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 (NaD oBS) in dilute aqueous solution can be transformed into lamellar aggre gates by the addition of alkali metal chloride (LiCl to CsCl) and tetr aalkylammonium chloride (alkyl is methyl or n-butyl) salts. Depending on the type of cation, concentration of salt, and isomeric purity of t he alkyl chain of the surfactant, different types of phases are observ ed: large unilamellar vesicles, multivesicular vesicles, and flocculat ed multilamellar vesicles (lamellar droplets). Over limited concentrat ion ranges, some salts induce phase separation in a surfactant-rich an d a surfactant-lean phase. The formation of the different phases was m onitored by turbidity and fluorescence depolarization measurements, wh ereas the phases were characterized by light microscopy, freeze-fractu re electron microscopy, and confocal scanning laser microscopy. Thermo dynamic aspects of aggregation, in particular the counterion binding c haracteristics, were studied by microcalorimetry and conductivity. On a molecular level, the packing in a lamellar array can be explained la rgely in terms of a change in counterion binding and, to a lesser exte nt, by a decrease of the hydration of the headgroup and the. counterio n. A better counterion binding is facilitated by a less hydrated catio n or by an increase of the electrolyte concentration. The formation of different types of lamellar aggregates is due to different types of i nteractions between lamellar layers or between aggregates: largely rep ulsive for stable dispersions of unilamellar vesicles to attractive do wn to short distances for the flocculated lamellar droplets.