Characterization of the aggregates of N-alkyl-N-methylpyrrolidinium bromide surfactants in aqueous solution

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

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 (bmimPF6). It is shown that the solubility of 'hydrophobic' bmimPF6 in aqueous TX-100 increases with TX-100 concentration. This observation combined with the conductivity data strongly indicates partitioning of bmimPF6 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% bmimPF6. Based on the overall data, it is inferred that ionic liquid bmimPF6 partitions into the TX-100 micellar phase; presence of bmimPF6 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 bmimPF6 and TX-100 is proposed to be the reason for these observations.

Rapid exploration of phase behavior in surfactant systems using flow in microchannels

The paper describes a study for the determination of the phase behavior of a self-assembling dilute aqueous cetyl trimethylammonium bromide (CTAB) and dodecyl benzene sulfonic acid (HDBS) system using flow in microchannels. The diffusional length scales of approximately 10-100 microm and volumes on the order of a few tens of nanoliters allow fast composition and temperature homogeneity compared to "bulk" experiments, where characteristic volumes and length scales are on the order of milliliters and centimeters, respectively. Fluorescence emission of a polarity-sensitive fluorophore was used with the surfactants for phase characterization. To demonstrate the validity of the new approach, the critical micelle concentrations (cmc) for CTAB and HDBS were first shown to agree with the cmc obtained in the literature under bulk conditions. Subsequently, the microstructures of dilute (less than 0.8 wt % total surfactant) aqueous mixtures of CTAB and HDBS were examined. The range of desired concentrations and accurate flow dilutions of the samples were achieved by imposing controlled pressure gradients across the channel network. Marked changes in slopes of fluorescence emission intensity versus composition were used to demarcate phase boundaries. A series of microstructures ranging from mixed micelles (M), vesicles (V), and giant vesicles (GV) was observed in the ternary CTAB/HDBS/water system. Experimental data from the microfluidic method was found to be consistent with the results obtained from bulk phase experiments using fluorescence, turbidity, dynamic light scattering, and cryogenic transmission electron microscopy.