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

The interaction between the cationic surfactant Hexadecyltrimethylammonium bromide (CTAB) and the ionic liquid 1-butyl-3-methylimidazolium octyl sulfate (bmim-octylSO(4)) in aqueous solution has been investigated at two total concentrations (0.5 and 10 mM) and different CTAB mole fractions (alpha(CTAB)). Samples of different physical appearance were obtained depending on a alpha(CTAB) suggesting different kind of aggregates. Bluish solutions (alpha(CTAB) from 0.1 to 0.6) were assigned to vesicles of the catanionic surfactant formed (CTA-octylSO(4)), whereas clear and isotropic solutions (alpha(CTAB) >0.6) corresponded to mixed micelles. Surface tension, electrical conductivity, absorbance and dynamic light scattering (DIS) 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 (gamma(cac)) 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-octylSO(4) catanionic surfactant. Once the catanionic surfactant is formed it makes no sense to consider anymore the initial CTABIbmim-octylSO(4) binary system but a new one composed by CTA-octylSO(4) and the excess of bmim-octylSO(4) or CTAB. On the basis of this consideration the surface activity parameters have been recalculated. (C) 2015 Elsevier B.V. All rights reserved.