Aerosol-OT in water forms fully-branched cylindrical direct micelles in the presence of the ionic liquid1-butyl-3-methylimidazolium bromide

Abstract

A recent investigation on the sodium bis(2-ethylhexyl)sulfosuccinate/water/1-butyl-3-methylimidazolium tetrafluoborate (NaAOT/W/bmimBF₄) system showed that the anionic two-tailed surfactant NaAOT, that is known to form reverse micelles or planar interfaces (typically lamellar liquid crystals), can originate discrete spherical micelles of normal curvature because of strong interactions with the ionic liquid. The goal of the present paper was to detect macro- and microscopic modifications within such a system upon substitution of the ionic liquid's counter-ion tetrafluoroborate with bromide. Firstly, the phase diagram of the NaAOT/water/bmimBr system was determined. Then, the monophasic regions were investigated by means of NMR self-diffusion and SAXRD experiments. The results obtained proved this system to be surprisingly different from that containing bmimBF₄. This study focused mainly on the characterization of the micellar region, which turned out to be constituted of a bicontinuous nanostructure. This finding can be accounted for suggesting a decreasing of the NaAOT effective surfactant packing parameter, as in the case of NaAOT/water/bmimBF₄ system, although the effect in the presence of Br⁻ is less pronounced. Data modeling showed the same degree of interfacial adsorption for the bmim⁺ cation in both systems, regardless of the particular counterion used—either BF₄⁻ or Br⁻. Thus, the remarkable differences between the two systems appear to be mainly due to a specific counterion effect. This result highlights once again the ions specificity, which is found ubiquitously in chemistry and biology.