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About the nanostructure of the ternary system water - [BMIm]PF6 - TX-100
Harun Bilgili1, Matthias Bürger1, Cosima Stubenrauch1
1Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany.
Hypothesis:
Many efforts have been made to formulate water-IL microemulsions. One of the most intensely studied systems is H2O - 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF6) - 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]PF6. However, the high polarity of [BMIm]PF6 and the observation that [BMIm]PF6 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 H2O - [BMIm]PF6 - 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]PF6, 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.
