ELECTROSTATIC EFFECTS ON THE PHASE-BEHAVIOR OF AQUEOUS CETYLTRIMETHYLAMMONIUM BROMIDE AND SODIUM OCTYL SULFATE MIXTURES WITH ADDED SODIUM-BROMIDE

Electrostatic interactions play a prominent role in setting the phase behavior of mixtures of oppositely Charged surfactants and lead to the formation of a variety of microstructures, including a novel equilibr ium vesicle phase. Addition of a monovalent salt alters the electrosta tic contribution to the free energy of aggregation and thus changes th e equilibrium phase behavior and aggregate properties. Here we present observations for the catanionic mixture cetyltrimethylammonium bromid e (CTAB) and sodium octyl sulfate (SOS) with added sodium bromide (NaB r). The phase behavior of this surfactant system changes markedly when electrolyte is added. At certain compositions, there is a vesicle-to- micelle transition with increasing salt concentration, and surface cha rge density measurements, deduced from electrophoretic light scatterin g experiments, show that aggregate composition changes with added elec trolyte. A thermodynamic cell model has been developed, supported by a new method of calculating vesicle electrostatic free energy, to quant ify the various free energy contributions to aggregation. The cell mod el provides insight into how solution conditions such as composition a nd ionic strength affect phase behavior and related properties.