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

Halogen-free surface active and biamphiphilic ionic liquids 1-alkyl-3-methylimidazolium octylsulphates (C(n)mim C8SO4, n = 4, 6, 8, 10) were synthesized and their aqueous solution behaviour was studied using surface tension, conductance, H-1 nuclear magnetic resonance (H-1-NMR), and small angle neutron scattering (SANS). These catanionic type surfactants demonstrate high surface activity. SANS data revealed that C(4)mim C8SO4 and C(6)mim C8SO4 form spherical and ellipsoidal micelles in water, while those with long chain imidazolium cations (C(8)mim C8SO4 and C(10)mim C8SO4) depict limited aqueous solubility and form vesicles. The effect of addition of these ionic liquids on the phase and aggregation behaviour of nonionic surfactant Triton X-100 (TX-100) solution was investigated using cloud point (CP), H-1-NMR, zeta potential and scattering (DLS and SANS) techniques. These C(n)mim 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(n)mim C8SO4 in TX-100 micelles was investigated by NMR. The SANS data for the size/shape of aqueous TX-100 with C(4)mim C8SO4 and C(6)mim C8SO4 systems showed an ellipsoidal to smaller ellipsoidal micelle transition. Addition of C(8)mim C8SO4 and C(10)mim 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 C(10)mim 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(n)mim C8SO4 and TX-100 micelles desirably tailored by adding appropriate concentrations of ionic liquids.