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 (C13-15E(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 mum) 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 mum) 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.