What does an activity coefficient of 1 mean?

γ = 1 indicates ideal behavior (Raoult's law). The component behaves as if interactions with unlike molecules are identical to like-molecule interactions.

When is the van Laar model better than Margules?

Van Laar handles strongly asymmetric systems more accurately. For nearly symmetric mixtures (A₁₂ ≈ A₂₁) the two-suffix Margules is adequate; when the constants differ significantly, van Laar generally fits better.

What is excess Gibbs energy gᴱ/RT?

It is a dimensionless measure of non-ideality for the mixture: gᴱ/RT = x₁ ln γ₁ + x₂ ln γ₂. Positive values indicate positive deviation from Raoult's law; negative values indicate negative deviation.

How do I find A₁₂ and A₂₁ for my mixture?

These constants are fitted from experimental vapor-liquid equilibrium (VLE) data. Published sources like the DECHEMA data series, Perry's Chemical Engineers' Handbook, or the reference table below provide common values.

Are these models valid over the full composition range?

Yes, but accuracy depends on how well the constants were fitted. Margules and van Laar are two-parameter models best suited to moderately non-ideal mixtures. For highly non-ideal or associating systems, NRTL or UNIQUAC are preferred.

Activity Coefficient Calculator

Name: Activity Coefficient Calculator
Availability: InStock
Author: Nham Vu

Calculate activity coefficients (γ₁, γ₂) for binary non-ideal liquid mixtures using Margules one-suffix, Margules two-suffix, or van Laar models.

Inputs

Model

Mole Fraction x₁ (0 to 1)

x₂ = 1 − x₁ is computed automatically.

Margules Constant A (dimensionless)

Enter values and click Calculate.

Common Binary Mixture Constants

Values are for the van Laar / two-suffix Margules model at approximately 25 °C unless noted. Component 1 is listed first.

Component 1	Component 2	A₁₂	A₂₁	Model
Ethanol	Water	1.6798	0.9227	Van Laar
Acetone	Chloroform	0.8206	1.3376	Van Laar
Ethanol	Benzene	1.7500	1.7500	Margules (sym.)
Acetone	Water	0.5990	0.7480	Van Laar
n-Hexane	Ethanol	2.0800	1.9200	Van Laar
Benzene	Toluene	0.4200	0.4200	Margules (sym.)
Methanol	Water	1.5240	1.6348	Van Laar

Click any row to load its constants into the calculator. A₁₂ = A₂₁ for symmetric systems (use Margules one-suffix with A = A₁₂).

Summary

Calculate activity coefficients (γ₁, γ₂) for binary non-ideal liquid mixtures using Margules one-suffix, Margules two-suffix, or van Laar models.

How it works

Select a model: Margules one-suffix (symmetric), Margules two-suffix (asymmetric), or van Laar.
Enter mole fraction x₁ (0 to 1). x₂ is computed automatically as 1 − x₁.
Enter the required model constant(s): A for one-suffix, or A₁₂ and A₂₁ for two-suffix/van Laar.
Click Calculate to get γ₁, γ₂, activities a₁ and a₂, and excess Gibbs energy gᴱ/RT.
Compare your result against the reference table of common binary mixture constants.

Use cases

Verify non-ideality of ethanol-water or acetone-chloroform mixtures in VLE calculations.
Determine activity coefficients for distillation column design.
Estimate excess Gibbs energy for thermodynamic modeling of solution behavior.
Check whether a mixture exhibits positive (γ > 1) or negative (γ < 1) deviation from Raoult's law.
Validate experimental data against Margules or van Laar model predictions.

Frequently Asked Questions

Last updated: 2026-07-08 · Reviewed by Nham Vu