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van Oss-Chaudhury-Good method (SFE model)

Pronunciation: /væn ɒs ˈtʃɔːdəri ɡʊd ˈmɛθəd/

The van Oss-Chaudhury-Good method is a surface free energy model that splits the surface energy of a solid into a Lifshitz-van der Waals part and a Lewis acid-base part. The acid base part is built from separate electron acceptor and electron donor parameters, found from the contact angles of three test liquids.

Also called

vOCG method Acid base method van Oss method LW/AB approach

Related terms

Equation of State (SFE model) Fowkes method Critical surface tension (Zisman)

Topics

Surface energy Acid base Contact angle
Book a 15 minute demo Read the surface energy measurement guide

At a glance

What the van Oss-Chaudhury-Good method adds to a dispersive and polar split.

What it is

Proposed by Carel van Oss, Manoj Chaudhury and Robert Good in the late 1980s. It keeps a geometric mean for the apolar part but treats polar interactions as acid meeting base.

  • γS = γSᴸᵂ + γSᴬᴮ
  • γSᴬᴮ = 2√(γS⁺ γS⁻)
  • γ⁺: electron acceptor (Lewis acid); γ⁻: electron donor (Lewis base)
  • Three unknowns, so three liquids, at least two of them polar

Why it matters

Some surfaces only donate electrons and some only accept them. A single polar number hides that, yet it decides how proteins, cells and water based coatings interact with the surface. That is why the method is popular in biomaterials and colloid science. Standards for coatings such as ASTM D7490 on surface tension of coatings and substrates describe the simpler two liquid approach it extends.

Worked example

Same hypothetical polymer as our other model pages, 20 °C: diiodomethane 50°, water 75°, formamide 60°.

  • Diiodomethane gives γSᴸᵂ = 34.3 mJ/m²
  • Water: 45.82 = 27.34 + 5.050 (√γS⁺ + √γS⁻), so √γS⁺ + √γS⁻ = 3.659
  • Formamide: 43.50 = 36.57 + 6.293 √γS⁺ + 1.510 √γS⁻
  • Solving: √γS⁺ = 0.294, √γS⁻ = 3.365, so γS⁺ = 0.09 and γS⁻ = 11.3 mJ/m²
  • γSᴬᴮ = 2 × 0.294 × 3.365 = 2.0; total 36.3 mJ/m²

Expert view

“Droplet adhesion is three-dimensional. Two side-view angles give an estimate; the whole story is how the angle and the contact line vary all the way around the drop.”

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

What the terms mean

The three parameters and the surface types they describe.

Apolar surface (γ⁺ and γ⁻ near zero)

Only Lifshitz-van der Waals forces act. Polyolefins and fluoropolymers sit here, and the result matches a dispersive only Fowkes value.

Monopolar basic surface (γ⁻ large, γ⁺ near zero)

The surface donates electrons but accepts few. It interacts strongly with water yet has almost no acid base self attraction, so γSᴬᴮ stays small.

Bipolar surface (both γ⁺ and γ⁻ sizeable)

The surface both donates and accepts electrons, like water itself. γSᴬᴮ becomes a real share of the total surface energy.

Liquid values (van Oss, 20 °C): water 72.8 = 21.8 LW + 51.0 AB with γ⁺ = γ⁻ = 25.5; diiodomethane 50.8 LW; formamide 58.0 = 39.0 LW + 19.0 AB with γ⁺ = 2.28, γ⁻ = 39.6.

γSᴸᵂ Lifshitz-van der Waals (apolar) component of the solid, mJ/m²
γS⁺ Electron acceptor (Lewis acid) parameter of the solid, mJ/m²
γS⁻ Electron donor (Lewis base) parameter of the solid, mJ/m²
γSᴬᴮ Acid base component, 2√(γS⁺ γS⁻), mJ/m²
γL, θ Liquid surface tension (mN/m) and its contact angle (°)

Key equations

The component split and the working equation used for each liquid.

Equation: γS = γSᴸᵂ + 2 √(γS⁺ γS⁻)

Variable legend:

  • γSᴸᵂ: Lifshitz-van der Waals component (mJ/m²)
  • γS⁺, γS⁻: acid and base parameters (mJ/m²)

Interpretation: The acid base part needs both an acid and a base parameter. A surface that is only basic has γSᴬᴮ near zero even if γS⁻ is large.

When to use: Use it to report and compare the acid base character of polar surfaces.

Assumptions:

  • Apolar forces combine by a geometric mean
  • Acid base forces combine crosswise, acid with base

Common mistakes:

  • Reading a large γS⁻ as a large polar component

Equation: γL (1 + cos θ) = 2 [√(γSᴸᵂ γLᴸᵂ) + √(γS⁺ γL⁻) + √(γS⁻ γL⁺)]

Variable legend:

  • One equation per liquid; three liquids for three unknowns
  • Liquid parameters γLᴸᵂ, γL⁺, γL⁻ from a published table (mN/m)

Interpretation: Solid acid pairs with liquid base and solid base pairs with liquid acid. That crosswise pairing is what separates this model from OWRK.

