Adsorption of ions on aqueous surfaces of NaBr solutions of dodecyldimethylammonium chloride

Abstract

The surface tension of aqueous NaBr solutions of dodecyldimethylammonium chloride has been measured by the drop weight method at different surfactant and NaBr concentrations. In order to apply the Gibbs adsorption isotherm for this indifferent system, the surface tension of these solutions containing differentially added NaCl has also been measured. From these results the surface excess densities of surfactant ion (D +), Na +, Cl −, and Br − are derived as functions of surfactant concentration at different NaBr concentrations, assuming the ideal solutions below the critical micelle concentration. When the NaBr concentration is lower than 0.05 M, D + (or Br −) is more preferentially adsorbed on the surface than Na + (or Cl −), and Br − (or D +) penetrates into the surface, while Cl − (or Na +) is expelled from there. When the NaBr concentration is higher than 0.02 M, the composition of anion on the surface is identical with that in the bulk, and D + and Na + are intrinsically adsorbed on the surface, in which the adsorption of Na + is negative. The specific adsorption of ions in the common anion ternary system is also derived from the present results and compared with the results of previous experiments.