MICELLE TO LAMELLAR AGGREGATE TRANSITION OF AN ANIONIC SURFACTANT IN DILUTE AQUEOUS-SOLUTION INDUCED BY ALKALI-METAL CHLORIDE AND TETRAALKYLAMMONIUM CHLORIDE SALTS

Micelles of the anionic surfactant sodium dodecylbenzenesulfonate (NaDoBS) in dilute aqueous solution can be transformed into lamellar aggregates by the addition of alkali metal chloride (LiCl to CsCl) and tetraalkylammonium chloride (alkyl is methyl or n-butyl) salts. Depending on the type of cation, concentration of salt, and isomeric purity of the alkyl chain of the surfactant, different types of phases are observed: large unilamellar vesicles, multivesicular vesicles, and flocculated multilamellar vesicles (lamellar droplets). Over limited concentration ranges, some salts induce phase separation in a surfactant-rich and a surfactant-lean phase. The formation of the different phases was monitored by turbidity and fluorescence depolarization measurements, whereas the phases were characterized by light microscopy, freeze-fracture electron microscopy, and confocal scanning laser microscopy. Thermodynamic aspects of aggregation, in particular the counterion binding characteristics, were studied by microcalorimetry and conductivity. On a molecular level, the packing in a lamellar array can be explained largely in terms of a change in counterion binding and, to a lesser extent, by a decrease of the hydration of the headgroup and the. counterion. A better counterion binding is facilitated by a less hydrated cation or by an increase of the electrolyte concentration. The formation of different types of lamellar aggregates is due to different types of interactions between lamellar layers or between aggregates: largely repulsive for stable dispersions of unilamellar vesicles to attractive down to short distances for the flocculated lamellar droplets.