Micellar transitions in the aqueous solutions of a surfactant-like ionic liquid: 1-butyl-3-methylimidazolium octylsulfate

Abstract

We have observed dual transitions in various physical properties while investigating the surfactant-like behavior of the ionic liquid (IL) 1-butyl-3-methylimidazolium octylsulfate [C₄mim][C₈SO₄] in aqueous media. Interestingly, in conductivity measurements, it was found that the second transition, which is comparatively weak in pure water, can be modulated and tuned to the desired concentration without affecting the first transition by the addition of a suitable electrolyte. Using an array of techniques, such as conductivity, ultrasonics, TEM, cryo-TEM, DLS, ¹H NMR and 2D ¹H–¹H NOESY, it has been revealed that the first transition corresponds to the anionic aggregation with imidazolium cations adsorbed as counterions, and the second transition corresponds to the restructuring of the initially formed aggregates in a way that the alkyl chain of the imidazolium cation is incorporated towards the micellar core, leading to a unique mixed micelle-type structure in a single IL system.