Aggregation of 1-alkyl-3-methylimidazolium octylsulphate ionic liquids and their interaction with Triton X-100 micelles

Abstract

Halogen-free surface active and biamphiphilic ionic liquids 1-alkyl-3-methylimidazolium octylsulphates (C_nmim C₈SO₄, n = 4, 6, 8, 10) were synthesized and their aqueous solution behaviour was studied using surface tension, conductance, ¹H nuclear magnetic resonance (¹H-NMR), and small angle neutron scattering (SANS). These catanionic type surfactants demonstrate high surface activity. SANS data revealed that C₄mim C₈SO₄ and C₆mim C₈SO₄ form spherical and ellipsoidal micelles in water, while those with long chain imidazolium cations (C₈mim C₈SO₄ and C₁₀mim C₈SO₄) depict limited aqueous solubility and form vesicles. The effect of addition of these ionic liquids on the phase and aggregation behaviour of non-ionic surfactant Triton X-100 (TX-100) solution was investigated using cloud point (CP), ¹H-NMR, zeta potential and scattering (DLS and SANS) techniques. These C_nmim C₈SO₄ ionic liquids show remarkable changes in the CP of TX-100 aqueous solutions and micelle size depending on the alkyl chain of the cation and anion. The site of C_nmim C₈SO₄ in TX-100 micelles was investigated by NMR. The SANS data for the size/shape of aqueous TX-100 with C₄mim C₈SO₄ and C₆mim C₈SO₄ systems showed an ellipsoidal to smaller ellipsoidal micelle transition. Addition of C₈mim C₈SO₄ and C₁₀mim C₈SO₄ in TX-100 showed a micelle transition of ellipsoidal to smaller ellipsoidal at lower concentration and ellipsoidal to extended ellipsoidal at higher concentration, while addition of 100 mM C₁₀mim C₈SO₄ in TX-100 formed vesicle structures. The obtained results are discussed in terms of the formation of charged mixed micelles of TX-100 with C_nmim C₈SO₄ and TX-100 micelles desirably tailored by adding appropriate concentrations of ionic liquids.