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Interfacial tension (IFT)

Pronunciation: /ˌɪntəˈfeɪʃəl ˈtɛnʃən/

Interfacial tension (IFT) is the energy needed to create a unit area of boundary between two liquids that do not mix, such as oil and water. It is reported in mN/m and sets how easily one liquid breaks up, spreads or emulsifies in the other.

Also called

IFT Liquid liquid interfacial tension Boundary tension

Related terms

Capillary number Spreading coefficient Surface tension

Topics

Surface tension Emulsions Interfaces
Book a 15 minute demo Read the surface tension measurement guide

At a glance

What interfacial tension is, what values to expect, and where it shows up in practice.

What it is

Molecules at the boundary between two liquids have fewer like neighbours than molecules in the bulk. That missing attraction costs energy, and the boundary pulls itself as small as it can. Interfacial tension is that cost per unit area.

  • Same units as surface tension: mN/m, equal to mJ/m² and dyn/cm
  • Surface tension is the special case where the second phase is air or vapor
  • Lower IFT means the boundary is cheaper to stretch

Typical values

  • Water against straight chain alkanes such as decane or hexadecane, room temperature: roughly 50 to 53 mN/m
  • New mineral insulating oil against water: usually above about 40 mN/m, falling as the oil ages
  • Oil and water with surfactant: often a few mN/m
  • Ultralow systems for enhanced oil recovery: below 0.01 mN/m

For liquid air values, see our surface tension of common liquids dataset.

How it is measured

The usual route is a pendant drop of the denser liquid hanging in the lighter one, with the drop shape fitted to the Young-Laplace equation. Force methods such as the ring are also used, and standards such as IEC 62961 for insulating liquids describe one. The surface tension measurement guide compares the methods.

Expert view

“Plot surface tension against surfactant concentration and the interface tells you when it is saturated: the curve bends at the point where the surface can hold no more.”

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

What the terms mean

Three IFT regimes and what each one looks like in a real oil and water system.

High IFT (about 30 mN/m and above)

Clean oil and water. Drops stay round, coalesce quickly and separate soon after mixing stops.

Moderate IFT (about 1 to 20 mN/m)

Surfactant or polar impurities sit at the boundary. Mixing makes smaller drops, and emulsions last longer.

Ultralow IFT (below 0.01 mN/m)

The boundary costs almost nothing to stretch. Drops deform easily, and trapped oil can be pushed through narrow pores.

IFT always belongs to a pair of liquids at a stated temperature. Quote both phases whenever you report a value.

γ12 Interfacial tension between liquid 1 and liquid 2, mN/m
γ1, γ2 Surface tension of each liquid against air, mN/m
γd Dispersive (London) component of a surface tension, mN/m
Δp Pressure difference across a curved liquid liquid boundary, Pa
R Radius of a spherical drop, m

Key equations

How IFT relates to the surface tensions of the two liquids and to the pressure inside a drop.

Equation: γ12 = γ1 + γ2 − 2 √(γ1d γ2d)

Variable legend:

  • γ12: interfacial tension between the two liquids (mN/m)
  • γ1, γ2: surface tension of each liquid (mN/m)
  • γ1d, γ2d: dispersive components (mN/m)

Worked example, water and hexadecane at 20 °C: γ1 = 72.8, γ1d ≈ 21.8, γ2 = γ2d ≈ 27.5 mN/m. √(21.8 × 27.5) ≈ 24.5, so γ12 ≈ 72.8 + 27.5 − 49.0 ≈ 51.3 mN/m.

Interpretation: Only the attractions the two liquids share across the boundary lower the cost of the interface. Water bonds strongly to itself but only weakly to an alkane, so IFT stays high.

When to use: Use it to estimate IFT for water against a nonpolar oil, or to check whether a measured value is plausible.

Assumptions:

  • One liquid interacts only through dispersion forces
  • Pure liquids, no surfactant at the boundary

Common mistakes:

  • Applying it to two polar liquids, where it underestimates the interactions
  • Using it once surfactant is present

Equation: Δp = 2 γ12 / R

Variable legend:

  • Δp: pressure inside minus outside the drop (Pa)
  • γ12: interfacial tension (N/m)
  • R: drop radius (m)

Example: a 10 µm radius oil drop in water with γ12 = 50 mN/m holds Δp = 2 × 0.050 / (10 × 10−6) = 10,000 Pa.

Interpretation: Small drops and high IFT both mean high internal pressure, which is why fine emulsions need low IFT or a lot of mixing energy.

