Effect of added ionic liquid on aqueous Triton X-100 micelles

Abstract

Addition of ionic liquids to aqueous surfactant solutions can alter/modify physicochemical properties of such systems in favorable manner. Changes in the properties of aqueous solutions of a useful nonionic surfactant Triton X-100 (TX-100) are assessed upon addition of 2.1 wt% of a common and popular ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF 6). It is shown that the solubility of ‘hydrophobic’ bmimPF 6 in aqueous TX-100 increases with TX-100 concentration. This observation combined with the conductivity data strongly indicates partitioning of bmimPF 6 into TX-100 micellar phase. Behavior of a variety of molecular absorbance [methyl orange, phenol blue, and N, N-diethyl-4-nitroaniline] and fluorescence [phenyl on the TX-100, pyrene, pyrene-1-carboxaldehyde, 2-( p-toluidino)naphthalene-6-sulfonate, and 1,3- bis-(1-pyrenyl)propane] probes further confirm this observation. Statistically insignificant increase in critical micelle concentration (cmc) and decrease in aggregation number ( N agg ) of TX-100 micelles are observed upon addition of 2.1 wt% bmimPF 6. Based on the overall data, it is inferred that ionic liquid bmimPF 6 partitions into the TX-100 micellar phase; presence of bmimPF 6 both close to the core as well as in the palisade layer of TX-100 micelles is suggested. Presence of favorable interactions (e.g., H-bonding, dipole-induced dipole, among others) between bmimPF 6 and TX-100 is proposed to be the reason for these observations.