Phase behavior and microstructure of aqueous mixtures of cetyltrimethylammonium bromide and sodium perfluorohexanoate

The phase behavior and microstructure of aqueous mixtures of cetyltrimethylammonium bromide (C₁₆TAB) and sodium perfluorohexanoate (FC₅) were investigated as a function of total surfactant concentration, mixing ratio, and temperature. The aggregation behavior of both the pure components and several mixtures was studied via surface tension experiments, and analysis of the results using the regular solution model yielded a β value of -19.4, indicating highly nonideal mixing. The evolution of phase behavior from the FC₅-water binary axis with the addition of C₁₆TAB was of particular interest. Below the critical micelle concentration or cmc of FC₅ (5.70 wt %), the addition of small amounts of C₁₆TAB leads to the formation of large structures, including vesicles, between surfactant concentrations of 2 and 4 wt %. Above the cmc, FC₅ forms small globular micelles in solution. The addition of C₁₆TAB to the FC₅ micelles leads to an increase in viscosity and eventually phase separation into two rodlike micellar solutions, one enriched in FC₅ and the other enriched in C₁₆TAB. The microscopic structure of the vesicles and micelles was probed using small-angle neutron scattering, and the experimental data was analyzed with two complimentary methods. The vesicles were found to be polydisperse with average sizes ranging from 500 to 800 Å (determined from light scattering) and a bilayer thickness of about 29 Å. Consistent with the viscosity results, neutron-scattering measurements showed that the rodlike micelles undergo one-dimension growth and become more flexible near the phase boundary. Results from an indirect transform of the scattered intensity indicate that near the center of the aggregate cross section there is a region consisting solely of the hydrocarbon surfactant.