Effect of cation substituent of dodecanesulfate-based anionic surface active ionic liquids on micellization: Experimental and theoretical studies

Abstract

Anionic surface active ionic liquid (SAIL), 1-butyl-3-ethoxycarbonylimidazolium dodecanesulfate ([Etbim][C12H25SO4]) was synthesized. The dependence of micellization of dodecanesulfate-based anionic SAILs on cation substituents of anionic SAILs was analyzed using experimental and theoretical studies. Critical micelle concentration of [Etbim][C12H25SO4] was 1.17 mmol/L at 298.15 K. [Etbim][C12H25SO4] showed superior surface property than [Bmim][C12H25SO4] with same dodecanesulfate anion.Theoretical density functional theory (DFT) calculation was used to determine the micellization tendency of anionic SAILs ([Bmim][C12H25SO4] and [Etbim][C12H25SO4]). CMC difference of [Etbim][C12H25SO4] and [Bmim][C12H25SO4] was ascribed to the electrostatic interaction (repulsion or attraction) and hydrophobic interaction of cations ([Bmim]+ and [Etbim]+), dodecanesulfate anion, anionic SAILs ([Bmim][C12H25SO4] or [Etbim][C12H25SO4]), and the interactions toward water molecules in aqueous solution. The hydrocarbon chain lengths of [Bmim]+ and [Etbim]+ reflected the different hydrophobic interaction of cations, which clarified that the important role of cation substituents on micellization of [Etbim][C12H25SO4] and [Bmim][C12H25SO4]. Atomic charge distribution of cations will play important role on the further hydration toward water molecules or electrostatic attraction toward [C12H25SO4]− anion. Typical hydrogen bonds and charge changes of typical atoms were beneficial for electrostatic attraction and electrostatic repulsion between polar groups of SAILs. The electrostatic repulsion and electrostatic attraction between polar groups of SAILs affected the CMC values of [Bmim][C12H25SO4] and [Etbim][C12H25SO4]. Changes of binding energy (D0) and difference of binding energy (ΔD0) confirmed the tendency of [Etbim][C12H25SO4] micelle formation was higher than that of [Bmim][C12H25SO4].