Hexadecyltrimethylammonium bromide (CTA-Br) and 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF 4 ) in aqueous solution: An ephemeral binary system.

HYPOTHESIS: Mixtures of the cationic surfactant hexadecyltrimethylammonium bromide (CTA-Br) and the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF4 ) in aqueous solutions are expected to behave as typical binary cationic surfactant system taking into account the surface activity displayed by the ionic liquid, instead of considering the IL as a water cosolvent.

EXPERIMENTS: Surface tension and conductivity measurements have been conducted as a function of the total concentration of the mixtures at different surfactant mole fraction (αCTA-Br ) to investigate the surface active properties.

FINDINGS: Turbidity immediately appearing when the compounds are mixed in water suggests the spontaneous formation of the low soluble compound hexadecyltrimethylammonium tetrafluoroborate (CTA-BF4 ), together with the salt formed by the respective counterions bmim+ and Br- in solution. For αCTA-Br ≠0.5, furthermore of the mentioned compounds, the spare bmim-BF4 (for αCTA-Br <0.5) or CTA-Br (for αCTA-Br >0.5), are also present in the aqueous solution. Systems containing excess of bmim-BF4 show a low critical aggregate concentration (cac), but an unexpected high surface tension at cac (γcac ≈53-56mN/m), as pure CTA-BF4 . For systems containing excess of CTA-Br, cac increases but γcac decreases up to 36mN/m. Mixtures of pure CTA-BF4 and bmim-BF4 or CTA-Br behave as typical binary surfactant systems.