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

The M^ vs. X^2H curve (solid line) is shifted to the left hand side from the corresponding ideal mixing (dashed line) curve, which indicates a better miscibility of Br−1 and BF4- counterions in the adsorbed film. [Display omitted] ► 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. To investigate the counterion effects, we employed dodecyltrimethylammonium bromide (DTABr)–dodecyltrimethylammonium tetrafluoroborate (DTABF4) mixed aqueous solutions and measured their surface tensions, then analyzed these data in a thermodynamic treatment. The tensiometry showed that DTABF4 was more effective in lowering the surface tension of water. The phase diagram of adsorption demonstrated that the surface was enriched with BF4- ions, but the composition of Br− ions in the adsorbed film was slightly enhanced compared to the ideal mixing criteria. These were explained in terms of the size and polarizability of counterions. Moreover, the distribution of counterions of the DTABr–DTABF4 mixtures in the adsorbed film is greatly different from that of the 1-hexyl-3-methylimidazolium bromide (HMIMBr)–1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4) mixtures, where a stronger hydrogen-bonding exists between BF4- and HMIM+ ions. These findings suggest that the adsorption of counterions in electric double layers is likely subject to two factors: the nature of counterion and their interactions with surfactant ions.