Specific counterion effect on the adsorbed film of cationic surfactant mixtures at the air/water interface

► BF4- ions were richer in the surface than in the bulk solution. ► The different size of counterions caused their better miscibility in adsorbed films. ► Composition of Br− in the surface was higher than predictions based on ideal mixing. ► Dehydration free energy may be the principal factor governing the adsorption of ions. ► Specific forces and natures of counterions should be added to the Poisson-Boltzmann theory.