Hexadecyltrimethylammonium bromide (CTA-Br) and 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF) 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-BF) 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 (α) 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-BF), together with the salt formed by the respective counterions bmimand Br in solution. For α≠0.5, furthermore of the mentioned compounds, the spare bmim-BF (for α<0.5) or CTA-Br (for α>0.5), are also present in the aqueous solution. Systems containing excess of bmim-BF show a low critical aggregate concentration (cac), but an unexpected high surface tension at cac (γ≈53-56mN/m), as pure CTA-BF. For systems containing excess of CTA-Br, cac increases but γ decreases up to 36mN/m. Mixtures of pure CTA-BF and bmim-BF or CTA-Br behave as typical binary surfactant systems.