Contents

Surfactant

Pronunciation: /sɜːˈfæktənt/

A surfactant, short for surface active agent, is a molecule with a water loving head and an oil loving tail. It collects at surfaces and interfaces, where it lowers surface and interfacial tension and changes how liquids wet, foam and emulsify.

Also called

Surface active agent Tenside Amphiphile

Related terms

Spreading Dynamic surface tension Critical micelle concentration (CMC)

Topics

Surfactants Surface tension Formulation
Book a 15 minute demo Read the surface tension measurement guide

At a glance

What a surfactant is, how it works at an interface, and the values you are likely to meet.

What it is

A surfactant has two parts that want different things. The head prefers water and the tail avoids it. At a surface, the molecule can satisfy both by standing with its head in the water and its tail pointing out. That arrangement lowers the energy of the surface.

  • Anionic: negative head, for example sodium dodecyl sulfate (SDS)
  • Cationic: positive head, for example CTAB
  • Nonionic and zwitterionic: no net charge

Typical values

  • SDS in water at 25 °C: CMC about 8 mM
  • CTAB in water at 25 °C: CMC about 1 mM
  • Surface tension at and above the CMC for many common surfactants: roughly 30 to 40 mN/m, compared with about 72 mN/m for water

More values are in our CMC of common surfactants dataset.

Expert view

“The critical micelle concentration is not a number you look up; it is where the surface-tension curve flattens, and the only honest way to know it is to measure it.”

Dr. Alidad Amirfazli, Co-founder and Scientific Advisor, Droplet Lab

What the terms mean

What a surfactant does as its concentration rises.

Below the CMC: filling the surface

Molecules adsorb at the surface and surface tension falls steadily as concentration rises.

At the CMC: surface saturated

The surface is packed. Extra molecules start forming micelles in the bulk instead.

Above the CMC: micelles in solution

Surface tension stays almost constant. Micelles can dissolve oils, which is the basis of detergency.

The head and tail picture is a simplification. Real molecules may have branched tails, two heads or polymer chains, but the logic is the same.

γ Surface tension of the solution, mN/m
c Surfactant concentration, mol/L or mM
Γ Surface excess: amount adsorbed per unit area, mol/m²
R, T Gas constant 8.314 J/(mol K), absolute temperature K
CMC Critical micelle concentration, mM

Key equations

How the surface tension curve tells you how much surfactant sits at the surface.

Equation: Γ = −(1 / R T) × dγ / d ln c

Variable legend:

  • Γ: surface excess (mol/m²)
  • γ: surface tension (N/m)
  • c: concentration; only its logarithm matters
  • R: 8.314 J/(mol K); T: temperature (K)

Worked example at 25 °C: just below the CMC, γ falls by 10 mN/m per unit of ln c. RT = 8.314 × 298.15 ≈ 2479 J/mol, so Γ = 0.010 / 2479 ≈ 4.0 × 10−6 mol/m², about 0.41 nm² per molecule.

Interpretation: A steeper fall of γ with log concentration means more molecules packed at the surface.

When to use: Use it on the falling part of the curve, just below the CMC, to estimate packing and area per molecule.

Assumptions:

  • Dilute solution, activity close to concentration
  • Single pure surfactant at equilibrium

Common mistakes:

  • Using log base 10 without the factor 2.303
  • Ignoring the factor of 2 needed for a 1:1 ionic surfactant without added salt

Equation: Π = γ0 − γ

Variable legend:

  • Π: surface pressure (mN/m)
  • γ0: surface tension of the pure solvent (mN/m)
  • γ: surface tension of the solution (mN/m)

Example: a surfactant solution at its CMC with γ = 38 mN/m, against water at 72 mN/m, gives Π = 72 − 38 = 34 mN/m.

Interpretation: The amount by which the surfactant has lowered the tension. It is a direct measure of how effective a surfactant is.

When to use: Use it to compare surfactants on the same scale, or to report effectiveness at the CMC.

Assumptions:

  • Both values measured at the same temperature

Common mistakes:

  • Comparing a dynamic γ with a static γ0

How to read it

What a surface tension versus concentration curve tells you about a surfactant.

Plot surface tension against the logarithm of concentration. The slope, the break point and the plateau each tell you something different. Students can build this curve themselves in experiment 2 on the effect of surfactants on surface tension.

From curve to meaning

If you see It usually means Check next
A sharp break followed by a flat plateau A clean CMC for a reasonably pure surfactant Read the CMC at the intersection of the two lines
A dip below the plateau just before the break A more surface active impurity, such as dodecanol in SDS Purity of the surfactant batch
Plateau higher than expected A weaker or more soluble surfactant Chain length and head group
Curve shifts with polymer or salt added Interactions are changing adsorption or micelle formation Compare with the surfactant alone

Common misconceptions

Where surfactants are most often misunderstood.

More surfactant always means lower surface tension

Only up to the CMC. Above it, extra molecules form micelles and the surface tension barely changes. Adding more can still help cleaning or solubilising, but not wetting.

Two surfactants with the same static surface tension behave the same

In fast processes the speed of adsorption matters as much as the end value. Our article on why you need dynamic surface tension explains why sprays and inks need the dynamic picture.

Surface tension tells you everything about wetting

Wetting also depends on the solid. A surfactant can lower γ and still adsorb on the solid in a way that raises the contact angle. Check both, for example with our free contact angle tool. Teaching labs can compare instruments in our guide to the best tensiometer for teaching labs.

Related terms

Where surfactants sit among other surface tension concepts.

Broader term

  • Surface tension
  • Interfacial tension (IFT)

Related terms

  • Critical micelle concentration (CMC)
  • Dynamic surface tension (time-resolved pendant)
  • Spreading
  • Static surface tension

Often confused with

  • Emulsifier
  • Detergent

FAQ

Short answers to the questions people ask most about surfactants.

How does a surfactant lower surface tension?

Water molecules at the surface pull strongly on each other, which creates surface tension. Surfactant molecules adsorb at the surface with their heads in the water and their tails outward. They replace some of the high energy water surface with a lower energy layer, so less energy is needed to create new surface.

What are the main types of surfactant?

Surfactants are grouped by the charge on their head. Anionic surfactants, such as SDS, carry a negative charge and are common in detergents. Cationic surfactants, such as CTAB, carry a positive charge. Nonionic surfactants carry no charge, and zwitterionic ones carry both charges and are often milder.

Is a surfactant the same as a detergent or an emulsifier?

Not exactly. Surfactant is the general term for any surface active molecule. A detergent is a surfactant chosen for cleaning, and an emulsifier is one chosen to stabilise drops of one liquid in another. The same molecule can play different roles depending on the formulation and concentration.

References

Primary sources and further reading.

1. Rosen, M. J., and Kunjappu, J. T. (2012). Surfactants and Interfacial Phenomena, 4th edition. Wiley, Hoboken. https://doi.org/10.1002/9781118228920
2. Mukerjee, P., and Mysels, K. J. (1971). Critical Micelle Concentrations of Aqueous Surfactant Systems. NSRDS-NBS 36, National Bureau of Standards, Washington. https://nvlpubs.nist.gov/nistpubs/Legacy/NSRDS/nbsnsrds36.pdf
3. Eastoe, J., and Dalton, J. S. (2000). Dynamic surface tension and adsorption mechanisms of surfactants at the air water interface. Advances in Colloid and Interface Science, 85(2 to 3), 103 to 144. https://doi.org/10.1016/S0001-8686(99)00017-2
4. Surface tension measurement guide
5. Critical micelle concentration (CMC)
6. Dynamic surface tension
7. Spreading