When to use: Use it to judge how hard it is to break a drop into smaller ones, or to force it through a pore.

Assumptions:

  • Spherical drop, small enough that gravity does not distort it

Common mistakes:

  • Mixing mN/m with metres without converting to N/m

How to read it

What a measured IFT tells you about your oil and water system.

An IFT number only makes sense next to its two phases, temperature and drop age. Our article on why water forms droplets in oil but oil spreads in water shows how the same tensions explain everyday behaviour.

From measured IFT to meaning

If you see It usually means Check next
Pure water and alkane well below 50 mN/m Polar impurities or surfactant are adsorbing at the boundary Purity of the oil and cleanliness of the glassware
IFT drifting down over minutes Surface active species are still arriving at the interface Report the value with its drop age, or wait for a plateau
Insulating oil lower than the last sample Oxidation products are building up as the oil ages Acidity and the trend across previous samples
Values below about 0.1 mN/m Drops are barely deformed by gravity, so shape fits lose precision Whether a spinning drop measurement suits the range

Common misconceptions

Where interfacial tension is most often misread.

IFT is just the difference between the two surface tensions

That shortcut, known as Antonow's rule, works only for some mutually saturated pairs. For water and an alkane it gives about 45 mN/m, while measured values sit closer to 50 to 53 mN/m. Measure the pair directly, as in experiment 10 on oil and water interfacial tension.

Any tensiometer can measure any IFT

Pendant drop and ring methods work well from tens of mN/m down to around 1 mN/m, and somewhat lower with care. Ultralow values need a spinning drop, because a hanging drop barely deforms. Our comparison of pendant drop and spinning drop, including ultralow IFT limits, covers where each fits.

IFT is a fixed property of two liquids

Temperature, dissolved surfactant, oxidation and drop age all shift it. That is why IFT works as a routine quality check on oils, as a lubricant base oil case study illustrates. Always report the conditions with the number.

IFT and contact angle tell you the same thing

IFT describes two liquids. Contact angle describes how a liquid meets a solid, and depends on the solid too. You often need both, and our free contact angle tool reads the angle from a drop photo. Labs choosing equipment for surfactant work can compare options in the best tensiometer for surfactant and CMC research guide.

Related terms

Where interfacial tension sits among other surface science concepts.

Broader term

  • Surface tension

Related terms

  • Capillary number
  • Spreading coefficient
  • Surfactant
  • Critical micelle concentration (CMC)
  • Pendant drop method

Often confused with

  • Surface tension
  • Surface free energy
  • Contact angle

FAQ

Short answers to the questions people ask most about interfacial tension.

What is the difference between surface tension and interfacial tension?

Both measure the energy cost of a boundary per unit area, in mN/m. Surface tension refers to a liquid against air or its own vapor. Interfacial tension refers to the boundary between two liquids that do not mix, such as oil and water. Some texts use interfacial tension for any boundary, with surface tension as the special case.

What is a typical oil and water interfacial tension?

For pure water against straight chain alkanes such as decane or hexadecane at room temperature, values sit roughly between 50 and 53 mN/m. Crude oils and formulated oils are usually lower because they contain polar compounds. Adding surfactant can bring IFT down to a few mN/m, and specially designed systems reach below 0.01 mN/m.

Why does interfacial tension drop when surfactant is added?

Surfactant molecules have a part that prefers water and a part that prefers oil, so they collect at the boundary. Each adsorbed molecule replaces costly water and oil contact with more favourable contacts. The more molecules pack into the interface, the lower the IFT, until the interface is saturated.

References

Primary sources and further reading.

1. Fowkes, F. M. (1964). Attractive forces at interfaces. Industrial and Engineering Chemistry, 56(12), 40 to 52. https://doi.org/10.1021/ie50660a008
2. Zeppieri, S., Rodríguez, J., and López de Ramos, A. L. (2001). Interfacial tension of alkane + water systems. Journal of Chemical and Engineering Data, 46(5), 1086 to 1088. https://doi.org/10.1021/je000245r
3. Berry, J. D., Neeson, M. J., Dagastine, R. R., Chan, D. Y. C., and Tabor, R. F. (2015). Measurement of surface and interfacial tension using pendant drop tensiometry. Journal of Colloid and Interface Science, 454, 226 to 237. https://doi.org/10.1016/j.jcis.2015.05.012
4. Surface tension measurement guide
5. Capillary number
6. Spreading coefficient