Study on surface adsorption from cationic surfactant-electrolyte mixed aqueous solution including BF 4 - ion

The surface tension of aqueous mixtures of dodecyltrimethylammonium tetrafluoroborate (DTABF(4)) and sodium tetrafluoroborate (NaBF4) was measured as a function of total molality and composition of DTABF(4) 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 DTABF(4)-NaBr mixed system was also measured. The data analysis revealed that BF (4) (-) 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 BF (4) (-) ions.