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.