Salt-Induced Transition from a Micellar to a Lamellar Liquid Crystalline Phase in Dilute Mixtures of Anionic and Nonionic Surfactants in Aqueous Solution

In dilute mixtures of anionic surfactant, sodium dodecylbenzenesulfonate (NaDoBS), and nonionic Poly(ethylene oxide) alkyl monoether (C_13–15E_〈7〉) a transition from a micellar to a lamellar phase is found at high salting-out electrolyte (NaCit) concentrations. With an increase of the salt concentration, different types of lamellar aggregates are formed. At intermediate salt concentrations large uni- and multilamellar vesicles (with diameters up to 40 µm) with flexible bilayers are formed spontaneously, next to smaller aggregates. At still higher salt concentrations, this is followed by a stage of only small aggregates (with diameters of several micrometers), finally leading to a lamellar phase in the form of droplets (up to 10 µm) at high (more than 16 wt %) salt concentration. The existence of different types of aggregates is reflected by changes of the turbidity of the solutions. Light and fluorescence microscopy, freeze-fractured electron microscopy, confocal scanning laser microscopy (CSLM), and fluorescence depolarization were employed to characterize the aggregates and to deduce a mechanism for the transition from a micellar to a lamellar phase.