Experimental and DFT studies on micellization features of anionic surface active ionic liquid and nonionic surfactant mixtures: Effect of imidazolium cations

The micellization characteristics of nonionic surfactant (Triton TX-100, p-octyl polyethylene glycol phenyl ether) and two anionic surface active ionic liquids (SAILs) were investigated by surface tension measurements, steady state fluorescence tests, dynamic light scattering (DLS), H Nuclear magnetic resonance, and density functional theory (DFT) calculations. The anionic SAILs were 1-ethoxycarbonyl-3-methylimidazolium dodecyl sulfate ([E tmim ][C 12 H 25 SO 4 ]) and 1-ethoxycarbonyl-3-butylimidazolium dodecyl sulfate ([E tbim ][C 12 H 25 SO 4 ]). CMC mix values of SAIL/TX-100 mixtures were between CMC values of individual surfactants, and decreased with the increase of the molar fraction of TX-100. The negative β values (interaction parameter) of SAIL/TX-100 mixtures indicated good synergistic effect amid SAIL/TX-100 mixtures. [Etbim][C 12 H 25 SO 4 ] showed stronger attractive interactions than [Etmim][C 12 H 25 SO 4 ] during SAIL/TX-100 micellization. The mixed micelles contained a larger molar fraction of TX-100 over the entire range of composition. At same molar fraction of TX-100, X 1 values of [Etbim][C 12 H 25 SO 4 ]/TX-100 mixtures were lower than those of [Etmim][C 12 H 25 SO 4 ]/TX-100 mixtures. With addition of SAIL, the particle sizes of SAIL/TX-100 micelles decreased. With the increase of SAIL concentrations, the particle sizes of SAIL/TX-100 micelles decreased. In presence of SAILs ([E tmim ][C 12 H 25 SO 4 ] or [Etbim][C 12 H 25 SO 4 ]), all protons of TX-100 moved downfield. The chemical shifts of protons in [Etmim][C 12 H 25 SO 4 ] or [Etbim][C 12 H 25 SO 4 ] slightly moved upfield. DFT simulations demonstrated the presence of hydrogen bonds between the imidazolium cation of SAILs and EO chain of TX-100, and between sulfate anion group(-SO 4 -) of SAILs and hydrogen atoms in TX-100. The information will shed light on the rapid screening of novel anionic SAIL/surfactant mixtures for practical applications.