Self-organization of mixtures of sodium oleate and imidazolium based surface active ionic liquids studied by tensiometry, rheology and neutron scattering

We report on the aggregation behavior of the mixed systems composed of sodium oleate (NaOl) with various imidazolium based ionic liquids (ILs) using tensiometry, rheometry and small angle neutron scattering (SANS). The effect of alkyl chain in 1-alkyl-3-methylimidazolium chloride, ([Cnmim][Cl], where n = 4 to 10) showed that ILs with long chain length (~ C10) exhibit spontaneous transition of micelles into vesicles. The comparison in ILs with same alkyl chain (C8) but with different counterions viz. chloride [Cl−], tetrafluoroborate [BF4−] and hexafluorophosphate [PF6−] showed that IL with hydrophilic ion Cl− exerts micellar transition while hydrophobic PF6− showed no noticeable effect. However, an interesting finding in this paper is the unusual behavior of IL with moderately hydrophobic BF4− that transforms micelles into vesicles. Moreover, mixed micellar systems of NaOl and IL transform into vesicular solution when pH condition turns into acidic. Tensiometric measurements disclosed strong synergistic interactions between the oppositely charged ILs and NaOl. The information on the modification of micellar characteristics of NaOl/ILs mixed system is useful for industrial applications.