The Van Laar equation is a thermodynamic activity model, which was developed by Johannes van Laar in 1910-1913, to describe phase equilibria of liquid mixtures.
The original van der Waals parameters didn't give good description of vapor-liquid equilibria of phases, which forced the user to fit the parameters to experimental results.
Since these parameters didn't lead to good phase equilibrium description the model was reduced to the form: In here A12 and A21 are the van Laar coefficients, which are obtained by regression of experimental vapor–liquid equilibrium data.
The activity coefficient of component i is derived by differentiation to xi.
The model can not describe extrema in the activity coefficient along the concentration range.
, which implies that the molecules are of equal size but different in polarity, then the equations become: In this case the activity coefficients mirror at x1=0.5.
An extensive range of recommended values for the Van Laar coefficients can be found in the literature.