(350f) Rapid Exploration of Phase Behavior in Surfactant Systems Using Microfluidics

A new method for rapidly determining the phase behavior of surfactants using flow in microchannels is described. The small volumes (nanloliters), lower length scales (10-100?Ým) and delicate control of dilution characteristic of microfluidic flow implies fast equilibration times and temperature homogeneity as well as accurate composition determination. Thus, compared to °¥bulk' experiments where characteristic volumes and length scales are of the order of milliliters and centimeters respectively, the determination of phase behavior is rapid and accurate. As an alternative approach, fluorescence measurements were performed to examine the microstructures of aqueous cationic(cetyl trimethyl ammonium bromide, CTAB)/anionic(dodecyl benzene sulfonic acid, HDBS) surfactant system in the dilute region. It is demonstrated that a polarity sensitive fluorophore prodan can be used to characterize the supramolecular aggregates of the ternary system of CTAB-HDBS-H2O formed in microchannels. The CTAB and HDBS solutions are diluted with water in the microfluidic chip by imposing pressure gradients across a channel network. The concentration and flow dilutions of the samples are evaluated using mass and momentum balances in the pressure driven microchannel flow. The CMC for each of these surfactants is measured and confirmed. A wide range of microstructures ranging from spherical micelles (M), vesicles (V) and giant vesicles (GV) were observed in the ternary CTAB/HDBS/water system. Fluctuations at phase boundaries cause distinct changes in fluorescence intensities. Experimental data is compared with the results obtained from °¥bulk' phase experiments using fluorescence, time resolved turbidity, light scattering and cryogenic transmission electron microscopy. Experiments were also performed to understand the effect of surface charge and fluorophore concentration on aggregation process.