Study on surface adsorption from cationic surfactant-electrolyte mixed aqueous solution including BF₄^- ion

The surface tension of aqueous mixtures of dodecyltrimethylammonium tetrafluoroborate (DTABF₄) and sodium tetrafluoroborate (NaBF4) was measured as a function of total molality and composition of DTABF₄ at 298.15 K. The results were analyzed by originally developed thermodynamic equations and compared with those of dodecyltrimethylammonium bromide (DTAB)-sodium bromide (NaBr) mixed system. It was indicated that BF ₄^- ions reduce the repulsion between DTA⁺ ions more effectively than Br^- ions in the adsorbed film. To investigate this difference more closely, the surface tension of DTAB-NaBF₄ and DTABF₄-NaBr mixed system was also measured. The data analysis revealed that BF₄^- ions are adsorbed positively even for the pure NaBF₄ system and preferentially to Br^- ions in these mixtures. Furthermore, it was concluded that the side-by-side arrangement suggested in the adsorbed film of 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF₄) is due to not only the positive adsorption of BF ₄^- ions but also the capability of hydrogen bond formation between imidazolium ion and BF₄^- ions.