When to use: Use it with one apolar liquid, usually diiodomethane, and two polar liquids such as water and formamide or ethylene glycol.

Assumptions:

  • Equilibrium contact angles on a smooth, uniform solid
  • The liquids do not dissolve or swell the solid

Common mistakes:

  • Choosing two polar liquids with similar γ⁺/γ⁻ ratios, which makes the system ill conditioned

Equation: γSL = (√γSᴸᵂ − √γLᴸᵂ)² + 2 (√γS⁺ − √γL⁺)(√γS⁻ − √γL⁻)

Variable legend:

  • γSL: solid liquid interfacial tension (mN/m)

Interpretation: The acid base term can be negative, so the model allows a negative interfacial tension, which signals spontaneous mixing or dispersion.

When to use: Use it to predict whether particles or cells will attract or repel each other in water.

Assumptions:

  • Same component values used for every phase

Common mistakes:

  • Mixing component scales from different reference tables

How to read it

What the three parameters tell you and when to be cautious.

The water scale sets γ⁺ = γ⁻ = 25.5 mN/m by convention, not by measurement. That choice makes most solids look strongly basic, so compare γ⁺ and γ⁻ between samples rather than reading them as absolute values. Our note on reproducible surface energy calculation from contact angles covers how to keep the liquid set and tables fixed.

Reading a van Oss-Chaudhury-Good result

If you see It usually means Check next
γ⁻ much larger than γ⁺ A monopolar basic surface, the most common result Whether γ⁻ rises after oxidation or hydrolysis
γ⁺ and γ⁻ both near zero An apolar surface; the LW part is the total Agreement with a Fowkes dispersive value
Negative value for √γS⁺ or √γS⁻ The liquid set or angles do not fit the model Liquid purity, swelling and the conditioning of the liquid set
Results change with the polar liquid pair Ill conditioning or a liquid interacting with the solid Repeat with a different second polar liquid

Common misconceptions

Where the van Oss-Chaudhury-Good method is most often misread.

Three liquids make it more accurate than OWRK

Three liquids give more parameters, not automatically better ones. Small angle errors can swing γ⁺ widely because the equations are close to parallel. Methods with fewer unknowns, which many compact instruments in our guide to the best surface energy analyzer under $5,000 support, can be more robust for routine checks.

γ⁻ is the polar surface energy

The polar contribution is 2√(γ⁺ γ⁻), not γ⁻. In the worked example γS⁻ is 11.3 mJ/m² but γSᴬᴮ is only 2.0 mJ/m², because γS⁺ is tiny. Our surface tension of common liquids dataset lists the liquid parameters this depends on.

Acid base values are absolute properties

They rest on the water convention γ⁺ = γ⁻, so they are relative to that scale. Compare samples within one study and one liquid set. On coatings, as in thin film and nanocoating surface characterization, trends are more trustworthy than single values.

Related terms

Where the van Oss-Chaudhury-Good method sits among surface energy models.

Broader term

  • Surface free energy models
  • Surface energy components

Related terms

  • Fowkes method
  • Equation of State (SFE model)
  • Critical surface tension (Zisman)
  • Young equation

Often confused with

  • OWRK method

FAQ

Short answers to the questions people ask most about the van Oss-Chaudhury-Good method.

Which three liquids are used for the van Oss method?

One apolar liquid and two polar ones. Diiodomethane is the usual apolar choice. Water is almost always one polar liquid, with formamide, ethylene glycol or glycerol as the other. Pick a second polar liquid whose acid and base balance differs clearly from water, and check it does not swell or dissolve your solid.

What is the difference between OWRK and van Oss-Chaudhury-Good?

OWRK lumps all nondispersive forces into one polar part that combines with a geometric mean. van Oss-Chaudhury-Good splits them into an acid and a base parameter that combine crosswise. It needs three liquids instead of two, and it can show a surface that is basic but not acidic, which OWRK cannot.

Why does my sample always come out basic?

Most solids do, and part of the reason is the scale. Water is assigned equal acid and base parameters of 25.5 mN/m by convention. Against that reference most surfaces show a large γ⁻ and a small γ⁺. Treat the values as relative and compare samples measured with the same liquids and tables.

References

Primary sources and further reading.

1. van Oss, C. J., Chaudhury, M. K., and Good, R. J. (1988). Interfacial Lifshitz-van der Waals and polar interactions in macroscopic systems. Chemical Reviews, 88(6), 927 to 941. https://doi.org/10.1021/cr00088a006
2. Good, R. J. (1992). Contact angle, wetting, and adhesion: a critical review. Journal of Adhesion Science and Technology, 6(12), 1269 to 1302. https://doi.org/10.1163/156856192X00629
3. van Oss, C. J. (2006). Interfacial Forces in Aqueous Media, 2nd edition. CRC Press, Boca Raton.
4. Kwok, D. Y., and Neumann, A. W. (1999). Contact angle measurement and contact angle interpretation. Advances in Colloid and Interface Science, 81(3), 167 to 249. https://doi.org/10.1016/S0001-8686(98)00087-6
5. Surface energy measurement guide
6. Fowkes method
7. Equation of State (SFE model)
8. Critical surface tension (Zisman)