About the nanostructure of the ternary system water – [BMIm]PF6 – TX-100

Abstract Hypothesis Many efforts have been made to formulate water-IL microemulsions. One of the most intensely studied systems is H 2 O – 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF 6 ) – 4-octylphenol polyethoxylate (TX-100) and it is not questioned that this system forms microemulsions. The nanostructures observed for traditional microemulsions are postulated with the surfactant being adsorbed at the interface such that the hydrophilic EO sides intrude into the water domains, while the hydrophobic hydrocarbon chains are immersed into [BMIm]PF 6 . However, the high polarity of [BMIm]PF 6 and the observation that [BMIm]PF 6 mixes well with oligoethylene oxides but hardly with non-polar solvents like toluene or alkanes are not in line with this picture. Experiments We re-studied the ternary system H 2 O – [BMIm]PF 6 – TX-100 by measuring phase diagrams, determining tie-lines, and carrying out ROESY NMR and PFG NMR measurements. Findings We found that the hydrophobic part of the surfactant interacts neither with water nor with [BMIm]PF 6 , while both solvents interact with the hydrophilic part of the surfactant. We suggest that the surfactant is not adsorbed at the interface between water and the IL, but forms normal spherical or elongated micelles or even continuous aggregates with the hydrocarbon chains forming the interior of the aggregates.