Catanionic surfactant formation from the interaction of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and the ionic liquid 1-butyl-3-methylimidazolium octyl sulfate (bmim-octyl SO4) in aqueous solution

Abstract

The interaction between the cationic surfactant Hexadecyltrimethylammonium bromide (CTAB) and the ionic liquid 1-butyl-3-methylimidazolium octyl sulfate (bmim-octylSO4) in aqueous solution has been investigated at two total concentrations (0.5 and 10mM) and different CTAB mole fractions (αCTAB). Samples of different physical appearance were obtained depending on αCTAB suggesting different kind of aggregates. Bluish solutions (αCTAB from 0.1 to 0.6) were assigned to vesicles of the catanionic surfactant formed (CTA-octylSO4), whereas clear and isotropic solutions (αCTAB>0.6) corresponded to mixed micelles. Surface tension, electrical conductivity, absorbance and dynamic light scattering (DLS) measurements were performed with the aim of correlating properties changes with the aggregation structures. The critical aggregation concentration (cac) and the surface tension value at the cac (γcac) were much lower for the mixtures than for the individual components. This finding reveals the exceptional surface activity of these mixtures, attributable to the formation of the CTA-octylSO4 catanionic surfactant. Once the catanionic surfactant is formed it makes no sense to consider anymore the initial CTAB/bmim-octylSO4 binary system but a new one composed by CTA-octylSO4 and the excess of bmim-octylSO4 or CTAB. On the basis of this consideration the surface activity parameters have been recalculated.