Interaction of imidazolium based ionic liquids with Triton X-100 micelles: investigating the role of the counter ion and chain length

Abstract

The micellar characteristics of an aqueous solution of the polyoxyethylene based alkyl aryl type non-ionic surfactant Triton X-100 (TX-100) in the presence of several 1-alkyl-3-methylimidazolium (C_nmim⁺) based non-amphiphilic, amphiphilic and biamphiphilic ionic liquids (ILs) varying in the alkyl chain length of the cation (C₄–C₁₀) and different anions viz. chloride [Cl⁻], tetrafluoroborate [BF₄⁻], trifluoromethanesulfonate [CF₃SO₃⁻], hexaflurophosphate [PF₆⁻] and octylsuphate [C₈SO₄⁻] have been systematically investigated by cloud point (CP), viscosity, dynamic light scattering (DLS), nuclear magnetic resonance (NMR), fluorescence and small-angle neutron scattering (SANS) measurements. These ILs interact with TX-100 micelles on account of their hydrophobicity; ILs with a hydrophilic counter ion (Cl⁻) favour demicellization, increase CP, decrease the solution viscosity and apparent hydrodynamic diameter (D_h) of micelle, whereas ILs with a hydrophobic counter ion (PF₆⁻) increase the solution viscosity and exhibit micellar growth. SANS data on the size/shape of TX-100 micelles in the presence of different ILs are reported to provide information on microstructural changes in the micelles. The location of ILs, which has a dominating effect on the structure of the micelles, was evaluated using fluorescence and ¹H NMR studies, and these results are discussed in terms of the insertion of the ILs in TX-100 micelles. IL molecules with long alkyl chains penetrate deeper in the TX-100 micelles. The present study provides useful information on tuning the solution characteristics of TX-100 in the presence of ILs that may have industrial applications.