of Additives

Salts, co-surfactants, and polymers can markedly influence surfactant behavior:

• Electrolytes reduce electrostatic repulsions, affecting CMC, micelle size, and interfacial adsorption.
• Short-chain co-surfactants can fit between larger molecules at the interface, improving packing and modifying interfacial properties.
• Polymers can interact with surfactants (e.g. through hydrophobic or ionic interactions), forming complexes that alter viscosity, foaming, and stability.

Environmental and Biological Aspects of Surfactants as Surface-Active Species

Although detailed environmental and biological chemistry is treated elsewhere, certain points are directly tied to the surface-active nature of surfactants:

• Surfactants can alter natural interfaces, for instance at air–water or soil–water boundaries, affecting:
• Transport and availability of other chemicals.
• Biological membranes, due to their similarity to lipid surfactant systems.
• Their amphiphilic structure allows them to:
• Interact with cell membranes (which are essentially lipid bilayers).
• Disrupt membrane integrity at sufficient concentrations (basis for some antimicrobial effects and also potential toxicity/irritation).
• Biodegradability often depends on:
• The structure of the hydrophobic tail (e.g. branching, chain length).
• The nature of the head group (e.g. sulfonate vs sulfate vs sugar).

These aspects must be considered in the selection and design of surfactants for specific uses, especially in large-scale applications where they are released into the environment.

Summary of Key Characteristics of Surfactants as Surface-Active Substances

• Surfactants are defined by their amphiphilic structure, leading to preferential accumulation at interfaces.
• They lower surface and interfacial tension, improving wetting, spreading, and mixing of otherwise immiscible phases.
• Above a critical micelle concentration, they form micelles and other aggregates, providing a hydrophobic microenvironment in aqueous solution.
• Different head group types (anionic, cationic, nonionic, amphoteric) determine charge, solubility, and interactions with other components.
• Their interfacial activity underlies:
• Wetting of solids.
• Emulsification and stabilization of dispersions.
• Foam formation and stability.
• Structural details (head group size and type, tail length, branching) and conditions (temperature, ionic strength, co-solutes) govern quantitative behavior such as CMC, micelle structure, and HLB, which must be tuned for specific applications.