For the first time, the Sprow and Prausnitz equation has been used to describe quantitatively the nature, properties and compositions of the surface layers in imidazolium-based ionic liquid binary mixtures. Bulk mole fraction, surface molar area and surface tension of pure component are necessary inputs for this equation. The UNIFAC group contribution method is also applied into account for the non-ideality into the bulk and surface of the solution. The average relative errors obtained from the comparison of experimental and calculated surface tension values for 16 binary systems are less than 4%. Therefore, the model has good accuracy in comparison with other predictive equations.
