Self-aggregation of ionic liquids: micelle formation in aqueous solution

Abstract

Interfacial tension (using a drop-shape analysis technique), fluorescence (of a widely used spectroscopic molecular probe, pyrene), and ¹H NMR measurements were used to monitor the adsorption at the aqueous solution–air interface and self-aggregation behaviour (critical micelle concentration, CMC) of room-temperature ionic liquids (ionic liquids) of the 1-alkyl-3-methylimidazolium family of cations, [C_nmim]⁺, with different linear alkyl chain lengths, C_nH_2n+1 (½n = 1–7), and different counter-ions, namely [C_nmim]Cl (n = 2–14), [C_nmim][PF₆] (n = 4 or 10), and [C₁₀mim][NTf₂]. Only [C_nmim]Cl with n > 8 unambiguously form aggregates in solution and the nature of this self-aggregation is discussed in terms of the electrostatic vs. hydrophobic contributions of the isolated cation. In contrast, the shortest chains behave, as anticipated, as simple salts. In turn, the transitional ionic liquid, [C₆mim]Cl is able to develop a monolayer at the aqueous solution–air interface but shows no noticeable self-aggregation in the bulk fluid. Moreover, the micellar characteristics of the well-studied sodium dodecyl sulfate (SDS) aqueous solutions as a function of the total concentration of [C_nmim]Cl (½n = 1–7) showed a clear change in the behaviour of the mixtures [C_nmim]Cl + SDS for n ≈ 6–8, with a characteristic mixed-micelle formation for the longer and a pure salt effect for the shorter chain lengths of [C_nmim]Cl.