Self-aggregation of nonionic surfactants in imidazolium-based ionic liquids with trifluoromethanesulfonate anion

Abstract

Self-aggregation of polyoxyethylene (POE)-type nonionic surfactants in ionic liquids, 1-butyl- and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (bmimCF3SO3 and emimCF3SO3), was investigated by means of 1H-NMR chemical shift, dynamic light-scattering (DLS), and surface tension measurements. The surfactants showed no definite aggregate formation in bmimCF3SO3. This shows a remarkable contrast to the previous observation in bmimBF4 and bmimPF6, and demonstrates an importance of anion species to determine the property of ionic liquids as a solvent to support the self-assembly of amphiphilic compounds. On the other hand, the surfactants formed micelles in emimCF3SO3, which shows an importance of alkyl chain attached to imidazolium ring to determine the solvophobic interaction between surfactant hydrocarbon chains in imidazolium-based ionic liquids. The low solvophobicity of the surfactants to the ionic liquid composed of imidazolium cation with long alkyl chain is attributed to an affinity of the surfactant hydrocarbon chain to the imidazolium alkyl chain. The values of micellization parameters and surface adsorption parameters obtained for the surfactant solutions in emimCF3SO3 are reported.